System Voltage Regulation

CHAPTER 7 SYSTEM VOLTAGE REGULATION

H. E. LOKAY

The primary objective of system voltage control is to system are discussed, as well as the characteristics of economically provide to each power user voltage that each application. The equipment is discussed by de- conforms to the voltage design limitations of the utiliza- scribing its method of operation and how it affects an tion equipment. Almost all utilization equipment is de- application. signed for use at a particular, definite terminal voltage: I. DEFINITIONS the nameplate voltage. It is economically impossible to provide each and every consumer on a distribution In discussing system voltage control, certain terminol- system with a constant utilization voltage correspond- ogy is naturally used. Following are common terms and ing to the nameplate voltage of the utilization devices. definitions used throughout this chapter. Other terms Voltage drop exists in each part of the power system that refer only to a particular section of this chapter are from the source to the consumer's service drop. Voltage defined in that particular section. drop also occurs in his interior wiring. Voltage drop Voltage drop—"Voltage drop (in a supply system) is is proportional to the magnitude and phase angle the difference between the voltage at the transmitting of the load current flowing through the entire power and receiving ends of a feeder, main, or service."' system. This essentially means that the consumer elec- The voltage drop is not necessarily the impedance trically closest to the source would receive a higher drop (IZ) of a feeder, main or service, but the difference voltage than the consumer most remote from the source. in absolute magnitude of the sending and receiving end Each of the consumers has practically the same utili- voltage. The IZ drop when added vectorily to the re- zation devices; therefore it is necessary to provide each ceiving end voltage equals the sending end voltage. with almost equal supply voltage. A compromise has Maximum voltage—"Maximum voltage is the great- been required between the allowable deviation from est five-minute average or mean voltage."2 utilization equipment nameplate voltage that is sup- Minimum voltage—"Minimum voltage is the least plied by the power system and the deviation above and five-minute average or mean voltage."2 below the nameplate voltage at which satisfactory Voltage spread—"Voltage spread is the difference equipment performance can still be obtained. If too between maximum and minimum voltages."3 broad limits were maintained by the power companies, With regard to power systems, voltage spread is the the cost for appliances and equipment would be high, difference between maximum and minimum voltage because they would have to be designed to operate which exist in a single voltage class under steady-state satisfactorily at any voltage within the limits. If the conditions. Voltage spread does not generally include voltage limits maintained were too narrow, the cost of the momentary voltage changes resulting from motor providing power would become too high. The com- starting—frequently called voltage dip. Voltage spreads promise that has resulted has proved satisfactory over at certain locations within a power system are discussed the past years, although it must be re-evaluated from further in Section 1. time to time as new utilization equipment or appliances Nominal voltage—"The nominal voltage of a circuit are made available. The widespread acceptance and or system is a nominal value assigned to a circuit or high saturation of television has resulted in a check of system of a given voltage class, for the purpose of con- the permissible limits previously held. The operating venient designation."2 voltage limits as determined by the compromise are The nominal voltage is intended to be a common listed in Table 1. Further discussion of the voltage designation for all systems whose operating voltages lie limits in the tables is in Sections 2-4. within the same general class. The value lies, in gen- System voltage regulation is essentially no more than eral, a little above the midpoint of the band of voltages maintaining the voltage at the consumer's service en- which fall within the same general class. trance within permissible limits by the use of voltage Rated voltage—"The rated voltage is the voltage at control equipment at strategic locations within the which operating and performance characteristics of system. This chapter discusses the application of this equipment are referred."' voltage control equipment within the distribution sys- The rated voltage of equipment is normally the name- tem. Application of voltage control equipment (regu- plate voltage • and the voltage at which optimum per- lating transformers, synchronous condensers, etc.) ap- formance would be realized. plied in the transmission system either for voltage Service voltage—"Service voltage is the voltage control or load flow is not included. The various measured at the terminals of the service entrance methods of controlling voltage in the distribution equipment."2 247

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The terminals of the service entrance equipment is 11. UTILIZATION VOLTAGES essentially the last point of the distribution system. The service entrance equipment is normally owned by the Utilization voltages—the voltage as measured at the consumer. This service point is not necessarily the kwhr terminals of the utilization equipment or at the con- meter socket, although for rural distribution systems it venience outlets—have a voltage spread which is inherent is often considered as that. to distribution system operation. A voltage band occurs Utilization voltage—"The utilization voltage is the at each point of utilization. The width of the band and voltage measured at the terminals of the machine or the location of the band with respect to the base voltage device."2 depends upon the location of the consumer with respect It is the voltage at any convenience outlet where an to the physical layout of the distribution system. Before appliance or device would be connected, or the voltage going into the acceptable voltage variations at the point at the terminals of large permanently located equip- of utilization for proper equipment operation and how ment. The utilization voltage is not to be confused with the variations affect equipment performance, the con- the service voltage, as it is a voltage less than the service cept of voltage spread should be clearly understood. voltage by the amount of the consumer's interior wiring voltage drop to the point of utilization. 1. Voltage Spread Base voltage—"Base voltage is a reference value Voltage spread—the difference between maximum and which is a common denominator to the nominal voltage minimum voltage at a particular point in the distribu- ratings of transmission and distribution lines, transmis- tion system—will vary in magnitude depending upon sion and distribution equipment, and utilization equip- the particular location within the system where the ment."4 spread is measured. Not only will it vary in magnitude, For example, the base voltage of a subtransmission but the relation of spread with reference to the base line having a nominal voltage rating of 34.5 kv is 115 value will vary depending on the point of measurement. volts; the base voltage of a distribution line having a An illustration of the voltage spreads occurring at the nominal voltage rating of 4.8 kv is 120 volts. A survey3 utilization point is shown in Fig. 1. Consumer A, which of the existing operating power companies showed that is the first consumer served by the feeder, has a voltage distribution lines and associated equipment having a spread of just one volt when going from light-load nominal voltage rating of from 2.4 kv to 14.4 kv have a (123v.) to heavy-load (122v.) conditions. Consumer B, 120-volt base while subtransmission or transmission which is the last consumer served by the distribution lines and associated equipment having a nominal volt- feeder, has a voltage spread of seven volts: 111 volts at age rating of from 23 kv to 230 kv have a 115-volt base. Voltage regulation—Voltage regulation is the per cent sues/yawl BUS VOLTAGE voltage drop of a line with reference to the receiving rC REGULATOR PRIMARY FEEDER end voltage. W:1841M 100(14 HE'D where, Per cent regulation — SECONDARY 4 l: 4 1Er1 SERVICE DISTRIBUTION FEEDER ONE-LINE DIAGRAM

E. = sending end voltage. 13

PRIMARY = receiving end voltage. 12

DISTRIBUTION 171ANSPOWER Voltage drop in any system component of a distribu- O ISO $225V FIRST tion system is often referred to as per cent voltage drop. CUSTOMER Normally the per cent value would be the same as cal- ,J$ 14 SERVICE B culated using Equation (1), but when referring to per HOUSE WIRING g cent voltage drop of the various system components in I ID III V distribution systems, they are all referred to the same VOLTAGE PROFILE AT LAST HEAVY LOAD CONDITIONS CUSTOMER base. Systems which have a base voltage of 120 volts (ILLUSTRATIVE ONLY) use this value as the per cent reference, and systems with a base voltage of 115 volts use that value. For example, a distribution feeder with a three per cent voltage drop would have an absolute voltage drop of 3.6 volts (3 X120/100) on a 120-volt base. If the feeder voltage was actually 2400 volts, the absolute voltage drop would be 72 volts (3.6 X 2400/120). Equation (1) would produce the same results only if the receiving end VOLTAGE PROFILE AT voltage is the base voltage. That is, if 'Ed is 120 volts LIGHT LOAD CONDITIONS for the 3.6 volts drop or 2400 volts for the 72 volts drop. (ILLUSTRATIVE ONLY) Throughout this chapter per cent voltage drop always Fig. 1—Diagrammatic illustration of utilization voltage refers to a 120-volt base. spreads on a typical primary feeder distribution circuit.

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heavy-load conditions to 118 volts at light-load condi- NOMINAL SYSTEM tions. The utilization voltage at consumers A or B for \ VOLTAGE load conditions between heavy and light load would have values somewhere between the maximum and LOWEST VOLTAGE ON I HIGHEST VOLTAGE ON SYSTEM UNDER I SYSTEM UNDER minimum values of their respective voltage spreads. NORMAL CONDITIONS I NORMAL CONDITIONS The voltage spread at the utilization point of any other consumer on the same feeder would have a voltage VOLTAGE SPREAD IN A SYSTEM I spread width somewhere between one and seven volts, depending upon their location. Similar voltage spreads 12 I MODE VOLTAGE are found at the utilization points of every consumer on the system. The average voltage spreads at the utilization points are generally wider for rural distribution feeders than residential feeders or urban feeders. Con- sumers on urban feeders generally have the smallest ff,1 average voltage spread because the feeders are shorter in length and the conductor sizes are larger. There is also a utilization voltage feeder voltage VOLTAGE spread; that is, the difference between the maximum ILLUSTRATIVE DISTRIBUTION OF SERVICE ENTRANCE and minimum utilization voltage of every consumer on VOLTAGES WITHIN THE VOLTAGE SPREAD the feeder. For the feeder of Fig. 1, the utilization voltage feeder voltage spread is twelve volts (123v. minus 111v.). Fig. 1 shows the highest voltage at the first con- COMP ANY A 1 sumer occurring at heavy load conditions (123v.). This COMPANY B is not always the case, as the highest voltage at the first consumer could occur at light load or at any load condi- COMPANY C tion. While Fig. 1 illustrates the voltage at the point of COMPANY utilization, the point of main interest to an operating COMPANY E I system is the service entrance. The voltage spread principle is the same for the service entrance and utilization I - TOTAL RANGE point. COMPARISON OF SERVICE ENTRANCE VOLTAGE In a particular system, the feeder service entrance SPREADS IN DIFFERENT SYSTEMS voltage spread will vary in magnitude from feeder to Fig. 2—Relationship between range of service entrance feeder. Continuing further, the system service entrance voltage and voltage spread for many systems. (Reproduced voltage spread will vary from one system to another. from Reference 3.) Fig. 2 shows the relationship between the range of system service entrance voltages and voltage spreads for many systems. On individual systems the service entrance voltage spread ranges from 4 to 15 volts, with range of voltage for each voltage level, which included nearly two-thirds of the operating systems having all normal operating voltages that were considered spreads of 7 to 10 volts, very few being greater, and the satisfactory on operating systems, was studied by a remaining one-third having spreads lying between 4 and joint committee of EEI and NEMA. The logical extent 6 volts.' It should be clearly understood that the above and nature of these ranges were determined and defined. figures refer to voltage spreads existing on systems as a For each voltage level the total operating range has whole and do not refer to voltage variations of any been divided into three zones, each of different im- individual consumer's service. portance, to establish guides for rating equipment. The For residential feeders, the feeder service entrance three zones are classified as the favorable, tolerable, and spread is the greatest around the early evening period extreme zone. when there is a considerable amount of electrical cook- Favorable Zone—This zone contains the majority of ing and lighting. Where a considerable amount of air the existing operating voltages. Systems should be de- conditioning exists on residential feeders, the greatest signed so that most of their operating voltages lie within spread may occur in the afternoon. For rural feeders, the this zone, and equipment should be designed and rated voltage spread is greatest during the daylight hours due so as to give completely adequate and efficient perform- to a considerable amount of motor load, although some ance throughout this zone. The normal equipment oper- rural feeders also have the greatest spread occur during ating characteristics will be slightly different through- the early evening. out the zone, but it should be adequate and satisfactory. Tolerable Zone—This zone includes operating voltages 2. Voltage Zones slightly above and below the favorable zone. The toler- For any specific voltage level, designated by a cer- able zone is necessary because from practical field contain nominal system voltage, there exists on any one ditions voltages slightly outside of the favorable zone system, inherent with system design, a range of operat- often result, and they must be recognized as normal ing voltages within the same voltage level. The existing operation, although not entirely desirable. Operation

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Table 1—System Voltages (Reproduced from EEI-NEMA Reports)

System Preferred Preferred Designation Nominal Voltage Zones in Terms of Base System Voltage Base Voltage Voltage (a) (f) Favorable Zone Tolerable Zone (d) (BV) Minimum 1 Maximum Minimum I Maximum VOLTAGES AT POINT OF UTILIZATION BY EQUIPMENT 1 BV 120 120 110 125 107 127 1/2 BV 120 120/240 110/120 125/250 107/214 127/254(c) 1/1.73Y BV 120 120/208Y 114/197Y 125/217Y 111/193Y(c) 127/220Y 2 BV 120 240 210 240 200 250 4 BV 120 480 420 480 400 500

5 BV 120 600 525 600 500 625 20 BV 120 2400 2200 2450 2100 2540 20/34.6Y BY 120 2400/4160Y 2200/3810Y 2450/4240Y 2100/3630Y 2540/4400Y 40 BV 120 4800 4400 4900 4200 5080 60 BV 115(b) 6900(b) 6300 6900 6000 7200 120(b) 7200(6)

PRIMARY VOLTAGES AT DISTRIBUTION TRANSFORMERS 20 BV 120 2400 2300 2500 2200 2600 20/34.6Y BV 120 2400/4160Y 2300/4000Y 2500/4330Y 2200/3810Y 2600/4500Y 40 BV 120 4800 4600 5000 4400 5200 60 BV 120 7200 6900 7500 6600 7800 40/69.3Y BV 120 4800/8320Y 4600/8000Y 5000/8660 4400/7620Y 5200/9000Y

100 BV 120 12000 11000 12500 10500 13000 60/103.9Y BV 120 7200/12470Y 6900/12000Y 7500/13000Y 6600/11450Y 7800/13500Y 63.5/110Y BV 120 7620/13200Y 7270/12600Y 7960/13800Y 7000/12100Y 8250/14300Y 110 BV 120 13200 12600 13800 12100 14300 120 BV 120 14400 13000 14500 12600 15000

VOLTAGES AT SUBSTATIONS AND ON SUBTRANSMISSION SYSTEMS 20 BV 120 2400 2300 2600 2200 2750 20/34.6Y BV 120 2400/4160Y 2300/4000Y 2600/4500Y 2200/3810Y 2750/4760Y 40 BV 120 4800 4600 5200 4400 5500 60 BV 120 7200 6900 7800 6600 8250 40/69.3Y BV 120 4800/8320Y 4600/8000Y 5200/9000Y 4400/7620Y 5500/9520Y

100 BV 120 12000 11000 13000 10500 13200 60/103.9Y BV 120 7200/12470Y 6900/12000Y 7800/13500Y 6600/11450Y 7920/13700Y 63.5/110Y BV 120 7620/13200Y 7270/12600Y 8250/14300Y 7000/12100'Y 8320/14500Y 110 BV 120 13200 12600 14300 12100 14500 120 BV 120 14400 13000 15000 12600 15500

200 BV 115 23000 (e) (e) 20400 25800 240 BV 115 27600 (e) (e) 24500 31000 300 BV 115 34500 (e) (e) 30600 38000 400 BV 115 46000 (e) (e) 40000 48300 600 BV 115 69000 (e) (e) 60000 72500 (Y) designates wye-connected. novels: (a) Except for the first and second lines. which indicate the usual single-phase Preferred Nominal Other Designations two- or three-wire ayatems, the figures in the third column refer to three-phase System Voltage for Identical Systems systems. Single values designate systems on which single-phase equipment is ordinarily connected from phase-to-phase; double values designate systems on which singlephuee equipment is ordinarily connected from phase-to-neutral. 120 110, 115 or 125 (b) Since utilization at this level is confined principally to 6600-volt motors in 120/240 110/220 or 115/230 large steel mills, minors, etc., a nominal designation of 6900 volts is commonly 120/208Y 115/199Y used and is included herewith, even though it is not consistent with other 240 220 or 230 nominal voltages in the utilization group. The voltage zones for this level are 480 440 or 460 likewise related to 6900 volts rather than to 7200 volt.. (a) Equipment to be used on both the 120/208Y and the 240-volt systems 600 550 or 575 must recognize the minimum voltage of the former and the maximum voltage 2400 2200, 2300 or 2500 of the latter. 2400/4160Y 3810 or 4000 (d) Equipment designed for the Favorable Zone will in general give fairly 4800 4400, 4600 or 5000 satisfactory operation throughout the Tolerable Zone, except that modification 7200 6600, 6900 or 7500 of loading or alternate designs may be required for certain types of equipment. 4800/8320Y 8000 (e) For systems with nominal voltage. above 14400 volts, designation of a "Favorable Zone" has been omitted as being somewhat less necessary than for 12000 11000 or 11500 the lower voltages. It should be considered, however, that the relationships and 7200/12470Y 11450, 11950 or 12000 definitions pertaining to the lower voltage, apply equally well to the higher 7620/13200.Y" vol tagea . 13200 13500 or 13800 (1) In order that other numerical designations for system voltages which are 14400 13200, 13800 or 14000 sometimes used may be interpreted In terms of the "Preferred Nominal System 23000 22000 or 24000 Voltages" values shown in Table 1, the following tabulation has been prepared. It should be emphasized that all of the different values on any ono line refer to 27600 26400 the same system as defined by its zone values in Table 1. Apparatus ratings 34500 33000 or 36000 indicated in other tables in this report in connection with the "Preferred 46000 44000 or 48000 Nominal System Voltages" apply, therefore, equally well where these other 69000 66000 or 72000 designations are used.

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within the extreme limits of this zone is generally min- Permissible system voltage spreads at the service imized on a particular system, both as to the number entrances set by the various Public Utility Commissions of locations within the system where such voltages throughout the country generally fall within the favor- appear and to the time duration in which they exist. able or tolerable zones. In many instances the permis- Equipment should give fairly satisfactory operation sible spread does not include the entire zone. Public throughout the entire zone, although at the low and Utility Commission requirements vary throughout the high ends of the zone, equipment operating characteris- country, with permissible voltage spreads ranging from tics may not be as good as obtained throughout the 6 to 10 per cent in the residential areas and from 8 to 12 favorable zone. per cent in the rural areas. The maximum permissible Extreme Zone—The extreme zone does not have any deviation from the nominal system voltage is generally set boundary limits, but it normally extends two or three limited to no more than 5 per cent above nominal or 5 per cent above and below the tolerable zone. Operating per cent below nominal; a few commissions permit a voltages occurring in this zone, above and below the maximum of 8 per cent above or below nominal. Com- tolerable zone, should be only temporary. That is, they mission limits for power customers (industrial) gen- should occur only during emergency periods such as erally permit a variation of 10 per cent above or below during fault conditions where power re-routing is nominal. necessary, or as a temporary measure during periods of construction. Utilization equipment should be able to 3. Utilization Equipment Voltage Ratings operate throughout the extreme zone, although it will The design (nameplate) voltage of utilization equip- likely involve exceeding the design limits of the equipment is generally matched to the utilization voltage ment. that occurs during the time when it is most frequently Table 1 shows the boundaries of the favorable and used. Appliances which are used for long duration and tolerable zones for various system voltages and at dif- during any load condition, light or heavy, have design ferent locations within a system. The boundaries have voltages the same as nominal system voltage. Appliances been based on surveys of existing system voltages, al- of shorter time usage and relatively high demands have though the voltage spread associated with any indi- a design voltage slightly less than nominal. Also, the vidual system may not cover the whole zone. voltage spread used as a basis of design for utilization Nominal system voltages are also included in Table 1. equipment will vary depending upon the time usage and They are listed as the preferred value and are even demand. For example, the small household cooking ap- multiples of the base voltage. The nominal voltage lies, pliances have a design voltage spread of 110 to 125 volts, in general, slightly above the midpoint of the voltage while the large household cooking appliances have a zones for the corresponding voltage level. Note (f) of design voltage spread of 107 to 122 volts. A table listing Table 1 lists other designations of system voltages the design voltage spread for most household, commer- which are in use throughout the country and differ from cial, and industrial appliances or utilization devices is the preferred nominal system voltages. included in Reference 3. The voltage zones in Table 1 are for various locations within the distribution system: the point of utilization, 4. Effect of Voltage Spread on Utilization Equipment high-voltage side of the distribution transformers, dis- Whenever the voltage applied to the terminals of a tribution substations and subtransmission. It is to be utilization device varies from the rated or nameplate remembered that the zone voltage boundaries for the voltage of the device, performance characteristics and first group in Table 1 are for the point of utilization and equipment life will also change. The extent of the change not at the consumer's service entrance. The consumer's may be minor or serious depending upon the device, interior wiring drop should be added to the lowest volt- how it is applied, and how much the terminal voltage age boundary value to obtain the favorable and tolerable deviates from the nameplate rating. NEMA Standards zone at the consumer's service entrance. The average for utilization equipment provide certain voltage toler- interior wiring drop is about three volts' during heavy ances, in that if operation takes place within the voltage load conditions. Therefore, for a nominal system voltage spread of the maximum and minimum tolerances little of 120 volts, the favorable zone at the consumer's serv- change in operating performance results. For utilization ice entrance would be from 113 to 125 volts, and the voltages applied in precise operations, however, there tolerable zone from 110 to 127 volts. may be a major sacrifice in performance even if the ter- Distribution systems should be designed and op- minal voltage is considerably less than the tolerances erated so that the service entrance voltage spread given in the NEMA Standards. will be within boundaries which will provide proper Incandescent Lamps—The light output and theoretical utilization voltage at the consumer's appliances and life of incandescent filament lamps are considerably equipment. The voltage zones suggested by the EEI- affected by variations in applied voltage. Characteristic NEMA joint committee represent much investigation curves for the commonly used, tungsten-filament, gas- of system operating practices, equipment performance, filled incandescent lamps are shown in Fig. 3. As and design characteristics, and are a compromise or shown, a 10 per cent reduction in rated lamp voltage re- balance between economical distribution system design sults in a light output (lumens) reduction to 70 per cent and utilization equipment design to provide satisfactory of rated output and a power consumption reduction to equipment operation at minimum cost. about 85 per cent of rated. A 15 per cent reduction in

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1- 18 / ,« corresponding one per cent change in lumen output results. With fluorescent lamps, voltage is also a factor in E

16 1 644 F starting, in that unsatisfactory or uncertain starting o. CURVES ARE BASED ON 6 LUMENS co) PER WATT At 100% VOLTAGE LI may result for an applied voltage of about ninety per la 14 / 564 cent or less of rated voltage. Excessive high voltage, ten

LUMENS TICAL LUMENS, per cent above rated, will cause overheating of the PER -- RE / mot- 481 ballast. a 12 , ' THEORETICAL .. LIFE ...... „,- ATTS THEO The theoretical life of the fluorescent lamp differs

AL from the incandescent lamp in that the applied voltage / r "•-• 401

RM both above and below rated voltage will reduce life. z Also, frequency of starting is a factor in the life of a fluo- 4 324 .J I,..- - rescent lamp; this differs somewhat from incandescent

-/ T OF NO 4 lamps. Generally, rated life of a lamp is based on three M 6 241

CEN hours of operation for each start. If rated voltage is 0 R being applied and the hours of operation per start in- PE 0 4 161 creased to ten, the life of the lamp is increased about 35 I-z per cent. Because of the many other factors, which also IFE IN u 2 al L include the ballast, involved in estimating fluorescent lamp life, no data is generally available for lamp life as a a. function of applied voltage. 85 90 95 100 105 110 115 Mercury Lamps—Mercury vapor or mercury fluores- PER CENT OF RATED VOLTAGE cent lights are used in industrial plants, streetlighting, Fig. 3—Characteristics of tungsten-filament, gas-filled in- and floodlighting. Characteristic curves for mercury candescent lamps. (Reproduced from Standard Handbook lamps are shown in Fig. 5. for Electrical Engineers, 8th Edition.) Resistance Heating Devices—Energy input to resistance heaters varies as the square of the applied volt- revenue would thus result to the power company. The age. This is true over the range of operation where the theoretical life of the lamp will increase to about 350 resistance remains constant. Therefore, the heat output per cent. With a 10 per cent increase in rated lamp volt- of the device will -vary as the square of the applied voltage, the theoretical life reduces to about 30 per cent, and age. The time needed to heat the device will be inversely the lumen output and power consumption increases to proportional to the rate of applied energy. Fig. 6 shows 140 and 115 per cent, respectively. how power input to a resistance load varies as the ap- Fluorescent Lamps—Characteristic curves for fluores- plied voltage. cent lamps as a function of voltage applied to the Voltage variations in residential use of resistance heat- ballast are shown in Fig. 4. The effect of voltage on the ers are not quite as important as in industrial use. Heat- lumen output is not as great with fluorescent lamps as ing devices in the home are generally used for some form it is with incandescent lamps. In general, for a one per of cooking, and only the time needed to do a cooking cent change in voltage above or below rated voltage, a task is affected due to the increased warm-up time. The

120 120 /1 ,,//e CURRENT ter.LAM P WATTS ..,.\

E 110 ..- 110 / LAMP CURRENT E i LUMENS ALU

„--"" V TED VALU 100 D 100 RA

TE `'LAMP RA F ...)/ O F

O /(/' T ,..' 90 ....- , 90 T EN

/ EN R C /,' LAMP WATTS R C PE 80 80 PE /LAMP LUMENS

LA LA 90 95 100 105 110 115 90 95 100 105 110 1 1 PER CENT OF RATED LINE VOLTAGE PER CENT OF RATED LINE VOLTAGE Fig. 4—Characteristics of fluorescent lamps as a function of Fig. 5—Characteristics of mercury lamps as a function of voltage applied to the ballast. (Reproduced from Standard voltage applied to the ballast. (Reproduced from Westing- Handbook for Electrical Engineers, 8th Edition.) house booklet A51 12, 1956.)

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120 1 120 POWER AS A FUNCTION OF VOLTAGE IN A RESISTOR LOAD 110 ., .., V e I Tot Ts, R V I E 110 2 V ....sPLEo....". P * P V POW e 100 4 I SURF • - • __.____ V I V 2 AL FULL LOAD 0. NT RM 9PF-E0 • I- SLP 100 90

F NO IXPEN05 ON DESIGN

* SL P • 5% T O N SO CE 90 95 100 105 110 PER CENT OF NORMAL VOLTAGE ER 90 P Fig. 7—Characteristics of standard induction motors as a function of applied voltage.

80 applications where precise torque requirements are im- 90 95 100 105 I10 portant, less voltage deviation may be required. PER CENT OF NORMAL VOLTAGE Synchronous Motors—The general effects of voltage Fig. 6—Power as a function of voltage for a resistor load. variations on the performance of synchronous motors are very similar to the effects just discussed for induction motors. However, the maximum or pull-out torque will vary directly with the voltage, as compared to the same would apply for electric clothes drying or house starting torque for induction motors, which varies as heating. In industrial plants, low voltage with the cor- the square of the voltage. responding increase in heating time can definitely affect Electronic Equipment—The life and current-carrying production. In any case, excessive overvoltage will ability or emission of electronic tubes are affected affect the life of the resistance heating element. rather seriously by deviation in voltage from the tube Induction Motors—Characteristics of standard induc- rating. Each type of electronic tube has different cath- tion motors as a function of applied voltage are shown ode emission characteristics with varying cathode heater in Fig. 7. As voltage falls below rated voltage, the volts. No attempt is made in this chapter to show the starting torque reduces substantially, because starting tube operating characteristics for varying voltage, due torque varies as the square of the applied voltage. For to the many types of electronic tubes. Such information an applied voltage 10 per cent below rated voltage, the is available in any good book on electronics or electronic starting torque decreases to 81 per cent of normal. The tubes. The cathode life of electronic tubes is reduced by reduction in starting torque with low voltage may be one-half for each five per cent rise in cathode voltage. significant and costly in motor applications driving high This is due to reduced life in the heater element and the inertia equipment. With low applied voltage and oper- higher rate of evaporation of the active material from ating at full load, the full-load temperature rise increases, the cathode surface. For certain tubes, tube-life can also thus causing reduced insulation life. Full-load current, be reduced with a decrease in voltage below rating. the magnitudes depending upon the particular design, Standard industrial tubes are normally designed to increases with low applied voltage. It increases about 10 operate with a voltage tolerance of ±5 per cent; opera- to 15 per cent with an applied voltage 10 per cent below tion with a closer tolerance increases life. rated. Starting current varies directly as the applied Deviations in voltage also affects residential electronic voltage. appliances such as radios and television. Generally, the With excessively high voltage the starting torque is only effect experienced by the radio is longer warm-up increased, the starting current is increased, and the run- time with low voltage, and shorter tube life with high ning power factor is decreased. The increase in starting voltage. Volume and tone quality vary only slightly current will cause a greater voltage dip when starting with reasonable voltage variations. and, depending upon the frequency of starting, it may Television can be more seriously affected by devia- increase the annoyance of lamp flicker. The increased tion from the voltage rating, although, if the TV set starting torque obtained with a high applied voltage has been adjusted for the average applied voltage, the would only be a factor if damage could result to any normal amount of voltage variation has little effect. If driven equipment or the shearing of couplings between the set is not adjusted to the average voltage (for in- equipment. Speed changes vary little with voltage de- stance, the set was adjusted when the utilization voltage viations as high as ±10 per cent. Table 2 lists the general was at the top of its voltage spread), the picture may be effects of voltage variation on induction motor charac- reduced during low-voltage periods (heavy-load periods). teristics. Generally satisfactory performance results for There is also the possibility of brilliance and sensitivity voltage deviations of ±10 per cent, although in certain becoming noticeably affected.

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Table 2—General Effect of Voltage Variation on Induction Motor Characteristics* (Reproduced in part from Reference 8 or 9)

Voltage Variation - 90% Voltage Function of Voltage 110% Voltage 120% Voltage Starting and Maximum Running Torque Decrease 19% (Voltage)2 Increase 21% Increase 44% Synchronous Speed No Change Constant No Change No Change Percent Slip Increase 23% 1/ (Voltage)2 Decrease 17% Decrease 30% Full Load Speed Decrease 1.14% (Syn. Speed—Slip) Increase 1% Increase 1.5% Efficiency Full Load Decrease 2 points .... Increase to 1 pt Small increase % Load Practically No Change .... Practically No Change Decrease M to 2 pts M Load Decrease 7 to 20 pts Increase 1 to 2 pte • •• • Decrease 1 to 2 pts Power Factor Full Load Increase 1 point .... Decrease 3 points Decrease 5 to 15 pts VI Load Decrease 10 to 30 pts Increase 2 to 3 pts •• • • Decrease 4 points M Load Increase 4 to 5 pts .... Decrease 5 to 6 pts Decrease 15 to 40 pts Full Load Current" Increase 11% .... Decrease 7% Decrease 11% Starting Current Decrease 10 to 12% Voltage Increase 10 to 12% Increase 25% Temp. Rise, Full Load Increase 6° to 7°C •• • • Decrease 3° to 4°C Decrease 5° to 6°C Max. Overload Cap Decrease 19% (Voltage)2 Increase 21% Increase 44% Mag. Noise—No Load in Particular Decrease Slightly • • • • Increase Slightly Noticeable Increase *This table shows general effects, which will vary somewhat for specific **Same ratings will vary somewhat depending upon the particular design. ratings.

Ill. VOLTAGE DROPS IN SYSTEM COMPONENTS feeder is then 113 to 125 volts, or a width of 12 volts. The 12-volt drop permitted is then apportioned to the feeder The system component voltage drops will be discussed components between the first and last consumer's serv- only for the various types of feeders from the location ice entrance. on the feeder of the first consumer served to the last. Also, to completely insure that the service entrance Voltage drop also exists ahead of the first load, but it is voltage will not go beyond the limits of the favorable assumed that voltage control has been included in the zone (excluding transient voltage dips due to motor system up to the location of the first consumer, so that starting), the relay bandwidth setting for any voltage the voltage magnitude at the first consumer is at the regulating equipment on the system should also be in- top of the permissible voltage zone (Table 1). Voltage cluded. The relay bandwidth is the difference between regulating equipment is generally located in the sub- the voltage limits set in the voltage regulating relay of stations serving the various feeders to attain the maxi- the regulating equipment. That is, the relay voltage can mum voltage at the first consumer. vary within this band and no regulator operation will take place, but if the relay voltage went out of this bitnd, 5. Residential Feeders an operation would take place, raising or lowering the The voltage at the point of utilization when keeping regulator output voltage (and hence the relay voltage), within the favorable zone, Table 1, can be 110 to 125 bringing the relay voltage back within the band. The volts. A 120-volt base is assumed throughout. The logi- minimum bandwidth usually used with regulatine, cal primary feeder design to permit maximum loading equipment on power systems is ±1 volt or 2 volts. 'rwo and area coverage is to permit the first consumer electri- volts of the 12-volt permissible drop must then be allo- cally nearest the source (substation low voltage bus) to cated to the bandwidth leaving ten volts drop permitted have the maximum voltage permissible, 125 volts, dur- in the feeder components. ing maximum load conditions, and the most remote It is often found in practice that the bandwidth is not consumer electrically from the source to have the mini- included when determining the permissible voltage drop, mum permissible voltage, 110 volts. The nearest con- and the voltage is permitted to vary out of the voltage sumer physically to the source may not be the nearest zone by a fraction of a volt. This is especially true when electrically, because it is often economical to back feed a power company uses the limits of the favorable zone. in order to permit a higher substation bus voltage than With the bandwidth neglected, it means that occasion- 125 volts during maximum load conditions. ally some consumer's voltage, those near the start or end The average voltage drop for residential interior wir- of the feeder, will go out of the favorable zone and into ing during maximum load conditions is approximately the tolerable zone. Where power companies use the volt- three volte; hence, to have the utilization voltage no age limits of the tolerable zone for determining permis- lower than 110 volts, the voltage at the consumer's serv- sible feeder voltage drop, the bandwidth is generally ice entrance or meter socket must be 113 volts or above. included. The bandwidth will be included in the follow- The service entrance voltage spread for a residential ing discussion in order to be complete.

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3 PHASE drop reaches the upper limit, another distribution trans- PRIMARY MAIN former is added between existing transformers, and the S secondary line is split between the new and existing

EXPRESS DISTRIBUTION units. Such a procedure reduces the secondary voltage PRIMARY TRANSFORMER drop to less than one volt. The amount of voltage drop FEEDER La TATION BU 2 apportioned to the secondary line out of the permissible QF- 12-volt spread is generally three volts. SUBS Distribution Transformer—At the time of installation in a developed residential area, the transformer loading during peak periods is generally 80 to 100 per cent. For the average distribution transformer rating, this represents a voltage drop of 1.75 to 2.5 volts. The transformer remains in service until the peak load increases to about I PHASE FR MARY 140 to 160 per cent, when it is then replaced with a LATERALS larger rated unit. This represents a voltage drop of 3.25 to 4 volts. The voltage drop for a 25-kva, single-phase LAST CUSTOMER distribution transformer as a function of load is shown in Fig. 9. This transformer rating represents almost Fig. 8—One-line diagram of a residential feeder showing exactly the voltage drop of the average distribution feeder components and location of first and last consumer. transformer for kva ratings up to 50 kva and high voltage ratings up to 7.62 kv. The amount of voltage drop The feeder components of a residential feeder are allocated to the distribution transformer out of the shown in the one-line diagram of Fig. 8. Studies of resi- permissible 12-volt spread is generally three volts. dential feeder design have shown that a definite amount Primary Feeders Including Laterals—The voltage drop of voltage drop can be allocated to each component for allocated to the primary portion of the residential maximum economy. A voltage drop breakdown for the feeder is three volts, on a 120-volt base, and is as meas- various components of a residential feeder, when keeping ured from the primary terminals of the first distribution within the favorable zone including bandwidth, is shown transformer on the feeder to the last or most remote in Table 3. transformer electrically. Where single-phase laterals are Service Drop—The voltage drop most generally found tapped off the three-phase main (Fig. 8), they generally for service drops during heavy load conditions is one have a voltage drop from one to three volts, with the volt. This value is seldom exceeded unless the service last lateral having about one-volt drop, and the lateral drops are abnormally long. tapped off near the first distribution transformer on the Secondary Line—Secondary conductors, when in- feeder three volts. stalled, generally have a voltage drop of approximately The sum of the voltage drops allocated to each por- 2 to DA volts, and as load grows the voltage drop is per- tion equals ten volts; including a two-volt bandwidth, mitted to increase to 3 or 3% volts. When the voltage this completes the 12 volt spread. In the above reasoning, the amount of voltage drop permitted for each Table 3 component is based on the assumption that the first distribution transformer is at zero load, and that no Residential Feeder Rural Feeder drop exists in the service drop of the first consumer on Feeder Component Maximum Light Load Maximum Light Load the feeder. With such a condition, the voltage at the Lod . Lod . Co nditko . Condition first customer will be the same as the voltage at the Conditiontion Condition

Primary Feeder 3.5 70% From First 40 I 1 1 Boos 25 KVA DISTRIBUTION inionsoman Distribution 30 SINGL E PHASE . 95% _., 35 2400 / 4160 Y Transformer to 120 /24 0 VOLTS Last Distribution 3.0 g 2.5 BASE ) Transformer 3.5 1.0 6 2.0 100% POWER TS 23 1:4, 2.0 fAGTCR Distribution 0 120 VOL 20 Transformer 3 1.0 3 1.0 I ?, 1.5 P ( RO Secondary Line . . . . 3.5 1.0 . .. I. .0

TS D 10 Service Drop 1 .3 2 1.0 VOL — _ — TOTAL 11.0 volts 3.3 volts 11.0 volts 4.0 volts - T O 40 60 BO 100 120 140 160 ISO PER GENT TRANSFORMER LOADING NOTE: Table assumes 120-volt secondary base and utilization voltage to keep within EEI-NEMA favorable zone. Values are not rigid, in that other combinations may be possible. Fig. 9—Voltage drop in a 25-kva, single-phase, distribu- During full load conditions the first distribution transformer is at light load tion transformer as a function of loading. conditions.

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primary of the first transformer, or 125 volts. With all rural feeder is shown in Fig. 11. The rural feeder is much the remaining feeder components at heavy load, the longer than a residential feeder, often five to ten times voltage at the most remote consumer is 113 volts. The longer. In some instances the entire feeder will be single condition under which the first transformer is at no phase, in that each phase after the substation feeder load while the rest are at heavy load is very remote, breaker goes out into the load area in different directions. but the chance of its being at a lighter load while the Table 3 shows typical voltage drop allocations for remaining are at heavy load is possible. Light load on rural feeder components. The table values keep the feeders will range anywhere between 10 and 40 per service voltage within the favorable zone. While Table 3 cent, so it is permissible to let the primary voltage of keeps within the favorable zone, utility companies often the first transformer be slightly higher than 125 volts. permit their rural feeders to extend into the tolerable With average light-load conditions being about 33 per zone. Any increase in permissible voltage drop when cent, the voltage at the primary terminals of the first going into the tolerable zone would be added on to the transformer can be raised by one-third of the trans- drop allocated to the primary line component. Light former peak-load drop. A value of 126 volts is then load conditions are generally less than residential feed- permitted. The first consumer off the transformer is ers, approximately 25 per cent; although for some rural still assumed to be at essentially no load. Table 3 shows feeders where electric heating is used on the farm (e.g., the allocation of voltage drop to the various compo- poultry farms), the per cent is higher. nents, assuming the first transformer at light load when Even with the increased primary line drop as com- the remaining feeder components are at peak load. The pared to the residential feeder, it is often necessary to values shown in Table 3 are not rigid, in that other,com- add some supplementary voltage boost out on the feeder, binations may be possible. The intent is to remain with- because to pick up enough load on the feeder requires in the favorable zone and show typical allocations. The that the feeder length becomes great and the single- feeder voltage profile in Fig. 10 illustrates the voltage phase laterals long. drop values listed in Table 3. 7. Industrial Feeders 6. Rural Feeders Industrial feeders are relatively short feeders and Rural feeders differ somewhat from residential feed- serve anywhere from one to several consumers. They are ers. There are no secondaries as a rule (i.e., run and similar to rural feeders, in that there are generally no owned by the power company), because of the distance secondaries, as each consumer has his own transformer. between consumers. Each consumer has his own dis- Many industrial consumers purchase energy directly tribution transformer. The distribution transformer rat- at the primary voltage level and own the step-down ings are smaller, and since the transformer pole is cen- transformer. trally located between all the farm buildings requiring There are no recommended allocations of voltage drop electric power, the service drops are longer than for a for industrial feeder components. Each industrial con- residential customer. A one-line diagram of a typical sumer on a feeder should have the supply voltage to the transformer or transformers which serve the plant fall within the zone shown in Columns 2 and 3 of Table 4. PRIMARY LINE The voltage spread for the primary supply should be SULISTATION four per cent and should fall within the recommended BUS m/rr SECONDARY SERVICE DROP

°TRISATABF0U RV&I (CUSTOMER Table 48—Zone and Spread of Primary Supply System tatOMER Voltage for Industrial Plants in Which the Standaild Transformers can be Operated Without Automatic

126 1 Voltage Regulation and Meet the Limits of Table 1 127 1 126 Nominal Zone of Voltages 125 r 1 Primary Voltage 124 15 VOLTS System of Primary on Trans- 123 UTILIZATION PRIMARY Spread** VOLTAGE-FIRST USE Voltage formers* 122 CUSTOMER 3 VOLT 121 Column 2 Column 3 Column 4 120 - „, 2 VOLT Buri014 z BANDWIDTH Column 1 Column 5 119 0 TRANS- Minimum Maximum Percent of FORMER. 118 Volts Volts 15vOLTS Volts Volts Column 1 O SECOND- :167 - 3 ART USE 115 I VOLT 2400 2130 2520 4% 100 114 SERVICE 4160 3680 4360 4% 170 113 2 VOLT 3 VOLTS BANDWIDTH INTERIOR 4800 4260 5040 4% 190 112 WIRING 11I 6900 6100 7250 4% 280 110 UTILIZATION VOLTAGE-LAST 11500 10200 12100 4% 460 CUSTOMER 13800 12200 14500 4% 550 Fig. 10—Voltage profile of a residential feeder. Feeder at 'Based on plant power factor 0.85. transformer reactance 5%. and 2% full-load conditions except first transformer which is at light maximum secondary feeder drop. load and first customer which is at no load. Full-load voltage Should power factor be higher than .85 or feeder drop lees than 2%. a lower value can be tolerated for column 2 and greater primary spread for column 4 drops allocated to each feeder component are indicated at without exceeding values given in Table I and without using voltage regulators. "The primary voltage spread must lie wholly within the sone of primary the right. voltages.

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FIRST PRIMARY FEEDER CUSTOMER 6. Changing of feeder sections from single-phase 1/4zAjO A Lkj to multi-phase. CO 7. Transferring of loads to new feeders. CO z EXPRESS DISTRIBUTION 8. Installing of new substations and primary 0 PRIMARY RANSFORMER feeders. g FEEDER 9. Increasing of primary voltage level. k.a.k., /F/ 10. Application of voltage regulators out on the SERVICE DROP primary feeders. rN''M 11. Application of shunt capacitors on the primary B feeders. fT LAST CUSTOMER 12. Application of series capacitors on the primary Fig. 11—One-line diagram of a rural feeder showing feeders. feeder components and location of first and last rural consumers. The selection of which method or methods is most NOTE The primary feeder portion can be three-phase and the laterals applicable and which voltage regulator is best to use single-phase or the entire feeder can be single-phase. will depend upon the particular system or problem involved. There are no clear-cut rules for selecting the best method. The size of system, type of load served, voltage zone of Table 4. The voltage zones have been existing equipment location, amount of voltage cor- determined so that the standard transformers normally rection necessary, area served, future system expansion, used for serving industrial power systems can be used, and load growth are all factors which must be studied. utilizing the standard taps if necessary, with no automatic voltage regulating equipment, and with the sum 8. Generator Voltage Control of the transformer and interior wiring voltage drop Where the generator feeds directly into the distribu- equal to six per cent at heavy-load conditions. Where tion system, the generator bus voltage can be regulated the primary supply voltage is at the maximum voltage economically to keep a constant voltage at the load end of the zone, the above taps of the transformer will center for any load condition. In large systems, genera- have to be used, and where the primary supply voltage tor-voltage regulators usually are used only to maintain is at the minimum voltage end of the zone, the below the desired bus voltage for prevailing load conditions taps will have to be used. and reactive power-flow requirements. Varying the generator bus voltage as load conditions vary by changing the generator field offers one of the IV. METHODS OF IMPROVING VOLTAGE least expensive methods of obtaining voltage control. REGULATION It has the advantage of extreme simplicity and can There are several methods of improving voltage regu- eliminate the initial and operating cost of many small lation throughout the distribution system. Some meth- regulators. Depending upon the particular system and ods raise the voltage at the beginning of a distribution location of the power plant, a voltage spread of eight to feeder as load increases, thus reducing the average volt- ten per cent can often be utilized at the generator bus. age difference between light-load and heavy-load condi- Voltage increases as load increases, and conversely. tions (voltage spread) for all the consumers on the feeders. Other methods decrease the impedance between 9. Substation Voltage Regulation the feeder source and load, thus reducing the voltage Economic system design usually dictates use of volt- drop and the voltage spread. Also, the load current can age regulating equipment in substations. The voltage be reduced, thus reducing the voltage drop and voltage SUBSTATION spread. Voltage regulating equipment can also be ap- OUTGOING PRIMARY FEEDER INCOMING plied out on a feeder where the voltage becomes too low LINE or too high, in order to reduce the voltage spread. The various methods of improving voltage regulation MAXIMUM ALLOWABLE VOLTAGE I throughout a distribution system are listed below. Each I} }FEEDER I SERVICE ENTRANCE method has its own characteristics concerning amount VOLTAGE of voltage improvement, cost-per-volt improvement, SPREAD and flexibility. MINIMUM ALLOWABLE VOLTAGE 1. Use of generator voltage regulators. MAXIMUM ALLOWABLE VOLTAGE 2. Application of voltage regulating equipment in the distribution substations. ALLOWABLE ,VOLTAGE IN DROP 3. Application of capacitors in the distribution MINIMUM ALLOWABLE VOLTAGE substation. Fig. 12—A heavy-load voltage profile of the same feeder 4. Balancing of the loads on the primary feeders. with and without substation regulation. Note that substation 5. Increasing of feeder conductor size. regulation increases allowable feeder-voltage drop.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 265/576 258 System Voltage Regulation regulating equipment can be LTC (Load Tap Changing) tire feeder length and not just according to the feeder mechanisms in the substation transformer, a separate output line currents at the substation. If a feeder serves regulator between the transformer and low-voltage bus, only three-phase loads, balance is not a problem. switched capacitors at the low-voltage bus, or separate Where the phase area method (each phase of a feeder regulators located in each individual feeder in the sub- serves all the distribution transformers in one area) of station. Without regulating equipment in the substa- primary distribution is used, it is necessary to select tion, the voltage spread on the incoming side of the phase areas such that the load on each phase is about substation would be passed along the feeders. This is the same. illustrated in Fig. 12. Since the allowable feeder-volt- The calculation of feeder voltage drop for unbalanced age drop is the difference between the feeder service- loading is also a cumbersome task, depending on whether entrance spread and the substation-output spread, or not the system is a three- or four-wire, three-phase substation regulation permits an increased allowable system and on the method of grounding the feeder, if feeder-voltage drop and greater load-carrying ability. grounded. Maximum output voltage of the substation is limited by the nearest consumer. More detail concerning the 11. Increasing Conductor Sizes application of voltage regulating equipment at the bus Increasing feeder conductor sizes decreases the line or on individual feeders in the substation is discussed impedance and hence, for the same feeder load, de- later in this chapter. creases voltage drop. Re-conductoring lines is one of the most expensive means of improving voltage regu- 10. Balancing Load on the Primary Feeder lation when considering dollars-per-volt-improvement, One of the first items to check, if a primary feeder but it is often necessary where extensive load growth is has poor voltage regulation conditions, is the loading encountered and long range planning is a major con- on each phase of the three-phase feeder. If the load on sideration. Fig. 13 shows the reduction in voltage drop the feeder is not balanced between the phases, means for a feeder when changing to a large conductor. In Fig. should be taken to achieve balance. For a given feeder- 13(a), the present conductor is copper and the change load, balanced conditions mean equal current in each in voltage drop is shown for the same feeder loading with phase with corresponding minimum regulation. In the new conductor being copper or stranded all-alumi- addition to the possibility of poor voltage regulation if num. Fig. 13(b) has stranded all-aluminum for the the load is badly unbalanced, substation facilities (volt- present conductor, and again the new conductor is cop- age regulators, transformers, etc.) may be overloaded per or aluminum. The curves are accurate for reasonable in the heavily loaded phase. This is possible even if the equivalent conductor spacings up to 56 inches, even if total three-phase feeder load is not excessive. changing the spacing when re-conductoring. Load balance should be achieved throughout the en-

VOLTAGE DROP (EXISTING\ RATIO k NEW / 5.0 40 3.0 e.0 15 12 1.0 VOLTAGE DROP AMONG, RATIO NEW

5.040 3.0 20 1.5 12 1.0

ii A 3/0 % 46 44p irk zb 44 — i

343 ISO 12 % ; 2-§ 1t -- R,C0S• •- VOLTAGE MCP R, COS. X. sm. oP" 0, - 2 RATIO R. COSO 714 SRO P .°- 2 I 4 . 6 4 — 6

1- 4 o 2 I yo % 3 .t0 PRESENT CONDUCTOR SIZE (ATM ALUMVLIA PRESENT COPPER COMUGTOR SIZE 1AWG) (a) (b)

(a) Present conductor—copper. (b) Present conductor—stranded all-aluminum.

Fig. 13—Reduction in circuit voltage drop when increasing conductor size with the same circuit load. Load power factor 90%. Curves valid for reasonable conductor equivalent spacings of 8" to 56". Same load assumed on both present and new conductors.

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voltage phasors for the single-phase case are shown in 12. Changing Feeder Sections from Single-Phase Fig. 14 (a) and for the two-phase cases in Figs. 14 (b) to Multi-Phase and 14 (c). Positive phase rotation is assumed with Fig. Many laterals on a primary feeder are single-phase, 14 (b) showing phases a and b, and Fig. 14 (c) showing and the voltage drop results from load current in the phases a and c. In each of the two-phase cases, the return path as well as the phase conductor. When load was divided evenly between the two phases ; hence adding two conductors to a single-phase lateral to make Is for the two-phase case was one-half the magnitude of it four-wire, three-phase and dividing the existing load Ia for the single-phase case. The effect on the reduction evenly between the three-phases, the voltage drop will in voltage regulation for phase a when going from phase be reduced to one-sixth of the drop that occurred when a alone to phases a and b as compared to phases a and the lateral was single phase. This is shown in the follow- c is clearly shown in Figs. 14 (b) and 14 (c). The actual ing equations: ratios of two-phase voltage regulation to single-phase For a single-phase circuit regulation for the individual phases are shown in Fig. 15 for different load power factors and different r/x VD(4) =2/00) Z (2) ratios for the circuit conductors. The curves assume where, positive phase rotation. For negative rotation the VD = Voltage drop in volts per unit circuit length. curves of Fig. 15 (a) would be interchanged with the I = line current in amperes. curves of Fig. 15 (b). Z = impedance in ohms per unit conductor length. An example using the curves in Fig. 15 is as follows: The subscript (14.) refers to the single phase circuit. Assume a phase a, single-phase lateral with #4 copper

2/ (igs,Z Per cent VD (ph= (100) (3) where, A

E = Line to neutral voltage in volts. Eau IERCI For a three-phase circuit serving the same load, N VD(30) =/(30 Z (4) The subscript (3 cb) refers to the three-phase circuit. (a) /(395)Z ,,rtnN Per cent VD (34,) =—. kluu ) (5) (a) Single-phase lateral—Phase and neutral wire.

where, ERU E = Line to neutral voltage in volts. With the same load that was served with the single- phase circuit divided evenly between the three phases of the new three-phase circuit, ERb Esb I(3) _ 3 (6) Therefore, the ratio between per cent voltage drop using Equations (3) and (5) is: (b) (b) Two-phase lateral—Phase a and b and neutral wire. 2/(10,Z(100) %V D (195) E 2I Go) - 6 (7) ER %VD(30) I (30)Z I (34) 7310) (100) e N Reducing the voltage drop by six when adding the ERa additional wires to give a three-phase line permits a Esc considerable amount of load growth unless the original single-phase circuit voltage drop was excessively high. When adding only one more phase conductor to a single-phase lateral off of a four-wire, three-phase circuit and dividing evenly the existing load between the two (c) phases, the voltage drop will be reduced, but not by a simple ratio as it was when changing from single-phase (c) Two-phase lateral—Phase a and c and neutral wire. to three-phase. The amount of reduction in voltage Fig. 14—Circuit and phasor diagrams of single-phase and regulation over the single-phase case depends upon two-phase laterals off a 4-wire, three-phase circuit showing which two phases are involved, the load power factor, the relationship between load current in the phase conductor and the r/x ratio of the circuit conductors. This is and the neutral conductor and the relationship between the illustrated in the phasor diagrams of Fig. 14. The sending and receiving end voltage for phase a.

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.3 I 1 I z z 5 a C 4 oc La -1 T1.0 rc O b u. .9 rc . 9

0 a. ID , r 0tl a q 0 .8 sn 5

! itl .7 .7

CIRCUIT / CIRCUIT CONDUCTOR CONDUCTOR .6 R /:1 ----- .6 R 1 RATIOS 3 vt 2 15 1 I .75 .5 RATIOS .5 .75 I 1.5 2'Ri 3 oc X ? 1 I I , t.__ I 1 I .1 .2 .3 .4 .5 .6 .1 .2 .3 .4 .5 .e TWO PHASE VOLTAGE REGULATION RATIO VOLTAGE REGULATION RATIO , TWO PHASE 1 SINGLE PHASE/ `SINGLE PHASE J (0) (b) (a) For phase a when using phases a and b. (b) For phase b when using phases O and b. For phase b when using phases b and c. For phase c when using phases b and c. For phase c when using phases a and c. For phase a when using phases a and c. Fig. 15-Voltage regulation ratio of a two-phase circuit to a single-phase circuit serving the same total load. The load is evenly divided between the two phases for the two-phase cases. NOTE: For negative phase rotation, the curves of (a) & (b) would be interchanged. conductors (r/x ratio -= 2.28) and a third wire of the sive means of improving voltage regulation. Long range same size is added to the lateral from phase b. After planning, availability of substation sites, and load the single-phase load of 0.8 power factor lagging is di- growth are conditions which often dictate that an vided between the two phases, the voltage regulation increase is necessary. on phase a is reduced to .32 of the original two-wire, single-phase regulation. The voltage regulation on 15. Installing Supplementary Voltage Regulators phase b is .42 of the original two-wire, single-phase Voltage regulators installed out along the primary regulation. This is shown by the dotted line in Fig. feeders correct excessive feeder voltage drop and reduce 15 (a) and Fig. 15 (b). Had the additional phase wire the feeder service entrance voltage spread. They are been phase c, the regulation would have been reduced located at the point where heavy-load feeder voltage to .42 on phase a and .32 on phase c of the original falls below the minimum allowable value, with con- single-phase value. sideration for load growth. A heavy-load feeder voltage profile with excessive voltage drop and the supple- 13. Reducing Feeder Loading mentary regulator location to correct the voltage con- Reduction in feeder loading as a result of adding new dition are shown in Fig. 16. This figure illustrates a case feeders in the load area will reduce the feeder current in which a voltage boost was necessary. On some and hence the voltage drop. Feeder loading would feeders a lowering of voltage is necessary, as when a generally not be reduced specifically to improve voltage feeder contains several banks of fixed shunt capacitors regulation, but as part of an overall system growth plan. and a voltage higher than maximum permissible occurs during light-load conditions. 14. Increasing Primary Voltage Level Supplementary regulators can be applied in series When changing the voltage level of a primary feeder along a feeder (Fig. 17), but feeder thermal capacity and maintaining the same load, the line current of the and line losses limit their number. On long rural feeders feeder will be changed by the inverse ratio of the volt- there are often two, sometimes three, in series, but age change, and the voltage regulation will change by seldom more. Where two or three supplementary regu- the square of the voltage change. Changing to a four- lators are required in series, a fixed boost is often pos- wire wye system from a three-wire delta system (in- sible instead of using one automatic voltage regulator. creasing the voltage level by -V3) reduces the voltage This is possible if the fixed boost does not cause exces- drop of a feeder to one-third of the original drop. sively high voltage during light-load conditions. The Voltage dip or flicker due to motor starting will also be fixed boost can be obtained with the no-load taps on reduced. the distribution transformers or by adding a transformer Raising the primary voltage level is another expen- which provides a certain per cent fixed voltage boost.

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SUBSTATION ,..VOLTAGE creasing to a constant source voltage. The per cent OUTGOING fe,../ R EGULATOR voltage rise at a capacitor installation is: PRIMARY FEEDER (c kva)(d)(X) Per cent voltage rise = (8) b 6 6 6 6 (10) (kv)2 o where,

I I ALLOWABLE VOLTAGE ckva= Three-phase capacitor kva or kvar. d= Distance in unit lengths. X = Line reactance per conductor in ohms per unit length. kv =Line-to-line voltage in kilovolts. MINIMUM If the capacitor installation is a single-phase bank, the 'ALLOWABLE VOLTAGEI FORLL6A'AD LOANCE quantity ckva would be the installed single-phase kvar, I GROWTH the kv would be the voltage in kv between the two con- I I MAXIMUM ALLOWABLE VOLTAGE ductors, and X would be twice the value per conductor. For example: Given: Three-phase feeder two miles in length 135 ckva capacitor bank installed at end of feeder MINIMUM ALLOWABLE VOLTAGE Line reactance = .66 ohms/mile/conductor Line-to-line voltage = 4160 volts Fig. 16—A heavy-load voltage profile of a primary feeder with excessive voltage drop and the method of correcting Using Equation (8) using supplementary voltage regulators. (135)(2) (.66) Per cent voltage rise = = 1.03% (10)(4 .16)2 and for a single-phase feeder: Given: Single-phase feeder two miles in length 16. Installing Shunt Capacitors 45 ckva capacitor bank installed at end of Shunt capacitors installed on a distribution system feeder will cause a voltage rise from the capacitor bank lo- Line reactance = .66 ohms/mile/conductor cation back to the source. Capacitors draw a leading Line-to-line voltage = 2400 volts power-factor current, and this leading current flowing Using Equation (8) through the series reactance of the circuit causes a (45)(2) (.66)(2) voltage rise equal to the circuit reactance times capaci- Per cent voltage rise = — 2.06% tor current. The voltage rise is independent of load con- 2 ditions and is greatest at the capacitor location, de- Fixed capacitors will not appreciably improve voltage regulation, but they provide a constant increase in voltage level. The effect that fixed capacitors have when SUBSTATION -VOLTAGE installed out on a primary feeder is shown in Figs. 18(a) OUTGOING PRIMARY REGULATOR and 18(b). Note the voltage increase at the capacitor FEEDER location is the same for both light-load and heavy-load z bbb bbbb bbb feeder voltage profiles. If the shunt capacitors are installed in a bank that z can be switched on and off with changing feeder load MAXIMUM ALLOWABLE VOLTAGE I conditions—capacitors on during heavy-load and off during light-load conditions—feeder voltage regulation will be improved. The effect that switched capacitors produce when installed out on a primary feeder is shown in Fig. 18(c). The voltage regulation can be shown to MINIMUM ALLOWLI improve by comparing Fig. (18(c) with Fig. 18(b). ABLE VOLTAGE' With capacitors located out along the primary feeders, the feeder voltage can be made relatively flat or even rising at light-load. The amount of ckva added to a MAXIMUM ALLOWABLE VOLTAGE feeder and where it should be located on the feeder will depend upon the distribution of loads, feeder conductor size, load power factor, and voltage conditions. Light- load conditions will determine the amount of ckva that can be fixed and the amount that will have to be MINIMUM ALLOWABLE VOLTAGE"— --4 switched. The light-load voltage profile of Figs. 18 (b) and Fig. 17—Supplementary voltage regulators applied in series 18(c) are the same because the switched capacitors are on a primary feeder with excessive voltage drop. off during light-load conditions.

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SUBSTATION the point of application. In addition, they release some 1/4 I PRIMARY FEEDER distribution transformer capacity. I I 17. Installing Series Capacitors FIRST ILAST CUSTOMER SWITCHED FIXED CAPACITORS= =CAPACITORS I CR Series capacitors when installed out on a primary T feeder will reduce voltage drop. Voltage drop through a feeder is approximately IRL cos 0±/XL sin 0 (9) where RL is line resistance, XL is line reactance, and 0 I Liam- Leiy, I is the power factor angle. With a series capacitor inserted in the primary feeder, the voltage drop becomes MINIMUM ALLOWABL VOLTAGE IRL cos 0±/(XL— Xc) sin 0 (10) RN

MAXIMUM ALLOWABLE VOLTAGE where Xc is the capacitance reactance. If Xc equals XL, the feeder voltage drop is simply IRL cos 0. Hence, I LIGHT LOAD I I I the effect of series capacitors is to reduce circuit react- I I i I I ance. N.44ey I 4CI40 The power factor of the load current through the I I MINIMUM ALLOWABLE VOLTAGE feeder must be lagging for a series capacitor to decrease

(b) the voltage drop appreciably between the sending and receiving ends. If power factor is leading, the receiving XIM end voltage may be decreased by the addition of a series LIGHT LOAD capacitor. If the power factor is near unity, sin 0 and

I Cs consequently the second term of Equation 9 is near zero. In such cases, series capacitors have comparatively little value. INIMUM ALLOWABLE VOLTAG When applied on a primary feeder serving a lagging (C) power factor load, series capacitors will cause a voltage Fig. 18—Light- and heavy-load voltage profiles showing rise as load increases. The voltage on the load side of the improvement of feeder voltage conditions when adding series capacitor is raised above the source side similar shunt capacitors out on feeder. to an automatic step or induction voltage regulator. (a) Voltage profiles with no capacitors added. Since the voltage rise increases with load, a series capacitor will improve voltage regulation of a feeder. The (b) Voltage profiles with only fixed capacitors added. voltage rise increases instantaneously with increasing (c) Voltage profiles with fixed and switched capacitors added. load; therefore their application is particularly suited to Switched capacitors not on for light-load conditions. feeders where lamp flicker is encountered due to rapid and repetitive load fluctuations from frequent motor starting, varying motor loads, electric welders, or electric Shunt capacitors can also be installed in the distribu- furnaces. tion system on the low-voltage bus in the distribution substation. When installed at substation locations, the V. TYPES OF AUTOMATIC VOLTAGE REGULATORS capacitor banks are relatively large, and it is generally necessary for part of the bank to be switched to prevent There are four basic types of automatic voltage regu- overvoltage during light-load conditions. The voltage lators used in distribution systems; they are drop of a primary feeder served from a substation which 1. Generator voltage regulators contains capacitors is unchanged. The power factor of the feeder remains the same, as the total reactive portion 2. Load-tap-changing (LTC) mechanisms in sub- of the load is still served from the substation. The volt- station transformers age level of the entire feeder is raised depending upon 3. Feeder voltage regulators the amount of capacitors added to the substation, but 4. Switched shunt and series capacitors the feeder voltage spread remains the same. Generally, the primary purpose of installing capacitors at substa- Each basic type changes voltage conditions as load con- tions is not necessarily to control voltage, but to supply ditions change in order to minimize the voltage spread kvars and release substation and transmission-line at particular points throughout the distribution system. capacity. In some installations they reduce high-voltage condi- Secondary 240-volt capacitors may also be applied on tions, although the greatest use of automatic voltage the low-voltage side of the distribution transformers at regulators in distribution systems is to compensate for the transformer or out along the secondary line. They line voltage drop by increasing voltage with load. are generally fixed capacitors, and most applications Feeder voltage regulators include both induction and provide a voltage rise of about one to three per cent at step-type voltage regulators.

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regulation is plus 10 per cent and minus 10 per cent of 18. Generator Voltage Regulators the rated line voltage, with plus and minus 7% per cent There are five types of voltage regulators being used and plus and minus 5 per cent being second and third, to control the excitation of synchronous machines: respectively, in popularity. For distribution substation 1. Direct-acting rheostatic type transformers, the plus and minus 10 per cent regulation range is by far the most popular. The 32-step, plus and 2. Indirect-acting exciter-rheostatic type minus 10 per cent regulation range has such wide ac- 3. Impedance-network or static-network type ceptance that it is considered standard for most load- 4. Electronic type tap-changing equipment on power transformers. A 5. Static network—magnetic amplifier type typical single-feeder distribution substation, CSP power Each type varies the generator output voltage by transformer, with plus and minus 10 per cent load-tap- changing the exciter field strength. The direct-acting changing is shown in Fig. 19. type controls the voltage by directly varying the resist- The 32-step, ± 10 per cent unit has 16 steps in each ance in the generator main exciter field circuit. The direction, and each step represents a 5/8 per cent change indirect-acting type controls the voltage by varying the in voltage. With a 32-step, ±5 per cent unit, each step resistance in the field circuit of the exciter that excites would represent a 5/16 per cent voltage change. The tap- the synchronous machine. The remaining types (Num- changing mechanisms for units with different regulation bers 3, 4, and 5 in above list) all vary in some way the ranges, but with the same number of steps, may be the field strength of the exciter that excites the generator. same depending upon the kva of regulation; the differ- The operation of each type of generator voltage regu- ence is only the number of turns per winding section lator will not be discussed in detail in this chapter. The between steps. The schematic diagram and sequence of method of operation and characteristics of each are fully operation for the simplest type of load-tap-changing discussed in References 12 and 13. mechanism, type URS, are shown in Fig. 20. Schematic Occasionally it is desirable to regulate so that a diagrams and sequences of operation for other auto- constant voltage is maintained at some point on the matic tap-changing mechanisms containing different system external to, or distant from, the station where the schemes of transfer and reverse switching are shown in generator and its regulator are located. A line drop Reference 12. compensator is then used in conjunction with the gen- The URS load tap-changer is applied to small power erator voltage regulator. The constant voltage at the transformers and large distribution transformers. For a remote point is accomplished by supplying the generator +10 per cent regulation range, the URS mechanism is bus voltage to the regulator after subtracting the line used on transformers 15,000 kva and below and at 15 IR and IX drop artificially with the compensator. The kv and below. The tap-change is made without opening generator will then maintain a bus-voltage value equal the circuit by using a mid-tapped autotransformer, to the desired remote point voltage plus the line drop to frequently called the preventive autotransformer. The that point. The wide use of interconnected power sequence of connections in going from one step to the systems has eliminated to a large extent the need for line next step for the URS tap-changer, showing the function drop compensators. of the preventive autotransformer, is illustrated in Fig. 19. Load-Tap-Changing (LTC) The modern load tap-changer had its beginning in 1925. Since that time the development of more complicated system networks and interconnections has made load-tap-changing, often called tap-changing-under- load, more and more essential to control the in-phase voltage of power transformers, and in other cases to control the phase-angle relation. In distribution systems, load-tap-changing equipment is applied to power transformers to maintain a constant low-side or secondary voltage with a variable primary voltage supply, or to hold a constant voltage out along the feeders on the low- voltage side for varying load conditions on the low- voltage side. For power systems as a whole, LTC is also used to control the flow of reactive kva between two generating stations or branches of loop circuits, and to control division of power between branches of loop circuits by shifting the phase-angle position of transformer output voltages. There are various types of tap-changing equipment and circuits depending upon the equipment voltage rating and kva. They are built with the regulation range Fig. 19—Installation photo of CSP, 1-feeder distribution divided into 8, 16, and 32 steps. The usual range of substation with L.T.C.

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264 System Voltage Regulation

REVERSING SWITCH PREVENTIVE AUTOTRANSFORMER

SELECTOR-TRANSFER ---1) SWITCH PREVENTIVE AUTOTRANSFORMER

SEQUENCE OF OPERATION

_i_l_o_k_I—l_o_J—JLA-1 J mwrrawaremawm=mmicew """"-ckmmP-to.dyer.lm-z-w~www....mm°:N""m°....._ POSITION T T —_ (a) (b) (c) SW D-I10 00 POSITION I POSITION 2 " G-Il 00 00 " 0-10 00 00 " C-10 00 00 - D-9 00 00 " C-9 00 00 " D-8 00 00 " C-8 00 00 ' D -7 00 00 " G-7 00 00 ' D -6 00 00- " C -6 00 00 " 0-5 00 00 00 ' C -5 00 (d) (e) ' D -4 00 00 POSITION 3 " D-4 00 0 " D- R 00 Fig. 21—Sequence of connection in going from one position ' C-R 00 to another for a URS tap changer. " A__. 00000000000000000 - 8 0 0000p 0000000000 0= SWITCH CLOSED R = RAISE L LOWER because as the tap-changer capacity is increased, the Fig. 20—Schematic diagram and sequence of operation for circuit kva which each selector switch must interrupt the 33 position, single-phase, Westinghouse Type URS tap increases, and a point is reached where it is not economi- changer. cal to build so many switches with the large interrupting capacity. The transfer switches are then added and the 21. Actual tap-changer positions are shown in Figs. sequence of operation is made so that all the inter- 21(a), (c), and (e). The preventive autotransformer is rupting and arcing is limited to the transfer switches. capable of carrying full transformer winding current When changing from one position to another, the trans- continuously through either leg in case a mechanical fer switches open just before the selector contacts open, failure stops the mechanisms in the middle of an opera- perform the interruption, and close just after the selection, Figs. 21(b), or (d). On all odd-numbered positions, tor contact has moved to its new position. A schematic shown on the chart of Fig. 20, the preventive autotrans- diagram of the larger rating tap-changer, Type URT, is former bridges two transformer taps, Fig. 21(c). In this shown in Fig. 22. The reversing switch operates in a position, the high reactance of the preventive auto- similar manner as the URS. transformer to circulating current between adjacent taps prevents damage to the transformer winding, while 20. Step-Type Feeder Voltage Regulators its low impedance to load current permits operation on The step-type voltage regulator consists of an auto- this position to obtain voltages midway between trans- transformer and a load-tap-changing mechanism built former taps. Physically, the stationary selector switch contacts, REVERSING (Contacts 4 to 11, Fig. 20), are arranged in circles,•one SWITCH% for each phase. The moving selector switches, as they 0 \ rotate about a center shaft, both select and contact the B A taps. The reversing switch operates when the selector switches are on position zero (no voltage _buck or boost), R at which time there is no current through the reversing SELECTOR switches, and therefore no arcing occurs. SWITCHES Tap-changers can be equipped for hand operation, TRANSFER remote manual operation, or for full automatic opera- SWITCHES tion with relay control. PREVENTIVE Large rating tap-changers differ from the simple URS AUTOTRANSFORMER tap-changer in that transfer switches are used. The Fig. 22—Schematic diagram for the single-phase, Westing- construction is similar—just larger and more rugged to house Type URT tap changer showing the location of the carry the greater currents. Transfer switches are used transfer switches.

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into an integral unit. The voltage change is obtained by SERFS REVERSIVG TRANSFORMER changing the taps of the autotransformer. Standard SWITCH WRIES step regulators have a regulation range of 10 per cent. WINDING Some units contain a reversing switch enabling a voltage regulation range of plus and minus 10 per cent. Modern R. step voltage regulators have the 10 per cent regulation IN., MIME 2i range divided in either or 11/1 per cent steps. For a PREVENTIVE AUTO TFIANSFORkER plus and minus 10 per cent, they will have 32 or 16 steps, soliacE LOAD respectively. SL Step voltage regulators differ from autotransformers SINGLE CORE - TWO WINDING SL TWO GORE -TREE WINDING in that they are rated on kva of regulation rather than (0) (b) on circuit or through kva. For example, a three-phase, +10 per cent, 750-kva step regulator would be applied on a three-phase circuit rated 7500 kva and would vary the line voltage plus or minus 10 per cent from rated. Also, a single-phase, ± 10 per cent, 100-kva regulator would be applied on a single-phase circuit rated 1000 kva and would vary the single-phase voltage plus or minus 10 per cent from rated. Voltage regulators are also rated in amperes, and for a regulator application the ampere rating of the unit should be the same as or more SL than the circuit line amperes. TWO GORE - FOUR WINDING There are two basic types of step regulators, the (G) station-type and distribution-type. The station-types are either three- or single-phase and are applied in sub- Fig. 23—Various connections of tap changers in station type stations for bus regulation or individual feeder regula- voltage regulators. One phase shown only. tion. The distribution-types are only single-phase and (a) Single core—two winding—used when both line voltage most are adaptable for pole-mounting out on overhead rating and line current rating are within tap changer rating. primary feeders. The distribution type represent smaller (b) Two core—three winding—used when line current rating ratings than the station type. The distribution-type exceeds tap changer current rating. units not adaptable for pole-mounting are used either (c) Two core—four winding—used when line voltage rating platform-mounted out on overhead primary feeders or exceeds tap changer voltage rating. in substations as individual feeder regulators, depending upon the feeder kva size. Station-Type—Voltage regulators for application in substations are rated 2500 kva and below, three-phase, self-cooled, and 69 kv and below. Standard single-phase units are 250 kva and below, self-cooled, and 69 kv and below. Forced air cooling can be applied to some of the standard units, increasing the ratings 25 to 33 per cent. The three-phase regulators are operated from one control, and the intelligence is obtained from one of the phases. Standard station regulators have 32 steps for plus and minus 10 per cent voltage regulation, with a per cent voltage change per step. Two Westinghouse types of station, step voltage regulators are the type URS and URT. The tap-changer for the URS- and URT-type station regulators and their sequence of operation are the same as discussed in Section 19 for the URS and URT tap- changers. The schematic diagram for tap-changers used in station-type regulators showing the location of the tap-changer with respect to the windings is illustrated in Fig. 23. Any of the three arrangements can be used, depending upon the circuit current and voltage. Station regulators up to 1500 kva ratings will use any of the connecting schemes shown in Fig. 23, but beyond 1500 kva, three-phase, for economy reasons, a larger rated tap-changer is used. Most of the station URS voltage Fig. 24—Installation photo of a station Type URS step regulators used in distribution substations have connec- regulator.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 273/576 266 System Voltage Regulation tions similar to Fig. 23(a). An installation of a URS station regulator in a distribution substation is shown in Fig. 24. The type URT station regulator (Fig. 25), which has the larger rated tap-changer, is used for applications requiring ratings above 1500 kva, three-phase. Distribution-Type—The distribution type voltage regulators, often called distribution-feeder voltage regulators, are single-phase, rated 12.5 to 167 kva, and rated 14.4 kv and below. The smaller units, frequently called line type, are made available for pole-mounting. The larger units are not adaptable for pole-mounting and are located in substations or on platforms. All the units are completely automatic and self-contained. The first small, single-phase step voltage regulators were used around 1933 and were considered as 2- or 4- step boosters with a 5 or 10 per cent regulation range. Present standard distribution-type regulators have a plus or minus 10 per cent regulation range in either 16 or 32 steps; I WI per cent or EA per cent per step, respectively. Fig. 26 is a picture of the URL distribution-type step regulator adaptable for pole-mounting. A large distribution-type unit, Type URF, generally used for feeder application in substations, is shown in Fig. 27. The tap-changer for the 16-step URL or URF step regulator is similar in principle to the type UM used in station regulators, although much smaller in size. The sequence of operation is also similar, but the number of steps are fewer. The tap-changer for the Type URF is also slightly larger than the Type URL. A schematic diagram of the URL regulator showing the contact and winding arrangement is shown in Fig. 28. If the distribution-type regulators are operated at a reduced regulation range, their capacity can be increased. By reducing the range to plus or minus 5 percent, the capacity is increased to 160 per cent. Table 5 lists

Fig. 26—Installation photo of a URL step regulator.

the increase in capacity permitted for various reductions in operating regulation range. The advantage of the increased capacity at a reduced range is the increase in service time for an application before load growth requires a change-out, that is, providing a smaller range is permissible. Also, when line-type units are located near capacitor installations, such as in substations, and too high a voltage occurs during light-load conditions, the regulator can buck down the voltage the entire 10 per cent range, if necessary ; while at full load a 5 per cent boost may be sufficient, thus allowing a reduced regulator rating and a lower cost. When reducing the regulation range, the short-circuit capability is also increased, providing the time duration is limited. The short circuit capabilities for the reduced ranges are also listed in Table 5. Standard Ratings—A list of the standard ratings for station-type and distribution-type voltage regulators is not included in the chapter; however, they can be Fig. 25—URT station regulator. identified on the equipment cost curves of Fig. 37.

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System Voltage Regulation 267

Table 5-Table of Permissible Regulation Range, REVERSING TAP WITC A Continuous Load Current, and Fault Current Capability for CHANGER Single-Phase, Distribution-Type Step Regulators

Greatest Regula- Continuous Rated Fault Current tion Range to be Load Current in Capability in per unit Used per unit current

10% Boost 1.00 25 x 1.0 for 2.0 seconds 8Y1% Boost 1.10 25 x 1.1 for 1.7 seconds 7M% Boost 1.20 25 x 1.2 for 1.4 seconds 6%% Boost 1.35 25 x 1.35 for 1.1 seconds 5 % Boost 1.60 25 x 1.6 for 0.8 seconds

10% Buck 1.00 25 x 1.0 for 2.0 seconds REGULATING CONTROL 8Vi% Buck 1.10 25 x 1.1 for 1.7 seconds TRANSFORMER CIRCUIT 7%% Buek 1.20 25 x 1.2 for 1.4 seconds PREVENTIVE 63% Buck 1.35 25 x 1.35 for 1.1 seconds AUTOTRANSFORMER 5% Buck 1.60 25 x 1.6 for 0.8 seconds S2 La 21. Induction-Type Feeder Voltage Regulators Fig. 28-Schematic diagram showing contact and winding Induction regulators were the first type of automatic arrangement of the URL step regulator. voltage regulators used on distribution systems. Their use has become lessened in the last several years due gradually varies from the maximum positive, through to the acceptance of the lower cost and equally reliable zero or neutral, to the maximum negative value. The step regulators, and because of the increase in feeder induced voltage in the series winding is always in voltages and capacities. Many induction regulators are phase with excitation voltage and adds to or subtracts presently in use and will be for a good many years. Induction feeder-voltage regulators operate on the transformer principle, although their internal construction resembles that of an induction motor (Fig. 29). Induction regulators have a stator and a rotor, with a primary or shunt winding on the rotor and a secondary or series winding on the stator. The secondary winding is connected in series with the line to be regulated, while the shunt or rotor winding is connected across the line and supplies excitation for the regulator. As the shunt winding (rotor) is rotated, the direction of flux which links the series winding (stator) varies. This produces in the series winding a voltage which

Fig. 27-Installation photograph of the URF, distribution- type step regulators used for feeder application in sub- Fig. 29-Cutaway view of a single-phase induction stations-Open-delta connections used for both feeders. regulator.

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directly from the line or regulator input voltage. The automatic voltage regulator. Most applications in a position of the rotor with respect to the stator will distribution system are single-step similar to Fig. 31 therefore determine the amount of buck or boost pro- and are located out on the primary feeders near the loads. vided by the regulator at any particular time. Multi-step banks are much larger and generally located Since the circuit is not open at any point, the voltage at distribution substations or on subtransmission change is smooth and gradual in either direction. Opera- systems. tion is initiated by the voltage-regulating relay, where- upon a driving motor automatically moves the rotor C2 in the proper direction to restore the output voltage to the desired value. The design of standard induction regulators is such that there is always an equal amount of buck and boost. The regulation range is plus or minus 10 per cent, with the possibility of reconnecting without un- tanking for a plus or minus 5 per cent range. Reducing the regulation range permits operation at twice nameplate current or kva rating. Their single-phase kva ratings have been 19.1 to 125 kva at 7620 volts and below. Three-phase ratings were available from 112 to 300 kva at 4330 volts. There are two types of three-phase induction regulators, the single-core and the triplex type. The single- core type has a winding for each phase on the rotor and one for each phase on the stator similar to a generator. A 10 per cent voltage is always induced in the series windings as long as the shunt winding is energized. The regulator output voltage will depend upon the phase relationship of the induced series REGULATOR INPUT VOLTAGE- LINE TO LINE AB, BC, CA (stator) voltage and the shunt (rotor) voltage, as the LINE TO NEUTRAL OA, OB, OC output voltage is their vector sum. This causes a phase shift between the regulator input and output REGULATOR OUTPUT VOLTAGE- voltage except when the regulator is at the full boost NEUTRAL POSITION - LINE TO LINE A, B,, B, CI , C, A, or buck position. This is illustrated in Fig. 30(a). The LINE TO NEUTRAL OA, , OBI , OC, amount of phase shift for various buck and boost regulator positions is shown in Fig. 30(b). The phase TEN PER CENT BOOST -LINE TO LINE Az ez, 82 62 , 62 Az shift causes no harm if the single-core regulator is LINE TO NEUTRAL 0A2 , 082 , 0C2 installed in a radial feeder; such applications are found TEN PER CENT BUCK - LINE TO LINE A3 B3 , B3C3 , C3 A3 on many old feeders. A feeder containing this type LINE TO NEUTRAL 0A3 , OB3 , 063 of regulator should not be operated in parallel with another feeder (both feeders connected on the output (a) Input and output voltage phasor relationship for single- side of the regulator) as circulating currents can result, core, three-phase induction regulators. possibly causing incorrect operation. Single-core, three-phase induction regulators are no . longer being purchased by utility companies for application on distribution systems. Many are still being pi REGULATOR used and will be for some time on old existing feeders. Where the three-phase induction regulators are to be : I applied, the triplex design is predominately used. The triplex, three-phase induction regulator consists essentially of three single-phase induction regulators, mechanically connected together with a flexible shaft 2 ,%..A. assembly and mounted in one tank. The three-phase unit is operated from one control. No phase shift between regulator input and output voltage exists in the triplex design because three single-phase units are used. This is in contrast to the single-core, three-phase de- -10 -a -6 -4 -2 6 10 % VOLTAGE GUM % VOLTAGE 6005T sign, where a phase shift does occur. NEUTRAL

22. Switched Shunt Capacitors (b) Phase shift in degrees for three-phase, single-core in- If a shunt capacitor application includes automatic duction regulators. switching equipment, it can also be considered as an Fig. 30.

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practical. The details of the undesirable phenomena or operating problems and the methods and cost of protecting the series capacitor are beyond the scope of the chapter; further discussion is included in Chapter 9 and in References 14 and 15. The cost of applying series capacitors plus the cost of the protective devices, in general, is more expensive for steady-state feeder 'voltage control than the use of fixed and switched shunt capacitors or distribution-type voltage regulators. For applications where excessive voltage dip and objectionable lamp flicker are encountered, series capacitors are often the most economical solution. Construction-wise, shunt and series capacitors are identical. If there is no longer a need for a particular series capacitor installation, the capacitor units can be removed and reinstalled as shunt units.

24. By-Passing Voltage Regulators Voltage regulators are taken out of service for inspection and maintenance at periodic intervals. To maintain continuity of service while the voltage regulator is being connected or disconnected and to prevent damage to the voltage regulator, a certain sequence of steps must be followed. The series winding of the voltage regulator is, as its name describes, in series with the line or feeder. To maintain continuity of service while connecting or dis- Fig. 31—Installation photo of an autotrol capacitor connecting, it will be necessary to short across the series installation. winding at one time or another during the work procedure. At that time it is necessary that no voltage appears across the series winding; otherwise a high cir- Most switched banks are three-phase and made up culating current will result. This circulating current can of individual capacitor units rated 25, 50, or 100 kvar. be several times nameplate rating, depending upon the Standard bank ratings are available up to 6000 kvar. voltage induced in the series winding and the type of For large banks, the amount of kvar to be switched voltage regulator (step or induction). The high current per step is not standard and is directly dependent upon could cause considerable damage to the voltage regu- the application. The smaller switched banks (Fig. 31) lator and possibly to the operator closing the disconnect are generally factory assembled and come in standard switch. kvar ratings of 225, 300, 450, and 600 kvar and voltage Single-phase step and induction regulators, three- ratings of 2.4 to 13.8 kv. phase step regulators and three-phase triplex induction The amount of voltage change permissible when regulators present no problem when by-passing, pro- switching one step depends to a large degree on the viding the proper sequence of operations is followed. individual utility company. A voltage change of two Single-core, three-phase induction regulators do present or three per cent is generally very acceptable and often a problem, because there is no neutral position within where a switching operation occurs only two to five the regulation range where no voltage is induced in times per day, a four or five per cent voltage change the series winding. The sequence of operation will first is permissible. be described for the single-phase regulators and three- phase step and triplex induction regulators. 23. Series Capacitors The voltage induced in the series winding of the Probably the least used automatic voltage regulator single-phase step and induction regulators is always is the series capacitor. This has occurred because the in phase with the primary voltage, but it may vary in behavior of the series capacitor in particular applica- magnitude from full value boost to full value buck. tions has caused some unsolved problems. The principle When the regulator is in the neutral position, there is of the series capacitor as a voltage regulator is generally no voltage induced in the series winding, because the accepted and understood. The operating problems and series winding is not in the circuit for the step regulator method of protection have caused the infrequent use. and because of the relative position of the primary When series capacitors are applied on feeders, there is and series winding in the induction regulator. When the possibility of undesirable phenomena occurring, shorting the series winding, it is then necessary to have usually involving some kind of resonance. For many the regulator in the neutral position. Following Fig. 32, applications the difficulties can be anticipated and the sequence of operation for disconnecting a voltage required precautions taken to make the installation regulator can be summarized as follows:

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1. On the voltage regulator control panel, switch breakers, combinations of circuit breakers and inter- the regulator control from automatic to manual locking switches, open type disconnect switches, or and operate the regulator to the neutral posi- cutouts with disconnecting blades. Standard voltage tion. The neutral position is observed on the regulator by-pass switches are also available which regulator position indicator. include switches "B", "C", and "D" (Fig. 32) as an integral unit, and when the by-pass switch is operated 2. Open the control power breaker. the correct switching sequence occurs. When discon- 3. Close switch "D" shorting the series winding. necting, the first opening movement of the switch closes by-pass "D", and the continuing movement opens 4. Open switch "C" and then open switch "B" switches "C" and "B". 5. Open switch "A" if used. In substations for economic reasons, the open type disconnects are usually preferred, while out on primary Again following Fig. 32, the sequence of operation for feeders pole-mounted cutouts with disconnecting blades connecting a regulator into an energized circuit can be are preferred. The special regulator by-pass disconnect summarized as follows: switch is a relatively new device, and it is beginning to 1. Be sure the regulator is in the neutral position. gain greater acceptance as it helps eliminate some of If not, from an external control source operate the possible errors which can occur. the regulator to neutral. Leave the control Switches "B", "C", and "D" (Fig. 32) must be switch in the manual operation position. capable of carrying, but not interrupting, line current. Switches "B" and "C" must be capable of interrupting 2. Close switch "A" if used. regulator magnetizing current. 3. Close switch "B" and then switch "C". The problem is somewhat different when by-passing a single-core, three-phase induction regulator. There is 4. Open switch "D". no neutral position within the regulation range where no voltage is induced in the series winding. One cannot 5. Turn control switch from manual to automatic. directly short the series winding when the regulator is The voltage regulator will then operate to in the neutral position, as was done with the single-phase satisfy voltage requirements if necessary. regulators. When disconnecting, it is first necessary Fig. 32 is the schematic diagram for a single-phase to shunt the series winding with a resistor or reactor, regulator or for one phase of a three-phase regulator. and then after the primary or exciting winding of the Hence, for a three-phase regulator the above sequence regulator has been opened, the resistor or reactor is applies for each phase. Three identical sets of switches shorted. The resistor or reactor would carry load current would be used. for the length of time required to complete the by-pass There are several types of equipment which can be switching sequence. Fig. 33 illustrates the schematic used for by-passing regulators. Some are circuit diagram of one phase of the single-core, three-phase regulator with its associated by-pass switches and shunting resistor or reactor. The arrangement would be the same for each phase, and corresponding switches of each phase would likely be a three-pole, gang- operated switch so that all phases can be by-passed simultaneously. No switch is shown in the common leg (Fig. 33), because it would be connected to the neutral S I LI or ground. Often the neutral point of the three phases SERIES WINDING is not brought out to a neutral terminal or bushing. Following Fig. 33, the sequence of operation for PRIMARY SOURCE OR LOAD disconnecting a single-core, three-phase induction regu- EXCITING lator can be summarized as follows: WINDING 1. Switch the regulator to manual control and S2- L2 bring the regulator to the neutral position. 2. Open the control power breaker. 1 3. Close switch "E" shunting the series winding with a resistor or reactor. FOR REGULATOR IN CIRCUIT - O 0 © CLOSED OPEN 4. Open switch "C". Close switch "D" shorting out the resistance FOR REGULATOR OUT OF CIRCUIT - 0 0 © OPEN 5. 0 CLOSED or reactor. 6. Open switch "E" (There may be a switch on Fig. 32—Schematic diagram of a feeder voltage regulator both sides of the resistor or reactor to com- illustrating switches used for by-passing. Switch "A" is re- pletely de-energize the resistor or reactor). quired only when the terminal S2-L2 is not connected to the neutral or ground wire. 7. Open switch "B".

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when the primary winding is first disconnected from the feeder circuit.

VI. SUBSTATION REGULATION

SHUNT 25. Bus Regulation RESISTOR OR SOURCE REACTOR LOAD Bus regulation is the simultaneous voltage control of two or more feeders served from the same substation SI LI SERIES bus; the voltage at the bus is held within fixed pre- WINDING determined limits. A simple-radial substation schematic diagram with bus regulation is shown in Fig. 34. For PRIMARY OR most applications bus regulation is accomplished with EXCITING load-tap-changing mechanisms (LTC) within the sub- WINDING station transformer or with step-type voltage regulators SO-LO applied between the low-voltage bus and the substation COMMON POINT OF THE THREE PRIMARY WINDINGS transformer. Separate voltage regulators, not LTC, applied for bus regulation are predominately three- FOR REGULATOR IN CIRCUIT- ® © CLOSED phase. Single-phase regulators applied in a three-phase 0 T) OPEN bank are used only where unbalance in the bus phase voltage is excessive, or where power company require- FOR REGULATOR OUT OF CIRCUIT- © 0 OPEN ments demand a spare regulator to be included in a 0 CLOSED substation for emergency and maintenance purposes. NOTE: Where a spare regulator is required, four single-phase SWITCH 0 FOR SHUNTING SERIES WINDING DURING regulators are lower in cost than two three-phase units. CONNECTING a DISCONNECTING SWITCHING OPERATION ONLY Whether LTC equipment is used or a separate voltage regulator will depend upon the particular situation. It Fig. 33—Schematic diagram of one phase of a single-core, is necessary to take the transformer out of service to three-phase induction regulator showing by-passing switches perform maintenance on the LTC equipment, and un- and resistor or reactor used for shunting the series winding less an alternate source is available from a portable during by-passing operation. substation, or other transformers in the same or other substations, a service interruption is necessary. If Again following Fig. 33, the sequence of operation for separate voltage regulators are used, they can be by- connecting the regulator into an energized circuit can passed temporarily, and no provisions for alternate be summarized as follows: sources are required. 1. Be sure the regulator is in the neutral position. A bank of switched capacitors located in a substation If not, from an external control source operate and connected to the low-voltage bus is also considered the regulator to neutral. Leave the control bus regulation. Rarely are switched capacitors used switch in the manual operation position. alone for bus regulation. The voltage change per step is 2. Close switch "B".

3. Close switch "E". (There may be a switch on INCOMING both sides of the resistor or reactor.) H V SUPPLY 4. Open switch "D". 5. Close switch "C". 6. Open switch "E". Another method of by-passing single-core, three-phase induction regulators without interrupting service is to use another three-phase regulator instead of the resistor or reactor. This is frequently done in substations where a spare regulator is available. The spare regulator is paralleled with the first regulator with its rotor in the same position. The first regulator is then removed. Still another scheme of disconnecting a single-core, PRIMARY FEEDERS three-phase induction regulator is to shunt the primary ( b) or exciting winding with a reactor, open the primary (a) circuit, by-pass the series winding with a direct short- Fig. 34—A schematic diagram of a simple-radial, four circuit, and then disconnect the regulator. The shunting feeder substation with bus regulation. reactor across the primary winding prevents the possi- (a) LTC in substation transformer. bility of high induced voltages in the primary winding (b) Using separate step-type voltage regulators.

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generally too large and the number of steps, due to cost, are too few for fine voltage control. Switched capacitors located in substations generally supplement the substation voltage control equipment. The bus voltage regulator will correct for any varia- Mfr cur tions in the incoming supply voltage to maintain the bus voltage within the predetermined voltage limits; in addition, its controls can be set so that a high bus voltage is held during peak-load periods and a lower bus voltage during light-load periods. Bus regulation cannot correct for low primary voltage out on a feeder that is due to 1 excessive primary feeder drop. Its primary purpose is ) to provide adequate voltage at the start of each pri- NO mary feeder to permit economic primary feeder design and maintain the specified primary feeder voltage spread. (a) (b) The regulation range of voltage regulators and LTC Fig. 35—Single line diagrams of other substation arrange- equipment used for bus regulation is plus or minus 10 ments utilizing bus regulation. per cent, although in some cases a smaller range is (a) Two regulated buses served from one transformer. adequate. The choice of regulation range should be (b) Two regulated buses served from two transformers in the based on the maximum probable variation in the in- same substation site. coming voltage and the buck and boost required to maintain the predetermined bus voltage limits. Further discussion on selecting the required regulation range for 35(b). The residential feeders are served from one bus a particular application is included in Section 29. The regulated section and the industrial feeders from the regulation range for most voltage control equipment other. One advantage for such an arrangement is that used for bus regulation is divided into 32 steps giving a the industrial load, which often causes objectionable % per cent voltage change per step. On a 120-volt base voltage flicker, can be segregated from the residential this is % of a volt change per step. load except during emergency conditions. The control intelligence of the tap-changing mecha- Common applications of bus regulation are in sub- nisms is usually taken from one of the phases at the bus; stations serving subtrarismission circuits, secondary net- therefore, the voltage held at the bus coincides with the work primary feeders, and industrial feeders serving loading or voltage change occurring on the controlling concentrated loads. Also, small substations located at phase, regardless of the requirement of the other two the load centers of high load density areas, primary phases. For good bus regulation with three-phase regu- network substations, and spot networks use bus regula- lators, loads on each phase at the bus, as well as along tion. Present trends indicate that the number of substa- each of the feeders, should be balanced. To simultane- tions with three-phase bus regulation is increasing. This ously obtain adequate voltage control for two or more is due to the greater use of small capacity substations feeders, certain requirements are necessary for the feed- located at the load centers, the increased engineering ers. They should have similar characteristics: feeder kva size; coincidental load cycles; equi-distant to the first load; equi-distant to the feeder load center. Generally, if all of the feeders served from one bus are the same type (residential, rural, industrial, or commercial), bus regulation is possible. If the feeders are not of the same nrYThrlrY1 type, it may then be necessary to use individual feeder regulation. In substations containing a number of feeders, it is often feasible to divide the feeders into two or more groups of similar or equal regulation requirements and ) ) ) ) have a voltage regulator for each group. An example of two regulated buses in the same substation is shown in Fig. 35. Also, in substations where one feeder might be dissimilar to the other feeders and not conform to bus regulation, an additional voltage regulator can be located in the dissimilar feeder to supplement the bus regulator. The additional regulator could be located in the feeder at the substation or out along the feeder away from the substation. When residential and industrial loads are served from the same substation location, a common substation Fig. 36—Simple single-lineI IT diagram1 of a 4-feeder1 sub- arrangement utilizing bus regulation is shown in Fig. station with individual feeder regulation.

EN; Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 280/576 System Voltage Regulation. 273 time toward grouping loads and system planning, and the increased use of supplementary regulators and shunt 27. Initial Investment capacitors out on the primary feeders. A comparison of installed costs for voltage control equipment is shown in Fig. 37 for a single-feeder sub- 26. Individual Feeder Regulation station. The comparison also applies for the cost of volt- Individual feeder regulation is maintaining the voltage control equipment in an individual feeder in a multi- age of a single feeder within predetermined limits. Reg- feeder substation, with exception of the LTC cost curve. ulating each feeder is in contrast to bus regulation, where The cost curves are for standard-type voltage regulators two or more feeders are regulated by a single regulator and include the first cost of the equipment plus general bank. Individual feeder regulation is also considered a installation costs. The costs of by-passing equipment, method of substation voltage control, in that it corrects maintenance, losses, and real estate space are not for any varying of voltage in the substation high-voltage included. supply, and its controls can be set to maintain a high Where a +10 per cent regulation range is required feeder source voltage during heavy feeder loading and a and for the common primary feeder voltage levels, lower feeder source voltage during light-load periods. the use of LTC within the substation transformer The feeder regulators are located in the substation as represents the lowest initial investment. However, shown in Fig. 36. With voltage regulators located in where two, single-phase regulators can be applied each feeder, no voltage control equipment is necessary in open-delta for three-phase substation regulation, in the susbsta Lion transformer. Occasionally the voltage the use of separate distribution-type regulators spread at the substation bus and the load and feeder may offer the lowest initial cost. This is the case characteristics are such that voltage regulators are not for single feeder substations with a low-side voltage required in every feeder. of 4.8 kv. (Fig. 37(b)). In contrast to bus regulation, the regulation required Where a regulation range of + 5 per cent is permitted, on each feeder is entirely independent of the regulation the distribution-type step regulators, with the permis- necessary on the other feeders served from the same sible 160 per cent loading, are least expensive for the com- bus. The voltage regulator is individually controlled mon single-feeder substation ratings. If, however, all from the feeder in which it is located. Therefore, of the substation components and engineering substa- various types of feeders with different load cycles, tion design time were included in a cost comparison, it feeder lengths, feeder sizes, and line distances to the is very probable that standard factory-assembled CSP first load (express feeder portion) can be served from power transformers with ± 10 per cent LTC mechan- the same substation. isms would prove most economical. Individual feeder voltage regulators will not correct The initial investment for voltage regulating equipment for excessive primary feeder voltage drop. They will in multi-feeder substations is shown in Fig. 38 for four only maintain the required feeder source voltage to have common primary feeder voltages. In multi-feeder sub- a constant voltage at the feeder load center for all load stations, whether the regulation range is ±5 or ±10 per conditions or to have the desired voltage spread at the cent, bus regulation utilizing LTC is the least expensive. first distribution transformer. Also, in many applications where a + 10 per cent range is Both three-phase step or induction regulators, or required, bus regulation using a separate station regula- three-phase banks of single-phase step or induction tion is lower in initial cost than individual feeder regula- regulators, are used for individual feeder regulation. tion with distribution-type regulators in each feeder. Whether a three-phase regulator or a bank of single- Where only a ±5 per cent regulation range is required, phase regulators are used will depend upon the partic- individual feeder regulation is generally lower in initial ular application. Factors such as initial investment, de- cost unless the number of feeders served by the substa- gree of unbalance in phase loading, availability of a tion is large. spare regulator for maintenance periods, substation space, losses (three-phase regulators are slightly lower 28. Effect of Dissimilar Feeders than three equivalent single-phase regulators), location Substations often serve various types of feeders—in- of feeder load, and the type of feeder served should be dustrial or residential—with different load cycles. Also, studied for the proper equipment choice. Use of single- feeders could be different in capacity or have different phase regulators is the most common. A comparison of express portions of primary line out to the first load. the initial investment for bus regulation and individual With feeder conditions such as this, it may not be pos- feeder regulation, with the different types of equipment sible to provide an adequate voltage level for every available, is shown in Figs. 37 and 38. The economics are feeder when using bus regulation. Fig. 39 illustrates the further discussed in Section 27. effect that one dissimilar feeder has on the load-carrying Individual feeder regulation is generally used in sub- ability of other similar feeders in multi-feeder substa- stations where the number of feeders served by the sub- tions. The figure shows the per cent loading allowed on station is about four or more. When the number of feed- the remaining similar feeders in the substation to pro- ers served by a substation is more than four, the feeders vide adequate voltage at every load when the loading on generally become dissimilar and individual feeder regu- the dissimilar feeder changes. The curves assume that lation is required unless it is possible to group feeders when all feeders are at 100 per cent load, the last load on similar to Fig. 35(a). each feeder has the minimum permissible voltage. The

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PRIMARY FEEDER VOLTAGE PRIMARY FEEDER VOLTAGE 4.16 KV WYE 13.2 KV WYE 12 12 1-3#,-1- 10% STATION TYPE 1 II STEP REGULATOR _ II

I0 3- 1 # , +10% STATION 10 TYPE STEP REGULATORS 9 9 • 3-1#, + 10% STATION TYPE LTC STEP REGULATORS

8 8 I 1 • 7 7 \ I-34',+10% 6 DISTRIBUTION TYPE 6 ..- STATION TYPE VOLTAGE REGULATOR STEP REGULATORS CC • ARS PER KVA 5 -I 5 haw.- 11111111%i- #,±10%

DOLL s•••••, DISTRIBUTION TYPE 4 4 -,... --, '---,,, STEP REGULATORS 1-30,+10% LTC — 3 3 30,4-10% LTC 3-I¢,±5°/ DISTRIBUTION 2 3-14', +5% DISTRIBUTION - - TYPE STEP REGULATORS TYPE STEP REGULATORS 1 NOV Nay --0---- STANDARD RATINGS --0-- STANDARD RATINGS 1 1958 11958 0 500 1000 1500 2000 2500 3000 0 1000 2000 3000 4000 5000 SUBSTATION KVA RATING SUBSTATION KVA RATING (a) (c)

(a) 4.16-kv, four-wire wye. (c) 13.2-kv, four-wire wye.

PRIMARY FEEDER VOLTAGE PRIMARY FEEDER VOLTAGE 4.8 KV DELTA 13.8 KV DELTA 12 12

II 11 • 1 i 1 + 2-I# - 10% 2-1#, ± 10% STATION TYPE I0 I0 STATION TYPE REGULATORS STEP REGULATORS • OPEN DELTA OPEN DELTA 9

8 8 A

7 ill• KVA I-3#, ± 10% 7 ER KV

ER STATION TYPE P 6 STEP REGULATORS 6 \ lk 1- 3#, ± 0% STATION ._ • TYPE STEP

ARS REGULATOR ARS P •.-. 5 1-3#, - 10• , LTC 5 111 -•• • 2-4, ± 10% DISTRIBUTION DOLL DOLL 4 - 2- 4 i . TYPE STEP REGULATORS DISTRIBUTION TYPE STEP OPEN DELTA 3 REGULATORS OPEN DELTA .1

2-1.,- V% DISTRIBUTION 2 2- I4'. + 5% DISTR BUTION I - 30. 1:10% LT TYPE STEP REGULATORS TYPE STEP REGULATORS OPEN DELTA 1 - OPEN DELTA STANDARD RATINGS NOV NOV --.-- STANDARD RATINGS r 1958 1958 500 1000 1500 2000 2500 3000 0 1000 2000 3000 4000 5000 6000 SUBSTATION KVA RATING SUBSTATION KVA RATING ( b) (d)

(b) 4.8-kv, three-wire delta. (d) 13.8-kv, three-wire delta.

Fig. 37—Approximate installed cost of voltage control equipment in a single feeder substation.

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System Voltage Regulation 275

PRIMARY FEEDER VOLTAGE PRIMARY FEEDER VOLTAGE 13.2 KV— 4.16 KV-4 WIRE 4 WIRE 2 4 6 2 5 4 -1-EEDERC FEEDERS FEEDERS FEEDERS FEEDERS FEEDERS FEEDERS

10 > to a 9 r

J 9 g a pp 4 9- 3 A A E t 1 I 4 1111 1111 II 2 3 4 5 3 • 5 6 7 6 4 6 8 10 12 0 10 12 14 16 116 3 5 7 9 II IS le 6 10 15 90 5 10 119 tA Z6 DO SUBSTATION MVA RATING SUBSTATION MVA RATING (a) (e)

4.8 KV-3 WIRE 13.8 KV-3 WIRE

3 4 2 3 -• 4 FEEDERS FEEDERS FEEDER FEEDERS FEEDERS FEEDERS FEEDERS k el 10

c. .7r - 1 g 5 8 6 S5 4 0 4

r 3 C 3 ▪ M2 M 2 A A A A

0 • It 6 3 10 15 20 25 30 2 3 4 5 3 4 5 6 7 6 4 6 8 M 12 10 12 14 16 16 3 6 13 IS IS S 10 15 20 SUBSTAT ON MVA RATING SUBSTATION MVA RATING ( b) (d) NOV 1958

Fig. 38—Approximate installed cost of voltage control equipment in a multi-feeder substation. Primary feeder voltage— (a) 4.16-kv, four-wire wye. (c) 13.2-kv, four-wire wye. (b) 4.8-kv, three-wire delta. (d) 13.8-kv, three-wire delta.

Key: (A) 1-3 phase, ± 10% Load Tap Changing (LTC); feeder; (E) 3*- I phase, ± 10% Distribution-Type Step (B) 1-3 phase, ± 10% Station-Type Step Regulator; (C) 1- Regulators per feeder; (F) 3*-1 phase, ± 5% Distribution- 3 phase, ± 10% Station-Type Step Regulator per feeder; Type Step Regulators per feeder. (D) 3*-1 phase, ± 10% Station-Type Step Regulators per *2 units connected open delta for 4.8x13.8 kv delta systems.

bus voltage is regulated so that the maximum per- stalled on the dissimilar feeder or feeders (possibly at missible voltage is held at the nearest load to the substa- the substation) and the bus voltage maintained for the tion during heavy-load periods. A reduction in load is other feeders served from the substation. Combinations therefore required on the similar feeders, because the bus of bus and supplementary regulation should be studied voltage is reduced when the dissimilar feeder reduces its before a design decision is made to use individual feeder loading. It is reduced because the load current in the regulation. Previous studies have shown that, depending regulator line drop compensator circuit is reduced. upon the type and rating of regulators used and on the Fig. 39 assumes no supplementary voltage control number of feeders, it may be possible to re-regulate 50 equipment out on any of the feeders or in the substa- to 75 per cent of the total substation kva with supple- tion. While the curves represent hypothetical feeders mentary regulators before the cost of bus regulation plus and feeder conditions, it does show the effect of bus regu- supplementary regulation equals the cost of individual lation alone and how it could impose limited feeder feeder regulation. loading conditions. If a multi-feeder substation will have limited feeder VII. SUPPLEMENTARY REGULATION load conditions with bus regulation alone, it may be economical to use a combination of bus regulation and Supplementary regulation is the use of any voltage supplementary regulation instead of individual feeder control equipment applied to primary or distribution regulation. The supplementary regulators could be in- feeders remote from the substation. Supplementary

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276 System Voltage Regulation

I00 is located near a capacitor installation which causes a et voltage rise above maximum permissible voltage, or 8 90 when the regulator is located in a lateral off the main 6 14- three-phase portion of the feeder near a substation. The 80 main portion of a feeder near a substation may have a high voltage during heavy loading in order to maintain •a 70 2 the maximum permissible voltage at some point farther ce out on the feeder. Most modern distribution-type step 60 regulators for pole-mounting have a bucking range equal 2 to their boost range, ± 5 or ± 10 per cent, because of the w 50 _NUMBER OF FEEDERS IN BUS REGULATED relative small increment in cost to obtain a plus or min- SUBSTATION -3 40 us range and for interchangeability within the system. Because of the lower cost for the distribution-type,

30 single-phase step regulators compared to the station- type regulators, they are beginning to get frequent use 20 in substations as individual feeder regulators, and often a bucking range is required. I0 Switched capacitors used for supplementary regula- Ui tion are single-step. They are located at the feeder load 0 center; near the end of the main three-phase portion of 10 20 30 40 50 60 70 80 90 100 the feeder; or near the end of a three-phase branch off PER CENT LOADING ON DISSIMILAR FEEDER the main feeder portion. Switched capacitor banks are Fig. 39—Curves showing the effect a dissimilar feeder will generally three-phase so that a single intelligence and have on load carrying ability of the remaining feeders in a control power source can be used for each phase. If a bus regulated substation. Line drop compensator settings: separate intelligence source and control power source R = 5, X = 1.5. are used for each phase, which would be the case if switched banks were located on single phase laterals, the overall cost becomes prohibitive when compared to voltage control equipment is used to supplement any other methods of voltage control or to a three-phase regulator that may be applied at the substation. The bank located on the main three-phase feeder portion. voltage control equipment used is predominately single- The voltage rise for switched capacitor banks located on phase, pole-mounted step regulators (Fig. 26) or small feeders is generally two to four per cent, depending upon installations of switched capacitor units (Fig. 31). the bank rating and location. The amount of voltage The pole-mounted step regulators would be located rise should not cause the feeder voltage to go above the out along the feeder just ahead of the point on the feeder maximum permissible voltage at any load. where the feeder voltage falls below minimum permissible (Fig. 16). The unit is located ahead of the minimum permissible voltage point to allow for feeder load VIII. REGULATOR RANGE AND KVA RATING growth. The maximum amount the supplementary step regulator can raise the voltage is the amount necessary 29. Voltage Regulator Range Requirements to obtain the maximum permissible feeder voltage at the To adequately perform its function, the regulation first load beyond the regulator location. The per cent range of a voltage regulator should be large enough to boost possible would be the permissible primary feeder correct for any variations in the regulator input voltage voltage spread plus the per cent voltage drop of the line and to compensate for a portion of feeder line drop be- from the regulator location to the first distribution trans- yond the regulator location. It has not proved economi- former beyond the regulator location. If a portion of the cal to design a voltage regulator for every application; first distribution transformer voltage drop is also to be hence a standard 10 per cent regulation range in either compensated for, the maximum permissible feeder vol- direction has been accepted by the industry. Almost all tage is then held at a point beyond the first transformer voltage regulators applied on distribution systems to-. after the regulator. In general, the maximum boost for a day, both station and distribution type, have a +10 per supplementary step regulatOr would be four to eight per cent regulation range. Some standard + 10 per cent cent. On long rural feeders or on feeders where a large regulators permit greater load carrying ability if the primary feeder voltage spread is permissible, a boost of operating range is reduced. Therefore, for economy in ten per cent may be required; however, such a situation applying regulators, it is necessary to check the voltage would be exceptional. In situations where an eight to range that is required for an application. ten per cent boost is required, the regulator is likely in Station Regulators—For voltage regulators applied in some boost position throughout all load conditions. This distribution substations, either as bus regulators or in- indicates the voltage level of the entire feeder is low. dividual feeder regulators, the regulation range will A bucking range is very seldom required for regula- have to be large enough to correct for incoming sub- tors located out along the feeder. The only applications transmission voltage variations plus the substation for the bucking range would occur when the regulator transformer voltage drop, and plus any compensation

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 284/576 System Voltage Regulation 277 required of voltage drop in the express portion of feeders sible substation incoming voltage variation when using served by the substation. Equations to determine the a ± 10 per cent voltage range station regulator. A de- permissible incoming subtransmission voltage variation sired regulation point voltage of 121 volts was assumed. are: Figure 40(b) gives values of the maximum permissible input voltage variation for different regulation ranges of I Einput mine = I Erp I.+ I VDrirt (peak load)! + I V De. (peak load) ! (11) station regulators and for different values of peak-load (lEml+ VDrp-t (peak lead)I 100+ ft SUSSTAIXAS TRANSFERRER IV De. (peak load) I) ± I ((peak load)! VOLTAGE VD. sue TRANsmsso, REXILATOR

INCOMING REGULATING PON IEinput maxl = I Erp I+ I V Dr p_t (light load) I+ I V.D.. (light load) I SUPPLY (12) EMPRESS PR !MART Vj FEEDER + (lErpl+ I VDrp-t (light load)I F FIST L AST 100 — R 9 TRANSFORAER TRAMS 140 I VDm (light load)I) VD. (light 1.4) 1 where

Einput =Input or high-side voltage of the substation. 13S HEAVY LOA Q = Constant voltage to be held at a point (regulating point) on a feeder for all load conditions. The regulating point is almost always beyond the first distribution transformer on the feeder $30 as shown in Fig. 40(a). VDtp_t =Primary feeder voltage drop between the regulating point and the first distribution trans- 125 former on the feeder. VD. = Voltage drop in the express portion of the 120 primary feeder. NL SS TROIS = Regulation range in per cent of the station DROP voltage regulator in one direction only. VD. --Substation transformer voltage drop. and (a) Permissible Substation (a) Peak-and light-load voltage profiles showing substation high-voltage supply requirements. High-side Voltage Einput max I — I Einput min 1(13) Variation 20 I - 1 1 Ce 1 I Ce Since it is not always possible to have a distinct regu- SS TRANSFORMER Z • 7% =0 18 lating point, Equations (11) and (12) can be revised to — LIGHT LOAD a PEAK LOAD U/ 4 (7 4 include desired voltage limitations at the first distribu- 4E 16 - 1 tion transformer on the feeder. For such feeders the Or 14 2 L'; maximum permissible primary voltage is held at the w 7 cc' first distribution transformer during peak-load condi- L9 7, 5 12 tions, and at a voltage value nearer the nominal value dur- H 15 9g O .4 ing light-load conditions. The equations would then be: >> to e- g Ei.put mini = I Et (peak load) I+1 V Den O (peak load) I (14) N 8 (I Et (peak load 1+ I VA. /7/ 100+R ) (Peak load)I) + I VD's (peak load)I 4 5 Einput manl = I Et (light load) I + I V Den (light load a. 2 if/ l ) I (15) R L., 0 I t (light load) (light load) I) 0 2 3 4 5 6 7 8 9 I0 ▪ 100 —R ± PER CENT VOLTAGE REGULATOR RANGE +I VD.. (light load) I where Et .= The voltage to be held at the primary of the first (b) distribution transformer on the feeder. (b) Permissible high-voltage supply requirements for different Figure 40(a) shows a voltage profile of a typical substa- station regulator ranges and express-feeder voltage drop. tion and primary feeder showing the maximum permis- Fig. 40—Station voltage regulator range requirements.

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278 System Voltage Regulation voltage drop from the substation to the first distribution portion connected to an adjoining feeder, the system transformer. The curves assume a three-volt difference reactance from the capacitors back toward the source between light-load and peak-load conditions, such as 125 can be increased, causing a possibility of high voltage volts at peak-load and 122 volts at light-load, at the first during light-load periods. To take care of emergency distribution transformer. For voltage differences other conditions and to maintain flexibility of location within than three volts, the permissible high-voltage variation a system, most modern supplementary regulators have would be decreased by approximately the same amount a ± 10 per cent range plus the ability to increase their the difference is increased, or the permissible variation capacity when reducing their regulation range. would be increased by approximately the same amount the difference is decreased. 30. Determining Regulator Kva Rating Requirements The curves assume that the lowest substation input In three-phase circuits, voltage regulators can be ap- voltage occurs during peak-load conditions and the plied using various types of connections. They are wye highest substation input voltage occurs during light-load or delta for three-phase regulators and wye, open-delta, conditions. Also, a 7 per cent impedance substation and closed-delta for single-phase regulators forming a transformer and a light-load of one-fourth of peak-load three-phase bank. Each connection has its own charac- was assumed. teristics regarding regulator kva rating with respect to Equations (11) and (12) or (14) and (15) give the circuit kva and also to voltage phase shift. The kva maximum and minimum permissible substation input nameplate rating of a step or induction voltage regula- voltage; therefore, even if the permissible amount of in- tor is only the kva which is transformed within the regu- put voltage variation for a specific condition is the same lator and not the carrying kva capability of the regula- as the actual variation, it is also necessary that the tor or the connected kva load. variations occur between the maximum and minimum From the simplified circuit of a voltage regulator values. If the variations do not occur between the maxi- shown in Fig. 41, general equations can be obtained. mum and minimum values, the general voltage level of When in the maximum boost or buck position, the incoming voltage will have to be changed. It can be changed by the manual no-load taps of the distribution Per cent regulation range — E.1 — 1 E.1 (100) (16) substation transformer or of the -subtransmission substation transformer. If the voltage variations cause the where E. and E,: are the regulator input and output input voltage to fall below the minimum permissible or voltage respectively; then, above the maximum for a specific condition, the voltage regulation range of the station regulator will have to be Kva of regulation =(EC! rEar) (Circuit kva) (17) IE.' increased by the amount that the input voltage went and above or below the permissible spread. For example, a substation with an incoming voltage variation of 12 Regulator kva rating = %R(circuit kva) %RIE.11/.1 volts (120-volt base) and a voltage drop at peak-load to (18) the first distribution transformer of 5 volts can utilize a 100 100 regulator with a regulation range of ± 8% per cent, Fig. where 40(b). If the lowest voltage of the incoming voltage spread is 1.5 volts below the minimum value permissible %R = Maximum per cent regulation range of regulator (Equation 14), the regulation range of the station regu- in the buck or boost direction, Equation (16). lator would have to be increased 1.5 volts to +10 per For a single-phase regulator applied on single-phase cent. circuits, Equation (18) would be used, and E. would be Supplementary Regulators—For distribution-type step the single-phase voltage and I. the line current, similar voltage regulators applied on primary feeders, the per to Fig. 41. cent boost range should be the sum of the permissible Three-Phase, Four-Wire Wye Circuits—Three-phase reg- primary feeder voltage spread and the feeder drop from ulators for four-wire, three-phase circuits are always the regulator location to the first distribution trans- connected wye, and the three-phase nameplate kva rating former beyond the regulator. Occasionally part of the is determined using Equation (18), with the circuit kva first distribution transformer drop is also compensated for by the supplementary regulator; if so, it should also be included in the boost range. The buck range is seldom required. When it is needed the range is generally less than the boost range required for the common supplementary regulator applications. Ia Supplementary voltage regulators generally have a E ra LOAD regulation range greater than required for normal use in SOURCE 1E 0 order to help maintain correct voltages during emergency conditions. During feeder fault conditions, portions of the faulted feeder are often connected to adjoining feeders. This can cause low-voltage conditions. Fig. 41—Simple schematic diagram of a single-phase voltage Also, where fixed capacitors are located on the feeder regulator.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 286/576 System Voltage Regulation 279 being the three-phase circuit kva. Load is assumed balanced on each of the phases. 3-Phase regulator kva rating = REGULATOR I %R(3-phase circuit kva) = %R(VIEI 111) (19) 100 100 where E= Three-phase circuit line to line voltage in kv. Is I = Line current. REGULATOR 2 For example, on a 2400/4160-volt primary feeder, where /1..1=278 amperes Regulator range required = + 10 per cent. VOLTAGE CURRENT The regulator nameplate rating would be: REGULATOR I Ew a I A REGULATOR 2 EcB i (10)(N) (4.16) (278) Regulator kva rating = = 200 kva 100 Fig. 43—Schematic diagram of two single-phase voltage regulators connected open-delta on a three-phase, three- Equation (19) would apply for all three-phase step or wire circuit. induction regulators. A three-phase, four-wire circuit can be regulated by a diagram of Fig. 44, because a given equal per cent voltage bank of three single-phase units connected in wye as increase of voltages E BC and E AB to E Etc, and EA-B, respec- shown in Fig. 42. The kva rating of each unit is deter- tively, will cause an equal voltage increase of &A t° EC' A'. mined using Equation (18). Using the same example as When using two regulators in open-delta, care must be above, the kva rating of each regulator would be: taken in selecting the regulator kva ratings to obtain suf- 1-phase regulator kva rating = ficient kva of regulation. The two regulators are con- (10)(2.4)(278) nected from one phase conductor to another; hence, the — 66.67 kva. 100 regulator input voltage is the three-phase line-to-line voltage and not the line-to-neutral voltage. Using Equa- The kva rating of a single unit in a wye connection is tion (18), the kva rating of each single-phase regulator equal to one-third of the rating for a three-phase regula- would be, tor to be applied on the same circuit. EI Three-Phase, Three-Wire Circuits—The kva ratings of Regulator kva rating —%RI III (20) three-phase regulators applied on three-phase, three- 100 wire circuits are determined the same as for four-wire and E would be the line-to-line voltage and I the three- circuits. Hence, Equation (19) can also be used. Three- phase line current. phase regulators applied on three-wire circuits are the Since same as applied on four-wire circuits. They are normally Three-phase circuit kva = 1E1111 wye connected, but with no line connection to the neutral. then A common method of obtaining three-phase regula- Regulator kva rating .-- tion in three-wire circuits is to use two single-phase %R(-V-31E1 _%R (Circuit kva) regulators connected open-delta, as shown in Fig. 43. III) (21) Three-phase regulation is obtained, following the phasor (100)(Vg) (100)(Vd) When using standard +10 per cent single-phase regulators in open-delta, the kva rating required of each unit is equal to the three-phase circuit kva divided by I_A.. (T. cic0 (100-) or 17.32. For example, on a three-wire 4800-volt, three-phase primary feeder where the line current is 240 amperes 15 REGULATOR 3 with a three-phase load of 2000 kva, the kva rating of w 8 each ± 10 per cent range, single-phase unit would be 0 N (10)(4.8)(240) kva= =116 kva ti 100 or Ea 2000 kva= -116 kva 10-0 Three-phase regulation can also be obtained with a N N closed-delta arrangement using three single-phase regu- Fig. 42—Schematic diagram of three, single-phase voltage lators. The addition of the third regulator and closing regulators wye connected on a four-wire, three-phase circuit. the delta does not permit the regulator bank to carry

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0 10

0 8 5 7 A ,B ,C REGULATOR INPUT cp 6 VOLTAGE cc 1- A',8 ,C. REGULATOR OUTPUT VOLTAGE c.) w 2

-6 -2 -8 -4 4 8 12 16 PER CENT DECREASE IN PER CENT INCREASE IN LINE VOLTAGE LINE VOLTAGE 0 03 03

Fig. 44—Phasor diagram of two, single-phase voltage 0 regulators connected open-delta. .J 6 more load, but it does increase the regulation range of the bank by approximately 50 per cent. That is, when 1- 8 wz installing three, ± 10 per cent single-phase regulators in 10 closed-delta, the regulation range of the three-phase ca bank is approximately + 15 per cent. The voltage phasor a. diagrams in Fig. 45 illustrate the 1.5 ratio between the Fig. 46—Curve of per cent change in line voltage for in- per cent change of the system voltage and the per cent dividual regulator buck or boost positions for closed-delta voltage change of the individual regulators. The 1.5 connections using three single-phase voltage regulators on a ratio is not exact, but is extremely accurate for voltage three-phase, three-wire circuit. regulators with small regulation ranges. Fig. 46 shows the per cent change in line voltage for different per cent shown in Fig. 47(b), has the current lagging by 30 de- voltage boost or buck positions of the individual regu- grees. The phasor diagrams showing the current and lators. voltage relationships for each arrangement are also As in the open-delta connection, each regulator in the shown in Fig. 47. closed-delta connection carries line current, and the exciting or primary winding is connected line-to-line. Eat Therefore, the kva rating of each individual unit is determined using Equations (20) or (21), and they are the same rating as for an open-delta connection. There are two arrangements for connecting three single-phase regulators in closed-delta. One arrangement, as shown in Fig. 47(a), has the line current associated with each regulator, at unity power factor loads, leading REG 2 the line-to-line voltage across the regulator exciting winding by 30 degrees. The other arrangement, as

(o) CLOSED-DELTA LEADING CONNECTION

A,13.0 — SOURCE LINE VOLTAGE

N,11: — REGULATOR OUTPUT VOLTAGE (115 E REG T INDIVIDUAL Err REGULATOR INPUT VOLTAGES REG 2 INPUT LINE VOLTAGE

Err REG 3

B. ID% CA .

(b) CLOSED-DELTA LAGGING CONNECTION Fig. 45—Phasor diagram of three, single-phase voltage regulators connected closed-delta. Each regulator is in the Fig. 47—Closed-delta regulator connections and phasor 10 per cent voltage boost position. diagrams using three single-phase voltage regulators.

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Generally there are no advantages in using a leading 16 connection over a lagging connection, or vice versa. I4 However, when the regulator bank is to compensate for 0 0 12 part of the feeder voltage drop beyond the regulator 10 location, one connection may favor the other. This is co because modified line drop compensator settings are re- 4 8 - 1 quired to correct for the 30 degree lead or lag between O 6 the current and voltage in each regulator. Depending upon the R and X of the feeder, negative settings may W 4 be required, or it may be necessary to go beyond the 3 2 range of the line drop compensator elements for one iP -4 - -2 -I 3 4 connection as compared to the other. Further discussion T IN DEGREES ANGLE SHIFT IN DEGREES -2 ANGLE SHIF on determining line drop compensator settings for delta (BUCK) (BOOST) or open-delta regulator connections appears in Sections g-4 39 and 41. 6

With the closed-delta arrangement, a phase shift re- F - sults between the input and output line voltage of the voltage regulator. This phase shift is shown in the 10 w 12 phasor diagram of Fig. 45, and the angle of phase shift z is different for each buck or boost regulator position. 14 The actual angle of phase shift is shown in Fig. 48. 16 In three-phase, three-wire circuits, the use of three single-phase regulators connected in wye is not recom- Fig. 48—Curve of l'ne voltage phase-angle shif when using mended. The neutral or common point of the wye con- three single-phase voltage regulators in closed-delta. nection would be floating, and unbalanced load conditions or different regulator control response characteristics could cause this point to shift. With individual regula- 31. Voltage Regulating Relay—Bandwidth tor controls, unstable operation could result. Even if one The voltage regulating relay (often called the primary set of controls were used for all three single-phase units, relay) is the receiver of the intelligence from the cir- differences in excitation characteristics of the three cuit in which the regulator is located, and it initiates the cores and differences in response characteristics may operation of the regulator. It may be a definite relay cause neutral displacement. with contacts initiating the required regulator operation, or it may be a static device utilizing magnetic amplifiers for initiating the required operations. The IX. CONTROLS FOR STEP AND INDUCTION voltage as measured by the voltage regulating relay is VOLTAGE REGULATORS the regulator output voltage less the voltage drop in the The control of the step or induction voltage regulator line drop compensator. If the voltage as seen by the receives its signal or intelligence of the circuit voltage relay is below a preset value, contacts will close (when conditions from the circuit in which the regulator is lo- a definite relay is used) energizing the means for the cated, and it initiates the operation of the regulator to regulator to operate and raise its output voltage; or if either raise or lower the circuit voltage as necessary. Voltage is the only main signal used for the control LINE CURRENT intelligence in step and induction regulators, while switched capacitors used for controlling voltage may use many types of signals—voltage, current, time, CURRENT temperature. Switched capacitor controls are briefly TRANSFORMER discussed in Section 34; they are discussed in detail in Chapter 8. LINE The control components for the step and induction DROP regulators that are of prime importance to a distribu- 0 COMPENSATOR I- 4 5 -1 tion engineer are the voltage regulating relay, the 0 line drop compensator, and the time delay relay. A 0 POTENTIAL VOLTAGE control circuit block diagram is shown in Fig. 49. 0_ TIME Other control components such as auxiliary relays, TRANS- REGULATNG DELAY limit switches, and protective relays may be part of 0 RELAY the control circuit, but they have no bearing in the 0 FORMER RELAY application of the regulators and, hence, will not be O 3 discussed in this chapter. The voltage regulating relay and time delay relay are discussed in this section; CC 1 the line drop compensator, its theory and application, Fig. 49—Step and induction voltage regulator control circuit is discussed in Part X. block diagram.

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282 System Voltage Regulation

the voltage as seen by the relay is above the preset of relay are shown in Fig. 50. In the solenoid-operated value, other contacts will close energizing the means type, Fig. 50(a), the main coil is energized from the for the regulator to operate and lower its output regulator output voltage less the drop in the line voltage. The voltage difference between the raise and drop compensator, and the armature is so adjusted to lower preset values is the relay bandwidth. The mid- balance at a preset voltage. The relay bandwidth is set point voltage value of the bandwidth is considered the by varying the tension required to move the armature relay balance voltage. in the solenoid. At the end of the armature, the left The voltage regulating relays of most common use contacts run the regulator in the raise direction, while are essentially contact-making voltmeters, and they the right contacts run the regulator in the lower direc- can be either a solenoid-operated balanced beam type or tion. The above description is for one type of solenoid of the induction disc principle. Illustrations of each type operating relay. There are various component arrangements, but they all follow the same principle. In the induction disc type of relay, Fig. 50(b), the disc maintains a certain position for an applied voltage and rotates in one direction or the other to a different position as voltage changes. The voltage corresponding to each disc position is marked on a scale, and the raise and lower contacts are set at the desired bandwidth by simply moving the two pointers associated with the voltage scale of the voltage sensitive element to the upper and lower band limit, respectively. In the most modern station regulators, static devices are replacing the voltage regulating relay." The static devices, commonly called MAGAMP control, consist is of a voltage-sensing circuit and a voltage regulating 1: magnetic amplifier. The regulator output voltage less the drop in the line drop compensator is monitored by a (a) Type Si—Solenoid operated, balanced beam relay. circuit composed of non-linear static devices, and if the voltage goes outside the preset bandwidth, magnetic amplifiers energize the time-delay circuit. The time- delay device is also static, and it in turn operates the motor to change the regulator output voltage in either a raise or lower direction. The bandwidth setting of the voltage regulating relay or MAGAMP control—the difference between the voltage raise and lower setting—is an important factor in determining the load capability of a distribution feeder when maintaining a certain feeder utilization voltage spread. Since the relay voltage can be just above the raise relay contact setting or just below the lower contact setting, the bandwidth must be part of the distribution feeder utilization voltage spread. This is illustrated in Fig. 51. If the voltage drop in the feeder components between the first and last customer is equal to the permissible spread, shown as the favorable zone in Fig. 51(a), the voltage near the first consumer's utilization device can go above maximum permissible, or at the end of the feeder the utilization voltage can go below minimum permissible. To stay within the permissible limits, it is necessary to reduce the feeder voltage drop by the control bandwidth, as shown in Fig. 51(b). Therefore, on distribution feeders where voltage drop determines the feeder length and kva loading, the size of the control bandwidth is an important factor. This is shown in Fig. 51(c), where the permissible feeder drop to stay within limits is 12 volts for a three-volt bandwidth, while in Fig. 51(b) a 13-volt drop is permissible with a two-volt bandwidth. The bandwidth should be as small as pos- (b) Type CVR—Induction disc type. sible without obtaining too many operations and ex- Fig. 50—Typical voltage regulating relays. cessive maintenance.

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The bandwidth for step-type voltage regulators must as be larger than the voltage change due to the operation 50 of one step to prevent hunting. Small bandwidths are then possible only if the voltage steps are reasonably 45 small. The conventional % per cent steps of the station- V ERAGE BAND WIDTH I 3.17 VOLTS type step regulators can theoretically use a bandwidth 40 rcEn as small as one volt, but in actual use the number of 35 operations becomes excessive and a two-volt (± 1 volt) band in conjunction with a time-delay relay is generally 30 used. For 114 per cent steps of the distribution type step regulators, a two-volt band can also be satisfactorily 25 used. Pole-mounted applications of both % and lYt a 20 I 1 per cent steps generally use a DA or 3-volt bandwidth, a.re a. however, a two-volt bandwidth can be and is beginning I 5 to be used. For induction regulators the bandwidth need be only 10 wide enough to limit the number of operations and 5 I allow for the necessary tolerance in the setting of the voltage regulating relay. A minimum bandwidth of two volts is generally used. BAND WIDTH IN VOLTS In 1946 a survey of 234 utility companies was taken of the bandwidths used for station-type step regulators. Fig. 52—Distribution curve of bandwidths for station type The results of the survey are shown in Fig. 52. The step regulators—survey coverage of 234 users. minimum bandwidth as shown by the graph is one volt, with the average being 3.17 volts or approximately + 1% volts. The survey further indicates that about 29 per cent of the utility companies reporting operate with a bandwidth of two volts; about 41 per cent use a

DISTRIBLITlel FEEDER bandwidth of three volts; and about 25 per cent of the companies operate with a four-to five-volt band.

VOLTAGE In the instances where a one-volt bandwidth was used, REGULATOR 1" LAST the total regulation range of the station regulator was CUSTOMER CUSTOMER plus or minus five per cent using 32 steps---16 in each 126 1A426261 ALLOWABLE VOLTAGE direction. 124 122 With the present day controls and regulators, the 120 trend is toward using a two-volt ( + 1 volt) bandwidth Il e 116 for all station regulators. Where the load fluctuations 114 are frequently causing voltage changes of two volts 112 110 or more, excessive operations can be prevented with the 1.11WV1A1 ALLOWABLE VOLTAGE time-delay relay and only occasionally is a larger band (0) required. In induction regulator applications where modern controls are used and the frequency of regulator 126 MAXIMUM ALLOWABLE VOLTAGE operation is not too important, the trend may be 124 2 az toward bandwidths less than two volts. 520 Tx It 112 32. Time Delay Time delay is required for feeder voltage regulators to prevent excessive operations due to momentary dis- ANNUM ALLOWABLE VOLTAGE turbances that cause the voltage to go out of the control (b) bandwidth. The time delay can be obtained from a separate time-delay relay, from the inherent time re- 126 sponse characteristics of the voltage regulating relay, 124 122 or from the inherent mechanical time delay of the 120 regulator operating mechanism. The time delay, in 11 0 effect, is in the control circuit between the voltage II 6 114 regulating relay and the motor control circuit, as shown 112 in the block diagram of Fig. 49. Excess operations could 110 14 MIWOJA1 ALLOWABLE VOLTAGE be reduced by increasing the relay bandwidth, but (C) increasing the bandwidth can affect the feeder loading Fig. 51—Voltage profiles showing the effect the regulator or feeder length, as shown in Fig. 51. Using a time delay bandwidth has in keeping within voltage limitations and on instead of increasing the bandwidth is generally the feeder load capability or feeder length. most economical method.

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In station-type step regulators, separate time-delay response for the time delay. The time response of the relays are used in the control circuit. A common relay relay to a voltage change is inversely proportional to is a hi-metal thermal time delay, Type TH, which has the magnitude of the voltage change. Typical time- an adjustable setting of 15 to 60 seconds. On modern voltage curves for the induction-disc type relay used station-type step regulators where the control circuit in the URL and URF regulator (Figs. 26 and 27) are is entirely static, a timing circuit and time-delay mag- shown in Fig. 53. The solid-line curves shown in the netic amplifier is used which has an adjustable setting figure are for minimum damping, and by moving the of 5 to 90 seconds. magnet keeper, shown on Fig. 50(b), different time- Induction voltage regulators do not use a separate delay characteristics can be obtained. Changing the time delay. The only time delay is the inherent mechan- magnet keeper changes the speed of the disc. The time ical time delay of the operating mechanism. It is pos- coordinate in Fig. 53 is multiplied by a factor, depend- sible to omit a separate relay because there is no distinct ing upon the speed change, ranging from 1.0 to 4.4. per cent voltage change for each operation as there is For example, the solid curve "a" for a two-volt band- in a step regulator. The voltage change is small and width would be changed to the dotted curve "a" when gradual, and the unit only operates until the voltage the magnet keeper was adjusted so that the multiple is brought back within the bandwidth. However, in of time at minimum damping is three. As the speed of applications serving rapidly fluctuating loads, time- the disc is decreased for the same voltage change, the delay relays may be needed and can be easily added. time-voltage curves become less inverse. If the induction regulators have static controls, there A survey was made in 1946 of 230 utility companies is no need to limit the number of regulator operations, to determine the various time-delay settings used for as there are no contacts to corrode and get out of ad- station-type step regulators. The results of this survey justment. Hence, regardless of the circuit no time- are shown in Fig. 54. It can be seen that the most com- delay relay would be required. mon settings are: first, 30 seconds; and second, 60 In distribution-type step regulators, any of the same seconds, with an overall average of 36.2 seconds. It is relays used with station-type regulators could be used. difficult to determine what the actual time-delay set- Induction-disc type voltage regulating relays are fre- ting should be for an application, because it will depend quently used on distribution-type step regulators, and upon the bandwidth setting of the voltage regulating it is possible to use the inherent characteristics of relay and on the load or voltage fluctuations of the circuit in which the regulator is applied. An estimate can be made with the aid of the survey results when in- 28 stalling the regulator, and the regulator performance I 26 can be watched for a short period after installing. `A (SEC x3) 24

22 45

g 20 40 1 8 18 36.2 SEC 16 35 14 6 A 12 ct 30 0 C F- I0 7 "- 25 ir 8 O

6 T

EN 20

4 C

2 PER 15 O 2 3 4 5 6 7 8910 1214161820 VOLTS EITHER ABOVE OR BELOW 120 I0 RAISE BLOWER CIRVE CONTACT SETTING 5 A 119 — 121 8 118 — 122 C 117 — 123 0 0 10 20 30 40 50 60 70 80 90 100 110 Fig. 53—Typical time-voltage curve for CVR voltage regu- TIME IN SECONDS lating relay. Solid curves are for minimum damping. Dotted curve is for a magnet keeper setting to reduce the disc speed Fig. 54—Distribution curve of time delay for station-type by a multiple of three. step regulators—survey coverage of 230 users.

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The time-delay setting, if distinct settings are made, The errors which will be due to application and ad- of pole-mounted step regulators should be slightly justment are (1) line drop compensator settings; (2) greater than regulators located in substations. It should voltage-regulating relay bandwidth; (3) the change in be greater to enable the station regulator to operate calibration of the voltage-regulating relay; this type of first for any variations that may occur in the source error occurs due to mechanical wear, dust, moisture, voltage; this assumes the bandwidth of the station and friction and handling and is associated with use and life line regulators are the same or approximately the same. of the equipment. However, if the inverse-type voltage regulating relays Fig. 51 showed that the band in which the voltage canes are used with the pole-mounted units, it is not necessary vary without a regulator operation taking place is a fac- to use a greater time-delay setting. The inverse response tor which determines the permissible feeder voltage characteristics will cause the unit to operate where the drop when staying within permissible limits. These voltage change is needed most. voltage profiles do not consider regulator accuracy. If For the old single or two-step boosters where the in Fig. 51(b) the regulator has Class 1 controls, it is voltage change per step is 2% or 5 per cent, the time possible that the voltage at the first consumer could be delay should be longer, and they generally range from one per cent high (126.2 v) or the last consumer could time settings of one to five minutes. have a utilization voltage one per cent below minimum (108.8 v). If the control accuracy was included, the 33. Control Accuracy feeder drop would have to be decreased by 2.4 volts Voltage regulator controls directly determine the (2%) from 13 volts to 10.6 volts. In generalizing the operation of the equipment, therefore, any in- effect of control accuracy, a ±n per cent error in the herent inaccuracies in the control equipment may cause regulator controls will decrease the minimum consumer incorrect consumer supply voltage and may result in utilization voltage by zero to n per cent, and will in- excessive system investment. The errors affecting the crease the maximum consumer utilization voltage from control system of voltage regulators can be given two zero to n per cent, depending upon the variations of the classifications.18 The first involves errors due to design regulator input voltage. Ideally for the basis of feeder and manufacturing. These errors are all given limits design, the feeder utilization voltage spread should be which are upheld through factory tests. The second decreased by an amount between zero and 2n per type of error occurs in the application of control equip- cent. ment. These errors cannot be completely controlled by The control errors vary slowly with frequency, tem- the manufacturer, as they are errors in adjustment of perature, and load, and the coincidence of the maximum the control equipment. errors caused by each factor occurring in the same The errors that are incident to the design and manu- direction at the same time is problematical. The guar- facture are (1) ratio errors in the potential transformers, anteed accuracy class of the regulator controls by the including the ratio error with no load on the regulator manufacturer is based on the cumulative occurrence of and the effect on the ratio error due to regulator loading all errors in one direction and then the other, so the and regulator position; (2) ratio errors in the current distribution engineer designing the feeder has the choice transformer used to energize the line drop compensator; of planning his design on the control class of the regu- (3) errors in the voltage regulating or primary relay lator or less. Generally, the control accuracy is not con- and the associated control circuits. sidered in the feeder design even though the utilization For the above mentioned errors, the limits to be upheld voltage may go out of the permissible zone. This is are based on the ASA Standards" for step and induction especially true where regulators with Class 1 accuracy voltage regulators (C57.15), and the limits should not are applied, although many regulators due to years of be exceeded over a frequency range of ±0.25 per cent operation have control accuracies of ±3 per cent or (±0.15 cycle on a 60-cycle system) and over a range of higher and consideration should be given to the control external ambient temperature of —30 to +40 degrees, error. Centigrade. Effective Bandwidth—Effective bandwidth is the Accuracy classes of control devices for step and in- actual bandwidth as measured at the output of the duction voltage regulators according to Industry (ASA regulator. It will differ somewhat from the preset & NEMA) Standards are: bandwidth because of time delay and control error. An Accuracy Class Over-All Per Cent Error illustration of effective bandwidth is shown in Fig. 55, where the preset bandwidth is two volts (± 1v), but the Class 1 ± 1% actual regulator output band is 2.6 volts. The effect of Class 2 ± 2% time delay and control error are both shown in the Class 5 ± 5% figure. While the effective bandwidth as shown in the The particular class associated with the regulator figure appears to be not much greater than the preset control device is determined from the summation or bandwidth, it represents a 30 per cent bandwidth in- maximum of all the plus errors or the maximum of all crease; with a longer time delay setting or less accurate the minus errors. The largest per cent value determines the controls, it could be substantially greater. class of the control devices. The per cent error is based When extremely small bandwidth settings are used on a reference point of 120 volts at rated frequency and with longer time-delay settings to prevent excessive reg- an ambient temperature of 25 degrees Centigrade. ulator operations, the effective bandwidth often ap-

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REGULATOR TI ME DELAY necessary to know the daily voltage variations at the L proposed capacitor location, because the feeder voltage at that location must decrease as feeder load increases. The control bandwidth must be larger than the volt- REGULATOR OPERATION age change due to the switching operation of one step. Since the voltage change due to one step will depend upon the system characteristics back toward the source,

A it is also necessary to check the voltage change during

pm A REGULATOR OUTPUT possible abnormal conditions. The bandwidth settings VOLTAGE are wider than used with voltage regulators and normally range from four to ten volts. The relationship CONTROL. between voltage change and control bandwidth as exist- ERROR ing in actual installations is shown in Fig. 56. With voltage control a time delay is also required between the time when an operation is called for and when Fig. 55—Voltage-time profile of voltage regulator output the switching operation actually takes place, to prevent voltage illustrating effective bandwidth. unnecessary switching due to momentary disturbances. If an induction-disc type voltage relay is used, the in- proaches the same effective bandwidth as would occur verse time characteristic of the relay will provide the if a slightly larger bandwidth and smaller time-delay time delay. If a solenoid operated, balance-beam volt- setting were originally used. Therefore, if the original age relay is used, a separate time-delay relay is required. purpose in using the extremely small bandwidth was to With separate relays the most common delay setting permit a greater feeder voltage drop, the additional used is one minute.20 drop cannot actually be gained without the feeder volt- The use of voltage control with the switched capaci- age going above or below permissible voltage limits. tors will require coordination with other voltage regu- Generally, the increase in bandwidth from the preset value to the effective value is not considered in distribution system design; however, if it becomes too wide and results in frequent low voltage conditions, it should 60 be considered. AVERAGE 1.73 34. Switched Capacitor Controls 50 The requirement of the switched capacitor controls is to initiate the switching on the basis of intelligence

received from some source. The intelligence source used

S• 40 for a specific application will depend upon the primary purpose of the switched capacitor bank and upon the SER daily load and voltage variations of the circuit in which F U the bank is to be applied. Time, voltage, current, tem- O 30 perature, kvar, manual, and .combinations of each are NT all sources of intelligence used to switch capacitors. R CE Time and voltage are the most common intelligence 20 sources used on distribution systems when voltage con- PE trol is the primary purpose of the switched capacitor application. The various switched capacitor controls 10 are discussed in detail in Chapter 8; brief remarks only are included in this section. With time-switch control the capacitors are switched rJ on at a certain time of day and switched off at a later 0 0 .5 1.5 2 25 time. Its greatest use is with small single-step banks rated 150 to 600 kvar located out on primary feeders. BANDWIDTH The feeder daily load cycle must be consistent and the RATIO OF CALCULATED BANK VOLTAGE RISE daily voltage variations must be known before the capacitor bank is applied, in order to study the effect of the Fig. 56—Survey" results of ratio of control bandwidth to voltage rise due to the capacitors and to select the calculated voltage rise per step for switched capacitor proper switching times. applications. Coverage of 39 users. Voltage control for initiating the capacitor switching Five users reported no ratio but instead, is very similar to the voltage control method used with (2) Bandwidth = Voltage rise + 1 volt feeder voltage regulators. The relays used are very (1) Bandwidth = Voltage rise + 1 1/2 volts similar, and in substation capacitor applications the re- (1) Bandwidth = Voltage rise + 2 volts lays are often the same. To use voltage control it is (1) Bandwidth = Voltage rise + 3 volts

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lating equipment so that the operation of one device VOLTAGE REGULATOR REGULATION (switched ,capacitor or regulator) will not cause an POINT operation of another device, resulting in excessive oper- II. ations and possibly control instability or pumping. U a Equipment coordination is discussed in Sections 45 0 _LINE DROP and 46. J5OMPENSATOR It is not the intent for switched capacitor applications on distribution systems to give a fine voltage control; VOLTAGE hence, as large a voltage change as permissible should PT REGULATING be obtained with each step. Most applications supple- RELAY ment voltage regulators applied in the distribution substations or on the feeders, and have only two to four switching operations per day. The few operations per day permit the voltage change to be much greater than voltage regulators—in practice, voltage changes of three to five per cent are not uncommon.

X. LINE DROP COMPENSATION In order that voltage regulators applied in substations ER0 2 REGULATOR OUTPUT VOLTAGE or out on a feeder can automatically maintain a prede- ER =RELAY VOLTAGE termined voltage at some point remote from the regu- I L =LOAD CURRENT lator location, line drop compensators are required in RL • LINE RESISTANCE the regulator control circuit. The voltage at this remote XL • LINE REACTANCE point, commonly referred to as the regulation point, is = LOAD POWER FACTOR ANGLE to be kept constant regardless of the magnitude or Fig. 57—Simple schematic diagram and phasor diagram of power factor of the load. This is accomplished by dial the control circuit and line drop compensator circuit of a step settings of the adjustable resistance and reactance ele- or induction voltage regulator. ments located on the control panel of the regulator. The adjustable resistance and reactance elements as a unit The resistance in the potential transformer second- is the line drop compensator. ary circuit is relatively high compared to the reactance of the circuit; thus the current supplied by the po- 35. Theory of Operation tential transformer is almost in phase with the voltage. The theory of operation of the line drop compensator The current transformer produces an additional cur- can be simply explained with reference to Fig. 57. The rent through the resistance and reactance element voltage regulating relay has complete control of the which is directly proportional to the line current and regulator operations; that is, a change in the voltage in the same phase relation. across the relay causes the relay to actuate the regula- The voltage regulating relay is adjusted so that with tor in such a way as to return the voltage across the zero load current, the output voltage of the regulator relay to the predetermined value. If the voltage regu- is equal to the voltage desired at the regulation point. lating relay were connected across the secondary of the Taking into account the instrument transformer ratio, potential transformer, the voltage across the regulator the compensator is adjusted so that its elements are output terminals would be held constant (i.e. within respectively proportiona' to the resistance RL and re- the relay bandwidth). In order for the voltage regulator actance XL of the feeder between the regulator and the to compensate for the line voltage drop between the regulation point. The regulator will then maintain the regulator and the regulation point (the point where a predetermined voltage at the regulation point. The constant voltage is to be held), a voltage must be intro- similarity between the feeder circuit and the line drop duced between the potential transformer and the volt- compensator circuit is illustrated by the phasor diagram age regulating relay which will subtract from the po- also in Fig. 57, where ERO is the feeder voltage at the tential transformer voltage, be proportional to load regulator output terminals as well as the secondary current, and be dependent upon the load power factor. voltage of the potential transformer. Similarly, IL rep- This is the function of the line drop compensator. resents either the feeder load current or the secondary The diagram of the line drop compensator shown in current of the current transformer; ILRL is the voltage the dotted square of Fig. 57 is only a simple schematic drop over the line resistance or that across the compen- diagram. Actually the compensator circuit may contain sator resistance element caused by the current from the additional current transformers for further reduction current transformer; and ILXL is the voltage drop over of the current in the compensator elements, ballast the reactance of the line or the voltage introduced into reactors, and auxiliary switches for reversing the volt- the relay circuit by the reactance element. ER is there- age polarity across the compensator elements. The fore the feeder voltage at the regulation point and the exact components of the compensator circuit will de- voltage across the relay. ER would be within the band- pend upon the manufacturer. width around the preset balance voltage of the relay.

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The compensator elements are located on the regula- From Equation (22) tor control panel, and the dials are calibrated in volts. R 120 ) 481)(3)(. =4.35 volts When the element is set at a voltage value, it indicates (200) \ 7960 the volts compensation obtained for that element when rated current is flowing in the secondary of the current From Equation (23) = (120 ) . transformer. If the current transformer (primary rat- X (200) ( 7178)(3) =6.50 volts ing) and the regulator nameplate current rating are the 7960 same, the dial setting would be the volt compensation It is to be noted that in Equations (22) and (23) and obtained at regulator nameplate loading. Such is the in the above example, no recognition is given to the case for distribution-type regulators and induction regulator current rating or the actual load current. This regulators. is because the compensator dials are calibrated for rated 36. Determining L.D.C. Settings current in the current transformer (200 amperes in the CT primary for above example). Design standards for The voltage drop introduced in the voltage regulator station type step regulators require that if the regulator control circuit by the line drop compensator must be nameplate current ratings are not equal to a standard equal, on the control circuit voltage base, to the voltage current transformer primary current rating, the next drop of the feeder from the regulator location to the larger current transformer rating is used. Such was the regulation point. If no load is tapped off the feeder be- case in the above example. Although for the distribu- tween the regulator and regulation point, it is relatively tion-type step regulators and for induction regulators simple to determine the settings. The proper line drop the primary rating of the current transformer and the compensator settings can be derived by using the regulator nameplate current rating are the same, there following equations: the factor C Ty in Equations (22) and (23) could be CTp changed to the regulator nameplate current rating. R setting = (rL) (d) (22) 1V PT If load is tapped off the feeder between the regulator location and the regulation point, Equations (22) and C Tp (23) are not applicable because the entire load current X setting = —(xt)(d) (23) NPT which flows in the primary of the current transformer where does not flow through the line resistance and reactance R= Dial setting for resistance compensation in out to the regulation point. Equations (22) and (23) volts would have to be modified as follows: X = Dial setting for reactance compensation in C Tp , volts R setting — , (REEF) (24) iv PT CTF =Primary rating of the current transformer where NPT =Potential transformer ratio /Primary voltage I VDRI (25) \Secondary voltage) REFF —1=1 (IL I II= Resistance per conductor from regulator to En I VDR I= I /Li I rich+ I /1.21r2d2-1- I hal r3d3.... I /L. I ruda (26) regulation point in ohms per mile. i-1 xL = Reactance per conductor from regulator to and regulation point in ohms per mile. REEF = Effective line resistance from the regulator d= Distance in miles from the regulator to the location to the regulating point, or the re- regulation point. For single-phase circuits sistance value that when multiplied by 1-1. the distance would be twice the distance to will give a voltage drop value equal to the the regulation point. actual line resistive voltage drop out to the A typical example is as follows: regulation point. E I TTDR I = Total voltage drop due to the line re- Three-phase, station-type, step regulator rated sistance out to the regulation point. 13.8 kv, 375 kva, 157 amperes, and +10% regulation range. /L = Load current at the regulator location. 1=Load current in the first section of line be- Current transformer ratio 200/5. IL fore any load is tapped off (IL= /L1) Potential transformer rating 7960/120. 1-1,2 = Load current in the line section after the Three miles from regulator location to regulation point. first load and before the second load. = Load current in the last section of line be- Load current = 150 amperes. ./La fore the regulation point. Conductor size = #2/0 AWG Copper, 7 strands. = Conductor resistance per mile in the first Three conductor, 44 inch flat spacing (55.5 inch equivalent spacing) section. d1= Distance in miles of the first section. The From Table I, Appendix, distance is doubled for single-phase circuits. r = .481 ohms per mile n= Number of line sections out to the regula- x= .7178 ohms per mile tion point.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 296/576 System Voltage Regulation, 289 Also for the reactance compensator setting: As expected, when comparing the compensator set- C Tp tings of the two examples, they are less for the feeder X setting = 7„. (XEFF) (27) where part of the load is tapped off before the regulation IV PT point. The current /L in Equations (25) and (28) does where not have to be peak load current, but it can be the VDxI current at any loading. The current in each line section, XEFF — (28) as used in Equations (26) and (29), are assumed to be n I/ LI at the same power factor angle from the regulation point VDx I = I hi I xidi-1- I IL2 I X2d2.... I ILn rnel. (29) voltage. Also as loads are tapped off the main feeder, i-1 conductor sizes generally decresise. Therefore, the indi- and vidual section line reactances and resistances in Equa- XEFF = Effective line reactance from the regulator tions (26) and (29) will change. location to regulation point, or the reactance In actual practice it is often too tedious to determine value that when multiplied by IL will give the voltage drop in each section. An alternate is to a voltage drop value equal to the actual line measure the current at the regulator location and simul- reactive voltage drop out to the regulation taneously measure the voltage at the desired regulation point. point and at the regulator location. The difference be- E VDx =Total voltage drop due to the line react- tween the two voltage values would be the voltage drop ance out to the regulation point. between the two measuring points or: = Total line reactance in the first section of line. Voltage Drop = I /L I REFF cos 0+II L I XEFF sin 0 (30) An example using the same feeder as the previous ex- where IL is the current at the regulator location. Then ample with loads tapped off before the regulation point by knowing the feeder power factor and the average (Fig. 58) is as follows: r/x ratio of the conductor out to the regulation point, REFF and XEFF can be determined. The compensator E I vp. I =(150) (.481) (1) (130) (.481)(1) setting are then calculated using Equations (24) and +(120) (.481) (1) =192.2 volts (27). I VDa On primary feeders where the load is distributed along the entire feeder, it is often found for both sub- REFF =1 I/L1 —192.02 — 1.282 ohms 15 station and supplementary voltage regulator applica- using Equation (24), tions that there is no load center or any particular good 102 ) point where the primary voltage should be kept con- R setting = (200) (1.282) = 3.96 volts. (7 60 stant. For such feeders it is desirable to set the line drop and compensators so that maximum permissible voltage is held during peak load at the primary voltage terminals I VDx I =(150) (.7178) (1) + (130) (.7178) (1) of the first distribution transformer beyond the regula- +(120) (.7178) (1) = 287.5 volts tor location and a voltage slightly lower during light- load conditions. For instance, during peak-load conditions the primary voltage at the first transformer would I VDxI 287 5 XEFF = t-1I ILI — ' —1.917 be 126 volts (on a 120-volt base) and, say, 122 volts dur- 150 ing light load. Knowing the voltages that are to be held using Equation (27), at the first transformer, it is then possible to determine a fictitious regulating point where the voltage will X setting = (200) ( 0.- (1.917) = 5.76 volts. 120 ) be held constant. For example: for the primary feeder single-line diagram of Fig. 59 (a),the peak-load and light- 3 load voltage profiles were determined by calculation or VOLTAGE REGULATOR measurement. The two voltage profiles were then REGULATION plotted on a voltage vs. distance chart as shown in Fig. POINT 59 (b), with the voltage at the first transformer equal to I= 150 —""I= 130 _ the desired value for each load condition-126 volts at I MILE I MILE I MILE peak load and 122 volts at light load. Since the portion 2/0 CU. 2/0 CU. 2/0 CU. of the voltage profile from the regulator location to the first transformer is linear, the linear portion for both 13.8 KV, - 10% profiles can be extended until they cross. The crossing 375 KVA, 157 a point will be the fictitious regulation point. This is shown dotted in Fig. 59(b). Then using Equations (22) LOAD I LOAD 2 and (23), the compensator settings can be determined as 20° 10° shown in Fig. 59(c). The balance voltage setting of the Fig. 58—One-line diagram of a 13.8-kv, three-phase voltage regulating relay would be the voltage at the feeder with load tapped off between the voltage regulator fictitious regulation point. and regulation point. If, when locating the peak- and light-load profiles on

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VOLTAGE 38. Settings for Three, Single-Phase Regulators REGULATOR Wye Connected PRDAART FEEDER I 1 1 1 1 1 I I 1 Line drop compensator settings for each of the three, FIRST single-phase regulators when wye connected are deter- DISTRIERJTKO4 TRANSFORMER (0) mined as described in Section 36. Generally, the settings are the same on each of the regulators in the three- phase bank. For feeder applications where immediately 130 after the regulators each phase goes in a separate direc- 120 tion, it is possible for each unit to have different compensator settings. This would occur if the regulation 126 point for each phase is at a different distance from the 124. regulator. Each regulator setting would be determined separately based on the single-phase circuit. Again, the 122 method of determining the settings is described in Sec- tion 36.

39. Settings For Two, Single-Phase Regulators

tie Connected Open-Delta -I When single-phase step or induction regulators are 2 3 4 5 6 connected in open-delta in three-phase circuits, the FEEDER LENGTH MIEN, REGULATOR — MILES regulator control voltage and the current in the line

(0) drop compensator circuit do not have the same phase relationship as a single-phase regulator in a wye con- Voltage Regulator Conductor size between 7960 regulator and first nected bank or a single-phase regulator in a single- PT Ratio distribution transformer—No. 2/0 120 phase circuit. In a wye connection, following Figs. 42 0 44' flat spacing CT Ratio=2 0 5 r---.481 ft/mile x=.718 0/mile and 60, the control voltage (secondary voltage of the PT) and the line drop compensator current (secondary Distance to fictitious regulating point=3.9 miles current of the CT) are EA and /A for Regulator 1, EB Line drop compensator settings: /-)3 for Regulator 2, and Ec and Ic for Regulator 3. 120 and R setting=(200) (iif2 -1) 481) (3.9)=5'56 In each case the voltage and current are separated by / 120 \ ,,R1 Ac the power factor angle. In the open-delta connection X setting =200) 7960)718). 91 (7960i "..." following Figs. 43 and 60, the control voltage and com- Voltage regulating relay pelting=120.1 volts pensator current are EAB and /A for Regulator 1 and Ec)3 and Ic for Regulator 2. Each regulator has its voltage and current phase angle changed by 30 degrees from Fig. 59—One-line diagram and voltage profiles of a feeder with distributed load beyond a voltage regulator location.

(a) One-line diagram. cb (b) Peak- and light-load profile showing fictitious regulating point for line drop compensator settings. (c) Example of determining compensator settings. Ec Eab Eca

the voltage-distance chart (Fig. 59(b)), the two profiles cross, the crossing point can be taken as the regulation point and the compensator settings obtained using Equations (24) and (27). When determining the compensator settings by the method where peak and light load voltages at the first transformer are established, as in the last two paragraphs, it is necessary to periodically check the settings as load grows because the peak- and light-load profiles EOC will change. Each new set of light- and peak-load voltage profiles will have a new regulation point.

37. Settings For Three-Phase Regulators Ebc

Line drop compensator settings for three-phase regii- Fig. 60—Typical three-phase phasor diagram showing lators, whether they are internally connected wye or phase relationship of line voltages, line-to-neutral voltages, delta, are determined as described in Section 36. and line currents.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 298/576 System Voltage Regulation 291 the conditions which would exist if the regulators were of the current and voltage for the leading regulator at wye connected. One regulator has its current lagging the unity power factor. If current le flowed through the voltage by 30 degrees plus the power factor angle (regu- compensator with the setting determined by the resist- lator 1), and the other regulator has the current ance and reactance of the line out to the regulation leading the voltage by 30 degrees minus the power fac- point, the voltage introduced in to the voltage regulat- tor angle. In order to obtain correct line drop compensa- ing relay circuit would be OE' in Fig. 61(b). The voltage tion, some means of compensating for the 30 degree OE' is made up of the resistance drop OF and the react- phase shift must be provided. The regulator that has ance drop FE'. If /c was in phase with the control volt- the current lagging the voltage by 30 degrees plus the age Ecg at unity power factor, which it should be to power factor angle is often called the "lagging unit", and obtain proper regulation, when the settings are the R the unit which has the current leading the voltage by 30 and X drop of the line, instead of leading by 30 degrees, degrees minus the power factor angle is known as the the voltage introduced into the relay circuit would *be "leading unit". OE. It can be seen from the phasor diagram that OE' Early methods for correcting for the phase shift in- is 30 degrees ahead of the voltage OE, the voltage that cluded the use of a third current transformer in the should be inserted in the relay circuit to obtain proper common phase, and by interconnecting the current compensation. If, however, the compensator was set for transformers, currents were obtained having the proper an impedance having an angle 30 degrees less than that phase relation. Such a scheme was expensive and in- of the line impedance angle and of the same magnitude volved inconvenient interconnections. Present methods as the line impedance, then the same voltage OE would use either phase shift networks in the control circuit or be introduced into the relay circuit. The voltage drop modified compensator settings. across the compensator with the new settings would On single-phase induction regulators, phase shift net- then be made up of the resistance drop OG and the works are always used. A terminal block is generally reactance drop GE. The new or modified compensator located on the back of the control panel of the regulator settings for the leading unit are obtained by multiplying and simple connection changes can be made for either the settings that would normally be made, as described no shift, a 30-degree forward shift or a 30-degree back in Section 36, by (cos 30°--j ,sin 30°) or (0.866—j 0.5). shift. After the correct terminal connection has been made for the regulator in the open-delta connection, the R' A-jX' = (0.866 —j 0.5) (R-I-jX) (31) compensator settings are determined as described in where R and X are the normal compensator settings and Section 36. R' and X' are the modified settings. The modified set- For single-phase, distribution-type step regulators, tings for the leading unit would be: there is no phase shift network in the control circuit and the modified compensator setting method is used. In R'= (0.866 R+0.5X) (32) large station-type, single-phase step regulators, a phase X —0.5R) (33) shift network is optional equipment and generally used X' = (0.866 for open-delta connections, although the modified com- For the lagging unit in the open-delta connection, the pensator setting method can be used. The method of compensator would have to be set for an impedance modified compensator settings, often called the "Wag- having an angle 30 degrees greater than that of the line ner Method" for Charles F. Wagner who patented the impedance angle. Hence, scheme in 1934, obtains correct line drop compensation by applying a 30-degree phase shift to the settings of the R'+j X' = (0.866+j 0.5) (R-i-jX) (34) line drop compensator. The modified settings for the lagging unit would be Applying a phase shift to the setting can be illustrated by referring to Fig. 61. Fig. 61(a) is the phasor diagram R' = (0.866 R— 0.5X) (35) X' = (0.866 X+0.5R) (36) E cs While the above derivation assumed unity power fac-

Ec 8 tor, Equations (32), (33), (35), and (36) hold true for any power factor, as the method still permits the voltage that is introduced into the relay circuit to correctly vary with the phase position of the current. Nomographs for determining the modified settings from the normal R and X settings are in Fig. 62. Since the modified setting method requires the phase angle of the line impedance to be rotated either 30 degrees forward or backward, negative R settings may be required when increasing the phase angle and negative X settings may be required when decreasing the (a) (b) phase angle. The negative R setting will be required on Fig. 61—Phasor relationship of the voltage and current in the lagging unit when the r/x ratio of the line imped- the "leading regulator" of an open-delta connection. ance is less than 0.6. The negative X setting will be re-

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292 System Voltage Regulation

+24 432 .12 424 - 0 observing the reaction of the regulator. After the units +20 29 - 9e +20 4. 4! are installed, the following procedure 23 can be used for • 24 - 4 proper identification. • 16 r .20 0 e e ~ 4 With the regulator control set for automatic opera- • 12 • 4 - Js •16 § 412 .12 g 5 tion, the resistance compensator dial set at zero, and the • 8 'x_ .12 0 zd • i2 reactance setting of both regulators raised to the same

4 • 16 +4 +20 value, the regulator which tends to go to the higher 420: 0 0 24 raise position is the lagging unit. If the incoming voltage 0 • LAE RESISTANCE REACTANCE LIFE to the regulators is unbalanced at zero line-drop com- RESISTANCE DIAL DIAL REACTANCE DROP SETTING SETTNG DROP (VOLTS ) IVIXTS) pensator settings, the regulators will assume different positions. In this case, after raising the reactance com- (a) pensator dial of both units to the same value, the lagging (a) Leading regulator. unit will operate more steps in the raise direction than the leading unit. The time required for the trial will be 0 :424 --12 • 32 +24 short and will last only until a clear indication is - 21. 4 :•XO 21 •26 +20 observed. Generally the X setting will not have to be •-• - 4 +24 -•16 12101---- .16 raised any higher than 12 volts to obtain a positive •20 identification. This is shown by the output voltages of • I2 •I2 g -._ .15 8 412 each unit in Fig. 63. .1, *16 • 9 yg • I .12 -• 8 §j i+20 .16 • 4 4L Settings for Three, Single-Phase Regulators • • +20 Connected Closed-Delta .24 - 0 0- - 0 LINE RESISTANCE REACTANCE LSE RESISTANCE DIAL DIAL. REACTANCE When three single-phase regulators are connected DROP SETTING SETTING DROP 1 VOLTS) (VOLTS) closed delta in three-phase circuits, a similar condition exists as in the open-delta connection, Section 39. That is, the regulator control voltage and the current in the (b) Lagging regulator. line drop compensator circuit do not have the same Example: phase relationship as a single-phase regulator in a wye Actual Line Leading Regulator Lagging Regulator connected bank, or a single-phase regulator in a single- Drop Settings Settings

ILRL ILXL IV X' R' X' FEEDER 4 v. 7 V. 7 v. 4.2 v. 0 8 v. POWER FACTOR 132 .6 LAG Fig. 62—"Wagner Charts" for determining actual line drop .8 LAG compensators dial setting for voltage regulators connected 130 .9 LAG open-delta. TS L .95 LAG

VO 128 - R) 126 TO 1.0

quired on the leading unit when the r/x ratio of the A

line impedance is greater than 1.73. Reversing switches UL 124 6 LAG

for both elements are part of the line drop compensator REG

circuit in modern, single-phase, distribution-type step F regulators. On station-type, single-phase step regulators 122 .95 LEAD DE O only a switch for reversing the reactance element is in- I .8 LAG .9 LEAD cluded in the compensator circuit, as a reversed R setting 120 AD S ...... --..__ ...... -- — _ _ _ is rarely required. O L -. _ .9 LAG 118 ...... ,;.... -..... , ..... __ 40. Identifying Leading and Lagging Unit in an .4., GE ( .95 LAG N... -...... -...... Open-Delta Connection TA 116 Ns, L 4.... '..., .... -...... VO -.... When two single-phase regulators are connected ".... `.... -... open-delta in a three-phase circuit, it is necessary to 114 N... -.... ••• .... I.0

TED 's...

know which unit is the leading unit and which unit is LA \ s. ss the lagging unit, so that the correct modified compensator U 112 s. — LAGG NG REGUL A TOR `s. REG N. "...s. .95 LEAD setting or the correct phase shift network terminal con- --- LEADING REGULATOR .... nection can be made. If the phase rotation of the three- 110 r's .9 LEAD phase circuit is known, a simple phasor diagram such as 108 Fig. 60 can be drawn and the lagging and leading unit 25 50 75 100 easily determined. If the phase rotation is not known, PER CENT LOADING OF REGULATOR which is often the case at a regulator location out along Fig. 63—Regulator output voltage for various line load a primary feeder, the leading and lagging unit can be power factors with line drop compensator settings of R = 0 determined by adjusting the line drop compensator and and X = 12. Voltage regulating relay setting is 120 volts.

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phase circuit. The closed connection does differ, how- and had a different relay setting been chosen, positive ever, in that every regulator in the connection either has settings could have been used. If the X setting comes the unity power factor current leading the voltage by 30 out negative, the relay setting can be raised. This will degrees (leading units) or has the unity power factor result in the X setting becoming positive, although at current lagging the voltage by 30 degrees (lagging the same time R decreases. If the R setting was nega- units). The two possible connections and their vector tive in the original calculations, the relay setting should diagrams are shown in Fig. 47. Again, to obtain correct be decreased. line drop compensation, it is necessary to correct for the If the power factor is the same for peak- and light- 30-degree phase shift. Either a phase-shift network can load conditions, VIA3c in the peak-load equation and be used in the control circuit or the modified compensa- the light-load equation must be in the same proportion tor settings can be used. Both methods are described in as the per cent load current. The relay setting can be Section 39. varied to obtain the same proportion if the V1,DC ratio of peak load to light load differs. Also with the power factor the same for both conditions, many R and X 42. Settings for Bus Regulators setting combinations are possible. A satisfactory com- For bus regulators in multi-feeder substations, it is bination can be obtained by deviating slightly either generally difficult to determine the line drop compensa- the peak- or light-load power factor in the voltage tor settings based on holding a voltage constant at a equation toward the average power factor for all load load center. If all of the feeders served from the substa- conditions, and then solving for the R and X settings. tion bus are similarly loaded, have the voltage drop to Since the method of determining the R and X set- their load centers approximately the same, and have tings using Equation (38) is based on the desired bus similar load cycles, it would be possible to determine the voltages for different load conditions and load power settings as described in Section 36, with the quantities factor, it will be necessary to periodically check the (rLd) and (xLd) in Equations (22) and (23) changed to settings as substation load grows. r,c1IN, and xLd/NF respectively, where NF is the num- Fig. 64 shows curves of regulator output voltage as ber of feeders served from the bus. Generally, feeders a function of load power factor for various compensator served from the same regulated bus have similar load and relay settings. These curves represent typical characteristics, but in most cases the load centers for each voltages that are maintained at the substation bus for feeder are at different distances from the substation and peak- and light-load conditions. The R and X settings the distances to the first load are different. Therefore, on and relay setting maintain these voltages reasonably bus regulators it is difficult to use line drop compensa- constant over the typical power factor range. Light-load tors as easily and efficiently as with individual feeder conditions were assumed to be about one-third of peak- and supplementary regulators. load conditions. The per cent load conditions in Fig. 64 When no specific line impedance is known, the set- are based on the current transformer primary rating tings can be determined by knowing what bus voltages of the bus regulator. If peak-load conditions are less should be maintained during peak load and light load. than 100 per cent, the R and X settings shown would The maximum voltage held during peak load will be be proportionally increased. For example, if peak load dependent on the location of the first load served by was 80 per cent of the CT primary rating and the bus the substation. The light-load voltage would be slightly voltages required correspond to the curves of Fig. 64(c) below the peak-load voltage to minimize overexcitation the compensator settings would be: of distribution transformers. It is also necessary to know the per cent substation loading at peak- and light,- R setting — (6) 100 load conditions and their respective power factors. The 7—0 Actual Peak Loading (6) 80 (100) — _ 7.5 voltage equation for the voltage regulating relay cir- X setting = (2) 80 = 2.5 cuit at any substation loading following Fig. 57 is: 0

Ego = ER+ (WO (RCOS0+ XsinO) (37) 43. Overcompensation where (%IL) is the load current in per cent of the cur- Overcompensation is the increasing of the R and X rent transformer primary rating, R and X are the settings over and above the settings required to hold the compensator dial settings in volts, and 0 the power voltage constant at the regulation point. Naturally, factor angle. Subtracting the relay setting voltage from when increasing the compensator settings, the voltage the desired regulator output voltage gives the voltage will no longer remain constant throughout all load that should be introduced into the relay circuit by the conditions at the regulation point, but rather at some compensator or: point beyond it. In effect overcompensation is just mov- VLE)c = (%IL) ( Rcos0+ Xsin0) (38) ing the regulation point farther out along the feeder. The increased R and X settings will cause the voltage After selecting the relay setting the values of Vim°, at the primary terminals of the first distribution trans- IL, and 0 are known for both peak- and light-load con- former to be increased during peak load, as compared to ditions, resulting in two equations solvable-for R and X. the normal compensator settings. Therefore, the maxi- The relay setting generally used is 120 volts although mum permissible voltage at the first transformer will for particular cases negative R or X settings may result, determine the amount of overcompensation permissible.

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294 System Voltage Regulation

130 i 1 130 I I 130 I R • 5 R • 6 R • 9 X • 1.5 X • 2 '----- X • 2 E, •119 E,•120 Er., WI 100% LOAD 128 128 ...----•-- DESIRED VOLTAGE RANGE DESIRED VOLTAGE RANGE 128 DESIRED VOLTAGE RANGE En_ • 124 EFL • 126 E LL• 122 E,L 129 100% LOAD EL, 121 ...... _...... ,1 ELL 123 126 126 126 75% LOAD ^.--•-•--...... , -, --1-1-2rts...... „5% LOAD . ,_ -..----.••••"-- 100% LOAD co 124 h....--- 124 124 co 50% LOAD co -775:L1"...... %ORD ‘ 50% LOAD --, .. 122 25% LOA .....S r...LOAO 122 122 1.• -•"'-'-'--•-•-• ..-.--..--...... 25% LOAD •-...... 25% LOAD 120 -... 120 120

.8 .85 .9 .95 1.0 .95 .6 .85 .9 .95 1.0 .95 .8 .85 .9 .95 1.0 .95 LAG LEAD LAG - I -LEAD LAG -1- --LEAD POWER FACTOR POWER FACTOR POWER FACTOR (a) (C ) (e)

130 130 II I I I 130 I 1 100% LOAD R • 6 R • 7 X • 2 E,•120 "--•-•-...._ X • 2 DESIRED VOLTAGE RANGE E,•119 R II X•4 E,• IS 128 128 E,L• 127 E LL• 123 DESIRED VOLTAGE RANGE 128 DESIRED VOLTAGE RANGE 100% LOAD ‘...... \\ tal ...... , -. 7-- 1,...... c, E, • 129 ELL 123 E,L • 125 75% LOAD ELL 121 126 126 126 ▪ LOAD - 100% LOAD 75% "---•...... _ _. . • 124 15% LOAD 27"•- -.. 124 1"- ...._....-.-n-^1 ---,-..,______„__._..„...... ,.50% LOAD 124 ---- 5 % LOAD In "'---...... ,...... „.

50% LOAD 122 si. -----..„- 122 25% LOAD .-..--...... 122 25% LOAD 25% LOAD - .-.-..-.- 120 120 120

.8 .85 .9 .95 LO .95 .8 .85 .9 .95 1.0 .95 .85 .9 .95 1.0 .95 LAG -1 LEAD LAG - -LEAD LAG I --LEAD POWER FACTOR POWER FACTOR POWER FACTOR (b) (d) (f ) Fig. 64-Regulator output voltage (bus voltage) as a function of load power factor for various compensator and relay settings. The settings shown are for typical bus voltages held at peak- and light-load conditions. Light-load conditions assumed approximately one-third of peak load.

When overcompensation is used, the additional R former load grows. If additional overcompensation is and X settings generally correspond to the R and X permissible, additional R and X settings could be made voltage drop of the distribution transformer. Therefore, corresponding to part of the secondary line drop. But at the regulation point, the voltage will be held reason- it must be remembered that when the line drop com- ably constant throughout all load conditions at the pensator setting is increased, the voltage at the primary secondary terminals of the distribution transformer terminals of the first distribution transformer also rather than the primary terminals. The additional increases. settings are generally determined as follows: Assume at peak load the distribution transformer drop is 3.0 Xl. COORDINATION OF REGULATING volts (120-v. base), the load power factor is 90 per cent EQUIPMENT and the x/r ratio of the distribution transformer impedance is 1.2. The additional settings would be: With economic voltage control, voltage regulating equipment will be applied in series throughout the Volts drop 3.0 R - -2.1 (39) distribution system. On feeders with excessive voltage x -.9+ (1.2)(.436) cos 0±- sin drop, several step regulators can be applied in series, r as shown in Fig. 17, or a combination of step regulators Volts drop 3.0 and fixed or switched capacitors can be applied. X= =2.53 (40) 1 Switched or fixed capacitor banks and voltage regu- o+sin 436 z7 cos 6 -( 1)(.9)+' lators are often located in the same substation. It is therefore necessary that proper coordination be It is necessary that the distribution transformer load- achieved between the voltage regulating equipment ing follow the feeder loading to insure correct over- so that the operation of one unit will not cause an compensation. Periodic checks must be made as trans-- operation of the other unit or units, and result in ex-

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cessive operations. A general rule of application is that especially those which are heavily loaded near the end, the control settings of voltage regulating equipment it is possible for some excessive operations to occur. applied on the game feeder (units in series) should be If so, they would be due to load changes near the end. such that an operation of any one unit will not cause The last regulator may operate before the regulator a voltage change at a second unit greater than the ahead and then recorrect after the ahead regulator bandwidth setting of the second device. Following this operates. Depending upon the magnitude of voltage rule is not always possible when applying voltage change, it is often desirable to have the last regulator regulators and switched capacitors on the same feeder operate first to rapidly correct the worst voltage condi- because of the relatively large voltage change when tions. As a whole, satisfactory performance is obtained switching the capacitors. However, since the band- with few extra operations. Severe cases require special width of the capacitor controls is much larger than the consideration, and proper operation can be achieved by bandwidth setting of the voltage regulator controls, and adjusting the inverse characteristics of the relay in the because the capacitor switching occurs only a few times last unit or in each of the units. a day, a regulator operation following a capacitor switching is not considered serious. 45. Regulators and Fixed Capacitors If the fixed capacitors are installed on the source side 44. Step Regulators Applied in Series of the regulator, there is no problem of coordination. The problem of series operation of two or more When the capacitors are added, the input voltage of regulators is one which involves many variables and is, the regulator changes and, if necessary, the regulator therefore, not subj ect to a simple solution. It is affected will operate to maintain the desired voltage at the by time-delay settings, relay bandwidth settings, and regulation point. The current through the line drop the size of the tap-changer steps. The less obvious compensator is only the load current; hence, the capaci- effects are whether circuit voltage changes are due to tors do not affect the compensator settings. With nor- supply voltage changes or to load changes, the rate mal settings proper voltage control is achieved. at which voltage changes, the size and location of loads, When the capacitors are located on the load side of the operating time of the tap-changer, and the accepted the voltage regulator, it is necessary to check whether criterion of proper operation. the regulator will perform correctly to achieve the de- The desired sequence of operation to give the min- sired voltage conditions. With the capacitors on the load imum number of regulator operations would be to have side, the capacitor current will flow through the line the first regulator complete its operation before the drop compensator. It is therefore necessary that the second one started, the second complete its operation voltage drop due to the capacitor current Io in the before the third started, etc. This could be accomplished compensator be equal to the voltage drop in the line if definite time delays were given for each regulator, due to 1-0 out to the regulation point. with the first having the least amount of time delay. Capacitors Located at Regulation Point or Beyond—If no For source voltage changes, it is desirable for the load is tapped off the feeder between the regulator and regulator nearest the source, normally considered to regulation point and the R and X settings are deter- be the regulators located in the substation, to operate mined using Equations (22) and (23), correct regulator ahead of any regulator located out on feeder. Also, for operation will occur with the capacitors located at the voltages changes which occur due to load changes out regulation point or beyond. The voltage drop due to on the feeder, the closest regulator toward the source, 1.0 in the line to the regulation point and in the com- where the voltage change demanded a regulator opera- pensator is the same. tion, should operate first. If there is load tapped off the line before the regula- Where fixed time-delay settings are used with each tion point and the R and X settings are determined of the regulators having the same bandwidth, the regu- using Equations (24) and (27), correct line drop com- lator nearest the source should have the smallest time pensation can not be achieved. This is because the delay. However, the time-delay setting of the most voltage drop (/0REFF) and (IOXEFF) in the compensator remote unit should not be so great as to cause long is not equal to (I0RLINE) and (L,XLINE), respectively. periods of over or under voltage in the last section of This is shown in Fig. 65(a). line. If such conditions exist, the time delay of the last To obtain correct compensation, the compensator regulator can be decreased at the expense of possible settings can be adjusted or the voltage regulating excess operations. relay setting can be adjusted. Either method will not With the inverse time-delay relay in which the time be absolutely correct for all load conditions (when going delay varies inversely with the magnitude of voltage from light to peak load) or for power factor varia- change, large over- and under-voltages can be cor- tions between line sections, or throughout the load cy- rected with little time delay. On feeders with units cle. The error for each method will be approximately with the same control bandwidth settings and where the same and will generally not be much more than a the load is distributed along the entire feeder, very few fraction of a volt (120 v base). That is, instead of hold- or no excessive operations occur. This is because the ing the predetermined voltage at the regulation point, voltage changes are generally gradual. Only one unit the voltage will vary from the desired voltage in going will operate at a time and at the location where the from light to peak load by the amount of the error. voltage correction is needed most. On some feeders, The exact error will depend upon the installed capacitor

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compensator due to /0 will not be equal to the actual line voltage drop due to /0. Again it is necessary to

Ic XodGal revise either the compensator settings or the relay ;ROOM setting to obtain correct regulator operation. 1Y RCA If no load is served between the regulator and the regulation point (excluding the capacitors), the com- Ayr/ pensator can be changed to the settings as determined IL ILRcn by Equations (41), (24) and (27). When determining

(0) the total line voltage drop in the Equation (41), there • RELAY SETTING APO REGULATING PONT VOLTAGE will be only two line sections. The first is the distance STORE APPLYING CAPACITORS EIro • REGULATOR OUTPUT VOLTAGE FOR RELAY, R AND X SETTING out to the capacitors, and the second is the distance AFTER APPLYING CAPACITORS from the capacitors to the regulation point. • REQUIRED REGULATOR OUTPUT VOLTAGE 10 RAVE !Ed AT REGULATION POINT The simplest method and the most commonly used r. • REVISED RELAY SETTING fE.1-11cf( -A,"] • REGULATING POINT VOLTAGE WITH REVISED RELAY SETTING is to change the relay setting and leave the compensator settings the same as used before applying the capac- Fig. 65—Phasor diagram showing the effect of the capacitor itors. Using the same reasoning as illustrated in Fig. current in the line drop compensator and the different 65, the relay setting will be increased as shown: regulator output voltages before and after adjusting the relay setting. The amount of error is shown in (b). Eaoiewi I = I &(old) I+ ILI (Xactual X1) (45) where La..' is the line reactance from the regulator rating, load distribution, load cycle, conductor size, and to the regulation point and X1 is the line reactance from the location of the regulation point. the regulator to the capacitor location. This equation When determining the new compensator setting can be revised to Equations (24) and (27) could again be used, although ickval IER(..voi= (46) it will be necessary to determine a new REFF and XEFF. Ikval (X)(1 --L ) In determining the new effective values, Equations where (25) and (28) will change to: ckva = Rating of the capacitor installation. kva = Through kva based on regulator CT pri- VDE I I VDIC) REFF = XEFF = (41) mary current rating. This would be the I /I.+ LI IIL-Fie through kva rating of the regulator if the where CT primary current rating is the same as the regulator nameplate current rating. = I VDE = (I/Ei /.1)ndi. (I/L2 /cDr2d2 • • • i -F (Ihn-Ficpriidn (42) X =Previous reactance compensator setting before applying the capacitors. E IVDxI= (1/. + + + /01)x2d, L = Circuit distance from regulator location to +(An+ (43) regulation point. The line reactance per In the revised relay setting method, the setting is unit distance assumed constant over entire slightly reduced and the compensator settings remain length. the same as determined before the capacitors were L1= Circuit distance from regulator location to applied. The new setting is: capacitor installation.

I ER (new) I = I ER (0 Id) I — IIGI (Xactual—XEFF) (44) If load is served between the regulator and regulation point, new compensator settings can be determined where X0a001 is the actual line reactance from the regu- using Equations (4.1) through (43) and Equations (24) lator to the regulation point and XEFF is the previously and (27). In Equations (42) and (43) /0 would be zero determined reactance, Equation (28). The extent of in the line sections beyond the capacitor location to the the error that results when changing the relay setting only is shown in Figure 65(b). regulation point. For changing the relay setting instead of the com- The new relay setting as determined from Equation (44) applies for the capacitors located at the regulation pensator settings, the new setting can be determined as follows: point or anywhere beyond it. If only the compensator settings are changed, the new settings will be the same I ER (new) I = I ER (o id) -I- 1/01(XEFF — X1) (47) regardless of where the capacitor bank is located be- where XEFF is as determined in Equations (28) and X1 yond the regulation point. Of the two methods, chang- is the actual reactance from the regulator location to ing the relay setting is the simplest and most widely the capacitor installation. The above equation can be used. revised to be Capacitors Located Between Regulator and Regulation Ickval Xl Point—For fixed capacitors located between the regula- I ER(new) = I ER (01d) I (48) tor and regulation point, the capacitor current LQ flows I kval (X) (1 XEFF only to the capacitor location and not completely to where ckva, kva, and X are defined with Equation (46). the regulation point. Hence, the voltage drop in the Using the same conditions as in the example immedi-

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ately following Equations (24) through (29), and as- VOLTAGE REGULATOR suming an original relay setting of 120 volts and a 900 kvar three-phase fixed capacitor bank located one mile from the regulator location, at Load #1, Fig. 58, the new relay setting using Equation (48) is: 900 (5 76 ) (1 (1) (.7178) \ 120+ I ER010101 = 3 (200) (13.8) 1.917

1ER (new)! = 120+ .68 120.68 volts or 120.7 volts.

Fixed Capacitors Located in Several Locations—When fixed capacitors are located at several locations on a feeder, each installation must be checked for its ef- Fig. 66—Circuit diagram showing interconnection of current fect. The simplest method of obtaining correct regu- transformers to obtain correct line drop compensation in the lator operation with several bank installations is to regulator control circuit. change the relay setting. The change in relay setting required for each bank is determined separately as described above. The total setting change is the sum of switched capacitor control circuit should be connected the changes required for each installation. on the source side of the regulator. Regulator operation will then not affect the capacitor operation. _If any 46. Regulators and Switched Capacitors other type of control is used besides voltage control Regardless of the type of control intelligence used for the capacitor switching, regulator operation will for the switched capacitor installation, the capacitor not affect the capacitor operation. bank when "on" will affect regulator operation similarly There are several methods possible for obtaining as fixed capacitor banks. When voltage control is used correct regulator line drop compensation. Three for the switched capacitor banks, it will also be neces- methods are discussed below. sary to study the effect of regulator operation on the 1. A current transformer can be inserted in the capa- capacitor controls. citor circuit and connected in the regulator control If the switched bank and the regulators are located circuit as shown in Fig. 66. As shown, the capacitor together, it is possible by interconnection of the two current will flow between current transformers and ie control circuits to attain correct regulator operation not in the compensator elements. Proper line drop as throughout all load conditions—i.e., with the capacitor compensation will result regardless of the number on or off. If the capacitors are remote from the regu- of capacitor steps. le- lators, it is not possible to attain correct regulator opera- 2. This scheme inserts an impedance in the regulator tion throughout the entire load cycle without undue control circuit of such a magnitude that the to expense. If the compensator or relay settings are set voltage drop across it is equal to the voltage rise )er assuming no capacitors, incorrect operation will result in the compensator elements due to the capacitor ire when the capacitors are on. Conversely, if the com- current. The circuit is shown in Fig. 67. A contact pensator or relay settings were revised taking into ac- is across the impedance, so that when the capa- count /0, incorrect operation will result when the capacitors are switched off, the impedance will be to citors are off. Reasonable coordination can be obtained by compromising between the regulating point voltage ala- AUXILLARY CONTACT at light- and peak-load. ON CAPACITOR ned Regulator and Switched Capacitors at the Same Loca- VOLTAGE SWITCH (24) tion—If the capacitor bank is located on the source side REGULATOR zero of the regulator, proper regulator operation will occur the because the capacitor current does not flow through the regulator. Each unit will be independent with re- 30M- gard to relay setting (if voltage control is used with the as capacitor bank), bandwidth, and time delay. The time- delay setting of one unit will not restrict the time-delay 104 (47) setting of the others. C1.. To reduce the current through the regulator, the 0 Ld capacitors should be located on the load side of the V.R. RELAY 3n to regulator; this is the case in most installations. The I7 in be capacitor current will then flow through the voltage Fig. 67—Circuit diagram showing method of obtaining cor- regulator and the line drop compensator circuit, and a rect line drop compensation in the regulator control circuit correction is necessary for proper regulator operation. (48) using an impedance inserted in the relay circuit. The voltage If voltage control is used for switching the capacitors, drop across the impedance is made equal to the voltage L (46). even though the capacitors are connected on the load rise in the line drop compensator due to the capacitor side of the regulator, the potential transformer of the current, I.

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shorted out of the relay circuit. This shorting For multi-step switched capacitor banks located contact is interlocked with the capacitor switch. beyond the regulator location (beyond reasonable con- This method may, in some instances, be less ex- trol interconnecting distances), no low-cost solution for pensive than the current transformer intercon- correct regulator operation is available. In distribution nection method (Fig. 66). For multi-step banks systems there would be very few applications where a the shorting contact arrangement may become multi-step bank is required beyond a regulator location. complicated, thus increasing its cost. Multi-step banks located out on distribution feeders should be avoided for economy reasons. However, if 3. As in the case using fixed capacitors, the relay or the banks are located beyond the regulation point, they compensator settings can be revised to give rea- may affect regulator operation very little, thus per- sonably accurate regulator operation. The change mitting any number of steps. in the voltage regulating relay setting is calculated No coordination is required with regard to regulator assuming the switched capacitors are on and as operation affecting capacitor switching, unless voltage described in Section 45. However, instead of the control is used with the switched capacitors. No full increase (it would be an increase for the regu- problems are generally encountered such as the capaci- lators and capacitor at the same location), as tor switching on or off, followed by operations of the required by the amount of capacitors being regulators, which in turn cause the capacitors to switch switched, only half of the increase would be used. back to their original position. Some reasons for this This will result in a slightly higher voltage than are (1) the capacitor voltage control relay bandwilth is desired at the regulation point during peak-load always greater than the regulator relay bandwidth; (2) conditions and a slightly lower value during light- the voltage change at the regulator location due to the load conditions. Had the full increase in relay capacitors switching on or off is less than at the capaci- setting been used, the peak load voltage at the tor location; (3) the time delay for the capacitor con- regulating point would be correct, but a much trols is generally greater than the time delay associated lower value than desired would occur at light load. with voltage regulator controls. If misoperations do With the smaller increase in relay setting, the occur, they generally can be corrected by slightly ad- average regulating point voltage throughout the justing the bandwidth setting or time-delay setting of entire load cycle will be approximately the desired the switched capacitors controls. value. For a specific application the increase in relay setting will not necessarily have to be one-half the full increase value as calculated. The exact XII. PARALLEL OPERATION OF amount will depend on the load cycle and the a- VOLTAGE REGULATORS mount of time the capacitors are on as compared to off. If they are on for the most part of a 24-hour 47. Voltage Control in Primary Networks day, the increase should be slightly greater than In primary networks the voltage control equipment the one-half value. A smaller value is used, if they is load-tap-changing (LTC) equipment in the primary are on for only short periods of time. network unit. The regulation range of the equipment The same reasoning as used for revising the is generally plus or minus ten per cent. The area served relay setting applies for revising the compensa- by one substation or primary network unit is relatively tor settings. Changing the relay setting will gen- small, and the feeders served from the network unit are erally prove more accurate than obtaining new about the same length, and each serve the same type of compensator settings. load. With these conditions bus regulation is adequate. Individual feeder voltage regulation is avoided, and For multi-step switched capacitor banks, it is difficult when used it is often expensive and complicated. The to determine new relay or compensator settings which principles and theory of the primary network are de- would be satisfactory for all step positions. Revised scribed in Chapter 4. settings are therefore not recommended and a coordi- Voltage-regulating equipment in primary networks nation scheme such as the first two described should essentially operates in parallel. To obtain satisfactory be used (Figs. 66 and 67). operation, it is therefore necessary that the bus voltage Switched Capacitors Located Beyond Regulator Loca- at each network unit or primary network substation tion—With the switched capacitors located remote from be the same throughout all load conditions to prevent the regulator location, interconnecting the control circulation of current between units. The magnitude of circuits is costly. Adjusting either the relay setting or circulating current depends upon the differential voltage compensator setting are then necessary for proper divided by the loop impedance. When one unit raises regulator operation. The simplest and, in general, its voltage above the general level of the network the most accurate is to adjust the voltage regulating system, undesirable circulating currents will occur relay setting. The new setting will again be a com- through the network units and tie feeders. If normal promised setting based on the length of time on and off. line drop compensation is used—that is, settings used The compromised settings were discussed in the third to raise the voltage during peak load and slightly reduce method in the paragraphs on regulators and switched voltage during light-load periods—the circulating cur- capacitors at the same location. rent may be increased, once it starts, and cause in-

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correct regulator operation. This is because the circu- The above mentioned method for obtaining stable lating current appears to be a load current at the unit operation may cause some error if the load power factor that establishes the higher voltage, and appears to be a varies over a wide range between peak- and light-load reduction of load at the unit or units holding the lower periods. Care must also be taken when the load power voltage. The result is that the unit which had the higher factor is high or leading to insure proper voltage control, voltage will increase its voltage due to the increase in especially if the reactance element is reversed. load current, and the unit that had the lower voltage If it is sufficient to maintain constant voltage at the will further decrease its voltage due to the reduction network unit buses throughout all load conditions, the in load current. Each unit will operate to the end of its line drop compensator need not be used. It can be regulation range—the higher voltage unit to the maxi- adjusted for zero resistance and reactance compensa- mum boost position, and the lower voltage unit to the tion. The regulator will then have stable operation, re- maximum buck position. This unstable operation and gardless of the loading on each primary network unit. high circulating currents could open several feeder breakers due to overcurrent, and could completely 48. Voltage Control in Secondary Networks interrupt the operation of the network system. In secondary networks, all of the voltage regulating The path of the circulating current for a primary equipment is located at the substation and not at the network is a closed loop involving the power source, network transformers. When possible, secondary net- subtransmission line to the network unit, the network works should be fed from one substation to eliminate unit, the primary network tie, the network unit, and any possible phase-angle difference in the network sup- the subtransmission line from the network unit back to ply voltage and to simplify the application and opera- the power source. The loop impedance will be pre- tion of any voltage regulating equipment which may be dominantly reactive; therefore, the circulating current necessary. Some large secondary networks are fed from will lag the system voltage by a much greater angle two substations with satisfactory regulating equipment than the load current. Because of this angle, a simple performance, but extensive studies and in some cases trial solution is possible to attain stable and correct opera- and error methods of setting compensators were neces- tion of the regulating equipment. By the simple ex- sary to obtain stable operation and minimize circulating pedient of reversing the reactance compensator ele- current. ments in the control system of the network units, Bus regulation should be used for network feeder stable operation of the tap-changing mechanisms can voltage control. The primary feeders serving networks be achieved.24 The reason is that a highly reactive cir- are about the same length and serve the same type of culating current flowing through the network unit loads, as interlacing of feeders is necessary for good net,- toward the network causes that unit to reduce its voltage work operation. This results in conditions suitable for because of the effect of the reactive current acting bus regulation. In large networks, as found in the major through the reversed reactance of the compensator. cities, primary feeders may not necessarily be the same Conversely, a reactive current from the network length, and individual feeder regulation may be required. through the network unit causes the regulator to raise However, secondary network size is often governed by the voltage. As a result the voltage difference between the amount of area that can be served satisfactorily the units that cause the circulating current is corrected, with bus regulation. With bus regulation the same volt- and stable operation of the regulators is maintained. age is applied to each feeder and a more equal division The resistance element of the line drop compensator of load current is obtained. is used for obtaining a rising voltage characteristic as When the network is served from two bus regulated load increases. A somewhat higher resistance setting substations, the line drop compensators are generally set will generally be necessary, as compared to the re- to hold a constant voltage at some multi-transformer in- sistance setting used with a normal reactance setting. stallation within the network area, where one unit in The most practical method of adjusting the compen- the multi-unit bank is served from one substation and sator elements is to start with a fairly high reactance a different unit from the other substation. However, to setting and a relatively low resistance setting, and then minimize circulating current requires more than obtain- by trial arrive at the best combination of settings. The ing equal voltage magnitude; their phase relationship reactance compensation can be gradually reduced until should also be the same. It may be necessary, as in the the minimum setting is found where stable operation is primary network system, to reverse the reactance com- positive. This can be determined by manually moving pensator settings at both substations to maintain the regulator away from the position corresponding to stable operation. Increased resistance compensation the desired system voltage. The regulator should auto- can also be used. matically return to the desired position instead of con- The amount of circulating current is more critical in tinuing, in the direction of the manual displacement, to secondary networks than it is in primary networks. This the end of its range. Small readjustments in the react- is because of the sensitive reverse tripping of the net,- ance element may be necessary, after the resistance ele- work protectors associated with the network trans- ment is increased, to obtain a rising voltage with load formers. The circulating current during the load cycle increases. These adjustments should be made during only tends to unbalance transformer feeder loading, with light load, because unstable operation is more likely at no actual current flowing from the network toward the times of light load than at heavy load. source. However, if the per cent loading during light

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 307/576 300 System Voltage Regulation load is low enough, protector tripping can occur due to out of service, no line drop compensation is avail- the circulating current. able. This method has its greatest application with units Where individual feeder regulation is used, extreme not initially designed for parallel operation and with care must be taken to minimize circulating current so units reasonably close in physical distance, as control stable operation can be obtained. Reverse reactance can wires must run between units. be used and, depending upon the number of feeders and Difference-Current Control—Control current networks their physical location, cross-compensation or even are used between units in the line drop compensator mechanical interconnection can be used to insure stable circuits in order to separate load current and circulating operation. On high-impedance loops it may be possible to current. The load current is diverted through the line use a bank of single-phase regulators or a three-phase drop compensator for normal control, and the cir- regulator (except single-core induction units) satisfac- culating current is diverted through paralleling reac- torily using the normal line drop compensator circuit. tors that apply voltages in the voltage regulating re- This will depend upon the size of the network as well lay circuit. The polarity of these voltages is such as to as the balancing of similar loads on each feeder. Oper- cause regulator operation, if necessary, to minimize cir- ating experience often determines the best procedure for culating current. Its main limitation is that only two satisfactory voltage control. units can be paralleled, although one unit can be taken 49. Parallel Operation Methods out of service without loss of line drop compensation. The most common application is with regulators in- There are essentially seven methods for parallel op- stalled in high-impedance circuits and reasonably close eration of voltage regulating equipment. The seven in physical distance, as control wires must run between methods can be, in turn, divided into three groups. They units. are: Current-Balance Control—This method is very similar control, except it extends the I. Use of standard line drop compensators and no to the difference-current auxiliary equipment. difference-control paralleling method to more than two 1. Reversed compensator reactance control. units. 2. Cross-current control. Mechanical Tie—The regulator operating mechanisms are interconnected by gearing and shafting to keep II. Use of current networks to separate load current all units on corresponding positions. The units must and circulating current. be located physically close to each other, and regu- 1. Difference-current control. lator operating characteristics must be identical. Main- 2. Current-balance control. tenance is difficult, as separate operation cannot be ob- III. Use of mechanical ties or step-switch controls. tained without a disconnection. 1. Mechanical tie. Out-of-Step Switch Control—The control circuits of 2. Out-of-step switch control. each unit are interconnected to operate all units simul- 3. Step-by-step switch control. taneously. If a step difference occurs between any units, a mechanical safety lock-out prevents further opera- The methods of parallel operation will not be dis- tion. Only one voltage regulating relay and one line drop cussed in detail. Further details can be obtained in Ref- compensator is needed to control the entire bank, which erence 25. A simple description, the limitations, and the can be any number of units. The units must each have applications of each method are listed below. A general corresponding taps and have similar operating charac- over-all comparison chart for the various methods is teristics. shown in Table 6. Step-by-Step Switch Control—In this parallel control Reversed-Reactance Control—The reactance compen- method, one unit is selected as a master unit, and sator element is reversed, and for an increase reactive after it operates it is locked out until all other units have current (circulating current is generally highly reac- operated the same single step. Each regulator will stop tive), the regulator will tend to operate in a lower itself after the one step operation. After all units have direction. Its main limitations are that it limits the line operated, the master unit is then reset. This method is drop compensation available, and its sensitivity will be an improvement of the out-of-step switch control in limited by reactive load current. This method of control that it does not require each unit to have similar oper- has its greatest use with LTC located in different sub- ating characteristics. Both step-switch control schemes stations and with primary and secondary network ap- permit each unit to operate independently if desired. plications of voltage control equipment. Their• greatest application is for large step regulators or Cross-Current Control—The compensator current trans- LTC substation transformers located in the same sub- former of one unit is connected to the line drop com- station. pensator circuit of the other unit, and vice versa. Successful parallel operation of voltage regulating Increased load current in one unit will cause the other equipment requires careful study. The operating condi- unit to operate and increase its voltage; correspond- tions to be met, number of units to be paralleled, loca- ingly, a decrease in load current in one unit will cause tion of units, flexibility of operation desired, similarity the other unit to decrease its voltage. Thus, circulating or dissimilarity of units being paralleled, and the im- current is minimized. Its main limitation is that only pedance of the circuits must be studied to determine the two units can be paralleled, and if one unit is taken best method.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 308/576 Table 6-Comparison Chart of Various Methods for Parallel Operation of Load-Tap-Changing Equipment Circulating-Current Methods iviebLIOU - - - Reverse Reactance Cross Current Difference Current Current Balance Point of Line-Drop Line-Drop Voltage- Voltage- Interlock Compensator Compensator Regulating Relay Regulating Relay Basis of Polarity of response to re- One unit responds to change Circulating current Same as Difference Operation active current is reversed in current in opposite unit. segregated and used Current. so that output voltage to operate voltage- decreases with increased regulating relay. circulating current. Advantages 1. Exactly corresponding 1. 1. 1. taps on all units not required. Same as Reverse 2 2. Same as Reverse Same as Reverse 2. Attachments on Reactance. 2 operating mechanism ' Reactance. ' Reactance. not required. 3. Tap-changermechanism 3. 3., 3. may have different operating characteristics. 4. No control wiring be- 4. Does not limit line-drop 4. Same as Cross 4. Same as Cross tween units. compensation. Current. Current. 5. No special control 5. No special control equip- 5. Provides inde- 5. equipment required. ment required. pendent adjustment of line-drop compensation and Same as Difference paralleling Current. sensitivity. 6. Can be used with any 6. Removing a unit 6. number of units. from service does not require changing line-drop corn- 7. For any number of pensator setting. units. Disadvantages" 1. Safety lock-out requires 1. 1. auxiliary over-current Same as Reverse 1.} Same as Reverse Same as Reverse relaying. Reactance. Reactance. Reactance. 2. Depends upon correct 2. 2. 2. operation of all control equipment on all units and upon properly cor- related settings of line- drop compensators, voltage-regulating relays, and paralleling reactors. 3. Limits line-drop corn- 3. Limited to two units. 3. Same as Cross pensation available. Current. 4. No independent adjust- 4. Same as Reverse ment of line-drop corn- Reactance. pensation and paralleling sensitivity. Table 6 (Continued) Tap-Changer-Position Methods Method Out-of-Step Switches Step-by-Step Switches Mechanical Tie

Point of Motor Control Relay Interlock Motor Control Relay Operating Mechanism Basis of All units operated simultaneously by parallel Controls operate master unit; Operating mechanisms are Operation circuits to motor control relay; interlocked mechanically operated switches mechanically synchronized by mechanically operated switches. operate follower units. by shafts between the units. Advantages 1. Eliminates voltage-regulating relay; line- 1. drop compensator, and time-delay relays from paralleling controls. 1.2 IsSamwitechaes0.Out-of-Step0 2. Does not restrict line-drop compensation. 2. 3. For any number of units. 3. 3. 4. Mechanically operated switches provide 4. 4. Paralleling is independent safety lock-out at one step difference in Same as Out- of-Step f St of the auxiliary and tap-changer positions. Switches. motor control relays. 6. Only one voltage-regulating relay and line- 5. 5. Only one set of auto- drop compensator needed to control entire matic and manual con- bankbut,i if desired, all units may be trol is required. suppl ed with these devices and the master unit selected by switch on control panel. 6. Tap-changer mechanisms may have dif- 6. ferent operating characteristics. Disadvantages* 1. Requires units designed with corresponding 1. Same as Out-of-Step 1. Same as Out-of-Step taps. 2. Switches. 2. Switches. 2. Requires mechanical attachments to 3. squires shafts between operating mechanisms. units. *All methods except Reverse Reactance require control wiring between unite. 301

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In general, the reverse-reactance scheme and the con- committee of the AIEE Committee on Transmission and trol-network schemes will be used on high-impedance Distribution, AIEE Transaction Paper 56-933. 21. "Combination Switching Fits Var Load," A. R. Milliken, loops and step-switch control schemes on low-impedance Electrical World, January 30, 1956, pp. 43 and 46. loops. Except for the mechanical-tie method, the step- 22. "Compensation of Open-Delta Regulators," C. F. Wagner, switch control schemes provide the most positive pro- The Electrical Journal, April 1933. tection against excessive circulating current. 23. "A Field Method for Determining the Leading and Lagging Regulator in an Open-Delta Connection," H. E. Lokay and REFERENCES R. L. Custard, AI EE Transactions, Volume 73, Part III, 1954, pp. 1684-1686. 1. American Standard Definitions of Electrical Terms, American 24. "Year of Experience Solves Primary Network Problems," Institute of Electrical Engineers, 35.20.110. R. J. Salsbury and H. S. Moore, Electrical World, Vol. 100, 2. Utilization Voltage Standardization Recommendations, November 5, 1932, pp. 624-626. October 1942, EEI Publication No. J-8. 25. "Parallel Tap Changing Under Load," H. L. Prescott, 3. EEI-NEMA Preferred Voltage Ratings for A-C Systems and Westinghouse Engineer, November 1951, pp. 180-183. Equipment, EEI Publication No. R-6, NEMA Publication 26. "Distribution System Economics," 0. B. Falls, Jr., Electrical No. 117, May 1949. West, Volume 95, No. 6, November 1945, pp. 80-83. 4. Voltage Levels on Rural Distribution Systems (a booklet) 27. "Extending the Use of Shunt Capacitors by Means of Auto- U.S. Department of Agriculture, Rural Electrification Ad- matic Switching," W. H. Cuttino, AI EE Transactions ministration, Technical Standards Division, Washington 25, Volume 63, 1944, pp. 674-8, discussion p. 1439. D.C., March 1952. 28. "The Development of the Feeder Voltage Regulator," E. R. 5. Utilization Voltages, Howard P. Seelye, AIEE Transactions, Wolf ert, The Electrical Journal, Volume XXV, September, Volume 61, 1942, pp. 147-151. 1928, pp. 441-446. 6. Service Voltage Spread and Its Effect on Utilization Equip- 29. "Capacitors and Automatic Boosters for Economical Correc- ment, H. G. Barnett and R. F. Lawrence, Paper 5-66, AIEE tion of Voltage on Distribution Circuits," L. M. Olmsted, Transactions, Volume 73, Part II, 1954, pp. 320-325. AI EE Transaction, Volume 58, 1939, pp. 491-496. 7. Quality Voltage—Why?, J. A. Sainz and D. J. Hubert, 30. "Basic Concepts of Voltage Regulation," C. J. Lake, Elec- Distribution, January 1956, pp. 5-7. trical World, February 26, 1949, pp. 72-76, 149. 8. Report on Industrial Voltage Requirements, AIEE Sub- 31. "Distribution System Voltage Study," H. L. Deloney and committee Report, AIEE Committee on Inustrial Power W. L. Peterson, Allis-Chalmers Electrical Review, First Applications, AIEE Miscellaneous Paper 47-157, May 1947. Quarter, 1956, pp. 12-15. 9. Standard Handbook for Electrical Engineers, A. E. Knowlton, 32. "Step-Type Feeder Voltage Regulators," L. H. Hill, AIEE Editor-in-Chief, McGraw-Hill Book Company, Inc., Eighth Transactions, 1935, Volume 54, pp. 154-158, discussion pp. Edition, 1949. 993-994. 10. "Busulation eg Plus Capacitors Takes Care of System 33. "Improve Regulation—Increase Revenue," F. A. McCrackin Voltage," D. R. Pattison and F. M. Reed, Electrical World and L. J. Flanigan, Electrical West, May 1954, pp. 69-71. May 19, 1952, pp. 147-149. 34. "Some Methods of Obtaining Correct Line Drop Compensa- 11. "Voltage Drops in Unbalanced Distribution Circuits," W. R. tion on Single-Phase Voltage Regulators Used on Three-Phase Bullard, H. L. Lowe, H. W. Wahlquist, Electric Light and Systems," H. L. Prescott, AIEE Transactions, Volume 70 Power. Part II, 1951, pp. 1598-1604. Part I February, 1944, pp. 42-48. 35. "Economic Rural Distribution System," R. W. Schlie, Part II April, 1944, pp. 60-66. Electrical World, April 19, 1954, pp. 122-125, 213-215. Part III May, 1944, pp. 62-70. 36. "Complaint Survey Basis for Higher Distribution Feeder 12. Electrical Transmission and Distribution Reference Book Voltage, H. E. Jung, Electric Light and Power, May 1951, (book). Westinghouse Electric Corporation, Pittsburgh, Pa., pp. 64-65. 4th Edition, Chapter 7, 1950. 37. "Bus and Individual Feeder Regulation," E. M. Hunter and 13. A New Regulator and Excitation System, J. T. Carleton, D. R. Sampson, General Electric Review, December 1951, pp. P. 0. Bobo, W. F. Horton. AIEE Transactions Volume 73, 47-51. Part III, April 1954, pp. 175-83. 38. "Economic Benefits in Reduced Bandwidths," H. C. Ander- 14. Electrical Transmission and Distribution Reference Book son and D. R. Sampson, Electric Light and Power, April 1952, (book). Westinghouse Electric Corporation, Pittsburgh, Pa., pp. 114-117. 4th Edition, Chapter 8, pp. 261-263. 39. "Fixed Boost vs Shunt Capacitors in Distribution System 15. Analysis of Series Capacitor Application Problems, J. W. Regulation," R. 0. Loomis, Electrical World, August 19, 1944, Butler and C. Concordia, AIEE Transactions, Volume 56, pp. 88-90. 1937, pp. 975-988. 40. "Overcompensation of Regulators Aids Voltage and Loading," 16. Feeder Voltage Regulator Application on Power Systems, P. J. Carlin and L. L. Stahler, Electrical World, May 17, 1941, J. N. Gosinski and J. R. Oberholtzer, AIEE Transactions, pp. 44-46. Volume 67, 1948, pp. 1458-1461. 41. "Bandwidth a Measure of Regulator Effectiveness," W. J. 17. Static Controls for Regulators, J. H. Chiles, Jr., and A. M. McLachlan, Electrical South, August 1940, pp. 27-29, 44. Harrison, Westinghouse Engineer, September 1955, pp. 162- 42. "Systematic Voltage Surveys—Procedure and Application to 164. Distribution Design," R. W. Burrell and W. E. Appleton, 18. Accuracy of Control Devices for Feeder Voltage Regulator, a AI EE Transactions 1938 Volume 57, pp. 535-540. report sponsored by the Transformer Subcommittee of the 43. "Voltage Regulation and Control in the Development of a AIEE Committee on Electrical Machinery, Electrical En- Rural Distribution System," G. H. Landis, AI EE Trans- gineering, June 1944, pp. 250-254. actions, 1938, Volume 57, pp. 541-547. 19. American Standards for Step-Voltage and Induction-Voltage 44. "Voltage-Regulating-Equipment Characteristics as a Guide Regulators, American Standards Association—C 57.15- to Application," P. E. Benner and G. S. Lunge, AIEE Trans- 1949. actions, 1938, Volume 57, pp. 548-553. 20. "Report of a Survey on Controls for Automatically Switched 45. "Automatic Boosters on Distribution Circuits," L. M. Olm- Capacitors," Working Group of the AIEE Capacitor Sub- sted, AI EE Transactions, 1936, Volume 55, pp. 1083-1096.

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MILES MAXWELL

I. POWER CAPACITORS 1. Construction of Capacitors The active parts of power capacitors are the paper, the foil, and the impregnating liquid. Most power capacitors are constructed with two sheets of pure aluminum foil separated by three or more layers of kraft paper and impregnated with a chlorinated aromatic hydrocarbon compound. The paper layers are in the 0.0005-0.00025 inch range of thickness. Extremely close quality control is used to limit the number of conducting particles per unit area and to hold the density and porosity of the paper to acceptable limits. The aluminum foil thickness is usually 0.00025 inch or 0.00035 inch. The aluminum and paper are wound into cylinders with many square feet of active dielectric in each cylin- der. For most power capacitors, several of these cylinders are compressed into a rectangular shape and inserted into the capacitor case. The assembly of the capacitor includes making internal connections, inserting the discharge resistors, evacuating in an oven to remove moisture, filling with impregnant, attaching bush- Fig. 2—Cutaway view of a secondary capacitor. ings, and finishing the case. Fig. 1 shows a cutaway 2. Ratings and Standards view of a power capacitor. Secondary capacitors are similar, except that for some ratings a cylindrical case is Power capacitors are generally rated in kilovars. This used and final form of the individual capacitor is cylin- is related to the farad unit by drical. Fig. 2 shows a cutaway view of a cylindrical E227rf C(10-1) kvar = (1) secondary capacitor. 1000 where E = rated rms voltage f = frequency, cycles per second C = capacitance, microfarads The kilovar is a convenient term for the power industry since load is usually described in terms of kilowatts and kilovars. There are comprehensive standards1 for power capacitors. These will not be discussed in detail, but some of the items more important from an application standpoint will be mentioned. The standard kvar and voltage ratings for non-enclosed and enclosed outdoor capacitor units are given in Table I. The kilovar manufacturing tolerance is minus zero and plus 15 per cent; that is, a 50-kvar capacitor will not be less than 50 kvar nor more than 57.5 kvar at rated voltage. Circuit breakers, disconnecting devices, and all current carrying parts associated with a capacitor bank shall have a current rating of at least 135 per cent of the Fig. 1—Cutaway view of a power capacitor. rated current of the capacitor bank.

*Aelmowledgement is given to Mr. A. A. Johnson for information used in this Chapter from the Westinghouse Electrical Transmission & Distraulion Reference Book, Fourth Edition, Chapter 8. 303

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304 Application of Capacitors

Table 1—Kilovar and Voltage Ratings for Non-Enclosed Table 3—Recommended Transient Voltage and and Enclosed Outdoor Capacitor Units Current Limits for Power Capacitors Voltage KVAR Number of Phases Probable Number of Permissible Peak Transient Values Switching Opera- Times Rated RMS 216 5, 7.5, 133. 1 and 3 tions Per Year Voltage Current 240 3 1 4 1500 240 5, 7.5, 15 1 and 3 7 10, 15, 20 1 and 3 40 5 . 8 1150 480 1 and 3 400 4 . 5 800 600 10, 15, 20 4000 3.25 400 2400 15, 25, 50 1 2770 25, 50 1 4160 15, 25, 50 1 High current and voltage transients are uncommon 4800 25, 50 1 in most power circuits, but capacitors may be subjected to both high voltage and high currents under certain 6640 25, 50 1 conditions associated with switching. The magnitude 7200 25, 50 1 of the permissible current and voltage depends on the 7620 25, 50 1 7960 25, 50 1 frequency of occurrence. In the case of frequently switched capacitors, current and voltage peaks must be 12,470 25 1 held to lower values, whereas higher values may be 13,800 25 1 tolerated occasionally. Table 3 provides a guide for the evaluation of service conditions from the standpoint of transient peak currents and voltages. For capacitors rated 1200 volts and above, the losses Extensive factory production line tests are made on at rated voltage and frequency, and at a capacitor tern- capacitors. Table 4 gives the factory test voltages used perature of 25C, shall not exceed 0.0033 kilowatts per to verify dielectric strength. kilovar. There are comprehensive test procedures for testing The maximum permissible working voltage of power capacitors in the field. Field tests are usually not made capacitors is 110 per cent of rated voltage. Daily opera- unless there is some indication of trouble, or the capaci- tion above this limit will shorten the life of the capacitor has been exposed to possible damage. Consult the tors. The maximum permissible operating kvar is 135 latest NEMA Standards or the capacitor manufacturer per cent of rated kvar. The maximum kvar includes for recommended tests and test procedures. kvar due to fundamental frequency voltage over 100 per cent but less than 110 per cent; kvar due to harmonic II. SHUNT CAPACITORS currents; and the kvar in excess of nameplate rating due to manufacturing tolerances. 3. Application of Shunt Capacitors Capacitors may be operated above 110 per cent of The function of a shunt capacitor applied as a single rated voltage under emergency and infrequently oc- unit or in groups of units is to supply lagging kilovars to curring conditions 200 to 300 times during the life of the the system at the point where they are connected. A capacitor. The recommended maximum rms overvolt- shunt capacitor has the same effect as an overexcited age without loss of life is dependent on the duration of synchronous condenser, generator, or motor. It supplies each overvoltage, with recommended limits listed in the kind of kilovars or current to counteract the out-of- Table 2. These overvoltage limits apply for power fre- phase component of current required by an induction quencies where high frequency transients are not super- motor, as illustrated in Fig. 3. imposed. The crest voltage should not exceed 1.41 times the foregoing rms values. Table 4—Factory Dielectric Test Voltages

Line-to-Line Line-to-Case Test Voltage, Volts Alternating-Current Voltage Rating of Table 2—Recommended Overvoltage Limits for Test Voltage, Volts Power Capacitors Capacitor Unit, Alternating Direct Volts Current Current Indoor Outdoor Multiplying Factor 240 500 1000 3000 10000 Duration Times Rated RMS Voltage 480 1000 2000 5000 10000 M cycle 4.8 600 1200 2400 5000 10000 1 cycle 4 . 2 2400 5000 15000 19000 19000 6 cycles 3.0 4160 9000 27000 19000 19000 15 cycles 2 . 6 4800 10000 30000 26000 26000 1 second 2 . 2 7200 15000 45000 26000 26000 15 seconds 1. 8 7960 16600 49800 26000 26000 1 minute 1. 7 12470 25000 75000 34000 34000 5 minutes 1. 5 13800 28800 86400 34000' 34000 30 minutes 1.35 Test voltage shall be applied for at least 10 seconds.

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Application of Capacitors 305

E0 IF EN IN CAPACITOR KVAR

• CIRCUIT LOADED AT LOAD FEEDERS INDUCTION MOTOR ALL TIMES TO RATED KVA T KVA GEN SHUNT CAPACITOR ••• (A) INCREASE IN LOAD KVA

SHUNT CAP. KVAR TOTAL KVAR IN KVAR FROM Fig. 3—Shunt capacitors supplying kvar required by an SYSTEM induction motor. (8) ADDING CAPS. 150 Shunt capacitors applied on the load end of a circuit 60 140 supplying a load of lagging power factor have several 7 VA effects, one or more of which may be the reason for the K application: 130

OAD 0 % LOAD P. F. 120 1. Reduces lagging component of circuit current. L

% 90 2. Increases voltage level at the load. 110 (C) 3. Improves voltage regulation if the capacitor units are properly switched. 100 0100 4. Reduces /2 R power loss in the system because of cr o 90 reduction in current. F 5. Reduces /2X kilovar loss in the system because 5 0 60 % LOAD P. F. u- o of reduction in current. z la 70 6. Increases power factor of the source generators. D) cL 7. Decreases kva loading on the source generators 60 4

and circuits to relieve an overloaded condition T 90% LOAD P. E or release capacity for additional load growth. UI 3 8. By reducing kva load on the source generators RC 2 7 additional kilowatt loading may be placed on the CI = 60 generators if turbine capacity is available. PLY (E)--- UP

9. Reduces demand kva where power is purchased. S Correction to 100 per cent power factor may be 0 5

economical in some cases. ) 90

VA 4 10. Reduces investment in system facilities per kilo- z watt of load supplied. cn 3 OAD K 7 To illustrate the effects of shunt capacitors, assume L that a 100-kva circuit or piece of apparatus has to carry 2 DSO I, P. F. 100 kva at various power factors. By adding shunt z capacitors at the load, the kva from the source is re- (F) duced materially. The lower the load power factor, the 0 0 20 more effective the capacitors are. This situation is illus- 40 60 80 100 trated in Fig. 4. CAP. KVAR IN % OF CIRCUIT KVA An increase in the capacitor kvar lessens the current Fig. 4—Fundamental effects of shunt capacitors on power carried by the supply circuit from the source up to the circuits. ultimate point at which capacitors supply all of the kilovars required by the load and the circuit supplies load can be increased from 100 kva to about 124 kva, as only the kilowatt component. For a constant load in the Fig. 4(c) suggests. (If the load should be 10,000 kva at circuit, adding various amounts of capacitors allows the 70 per cent power factor, then adding 4000 kvar of useful load to be increased. By adding 40 kva of capaci- capacitors permits the kw to be increased from 7000 to tors to a 100-kva load of 70 per cent power factor, the 8700 without increasing the circuit loading above 10,000

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 313/576 306 Application of Capacitors kva. The load kva can thus be increased to 12,400 kva at 70 per cent power factor.) Shunt capacitors can be viewed in two lights. Adding LOAD capacitors releases circuit capacity for more load, and adding capacitors relieves overloaded circuits. The capacitor kvar per kva of load increase, Fig. 4(e), LJ V is of particular interest, because multiplying this quantity by the cost per capacitor kvar, yields the average cost of supplying each additional kva of load. This cost, W rn 90% R E OF LOAD 3 ac 80 neglecting other advantages of the capacitor, can be 0 compared with the cost per kva of increasing the trans- U— Li. .1 former or supply circuit rating. Thus, if the load power 0 a- 80 X 60 factor is 70 per cent and a capacitor kvar of 40 per cent w 1— is added, the capacitor kvar per increase in kva of the Lai Z 70 load is 1.65. If installed capacitor cost is $10 per kvar, O cr w 0 then the increase n ability to supply load is obtained a. 4:( 40 at a cost of 1.65 times $10 or $16.50 per kva. The in- 60 cremental cost of adding transformer capacity may be En Occu. o much greater per kva of increased capacity. —3 es 20 The same data apply equally well to equipment other than transformers in which current might constitute a limiting factor, such as generators, cables, regulators, as n well as transmission and distribution lines. 0 20 40 60 80 100 In the example taken (Fig. 4), as the load through the CAPACITOR KVA IN PERCENT OF CIRCUIT KVA transformer approaches unity power factor, smaller and Fig. 5—Reduction in losses in the source circuit to shunt smaller incremental gains in load are obtained for in- capacitors. cremental increases in capacitor kvar. The incremental capacitor kvar required for an increment in kva of the load is Fig. 4(1). Expressed mathematically the ordi- permanently connected capacitors, then at light loads become smaller and the line is over-compen- d(Cap. kvar) /, and Ix nate in this curve is equal to sated, because I, is dependent only upon voltage and d(Increase in load kva)' not upon load. Regulation of the line is practically un- These curves show that the final increment is attained changed by the capacitor, because the capacitor effects at much greater expense than the initial increment. an increase in voltage both at light load and at full load. Capacitors applied to a given load reduce the /2 R and At light loads, the voltage rise might be so much in /2X loss in the supply circuit in accordance with Fig. 5. excess of normal as to represent an undesirable or even For a 70 per cent power factor load with 40 kvar of intolerable condition; a solution is to provide manual capacitors added for each 100 kva or circuit capacity, or automatic switching to add or remove groups of the /2 R and /2X loss will be 59 per cent of its former capacitors as desired. value. This loss in the particular circuit supplying the load can be calculated directly and may be important, especially if the circuit impedance is high. The resistance A and reactance losses are also reduced in all circuits and LOAD*I0 PER UNIT SOURCE transformers, back to and including the source genera- —I tors. SHUNT CAPACITOR The voltage drop in feeders or short lines can be ex- 'xi' (a) pressed approximately by the relation Voltage drop =RIr d-XIx (2) Rix where R is the resistance, X the reactance, I,. the power component of the current, and Ix the reactive compo- Rio — A nent as shown in Fig. 6. If a capacitor is placed in shunt XIr across the end of the line, the drop immediately de- RIr XI% creases or the voltage rises. The new voltage drop becomes approximately: x Voltage drop =RI,±XIx—XI, OA— VOLTAGE AT LOAD —ER (3) 08—VOLTAGE AT SENDING END WITHOUT CAPACITOR—Es where Lis the current drawn by the capacitor. Thus if 0C— VOLTAGE AT SENDING END WITH CAPACITOR—Es I, is made sufficiently large, both the RIr and the XIx (b) drops can be neutralized. This expression also shows Fig. 6—Effect of shunt capacitors on voltage drop in source that if the voltage drop is compensated at full load with circuit.

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An approximate formula which is often useful when useful since it shows the effect of the capacitor alone. If a applying capacitors to a feeder or radial line is particular increase in voltage is required at a given (kvar)(d)(X) point, Equation 4 allows direct calculation of required Per cent voltage rise — (4) capacitor bank size. 10 kv2 The curves of Fig. 7 show the amount of shunt capaci- where kvar is the total kilovars in a three-phase capacitor kvar required for loads of three power factors and tor bank, d is the distance in miles from the bus to the for 0, 5, and 10 per cent voltage drop over the supply cir- capacitor bank, X the reactance of the feeder in ohms cuit. To illustrate their use, assume a 20-mile, 33-kv line per mile and kv the line-to-line system voltage in kilo- of 2/0 copper conductors which steps down through a volts. Equation 4 gives the per cent voltage rise, caused 10,000-kva, 7 per cent reactance transformer to 13.8 kv. by the capacitor, from the bus to the capacitor. This Assume the full load is 10,000 kva at 80 per cent power must be superimposed on the drop caused by the load factor. Also assume: line impedance 9.62 +j15.36 ohms in order to find net voltage rise or drop. This equation is or 0.0883+j0.141 per unit on 10,000-kva base; trans-

Es=1.05 ER 90 Es.E R Es' 13° ER 120 80 ACV 80 ■ LOAD 110 70 P;E Aimem 70 LOAD P.F. LOAD RE 90% Oa ■ 90% Oa 100 60 ismonn 60 6:5 90 50 ■mossimm 50 80 40 ERISMEN 40 ■ o• 70 30 MAME 0 30

60 20 MEM,IN, TIIC 0 20 I00 50 NW/ .4m 10 90 0 I0 20 30 40 10 130 0 Eir IIMPIE 0 80

A I0 20 V 120 1° IIMISE: 70

OR K 0 110 '0 60 CIT

A ■ - 100 ErrizedLo..-Lo %P 50 CAP ■ ■ 02- 90 wris 40 LOAD I? F ■ 80% HUNT 03 rims 0 30 S 80 ■ ,, 70 UM -tr.- ic 0 100 LOAD E E. ■ 80 % 60 73:6 9' 90

>. 0 80 —E---' 5 0 !ril , 0 10 20 30 40 • 10 03 10 20 130 71 70 Ft AP. 0.5 120 0.4 6 0 60

110 61 50 0.3 100 02 41 40

90 31 30 ( 80 21 LOAD RE LOAD P E 20 70% 70% 70 10 1 if 60 Ifi 0 10 20 30 40 0 10 20 30 40 10 20 30 40 PER CENT CIRCUIT REACTANCE X PER CENT CIRCUIT REACTANCE X PER CENT CIRCUIT REACTANCE X

Fig. 7—Shunt capacitors required for various power factor loads to give 0, 5 or 10 per cent voltage drop in the source circuit. All per cent values are referred to full-load kva as 100 per cent base.

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former impedance 0.008+j0.07 per unit; total impedance 0.096+j0.211 per unit. Therefore, ratio R/X = 0.096/0.211 0.454. Referring to Fig. 7 for R/X ratio of 0.454 and a circuit reactance of 0.211 per unit, the shunt capacitor kvar required for a 10 per cent volt- FEEDER WITH UNIFORMLY DISTRIBUTED LOAD age drop on the line is 0.54 per unit. In this case, 1.0 per (a) unit is 10,000 kva, so that 5400 kvar of shunt capacitors are necessary. These data and the capacitor kvar required for 0 and 5 per cent voltage drop are given in Table 5. In addition, calculated losses in the circuit are RATED given, as well as the power factors at the sending (E.) VOLTAGE and receiving (Er) ends of the circuit with the selected capacitor kvar in use. To give a more complete view of the use of Fig. 7 curves, the shunt capacitor kvar re- WITHOUT CAPACITOR quired for 5000-kva, 80-per cent power factor load is HEAVY LOAD VOLTAGE PROFILE included in Table 5. (b)

Table 5—Data for 20 Mile, 33-KV Line with Transformation to 13.8-KV Load Bus WITH CAPACITOR RATED Capacitors Percent VOLTAGE Circuit At the Load Power Factor At Voltage Loss On, Conditions Per Unit I Kva KW Es ER WITHOUT CAPACITOR

10 000 Kva, 80% P.F. Load

ES = ER 1,07 10,700 826 95.1 lead 86.2 lead LIGHT LOAD VOLTAGE PROFILE Es =1 . 05ER 0.81 8,100 657 99.7 lead 96 , 7 lead Es= 1. 10ER 0.54 5,400 617 97.5 lag 99.7 lag Fig. 8—Capacitor effect on a feeder with uniformly distributed load. 5 000 Kva, 80% P.F. Load

ES = ER 1.01 5,050 194 94.0 lead 88.9 lead Es = 1 . 05ER 0.50 2,500 156 97.9 lag 99.2 lag transmission line and cable charging currents; and from Es=1.10ER 0.02 100 234 77.5 lag 81.0 lag shunt capacitors. For system planning, the transmission line and cable charging kvars are not usually considered as variables. For 5000 kva the circuit reactance is 0.106. The ratio The long-range kvar planning takes into account the R/X remains constant for all loads. Thus the capacitor available kvar from these sources. The kvar from syn- kvar can be determined, for a given voltage drop in the chronous motors is usually not under the control of the circuit, for any part of full load by using the per unit power company; hence this effect is often netted with reactance based on the partial load. the load. Synchronous condensers are seldom installed In the preceding discussion, a radial feeder with a load with kvar supply as the primary function. They are at the end of the feeder has been assumed. For most dis- usually installed to control the voltage on an extensive tribution feeders, the load is distributed along the feeder transmission network, although some are installed for in some manner. The effect of a fixed capacitor in this system stability or voltage flicker correction purposes. situation is shown in Fig. 8. As in the radial case, the However, if any synchronous condensers are available, capacitor does not change the voltage regulation; there the usable kvars should be considered in planning sys- is the same voltage spread between light-load and tem kvar supply. heavy-load voltage at the end of the feeder with or The generators on the system generally can supply without the capacitor. However, the voltage gradient kvars very economically. However, from an over-all sys- along the line from the substation to the end of the line tem standpoint, it is often more economical to install ca- is considerably reduced by the addition of the capacitor. pacitors out on the system even when excess kvars are This demonstrates the fact that a properly applied ca- available at the power stations. This is true because the pacitor can hold the voltage at the end of the line closer generators cannot always supply kvars with acceptable to the bus voltage. Since most feeders are designed not voltage levels at remote locations on the system. Kvars only for a certain ratio of heavy-load voltage to light- flow from a bus with higher voltage toward a bus with load voltage (regulation) but also for some minimum lower voltage. There will be no kvar flow unless a volt- heavy-load voltage, a fixed capacitor may make it pos- age difference exists between the busses. Excessive volt- sible to omit a voltage regulator in some situations. age drop may be required if an attempt is made to use System Kvar Supply—The average power system re- all available generator kvars. Kilovars supplied out on quires about one kvar per kw of peak load. This reactive the system reduce losses and release circuit capacity. power is supplied from d-c excitation of generators, syn- These factors usually make it economical to install ca- chronous motors and synchronous condensers; from pacitors even when generator kvars are available.

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The system kvar supply should be studied from an Although it is difficult to explain exactly how to de- over-all system standpoint. Studies of past experience termine the optimum capacitor kvar, it is possible to will show the system trend of required kvars as a func- list all the possible economic benefits of capacitors, so tion of peak load. These studies must take into account that the user can evaluate these factors for his particu- all presently installed capacitors and the kvars from lar situation. transmission lines and cables, as well as kvar loads on Released Capacity—A large percentage of the capaci- generators. Once the trend is established, the kvars re- tors now in service were installed because the economic quired in the near future should be easily predictable. benefits were many times the cost of the capacitors. It is The next step is to make a-c network calculator studies natural that the most obvious applications should come to determine the general areas where kvar will be re- first. Situations in which capacitors can be applied at quired. This will include determining the maximum savings far beyond the cost of the capacitors usually in- which may be supplied from generators. It must be re- volve overloaded equipment. If a $100,000 subtransmis- membered that emergency conditions generally deter- sion line can be postponed for a year by adding $5,000 mine the required location of kvars. The loss of major worth of capacitors, this is an obvious application. generators or transmission lines usually imposes severe There are many places in a power system to watch for conditions on kvar supply. System shutdown during this type of savings. In general, whenever studies show disturbance is most often due to improper kvar supply. that with the predicted peak load for the following year, The kvar forecasts show the required future kvars; a feeder, transformer (load or substation), or subtrans- the a-c network calculator load flow studies show the mission line is going to go slightly over the desired maxi- general areas where kvars are required. After the re- mum load, the application of capacitors should be con- quired kvar is determined, detailed studies of the indi- sidered. In many cases, large investments can be post- vidual areas will show whether the capacitors should be poned or even eliminated by relatively small invest- fixed or switched, and whether they should be located in ments in capacitors. Note that the overload may be substations, on feeders, or on subtransmission lines. either thermal or voltage limited; capacitors may cure either situation. There have even been cases in which 4. Economics of Shunt Capacitors the installation of capacitors to reduce system kilowatt When applying capacitors to power systems, an effort losses has postponed the installation of a generating must be made to determine the economic benefits re- unit. sulting from the installation of a capacitor. The opti- The situations discussed above involve the economic mum amount of capacitor kvar to use is always the benefit of released capability by means of capacitors. amount at which the economic gain derived from the This is still an economic benefit even when no very ob- addition of the last kvar exactly equals the cost of the vious gain exists, such as postponing a substation trans- kvar. It is usually easy to determine the total installed former changeover. Fig. 4 shows the released circuit kva cost of a kvar of capacitors; the problem lies in deter- for each kvar of capacitors for various load power fac- mining the exact economic value of the capacitor. tors. This released kva is effective in all the equipment There have been many papers and articles published between the capacitor and the source. This includes the on the economics of capacitors. Most of these are for feeder, the substation transformers, subtransmission special cases, or have so many qualifications that they lines, subtransmission transformers, transmission lines, are not of much use for other cases unless very careful power station transformers, and generators. Consider- attention is given to minute details. A general economic able disagreement exists in the industry with respect to analysis on a system-wide basis is a very complicated the value which should be assigned to released kva re- problem. This has been undertaken by several com- sulting from the addition of a kvar of capacitors. Some panies, and most of them have arrived at results which utilities use the average cost per kva of the entire sys- are satisfactory for their particular situation. Tomlinson tem from distribution transformer to power station and Bigelow2' have reported an excellent example of this transformer, not including the cost of either of these type of study. transformers. Other utilities arrive at some incremental The methods used for determining economic benefits kva cost for this part of the system. The average or in- differ for different companies. This makes it difficult to cremental system kva cost is often reduced by some fac- derive analysis methods which may be widely used. The tor to allow for diversity of load on the system. Still determination of optimum capacitor usage for a power other utilities evaluate a released kva of capacity at system involves the study of so many parameters and practically nothing, if the circuits involved are not load- the consideration of so many alternate combinations of ed to capability at that time. Very few utilities give equipment that a general solution applicable to any sys- credit for reduced loading on generators or generator tem would be extremely complicated. The advent of transformers. There are several reasons for this: most large high-speed digital computers may make it possible systems cannot use all the kvar available in the gener- to arrive at a solution to this problem. There are com- ators, because desired system voltage gradients cannot puter programs now available which make it possible to be maintained unless a large percentage of the kvars are obtain the most economic balance of fixed and switched supplied out in the system away from the power plants; capacitors and feeder voltage regulators for a particular and extra kvar capacity in generators is inexpensive and feeder. The uses of computers will certainly be expanded sometimes may be available at almost no cost just by in this field. raising the hydrogen pressure in the generators.

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Despite the large diversity of opinion, most of the in- The reduction in 12X loss which results in lower peak dustry agrees that there is concrete value in the released kilovar load is not given any economic credit by some capacity due to adding one kvar of capacitors to a power utilities. If credit is given for released capability and for system. In the author's opinion, the value to be attach- increased voltage levels, these companies believe that ed to this benefit is somewhere between 50 and 100 per adequate credit has thereby been given for reduced kvar cent of the latest cost of the system between distribu- peak. Some companies do allow credit for reduced kvar tion transformer and power station transformer, where peak. If credit is given for kvar peak reduction, the the system cost is expressed as dollars-per-kva of load maximum credit is the cost of the capacitors required capability. to furnish the PX reduction. Losses—One of the most important economic benefits The energy saving due to reduced kw loss is relatively of capacitors is loss reduction. As usually installed, ca- easy to evaluate once the loss reduction is known. Most pacitors reduce both 12R and 12X losses on the power utilities use the average bus-bar cost of energy for their system. V. J. Farmer has shown that loss reduction due system. However, there is considerable deviation among to capacitors is a function of reactive current only.2° The companies. In any event, the total yearly savings is real component of current need not be used in the cal- made up of the total kilowatt-hours of loss reduction in culation. a year multiplied by the cost per kwh used by the par- The reduction in PR loss in a circuit due to adding ca- ticular company in evaluating losses. pacitors is The reduction in kw demand on the system brought LRR=2I.I.R-1 about by loss reduction is an important economic bene- 2.R (5) fit. Again, the value used differs widely among com- The reduction in 12X loss is panies. Some use average cost per kilowatt of system LRx=21.1.X —1. generation,' others use average cost of last plant, and 2X (6) still others use incremental cost of last unit. Some multi- where I. is the capacitor current, I. is the reactive cur- ply the cost of system generation by a factor less than rent in the circuit before the capacitors are added, R the one, say 0.5. This line of reasoning maintains that there circuit resistance and X the circuit reactance. is a probability of one-half that reduction of the load on If peak load currents are used, Eqs. 5 and 6 give re- a feeder by one kilowatt reduces the required installed duction in peak load losses. If kilowatt-hours are de- capacity by one kilowatt. The author feels that a kilo- sired, I. must be the average reactive load current over watt of reduction of loss is worth more than 50 per cent some period. Use this average current in Eq. 5 and mul- of the average cost of generation on a system. The indi- tiply by the hours in the period to get kwh. The reac- vidual company must appraise the value of this benefit tive load factor is a good approximation to the ratio of in light of its own situation. average reactive current to peak reactive current, and Voltage Benefits—Capacitors have two primary volt- the value of the first term of Eq. 5, calculated for peak age benefits. Switched capacitors can be used to supple- current and multiplied by the load factors, gives proper ment or replace feeder voltage regulators, substation kw value for use in kwh calculations. Do not multiply bus voltage regulators, or LTC transformers. Fixed ca- the entire equation by the reactive load factor since, for pacitors can raise the average voltage levels on the sys- fixed capacitors, the average value of I. is the same as tem (See Fig. 8). the maximum, so the second term does not vary. It is By raising average voltage levels, fixed capacitors important to note that average current or load factor is may reduce regulation costs. A fixed capacitor is not a used in these equations. The loss factor is not used. voltage regulator and cannot be directly compared to When there is a simple radial circuit between the regulators. However, in addition to raising average source and the load, Equations 5 and 6 are easy to use. voltage levels, fixed capacitors may reduce voltage gra- In the usual case, however, there are loads between the dient along a feeder. In other words, the ratio of the end capacitor and the source which increase I.. In this case, of the feeder voltage to the bus voltage at heavy load is it is necessary to consider sections of the circuit individ- more nearly unity if fixed capacitors are properly ap- ually, calculating the loss reduction in each section, and plied. Fig. 8 also demonstrates this point. This may using the reactive load current and the R and X in the make it possible to eliminate regulators in some cases. particular section. The total loss reduction is the sum of Where fixed capacitors save the cost of regulators, the the loss reductions in the sections. capacitor should be credited with this saving. Using the above equations, it is possible to calculate By raising average voltage levels, a fixed capacitor to any desired degree of accuracy the reduction in losses may increase revenue from the loads. This is particular- due to adding capacitors. It is rather cumbersome to ly true for residential feeders where a one per cent in- attempt to find loss reductions in all of the system back crease in average voltage level may increase kwh con- to the source, and this is a fertile field for using digital sumed by the loads almost one percent. The fixed computers. capacitor installation should be credited with the in- After the loss reductions have been calculated, there creased revenue brought about by increased kwh con- are three economic benefits to evaluate. These are peak sumption. kilowatt load reduction due to reduction of peak-load When considering the reduction in regulator cost loss (demand) ; energy savings due to reduction in loss which switched capacitors can accomplish, a detailed kilowatt-hours; and reduction in peak kilovar load. study must be made to determine the most economic

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 318/576 Application of Capacitors 311 balance between regulators and switched capacitors. Note that Items 2, 3 and 4 are in dollars and Items 5 Some of the economic factors to be considered are: and 6 in dollars per year. All items must be on the same 1. Switched capacitors usually are not used for fine base before comparisons are made. voltage control on distribution feeders. For limits within For fixed capacitors, if the worth of Items 2, 3, 4, 5, ±2 volts or less (120-volt base), some combination of and 6, plus any possible credit for reduced regulator switched capacitors and voltage regulators is generally cost, is more than the installed cost of capacitors, capacitors should be installed. the most economic. For switched capacitors, Items 2, 3 and 4 can be used 2. Care must be used in evaluating annual or capital- as benefits credited to the capacitors only if the capaci- ized costs. Regulators and capacitors are very different tor will be on during system peak. This is usually the type devices with different average lives, different life case. When evaluating Item 5 for switched capacitors, dispersion curves, and different annual maintenance it is necessary to find the average reactive load current costs. while the capacitor is on the line, and the hours used 3. Regulators give some of the voltage benefits of must be the expected hours the capacitors will be on the switched capacitors, but not to such an extent in some system. Item 6 may not apply to all switched capacitor cases. A switched capacitor reduces kw peak if it is on installations. Each case must be studied. during the peak, and it usually reduces /2 R losses for the When comparing regulators and switched capacitors, hours it is on the system. A regulator affects these items the first step is to determine installed costs of the regula- to a lesser extent or not at all. The main point to note is tors and capacitors required to accomplish the desired that if a switched capacitor is compared to a regulator task. When all of the equipment is not to be installed at only on the basis of dollars investment to do a particular the same time, standard economic procedures must be regulating job, the economic choice may not be made. used to arrive at comparable present worths or annual The capacitor inherently has some other economic bene- costs. To find the economic solution, the switched ca- fits which are not apparent if investment alone is con- pacitor must be credited with the other economic bene- sidered. fits listed above.

Evaluation—To review the evaluation of the economic 5. Substation Capacitor Banks benefits of capacitors discussed in this section, some gen- Shunt capacitors are frequently installed in substa- eralized statements will be given. tions in large banks or groups of banks totaling as much 1. Capacitors can often defer or eliminate invest- as 60,000 to 100,000 kilovars. Individual banks on a ments in feeders, substations, subtransmission lines, single switching device may total 20,000 to 30,000 transmission lines, and generating units where over- kilovars. loads only slightly more than allowable occur. To receive maximum economic benefit from capacitors, they should be installed as near to the load as pos- 2. For the general case, the release of load capability sible. In general, this means that installations should be brought about by the installation of one kvar of capaci- out on the load feeders. When capacitors are installed in tors is evaluated as: substations, there is usually some special situation. This (Released load capability—kva/kvar) (average sys- may involve one or more of the following factors: tem cost between distribution transformer and power station transformer—$/kva) = $/kvar of capacitors 1. A large load is served directly from the substation. installed. 2. Due to excessive light-load voltages, it is impossi- 3. The reduction in system kw peak due to reduced ble to install more fixed capacitors out on the feeders. /2 R losses is worth: 3. It is impossible or uneconomical to put more (Reduction in peak loss—kw/kvar) (average system switched capacitors in small banks out on the feeders. generation cost—$/kw) = $/kvar of capacitors in- This may occur when a load peculiarity would require stalled. that more switched feeder banks have some expensive 4. The reduction in system kvar peak due to reduced special control. /2X losses is worth : 4. System considerations require more capacitors in (Reduction in peak loss—kvar/kvar) (average install- an area than the feeders can economically use. ed cost of capacitors—$/kvar) = $/kvar of capacitors 5. System considerations require a large block of ca- installed. pacitors in a certain area under the control of the system 5. The worth of /2 R loss energy saved by a kvar ca- dispatcher. pacitor is (See Eq. 5) : The above factors indicate why most substation ca- (Yearly loss savings—kwh/kvar) (average system pacitors are in switched banks. Very large banks of ca- energy cost—$/kwh) = $/year/kvar of capacitors in- pacitors can seldom be switched in a single step. The stalled. voltage change resulting from the insertion or removal 6. The worth of increased average voltage is: of the capacitors may be excessive. Fig. 9 shows typical (Percent increase in average voltage—%/kvar) (In- layouts for large capacitor banks. Fig. 9(a) shows a sin- creased revenue—$/yr/%) = $/yr/kvar of capacitors gle automatic breaker for a single bank and Fig. 9(b) installed. shows four automatic breakers for four banks. These

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 319/576 312 Application of Capacitors breakers must be able to handle the full system short- When installing large banks of capacitors in substa- circuit capability. Fig. 9(c) shows one full capacity auto- tions, future breaker maintenance can often be reduced matic breaker and two reduced interrupting capability by foresight in the physical layout of the capacitor non-automatic breakers for a three-step bank. Fig. 9(d) banks. This is especially true where parallel capacitor shows three automatic breakers in an arrangement to banks are switched. Relatively minor increases in the give effectively seven-step switching. For example, with inductance between the banks may greatly reduce breakers 1 and 3 closed 5/7 of the bank is in service. breaker maintenance by reducing the magnitude and Opening breaker 1 and closing breaker 2 would give 6/7 frequency of switching inrush currents. The leads be- of the total capacity. This scheme is little used because tween banks should be as long and on as large a spacing of the relatively large amount of capacity which is re- as practical. Single-conductor cable is preferable to moved or inserted during the switching operation. To go three-conductor cable if cable is used. When a separate from 3/7 to 4/7 requires switching off 3/7 in one step bus is used as a capacitor bus, the bus should be no and switching on 4/7 in one step. This would cause two larger than required for the current, and bus conductor fast voltage changes on the system, each much larger spacing should be as large as practical. Iron pipe has than would be caused by the 1/7 step which is ac- been used for capacitor busses at times in order to ob- complished. tain larger high frequency resistance and reactance. Fig. 9(e) shows an arrangement for using one full in- Substation capacitor banks may be either stack-type, terrupting capacity automatic breaker and several re- open-rack equipment, or metal-enclosed banks. Individ- duced interrupting capacity non-automatic breakers to ual fuses are generally used on the capacitors in substa- obtain a large number of switching steps. This scheme tion banks. The number of parallel units in a group or differs from that of Fig. 9(c) primarily in the use of fuses the available fault current may be such that group fuses to insure that the non-automatic breakers will never will not protect individual capacitors units against case open fault current. The schemes of Figs. 9(c) and 9(e) rupture in the event of internal fault. must be used with care. The non-automatic breakers Large capacitor banks located in substations are gen- must be able to withstand the full momentary currents erally switched. The bank is frequently controlled re- available, both fundamental frequency fault duty and motely over supervisory control or by similar means. If high frequency transient switching currents between the the bank is controlled automatically, voltage or time- capacitor banks. Frequently, momentary current duty clock control is often used. Other means, such as vars, (fundamental frequency or high frequency) on the amperes, power factor, temperature, or combinations of breaker will make it necessary to use breakers with full these may be used. interrupting capability, so that no savings result from the use of non-automatic breakers. Reactors may be 6. High-Voltage Applications used to reduce both momentary and interrupting duty Capacitors may be applied at almost any voltage. on capacity bank breakers. They are a building block type of device by which kvar can be added by adding units in parallel, and required voltage can be obtained by units in series. Capacitors AUTOMATIC" BREAKER should be operated at or near rated voltage for economic reasons. Since the kvar output varies as the square of AUTOMAT1C AUTOMATIC NON- the voltage, poor utilization is obtained at voltage well TBREAKER BREAKERS AUTOMATIC BREAKERS below rated. For example, a capacitor rated 7200 volts and 50 kvar could be used on a 4160-volt system in a wye-connected bank. The voltage across the capacitor T TTTT TTT would be 2400 volts and the kvar output would be (50) (2400)2/(7200)2 = 5.55 kvar. This would obviously (a) (b) (0) be very poor utilization of a 50-kvar capacitor and economics would certainly not favor such applications. However, 2400-volt capacitors could be used on a AUTOMATIC 12,470-volt system with 100 per cent utilization of ca- BREAKER pacity. They would have to be in a wye-connected bank with three 2400-volt capacitors in series per phase leg. FUSES It would also be necessary to insulate the capacitors I 2 3 AUTOMATIC NON-AUTOMATIC from each other and from ground. BREAKERS BREAKERS This ability to add capacitors in series to obtain any EQUAL KVAR desired voltage and to add capacitors in shunt to get de- PER STEP sired total kvar makes the application of high-voltage TTT TTTTT T capacitor banks a relatively simple matter. Fig. 10 X 2X 4X shows how capacitor units are connected for one phase of KVAR a wye bank. High-voltage banks are usually large banks (d) (e) and more elaborate protection and control schemes can Fig. 9—Schematic arrangements for switching large capaci- be justified. Switching may not be a problem since the tor banks. circuit breakers available for voltages above primary

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LINE units in parallel, connection of one bushing of capacitor to the insulated platform on which it rests, and means of detecting unbalance conditions before the unbalance becomes excessive. Each capacitor unit in a high-voltage GROUP I IFUSE 17. bank should be provided with a fuse of the indicating type. These fuses need not be of high interrupting ca- TT TI A_ pacity because there are always two or more capacitor groups in series, and, when a unit becomes short circuited for any reason, the current through the fuse is limited. With individual fuses, a faulty unit can be located T without resorting to the risky procedure of searching for GROUP 2 the source of noise or arcing, or making inconvenient TT 1 TI T. tests. It is also easy to make a check and determine if all units in the bank are operating properly. The individual MK fuses can be omitted, but at a sacrifice in the protection to the capacitor bank. The number of units in parallel in a single group is im- GROUP X 1111 portant. Several considerations affect the choice of correct number. First, the number should be sufficiently T T TIel large to insure that the fuse on a single unit blows when the unit becomes short circuited and the fuse is called NEUTRAL upon to carry the total phase current. Second, the volt- Fig. 10—Connection for fused capacitor units for one phase age on the remaining units in a group should not become of a three-phase bank. Symbols apply to Equations (7) excessive with the operation of one fuse in a group. If to (15). the number of parallel units is too small, the current X—Number of capacitor groups in series through the fuse may be so low that it will not blow, or M—Normal number of capacitor units per group take too long in doing so. An arc of 50 amperes inside a N—Number of units out of one group capacitor unit may rupture its case if allowed to continue el—Actual voltage across group 1 for a long time, and such a rupture may endanger other ea—Rated voltage across group 1 units in the bank. After considering the rating of fuses ea—Normal System voltage to neutral that must be used to avoid operation on switching transients, and taking into account the arc energy required feeder voltages are usually designed so that capacitor to rupture the capacitor case, it has been established switching ability is present or can be obtained at a that the current through the fuse, when a unit becomes slight increase in cost. The circuit breaker manufacturer shorted, should never be less than 10 times the normal should be consulted in all cases before a breaker is ap- capacitor current through the fuse. plied to switch capacitors. It is also desirable following the operation of one fuse Small high-voltage banks may have high switching to avoid voltages in excess of 110 per cent on the remain- cost on a per kvar basis. The smallest available switching units in a group. This assumes that, in the case of the ing device may be able to switch very large banks, and minimum size bank, not more than one fuse operation is the use of these devices on small banks makes the permitted. To accomplish this, periodic checks are nec- switching expensive. For subtransmission voltages, load essary. break disconnect switches have been used to switch The amount of current that flows through a fuse when small banks. In some situations this is an economical a unit is shorted is also affected by the number of series solution. However, as is the case with circuit breakers, groups and whether or not the neutral of the capacitor the manufacturer of the load-break switch should be bank is grounded. consulted before the switch is applied to capacitor Tables 6 and 7 show the recommended minimum switching. There will be limits on maximum bank sizes number of fused capacitor units that should be used in for single banks and restrictions on parallel bank switch- parallel for a given number of groups in series in each ing. Restriking in the switch may cause system prob- phase leg, for ungrounded or grounded-wye connections lems in some situations and this must be investigated. respectively, based on meeting the previously discussed Section 10 discusses capacitor switching devices in requirements. All capacitor units are assumed to be the more detail. same voltage and kvar rating. At one time, operation of capacitor units in series was Very often, large banks contain many more than the looked upon as risky because of the ever-present possi- minimum number of units in parallel. When this is the bility of subjecting capacitors to overvoltage as a result case, more than one fuse can operate and still not seri- of changes in voltage distribution, due either to a change ously raise the voltage across remaining units. In such in impedance of portions of the phase leg or to grounds instances, periodic checks of fuses are necessary to avoid at some point on the assembly. Most of these risks are abuse of good capacitors as result of a faulty one. The minimized or entirely eliminated, however, when proper voltage across the remaining capacitors can be deter- thought is given to such factors as fusing, number of mined from Tables 6 and 7, the-curves of Figs. 11 and

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Table 6-Ungrounded-Wye Capacitor Current and Table 7-Grounded-Wye Capacitor Current and Voltage Relationships with Shorting and Removal of Voltage Relationships with Shorting and Removal of One Unit in One Phase Leg One Unit in One Phase Leg

Current Voltage on Current Voltage on Number Minimum During Fault Remaining Number Minimum During Fault Remaining Groups Units Per Through Fuse Units in Group Groups Units Per Through Fuse Units in Group Series Group Times Normal Per cent Series Group Times Normal Per cent 1 4 12.0 109 1 1 Line Fault 2 8 12.0 109 2 6 12 109 3 9 11.6 109.6 3 8 12 109 4 9 10.8 110 4 9 12 109 5 10 11.5 110 5 9 11.2 109.8 6 10 11.2 110 6 9 10.8 110.0 7 10 11.0 110 7 10 11.7 109.4 8 10 10.9 110 8 10 11.4 109.5 9 11 11.9 Less than 110 9 10 11.2 Less than 110 10 11 11.8 Less than 110 10 10 11.1 Less than 110 11 11 11.7 Less than 110 11 10 11.0 Less than 110 12 11 11.6 Less than 110 12 10 10.9 Less than 110 13 11 11.6 Less than 110 13 10 10.8 Less than 110 14 11 11.5 Less than 110 14 11 11.8 Less than 110 15 11 11.5 Less than 110 15 11 11.8 Less than 110 16 11 11.5 Less than 110 16 11 11.7 Less than 110

12, or calculated from the equations given below. For all The current through the fuse for a completely short- equations, the system impedance up to the capacitor circuited capacitor unit in group 1 in times normal oper- bank was neglected. Refer to Fig. 10 for identification of ating current is symbols in the following equations. 2 2 2 Equations listed ( - a) apply to cases in which all ca- ea2 e ..- + + • + pacitor units are of equal voltage and kvar rating, and Mi M2 Mx (10) the number of parallel units is the same in each group. E_ (Ms) 2 2 2 ea a_ e.2 Ungrounded Neutral Capacitor Bank-Normal voltage 1 _L across group 1 is _3M M2 - • " I Mel 2 3M1X )(e0 '_ (10-a) (Ml) I 3X - 2 eihr - (7) 2 2 2 ec2 ecx Grounded-Neutral Capacitor Bank-Normal voltage -+ + • • +- M1 M2 M. elN across group 1 is same as for ungrounded neutral e. bank as given in Eq. (7). (7-a) eibr =-X With N1 units removed from group 1 the voltage el across the remaining units is With N1 units removed from group 1, the voltage el across the remaining units is 2 r/ eel \

2 WW1- N1))(e,) e.31 = ( (Mi - N I ) )(e.) eel 4 ecx el= (8) ± ± (3M1-N1) (4) ± e2c2 + !_ MI-Avi m. 3M1(MI -N1) M2 /-x Mie„ (11-a) 3Mie. Ni-FX(Mi-Ni) - 8 - 2N1 -I- 3X (MI N1) (8-a) el The current through the fuse of a completely short-cir- With N1 units removed from group 1, the voltage shift cuited capacitor unit in group 1 in times normal oper- of the neutral of the capacitor bank e No is ating current for a grounded-neutral capacitor is - - N1 41 2 2 2 ecl ec2 cox Ai ( )(e.) - --i- • • • -F - - -- I iv1--- 1 ex. =7- (9) M 1 M2 Mx 2 If = (M1) 2 2 (12) (3Mi-Ar1)eti + e.. ecx + + ec2 ± ... ± _ 3 3M1(MI -N1) MX M. M2 Mx. -

Nie. MIX (9-a) - (12-a) eN° 2N1-1-3X (M1-N1) 1

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Application of Capacitors 315

4, 16 2 -----NUMBER OF GROUPS The current in the fuse of a completely short-circuited IN SERIES capacitor unit in group 1 of one bank of two similar 9 banks, with the neutrals solidly connected and un- a grounded, in terms of normal current in one capacitor unit is 7 2 2 0 2 — CENT e01 2 %•atc +e' ± • • • • + 6 MI M2 If= (M1) (14) - PER 2 5 e.12 +€202 ec. + • • • • TAGE __6M1 M2 4 VOL / 777/ 6M1X 3 = (14-a) OVER 6X — 5 2 The current in the neutral connection between two 1 similar banks of capacitors, with N units out of group 1 in one bank, in terms of the normal current through one ) capacitor unit in the faulted group, is 0 4 8 12 16 g0 g 2 UNITS REMOVED FROM GROUP-PER CENT ecl [ Fig. 11—Ungrounded wye-connected shunt capacitor bank. (15) /N --- (6-1i) (4) 2 2 — Curves give the per cent overvoltage across the remaining 0 2 + e 02 + ee. units in a group. _6 (M1— N1) M2M x j 3M1N1 (15-a) 5N1+6X (MI — NI) Two Identical Capacitor Banks with Neutrals Solidly Tied Together and Ungrounded—The normal voltage across 7. Primary Feeder Application any group of capacitors in an installation consisting of A large percentage of the shunt capacitors now in use two similar groups, with the neutrals tied solidly to- are installed on primary feeders. This is because maxi- gether and ungrounded, is effsl as given by Equation (7) mum benefit is obtained if the capacitor is as near to the for any bank. With N1 units out of group 1 in one bank, load as practical. Most of these primary feeder applica- the voltage across the remaining units in group 1 is tions are in pole-mounted racks. These installations 1 may range from 45 to 1200 kvar on one pole. Most pres- — (e01) (") ent installations range from 300 to 600 kvar. — (13) The most economic size and location of capacitors on 6— N-1) 2 2 a particular feeder is usually determined primarily by ( MI (ecl) Z -I- + +m- consideration of reduction in losses. For a simple radial 6(M1— N1) M2 . circuit with loads only at the end of the circuit, loss re- 61111e. duction calculations are quite simple. If loads are dis- (13-a) el 5N 1+6X (111 N tributed along the feeder in some known pattern, it is %.+ possible to develop relatively simple equations for loss 16 : 4 3 2 reduction. However, most power systems have no simple 9 type uniformity of either feeder pattern or loads. This makes it necessary either to engineer each feeder in de- B T tail or to develop some "rules of thumb" which apply NUMBERS OF . CEN 7 reasonably well for most cases. The approximate appli- GROUPS IN SERIES cation rules are intended to be reasonably close to the

PER optimum without excessive detailed engineering. There

E- have been many approximate rules developed over the G years. Neagle and Samson22 have developed a general TA 46 rule which has some application. They state that a good approximation to the optimum size and location for a 3 VERVOL feeder with uniformly distributed load is a capacitor O 2 rated 2/3 of the total load kvars on the feeder located 2/3 of the distance from the source to the end of the I/7 feeder. This rule has merit and is particularly useful for feeders where the reactive load factor is high, say above • 21 El- 12 16 2 0.8. However, this rule must be used with care on feed- UNITS REMOVED FROM GROUP - PER CENT ers with low load factors. Fig. 13 shows this effect. If a Fig. 12—Grounded wye-connected shunt capacitor bank. fixed capacitor equal to 67 per cent of the maximum Curves give the per cent overvoltage across the remaining reactive load kvar is placed 2/3 of the way out on a units in a group. feeder with a yearly load factor of 0.4, the feeder reac-

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100 100

80 90 / \ x FACTOR I // 1.0 LOAD \ / / , / /1 60 80 -0 I- / i z / —OPTIMUM CAPACITOR I LOAD / I FACTOR= u_ LOCATION / 2R 1.0 0 0

40 Ix 70 1 z --- CAPACITOR LOCATED w 2/3 FEEDER LENGTH / AL i 0 FROM SOURCE END / NITI a. 20 I 60 /

z F 0 ----.. O 0 1 0 N. T 1 0 \‘. 50 lal / rc 0.4 LOAD RCEN ----- FACTOR /

CO - PE N \ ----../ ..J0 -20 ON 40 / \ 1 CTI / LOAD IFACTOR •

\ 1 REDU t 0.8 -40 30 0 02 04 06 08 I0 i I

CAPACITOR KVAR OSS I L 1 CORRECTIVE RATIO MAX.LOAD KVAR l I 20 LOAD Fig. 13—Optimum fixed capacitor location compared to i FACTOR= capacitor located by two-thirds rule. 0.6 , , 10 tive /2R losses are actually increased by about 20 per LOAD LOAD FACTOR' cent. Fig. 13 shows the loss reduction for any capacitor FACTOR• 0.4 corrective ratio with the capacitor located at the opti- 0.2 ---" 0 mum location for each ratio. It also shows the loss re- 02 04 06 08 10 duction for the capacitor located 2/3 of the feeder length away from the source. Fig. 14 shows the effect of load CORRECTIVE RATIO CAPACITOR KVAR factor on maximum possible loss reduction for a feeder MAX. LOAD KVAR with uniformly distributed load. Fig. 14—Effect of reactive load factor on maximum loss In every case the capacitor is located at the optimum reduction and optimum corrective ratio of fixed capacitors. distance from the source for that size capacitor and load Feeder with uniformly distributed load and a single fixed factor. Therefore, each point on a curve may represent a capacitor located at optimum location. different capacitor location. For example, for a corrective ratio of 0.8 and a load factor of 0.4, the optimum The conclusion from the study:" from which these location of the capacitor is in the substation, zero dis- curves were taken is: for fixed capacitors on feeders with tance out on the feeder. This gives zero loss reduction, uniformly distributed load, the maximum loss reduction of course, but if a capacitor of this rating is located any- occurs with a capacitor bank equal to 2/3 of the average where on the feeder the losses are increased, not de- kvar load located 2/3 the feeder length away from the creased. Obviously a fixed capacitor with 0.8 corrective source. The rating conclusion comes directly from Fig. ratio should not be used on a feeder with 0.4 load factor. 15, since the reactive load factor is the ratio of average The dashed curve in Fig. 14 connects the points of maxi- reactive load to maximum reactive load. mum possible loss reduction. This curve shows the opti- It is important to note that the above rule does not mum corrective ratio for each load factor. Fig. 15 was necessarily result in maximum possible loss reduction by taken from Fig. 14 and directly shows optimum correc- capacitors. It applies when only fixed capacitors are used. tive ratio as a function of reactive load factor. In all In general, some combination of fixed and switched cases the optimum location is 2/3 of the distance out on capacitors gives more total loss reduction. This is particu- the feeder. When both corrective ratio and capacitor larly true at lower load factors. Also, there is no rela- location are optimized for maximum loss reduction due tionship between the capacitor rating which gives maxi- to fixed capacitors on feeders with uniformly distributed mum loss reduction and the most economic capacitor load, the optimum location always is 2/3 of the feeder size. As discussed in Section 4, the most economic use of length away from the source. The optimum corrective capacitors results when the economic benefits from the ratio varies with the load factor, as shown in Fig. 15. last kvar installed exactly equal the cost of the kvar.

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nomic studies generally show that it is economical to correct power factor to near unity even under maximum load conditions. This is true whether individual primary feeder or overall system power factor is considered. In most cases, if the power factor of a feeder is corrected to near unity for heavy load conditions, the light load volt-

0.6 age will be excessive. For this reason, if for no other, it is O necessary to switch some of the feeder capacitors if max- ATI imum economic use of capacitors is approached. Any of the control methods discussed in Section 11 0.4

TIVE R may be used to control switched capacitors on primary feeders. The most frequently used methods are voltage control and time control (see Table 9). Time control is CORREC 0.2 usually the most inexpensive method and has a further advantage in that no consideration of coordination with feeder voltage regulators is necessary in most cases. A

OPTIMUM study of the load cycle on the feeder will show whether 0 02 04 06 08 10 time control will allow effective use of the capacitor REACTIVE LOAD FACTOR while still meeting all system requirements. Study of each individual feeder is usually not necessary. A few Fig. 15—Capacitor size for maximum loss reduction on a feeder with uniformly distributed load. If size shown on this representative feeders over the systems will show curve is used, optimum location is always two-thirds of whether the load characteristics are such that time con- feeder length from source end. trol can be used. Voltage control is an ideal method for controlling primary feeder banks in many situations. The capacitor However, from studies such as these plus numerous operates like a voltage regulator, but in many cases the studies of particular situations, it is possible to give operation of a voltage-controlled bank also gives opti- some rules which provide reasonably close to the optimum use of the other benefits of the capacitors. In de- mum economic benefit for most residential area distri- termining the voltage differential to be used for switch- bution feeders. ing the bank on and off, consideration of the operation 1. Fixed capacitors should be located 2/3 or more of any associated feeder voltage regulator is necessary. of the feeder length away from the source. The rating Operation of the capacitor switches causes a voltage should be the maximum possible value from a light- change. This voltage change may cause the regulator to load voltage standpoint. However, the rating should operate. Under some conditions, switching a capacitor not exceed that given in Fig. 15. bank on may cause enough voltage rise that the regula- 2. Switched capacitors should be added until the tor will operate to reduce the voltage. If the regulator ratio of total switched plus fixed capacitors to peak reduces the voltage to the point where the capacitor reactive feeder load is at least 0.70. The location of voltage relay switches the bank off again, a hunting sit- the switched capacitors will be determined primarily uation between the regulator and the capacitor bank by voltage regulation considerations, but in general will occur. This may damage the regulator and the ca- they should be located in the last 1/3 of the feeder pacitor switches. When voltage control of primary feed- away from the source. er banks is used, the voltage change due to capacitor When fixed capacitors are applied, it is necessary to operation must be determined and the operation limits determine the minimum load on the feeder for the fol- of the capacitor voltage relay adjusted such that inter- lowing year in order to calculate the maximum light- action between the capacitor bank and the feeder volt- load voltage. Equation 4 gives the voltage rise caused by age relay does not occur. The relationship must also be the capacitor, but the load drop and the effect of feeder such that the capacitor bank is in service as large a per- voltage regulators or bus regulators must also be known centage of the time as system conditions allow. in order to get the maximum light-load voltage. It is usually possible to use group fusing on primary In addition to fixed capacitors, many switched capaci- feeder banks. In most cases, fuses are available which tors are installed on primary feeders. There are relative- will allow for protection against case rupture in the ly low-cost switching devices available which make event of a shorted capacitor. If the bank size is such switched pole-mounted capacitor banks as economical that group fuses which will protect against rupture are as switched substation banks in many situations. The not available, individual fuses are recommended. For capacitors and switches are mounted in racks similar to most pole-mounted banks on primary feeders, fused the racks used for pole-mounted fixed banks. distribution cutouts are used as group fuses. There is The switched capacitor banks on primary feeders may generally no protection other than fuses on primary be considered primarily as voltage regulators, or as de- feeder capacitor banks. There is no fault relaying on vices to reduce the load on the system back to the switches used with switched banks. The group or indi- source. In most cases, a combination of these is the vidual fuses are the primary protection and the feeder reason for the switched capacitor application. Eco- relaying is the back-up protection. Fig. 16 shows a

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difficult to evaluate in most cases. However, if the savings in distribution transformer capacity justify the secondary capacitors, there is no question that some additional benefit results from the other factors. R. A. Zimmerman" has developed a set of curves which show whether secondary capacitors will be economical considering distribution transformer savings only. This implies that primary capacitors are economically justified in the case to be studied. The installed cost of secondary and primary capacitors and distribution transformers must be known, as well as the power factor of the load. With this information, the curves of Fig. 17 can be used to show whether secondary capacitors are justified. Assume that a primary feeder circuit is operating at a 70 per cent power factor. It is desired to raise the power factor to 90 per cent through the application of shunt capacitors. Starting at the initial power factor of 70 per cent and moving vertically to the intersection with the 90 per cent power factor curve, we find that 0.62 kva of capacity has been released for load at the initial power factor, for each kilovar of capacitors added. Fig. 16— Before proceeding with the example, some equipment Typical method of installing pole-mounted switched costs in terms of dollars per kva installed should be as- capacitors. sumed. The installed costs per kilovar for both primary and secondary capacitors should include the cost of fuses. Assume that the installed cost per kilovar for a typical installation of a switched primary feeder capacitor bank. primary capacitor is $10.50, the installed cost per kilovar for a secondary capacitor is $21, and for the 8. Secondary Capacitor purpose of simplifying the example, assume that the Applications installed cost per kilovolt-ampere for a particular dis- Shunt capacitors located at the load give maximum tribution transformer rating is $17.50. This gives a benefit to the power system. Compared to capacitors ratio of 2 to 1 for secondary and primary installed costs, nearer the source, capacitors at the load offer the most reduction in / and a ratio of 0.6 between the primary capacitor and 2X and PR losses, release capacity in the the distribution transformer. maximum number of transformers and circuits, and With these ratios established, the solution to the cause the maximum voltage rise for a given kilovar rat- problem may be obtained. From the point on the curves ing. However, from an economic standpoint, it is not al- corresponding to 0.62 kva released per kilovar of capaci- ways best to locate capacitors right at the load. This is tors, move to the right to the 0.6 ratio curve for the in- primarily because higher-voltage capacitors (2.4 to 8 kv) stalled costs of the primary capacitors and the distribu- cost less per kilovar than low-voltage capacitors (240 to tion transformers. The point of intersection with this 600 volts). Also, the load may be distributed in relative- curve shows that it is economical to pay 2.05 times as ly small increments at the utilization voltage, as in resi- much for secondary capacitor as for the primary capaci- dential areas, necessitating the use of smaller, more ex- tors for this application. Since the assumed ratio of ca- pensive capacitors. These factors tend to increase the pacitor-installed costs was 2.0, the secondary capacitors cost per kilovar of capacitors on the distribution second- are justified. aries. For these reasons, most capacitors located in the Had the installed-cost ratio of primary capacitor to distribution system are primary capacitors placed on distribution transformer been 0.7, the curves show that the feeders. Where capacitors must be switched, econ- only 1.9 times as much could have been justified for the omics favor primary capacitors to a larger extent, since secondary capacitors. Since the assumed cost ratio in the cost per kilovar of switching and control for the the example was 2.0, the curves indicate that primary relatively small secondary capacitors may be quite high capacitors would be the correct application. compared to equivalent costs with larger primary banks. Assume that it is desired to correct from the initial Despite the decided economic advantages held by power factor of 70 per cent to a resultant power factor of primary capacitors in the majority of cases, there are 100 per cent. The curves show that the released capacity cases which economically justify secondary capacitors. per kilovar of capacitors added is now 0.53. The curves The most obvious of these occur when the power factor also show that for both the 0.6 and 0.7 ratio of primary of the load is such that installation of secondary capaci- capacitors to distribution transformers, primary capacitors will save appreciable distribution transformer ca- tors are more economical. pacity. The additional benefits of reduced secondary It can be concluded from the curves that secondary main load, lower losses, and more voltage rise are more capacitors will have their greatest application when the

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 326/576 Application of Capacitors 319 0) tr 0.9 g as 1.0 0.9 v0.13 07 0.6 0.5 0

-...... Z1:11ESULTANT POWER FACTOR IN PER CENT

OS It) 0.6 OV 70 4?" 4 0.7 0 0.7 q /MA II, ' g 4. "" 0.6 Ot 0.6 CPI' 1 + ot, I P. rc — — — — — _ . cj w I '2" 0.5 OS 1 I .C4) 4- 4 90 0. 1 1 co _J I 1 .e. 1 1 ,0 - 0.4 0.4 Ifii f/ _I :1c. :I 95 03_ a 0 0.30.3 14 _ 0 u. 0 1,17 oe' 1 ii 1 e . , az 41W 0.2 4,6"7 I " I 0.1 w

100 - I- I 11 1 11 I 0.1 k() 0.1 /14" . - 4 II I 4- -1- ii I 0 I II I 0 Q 0 i 1 I I 50 60 70 80 90 100 .0 L25 1.50 1.75 2 0 2.25 2.50 2.75 3.0 INITIAL POWER FACTOR IN PER CENT SECONDARY CAPACITOR COST PER KVAR /PRIMARY CAPACITOR COST PER KVAR

Fig. 17—Secondary capacitor economics considering only savings in distribution transformer cost.

ratio of the installed cost per kilovar of primary capaci- ure caused by puncture of the dielectric in capacitor. tors to the installed cost per kilovolt-ampere of distribu- This is the most likely type of failure. However, a flash- tion transformers is low, and when the initial power fac- over to ground, either inside the capacitor or over a ca- tor of the circuit is low. Using estimated cost figures, it pacitor bushing, may not be cleared if only one fuse is appears that the secondary capacitors can be justified used. Such a flashover might be caused by lightning in for use with distribution transformers rated 50 kva and the secondary. The second fuse is used to protect against below. However, it should be recognized that most ap- this contingency. plications will be justified on circuits with distribution Secondary Network Capacitors—Secondary capacitors transformers rated 25 kva and below. are often used on underground secondary networks. The Secondary capacitors are not often switched, because high cost of networks on the basis of dollars per kva it is generally more economical to switch primary ca- of load served makes capacitors particularly attractive pacitors and use fixed secondary capacitors. However, in for this application in some situations. some cases, secondary capacitor switching is justified. There are capacitor designs especially tailored for use There is a relatively inexpensive temperature controlled in secondary networks. Fig. 18 shows a rack of secondary secondary capacitor switch available, and in cases where capacitors for use in network vaults. The individual the reactive load is temperature sensitive, these switches capacitor units are rated 13% kvar, 216 volts. Mounted may make it economical to switch secondary capacitors. and wired racks of 40, 80, and 120 kvar are available, Secondary capacitors should be installed with fuses if and these racks can be combined for still larger banks. the system is to be protected from capacitor failure. The Power capacitors are normally designed for ambient failure of an unfused capacitor may cause an outage, temperatures of 40 to 50C, depending on arrangement, and violent failure may cause damage in the immediate spacing, and ventilation. Network secondary capacitors

3 area of the capacitor. Since secondary capacitors are may be used in 55C air, if single tier arrangement is usually installed in areas where such damage is highly used and two inch spacing between adjacent units is undesirable, at least one capacitor fuse should be used. maintained. Ventilation and ambient temperature are Some users install two fuses on secondary capacitors, often severe problems in network vaults, and careful T one in each lead. This is considered to be a matter of consideration of spacing, arrangement, air movement, S preference. One fuse will protect against capacitor fail- and radiant heat must be used, even though network

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quired by the load. The load may be a single motor, or it may be a large industrial plant. The capacitor can be chosen to supply* the magnetizing current under peak load conditions, or it can be chosen only large enough to supply the reactive kva hours accumulated over the month. It can be located at the service entrance, thus removing magnetizing current from the utility system only; or units can be applied to the individual loads, thus removing magnetizing current from the plant circuit also, reducing their loss, increasing their load capacity, and maintaining better voltage at the loads. The selection of the capacitor rating and its location is dependent on what is to be accomplished. This varies with the power rates and local conditions. Location of Capacitors—Many factors influence the location of the capacitor, such as the circuits in the Fig. 18—Secondary capacitors for installation in a second- plant, the length of the circuits, the variation in load, ary network vault. the load factor, types of motors, distribution of loads, and constancy of load distribution. capacitors are designed for higher than normal ambient The capacitors can be located in many ways, as temperatures. follows: The network secondary capacitor case is designed to (a) Group correction—at primary of transformer. be quite corrosion resistant and will give satisfactory (b) Group correction—at secondary of transformer. service even when occasionally submerged in acid con- (c) Group correction—out in a plant, as for example taminated fresh water or sea water. The capacitor itself for one building. is hermetically sealed and a sleeve arrangement is avail- (d) Localized correction on small feeders. able to cover the terminals and fuse, so that a water- (e) Localized correction on branch motor circuits. tight connection to the capacitor is possible. (f) Localized correction direct on motors, or groups Most secondary capacitors are individually fused, of motors and switched with the motor. although group fuses may be used in some cases. The Group Correction—The two principal conditions under impregnating liquid used in the capacitors is non-in- which group correction is better are: flammable, so that no fire threat results from a failed 1. Where loads shift radically as to feeders. capacitor. However, mechanical damage to other capac- 2. Where motor voltages are low such as 230 volts. itors or equipment in the vault may result if a capacitor If the power flows from the service entrance to vari- case ruptures with violence. The permissible or recom- ous widely-separated parts of the plant and if the loads mended type of fusing may vary with available fault shift about a great deal from one feeder to another, the current, size of capacitor bank, and frequency of sub- correction may be needed first in one part of the plant mersion. The capacitor manufacturer should be con- and later in another. A centrally-located group capaci- sulted for recommendations for a specific case. tor in this case would be an advantage, since it would The economics of network secondary capacitors is tend to be the same distance from the loads at all times. quite simple in most cases. The capacitors are usually If a group capacitor remains connected during light associated with a particular transformer, and calcula- loads, the voltage rise is less if this capacitor is in- tions are based on the worth of released load capability stalled at or near the transformer bank, since the react- on this transformer. The released load capability per ance of the plant circuits does not contribute to voltage kvar of capacitors depends on the initial load power fac- rise. In this case, application of capacitors to individual tor. Some users find it economical to correct to more motors would represent a larger investment because of than 90 per cent. the diversity factor. It would be better, therefore, for Network secondary capacitors may be switched, and the operator to switch off portions of the central capaci- some users are switching vault banks. Both the control tor to meet the varying load conditions. Exceptions will and the switch involve problems. Some type of con- arise where feeders are long and where the gain from in- tactor seems to be the best solution to the switching dividual load application warrants the greater initial problem, particularly if the vault is never flooded. investment in capacitors. Because of the higher cost of Most standard control schemes will not work properly low-voltage capacitors, their application to 230-volt or are too expensive for this application, although time- motor circuits may more than double their cost. This clock control is satisfactory in some cases. Pilot wire gives considerable advantage to group installation if control by the dispatcher appears to be the most prom- this can be on the primary side, 2400 to 7200 volts. ising solution in most cases. Capacitors placed ahead of the main bank of transformers do not benefit the transformers; no transformer 9. Capacitors on Industrial Plant Circuits kva is released. Thus, use of the 230-volt capacitors on A capacitor can be installed in shunt with any load of the feeders or near the motors is frequently warranted. low power factor to supply the magnetizing current re- Localized Correction—Capacitors should be placed as

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 328/576 Application of Capacitors 321 near the load as possible, or near the ends of feeders, Reactive kva at 95 per cent power factor for three main reasons: 1. Losses are reduced in the circuits between the = -V12602 -1200' = 387 loads and the metering point. Kvar rating of capacitor is 1060 minus 387, which 2. Voltage is raised near the loads, giving better equals 673 kva. motor performance. The reduction in the kva demand from 1600 to 1260 3. Capacitor kvar can be reduced automatically as may result in a reduced kva demand charge, or it may the load drops off by installing some of the capaci- reduce the energy charge, depending on the rate struc- tors directly on loads so that they are switched off ture. Some rates involve several energy charges for suc- with the loads. cessive blocks of power, the size of the blocks depending The first point can be evaluated easily by investigat- on the kva demand. For example: ing the length of the circuits, and the transformations, if Size of block = (70) (kva demand) any. Whatever gains are found in released transformer 1st block-50 per kw hour capacity, and in the reduction in losses in transformers 2nd block-11/S per kw hour and circuits, are added gains. 3rd block-10 per kw hour The effect of the capacitor is to raise the voltage per- Additional Yi¢ per kw hour manently at any given point where it is connected. This In this case, the energy cost is reduced by a decrease voltage boost, superimposed on the normal voltage, is in kva demand, because if the blocks are smaller, the practically constant from no load to full load on the lower rate applies to a larger proportion of the energy feeder. consumed. Rates and Capacitor—For the purpose of analyzing the (c) Sometimes a check is made on the average power different types of rates, a typical application can be factor under day load conditions, and the billing there- considered, such as an industrial plant with a day load after based on this check until some future check is averaging 960 kw and 67 per cent power factor, with made. The energy charge, or the net billing, is adjusted peak loads running up to 1200 kw and 75 per cent power up or down according to this power factor. In such cases, factor. It is obvious that a large magnetizing current is it is necessary to determine how this check is to be drawn from the line, and considerable savings can be made, and under what conditions, in order to install ca- made by supplying this magnetizing current with capac- pacitors to raise the power factor as high as' warranted itors. The size of the capacitor or the merits of their use by the expected savings. Such a capacitor usually is can only be determined by systematic analysis. made proportional to day load requirements. In the One of the following conditions may exist. case above, the day load averaged 960 kw at 67 per cent (a) Power factor is not considered in the rates. power factor. Assuming this is to be brought up to 95 (b) Power factor is taken into account in demand per cent power factor, 720 kva of capacitors are required charge. as follows: (c) Power factor is checked by test and used to deter- 960 kw 1430 kva mine energy charge thereafter. 67 per cent (d) Power factor is determined by the ratio of kw Reactive kva at 67 per cent power factor hours and rkva hours and is used in different = -V14302 -9602 =1035 kvar ways to calculate the demand charge or energy 960 charge, or both. Kva at 95 per cent power factor = 1010 kva. = (a) If power factor is not taken into account in the 0.95 rate structure, the capacitor can be used only to secure Reactive kva at 95 per cent power factor savings in the plant, such as to reduce current in cir- = V10102 — 9602 = 315 cuits, reduce loads on transformers, and to reduce loads on customer-operated generators. The capacitor should Capacitor required is 1035 minus 315, which equals 720 kvar. usually be located near the loads of low power factor. (d) The size can be determined by calculating the reactive A method commonly encountered in industrial plants takes into account monthly power factor ob- kva. By using a capacitor large enough to supply all or tained by integrating kw hours and rkva hours. Assum- part of this reactive kva, the current in the circuit is ing the plant mentioned above is billed for 322,250 kw reduced to the desired figure. hours, and that the reactive kva hours equals 346,000, (b) If the rates include a kva demand charge, the kva can be reduced by raising the power factor during the this ratio amounts to a power factor of 68 per cent. demand peak. With a demand of 1200 kw at 75 per cent Assuming that rates indicate that it will be worth- power factor, the kva demand is 1200/0.75 =1600 kva. while to reduce these rkva hours to a point correspond- If the power factor is raised to 95 per cent, the demand ing to 95 per cent power factor, kva is 1200/0.95 =1260 kva. The size of the capacitor Kva hours at 95 per cent power factor required to accomplish this is determined from the re- 322,250 — =339,000 active kva at the two values of power factor as follows: 0.95 Reactive kva at 75 per cent power factor Reactive kva hours at 95 per cent power factor

= A/16002 -1200' =1060 =-- -/330,0002-322,2502=106,000

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20 tion motors and switched with the motor. The amount of kvar so connected should be limited to values that do not cause excessive voltage at the motor due to self- 1 excitation when the breaker is opened, as Fig. 19 shows. ) a MOTOR Table 8 gives the maximum permissible capacitor kvar / SATURATION for direct connection to the terminals of induction 4 / CURVE motors based on Rule 4606 of the National Electrical / 7 Code. 1 10. Capacitor Switching Devices I/ I / The device to be used for switching capacitors re- / / quires special consideration. Switching capacitors im- F / poses a particular type of duty on a switch or breaker, and a device which performs well under normal load and fault duty may be wholly inadequate for switching capacitors. Most conventional switching devices will / safely switch capacitor banks of moderate rating (see b SO 100 1 50 2C NEMA Standards); but the manufacturer's advice PER CENT RATED CURRENT should be obtained before making indiscriminate appli- Fig. 19—Self-excitation of induction motor with shunt cations on large banks or complex circuits. capacitors when supply breaker is opened. There are two phenomena involved which make capacitor switching severe duty on a switching device. A. Capacitor current less than motor current at no load The first is the 90-degree phase relationship between the rated voltage. system voltage and the capacitor current. The switch B. Capacitor current equal to motor current at no load interrupts the capacitor current at a normal current rated voltage. zero. This leaves a trapped voltage on the capacitor C. Capacitor current equals rated motor current. equal to the system crest voltage. One-half cycle later when the system voltage reaches crest of the opposite polarity, the voltage across the switch contacts is at Using 730 hours per month, the capacitor kvar re- least twice system crest voltage. Unless the switch quired equals (339,000 —106,000)/730 or 319 where the recovers insulation strength between the contacts very kvar meter has no ratchet, so that full credit results fast, a reignition or restrike is apt to occur (by defini- even if the power factor is leading at times. When the tion, a reignition occurs when the arc is re-established meter has a ratchet, the capacitor must be large enough between the contacts before 90 electrical degrees, a re- to build up accumulated kvar-hours while the power strike occurs when the re-establishment of the arc occurs factor is not leading. after 90 degrees). Capacitors on Induction Motor Terminals—Capacitors The second characteristic of capacitor switching is frequently are installed across the terminals of induc- the high frequency transient current which flows into a

Table 8—Maximum Permissible Capacitor Kvar for Use with Open-Type, Three-Phase Sixty-Cycle Induction Motors

3600 RPM* 1800 RPM* 1200 RPM* 900 RPM* 720 RPM* 600 RPM*

Motor Max. Max. Max. Max. Max. Max. Rating Capacitor ** Capacitor ** Capacitor ** Capacitor ** Capacitor ** Capacitor ** HP KVAR KVAR KVAR KVAR KVAR KVAR 10 3 10 3 11 3.5 14 5 21 6.5 27 7.5 31 15 4 9 4 10 5 13 6.5 18 8 23 9.5 27 20 5 9 6 10 6.5 12 7.5 16 9 21 12 25 25 6 9 6 10 7.5 11 9 15 11 20 14 23 30 7 8 7 9 9 11 10 14 12 18 16 22

40 9 8 9 9 11 10 12 13 15 16 20 20 50 12 8 11 9 13 10 15 12 19 15 24 19 60 14 8 14 8 15 10 18 11 22 15 27 19 75 17 8 16 8 18 10 21 10 26 14 32.5 18 100 22 8 21 8 25 9 27 10 32.5 13 40 17 125 27 8 26 8 30 9 32.5 10 40 13 47.5 16 150 32.5 8 30 8 35 9 37.5 10 47.5 12 52.5 15 200 40 8 37.5 8 42.5 9 47.5 10 60 12 65 14 *Synchronous Speed **Per Cent Reduction in Line Current.

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capacitor bank when it is energized, or when a restrike occurs during opening. The magnitude of the transient current may be especially severe when parallel capaci- PRIMARY tor banks are switched. Section 13 discusses the magni- RESISTOR IARCING tude and frequency of these transient currents in more T CONTACT detail. SECONDARY Restriking in a circuit breaker can lead to the buildup ARCING of high transient overvoltages on the power system. CONTACT However, field experience and laboratory tests have shown that dangerous overvoltages seldom result, even Fig. 20—Functional diagram of a resistor switching device. with multiple restrikes and reignitions, while switching capacitor banks. The more important aspect is the effect of restrikes on the circuit breaker itself. These be damaged by the restrike and the arc is extinguished effects may be indicated in different ways on different when the contacts separate far enough. types of circuit breakers. In an oil circuit breaker For larger banks, and for higher voltages, the prob- with interrupters for interrupting the current, the high lem is somewhat more difficult. In these cases, it is gen- frequency transient current associated with a restrike erally necessary to use some supplementary means, to a capacitor bank has an almost explosive effect such as a resistor, in the switching device. Oil breakers inside the interrupter. If an oil breaker is used to having such resistors are available. Fig. 20 shows a func- switch a capacitor bank beyond its capabilities, it is tional diagram of one pole of a resistor switching device. possible to have immediate failure. However, in the When the device is closed, the secondary arcing contact more general case, the result is a gradual mechanical closes first, energizing the capacitor bank with the deterioration of the interrupter. This results in increased resistor in series with the source. At a later time, the maintenance and, if the maintenance is not done, in resistance is shorted out. In the opening sequence, the eventual failure. Evidence of mechanical damage to the primary arcing contact opens first, inserting the resistor, interrupter of an oil circuit breaker used for capacitor switching usually indicates the breaker has been subjected to excessively severe duty. With proper precaution and the recognition of basic limitations, standard magnetic air breakers in general can be used for switching shunt capacitors. When so applied they have a long operating life with satisfactory performance and low maintenance. For a breaker of a particular make, the manufacturer should be consulted before applying the breaker on any kind of capacitor switching. Some magnetic air breakers can be applied to single bank switching up to their continuous capacitor current rating in amperes; i.e., their rated continuous current at 60 cycles divided by 1.35. In applications where there are two or more capacitor banks in parallel, with at least one being switched, some magnetic air breakers can be damaged by excessive high-frequency surge currents, especially when these currents result from restrikes. This can be true even for moderate size banks. In any case of parallel bank switching with magnetic air breakers, the manufacturer should be consulted. As a general rule, the maintenance on magnetic breakers will increase with an increase in capacitor bank size. Increased duty may particularly affect the contacts of breakers used in parallel switching applications since the high surge current is established across the arcing contacts every time the breaker is closed. Experience with capacitor switching has led to the development of devices specifically designed to switch capacitors. The oil switches used for pole-mounted banks and small substation banks are of this type. These devices are basically oil circuit breakers without interrupters. They have little or no fault-interrupting ability. They may restrike but there is no interrupter to Fig. 21—Resistor interrupter on a 34.5-lcv oil-circuit breaker.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 331/576 324 Application of Capacitors and the secondary arcing contact completes the inter- Voltage-time ruption later. Voltage-current The resistor switching device has definite advantages Temperature for capacitor switching. The addition of the series re- Manual sistance during opening or closing gives a critically The type of intelligence to be used for a specific appli- damped RLC, circuit and the transient currents and cation will depend upon cost, bank location, number of voltage are greatly reduced.". '2 The probability of a switching steps, daily load cycles, and the purpose of restrike is also reduced and if a restrike does occur, the the capacitor application. A survey showing the relative effect on both the system and the breaker is consider- usage of the various types of controls used for existing ably less. Fig. 21 shows a 34.5-kv oil circuit breaker with switched capacitor banks appears in Table 9. resistance switching devices. Compressed air breakers may be used when the ca- Table 9—Control Methods Used for Switched Capacitors26 pacitor banks are very large and restrike-free performance is desirable. Type of Pole Mounted Distribution When selecting a switching device for a capacitor Control Banks on Feeders Substation Banks bank, the complete details of the installation should be Per cent Per cent known. The maximum kilovar rating of the bank to be Voltage 16.6 30.8 installed, the present or future rating of any other ca- Current 4.9 2 . 4 pacitor banks in the immediate area, and the maximum Time 59.8 16.3 short-circuit capacity are all of importance when se- Voltage-Current 7.2 12.6 lecting a switch. The continuous current rating of the Voltage-Time 5.1 6.3 Manuals 6.2 28.4 switching device must be at least 135 per cent of the 0.2 3 . 2 rated current of the capacitor bank. With this informa- Other tion, devices capable of handling the specific job can be 100.0 100.0 selected. *Manual includes any switching directly or indirectly By careful and extensive testing, a manufacturer can caused by the dispatcher. determine maximum capacitor switching capabilities of a particular device, based on a reasonable number of Time-Switch Control—Time-switch or time-clock con- switching operations between inspection and main- trol is one of the most common types of control used tenance. The capacitor rating is based on a pessimistic with switched capacitor banks on distribution systems. transient current and frequency situation. However, it The control simply switches the capacitor bank on at a is usually advantageous to the user to design his circuits certain time of the day and takes it off at a later time. for less severe conditions. This is especially true for Its greatest application is with small single-step banks parallel capacitor banks. As discussed in Section 5, it is located out on primary feeders, where the daily load often possible to materially reduce the severity of the cycle is known and consistent. The banks generally duty on the switching device by spacing banks and bank range from 150 to 600 kvar. connections for increased inductance between banks. A carry-over device is required for each time-clock to Moderate increases in inductance between banks may keep the clock running during temporary power out- appreciably reduce the required maintenance on the ages. Most carry-over devices are of the mechanical switching device. spring type and can keep the clock running for about ten hours. The spring is continually kept in a wound 11. Controls for Switched Capacitors position by the small electric motor which runs the The purpose of the controls in switched capacitor clock. When a power outage occurs, the spring then banks is to receive intelligence from a source and initiate begins to unwind, and if power is restored before ten the switching of the capacitors into or out of the circuit. hours have passed, the motor restores the spring to its The components of the controls are the receiver of the wound position. If a carry-over device is not used, it intelligence, the circuit breakers or switches, and the will be necessary for a man to go to each capacitor loca- control power source. Of the three components, only the tion that is affected and reset the clock after a power receiver of the intelligence will be discussed in this sec- outage. tion. The control power source, often called the auxil- An omitting device is also required for each time- iary power supply, is required for operating the circuit clock to omit switching the capacitors on or off on days breakers or switches, and it is generally obtained from a where the known load cycle will change, such as Sun- small distribution transformer associated with the days and holidays. On commercial and industrial feeders transformer bank. For small banks located on a primary and somewhat on residential feeders, there is generally feeder, the power supply may come from the nearest a definite reduction in feeder loading on these days, and secondary line serving the load in the area. if the capacitors were switched on, high voltage could The various types of control intelligence used on dis- result. The same applies to capacitor banks with time- tribution systems are : clock switching located in substations which serve in- Time-switch dustrial and commercial load. Voltage One of the greatest advantages of time-switch control Current is its low cost. Its cost is about one-third of the cost

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using voltage or current as the intelligence source. anced-beam voltage regulating relay, a separate time Another advantage of time switch control is that co- delay relay is used. If an induction disc-type voltage ordination with other voltage regulating equipment is regulating relay is used, the inverse-time characteristic not required. of the relay will provide the time delay. On occasion, A disadvantage of time-switch control is that the separate timers are also used with induction disc relays. sequence of operation is the same throughout any un- Where separate timers are used, the most common usual load conditions. For example, if a power failure delay setting is one minute. occurs on a feeder and a portion of that feeder is tem- Coordination with other voltage regulating equip- porarily connected to a good adjoining feeder, it may be ment is required when using voltage control for switch- desirable for the capacitor to be in the circuit to main- ing capacitors, so that operation of one device (switched tain good voltage with the additional load. If this capacitor or regulator) will not cause an operation of occurred when the capacitors are normally off the cir- another device, resulting in excessive operations and cuit, it would require a special trip to the capacitor possibly pumping. Equipment coordination is discussed location to switch them on. Other abnormal conditions in Chapter 7. are possible where high voltage can occur if the capaci- Current Control—Current control alone is used only tors are in the circuit. Again, if the abnormal condition where voltage is not a satisfactory signal. Such applica- occurs at the time of day when the capacitors are on, it tions would be on a feeder or in a substation where volt- will require a special trip to the bank location to switch age increases with load, or at locations where the volt- them off. age reduction as load increases is not enough for effec- Voltage Control—Voltage alone can be used as a tive relaying. For effective current control, there must source of intelligence only when the switched capacitors be a load change such that the ratio of maximum de- are applied at a point where the circuit voltage demand to minimum demand is three or more. creases as circuit load increases. Generally where they The greatest applications of current control are with are applied, the voltage should decrease four to five single-step capacitor banks applied on feeders or in sub- volts (120-v base) with increasing load. If the capacitors stations where large intermittent loads are either on or are applied at a point where the circuit voltage in- off. The loads can be fluctuating loads if the capacitor creases with load, a second source of intelligence must "off" control setting is below the maximum dip. be used. Such types of control using two intelligence Current control relays are similar to voltage control sources are the voltage-current and voltage-time. They relays. The relays are either of the balanced-beam so- are discussed in subsequent paragraphs. lenoid type or of the induction-disc principle. The bal- Voltage is the most common type of intelligence used anced-beam type are used with large substation capaci- in substation applications, and it is becoming more and tor banks, while the induction-disc type are used with more common with small banks located out on primary small pole-mounted, single-step capacitor applications feeders. It has the advantage of initiating a switching on distribution feeders. The current transformer should operation only when the circuit voltage conditions re- always be connected on the load side of the capacitor quest an operation, and it is independent of the load bank in order to measure load current, and not load cycle. current plus capacitor current. The bandwidth setting of the voltage regulating relay When using current control, no recognition is given is wider than that used with step or induction regulators to the voltage conditions of the circuit. Therefore, cir- and normally ranges from about four to ten volts. The cuit voltage conditions throughout the load cycle must bandwidth setting will depend upon the rating of the be known in order to determine when the capacitors capacitor bank, the number of steps, and whether other should be switched on and off. Abnormal circuit condi- voltage regulating equipment is also applied on the tions which affect the circuit loading and the voltage same circuit. The setting must be larger than the volt- rise should be checked to be sure the capacitors do not age change due to the switching operation of one step. switch on if high-voltage conditions would occur. Since the voltage change due to one step will depend A time delay is always used before any capacitor upon the system characteristics toward the source, it is switching operation to prevent excessive operations due also necessary to check the voltage change for possible to momentary load disturbances. With the balance- abnormal conditions. From a recent survey" concerning beam relay, a separate time delay is used. With the switched capacitors, the average bandwidth used with induction disc type, the inverse-time characteristic of voltage control was 1.73 times the normal calculated the relay generally provides the time delay. voltage rise, per step. Most installations had a band- Voltage-Time—There are two types of voltage-time width to calculated voltage rise ratio of 1.5 or 2.0. control. One is time-clock control with voltage over- Switched capacitors are not used for fine voltage con- riding. The other is voltage control with the voltage trol. For best economy the voltage change should be as regulating relay balance voltage essentially set for two large as system conditions allow. Only two or four different balance voltages, and with the time switch switching operations should occur per day in most selecting the balance-voltage value. switched capacitor applications. Time-clock control with voltage overriding is used A time delay is always used before any switching with single-step substation and feeder applications operation to prevent unnecessary switching due to mo- where daily load patterns are generally known but mentary disturbances. With the solenoid-operated, bal- unusual load conditions often occur. If the unusual load

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 333/576 a voltage conditions.Thesecondarycurrentfromacur- able tocontinuallymonitorthefeederorsubstation spread atthecapacitorlocation. single-step substationandfeederapplicationswherethe voltage aloneisnotasatisfactorysignalanditdesir- resistor isusedinthecontrolcircuit,employedwhere light loadconditionsorhighvoltageduringheavy loading permitscurrentcontrol,butwhereunusualload The bandwidthusedisthepermissibleprimaryvoltage Fig. 22—Avoltage-currentcontrol forswitchingcapacitors. bandwidth voltagerelayisusedfortheoverridecontrol. or systemconditionscouldcauselowvoltageduring periods. Similarlywithtime-voltagecontrol,awide voltage coilcircuitsimilartothelinedropcompensator used withvoltageregulators. riding. Theotherisvoltagecontrolwitharesistorinthe rent control.Oneiscurrentcontrolwithvoltageover- and •takenoutofthevoltagecoilcircuitwithtime- the time-clockinvoltage-timecontrols. clock switch. setting ischangedbyinsertingaresistorinserieswith setting orrecalibratingthevoltageregulatingrelay.The high busvoltageisrequired.Theresistorinsertedin the relayvoltagecoil.Theresistorisincircuitwhen two voltagelevels.Thisisaccomplishedbychangingthe excessively high. needed most,andoffthecircuitwhenvoltageis insures thatthecapacitorswillbeoncircuitwhen location. Suchamethodoftime-voltagecontrolkeeps the switchingoperationstoaminimumandalways imum voltagespreadthatisdesiredatthecapacitor trol willagainoverrideandswitchoffthecapacitors.A wide-bandwidth voltagerelayisusedfortheoverride switch onthecapacitors.Also,ifunusualloadcon- ditions wouldcausethevoltagetobeexcessivelyhigh control. Thebandwidthusedisdeterminedbythemax- when thecapacitorsareoncircuit,voltagecon- 326 the voltagecontrolwilloverridetime-clockand mined valueandthecapacitorsarenotoncircuit, conditions causethevoltagetofallbelowapredeter- CI •CUIT POWER The secondtypeofvoltage-currentcontrol,wherea Current controlwithvoltageoverridingisused Voltage-Current—There aretwotypesofvoltage-cur-

;ITOR Omitting andcarry-overdevicesarealsousedwith Occasionally, asubstationbusisoperatedateitherof TRANSFORMER POTENTIAL TRANSFORMER CURRENT REGULATING VOLTAGE Gridco, Inc.v. Varentec, Inc.IPR2017-01134 RELAY +- CONTROL Application ofCapacitors TO SWITCH CIRCUIT LOAD GRIDCO 1004 Part 3 of 5 - 334/576 L

station hasseveralfeeders,amethodofobtainingtotal in theloadareaawayfromsubstation.Ifsub- drop compensatorcircuitforswitched capacitorsinsub- load thanatlightoraconstantvoltagethroughout all loadconditions,thusrequiringtheswitchedcapaci- tor's voltagerelaytomeasuretheatapointout Fig. 23—Amethodofobtaining totalfeedercurrentinline- bus regulatormaintainsahighervoltageatheavy drop compensationisinbus-regulatedsubstations.The sistance isallthatnecessaryforthelinedropcom- some pointbeyondthecapacitorlocationisused.The drop acrosstheresistorequalscircuitvoltage combination. compensation canbeobtainedwitharesistor-reactor from thecapacitorbanktodesiredlocation.Are- If theloadpowerfactorvariesoverawiderange, sation. Thatis,insteadofhavingthevoltageat resistor maynotbesatisfactory.Inthesecases,correct resistor ohmicratingisselectedsuchthatthevoltage pensator iftheloadpowerfactorisreasonablyconstant. light- tofull-loadconditions. former secondaryvoltageincreasedwhengoingfrom going fromlighttoheavyloadconditions.Theactual capacitor locationcontroltheswitching,avoltageat is verysimilartovoltagecontrolwithlinedropcompen- across therelay,plusamountpotentialtrans- sonably constantpowerfactor;therefore,withincreas- conditions wouldbethedesiredvoltagereduction voltage droprequiredacrosstheresistoratheavyload decreases. Forsingle-stepbanks,theresistorohmic crease acrosstherelayabouttwicebandwidthwhen rating shouldbeselectedtocausethevoltagede- ing loadthevoltageacrossregulatingrelay increasing load.Thevoltagedropacrosstheresistor locations wherethevoltageonlydecreasesslightlywith varies proportionallywithloadcurrent,assumingrea- 22. Thevoltageacrosstheresistorissubtractedfrom ducing thevoltageacrossregulatingrelay. the voltageincreaseswithincreasingload,orat the secondaryvoltageofpotentialtransformer,re- ble tousevoltagecontrolatacapacitorlocationwhere rent transformerlocatedontheloadsideofcapaci- tor ispassedthroughtheresistor.ThisshowninFig. The greatestapplicationofvoltagecontrolusingline The voltage-currentcontrolschemeshowninFig.22 When usingaresistorinthecontrolcircuit,itispossi- COMPENSATOR LINE DROP IMPEDANCE stations. 0 1 RELAY 1 " . SUBSTATION

BUS -- I BANK CAPACITOR IL 0 Application of Capacitors 327 feeder load current for the line drop compensator circuit 12. Fusing, Connection, and Protection of Shunt is shown in Fig. 23. Capacitors With voltage-current control, a separate current relay Fusing—Chapter 10 contains a detailed discussion of can also be used. When the load current exceeds a ,cer- capacitor fusing. This section will discuss the more gen- taM value, the current relay inserts a resistance in series eral aspects of the problem. with the voltage regulating relay: The resistance in- Most primary feeder capacitor banks and some smaller serted in the control circuit produces a constant voltage substation banks are protected with group fuses. For drop in the control circuit, which essentially recalibrates smaller banks, the detection of failed units is not as dif- the voltage regulating relay. ficult as on large banks, and when group fuses will pro- For single-step banks, both the separate current relay vide adequate protection against case rupture, they are method and line drop compensator method give satis- often used. factory operation. For multi-step banks, only the line When a capacitor develops on internal fault, the arc drop compensator method should be used, because a will evolve gases from reaction with the capacitor ma- separate current relay and resistor combination would terials and pressure will be built up inside the capacitor be required for each step. A control scheme with several case. The individual or group fuse must clear before current relays would be expensive compared to the line dangerous pressures are reached. The capacitor manu- drop compensator, voltage-current method. facturer usually recommends the individual fuses, and Temperature—The use of -outside temperature as the when they are used, coordination is not required by the control intelligence for switching capacitors started a user. However, group fuses are usually applied by the few years ago, mainly in the Southwest where air-condi- user, and a knowledge of the case rupture curves is tioning load became extensive. Some utilities found a required. definite correlation between outside temperature, load The curves of Fig. 24 are .4aken from NEMA Standard power factor, and low voltage conditions, thus enabling CA-1. The curves take into account the statistical na- temperature to be a satisfactory signal for some applica- ture of case rupture and allow for the fact that some tions. Temperature-switched capacitors can only be users are willing to take more chances on violent case used on feeders which have a large amount of temper- rupture at certain locations on their systems. ature-influenced load. Residential and mixed residential Connection—A three-phase capacitor bank on a dis- and commercial feeders may have such loads in areas tribution feeder may be connected in delta, in wye with where air conditioning is prevalent, but industrial feed- the neutral ungrounded, or in wye with the neutral ers generally do not. grounded. The preferred connection may depend on the It is not expected that temperature switching will type of system (grounded or ungrounded), fusing con- gain widespread use for switching capacitors, but it does siderations, the location of the capacitor bank on the offer a low-cost method of obtaining control intelligence system, and telephone interference. where applicable. The cost of temperature switching is Delta or ungrounded-wye banks always offer the pos- approximately the same as the cost of time switching. sibility of neutral inversion or a resonant condition when Other Controls—Manual switching is, of course, the one or two conductors on the source side of the bank are least expensive of all types of switching control. Where open. The capacitor bank will maintain some voltage on switching operations are required each day, manual the open phase, and this in effect energizes any trans- switching is usually used only in attended substations. formers on the load side of the open conductor through Where low voltage conditions are due to seasonal loads a series capacitor. There have been cases of trouble when and only a few switching operations would be required this situation occurred on four-wire systems with single- per year, manual switching is practical out along the phase distribution transformers connected line-to-neu- distribution feeders. tral. This condition may damage the transformers in- Some large capacitor banks are switched remotely by volved or may damage equipment connected to the load the load dispatcher over supervisory control. Generally side of the transformer or both. Because of this possible the main purpose for these large banks is kvar supply trouble, most companies do not locate delta or un- and not necessarily voltage control, although both may grounded-wye connected banks on the load side of single- be achieved. pole devices such as fuses or single-pole reclosers and Another method of switching capacitor banks located sectionalizers. in substations is by the step position of the bus voltage Grounded-wye capacitor banks are usually used only regulator. If the substation load and voltage conditions on four-wire systems. A grounded-wye bank on an un- are known, limit switches on the bus regulator can grounded or delta system provides a ground current switch the capacitors on at a certain voltage boost posi- source which may interfere with sensitive ground relays tion and switch the capacitors off at some voltage buck or detectors, and in some cases may contribute to tran- position. The limit switches can be located on the regu- sient overvoltages during ground faults on the system. lator position indicator or internally on the operating A survey of the industry27 indicates that no company mechanism. The voltage band between the "on" and uses grounded-wye capacitor banks on ungrounded or "off" limit switches must be wide enough so that after delta systems. the regulator position turns the capacitors on and then From the standpoint of group fusing, grounded-wye has to lower because of the voltage rise, it will not turn banks on four-wire systems or delta banks on any sys- off the capacitor bank. tem give maximum current through the fuse when a

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328 Application of Capacitors

20.00 20.00

_ 10.00 10.00 8.00 8.00 6.00 6.00 4.00 imissi, 4.00 \ \ PROBABILITY OF CASE RUPTURE PROBABILITY OF CASE RUPTURE ft IO% 150% 1 90% 10% 50°Ad 90% 2.00 2.00 SAFE ZONE ZONE HAZARDOUS .-SAFE ZONE ZONE I ZONE 2 HAZARDOUS ZONE-. III ZONE I ZONE 1.00 1.0 • 0.80 DS O N 0.8 c.) N 0.60 II CO Mk 0.6 E S 0.40 1

IN 0.4 E IM

- 0.20 T 0.2 1

0.10 0.I 0.0 1 0.06 0.0 k 0.04IEn I 1.•11111 M. 0.0 4 50-KVAR UNITS 25-KVAR UNITS

0.02 0.0 a

0.01 0.0 1 40 60 100 200 400 600 1000 2000 4000 40 60 100 200 400 600 1000 2000 4000 AVAILABLE SHORT CIRCUIT CURRENT IN RMS AMPERES AVAILABLE SHORT CIRCUIT CURRENT IN RMS AMPERES Fig. 24-Current versus time for a capacitor unit to rupture due to gas pressure caused by internal arcing. In times shorter than one cycle use asymmetrical rms amperes. Safe Zone-Safe for most applications. Usually no greater damage than slight swelling of the case. Zone 1-Suitable for locations when case rupture and/or fluid leakage would present no hazard. Zone 2-Suitable for locations which have been chosen after careful consideration of possible consequence associated with violent rupture of the case. Hazardous Zone-Unsafe for most applications. Case wil rupture with sufficient violence to damage adjacent units. capacitor is faulted, and allow fastest coordination. Un- currents to flow in feeders which cause interference with grounded-wye banks have the current limited to 300 per telephone lines. On most power systems, the triple har- cent of normal phase current by the impedance in the monics usually are responsible when interference occurs. other two phase-legs. The fuse must have a continuous Grpunded-wye capacitors are involved, since this con- current rating of at least 135 per cent of rated current of nection is the only one which allows triple harmonics to the bank and should clear in less than five minutes for flow. Interference is rare on most systems, and inter- reasonable coordination. It is sometimes difficult to select ference considerations usually do not determine general a fuse with a continuous rating of more than 135 per connection practices. When interference does occur, cent which will clear in less than five minutes at 300 per moving the bank will often cure the trouble. If not, the cent current. bank must be ungrounded, removed, or a filter added. As mentioned above, delta or grounded-wye banks To summarize, it is possible to state usual practices usually have sufficient current through a. shorted capaci- regarding connection of capacitor banks on distribution tor unit for positive fuse clearing. However, examina- feeders. tion of Fig. 24 shows that when the available fault cur- 1. For delta or ungrounded systems, delta connected rent exceeds 4000 amperes for 25-kvar units and 5000 banks are usually used, except that at system loca- amperes for 50-kvar units, the time to case rupture is tions where fault current is excessive, ungrounded- less than 0.8 cycles. Since this is the accepted minimum wye banks are most common. clearing time for fuses, group fuses cannot be coordi- 2. For grounded, four-wire systems, grounded-wye nated with case rupture curves for higher currents. In banks are used in most locations. Where system such cases, it is necessary to use ungrounded-wye banks fault current is excessive, ungrounded-wye banks or go to the more expensive current-limiting fuses in are used. Ungrounded-wye banks should not be order to get coordination with the ease rupture curves. used on the load side of any single-pole device such As mentioned in Section 14,- capacitors furnish low as a fuse or single-pole recloser. impedance paths for harmonic currents and may allow Capacitor banks in substations are usually wye-con-

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 336/576 Application of Capacitors 329 netted. On delta systems they are always ungrounded- switching operations or nearby system disturbances. wye, and on four-wire systems they may be either These transient currents may be particularly high when grounded or ungrounded. Most substation banks are parallel capacitor banks are switched. Transient cur- relatively large banks with individual fuses. It is usually rents may induce high voltages in the current trans- no problem to get fast fuse clearing with individual fuses former secondary. This makes it necessary to use surge on an ungrounded-wye bank. The fuse has a continuous protection on the current transformer secondary. rating some what above 135 per cent of the normal cur- Individual Unit Protection—When a capacitor unit de- rent through one unit, and the current through a faulted velops an internal fault, violent rupture of the case may unit is 300 per cent of the total rated phase current of result unless the faulted unit is quickly cleared. On larger the bank. capacitor installations, it is recommended that individ- High-voltage banks are also usually wye-connected. ual fusing of the capacitor units be used. These fuses The protection scheme of the bank determines whether remove the faulted unit before the case ruptures and it is grounded or ungrounded. also allow the unfaulted units to remain in service. Protection of Shunt Capacitor Banks—The protection of Individual fuses on the capacitor unit have an addi- large shunt capacitor banks must take several factors tional benefit, which is as important in many eases as into account: the primary function. The individual fuses make it pos- (1) Surge protection. sible to inspect the banks while energized and to deter- (2) Protection of the power system from capacitor mine if any units have failed. They quickly identify the bus or lead faults. failed units. This is an important factor on all capacitor (3) Protection of the individual capacitor units from banks and especially on very large banks. Without indi- damage due to violent failure of an adjacent unit. vidual fuses it may be very difficult to locate failed units. (4) Protection of remaining units from overvoltage Overvolta ge Protection—Figs. 11 and 12 show the volt- when one or more units in a parallel group are age on the remaining units when a portion of the units removed. in a parallel group are removed. Except for very short Surge Protection—Shunt capacitor banks should be periods of time, the voltage across capacitors should not protected from lightning and other transient overvolt- exceed 110 per cent of rated voltage. The failure rate in- ages. The insulation between internal elements and the creases very rapidly if capacitors are subjected to over- capacitor case may be damaged by surge voltages. On voltage. On large banks, it is recommended that some wye-ungrounded and delta banks, it is recommended type of protection be provided which will relay the bank that lightning arresters be installed to protect the capaci- off if the capacitors in a parallel group are subjected to tors. On grounded-wye banks on a solidly grounded sys- overvoltage because of the loss of a portion of the units tem, a large capacitor bank is self-protecting against in the group. This protection would also protect against voltage surges to a large extent. This is because of the an entire parallel group being shorted by a foreign object. tendency of a capacitor to slope off the wave front of a There are several methods for protecting against overvoltage surge and hold the crest voltage down. A surge voltage on a parallel group. Figs. 25, 26, and 27 show cannot build up voltage on a capacitor bank unless there three relaying schemes which might be used. Fig. 25 is is sufficient energy in the surge to charge the capacitor the best scheme theory-wise, particularly if potential to a high voltage. For these reasons, there is less need for transformers are used across each parallel group instead lightning arresters on a grounded-wye capacitor bank. However, surges of sufficient magnitude and duration may still damage the capacitor. POWER On very large banks, the energy stored in the capaci- CIRCUIT tor bank may be enough to damage lightning arresters TO OVER CURRENT if a situation arises in which the energy of the bank is RELAYS discharged through the arrester. It is recommended that the lightning arresters be placed on the source side of the CIRCUIT capacitor breaker on large banks, so that no possibility BREAKER exists for the arresters to discharge the stored energy in the capacitor bank. Overcurrent Protection—Faults on capacitor busses or associated leads should be cleared to protect the rest of 111 the system from disturbance. Ordinary overcurrent relaying is used for this purpose. Instantaneous trip at- II II II tachments are seldom used, since transient inrush current T-T-7 may cause operation of instantaneous overcurrent relays PT'S when the bank is energized. Current transformers associated with capacitor banks should have surge protection on the secondaries to pre- KrV)- vent relay or current transformer damage during tran- RELAY sient current conditions. The capacitor bank has high- Fig. 25—Protective scheme for single-wye-ungrounded frequency transient current flowing through it during capacitor bank.

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of across the entire phase leg as shown. The capacitor POWER banks in Figs. 25, 26, and 27 show two capacitors per CIRCUIT parallel group and two groups in series, but they are in- TO OVERCURRENT tended to represent any number of capacitor units in the RELAYS parallel groups and any number of groups in series. There are many other relaying schemes which may be used to protect parallel groups of capacitors from overvoltages. All of these schemes have certain advantages and disadvantages. It is felt that the three schemes shown here have considerable merit when all factors are considered, and that they are the more desirable schemes. As mentioned above, the scheme of Fig. 25 is the best scheme. In some applications it is possible to use the more economical capacitor potential devices rather than potential transformers. Potential transformers have an VOLTAGE RELAY advantage over capacitor potential devices in that they __Aurnish a discharge path for the capacitors, and will dis- / charge the capacitors in a much shorter time than the CT discharge resistors built into the capacitors. The internal RESISTOR discharge resistors built into each individual capacitor THIRD HARMONI unit discharge the bank in about five minutes. For auto- FILTER matically controlled banks this is too long a time in many cases, since the bank may be switched on while it Fig. 27—Protective scheme for single-wye-grounded capac- is still charged from a previous energization. This may itor bank. lead to higher than normal transient currents. The scheme shown in Fig. 26, known as the double- type fuses have limitations on the maximum number of wye scheme, has considerable merit. It offers protection parallel units. If these units are exceeded, it is necessary almost as good as that of Fig. 25 and is less expensive. to go to the more expensive current-limiting fuses. Be- For smaller banks, it may be expensive to split the banks cause of the fuse limitations, it is often desirable to make into two wye groups, or it may be impossible to do so large capacitor banks double-wye. Since double-wye and still meet the requirements of Table 6. However, in banks can be protected better than single-wye banks many cases it is no more expensive and may even be and since it is usually no more expensive for large banks, necessary to make the bank a double-wye bank. The in- the double-wye protection scheme is the recommended dividual fuses used on the capacitors in large banks may scheme. When conditions are such that single-wye banks have limitations on the number of parallel units which must be used, the scheme of Fig. 27 can be used for over- may be so located that they can discharge through a voltage protection of parallel groups. fuse into a faulted unit. All of the lower cost expulsion- The scheme of Fig. 27 allows sensitive protection for grounded-wye banks. The use of a voltage relay with a POWER third harmonic filter makes it possible to set the relay CIRCUIT pick-up at relatively low values. For small banks which cannot be made double-wye, this offers a low cost pro- TO OVERCURRENT RELAYS tective scheme. No overvoltage protection scheme will provide posi-

0 tive protection against overvoltage in all cases. The CIRCUIT basic theory of all of the schemes is the detection of BREAKER current or voltage unbalance. There is some inherent unbalance in capacitor banks, and the relaying must be set above the maximum inherent unbalance which can occur if false trip-outs are to be avoided. The inherent unbalance is due mainly to harmonic currents and voltages, and varying capacitance of the capacitors. The LJ tolerance of a power capacitor is minus zero to plus it tft fit L fifteen per cent. When individual units are placed in racks and stacked for high-voltage banks, the capacitance of the racks in each phase leg may vary consider- PT OR CT ably. This causes normal condition unbalance and makes it more difficult to detect small unbalance due to removal of faulted units in a parallel group. Since the RELAY capacitance is given on the rack nameplate, it is possible Fig. 26—Protective scheme for double-wye-ungrounded to reduce the unbalance from this effect by arrangement capacitor bank. of the racks.

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The unbalance due to removal of units from a parallel bank. If a capacitor bank is to be energized on a bus group can be determined from Equations (7) through where two other banks are already connected, (15). In cases where a large number of capacitor units, 1 say twenty or more, are used in a parallel group, it may C„= (17) 1 1 not be possible to detect the loss of one unit, since the + relays cannot be set too sensitive because of natural un- Cl C2+ C3 balances. This is not too important, since the loss of one where Cl is the capacitance of the bank to be energized unit in such a large group does not raise the voltage on and C2 and C3 are the capacitances of the two energized the remaining units above the allowable 10 per cent banks. Lc, is the inductance between the switched bank overvoltage. In most cases the relaying can be set to and the energized banks, including the internal induc- operate before dangerous conditions exist, without se- tance of the banks Note that in this case the source is rious danger of false operation. On large installations, it neglected. This is the usual assumption if the parallel is good practice to use two relays. One will sound an banks are close enough together that the natural fre- alarm when one or more units have failed but dangerous quency between banks is greater than five times the voltages are not yet present. The second relay will trip natural frequency between the total paralleled banks if allowable overvoltage is exceeded. Such a procedure and the source. The higher frequency transient current has the advantage of keeping the bank in service when between parallel banks will be reduced materially in a possible, but indicating when capacitors have failed few cycles of the transient frequency. By the time the while still protecting the capacitors from serious over- lower frequency transient current between the com- voltage. The natural unbalance in a bank can be mini- bined banks and the source reaches the first crest, the mized by properly matching the capacitance in each sum of the two transient currents will generally be lower phase leg. Normal unbalance can be reduced to as low than the first crest of the higher frequency transient as necessary in most cases by interchanging individual current between the paralleled banks. Therefore, even units in the field, or by proper phase leg matching when if the crests of the two currents coincide, there is little the bank is designed.'' likelihood the current will be increased above the value of the first crest. For this same reason, fundamental fre- 13. Inrush Currents quency current is generally neglected when the maxi- The magnitude and natural frequency of the transient mum magnitude of the transient current is calculated. currents which flow through the switching device when The way in which damping of the transient current is banks of capacitors are switched is of importance in de- measured is illustrated in Fig. 28. The peak-to-peak termining the severity of duty on the switching device. damping is: It is also important to know the magnitude and natural frequency of the transient current that will flow to the D= A-- =1— (0.5)x- (R \1-0 (18) combination of individual fuse and faulted capacitor, when evaluating the effects of the stored energy from a This is an approximate equation, which is nearly large number of capacitors closely connected in parallel. exact if I/ LC is more than five times R2 /4L2. Unless re- A third reason for knowing the natural frequency of sistance of appreciable magnitude has been purposely transient inrush current is the possibility of resonant introduced into the circuit, Equation (18) holds for effects with other parts of the power system. practical application. Tests on several installations have The calculation of transient current and natural fre- shown damping factors to range from 0.7 to 0.95. quency is relatively simple if the individual capacitor The crest current of the energizing transient is: banks are considered as a lumped capacitor, with capaci- .\iC, tance equal to the total capacitance of the bank. Since I= -V2(ELN) (19) the bank is made up of many individual capacitors, the Lc., assumption of lumped capacitance is an approximation. However, laboratory and field tests" have shown that -D there is one predominant transient frequency associated with a given capacitor bank. This indicates that the assumption of lumped capacitance is sufficiently accurate for normal calculations. The natural frequency of a capacitor bank is

1 fn an-N/LegC, (16) where L, and C„ are the equivalent inductance in henries and capacitance in farads for the particular case. If a single bank is switched with no other capacitor L. banks in the general area, L„ is the source inductance to Fig. 28—Damping of trans'ent inrush current to a capacitor the capacitor and C, is the total capacitance of the bank.

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where ELN is the line-to-neutral system voltage at the little affected by restrikes. For these devices the results bank and Cer, and Leq are as defined in Equation (16). of Eq. (19) should be used with no multiplier. This may be referred to as the normal maximum nat- When single capacitor banks are switched against the ural frequency crest current. If the bank added is al- source only, with no parallel banks, the ratio of the ready charged to crest voltage when paralleled and this transient current to the normal current is the same as charge is of opposite polarity to the source voltage, the the ratio of the transient frequency to the normal fre- crest transient current is twice that given in Equation quency. The crest value of the transient current for (19). This may occur if a bank is switched off and then 60-cps system is: back on before its charge has decayed. If a single re- / = -V2 (20) strike occurs at the crest of the voltage wave when a 60 (160) bank is being switched off, twice the normal current can again occur. Equation (20) is also true for parallel banks if the It is generally recommended that not less than 1.2 normal current (IN) is calculated for a bank with capaci- times the value given in Equation (19) be taken as the tance Ceq. For parallel banks, it is usually simpler to maximum transient current. Unequal pole closing in the calculate the transient current using Equation (16). switching device is almost certain to give currents some- Fig. 29 shows inrush current to a single switched ca- what higher than given by this equation. While restrikes pacitor bank as a function of bank size and system fault may be rare with a properly applied switching device, current. These curves assume that no initial charge most switched capacitor banks are installed with switch- exists on the capacitor bank, and there is no allowance ing devices which are quite likely to have reignitions. A for unequal switching device pole operation or restrikes reignition would give currents less than twice normal or reignition on opening. The values in Fig. 29 should value. Taking all factors into consideration, a transient be multiplied by 1.2 to 1.5 to make reasonable allowance crest current value of twice that given by Equation (19) for these factors. does not seem to be overly pessimistic when parallel When parallel capacitor banks are switched, the mag- capacitor bank switching is considered. For single bank nitude of the transient inrush current is usually much switching, a value 1.2 to 1.5 times the results of Eq. (19) larger than for single banks. For given bank sizes, should be used as maximum transient current. As dis- parallel banks of metal-enclosed capacitors will have cussed in Section 10, resistor equipped breakers are higher currents and natural frequencies than open-rack capacitor equipments in most applications. This is because of the closer bank spacing usually employed in 24,000 metal-enclosed banks and because the switching devices 40,000 A 2400 V SYSTEM that are incorporated into the metal-enclosed banks 20,000 20,000 A

16,000 METAL ENCLOSED BANKS ...... -/..------10,000 A 12,000 I. RESONANT FREQUENCY 13.8 KV BANKS 2. RESONANT FREQUENCY 12.47 KV BANKS 8,000 3. RESONANT FREQUENCY 7.2 KV BANKS MS) 3 PHASE S C R 4. RESONANT FREQUENCY 4.8 KV BANKS 5000A CURRENT FROM 4,000 SYSTEM AT 2 c CAPACITOR BANK AMP

( 0 2 D 10,000 20,000 30,000 40,000 NT

E I 10,000 40,000 A B RR 13.8 KV SYSTEM 2 CU 8,000 6 INTERCHANGE CURRENT E 4.8 TO 13.8 KV 4 LIN 6,000 H 4,000 2 US

INR 0 2,000 3,...... „______....__. M

U B --...... ,,,

IM 0 10,000 20,000 30,000 40,000 4 6

MAX 6,000 34.5 KV 40,000 A 20,000 A SYSTEM 4 - 4,000 10,000 A 2 2,000 5000 A- n 0 0 1000 2000 3000 4000 5000 6000 0 10,000 20,000 30,000 40,000 CAPACITOR BANK THREE-PHASE KVAR CAPACITOR KVAR (3 PHASE BANK) Fig. 30-Transient interchange current and frequency when Fig. 29-Inrush current from system when energizing a single energizing one capacitor bank with no initial charge in capacitor bank. parallel with an energized bank of the same rating.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 340/576 Application of Capacitors 333 have much less lead length than is possible with open- quencies and normal maximum transient current for rack banks. Figs. 30, 31, 32, and 33 show natural fre- metal-enclosed banks. The values in these curves assume no restrikes or reignitions, and no initial charge on the bank being METAL ENCLOSED BANKS switched. For reasonable estimation of the most severe switching duty, the current values on these curves I. RESONANT FREQUENCY 13.8 KV BANKS 2. RESONANT FREQUENCY 12.47 IN BANKS should be doubled. Results are shown for one bank 3. RESONANT FREQUENCY 4.8 KV BANKS switched against an equal size bank and one bank 22 switched against six equal size banks. These curves represent about the most severe practical installation. Very 20 low impedance was assumed for the capacitor bus and 18 quite close spacing between metal-enclosed equipments. (nxo For most applications of metal-enclosed equipments of 16 given bank sizes, the transient currents and frequencies co INTERCHANGE CURRENT w uj 4.8 TO 13.8 KV u) , 14 would be less than shown in Figs. 30, 31, 32, and 33. re to 2 Open-rack equipments would almost certainly have less cl- a. 12 severe currents and frequencies. Therefore, these curves a (A w are pessimistic, and if the switching device employed _Jo I0 o z can safely handle currents and frequencies shown in >- < 8 0 these curves, the actual application will offer less severe 0 D duty. 6 When actual calculations are performed, the bank 3 4 capacitance should be lumped as discussed above. For metal-enclosed banks with close spacings, the inductance 2 of the hank as viewed from the external bushings should 0 be included. For 4.8-kv, 7.2-kv, 12.47-kv, and 13.8-kv 0 1000 2000 3000 4000 5000 6000 equipments, measurements indicated that an inductance CAPACITOR BANK THREE—PHASE KVAR of 4.4 microhenries per phase is a reasonable value for Fig. 31—Transient interchange current and frequency when the inductance as seen from the roof bushings. For 2.4- energizing one capacitor bank with no initial charge in kv and 4.16-kv equipments, this inductance is approxi- parallel with six energized banks of the same rating. mately 2.6 microhenries per phase.

METAL ENCLOSED BANKS METAL ENCLOSED BANKS

13.8 KV 4.16 KV

I. CURRENT, ONE BANK AGAINST SIX. 1. CURRENT, ONE BANK AGAINST SIX. 2. CURRENT, ONE BANK AGAINST ONE 2. CURRENT, ONE BANK AGAINST ONE. 3. FREQUENCY, ONE BANK AGAINST ONE. 3. FREQUENCY, ONE BANK AGAINST ONE. 4. FREQUENCY, ONE BANK AGAINST SIX. 4. FREQUENCY, ONE BANK AGAINST SIX. 22 22

20 20

18 o rn 2 z 2 16 o 2 alto 14 w 12

.ct W 10 u) 0 >-z 8 ON 3 0 6 SC 0 4 I- 4 4 4 2 2

o 00 o 1000 2000 3000 4000 5000 6000 1000 2000 3000 4000 5000 6000 CAPACITOR BANK THREE —PHASE KVAR CAPACITOR BANK THREE—PHASE KVAR

Fig. 32—Transient current and frequency of 13.8-kv. metal- Fig. 33—Transient current and frequency of 4.16-kv. metal- enclosed capacitor banks. enclosed capacitor banks.

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For open-rack installations of parallel capacitors, only 19th, and 25th harmonics are positive sequence. The the inductance of the leads and bus used to connect the 5th, 11th, 17th, and 23rd are negative sequence. The phase groups and incorporate the switching device is 3rd, 9th, 15th, 21st, and 27th are zero-sequence har- used to find natural frequency and transient current. monics. The internal inductance of the individual racks is 0.5 In a symmetrical three-phase system, even harmonics microhenries or less, and can be neglected with satis- are absent. In order to generate even harmonics, a de- factory accuracy. General curves are not shown for vice which rectifies current must be present. Therefore, open-rack equipments, since there is a large range of even harmonics are usually associated with loads in- bank layout and spacing involved. volving rectifiers, such as metal reduction, traction, or welding. Calculations involving induced voltages due to 14. Operating Problems even harmonics are most often handled by considering Harmonic Effects—Capacitors do not generate har- each conductor separately. monic voltages under any condition. However, since the As discussed above, zero-sequence harmonics are impedance of a capacitor is inversely proportional to the most often involved in telephone interference problems. frequency a shunt capacitor offers lower than normal When shunt capacitors are involved by virtue of their impedance to higher than normal frequencies. There- tendency to furnish a low impedance path for harmonic fore, a shunt capacitor may cause harmonic currents to currents, zero-sequence harmonics can flow only if the flow by providing a low impedance path for the current. capacitors are wye-connected and the neutral is ground- Harmonic voltages, when present, are most likely gen- ed. This is one reason for the use of wye-ungrounded or erated by overexcited transformers, rectifiers, or gen- delta connected three-phase capacitor banks. Cases of erators, although other devices connected to the power telephone interference are rare, and if there is some system may generate harmonics in rare cases. other reason for grounding the wye point of a capacitor Harmonic currents flowing in portions of a system bank, the possibility of harmonic troubles should not may cause telephone interference. Excessive harmonic necessarily eliminate wye-grounded banks from con- currents may also cause capacitors to be overloaded, sideration. When harmonic troubles are associated with since there are definite limitations on the total rms a wye-grounded bank of capacitors, the trouble can current which may flow into a capacitor. often be cured by ungrounding the bank neutral. Since On modern power systems, telephone interference resonance with the power line at the harmonic fre- caused by harmonics is generally associated with areas quency is often associated with large harmonic currents where open-wire telephone lines run parallel to power through capacitor banks, relocation of the capacitors lines for appreciable distances. Harmonic voltages are may remove or reduce the problem. This solution is induced on telephone lines by harmonic currents flowing often used on pole-mounted capacitors. Locating the in the power lines through the inductive coupling be- capacitors one or two spans away from the location tween the power line and the telephone line. The fact which caused trouble often eliminates the problem. that inductive coupling is usually involved in telephone Shunt capacitors often have beneficial effects when interference problems makes the symmetrical com- harmonic voltages are present in a power system. Be- ponent sequence of the harmonics involved of impor- cause of the low impedance offered to harmonics by tance. If the power line and telephone line run parallel large capacitor banks, these banks often localize the for the length of a complete transposition of the power effects of harmonic currents. The capacitor bank in line, the positive- and negative-sequence harmonics will effect short circuits the harmonic voltage, allowing cur- induce no end-to-end voltage on the telephone line. rent flow only between the harmonic source and the Even if the lines are not parallel for a complete trans- capacitor bank. This reduces or eliminates the flow of position, the positive- and negative-sequence harmonics harmonic currents from this source in the rest of the are not too important in most cases. This is because the power system. Sometimes a capacitor bank is purposely positive- and negative-sequence components add to placed near a source of harmonics, such as a rectifier, to zero in the three phases of the power line. The net in- block the flow of harmonic currents into the power sys- duced voltage due to these components depends upon tem. A capacitor used in this manner is in effect a filter. the difference in distances between the three individual This type of filter is called a "brute-force" filter, since it phase conductors and the telephone line. The resultant is not tuned to a particular frequency. effect is generally much smaller than the induced volt- Resonant Effects—Under certain conditions, capacitors age caused by the positive- or negative-sequence har- may be involved in undesirable resonance effects. One monic current in one phase of the power line. The cur- phenomenon of this type may cause high voltage to rent in all three phases tends to give a net induced volt- appear when a capacitor bank is switched. These voltage of zero. ages are often in locations remote from the capacitor The zero-sequence harmonics are generally the most bank, such as a lower voltage circuit which is induc- troublesome. They are in-phase in all three power con- tively coupled through a transformer to the circuit on ductors and their induced voltages add arithmetically which the switched capacitor is located. The lower in the telephone line. voltage circuit usually has a fixed capacitor bank when The positive-sequence harmonics are all of the order this type of trouble occurs. For example, lightning ar- (6n+1) where n is any integer; the negative sequence, rester failure might be observed near a fixed capacitor (6n —1); and the zero sequence (6n —3). The 7th, 13th, bank on a 4160-volt circuit when a 33-kv capacitor

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Application of Capacitors 335

bank is switched. This may be due to the 4160-volt capacitors are a more specialized type of device and capacitor forming a resonant circuit with the 4160-volt have a much more limited range of application. The feeder and other inductance between the 4160 volt bank total kvar of shunt capacitors to be installed on a sys- and the 33-kv bank. This resonance must occur at some tem is often determined by a study of the overall sys- frequency which is associated with transient switching tem, with actual locations determined by detailed current to the 33-kv bank, if dangerous voltages occur study of small areas. Series capacitors in feeders and on the 4160-volt feeder when the 33-kv bank is switched. subtransmission lines are usually dictated by individual This type of trouble may result in failure of fuses or situations. Because of the special problems associated lightning arresters on the lower voltage circuits. It may with each installation, there is a large amount of en- also result in bushing flashover or failure of current or gineering and investigation required. For this reason, potential transformers. The switching device at the series capacitors of small size are usually not justified. 33-kv capacitor bank is often involved in this type of A series capacitor compensates for inductive react- trouble. It is most apt to occur if the switching device ance. When a series capacitor is inserted in a feeder or allows appreciable arcing time or several restrikes when transmission line, the inductive reactance, as viewed switching. between two points which include the capacitor, is re- Another phenomenon involves high transient over- duced by the amount of the capacitive reactance of the voltages at the same voltages as the capacitor bank capacitor. The effect of the series capacitor is to reduce which is switched. This is usually a switching surge the voltage drop caused by inductive reactance in the phenomenon rather than resonance effect, but either is feeder or line. This causes the feeder or line to appear to possible. Again, an improperly applied switching device be of lower inductive reactance. For certain application is usually involved in this type of trouble. it is best to think of the series capacitor as a voltage If reasonable care is exercised when selecting switch- regulator which gives a voltage boost proportional to ing devices for capacitor banks, cases of trouble involv- the magnitude and power factor of the through current. ing resonance or switching surge effects should occur This is the fundamental difference between the effects 1 very rarely. Consideration of the ability of the device of a series capacitor and a shunt capacitor. The shunt to switch capacitors without excessive restrikes should capacitor gives a constant voltage boost which is inde- e always enter into the selection of equipment for switch- pendent of the through current, as long as the through ing capacitors. current causes no appreciable voltage change. If the The most difficult facet of system troubles caused by load current does cause appreciable voltage drop, the .s resonant effects or switching surges associated with voltage boost of the shunt capacitor decreases. This is a IS capacitors is usually the recognition of the problem. change in the undesirable direction. Therefore, a shunt to The troublesome condition may occur rarely, and there capacitor may actually make voltage regulation worse. fn may be no particular reason for linking a failure of a It acts as a voltage regulator only if switched with metering transformer, lightning arrester, or fuse with proper controls. The series capacitor, on the other hand, the operation of a remote capacitor bank. When a series gives a voltage rise which increases as the load increases. - of otherwise unexplained failures of the above men- In addition, at lower power factors which would cause tioned equipment occurs in one location, an investiga- more line drop, the series capacitor gives more net volt-- ie tion should be made of the possibility that the failures age rise. Hence, the series capacitor may be considered in are coinciding with the switching of some remote capa- as a voltage regulator. ir- citor bank. The action of a series capacitor to reduce voltage drop he When it has been determined that a particular is illustrated in Fig. 34. Voltage drop through a feeder of switched capacitor bank is causing this type of trouble, is approximately he the capacitor switching device should be checked. Often, V = IR cos 0+ /XL sin 0 (21) an improperly applied switching device is at fault. If to the switching device appears to be adequate for the where R is feeder resistance, XL feeder reactance, and 0 particular duty, other solutions involve relocating the the power factor angle. If the second term is equal to er. capacitor bank or adding reactors in series with the or greater than the voltage improvement desired, a it capacitor bank. The addition of reactors is usually an series capacitor may be applicable. The magnitude of effective solution. The reactor changes the frequency of the second term is a relatively larger part of the total ors transients associated with capacitor switching and also voltage drop where power factor is low and where the )ne tends to reduce the magnitude of current and voltage ratio of feeder resistance to reactance is small. With a to transients. series capacitor inserted, Fig. 34(b), the voltage drop )lt- becomes tor III. SERIES CAPACITORS IR cos C-1-/(XL—Xc) sin 0 (22) uc- on 15. Application of Series Capacitors or simply IR cos 0 when Xc equals XL. In most applica- wer Series capacitors have been used to a limited extent tions, the capacitive reactance is made smaller than hen for many years on distribution and subtransmission cir- feeder reactance. Should the reverse be true, a condition ar- cuits. Shunt capacitors are almost universally applied of overcompensation exists. Overcompensation has been itor on power systems, since their beneficial effects are de- employed where feeder resistance is relatively high to itor sirable at practically all points in the system. Series make /(XL—Xc) cos 0 negative. However, overcompen-

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XL ER eN\ - IR Es (a) ER I I Es IR COS B IXL SIN 8

R XL x. ti (b) Es EIR

I IR COS 9 I(XL-Xe ) SIN

Fig. 34—Voltage vector diagrams for a circuit of lagging power factor (a) without and (b) with series capacitors. The series capacitor increases the receiving-end voltage, thus reducing voltage drop.

Fig. 35—The lagging current drawn by a motor while starting raises the receiving end voltage of a circuit which sation may not be a satisfactory condition if the amount is over-compensated with series capacitors. of capacitance is selected for normal load, because during the starting of a large motor the lagging current may cause an excessive voltage rise, as shown by Fig. larger thermal capability. Either of these solutions will 35. This is harmful to lights and introduces light flicker. allow for proper load division between the two lines. The power factor of the load current through a circuit This comes about because the power flow over a trans- must be lagging for a series capacitor to decrease the mission line is given by voltage drop appreciably between the sending and re- E Er sin i3 ceiving ends. If power factor is leading, the receiving- P— " (23) end voltage is decreased by the addition of a series ca- X where pacitor, as indicated by Fig. 36. If the power factor is E.= Sending end voltage near unity, sin B and consequently the second term of Er = Receiving end voltage Equation (22) are near zero. In such cases, series capac- = Angle between E. and Er itors have comparatively little value. X = Line reactance When properly applied, a series capacitor reduces the impedance of a line and thereby raises the delivered If the two transmission lines are bussed at both ends, voltage. This increases the kva capacity of a radial the angle 0 is equal for both lines. If the line lengths are feeder and for the same delivered load kva, slightly re- the same, the reactance will be approximately the same, duces line current. A series capacitor, however, is not a no matter what the conductor thermal capability if substitute for line copper. both lines are single conductor lines (Bundling the conductor reduces reactance appreciably, but changing 16. Series Capacitor Application in Subtransmission conductor size has little effect on inductance). With Circuits equal angles across both lines and equal reactance, the Series capacitors may be used in subtransmission lines will divide real power equally between the two lines to change the division of load between parallel lines. If a phase shifting transformer is placed in one lines or to reduce the voltage regulation. The induct- line, the angle across the line can be changed and the ance of a transmission line is little affected by conductor power flow increased independently of the other line. size. If two lines are terminated to the same busses on Likewise, a series capacitor in one line will decrease the both ends, the division of power flow between these two reactance of the line and cause it to carry a larger pro- lines will be inversely proportional to the relative length portion of the through power flow. For example, with of the two lines. Often a new transmission line with two parallel lines of equal impedance, the loads divide larger thermal capability will parallel an older line. It equally. If a series capacitor reduces the reactance of may be impossible to load the new line to its capability one line by one-third, this line will carry 50 per cent without overloading the old line. In a situation such as more real power than the other line for any condition of this, there are two possible solutions. A phase-shifting through power flow. transformer may be used to regulate the division of real The selebtion of the capacitor ohms for any desired power flow, or a series capacitor may be used to com- power division between parallel lines is a relatively pensate for a portion of the reactance of the line with simple matter. The lines will divide the real power in

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E s The 50-kvar 7200-volt unit has a rating of 6.95 amperes and 1035 ohms. Thirty in parallel have a rating of 208 amps and 34.5 ohms. Three of these groups in series would give 103.5 ohms per phase, and the total kvar would be 4500 per phase or 13,500 total. If 34 of these units were used in each parallel group, the three series groups would total 91.5 ohms, which is very close to the ER required 90 ohms. However, the through current rating would be 236 amps and the total kvar 5100 per phase or 15,300 total. Both these combinations are somewhat larger than necessary, and other capacitors should be investigated. A 50-kvar, 4160-volt unit is rated 12 amps and 347 ohms. Seventeen parallel units have a rating of 204 amps and 20.4 ohms. Four series groups would give 81.6 ohms per phase. The total kvar would be 3400 per phase or Fig. 36—When the load power factor is leading, a series 10,200 total. This combination might be used. The capacitor decreases the receiving end voltage. ohmic value is within 10 per cent of the desired value and this may be close enough. inverse proportion to their reactances. The capacitor A 50-kvar, 4800-volt unit is rated 10.4 amps and 462 should reduce the reactance of the line which is carrying ohms. Twenty in parallel give 208 amps and 23.1 ohms. insufficient power until the through reactance is such Four series groups give 92.4 ohms per phase. The total that the desired division of power with the other line is kvar would be 4000 per phase and 12,000 total. This is obtained. quite close to the desired values. Study of the specific Capacitor Rating—When the rating of series capaci- case would show whether the 4160-volt or the 4800-volt tors used in subtransmission lines is to be determined, unit should be used. The 4800-volt unit is closer to the the capacitor ohms are usually determined by system desired values but the 4160-volt unit is less expensive. considerations. The resultant reactance is fixed by The decision in a case such as this might be influenced studies which show what reactance is required to ac- by the standard shunt unit in use on the system. Using complish the desired result. The desired net reactance system standard shunt capacitors in a series bank sim- subtracted from the actual line reactance gives the re- plifies stocking of replacements and allows the series quired series capacitor reactance. For several reasons capacitor units to be used on the system as shunt capac- it is undesirable to compensate for more than 50 per itors, if the series capacitor is not required at some cent of the reactance of a tie line. A radial feeder may future date. have more than 50 per cent compensation in some cases, but a tie line will usually be difficult to operate and re- 17. Applications on Primary Feeder Circuits lay if more than 50 per cent compensation is used. In general, series capacitors are applicable to radial After the capacitor ohms are determined, the maxi- circuits supplying loads of about 70 to 95 per cent lag- mum load current through the line should be deter- ging power factor. Below 70 per cent, shunt capacitors mined. The capacitors should not be operated above are more advantageous (unless the power factor changes rated voltage except for very short periods of time, over a wide range, making it impossible to switch shunt hence, the maximum normal line current should be used capacitors fast enough to supply the kvar required by to determine the capacitor rating. With the capacitor the load). Above 95 per cent, the small value of sin 0 ohm and maximum through load current known, the limits the beneficial effect of series capacitors. Applica- capacitor rating is tions on radial circuits supplying loads of 70 to 90 per cent power factor are the most likely to be successful. 3/2Xo Kvar — (24) The application of series capacitors differs materially 1000 from that of shunt capacitors. Where voltage correction Equation (24) gives the total kvar rating of the series is the primary function of shunt capacitors, the correc- capacitor. Standard ratings of capacitors should be used tion is obtained by raising the power factor of the load. if possible, and examination of standard voltage ratings To determine the shunt capacitor kvar required, the will usually show that a combination of standard units most important data needed are the magnitude of the can be used. load, its power factor, and the impedance of the source For example, assume a 90-ohm series capacitor is re- circuit. While similar data are required for voltage cor- quired in a transmission line with a maximum through rection with series capacitors, the effect of series capaci- load current of 200 amperes. The approximate total tors is to reduce the reactance of the source circuit. kvar required is Series capacitors affect power factor to a limited extent as compared with shunt capacitors, because usually the (3) (200)2(90) 10,800 kvar kvar in a series capacitor is much smaller, being one- 1000 fourth to one-half of the shunt capacitor kvar for the or 3600 kvar per phase. same change in load voltage. In addition, the series

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2 60 capacitor contributes its kvar to the system as the E. • 105E. Es• 1.10E. R /X.0.5 0.4 square of the load current. g 50 - LOAD POWER LOAD POWER 818•0 5 FACTOR • 90% Capacitor Rating—The rating of a series capacitor for FACTOR. 90% 0.4 03 02 40 O. feeder applications may be determined from a desired 71, IVA' ohmic value, as in the subtransmission case. Frequently, 30 Aer r 0.1

however, the capacitor ohms are not as easily deter- 20 mined for the feeder. The desired division of load be- //I tween parallel tie lines or the desired voltage drop to /I between two busses are definite criteria. With these Ww a• 00 10 20 30 40 I 20 30 0 items determined, the required capacitor ohmic value is 4 usually easily determined. In the case of a series capac- 60 E.• 110 E. 5 E, • 105E. atx 0.8 0 RID •0.5• itor in a feeder, there is another approach to the capac- 50 - LOAD POWER - LOAD POWER FACTOR • 80i FACTOR • ROY itor rating which often has merit. 4 - 0 40 A three-phase circuit containing a series capacitor e2 consists of line resistance, line inductive reactance, and 30

capacitive reactance. The kva ratings of these com- 20 ponents are 3I 2R, 3/2XL, and 312X0. As a per cent of the total circuit rating these values are useful in considering 01 10 ../...6 the usefulness of series capacitors. The per cent rating E o o 0 20 30 40 10 20 30 40 is obtained by dividing the kva rating times 100 by the total circuit kva, rating (Va ER/), which must be CIRCUIT REACTANCE -PER CENT ON CIRCUIT RATING known. The per cent rating of the capacitor equals 300IXc/-✓ Fig. 37—Curves for determining kva and voltage rating of 3 ER or 173 /XWER, where I is full-load rating a series capacitor on a radial feeder. of the circuit and ER is the load line-to-line voltage. Calculation of kva ratings as a per cent of circuit rating can be extended to voltage. The voltage drops, IR, Other factors being equal, the ratio of R/XL has a IX L, and IXc times 100, are divided by the circuit volt- large effect on capacitor rating, as Fig. 37 indicates. age rating ER/ V3. The per cent of the capacitor again Higher ratios require more capacitors; this is seen equals 173 /XWER. Consequently, the per cent ratings vectorially in Fig. 38. of each component on a kva base and on a voltage base The current rating of the capacitor equals that of the are identical. Therefore, a series capacitor rated 20 per circuit, because the bank must be able to carry rated cent on the base of circuit kva is also rated 20 per cent circuit current continuously. In addition, when circuits on the base of circuit voltage. These ratings mean that supply relatively large motors, the capacitors must be at full load, the capacitor "consumes" 20 per cent of able to carry temporarily the starting current of the rated circuit kva, and the voltage drop across its ter- largest motor plus the current of other loads already in minals is 20 per cent of rated circuit voltage. service. The total of the transient and steady-state cur- The rating of a series capacitor (kilovars, voltage, and rents through the capacitors should not exceed 1.5 times current) for a radial feeder depends on the desired volt- rated. age regulation, the load power factor, and the amount Lamp Flicker—Series capacitors are suited particularly of resistance and reactance in the feeder relative to each to radial circuits where lamp flicker is encountered due other and to the circuit rating. The capacitor kilovar to rapid and repetitive load fluctuations, such as fre- rating can be determined for 80 or 90 per cent load quent motor starting, varying motor loads, electric power factor and 5 or 10 per cent circuit voltage drop welders, and electric furnaces. As discussed in Section from data given in the curves of Fig. 37. To use these 15, operating problems make it necessary to carefully data, the feeder rating is taken as 100 per cent kva, and study each application of series capacitors. all other figures are calculated in per cent on this base. A transient voltage drop, which causes lamp flicker, For example, assume a 10,000-kva feeder having an in- is reduced almost instantaneously in the same manner ductive reactance of 20 per cent and a ratio of resistance as a voltage drop due to a slowly increased load. To pre- to reactance of 0.3 supplying a load whose power factor dict accurately the reduction in voltage flicker by series is 80 per cent. From Fig. 37, to limit the voltage drop to capacitors, the current and power factor of the sudden 5 per cent at full load, the series capacitor must be load increment must be known. It is obvious that to rated 20 per cent of the circuit rating. This is 20 per cent of 10,000 kva or 2000 kilovars. The capacitor voltage rating is also 20 per cent of the rated circuit voltage. Thus, if the circuit is rated 20,000 volts, phase to neutral, the capacitor is rated 4000 volts. If a voltage regulation of 10 per cent is permissible, only 1100 kilovars (at 2200 volts) are required. Had load power factor been 90 per cent, 2200 kilovars (at 4400 volts) would be necessary for 5 per cent regulation and 900 kilovars (at 1800 volts) for 10 per cent regulation. Fig. 38—Effect of feeder R/X ratio.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 346/576 Application of Capacitors improve voltage conditions or to reduce lamp flicker at gap which will carry high currents indefinitely and still a given load point, the series capacitors must be on the be self-clearing at relatively high currents. The series source side of that point. The series capacitors must capacitor protective device has no control over the fault compensate for line inductance between the source and current. Line relaying and circuit breakers must detect the point where it is desired to reduce lamp flicker. This and clear the fault. If the fault is cleared by a breaker sometimes makes the application of capacitors difficult, on the load side of the capacitor, the line current may because one feeder from a bus with several feeders may never fall to zero but may go from fault value directly have a fluctuating load that produces sufficient voltage to load value. For these reasons, most applications re- change on the bus to cause lamp flicker on all feeders. quire some means for shunting the gap and transferring To use series capacitors to reduce the flicker, they must the current to another path. The capacitor may be re- be installed in the supply circuit or circuits to the bus. inserted by opening the by-pass device after the current Shunt capacitors cannot be switched fast enough to returns to normal. prevent lamp flicker. In fact, an attempt to use shunt There is a wide range of protective devices which capacitors for this purpose might aggravate the situa- may be used. On large high-voltage series capacitors, a tion. Step or induction voltage regulators also are not device operated by compressed air may be used. This sufficiently rapid to follow sudden voltage fluctuations. device is basically a gap and a compressed air circuit The voltage dip cannot be prevented by switched shunt breaker. The gap operates in one-half cycle or less and is capacitors or regulators, as the dip itself is used to cleared within a half cycle or a cycle after the line fault initiate the correction. is cleared. The expense of such a device is justified on large series capacitors where system stability is the 18. Protection of Series Capacitors primary reason for the capacitor. The capacitor is not Protection During Line Fault—For most circuits in which effective while bypassed, and the period immediately series capacitors are applied, the currents and corres- after a fault may be the most critical from a stability ponding capacitor voltages during fault conditions are standpoint. Therefore, a device for fast re-insertion several times the maximum working value. Standard must be used. capacitor units can withstand about 200 per cent of their On most series capacitors, such expensive equipment rated working voltage for brief periods without damage is not necessary. A device of the type shown function- to the dielectric; therefore, it is necessary to use capaci- ally in Fig. 39 may be used on relatively large series tors with continuous current ratings equal to 50 per capacitors, where system stability is not affected by the cent of the maximum current that may flow during a capacitor. fault, or to use a voltage-limiting device. For a given When the gap flashes over to bypass the capacitor, a reactance, the cost of capacitors increases approximate- solenoid in the gap circuit trips a spring loaded switch ly as the square of the rated current. Consequently, it which bypasses the gap. This extinguishes the gap cur- is usually more economical to use capacitors whose rat- rent. After the line current returns to normal, the relay ings are based on the working current, and to limit the energizes the motor to open the switch and reinsert the voltage that can appear across their terminals by means capacitor. For large series capacitors, this is probably of auxiliary apparatus. the least expensive device for providing line fault pro- The voltage rating of the series capacitor and its tection for the capacitor while still returning the capaci- associated protective equipment should be high enough tor benefits with moderate outage time. The capacitor to prevent the capacitor from being by-passed during may be out of service for a minute or so after a line fault working loads. To insure availability of the series capac- severe enough to require bypassing. itor during motor starting currents, when its effects are Protection Against Continuous Overload—Standard se- most useful, the capacitor rating must be at least 67 per ries capacitors should not be used for continuous oper- cent of the greatest motor inrush current that may be ation at more than 110 per cent of their rated voltage. imposed on the line, plus other operating load. With Consequently, maximum working current through a protective devices set to by-pass the capacitor at 200 series capacitor should not exceed the rated working per cent rating, the capacitor remains in service during such transient loads. MOTOR OPERATED SPRING LOADED The ideal device for protection of the series capacitor SWITCH during line faults must: -„ 1. Limit the voltage across the capacitor to about twice the rated value. 2. By-pass the capacitor in one-half cycle so that GAP excessive voltage is not present even for short times. 3. Keep the capacitor by-passed as long as the line SOLENOID RELAY TRANSMISSION current is excessive. I LINE OR 4. Re-insert the capacitor as soon as the line cur- FEEDER- rent drops to a safe value. 1( A properly designed gap will provide the first two requirements for all except the lowest voltage series ca- SERIES CAPACITOR pacitors. However, it is extremely difficult to design a Fig. 39—Series capacitor protective scheme.

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340 Application of Capacitors current by more than 10 per cent. The short-circuit pro- Three major phenomena may be encountered in a tective device is not designed to function at less than circuit employing a series capacitor: Sub-synchronous 200 per cent of the rated current; therefore, it is some- resonance of a motor during starting, ferroresonance of times desirable to provide overload as well as short- a transformer, and hunting of a motor during steady- circuit protection. The overload protective device state operation. One, two, or all these may occur. should have an inverse time-current characteristic that Sub-Synchronous Resonance During Motor Starting— can be coordinated with the capacitor to allow momen- When an induction or a synchronous motor is started, tary, but not continuous, overloads. Series capacitors (the latter as an induction motor) through a series ca- have a 30-minute rating of 1.35 times rated current and pacitor, the rotor may lock in and continue to rotate at a 5-minute rating of 1.5 times rated current. a speed below normal or synchronous. This condition is This special type of protection usually is not war- known as subsynchronous resonance. It is caused by the ranted except on large series capacitor banks. The ab- capacitor, whose capacitive reactance in conjunction sence of overload protection on small distribution in- with the inductive reactance of the circuit and motor stallations further emphasize the need for care in choos- establishes a circuit resonant at a frequency below that ing the continuous current rating. of the power supply. The rotor, in effect, acts as a stable Dielectric Failure Protection—Dielectric failure protec- asynchronous generator. It receives power at rated fre- tion rarely is used except on large banks. Dielectric pro- quency from the stator windings and transposes it to tection is a means of detecting a faulted capacitor unit the sub-synchronous frequency, which it returns to the in a series-capacitor assembly. In an unfused capacitor circuit containing the capacitor. This circuit, being res- bank, a short-circuited capacitor may sustain an inter- onant, imposes a minimum of impedance to the sub- nal arc, which causes gas to be generated in the unit. synchronous voltage and consequently conducts a large Continued operation causes the internal pressure to current. A motor operating under these conditions may reach a value that will rupture the case and possibly be damaged by excessive vibration or heating. damage other units and equipment. If the units are The sub-synchronous frequency is dependent on the equipped with individual fuses—and they should be—a relative sizes of the motor and the capacitor. The capac- fuse operation to remove a faulted unit increases the itor rating is determined by the circuit rating. (Other reactance of the bank, and operation at the rated cur- conditions remaining the same, the ratings are pro- rent of the original bank subjects the remaining units portional.) Consequently, the resonant frequency is re- to over-voltage. Protection is afforded by detecting with lated, indirectly, to the rating of the motor in proportion proper relaying when the currents become unequal in to that of the feeder. This frequency is usually 20 to 30 two equal branches of the capacitor. When the unbal- cycles for a 60-cycle motor whose rating equals half the ance in current exceeds the selected value, the capacitor circuit rating. is bypassed until the defective unit is replaced. As the motor size decreases with respect to the capaci- Circuit Relaying—On radial circuits, fault protection tor and circuit ratings, its reactance increases. During relaying is not affected by the addition of series capaci- resonance, capacitive and inductive reactance are equal. tors. Fault currents usually exceed twice rated current. Because capacitive reactance increases with decreasing Consequently, the parallel gap breaks down on the first frequency, the sub-synchronous resonant frequency is half cycle of fault current. This happens faster than lower when the motor is a smaller proportion of most types of relays operate and thus relay and circuit circuit rating. A motor requiring less than five per c,sat breaker operations are the same as without capacitors. of the circuit rating can be resonant at a sub-synchron- Relaying of line-to-ground faults is accomplished usual- ous frequency of five cycles or less if it starts under load. ly by residual or neutral current, which is not affected The most common method of preventing sub-sy n- greatly by a series capacitor. Fault protective relaying chronous resonance is to damp out this frequency by on a tie feeder, however, may be affected considerably placing a resistor in parallel with the capacitor. While by the installation of a series capacitor. Detailed studies the resistance to use can be calculated, the results thus must be made for each case prior to the installation of obtained are usually one-half to one-tenth the valiteo the capacitor. This is especially true if directional or that experience proves necessary. Field tests are neces- phase-comparison relays are used. Harder, Barkle, and sary to determine resistor size in most cases. Calcula- Ferguson" have studied the effects of series capacitors tions are inaccurate because of the difficulty of giving on tie line relaying on an analog computer, and their precise consideration to such variables as inertia of the results are useful when studying the problem. motor and load, starting load, speed of acceleration, the 19. Operating Problems type of starter, and other load on the circuit. For example, load elsewhere on the circuit, when a motor is Along with the desirable characteristics of series ca- started, reduces the possibility of sub-synchronous res- pacitors, there is the possibility of undesirable phenom- onance by providing a damping effect similar to that of ena, usually involving some kind of resonance, which parallel resistance. has deterred the installation of series capacitors even The resistance should be as high as possible in order where they otherwise could solve difficult system prob- to hold to a minimum its continuous losses, which are lems. In many cases the difficulties can be anticipated equal to the square of the voltage across the capacitor and suitable precautions taken to make an installation bank divided by the resistance. It is common practice practical. then to apply resistors that are adjustable over a pre-

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 348/576 Application of Capacitors 341 determined range, particularly in the larger installations. quency, the gap flashes over at rated current, since the When low ohmic resistance is used, the resistor can capacitive reactance is doubled. The lower the fre- be disconnected after the motor reaches full speed and quency, the smaller the current required to break down the risk of resonance has passed. Switching could be the gap. accomplished manually, or by remote control over a In general, the possibility of sub-synchronous reso- pilot wire or power-line carrier channel with electrical- nance should be checked for all circuits in which the ly-operated switching equipment. largest motor requires more than five per cent of the Sub-synchronous resonance can also be avoided by circuit rating. Experience indicates that standard mo- use of parallel resistors across only two phases of a tors rated less than ten per cent of circuit rating en- three-phase series capacitor. Such a solution is permis- counter no difficulty if started at no load. In fact, mo- sible where the omission of resistors from one phase tors rated up to 20 per cent usually accelerate satis- does not unbalance the voltage appreciably. The factorily if started at no load and across the line. How- amount of unbalance is determined by the resistance. ever, when high-inertia loads are involved, the circuit The higher the resistance, the less the unbalance. But must be checked for sub-synchronous resonance even the resistance necessary, not the degree of unbalance, if the power requirement of the largest motor is as low determines the ohmic value. At least one such installa- as five per cent of the circuit rating. tion is in service and is operating satisfactorily. Ferroresonance in Transformers—When a transformer Sub-synchronous resonance can exist only during bank is energized, a high transient exciting current may motor starting. Hence, resonance can be prevented by exist. This is known as magnetizing inrush current. The inserting resistance in series in the supply leads to the magnitude of the current may be as much as several motor instead of in parallel with the capacitor. A con- times full-load current. If a capacitor is in series with tactor is required to short circuit the series resistance the transformer when energization occurs, a resonant after the motor reaches full speed. If the circuit contains condition may occur which causes the high current to only a few motors, such a scheme may be more economi- persist. This condition is known as ferroresonance. cal than a single large resistance in parallel across the Ferroresonance is basically a sub-harmonic condition. capacitors. To be effective, the series resistance must be At times, even though the persistent ferroresonance in the stator circuit of the motor. Resistance in the rotor condition does not occur, a sub-harmonic current of circuit of a slip-ring motor does not give the desired high magnitude may flow in the circuit for a few cycles. damping, but affects primarily the amount of slip be- It is possible for this short duration to give rise to volt- tween the sub-synchronous frequency and the frequency ages across the series capacitor of enough magnitude to of the current through the rotor circuit. damage the capacitor. If motors are started infrequently, sub-synchronous Ferroresonance is automatically cured in most cases resonance can be avoided without using resistance by by the parallel gap. In practically all cases the ferrores- short circuiting the capacitor during starting. If a tem- onant condition will cause high enough voltage across porary unbalance is tolerable, the same result can be the capacitor to break down the gap. This shorts out achieved in some cases by short circuiting only one the capacitor and stops the ferroresonance. When the phase of the bank, which simplifies the switching current has dropped to safe values, the capacitor is re- equipment. inserted either by inherent characteristics of the gap or The reactance of a capacitor is inversely proportional by external means as discussed in Section 18. It is to frequency, while that of an inductor is directly pro- possible for a ferroresonant condition to exist which portional. Hence, in a series circuit consisting of capaci- causes currents above the capacitor rating and under tance and inductance, the voltage drop across the for- the gap breakdown, since the gap is usually set to mer increases as frequency is reduced. Therefore, a con- break down at a voltage corresponding to twice the dition of sub-synchronous resonance in a power circuit capacitor rating. This could damage the capacitor even causes an increase in the voltage drop across the capaci- though a gap is used. Experience has indicated that tor. This voltage may be large enough to cause the pro- little danger exists from this condition. First, there is tective gap in parallel with the capacitor bank to flash- very small probability that this condition will occur. over, thus short circuiting the capacitor. This halts the Field and laboratory tests and computer studies have resonant condition and permits the motor to accelerate shown that such a condition is possible, but the prob- normally to full speed. After a time delay, the capacitor ability is very great that when ferroresonance does oc- is automatically restored to the circuit. This sequence cur, it will be a severe case. No cases of moderate ferro- of operations may make it possible in some installations resonant conditions were found when cases in the range (particularly where motors are started rarely) to use of normal expected applications were studied. Second, the gap alone to prevent sub-synchronous resonance load on the secondary side of the transformer tends to and perhaps eliminate the need for parallel resistors. damp out ferroresonance. The less severe cases of ferro- However, heavy-duty gaps in series with resistors to resonance are apt to be prevented or else quickly dissipate the energy stored in the capacitors may be damped out by moderate secondary loads. For these required. reasons, a protective gap is considered to be excellent The gap is set to break down at twice rated current protection against ferroresonance in all cases. (twice rated voltage) at rated frequency. Consequently, A resistor shunting the capacitor is also effective in during sub-synchronous resonance at half rated fre- preventing ferroresonance. This is not considered to be

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 349/576 342 Application of Capacitors an ideal solution to the problem, since resistors with REFERENCES ohmic values low enough to prevent ferroresonance in all cases will be expensive and have considerable loss. GENERAL 1. Electrical Transmission and Distribution Reference Book If a parallel resistor is used across the capacitor to pre- (Book), Central Station Engineers, Westinghouse Electric vent sub-synchronous resonance of motors, this resistor Corporation, East. Pittsburgh, Pa., 1950. in combination with a gap gives excellent protection 2. Power Capacitors (Book), It. E. Marbury, McGraw-Hill against ferroresonance. Book Co. Inc., New York, New York, 1949. In some cases, such as 2300- or 4160-volt circuits, 3. NEMA Standards CA1-1955 and CP2-1957. the voltage rating of a series bank would be very low SHUNT CAPACITORS (and its cost high) if installed directly in the line. To 4. Uses of Capacitors, R. E. Marbury, Electric Journal, Vol. 33. permit application of a capacitor having a higher volt- July 1936, pp. 303-306. age rating, a transformer in series with the line is 5. Capacitors—Design, Application, Performance, M. C. Miller, Electric Light and Power, Vol. 16, October 1938, pp. 46-50. sometimes employed to step the voltage up from the 6. Shunt Capacitors Reduce KVA Loads, C. M. Lytle and required drop in the line to the capacitor rating. Such S. H. Pollock, Electric Light and Power, Vol. 15, November transformers must be designed carefully to prevent 1937, pp. 52-54. ferroresonance. 7. Capacitors Defer $135,000 Investment in Synchronous Unit, A series capacitor, when installed in a long circuit 4J2 ....F..Roberts,43 Electrical West, Vol. 83, October 1939, pp. supplying a transformer of abnormally high steady- 8. Shunt Capacitor Application Problems, J. W. Butler, Gen- state exciting current, may resonate during normal eral Electric Review, Vol. 43, May 1940, pp. 206-212. operation at a frequency corresponding to a harmonic 9. Current Control Broadens Capacitor Application, A. D. component of the exciting current. Fluctuating loads Caskey, Electric Light and Power, Vol. 18, February 1940, pp. 49-51. may cause such resonance, even though it does not 10. Seventeen Systems Report Smooth Capacitor Performance, appear when the transformer is energized. Resonance M. C. Miller, Electrical World, Vol. 113, January 27, 1940, in this case is eliminated by a parallel resistor, by chang- pp. 289, 339-340. ing the transformer winding, or by replacing the trans- 11. Facilities for the Supply of Kilowatts and Kilovers, Hollis K. Sets and Theodore Seely, A.I.E.E. Transactions, Vol. 61, former with another having a normal exciting current. May 1942, pp. 249-254. Hunting of Motors During Normal Operation—Hunting 12. Kilowatts, Kilovars, and System Investment, J. W. Butler, of a lightly-loaded synchronous motor can be caused by A.I.E.E. Transactions, Vol. 62, March 1943, pp. 133-137. disturbances such as switching on power circuits and 13. Mobile Capacitor Units for Emergency Loading of Trans- changes in load or excitation of the motor itself. Such formers in Open Delta, H. B. Wolf and G. G. Mattison, A.I.E.E. Transactions, Vol. 62, February 1943, pp. 83-86. hunting cannot be directly attributed to resonance. The 14. Extending the Use of Shunt Capacitors by Means of Auto- principal factor in predicting hunting is the ratio of matic Switching, W. H. Cuttino, A.I.E.E. Transactions, feeder resistance to total feeder reactance (including Vol. 63, 1944, pp. 674-678. the series capacitor) between the power source and the 15. Automatic Switching Schemes for Capacitors, W. H. Cut- tino, A.I.E.E. Transactions, Vol. 66, 1947, pp. 311-314. motor terminals. If the ratio is less than one and is not 16. The Why of a 25-KVAR Capacitor, M. E. Scoville, General negative, hunting is unlikely. Violent hunting of a syn- Electric Review, Vol. 52, No. 5, May 1949, p. 19. chronous motor was encountered upon application of a 17. Balancing Double Wye High-Voltage Capacitor Banks, 0. R. series capacitor in one instance because the ratio of Compton, A.I.E.E. Transactions, Vol. 74, 1955, pp. 573-580. 18. Natural Frequency of Parallel Capacitor Banks, W. H. feeder resistance to reactance was approximately four. Cuttino and Miles Maxwell, A.I.E.E. Transactions, Vol. 75, A synchronous motor, when fed through a long line 1956, pp. 662-666. excessively compensated by a series capacitor, may 19. Economic Merits of Secondary Capacitors, R. A. Zimmer- hunt if started during periods of light load. Such hunt- man, A.I.E.E. Transactions, Vol. 72, 1953, pp. 694-697. ing is avoided if the power factor angle of the load (after 20. Calculation of Loss Reduction by Capacitors, Victor J. Farmer, Electrical World, October 29, 1956, p. 100. the motor is started) is equal to or greater than the im- 21. An Evaluation of Power-Factor Correction on a System pedance angle of the circuit (including the capacitor). Basin, H. R. Tomlinson and R. 0. Bigelow, A.I.E.E. Trans- The tangent of this impedance angle is the ratio of total actions, Vol. 73, 1954, pp. 1677-1684. circuit reactance (feeder reactance minus capacitor re- 22. Loss Reduction from Capacitors Installed on Primary Feed- actance) to feeder resistance. ers, N. M. Neagle and D. R. Samson, A.I.E.E. Transactions, Vol. 75, Pt. III, 1956, pp. 950-959. Hunting is not limited to synchronous motors. Series 23. Report of a Survey on the Performance of Shunt Capacitors, capacitors should not be applied to circuits primarily A.I.E.E. Committee Report, A.I.E.E. Transactions, VoL 68, supplying either synchronous or induction motors driv- Pt. II, 1949, pp. 1200-07. ing reciprocating loads, such as pumps or compressors. 24. Report on the Operation and Maintenance of Shunt Capaci- In addition to problems of sub-synchronous resonance, tors, A.I.E.E. Committee Report, A.I.E.E. Transactions, the motors, once started, may hunt and cause objection- Vol. 68, Pt. II, 1949, pp. 1208-18. 25. Report on the Operation of Switched Capacitors, A.I.E.E. able lamp flicker. The frequency of hunting is some- Committee Report, A.I.E.E. Transactions, Vol. 74, Pt. III, times equal to, or a direct multiple of, the frequency of 1955, pp. 1255-61. power pulsation, which further aggravates the situa- 26. Survey on Controls for Automatically Switched Capacitors, tion. A cure for hunting may be the installation of a A.I.E.E. Committee Report, A.I.E.E. Transactions, Vol. 76, heavy flywheel to increase the rotating mass. However, 1957, pp. 1388-1393. 27. Report of a Survey on the Connection of Shunt Capacitor this solution may enhance the possibility of sub-syn- Banks, A.I.E.E. Committee Report, A.I.E.E. Transactions, chronous resonance, which is equally undesirable. Vol. 77, Pt. III, 1958.

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28. Capacitor Switching Phenomena, R. C. Van Sickle and J. 37. Design and Protection of 10,000-KVA Series Capacitor for Zaborszky, A.I.E.E. Transactions, Vol. 70, 1951, pp. 151-59. 66-KV Transmission Line, A. A. Johnson, R. E. Marbury, 29. Switching Capacitor Kilovolt-Amperes with Power Circuit J. M. Arthur, A.I.E.E. Transactions, Vol. 67, 1948, pp. 363- Breakers, N. E. Dillow, I. B. Johnson, N. R. Schultz, A. E. 367. Were, A.I.E.E. Transactions, Vol. 71, 1952, pp. 188-200. 38. 10,000 KVA Series Capacitor Improves Voltage in 66-KV 30. Oil Circuit Breakers for Switching 115-KV Shunt Capacitors, Line Supplying Large Electric Furnace Load, B. M. Jones, W. M. Leeds, J. H. Pehrson, C. F. Cromer, A.I.E.E. Trans- J. M. Arthur, C. M. Stearns, A. A. Johnson, A.I.E.E. Trans- actions, Vol. 72, 1953, pp. 1066-72. actions, Vol. 67, 1948, pp. 345-351. 31. Capacitor-Switching Phenomena with Resistors, R. C. Van 39. Design and Layout of 66-KV, 10,000 KVA Series Capacitor Sickle, J. Zaborszky, A.I.E.E. Transactions, Vol. 73, Pt. III, Substation, G. B. Miller, A.I.E.E. Transactions, Vol. 67, 1954, pp. 971-977. 1948, pp. 363-367. 32. High-Voltage Oil Circuit Breakers with Resistance-Equipped 40. Steady-State and Transient Stability Analysis of Series Interrupters for Capacitor Switching, W. M. Leeds, G. B. Capacitors in Long Transmission Lines, J. W. Butler, J. E. Cushing, A.I.E.E. Transactions, Vol. 73, Pt. III, 1954, pp. Paul, T. W. Schroeder, A.I.E.E. Transactions, Vol. 62, 1943, 1032-1036. pp. 58-65. 33. A New 115-Kv, 1,000-MVA Gas-Filled Circuit Breaker, C. F. 41. Series Capacitors Approach Maturity, A. A. Johnson, West- Cromer, R. E. Friedrich, A.I.E.E. Transactions, Vol. 75, inghouse Engineer, Vol. 8, July 1948, pp. 106-111. 1956, pp. 1352-1357. 42. Application Considerations of Series Capacitors, A. A. 34. Analysis of Capacitor Application as Affected by Load Cycle, Johnson, Westinghouse Engineer, Vol. 8, September 1948, R. F. Cook, A.I.E.E. Transactions, Vol. 78, 1959, Pt. III. pp; 155-156. 43. Series Capacitors During Faults and Reclosing, E. L. Harder, SERIES CAPACITORS J. E. Barkle, R. W. Ferguson, A.I.E.E. Transactions, Vol. 35. Series Capacitors for Transmission Circuits, E. C. Starr and 70, 1951, pp. 1627-1642. R. D. Evans, A.1 .E E .Transactions, Vol. 61,1942, pp. 063-973. 44. Fundamental Effects of Series Capacitors in High-Voltage 36. Characteristics of 400-Mile 230-KV Series Capacitors, B. V. Transmission Lines, A. A. Johnson, J. E. Barkle, D. J. Hoard, A.I.E.E. Transactions, Vol. 65, 1946, pp:1102-1114. Povejsil, A.I.E.E. Transactions, Vol. 70, 1951, pp. 526-536.

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Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 352/576 CHAPTER 9

VOLTAGE FLUCTUATIONS ON POWER SYSTEMS

D. M. SAUTER

problem. The most complete analysis is found in the I. INTRODUCTION report "The Visual Perception and Tolerance of Flick- The power systems of electric utilities are normally er," prepared by Utilities Coordinated Research, Inc. considered constant voltage types of systems. When the and printed in 1937, from which Figs. 1 to 3 of this voltage fluctuates, the problem generally lies with the chapter are reproduced.8 customer. A customer has caused the voltage fluctua- Fig. 1 shows the cyclic pulsation of voltage at which tion, and another customer has seen it or otherwise been flicker of 115-volt tungsten-filament lamp is just per- disturbed. The most common complaint is lamp flicker, ceptible. Flickers as low as IA volt were perceptible in and yet, with most distribution system designs today, ten per cent of the observations, when the rate of varia- this customer complaint is heard infrequently. tion was 8 cycles per second. In order for the variations The problem of voltage fluctuations upon power to be perceptible in 90 per cent of the observations, systems can be broken into two parts: (1) voltage fluc- however, the voltage change had to be over one volt at tuations caused by utilization equipment resulting in the same frequency. The range between 6 and 12 cycles lamp flicker, and (2) voltage fluctuations caused by per second was the most critical. transient system faults resulting in utilization equipment downtime. This chapter discusses the subject in 5.0 the same sequence. However, first it deals with the permissible amount of flicker in the power supply to elec- cn 4.0 tric lights. -J This latter element is very difficult to define concisely 0 d q0 .A. , because of the human element. The discretion and ex- 3.0 A• - periences of utility engineers must weigh the results so 66 95° .1" cc) 79 that the design of the distribution system solves the 151 6°0%) 2.0 0°' . fluctuation problem within economic boundaries. In- 5- IS\ 70 yo.j.0%04 dustry-wide standards on this measure of customer 7.0.1... -J 10 N\. ... 56 ... ei. ' 1.0 service are non-existent. Most data on voltage fluctua- 5- 44 '""*. 2 51--0 60 tion limits to lamps are given in curve form or exist ` D Ni simply as rules of thumb for use by distribution 0 engineers. 0 2 4 6 8 10 :2 14 16 18 20 This chapter presents a current review of flicker FREQUENCY OF VOLTAGE PULSATION (CYCLES PER SEC.) standards used by electric utilities. Consideration is Fig. 1—Cyclic pulsation of voltage at which flicker of 115- then given to the causes, results, and corrective meas- volt tungsten filament lamp is just perceptible—derived from ures to reduce voltage fluctuations in power systems. 1,104 observations by 95 persons in field tests of 25-watt, 40-watt, and 60-watt lamps conducted by Commonwealth II. PERMISSIBLE FLICKER LIMITS Edison Company. Figures on curves denote percentages of The permissible amount of flicker voltage cannot be observers expected to perceive flicker when cyclic voltage stated concisely for several reasons. There is first the pulsations of indicated values and frequencies are impressed human element; one individual may think objection- on lighting circuits. Plotted points denote medians of ob- able a flicker not perceptible to another. The lighting servations at various frequencies, number of observations fixture used is of considerable importance. Incandescent in each case being indicated by adjacent figures lamps and fluorescent lamps change illumination dif- ferently upon a change of voltage. The character of the Fig. 2 shows the minimum abrupt voltage dip to voltage change is also important. Cyclic or rapidly re- cause perceptible flicker in a 60-watt, 120-volt tungsten- curring voltage changes are generally more objection- filament lamp, as a function of intensity of illumination. able than non-cyclic. On non-cyclic changes, the annoy- Curves are shown for 5 and 15 cycles (60 cycles per ance due to the flicker is affected by the rate of change, second basis) durations of voltage dip. It should be duration of change, and frequency of occurrence of the noted that abrupt voltage dips of 1.5 to 2.0 volts were flicker. These and other factors greatly complicate the perceptible. problem of assigning limits to permissible flicker volt- Fig. 3 shows the effect of "duration of transition" of ages. voltage on the average threshold of perceptibility of Numerous investigators have studied the flicker flicker for tungsten-filament lamps. This curve shows 345

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O 25 u_ - • would be to flicker if he were not told what to look for cL 0 and were not aware of the flicker's impending occur- o > 2. • rence. Add to these complications the fact that flicker cc w w perception is influenced by a great many factors, such a 0 . as the intensity and color of illumination, size and type _a -1- 1I . o LL of lamps, rate of change of voltage, and many others. It o is obvious, therefore, that flicker limitation must be G. I.o p z rc o_ x 5 CYCLE DURATION OF VOLTAGE DROP based on compromise judgment. co 015 II n < a n n II Under these circumstances, a universally accepted w 0. i flicker rule was not established. In order to present the • cc 2 w most current flicker rules of the electric utility industry, ) 1 I• !I ...... a survey of the flicker limits or standards used by all ILLUMINATION ON READING MATTER (FOOT-CANDLES) utilities serving more than 100,000 connected customers was made in January, 1959. Of the 103 utilities who met Fig. 2—Minimum abrupt voltage drop for perception of this requirement, 98 replies were received from utilities flicker of 60-watt, 120-volt coiled-coil tungsten-filament lamps operated on 60-cycle alternating current. Each point who provide service to over 41 million metered cus- represents the means of the observation of 44 persons. tomers. Four questions were asked in this survey:

0- ow 1. Does your company have established voltage O 4 6 1-- flicker limits? /- 0 If yes, please enclose a copy of curve, table, or a..._ .....- --- le 2. 0_ 5 other information that you use. 2 ...-- -T.13% 0_ cc 2; ... --- k tS1 3. Do you differentiate between commercial and w -1 0 .... C., _I 4 .....• residential voltage flicker limits? CC < ...- UJ 2 o ....• Have established limits been revised recently? o_ / 4.

• z 3 , 0 3., x KEHOE,25 AND 00-WATT II5-VOLT,LAMPS The replies are summarized in Tables 1 and 2 and Fig. 4. 0 -J 0 / a BROOKLYN EDISON CO., 50-W, I15-V, LAMP 0 0 WERDENBERG, 40-WATT, 220-VOLT, LAMP X z Table 1—Tabulation of replies to flicker questionnaire • LINDBERG, 25 a 60-WATT, 115-VOLT, LAMPS 0 CC tr v ETL 25 TO 100-WATT, 120-VOLT, LAMPS No Percentage Yes X w I Question Yes w 1 59 39 60 • w I_ 2 See Table 2 and Figure 4 ole 3 4 5 w - 6 3 26 72 27 > DURATION OF TRANSITION OF VOLTAGE (SEC) U. 4 13 85 13

Fig. 3—Effect of duration of transition of voltage on average threshold of perceptibility of flicker of tungsten-filament lamps. In Table 1 the replies to questions 1, 3, and 4 above are indicated along with the percentile of affirmative answers. Of interest is the fact that slightly over 60 per quite clearly that whereas an abrupt change of about cent of the utilities employ limits, but only 22 per cent 1% volts is perceptible, a change of five volts or more of these utilities have revised their limits in the last ten is necessary before voltage variations requiring several years. seconds for completion can be perceived. Figs. 1 to 3 are of interest in showing the perceptibil- ities for various classes of flicker voltages. These are not Table 2—Responses to Questionnaire working limits, because a perceptible flicker is not necessarily an objectionable one. Further, the reduction of Question 2—Types of Voltage Flicker Limits information of this nature to universal rules is difficult. Types Number of Users The routine of evaluating the effect of voltage dip is to subject a number of observers to flicker produced by Own curve or table 22 varying magnitudes of voltage dips at a constant rate Composite curve of voltage flicker studies- T & D Committee, Edison Electric Institute, of repetition. It is not unusual to find that some ob- General Electric Company, and various other servers will object to a degree of flicker that is not even utilities 21 visible to others. Investigators used their own individual Maximum allowable voltage fluctuations- approach in reducing the statistical data to a usable set Westinghouse Transmission & Distribution of rules. Some use the voltage dip which represents the Reference Book, 4th edition, page 720 6 response of 40 or 50 or some other percentage of the ob- Service voltage flicker limits'-Consolidated servers, but other criteria were also employed. Edison Co. of N.Y., Inc 2 In staged tests the observers are more or less biased, Fixed value 8 as it is next to impossible to judge what one's reaction Total 59

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 354/576 Voltage Fluctuations on Power Systems 347 S 4 TUATION LUC

TAGE F 3 OL PERCENT V 2

.01 .02 .05 .2 5 I 2 5 10 20 50 100 200 500 1000 FLUCTUATIONS PER MINUTE

1 2 4 10 20 4 10 20 40 2 3 4 6 8 12 FLUCTUATIONS PER HOUR FLUCTUATIONS PER MINUTE FLUCTUATION PER SECOND 4° 11

Fig. 4—Comparison of flicker limits used by various utilities with over 100,000 connected customers each.

The replies to question 2 are summarized in Table 2. It is the author's opinion that the acceptable voltage While 22 companies reported their own curve, there flicker limit curve should lie above the heavy outline were only nine different curves represented in these area and close to the dashed curve. This recommenda- replies. Thus, of the 59 utilities with voltage flicker tion is predicated upon lack of limits reported by 39 limits, there were only twelve different sets of limits utilities, only 13 recent revisions of limits, and the small reported. number of flicker complaints received by utilities. No These twelve curves are presented in Fig. 4. Ten of attempt at drawing a new curve based on the survey the curves fall within the heavy outlined area. To avoid results has been made, because it would only serve to unnecessary detail, only the points for these ten curves complicate the picture. Rather it is stated that installa- are indicated. Of the two remaining curves, both re- tions resulting in flicker voltage within or below the sulted from recent revisions of the voltage flicker limits heavy outline curves will not be objectionable. Flicker by the respective utilities. These curves represented the voltages between the solid curve and the dashed curve revised limits of four of the 13 utilities that reported in all probability will not be objectionable. Until ex- limits revised recently. perience is gained with these higher limits, no further In order to utilize Fig. 4 in distribution system anal- upward revision appears practical. ysis and design, some interpretation must be given to the curves. The area outlined by the heavy lines repre- III. VOLTAGE FLUCTUATION CAUSES sents the limits used by the majority of the industry. Most of the voltage fluctuations and attendant lamp This relatively narrow band is predicated primarily flicker on distribution systems are due to customer's upon 1934 data, test, and analysis. The dashed curve utilization equipment. The following are some of the indicates the voltage flicker limits proposed recently to more common types of equipment known to cause flick- the EEI by one utility.' The points lying above this er. Some of the measures suitable for correcting the dashed curve represent the upper limit of permissible voltage fluctuation to permissible limits also are indi- flicker voltage as currently stated by two utilities. cated for this equipment.

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348 Voltage Fluctuations on Power Systems

extent of usage, they must continue to be low in cost, 1. Motor Starting economical, rugged and reliable. These requirements Probably most of the flicker problems are caused by have led to several classes of motors depending upon the starting of motors. For reasons of cost, efficiency, the service, with one class designed specifically for fre- and reliability, commercial general purpose motors re- quent starting with low starting current. This motor is quire a momentary starting current several times their used in great quantities in domestic refrigerators and full load running current, in order to produce sufficient oil burners, and the horsepower, 110-volt class usually starting torque. has a locked-rotor starting current of 20 amperes or less. Three general classes of motor installations are of Reference to the results of the survey indicates that importance in the flicker problem. most distribution systems when designed to meet volt- 1. Single phase fractional horsepower motors com- age regulation and load growth considerations can sup- monly used in homes and small stores. ply 20 amperes at 120 volts without objectionable 2. Integral-horsepower single and polyphase mo- flicker. Where single phase 120/240-volt systems are tors operated from secondary distribution cir- used, 40 ampere starting currents are permissible on the cuits, such as in small shops, large stores and 240-volt connection allowing larger motors to be used. buildings, and recently in an increasing number 2. Integral horsepower motors on secondary circuits of homes for air conditioning. are the largest potential source of flicker. The ever-in- 3. Large integral-horsepower three-phase motors creasing number of full air-conditioned homes has operated from primary lines, mostly by indus- placed three- and five-horsepower motors on residential trial concerns. area feeders. The location of this size of motors in the 1. Single phase fractional horsepower motors are past had been in areas of high-load concentration where manufactured in large quantities, and to maintain this flicker was a lesser consideration.

600' _ A I 0 230V DISTRIBUTION 1 ,,,,, 500' TRANSFORMV TO MOTOR

400'

CURVES FOR VARIOUS - DI STANCES D IN FEET 414N:5115V 21.1P(§) 230 300' I - i 200' r A 23o5 N3v: 30...Ai - loco A ..,100 111P@ 115V - 50' 7 HP @ 230 V -.

111111111%.„:, ,11, :11111111111111 1.11P.Il O 10 20 3.0 40 5.0 60 7.0 60 9.0 100 ..._,,- 3 , HP @ 230V PERCENT VOLTAGE DROP I ID t43 347 6 / 11 ," / -I.0 ,...._/...- 7i HP @ 230V I 1 % I _ 1 ______.— 5 HP (gb 230V \ % i — -0.9 ------v.- 2 HP @ 230V \ 1 I "..-\\...- *HP@ 115V \ I I - \ - 0.6 a i HP 0 1 15 8 230V \ I I \ li \ I DOTTED I CURVE . AUJMNUM \ 0.7 SOLD CURVE :COP ER \ 1-- I - I 1 1 1 1 R A 266.8 MCM t 1 I 1 -0.6 336-4 MCM -. \ \ t 1 \ 14 1 k 1 WIRE SIZE I I I I I 1 I I I‘ I i I I e , ft 6 4 2 IB toalb %

Fig. 5—Estimating curve for determining the voltage drop present upon starting various typical motors.

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Voltage Fluctuations on Power Systems 349

Table 3-Typical Motor Starting Characteristics power factor from Table 3; enter Fig. 5 at this power factor, continuing across horizontally to the correct HP Voltage Inrush Current Power Factor wire size, then vertically to the distance between the 3i 115 volts 27 amps 89.6% distribution transformer and the motor service entrance, WI 115 47 87.2% and then horizontally to the curve for the motor in Yi 230 23%3 87.2% question. This intersection will be read on the horizon- 2 230 61 90.0% tal scale as the percentage voltage drop. The actual 3 230 70 99.6% solution for the above example has been drawn on Fig. 92.5% 5 230 103 5 with a dotted line, and the percentage voltage drop 7% 230 160 95.0% was found to be 5.5 per cent using a 25-kva distribution transformer. In the early 1950's, with the mass production of room 3. The starting of large integral horsepower three- and home air conditioners, the advantages for improved phase motors by the industrial plant is one cause of starting power factor were recognized. Since that time flicker problems on utility supply lines. Usually this single-phase motors up to VA horsepower have been problem of supplying large motors from subtransmission built capacitor-start, capacitor-run. This design of or primary feeder power lines is not troublesome, be- single-phase motor has led manufacturers to supply cause such motors are usually located in an "industrial starting characteristics typical of those listed in Table 3. district" where the utility lines are inherently heavy, A more complete listing of motor characteristics is given few residential customers are served also from the same in Chapter 2-Load Characteristics. The fractional lines, and wider limits of voltage drop are permitted. horsepower motors with their slightly lower power fac- There are, nevertheless, a number of cases where motor tor are capacitor-start, induction-run motors. ratings are too high for the power facilities. Such prob- Fig. 5 may be used to estimate the percentage voltage lems exist when an industrial process involves a single drop which would be experienced at the motor service large motor with a number of smaller horsepower auxil- entrance upon starting a typical motor as listed in Table 3. Although this nomograph was designed specifically for the motor characteristics listed in Table 3, 2000 HP most motor starting characteristics fall close to those INDUCTION MOTOR listed for the various sizes. This voltage drop will in- (1760 KVA) clude the voltage drop through the distribution trans- MVA AVAILABLE FROM SOURCE former. The values of voltage drop which may be ob- DURING 3 PHASE FAULT tained from Fig. 5 are based on a 25-kva distribution (a) transformer and an eight-inch conductor spacing. For distribution transformers of other ratings, the value obtained from Fig. 5 must be added algebraically with a correction factor as listed in Table 4. For other conductor spacings, simply multiply the distance D as indi- Xe EQUIVALENT TO 400 MVA cated in Fig. 5 by the ratio of the impedance per unit length for the actual spacing divided by the impedance per unit length for the eight-inch spacing, and use this 1.0 pu 4000 KVA TRANSFORMER 5% IMPEDANCE new distance D when entering Fig. 5. Example-Calculate the percentage voltage drop at the 2000 HP MOTOR motor service entrance upon starting a five-hp. 230-volt 16% IMPEDANCE motor connected to the distribution transformer with 1/0 aluminum wire, when the motor service entrance is (b) located 200 feet from the distribution transformer. The solution is as follows: Determine the inrush current

Table 4-Transformer Impedance Correction Factors

Voltage Drop to Be Added to Results of Fig. 5 Motor Size .0.01 pu 1.0pu 230 v. 115 v. 0.05 pu 36 Transformer Vst(pu)."---=.86 OR 86% kva U hp 2 hp 3 hp 5 hp 7% hp % hp % hp .42 Vst 10 +.61 +1.64 +1.69 +2.65 +4.16 +2.48 +4.43 15 +.20 + .57 + .70 + .94 +1.50 + .80 +1.19 25 0 0 0 0 0 0 0 37% -.13 - .32 - .36 - .54 - .84 - 49 - .73 (c) 50 -.28 - .67 - .61 -1.09 -1.63 -1.14 -1.48 Fig. 6-Sample calculation to determine voltage (Vst) when 75 -.31 - .81 - .74 -1.31 -1.99 -1.35 -1.77 starting a large motor or group of motors.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 357/576 350 Voltage Fluctuations on Power Systems iary motors, or where the industrial plant is operating portion of the supply reactance. The various considera- with only a partial supply, due either to maintenance or tions necessary and the operating problems for a series a supply system failure. capacitor installation for motor loads are discussed in The largest motor or group of motors in the plants detail in this chapter and in Chapter 8, respectively. that may be started simultaneously should be calcu- Other remedies not associated with the supply design lated. Starting currents for both induction and syn- may be economically feasible. Some of these are reduced chronous motors with full voltage starting vary from voltage starters for starting large motors, or for recip- five to ten times full load current, depending upon the rocating or intermittent loads employing a driver design size and other application requirements, such as re- which incorporates special characteristics into the motor quired starting, pull-in, and pull-out torques. In order and provides an adequate inertia constant for the com- to arrive at a pessimistic answer, it is common to as- bined motor and load. Three examples indicating the sume that the starting current is all reactive. If specific need for studying these motor applications individually data is available on the power factor under locked rotor are given in Section 4—Miscellaneous. conditions, this information may be used when obtain-. able. 2. Electric Furnaces Fig. 6 indicates a sample calculation of the voltage Of the three types of electric furnaces—resistance, dip upon starting a 2000-hp induction motor or a group induction, and arc—the last type usually causes a volt- of motors simultaneously totalling this hp in a plant age fluctuation problem. With increased demands for supplied by a 4000-kva step-down transformer. A simple high-grade alloy steel, more utility companies are being voltage divider analysis is made assuming the starting asked to design a power system to supply these arc current is all reactive. The voltage drops to 86 per cent furnace loads. This load is considered to be a good rev- upon starting this amount of motor capacity, or starting enue load in spite of its fluctuations because of its high results in a 14 per cent dip. If starting is very infre- load factor and its high power factor during refining quent, this would be a permissible design. operation. Fig. 7 is an installation picture of an arc Motor loads which vary cyclically with each power furnace. stroke and produce a corresponding variation in line Most of the furnaces in the past have been relatively current can cause an objectionable variation in voltage small, but a number rated 25,000 kva are already in if the pulsation rate is around six to twelve times per service and larger ratings are being discussed. It is also second.g This type of load usually consists of large air significant that of these several 25,000-kva installations compressors and pumps. Even though these types of already operating, furnace loads as high as 45,000 kva loads are usually driven by induction motors, the slip have been reported. This step-up in production of the of the motor cannot prevent load fluctuations from showing up on the supply lines, unless the inertia of the load is high or the rate of the power pulsation is high. With this type of load it is not the magnitude of the load or current that is of concern, but rather the change in load. This change of load occurs not only as a current variation, but the power factor also pulsates when the motor load varies. Motor-driven intermittent loads are another category where the nature of the work calls for heavy overloads and for cyclic loads of long and irregular periods. Saw mills and coal cutters are typical examples of applications where heavy overload, sometimes to the stalling point, is common and difficult to prevent. The motor currents in such installations vary rapidly from light load through pull-out at heavy current and high power factor to the high locked rotor current at low power factor. Punch presses and shears are examples of applications where the load will show wide variations, but here industry has applied flywheels and other design features to limit both the application of load and magnitude of the load swings. In general, the three types of problems associated with motors (starting reciprocating loads and intermittent loads) can be alleviated by reducing the reactance of the supply system to these industrial loads. The most common method is to provide an additional supply circuit, or to provide additional transformer kva in the supply substation to the industrial plant. Another alternative is to employ a series capacitor to cancel out a Fig. 7—Three-phase steel melting arc furnace.

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VOLTS-PHASE AB 11,000 V.

IS WATTS-AMPS AS-VOLTS AB

6.0MW 745 MW 6.0MW U IBLE VOLTAGE HA E • r .-. • “••=1.••••••• 515 ---- 500 400 AMPS AS 480 480

AMPS BS -- 540 AMPS OS 1:38PM.

Fig. 8—Oscillogram at start of heat in a 10,000-kva three- phase arc furnace. A single phase arc struck and restruck 10 times in the space of 15 seconds before all three phases struck. After this initial period all three phases struck and restruck 10 times with currents in all three phases fairly well balanced before the arcs became generally stable. A portion of this performance is shown on this figure". furnaces is accomplished by raising the arc voltage, re- AMPERES -P ASE ducing the series reactance to the electrodes, raising the settings of the electrode regulator, or by a combination of several of these procedures. The operation of an arc furnace has two periods: the melting down period and the refining period. During the melting down period, pieces of scrap will, from time to time and at random, bridge the electrode and approximate a short circuit on the secondary side of the furnace transformer. Consequently, the melting down period is characterized by violent fluctuations of current at low power factor, single phase. When the refining period is reached, the steel has been melted down to a pool and arc lengths can be maintained uniformly by the automatic electrode regulators, so that stable arcs can be held on all three electrodes. The refining period is, therefore, characterized as mentioned above by a steady three-phase load of high power factor. Therefore, the utility's concern is confined to the melting down period of the arc furnace operation when the wide load variations cause objectionable voltage fluctuations and the resultant objectionable lamp flicker. Fig. 9—Graphic charts at time of same heat shown on The oscillogram of Fig. 8 represents a short part of a oscillogram of Fig. 8. Furnace swings occur approximately melting-down period of a 10,000-kva arc furnace. At once a second". times, the current variations occur at a periodicity ap- proximating the rate of the most objectionable flicker. twice normal at 50 per cent power factor. The effective A graphic chart illustrating the variation of load over a impedance of the arc (based on 11,500 volts in the pri- longer period of operation is shown in Fig. 9. These two mary) is plotted as the abscissa. For convenience, zero figures are reprints of figures from Reference 11. ohms, as plotted, represents the minimum arc resistance Calculated curves in Fig. 10 show the electrical char- as determined by the so-called short circuit condition. acteristics of a 10,000-kva, three-phase arc furnace. Actually, at this point there is appreciable voltage drop These curves were prepared on the assumption that the at the electrode tips, and considerable arc energy; the maximum attainable current would be approximately curves are plotted in this manner only to show the work-

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 359/576 352 Voltage Fluctuations on Power Systems

TAKEN FROM CURRENT CHART FOR 25 HR PERIOD (10.45 MRS FURNACE OPERATION ) 20,000 100 POWER FACTOR 25 000 KVA 400 VOLT TAP 4

In ---"------s- f:4 15,000 75 CI 0 KVA 0 N 0.0 q KW CC Li 10,000 50 1.1 0 3 O O -1 a. in -..

5,000 0 25 2 3 5 7 10 20 30 50 70 102 NUMBER OF SWINGS PER HR GREATER THAN CURRENT INDICATED

Fig. 12—Current swings for an electric arc furnace. 0 0 5 10 15 20 25 ARC RESISTANCE—OHMS Fig. 10—Electrical characteristics of a 10,000 kva, three- The current swings in a 25,000-kva electric-arc fur- phase arc furnace. nace versus frequency of occurrence are given in Fig. 12. This data was taken over a 25-hour period of time in which the actual time of furnace operation was 10.5 ing range. It is of interest that the point of maximum hours. (Data on the power factor during the heaviest power is not that of maximum kva. The usual melt- current flow is rather meager as an instantaneous watt- down range is probably between the points correspond- element oscillograph is required for the measurement.) ing to 0 and 10 ohms, the arcs fluctuating during this Available information indicates a range of 40 to 60 per period so that the heating refining range is probably cent power factor for this "short circuit" condition, the above 10 ohms. larger furnaces tending toward the higher power factors. It is difficult to obtain definite figures on the values of As illustrated above, the prediction of the frequency, instantaneous swings in current and power factor for magnitude, and power factor of load swings for a con- use in flicker determinations, because an oscillograph templated furnace installation is extremely difficult,. must be used and the maximum swings cannot always Designing a system to supply this load and still prevent be caught. On larger furnaces the short circuit impe- objectionable lamp flicker necessitates the use of em- dance from the high voltage bushings of the furnace pirical data which are, at best, doubtful. While a num- transformer to the shorted electrodes is approximately ber of excellent references on the subject are available, 30 to 40 per cent based upon the furnace transformer's there is usually some doubt about what flicker voltage maximum rated kva. The approximate distribution of limit should be used Many of the references on this this impedance among the various elements of the fur- subject use the short-circuit capacity of the bus supply- nace is shown in Fig. 11. ing the furnace transformer as a yardstick for judging the adequacy of the utility supply system. However, this FLEXIBLES ELECTRODES 0.15 0A yardstick is not valid except in the minority of cases 0.05 p where the electrically nearest lighting load is connected to the same bus which supplies the furnace transformer. In order to correlate the furnace size and system re- F F f, quirements from a voltage flicker standpoint, a different :4g15- 'I and more fundamental quantity than short circuit mva

FURNACE TRANSFORMER 0.05 5.0 4 SOURCE 1.0

FURNACE

A O 0 0 0 OA X I I fXX23+ X3 0 0 0 0 0 / 0 0 0 0 0 B OO 00 00a 0 0 0 0 0 X3 (Xi+X2) LIGHTING LOAD RANT PARALLELED ONE OR TWO PIPES CONNECTING X m m - CABLES PLEXIBLES TO ELECTRODES XI +X2 +X3 SPACING .2. 30 IN. X3 X1 Fig. 11—Approximate distribution of impedance among X mn +x2 +x3 various elements of arc furnace installation. Based upon three-phase symmetry and equal electrical lengths to elec- Fig. 13—Illustration of method to determine the mutual drop trodes. Base kva is the furnace transformer rating. coefficients.

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Reference to Fig. 13 shows that for a given load swing SINGLE FURNACE c0 at the furnace, the flicker voltage at the lighting load is •4 .04 proportional to X., and the flicker voltage at the fur- ONE nace bus is proportional to Xmm. Since the flicker volt- M• D3 —,•0c,so age at the lighting load is the important one, it is ap- 2 0141°' parent that Xmm, which is the reciprocal of short circuit 00 61' o D2 kva at the furnace transformer bus, is of no importance U) ElOki 2, z 2 Ea c•'0•• 1.•••• unless lighting load is tapped directly on that bus. In El i..ii,v'k locating point n, consideration should be given to the 09 166-0 possibility of having a future lighting load closer to the 0 DI LAG -tik 4 0 furnace installation than presently exists for any gen- z ,p0- CC 0 GI \°‘- eral three-terminal network. .007 .., 0 WO" t.) U- 0 The self and mutual drop constants can be deter- OS 0 005 0 0 mined analytically by network reduction or by the use 0 0 of a d-c or a-c calculating board, with the following 0 0 assumptions: voltage of generators and synchronous DO3 (D R condensers are equal and in phase, and the subtransient 10 20 30 2 5 reactance of all synchronous machines is utilized. Based RATING OF EACH FURNACE TRANSFORMER IN MVA on a recent survey by the AIEE Committee on System 0 NON-OBJECTIONABLE A BORDERLINE Engineering, Xma is plotted versus furnace transformer OBJECTIONABLE rating and limits are drawn above which objectionable Fig. 14—Plot of Xmn as function of furnace transformer flicker should be expected." This curve is shown in Fig. rating for single furnace installations. 14. Also, another curve is drawn to describe the region where objectionable flicker would not be expected, and the zone between these two curves indicates a border- should be used. This quantity is designated "mutual line area. Fig. 14 was drawn for single furnace installa- drop constant" and is identified by the symbol Xmn. tion. Figs. 15 and 16 are for two furnaces and three or Xmn is defined as the voltage drop from the internal more furnaces, respectively. It can be noted from an voltage of the system generators to the closest light- analysis of these curves that roughly a 10 to 15 per cent ing load with one per unit (based on furnace trans- reduction must be made in going from a single furnace former nameplate kva) drawn by the furnace trans- installation to a multi-furnace installation. former. The voltage drop to the furnace transformer The limit curves in Figs. 14, 15, and 16 are drawn as bus is designated Xmm. The self and mutual reactances smooth curves. However, there are indications that the or drop coefficients (Xmm and Xma, respectively) are larger arc furnace installations can exhibit a phenom- illustrated in Fig. 13 for a simple system configuration. enon termed "cyclic flicker," which is not observed in

to CO TWO FURNACES (.0 THREE OR MORE FURNACES D4 CO tr • D4

2 03 CC D3 La 6- (i) ....- Z D2 o• 02 s...*,_,. ip CC to 00 ENE Ct ; F- Z .01 id .01 CP A 1 ▪CC 00" z C A Z DO7 E .007 ,,,r..e4- 0 0 sc;c`°.- - DO5 • .005 ,,,oe-0 0 0"c” 0 cr 0 0- .003 .003 z 2 5 10 20 30 0 2 5 10 20 30 z RATING OF EACH FURNACE TRANSFORMER IN MVA E RATING OF EACH FURNACE TRANSFORMER IN MVA C O NON-OBJECTIONABLE E O NON-OBJECTIONABLE A BORDERLINE A BORDERLINE O OBJECTIONABLE O OBJECTIONABLE

Fig. 15—Plot of Xmn as function of furnace transformer Fig. 16—Plot of Xmn as function • of furnace transformer rating for two furnaces at the same location. rating for three furnaces at the same location.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 361/576 354 Voltage Fluctuations on Power Systems the smaller furnaces. The limit curves of Fig. 14, 15, and densers, series capacitors, and buffer reactors are shown 16 are shown as solid lines for furnaces from 1000 to in Fig. 17. 15,000 kva. Since above 15,000 kva there is some evi- The voltage dip on a power system resulting from a dence that "cyclic flicker" voltages at a rate greater suddenly applied or suddenly changed load, as in an arc than one per second will be superimposed on less fre- furnace, and its resulting flicker may be reduced by re- quent flicker voltages, the limit curves are shown dashed ducing the system impedance. Usually, the system im- above 15,000 kva. There are not a sufficient number of pedance is predominantly inductive, and flicker is caused these large furnaces which have experienced trouble by current of low power factor, so that most of the due to cyclic flicker to permit the development of definite voltage drop is due to the reactive component of the limits of X. for these large sizes. This will become an system impedance. This predominance of reactive com- increasingly important problem in the future as even ponents has led to frequent proposals to use synchronous larger furnaces are required for steel making; unfor- condensers in parallel with the system as a means of tunately, the problem cannot be resolved at this time, reducing system reactance and thus improving flicker and caution should thus be exercised in applying the conditions, as shown in Fig. 17 (a). This method, while data of these three figures for these large furnaces. feasible in principle, is not usually economical in prac- Engineering a satisfactory electrical supply system tice, since the system reactance based on the industrial far an arc furnace installation still cannot be described customer's kva demand will probably be very low. This as a rigid science. For the small and medium size fur- means that in order to reduce the system reactance by naces, it is believed that the curves illustrated in Figs. paralleling it with the reactance of a synchronous con- 14-16 are an excellent reference to determine whether denser, the kva-rating of the synchronous condenser any flicker problem should be expected. If flicker trouble must be at least several times the kva of the load. is incurred, the solution will probably require either The effectiveness of a synchronous condenser can be stiffening the supply system, or the installation of syn- much improved by the use of buffer reactors between chronous condensers, buffer reactors, or series capaci- the power system and the furnace load, and by operat- tors. Both of these methods can be, very costly, with ing the condenser from the furnace bus as shown by even a few tenths of a volt reduction of flicker possibly Fig. 17 (b). This scheme permits greater voltage fluctua- costing hundreds of thousands of dollars. tions on the condenser, and therefore causes it to bear a Where special corrective measures are required, the greater proportion of the fluctuating component of cur- separate radial line is used more often than any other rent. The industrial customer's bus voltage, of course, corrective means. This type of supply accomplishes two undergoes the same voltage fluctuation; and this fact, purposes: first, it makes other flicker sensitive loads plus the fact that only a limited amount of series re- electrically remote from the furnace bus, and second, it actance can be used without unstable synchronous con- brings the furnace electrically closer to the utility elec- denser operation, limits the extent of improvement. In trical supply source. The various remedies which are most instances it is likely that a reduction of flicker to used with arc furnaces that include synchronous con- one-half of its uncompensated value is the economic limit of correction by this means. Where this amount of correction is sufficient, the synchronous condenser and series reactor scheme may be the best economic solution f UTILITY SOURCE considering the power factor correction and the voltage control afforded by the machine. Another way to de- SYSTEM IMPEDANCE crease the system reactance is to use capacitors in series with the electrical supply line serving the furnace bus. BUFFER SYNCHRONOUS Fig. 17 (c) shows a layout for the application of series REACTOR CONDENSER capacitors.19 Capacitors in the line as shown in this FURNACE figure benefit only those customers beyond the point of (a) capacitor installation. Another alternative which uti- FURNACE lises the advantages of Figs. 17 (a) and (c) is shown in ( b) Fig. 17 (d), where a series capacitor is placed in series with the synchronous condenser leads. This capacitive reactance partially nullifies the synchronous condenser's inductive reactance, giving a lower net reactance which will be placed in parallel with the system reactance. This scheme theoretically should be quite effective and economical. While the series capacitor may cause the SERIES CAPACITOR synchronous condenser to hunt, several installations SYNCHRONOUS 11 FURNACE with this form of compensation have been made and TO OTHER FURNACE CUSTOMERS CONDENSER satisfactory operation obtained.''-20 This remedial measure of reducing the reactance of one of the two (c) (d) parallel paths supplying the sudden changes in furnace Fig. 17—Various schemes for improving the flicker perfor- load usually can be proved only for the very difficult mance by compensating for the system inductive reactance. voltage flicker problems.

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because of their large size and wide variation of the 3. Electric Welders loads. One of the foremost problems with these shovels Due to their load characteristics and wide spread use, occurs when the operator over-estimates the bite of the this is a class of equipment of great importance in volt- shovel and stalls the large induction motor driving the age fluctuations on power systems. Most welders have generator which supplies other motors on the shovel. a smaller "on" time than "off" time, and consequently, For the few seconds it takes the operator to back off the total energy consumed is small compared with the the shovel, a large reactive load is drawn from the sys- instantaneous demand. Fortunately, most welders are tem with very little useful power. The fast rate at which located in factories, where other processes require a this reactive load is applied to the system is usually in- large amount of power, and where the supply facilities jurious to the power system, principally by creating a are sufficiently heavy, so that no flicker trouble is ex- wide band of voltage fluctuation rather than flicker as perienced. In isolated cases, but nonetheless important, it is commonly encountered. The site of mining opera- the welder may be the major load in the area, and seri- tion is often at out-of-the-way locations where the ous flicker may be imposed on distribution systems ade- power requirements for general purposes are small. The quate for ordinary loads. normal power supply circuits are of low capacity, and Practically all welders in service are single phase and the lighting load is usually from the same bus or only the most commonly used welder is the resistance welder. slightly removed from the shovel and thus, very suscep- Various types of electric welders are: tible to the dimming of lights due to the load change a. Flash welders caused by stalling the shovel. Fig. 18(a) shows a typical electrical system supplying a shovel bus; (b) indi- b. Pressure butt welders cates the reactive load drawn upon stalling of the motor; c. Projection welders and (c) indicates the pronounced voltage dips caused by this sudden application of reactive load to the power d. Resistance welders system. 1. Spot The remedial measures proposed to minimize the 2. Seam voltage flicker at the substation were: (1) install a syn- The characteristics of all these welders are described chronous condenser with a capacity of one-half of the completely in the AIEE Publication S-45. Basically, the source voltage usually 230, 460, or 2300 volts is stepped down to a few volts to send high GENERATING currents through the parts to be welded. Although STATION 69 KV TRANSMISSION LINE SHOVELS bridge rectifier circuits are used in resistance welders, 30 MILES some saturation of the welding transformer usually will TO PROPOSED exist. This effect, together with the short successive REST CONDENSER OF f pulses of high current, creates the objectionable voltage SYSTEM SHOVELS dips.

Corrective measures for voltage dips caused by LIGHTING LOAD welders range from motor-generator sets installed in the customer's plant to supply circuit changes by the utility. Supply circuit changes may be accomplished by increasing the feeder size or the substation capacity. 2 Separate lines to the welding load or series capacitors ahead of the affected lighting and welding load also are 4 possible economic solutions. As with all fluctuating loads, the object is to decrease the reactance between 2 3 4 the source and the lighting load. TIME IN SECONDS (b) 4. Miscellaneous Under this category comes special equipment, such as electric shovels, strip mining equipment, stone crushers, heavy rolling mills, and similar installations. Most of these must be considered individually as to 80 special features and the type of power supply required. CURVE I - VOLTAGE WITH NO REMEDIAL EQUIPMENT The following paragraphs describe three cases of voltage CURVE 2 - VOLTAGE WITH SYNCHRONOUS CONDENSER fluctuations. Each problem required a different am CURVE 3 - VOLTAGE WITH IMPROVED EXCITATION SYSTEM proach. An outline of the method of solution is given to 3 4 5 6 7 8 provide an insight into the characteristics that must be TIME IN SECONDS determined, in order to calculate the flicker voltage and (c) the effect of remedial measures. Fig. 18—Comparison of reactive load and associated volt- Electric Shovels Strip mining shovels frequently cause age dips for various corrective schemes upon stalling a large severe voltage dips on the power system, principally electric shovel.

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 363/576 356 Voltage Fluctuations on Power Systems peak vars drawn by the shovel in the stalled condition. gives the best representation of the amount of satura- (2) replace the slow direct-acting voltage regulator on tion (S) in the machine. The basic equation is then the nearby generating station with an electronic voltage modified to include saturation regulator. The latter solution of improving the voltage de' regulator is usually not suited to correcting rapidly e x= ed-{- (S due to e„)+ Tido (2) dlt fluctuating loads. Since these devices operate only when the voltage changes, there is a time lag before voltage is Substituting for ed in terms of i and ei, we obtain restored to normal, as shown by the dashed curved in d(e +i(xd'—zp) ex= ep-1-i(xd— xp)+ 8+ T'do P (3 Fig. 18 (c). As shown in this figure, the abrupt change d1 ) in voltage cannot be eliminated by the voltage regula- or upon simplifying tor, and this may be the one to which the human eye is di dep more sensitive. However, in order to compare solutions e x — i(xd—x p) — do a (xd,--zos—=ep+r do at (4) to relieve the long period of low voltage, a mathematical where: analysis of the transient voltages was made utilizing graphical methods. i = the armature current In order to make the cases investigated not too xa = the machine synchronous reactance laborious to carry through analytically, the generators xi were assumed to be at no load when a load approximat- , = the Potier reactance ing the stalling of one electric shovel was applied. The The above equation (4) is now in a form to be handled assumed load is composed strictly of vars and was to be by the step-by-step process since constant or known of constant current magnitude from one second to five varying quantities are on the left and a first order seconds and possessed a rate of rise and decay of 2000 differential equation is on the right. The follow-up meth- kvar per second. The idea of a constant current load on od was followed exactly taking account of the fact that the generators appears reasonable since, as the voltage di drops due to the increase in vars because of the stalled dt .2 pu/sec from 0 < t <1 and 5

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was assumed with an effective excitation system re- The quadrature component of the above equation can sponse ratio of 0.5. Although the synchronous condenser usually be neglected because it causes a phase shift possessed a regulator for the results shown in curve 3 of between the sending end and receiving end voltages at Fig. 18, it was assumed that the exciters of the gene- negligible voltage drop. Thus, the equation may be rators held constant excitation as explained for curve 1. simplified to: In order to calculate the division of load between the sin 0 generator and condenser, the voltage behind transient lEei — lErl — =1/1R cos 0+IIIXL (8) reactance for each machine was determined at each step. The impedance of short, heavily-loaded distribution Using these voltages and the associated reactances, the feeders usually is so low that changes in current drawn current division between each machine was obtained by a motor causes a negligible flicker. However in the and a separate step-by-step calculation carried out for cases under consideration, the feeders are long and the each machine. relatively-high impedances to the load causes high The results of the mathematical analysis of the three flicker voltages. For these cases or others where the particular cases studied for this system are shown in welder or flicker producing loads are a large portion of Fig. 18. Curve 1 indicates that the system as it now the feeder loading and are electrically remote from a is equipped and operated could experience long voltage source, a series capacitor installation can reduce the dips of duration equivalent to the load cycles and a mag- fluctuations at the lighting load. If a series capacitor is nitude around 20 per cent at the 13.8-kv bus. With an added to the circuit, Equation 8 becomes: improved excitation system on the generators the voltage at the step-down substation is seen to dip only four ri = I/IR cos 0+ jil (XL — X.) sin 0 (9) to five per cent upon application or removal of severe If the capacitive reactance (X.) is made equal to the load cycles and the duration is only a fraction of the load inductive reactance of the circuit, the second term of cycle. The solution proposed by adding a synchronous Equation 9 becomes zero. Thus, the voltage drop due to condenser of 2500 kva (half of the reactive requirements reactance is zero, and the circuit behaves as though at the 13.8-kv bus) limits the voltage variation to six only resistance were present. If the swing power factor per cent and since the generators were assumed to still caused by the intermittent load is low and reasonably possess direct-acting, rheostatic regulators, these gener- constant, the capacitive reactance of the series capacitor ators tended to dampout any oscillations on the system. may be made larger than the line inductive reactance. Curve 2 gives evidence of this damping action. It This causes the second term in the right of the last equa- appears then that either solution will affect a consider- tion to become negative and thus subtract some of the able improvement in the system performance and the resistance voltage drop. By suitably choosing Xe, all of proper weighting in terms of economies, future plans the inductive voltage drop and all or part of the resist- and loads must be applied to each solution to evaluate ance drop may be cancelled. This cancellation, however, their relative merits. is complete at only one power factor and is less effective Crusher Motors—Other operations which subj ect the at higher and lower power factors. Therefore, to effec- distribution system to extremely varying loads are saw tively cancel the resistance drop, the power factor must mills, stone quarries, etc. These industrial operations be known accurately. The idea of cancelling out all or a utilize large motors and are commonly located on long portion of the resistance voltage drop due to the distribution feeders serving both residential, rural, and fluctuating load is pertinent on distribution feeders, industrial customers. An example is a crusher motor since for long feeders the resistance may equal the re- which may be operating at light load when a large stone actance of the feeder, and in order to effect any suitable suddenly dropped into the crusher raises its load to reduction in flicker the resistance voltage drop must be several times its rating. It then drops suddenly to light cancelled out. Suitable checks on the application may be load after the stone is crushed. These sudden changes in made by assuming various swing power factors for the load result in large changes in motor current. These load and determining the flicker voltage. varying currents cause the fluctuating voltage drop in Once the value is obtained for X0 in ohms, the various the distribution feeder supplying the motor. unit voltage and kva ratings available commercially This fluctuating voltage in one phase of a lagging should be checked to determine the nearest impedance power factor circuit is given by the following equation: value available. A check should then be made to determine whether this voltage rating is suitable, and the sin 0) lE.1-1E rl= (VIR cos e+Inx (7) two calculations given below should result in voltages +j(IIIXL cos e—iiiR sin 0) equal to or less than the voltage rating assigned the where capacitor. The first check should be obtained by multi- E.= sending end voltage (substation bus) plying the required capacitive reactance by the max- E r = receiving end voltage (voltage at resi- imum expected sustained current to determine the volt- dential tap on feeder) age across the capacitor. A second check should be made I = flicker current (momentary current minus by multiplying the required capacitive reactance by the steady load current) maximum expected momentary current and then R= total circuit resistance dividing by two. X L = total circuit inductive reactance The series capacitor installation must be made be- 0= power factor angle of the flicker current tween the substation transformer and the residential or

Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 365/576 358 Voltage Fluctuations on Power Systems lighting tap. Placing the capacitor between the residential load and the crusher motor will not reduce the flicker voltage at the lighting tap, since this latter installation does not reduce the impedance between the utility source and the lighting bus. This application of a series capacitor is shown in Fig. 19. Another point to be noted • from Fig. 19 is that the series capacitor must be large enough to carry all loads beyond its point of installation. Consequently, if the flicker producing load is small as (a) compared with normal load, the cost of the series capacitor becomes too high for the correction obtained. Series capacitors are, therefore, economical primarily where the flicker load is a large portion of the total, where the circuit resistance is equal or lower than the reactance, where the flicker producing load is of low power factor, and where the supply circuits are fairly long. There are a number of problems associated with the application of series capacitors, including ferroresonance, subsyn, (b) chronous motor operation, and the requirement for Fig. 20—Typical system voltage oscillographs (a) before reasonably accurate knowledge of the power factor of the addition of a series capacitor (b) after the addition of a the flicker load. These problems are discussed in series capacitor. The top chart in both (a) and (b) is a record Chapter 8 on capacitors. of the source side voltage, while the lower chart in each in- Consideration of single-phase loads indicates that the dicates the load side voltage. application of a series capacitor possibly could be made only to the two phases involved in supplying this single- in reducing voltage fluctuations, Fig. 20 indicates the phase load. Certainly if the power system can carry the source side and load side voltages of an installation sim- unbalance caused by the single-phase operation, the fact ilar to the schematic diagram of Fig. 19. Figure 20(a) that there will be some series unbalance generated in the consists of the voltage charts without the capacitor in transmission line due to series capacitors in only two of operation, and 20(b) with the capacitor in operation. the three phases should not cause any significant unbal- The series capacitor was located as shown in Fig. 19(b), ance problem. just ahead of the lighting load. Although the capacitor As pointed out in Chapter 8 on capacitors, the series was installed to correct a flicker condition, the records capacitor installation should be protected by a shunting of Fig. 20 show that the capacitor has also raised the protective gap which will protect the capacitor in the general voltage level at the load. event of system short circuits or the appearance of a Rolling Mills—The large continous rolling mills, now ferroresonance condition. Further, provisions may be being used extensively in producing wide metal strip, made to apply also a shunting resistor for the series have imposed a new problem on the power industry. capacitor to damp out any subsynchronous resonance. Like the electric shovel, these loads do not necessarily However, in most cases the high resistance of the dis- produce flicker in the customary sense of the word. tribution feeder will tend to decrease any sub-synchro- The power supply is usually through motor-generator nous operation. sets without added flywheel effect. The load comes on To demonstrate the effectiveness of a series capacitor and drops off in steps as the metal enters or leaves the rolls. The other loads in the plant if coincident with the POWER 1_0_41 strip rolling can produce severe voltage fluctuations SYSTEM fi not only on the in-plant electrical system, but also the RESIDENTIAL (CRUSHER) utility system. The load characteristics for a typical LOAD mill are described in the following paragraphs and FLUCTUATING LOAD Table 5. (a) Table 5 lists the load characteristics for six typical loads within a steel plant. Of these load characteristics the reactive demand upon the system is of primary importance in determining voltage drops as shown in the POWER 0 IL SYSTEM previous illustrations. The other load characteristics SERIES serve as a calibration point in any attempt to apply this CAPACITOR data to other installations. Shown is the d-c motor RESIDENTIAL FLUCTUATING load which is a direct indication of the power required LOAD LOAD from the m-g set or the rectifier. Where an a-c motor is used to drive a mill directly, the load characteristics are (b) in per cent torque. In other cases, the conditions of d-c Fig. 19—Distribution feeders without and with series capaci- voltage and current are given. The product of these tors, illustrating the best series capacitor location. two figures is an indication of the torque required on

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Table 5-Typical Characteristics of Various Steel Rolling Mill Loads.

Condi- D.C. Motor A.C. Input tion Number %I %V KW KVAR KVA %PF LI-Plate Mill-8000-HP D.C. Motor Reversing Rectifier-6400 KW 1 0 0 45.3 -125 133 34 2 200 0 755 -18,900 18,900 4 3 200 100 13,600 -13,100 18,900 72 4 3 100 234 -348 419 56 Synchronous M-G Set-9000-HP 4 3 100 298 4,440 4,440 6.7 90% P.F. Motor 3 200 100 13,650 13,650 100 *23,200 -15,000 28,300 82 L2-Blooming Mill-6000-HP D.C. Motor Flywheel M-G Set-5000-HP Wound Rotor with liquid slip 4 3 100 300 - 1,720 1,740 17.2 regulator set at 130% current 3 200 100 6,550 - 4,040 7,700 85

L3-Non-Reversing Plate Mill 2-2500-HP Wound Rotor Motors. 3 300 - 1,720 1,740 17.2 100 4,000 - 2,640 4,790 83.5

L4-Roughing Mill Ahead of Hot Strip Mill-1800-HP Wound 3 104 -590 601 17.2 Rotor (Not to be used with rectifier drive on hot strip mill) 100 1,440 -930 1,730 83.5

L5-Hot Strip Mill Synchronous M-G Set-8400-HP, 80% P.F 5 3 100 300 6,040 6,040 5 6 175/6 100 2,120 6,050 6,420 33 7 175 100 11,200 3,210 11,700 96 Rectifiers-6 x 3200 KW 5 3 100 700 -1,030 1,250 56 6 175/6 100 5,600 -3,030 6,600 85 7 175 100 33,600 -20,800 39,500 85 L6-Tandem Cold Mill Synchronous M-G Set-7000-HP, 80% P.F. Motor 8 0 25 100 5,180 5,180 19.2 9 100 25 1,420 4,500 4,730 30 10 175 25 2,420 4,790 5,370 45 11 175 100 9,350 2,680 9,750 96 12 100 100 5,380 5,080 6,730 80 *Approximate transient peaks.

the m-g set. Input a-c power characteristics including mill is running at no load and the steel is entered into kw, kvar, kva, and power factor are given for each of the mill at full voltage, the load in the d-c machines the mills. builds up in approximately 1/10th of a second. Since The first load (Li) considered was the plate mill. the flywheel set must slow down in order for the wound The load characteristics are given for a 6400-kw re- rotor motor to draw power from the line, the ac input versing rectifier (this includes a forward and a reverse to the wound rotor motor takes approximately 1% sec- rectifier so that the voltage on the d-c motor can be onds to build up. The condition 3 is shown after the set reversed without use of armature contactors or motor has had sufficient time to slow down to a speed suitable field reversal). Information is given on two different to draw the required power from the a-c line. types of operation. The normal operation would be to Two 2500-hp wound rotor motors would be used to bring the mill up to full voltage and then enter the drive the non-reversing plate mill (L3). If a flywheel is piece as shown in Fig. 21(b) under conditions 4-3. Even used with these motors, the same condition as de- though it would not be good practice, information is scribed under L2 would exist where it would take a given for entering at zero voltage and accelerating the considerable time for the motor to slow down and mill while rolling. This is condition 1-2-3 in Fig. 21(a). draw power from the a-c line. It was assumed that these This condition of entering at zero voltage is given only motors would not be used with flywheels and that the for the rectifier since it is quite significant. This is the load would build up very rapidly. worst condition as far as poor power factor (4% and It is assumed that a roughing mill would exist ahead very high lagging kvar) is concerned. This type of oper- of the hot strip mill if a synchronous m-g set were ation has very little effect on the input kva of an m-g used to drive the hot strip mill. This roughing mill set of either the synchronous or the induction motor drive does not include a flywheel, and consequently, plus flywheel type. a-c power changes rapidly as rolling load comes into Under L3 are given the load characteristics for a the mill. blooming mill using a 5000-hp flywheel m-g set. The Several hot strip mill characteristics are indicated in load condition numbers apply to Fig. 21(b). When the L. First, a synchronous m-g set is shown to provide

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ENTERING AT 0 DC VOLTAGE ENTERING AT 100% DC VOLTAGE

0 -1 0 5 SEC 1 a M

b0O- 1^-1 SECOND-1 0

0 I 2 4

(0) (b)

TIME (b)

200

TIME (b)

Fig. 22—Transient electrical swings in the input to a synchronous motor generator mill drive for one complete pass.

requirements from initial conditions. Of particular importance is the fact that the kvar requirements of the ( c ) synchronous m-g sets are peaks of a transient condition similar to that shown in Fig. 22. These kvar swings can (d) be approximated from a circle diagram analysis of the Fig. 21—D-C load characteristics versus time for the various motor. Entering the circle diagram with the peak kw steel mill drives as listed in Table 5. swings of the motor will determine the peak kvar a) Plate Mill c) Hot Strip Mill swings under these transient loadings. b) Plate and Blooming Mills d) Four Mond Tandem Cold Mill Once the combined kvar swings of all mills within the plant are known, a simple analysis similar to that the drive after the roughing mill. Next, a six-stand mill described in Fig. 6 can be used to determine the initial with 4000-hp motors driving each stand is illustrated voltage drop. The equivalent impedance of the utility to replace the roughing mill motor and the synchronous system looking back from the steel plant bus should m-g set. A 3200-kw rectifier is used to feed each of these be determined neglecting resistance and using transient stand motors. The load conditions are shown in Fig. reactances for all synchronous machines. By converting 21(c) for full voltage on the stand motors under three the kvars required to an impedance on a given base different conditions. Five (5) shows the load with and using the voltage divider principle, or by converting nothing in the mill, six (6) shows stand 1 loaded, and seven (7) shows all six stands loaded. Fig. 21(d) indicates the load conditions for a four- Table 6—Initial Value of stand tandem cold mill (L6). This mill is assumed to be Flicker Voltages at Motor Supply Bus threaded at 25 per cent voltage and then accelerated. IR Flicker The conditions shown are with 25 per cent voltage and Load Conditions n P.U. On 10 mva In Per Cent nothing in the mill, all four stands loaded at 25 per cent voltage, all stands loaded and beginning the The plate and hot strip mill acceleration from 25 per cent voltage to full voltage, are synchronous m-g set, drive. All mills assumed to the peak loading condition just before full voltage is start at same time.. 2.256 10.0 reached, and the steady rolling condition. Same as I, except roughin In order to simplify the calculation of the initial and hot strip mill are voltage drop only the quadrature component of the placed by rectifier drive . . 2.42 10.7 load characteristics were used. If the resistance of cir- Same as II, except pia cuits supplying the motors is significant, the real and mill is supplied by rectifier ,s quadrature components of load current could be in- with zero voltage starting 2.26 10.1 cluded in the conventional manner. All Ioads were Same as III, except recti considered as being coincident and the kvars required fier is assumed to start a 1.66 from the system were obtained by subtracting full load 100 per cent voltage 7.4

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Voltage Fluctuations on Power Systems 361

kvars to a quadrature current and multiplying this In analyzing the problem faced by various equipment current by the system impedance to the motor terminals in industrial plants, two types of momentary voltage to obtain voltage drop, the fluctuation may be calcu- fluctuations are discussed: voltage dips and momentary lated simply. interruptions. With a system impedance of 0.0442 per unit on a 10-mva base and the load characteristics of Table 5, 6. Voltage Dips the flicker voltages shown in Table 6 were calculated. Faults or short circuits can occur on the utility system The difference between rectifier and synchronous m-g for many reasons, and the short-circuit currents, be- set drives in the plate and hot strip mill is indicated. cause of their magnitude, produce large voltage drops. By a voltage divider calculation the percent flicker at Every fault on the subtransmission system will cause the utility supply bus may be calculated. a voltage dip on the other lines connected to the same step-down station bus. The duration of the voltage 5. Comparison Chart drop will consist of the relay plus breaker operating A reference chart showing at a glance the remedial time, and the magnitude of the drop will be a function measures available and those most promising for a of the "stiffness" of the source to the step-down station particular type of load is shown in Table 7. Inasmuch and the remoteness of the fault from it. Even the fastest as the best technical solution may not be the most and most modern relays and breakers have combined economical, the remedies are compared from both times in excess of three cycles from time of fault to points. breaker opening. A more average figure, however, for faults remote from the station bus might be around Table 7—Comparison Chart of Remedial Measures 20 cycles, and for these longer durations the mag- 1

nitude of the dip will be proportionately smaller.

o Therefore, from an economic standpoint alone, it will

e itor a) a)

it ly 4 bp 43. be many years before system growth and equipment u denser ac ng , z ies G n. ts r >, 73 If irc changes can materially reduce the duration of voltage M-

Source of Flicker Con C Se Sy Cap Supp Cha Se 4' g dips. Motor Driven Loads CC A B A D The trend toward increased use of automatic ma- Electric Furnaces C A C A DD chinery and continuous process lines has greatly mag- Electric Welders AB A B DD nified the importance of momentary voltage dips on the Miscellaneous utility systems. This automatic equipment is usually Electric Shovels D B B A D B encountered in the newer industrial plants. Due to the Sawmills, Crusher, etc CC A B DD congestion in existing heavily industrialized areas, more A Technically Suited and Frequently Economical and more of these new plants are growing up on sub- B Technically Suited and Possibly Economical urban locations. This usually results in the industrial C Technically Suited and Rarely Economical D Technically Unsuited customer being subjected to a larger number of voltage dips, because the utility circuits will not be as well protected against faults in the suburban and rural areas as are the circuits in a heavily industrialized bAation. IV. EFFECT OF VOLTAGE FLUCTUATIONS The out-lying circuits are almost invariably of overhead UPON UTILIZATION EQUIPMENT construction and are subject to faults due to lightning, Although customers have known and experienced ice, wind, and other natural and man-made hazards, voltage fluctuations for many years, it was not until while the more heavily concentrated areas might well the advent of the automatic industrial plant that the be supplied by cable circuits. The newer automated plant shutdown due to voltage dips changed from a plants are much more subject to trouble from voltage nuisance to a sizable economic loss. To avoid these dips than are the more usual semi-automatic plants. A shutdowns in automated plants merits effort on the tremendous amount of industrial control equipment is part of the industrial power salesman and additional required in one of these automatic plants, and the unit capital investment on the part of the industrial customer. cost of the control equipment must be kept low. This The assumption by industrial customers of abso- results in a large number of magnetic devices which lutely continuous power supply is a difficult situation are operated on application of a control voltage and to combat. However, the industrial power salesman which return to the normal position when control should impress on the industrial customer that occa- voltage is removed. Included in this category are such sional system disturbances are going to occur and devices as contactors, motor starters, control relays, explain that disturbances cannot possibly be avoided magnetic chucks, electrically operated values, etc. without a tremendous increase in the cost of power to Since these devices are spring-operated to their normal all customers. This explanation should be given before position, they are inherently very fast in operation. To a new plant goes into service and might be mentioned this control equipment a momentary dip in supply from time to time even though no trouble develops. voltage looks like an interruption of control voltage; The aftermath of one of these disturbances is less hence, the magnetic strength is removed and the device unpleasant if the warnings and explanations have been trips to its normal position. carefully given beforehand. If the industrial control equipment trips because of a

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voltage dip on the utility supply to the industrial plant, overcome this particular problem employ voltage regu- the consequences may be quite severe. Continuous lating transformers and motor-generator sets to supply process lines, such as paper mills, tin-plating lines or the control voltage for the contactors. The utility can automobile assembly lines may be shut down com- minimize the occurrence of faults and their attendant pletely. Some of these processes require considerable voltage dips by improvement in their relaying coordi- time to get back in operation again and substantial nation and by extending ground wire protection against production can be lost even if there is no damage to lightning for their subtransmission circuits. equipment. Extensive damage to tools may result due Another load affected by voltage dips is the mercury to unexpected tripping of the driving motor or to the lighting within large industrial plants. Once a mercury release of a magnetic chuck. Automatic sequencing light is extinguished, it takes approximately eight min- equipment, such as associated with industrial digital utes to get it started again. As with the simple motor control, may entirely lose sequence during one of these contactor, the effect of voltage dip on mercury lamps voltage dips. The consequences for the utility are also has considerable variation due to ballast, operating con- severe because of the loss of customer goodwill following ditions, lamp life, etc. However, with good ballast and one of these shutdowns. To date, this problem has been average lamp life, almost all high-intensity mercury primarily concentrated in those areas of the country lamps will drop out in two cycles upon complete loss of where automated factory have become so numerous. voltage. For dips of less than 100 per cent, there is con- However, as automation has spread into other parts siderable variation, but most lamps will normally with- of the country, troubles have begun to develop on stand dips of 25 per cent of rated voltage. The time fac- utility systems that had practically no complaints to tor for these partial dips has a wide diversity. Thus, the the voltage dips just a few years ago. A survey of operating characteristics of these lamps may or may troubles due to outages and voltage dips of short dura- not be compatible with the particular power system tion indicates that the largest source of complaints is supply. Regardless of the improvement in the supply from the effects of momentary disturbances on motor design, there will always be some circumstances where undervoltage releases and from loss of production rather the industrial plants may be faced with a complete loss than damage to equipment."-zi The number of com- of voltage or a sufficiently severe fault that the voltage plaints is certain to increase as more and more indus- dip will be adequate to extinguish the lights. As a con- tries convert to automatic machinery. sequence, in large industrial plants with straight mer- Perhaps one of the most important controls which is cury lighting, emergency incandescent lighting should subject to voltage dip is the simple motor contactor. be provided. Contactor drop-out has not been recognized in the past as a serious industry problem and no mention of 7. Momentary Interruptions drop-out voltage or time is made in the NEMA Stand- Many of the faults occurring on power systems are ards. As a result, no particular emphasis has been transient in nature, and if the circuit is opened momen- placed on this problem during the design of the tarily, permitting the arc to become extinguished, the contactors. circuit can be reclosed successfully. Over the years it Contactors may have a drop-out voltage of as low has been found that the number of successful reclosures as 30 per cent or as high as 70 per cent of rated voltage average around 85 per cent of the number of trip-outs. depending on the manufacturer, type and size, charac- This knowledge is used by the electric utility in a variety teristics of the particular batch of magnetic steel from of ways. Many of the radial subtransmission lines sup- which the armature is made, the degree of contact plying industrial plants are provided with reclosing re- between movable and stationary armatures, etc. This lays. A very common arrangement provides for one im- value varies greatly among different contactors of the mediate and several time delay reclosures. In the event same type and the same manufacturer. In fact, the of a trip-out after the third reclosure, the line is locked drop-out voltage for a particular contactor will also out until the relay is reset manually. However, if the vary from time to time, depending upon the change line holds even on the third try, the reclosing relay re- in armature seating and contamination conditions. De- sets automatically and is prepared to repeat the same pending on all these conditions, the contactors may performance at a later time. drop out in as short as three-quarters of a cycle to as "Single shot" reclosing is also widely used and, as in- long as 12 cycles. Assuming the voltage dip is due to a dicated by the data in the above paragraph, it takes fault on the transmission or subtransmission system, care of the large proportion of the cases. Thus, the use the short drop-out time makes it difficult or impossible of reclosing to supply lines serving important industrial for the fault to be cleared in a sufficiently short time loads keeps the systems from going out of step or pre- to prevent contactor drop-out. vents the loss of essential loads. However, the use of At present no completely satisfactory solution has high-speed reclosing (in the order of 20 to 25 cycles) on yet been devised for the voltage dip problem and its ef- subtransmission and distribution lines supplying indus- fect on automatic equipment. In some particular cases trial loads consisting primarily of motors can impose a latched-in relays and latched-in contactors have been severe duty upon the motor. specified. However, this feature may not be acceptable Consider, first, the induction motor. When the cir- to many industrial customers because of the safety haz- cuits supplying a running induction motor are opened, ards which might result. Various other means used to the motor will regenerate a voltage that will appear

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across its terminals because of the trapped flux and the REFERENCES shaft speed at the instant of power interruption. The 1. Flicker Effects of Single-Phase and Three-Phase Motors rotor flux will decrease exponentially according to the Compared—Bryce Brady, Electrical Engineering, January, open circuit time constant of the motor. Thus, the 1959, pp. 58-62. residual voltage appearing at the motor terminals will 2. Motor Starting Lamp Flicker on Open-Delta Transformer Banks—J. C. Neupauer and C. L. Smith, Jr., AIEE Trans- initially be almost equal to the supply circuit voltage actions, Vol. 77, pp. 1568-1576, 1958. and gradually decay with time, being at any instant 3. Chart for Secondary Voltage Drop with 230 V. Single-Phase proportional to the shaft speed at that instant and the Motors—J. M. Gilkey—Electric Light and Power, pp. 88-89, rotor flux. August, 1955. When power supply to an industrial bus is lost, mo- 4. Flicker Level Guides Distribution Design—W. A. Lindberg, tors with different time constants, inertias, and mechan- Electrical World, p. 69, July 2, 1951. ical loads are left connected together. Motors with a 5. The Effect of Cyclic Voltage Fluctuation on Television Re- slow decaying internal voltage will then supply power ceiver Operation—R. E. Clawson and W. F. Bloom, Pre- sented at Meeting of Transmission and Distribution Com- to motors having rapidly decaying internal voltage, and mittee, EEI, October 9, 1958. the voltage appearing on the bus will decay at a rate 6. Correlation of Voltage Dip Design Standards With Flicker which will be a composite of all the motors driving all Complaints—C. E. Bathe, presented at Meeting of Trans- sorts of equipment with varying torque characteristics. mission and Distribution Committee, EEI, October 9, 1958. The possibility that there may be power factor correc- 7. Permissible Flicker Limits—L. Brieger, presented at Meeting tion capacitors and cable capacitance involved further of the Transmission and Distribution Committee, EEI, aggravates the slow voltage decay rate, because this October 9, 1958. 8. The Visual Perception and Tolerance of Flicker—prepared shunt capacitance tends to self-excite the motors and in by Utilities Coordinated Research, Inc., New York, 1937. some cases over-excite the motors to the extent that 9. Power Company Service to Arc Furnaces—L. W. Clark, voltage may be higher for quite some time following AIEE Transactions, Vol. 54, pp. 1173-7, 1935. loss of power." These capacitors, besides delaying the 10. Arc Furnace Loads on Long Transmission Lines—T. C. residual voltage decay, will increase the asymmetrical LeClair, AIEE Transactions Paper, Vol. 59, 234-9, 1940. inrush current when the motors are re-energized. 11. Large Electric Arc Furnaces—Performance and Power Sup- Normally, motor inrush currents are determined by ply—B. M. Jones, C. M. Stearns, AIEE Transactions, Vol. 60, the motor locked rotor impedance and source imped- pp. 763-9, 1941. ance. In the case of reconnecting to the line or an al- 12. Considerations in Supplying Power to Arc Furnaces—C. M. Starr, 0. B. Falls, AISE Yearbook, pp. 463-74. ternate supply, motor inrush will be higher than normal 13. Transmission and Distribution Reference Book, fourth edition if the motor residual voltage is relatively large and con- —Westinghouse Electric Corporation, 1950, pp. 719-740. siderably out of phase with the alternate supply volt- 14. Electrical Supply for Arc Furnaces—R. F. Lawrence, R. L. age at the time the motor is re-energized. The highest Tremaine, AIEE Conference Paper, Summer General Meet- inrush will occur for the 180-degree phase difference being, 1952. tween the voltages. Since motors are designed to start 15. Voltage Dips and Flicker—A. A. Kroneberg, AIEE Trans- satisfactorily many thousands of times during their life, actions, Paragraph II, Vol. 75, pp. 349-52, 1957. commercial practice has indicated that a residual volt- 16. Survey of Arc Furnace Installations on Power System and Resulting Lamp Flicker—System Planning Subcommittee age of 25 per cent of normal can be safely permitted Report, AIEE Transactions Paper 57-9. upon re-energization of the motors. It is felt that this 17. The Application of A Series Capacitor to a Synchronous low value of motor residual voltage will result in inrush Condenser For Reducing Voltage Flicker—P. M. Black, L. currents only slightly larger than normal blocked rotor F. Lischer, AIEE Transactions, Vol. 70, pp. 144-50, 1951. current, and thus will not seriously jeopardize the fu- 18. Two Large Electric Arc Furnaces—Electrical Characteristics ture life of the motor. and Corrective Equipment—S. W. Luther, J. D. Ghesquiere, This inrush problem is predominant with the larger C. E. Quick, AIEE Transactions, Vol. 74, pp. 1401-1406, 1955. horsepower motors. Since most motors used in indus- 19. A 10,000 KVA Series Capacitor Improves Voltage on 66 KV Line Supplying Large Electric Furnace Loads—B. M. Jones, trial plants are of smaller size and have time constants C. M. Stearns, J. M. Arthur, A. A. Johnson, AIEE Trans- in the order of cycles, small and medium size motors in actions, Part Vol. 61, 1948, p. 345. general can be reconnected to the line without any pre- 20. Technical Problems Associated with the Application of a caution or significant reduction in motor life. Capacitor In Series With a Synchronous Condenser—R. L. When the circuit breaker for a line supplying power Witzke, E. L. Michelson, AIEE Transactions, Vol. 70, 1951, pp. 519-25. to a synchronous motor is lost and a negligible amount 21. PoNier Supply For Resistance Welding Machines—A IEE of other loads are connected to the motor bus, the syn- Publication S-45. chronous motor acts as a generator. Under these con- 22. Flicker From Single Phase Welder Load Predictable—S. ditions, the motor can convert stored rotational energy Freeman, Jr. and J. S. Francis, Electrical World, p. 56-7, into electrical output by gradually slowing down. If the June 24, 1944. feeder breaker were to reclose and re-energize the motor 23. Voltage Translator Scheme Cuts Light Flicker Due to Weld- while it was out of synchronism and field current still ers—R. 0. Askey, Electrical World, pp. 63-5, January 6, 1945. applied, severe current and torque oscillations would 24. Power Factor Correction of Resistance Welding Machines —L. G. Levoy, Jr., AIEE Transactions, Vol. 59, pp. 1002- occur. Synchronous motor applications should be re- 1009, 1940. viewed carefully and customers advised as to the haz- 25. Voltage Dip and Automation—C. E. Quick, EEI Bulletin, ards where automatic reclosing is employed. Vol. 23, pp. 348-9, 1955.

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26. Reduce Downtime by Counteracting Voltage Dip in In- 29. Appliance Motor Starting—Dips Call for Industry Analysis dustrial Controls—G. W. Heumann, Automation, pp. 60-65, —J. W. Anderson, Electrical World, pp. 103-105, March November, 1958. 12, 1957. 27. Industrial Plant Voltage Dips Can be Alleviated Four Ways 30. Effect of the Duration of Voltage Dip or Cyclic Light Flicker —B. G. Baily and A. H. Knable, Electrical World, March —L. Brieger, AIEE Transactions, Vol. 70, 1951, p. 685. 16, 1959, pp. 56-58. 31. Static Load to Cut Motor Self-Excitation Voltage—J. K. Dil- 28. Cyclic Flicker of Fluorescent Lamps—W. R. Weise, Electri- lard and C. J. Baldwin, Electric Light and Power, pp. 94-7, cal World, p. 80-1, October 27, 1945. August, 1954.

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