Gridco, Inc. V. Varentec, Inc. IPR2017-01135 GRIDCO

CHAPTER 5

SECONDARY NETWORK SYSTEMS

D. N. REPS

maintaining satisfactory voltage conditions. These I. DEVELOPMENT OF SECONDARY-NETWORK problems were serious obstacles to further development DISTRIBUTION of the d-c distribution system. From the earliest days of electric power usage, the As early as 1915, some forms of alternating current distribution of power in concentrated commercial areas systems patterned after the d-c network system began has been a major concern of electric utilities. Require- to appear in commercial areas. These systems used a ments and limitations generally not found in other grid of conductors operating at utilization voltages and areas have resulted in unique problems in the design energized from a number of step-down transformer and operation of commercial-area distribution systems. banks. Fuses were applied between the transformers and High concentration of commercial electric load, the re- the secondary grid in an attempt to provide some means quirements of this load for interruption-free service, and of isolating faulted cables or transformers. Experience the need for well-regulated utilization voltage have had with these early a-c networks proved to be unsatis- fundamental influence on the design of systems to serve factory because of the operating limitations of fuses. these loads. Flexibility to handle new loads where and This experience showed that detection of power-flow when they occur is another requirement which takes on direction was required to prevent a fault in a trans- added importance in the commercial area. The usual former or primary feeder from interrupting service to physical congestion in the downtown area and the dif- the system loads. Subsequent developments resulted in ficulties in locating heavy equipment and circuits over- an automatic switch and the necessary relays to pro-- head have resulted in the use of underground distribu- vide completely automatic operation of an alternating- tion systems in the downtown areas of most large cities. current network system. Such a scheme assured isola-

1. Early Network Systems From 1884 to about 1922, distribution in heavy load 300 density commercial areas became firmly established as a d-c network system. The very first electric power sys- 280 tems applied to serve the downtown office buildings in (1/ 260 cities such as New York and Chicago were underground d-c systems. Rapid growth and early improvement re- >-co 240 sulted in considerable investment in d-c facilities. While er 220 the pattern was established at an early date for d-c 0 systems in downtown areas of large cities, the develop- - 200 ment of practicable and efficient transformers soon per- ›. 180 mitted the application of the more efficient a-c system to rr urban and residential areas. 2 160 The d-c system in commercial areas was designed to 0 provide interruption-free service from a grid of con- U)hi 140 ductors. The d-c grid or network was energized from 120 multiple sources, usually several a-c to d-c converter stations operating in conjunction with large battery • 100 installations for emergency supply. While providing re- 1-1 liable service, the d-c network presented inherent dis- O 80 U- advantages which grew to serious proportions as expan- 0 60 sion was necessary to keep pace with load growth. rr Among these disadvantages were the following: the cost 2 40 of installing, operating, and maintaining large converter stations and battery installations; the problem of find- 20 ing locations for these installations in the downtown 0 area; the cost of copper conductors and the space re- 1922- 24 28 32 36 40 44 46 52 56 60 quired in underground ducts for the ever increasing YEAR number of circuits necessary to carry the low voltage Fig. 1—Growth in number of cities employing secondary- d-c power throughout the area; and the problem of network systems—including foreign installations. 149

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 157/576 150 Secondary Network Systems tion of a primary-system fault by a means which sensed SUBSTATION BUS • • CIRCUIT BREAKERS a reversal of power flow. In April, 1922, the first multi- NO 3 feed low-voltage a-c network using fully automatic net- PRIMARY FEEDERS work protectors was put in operation in New York City. 11111

SWITCH 2. Growth of Network Applications TRANSFORMER NET WORK The low voltage a-c secondary network has become PROTECTOR well accepted as the standard distribution system ar- SECONDARY rangement in large city commercial areas. Based on re- GRID cent census figures, there are only two cities in the United States with populations greater than 250,000 that do not use secondary network distribution. Appli- LOADS cation has steadily expanded into smaller cities and towns as increasing load density and electrical usage have necessitated improvements in commercial-area distribution. At present there are approximately 260 cities in the United States which use secondary network distribution in some form. While most of these systems Fig. 3—Simplified one-line diagram of a conventional a-c are underground, there are many localities operating secondary network. partially or completely overhead secondary network systems. Growth in the application of networks has been almost uninterrupted over the years. At the same II. ELEMENTS OF THE SYSTEM time, the increasing size and growing electrical load of In its conventional form, the network system, whether most cities have resulted in a continual increase in the underground or overhead, consists of an interconnected number of cities which could reasonably and profitably grid of low-voltage circuits operating at utilization apply network systems in their downtown areas. Esti- voltage and energized from a number of primary feeder mates indicate that networks are applied in only 40 per circuits and step-down transformers. A typical network cent of the total number of cities which could reason- system in which the low-voltage circuits are intercon- ably be considered to have use for this type of system. nected in the form of a grid or mesh is shown in Fig. 3. The system of multiple primary-feeder circuits, each supplying more than one step-down transformer feeding into the common secondary grid, is designed to provide uninterrupted service, except in the case of complete E 0-5000 KVA PEAK LOAD failure of the power supply to the primary feeders. The 5001-10000 KVA design of the system is based on the premise that failure of any one primary feeder circuit or step-down trans- g 10 001-20 000 KVA former causes no service interruption since the load con- 20 001-50 000 KVA tinues to be supplied over the remaining primary feeders and transformers. Automatic isolation of a faulty pri- 50 001-60 000 KVA mary circuit or network transformer is provided by the tripping of the feeder circuit breaker, and the circuit 200 breaker in each of the network protectors located in the

180 secondary leads of the network transformers supplied by the faulted feeder. The automatic network protector 160 was specially developed for this function of interrupting a back feed of power from the network grid to the trans- 140 former or feeder. Segregation of the basic network into 120 three parts facilitates discussion of basic system elements. These major segments are: 100 1. The secondary grid 80 2. The network units—consisting of step-down trans- 60 former and automatic network protector 3. The primary feeder circuits 40

20 3. Underground Networks—The Secondary Grid AL.1 1111 The secondary circuits which are tied together at various points to form the common grid generally sup- 1931 1943 1952 1955 ply all or most of the single-phase and three-phase loads Fig. 2—Evolution of the number of individual network in the network area. The disadvantages of separate systems of various sizes—including foreign installations. power and lighting transformer banks, and duplication

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of secondary mains and services, are avoided by the use Table 1—Secondary Cable Miles in Service and of a common three-phase, four-wire system at utilization Failure Rate voltage. The secondary grid operates at a utilization voltage which has varied over the years from 115/199 to Miles of Per Cent of Failures Per 100 Cable Total 125/216 volts. As a nominal system voltage, 120/208 Miles Per Year volts has become most widely accepted. It should be Types of emphasized that 120/208 volts is considered to be a Cable 1955 1952 1955 1952 1955 nominal system voltage, that is, the nominal voltage delivered at the customer's service. This voltage pro- Paper-Lead 4 449 16.68 16.13 3.70 3.84 vides a standard lamp voltage from line to neutral and Rubber- a three-phase line-to-line voltage generally satisfactory Lead 10 726 41.85 38.89 1.64 1.87 for power applications. The transformers supplying this system have a standard voltage rating of 216Y/125 volts. Miscel lane- Secondary Mains The secondary mains forming the ous Lead 272 0.84 0.99 6.95 0.13 grid from which consumers' services are tapped generally follow the geographical pattern of the load area Non-Lead and are located under the streets or alleys, so that the Including 12 132*** 40.63 43.99 3.57 9.79 service route to the consumers can be as short as possi- Neoprene ble. This arrangement facilitates access to the main for Rubber repairs, maintenance, and service connections. These 0.32 circuits are generally carried in duct systems, and the Neoprene 7 298 (18.77) (26 46) 0.42 service connections are made in manholes, vaults, or TOTAL 27 579 100.00 100 00 3.02* 3.36** shallow junction boxes. *Includes 174 Failures, Type Cable Unknown—**Includes 249 Cases Un- Single conductor cables are generally used in the known—***Includes Rubber Neoprene. secondary grid because of the many interconnections Table reproduced from Reference 1. and service taps required. These can be made more easily and less expensively on single conductor cables One of the major factors influencing the design of net- than on multi-conductor cables. Another reason for work secondary grids has been the problem of protect- using single conductor cables is that circuit continuity ing against cable failures in the grid. For many years, can be maintained after a cable fault where parallel network systems were designed on the basis that faults conductors per phase are employed. In the early net- in the secondary cables would be self-clearing. This de- work systems, practically all cable used in the secondary sign was premised on obtaining sufficient fault current grids consisted of rubber-- or paper-insulated, lead at a cable fault to burn the conductor free of the point sheath cable. With the development in recent years of of fault. This is feasible on low-voltage circuits such as ' newer, improved types of non-metallic sheath cable, 120/208-volt secondary network grids, because arcs are there has been an increasing trend toward eliminating not sustained at that voltage. For circuits operating at the problems associated with splicing and terminating lead sheath cables. At the present time, approximately 1.8 43 per cent of network cable in use on underground systems is non-leaded, as shown in Table 1. The size of the conductors in the secondary main de- ON 1.6 TI CURVE NO pends primarily on the required current carrying capac- A

UL $ ity. However, the voltage drop from a transformer to CURVE NO. 2 any load along the mains under normal operating con- REG 1.4 ditions, i.e., with all transformers in operation, should E

not exceed about two per cent. The carrying capacity of ATIV

a secondary network main should be one-half to two- REL 1.2 thirds of the rated capacity of the predominant size of network transformer unit. This relationship is based on the premise that a transformer is connected in the grid to so as to feed in at least two directions; and also that a 100 90 80 70 60 50 40 part of the total current of the transformer supplies load PERCENT POWER FACTOR at the transformer location itself. Fig. 4—Relative voltage regulation per unit length of three- Burning Faults Clear—Conductor sizes commonly used phase circuit at various power factors. in underground secondary networks are No. 4/0, 250, Curve No. 1—Three 500-MCM single-conductor cables 350, and 500 MCM, AWG. However, because of rela- in one duct. tively high voltage drop, difficulty in handling and in Curve No. 2—Two parallel branches of three 4/0 single- burning faults clear, parallel-conductor 4/0 or 250 MCM, conductor cables; one duct per branch. AWG circuit are preferred. Fig. 4 shows the reduction Curve No. 3—Two parallel branches of three 250 MCM in voltage drop obtained by using several parallel con- single-conductor cables; one duct per ductors of smaller size. branch.

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higher voltages such as 460 volts, this method of clear- 300 KVA 1" 5%2 ing faults is not always dependable. Use of "limiters" to (co ocan 0011 500 MC M- a0Z1 0101 FAVLY—It overcome this problem is discussed below. Tests and taw 4250 AMP. 05%! calculations have shown that the minimum currents 500 FT. 1 shown in Table 2 are required to burn clear the most 300 KVA 300 KVA L1-115% Z 5%2 severe type of fault. Such a fault is one in which the coon OHM TWO 4(01-0.0352 01431 !Aim 0 0070 OHM current and thermal capacity of the faulting mechanism (b) < AmP.•(1.3.2%) • AuLT-> are greater than that of the conductor itself, so that the

conductor on each side of the fault becomes fused. Such 300 KVA 300 KVA 5% 5%2 a fault might occur when a power shovel digs into a duct 0.001; OHM TWO 4/01-00350 OMM ZACH 0.0072 01131 (C) line. The probability of faults of this type is rather 4 3504. AMA CPT 1%1, F"LT small. Most faults on network grids clear with much smaller currents than those shown in Table 2. Fig. 5—Tying together paralleled conductors in a secondary main assists the burning-off of faults. Table 2—Minimum Current in Amperes Required in Each Conductor on Both Sides of a Solid Fault on Single- Conductor Cables to Burn Off the Fault fault at the end of the 500-foot section of 500-mcm cable will be 4250 amperes, or only 85 per cent of the 5000 Conductor Size Overhead Circuit Underground Circuit amperes necessary to clear a solid fault on that size con- 1 1000 1600 ductor. In Fig. 5(b), two paralleled 4/0 AWG conduc- 1/0 1200 1800 tors are substituted for the 500-mcm cable and the fault 2/0 1400 2100 clearing ability is improved. However, the amount of 3/0 1700 2500 fault current is still only 98.2 per cent of the 2900 4/0 2100 2900 amperes required for this size conductor. If, as in Fig. 250 MCM 2300 3200 5(c), the two paralleled 4/0 conductors are tied together 350 MCM 3000 4000 500 MCM 4000 5000 at the midpoint of the section, the available fault current is increased to 149.2 per cent of the 2900 amperes required for burn-off. This provides a wide margin of Parallel Conductors per Phase—The problem of burn- safety. The improvement in fault clearing ability is a ing off secondary faults influences design of secondary result of two factors: Both transformers supply energy grids by tending to limit the largest size conductor used, to the fault even after it is burned off on one side; and so as not to require an excessive amount of current for the effective impedance to the fault is less than that of burn-off. Since the load carrying ability needed in each either the single conductor or the two paralleled con- section of the main in the grid is directly related to the ductors without a tie point. The effectiveness of using size of network transformer used in the system, the parallel conductors and tie points is assured by carrying required current-carrying capacity is provided by using each set of three-phase conductors in a separate duct to two or more parallel conductors per phase tied together prevent a fault in one branch from affecting another. at the various junction points and service manholes in Limiters—While satisfactory clearing of secondary the grid. A 500-mcm conductor is about the largest con- cable faults by burn-off is often obtained, other cases ductor that can be expected to burn clear consistently. exist where faults would not burn clear, resulting in ex- Larger conductors require high minimum-fault currents, tensive cable damage, manhole fires, and interruption of which are difficult to obtain in network mains except service. Such cases require a fusible device for installa- where the transformer capacity is highly concentrated. tion at each end of each run of secondary cable. Fre- Also, the large amount of metallic vapor generated when quently there will be a few mains in a secondary net- a large conductor fuses makes it difficult to obtain arc work grid, particularly around the fringe areas, where extinction. With a parallel arrangement of secondary fault current is insufficient to insure clearing a solid main conductors, a fault opens only one of the branch fault. To avoid the possibility of faults failing to clear, conductors. To help maintain the maximum possible and to minimize the amount of cable damage resulting fault current available at the fault point, the parallel from secondary faults, limiters were first developed and circuit may be tied together at relatively short inter- applied. n New York in 1936. These high-capacity fuses vals, such as at the various service manholes between are called limiters because of their function of limiting main junction points. With such a circuit arrangement, damage to cable insulation as a result of carrying fault only a small section of main is affected by a fault, and current. It should be noted that these devices are not the magnitude of fault current at the fault point is designed to limit fault current, and should not be con- generally increased. fused with current-limiting fuses. The limiter is a re- The effects of parallel mains and tie points are illus- stricted copper section installed in the secondary main trated by Fig. 5. The fault location used in this example at each junction point. At the present time, limiters are is usually the most difficult location at which to com- being used to some extent in a majority of network sys- pletely clear the fault, because the current to one side tems. Some of the more recent systems have been in- of the fault is limited by the impedance of the entire stalled with limiters terminating all secondary cables. length of secondary main. In Fig. 5(a), the current to a In other systems, limiters have been installed in sections

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Secondary Network Systems 153

where doubt exists as to the availability of sufficient 10000 fault current for clearing secondary cable faults. 0 I INSULATION DAMAGE _ 5000 Since the limiter is designed to burn clear only on CHARACTERISTICS faults of sufficient magnitude to cause damage to the BALANCED CIRCUIT cable insulation, its time-current characteristic is co- %.‘ CONDITIONS 3 PHASES AND ordinated with the insulation-damage characteristic of NEUTRAL IN DUCT the cable. The insulation-damage characteristic has been 1000 %.\ found to be very similar for rubber-covered cable and \ \t"---- 1000 MCM oil-impregnated, paper-insulated cable. Curves giving 500 1:",‘ 750 MCM — \\‘\‘‘\.....--- 50 0 MCM the time-current characteristics of limiters and the in- 40 0 MCM sulation-damage characteristics of the cables ordinarily 350 MCM used in underground networks are shown in Fig. 6. 30 0 MCM \v"•-- 250 MCM 100 4. Network Units 4/0 MCM 0 co 50 The network units which supply power to the net- z 0 — z work secondary grid have been developed over the cra 00 -— 1000 MCM years into a unitized piece of equipment consisting of a w 750 MCM C.) three-phase step-down transformer and associated auto- 500 MCM - - 400 MCM matic network protector. In all early network installa- 1- 1" 10 0 M — 350 MCM tions, the transformers and network protectors were F: -— 300 MCM installed separately and not physically associated with 5 —250 MCM each other. These early installations invariably used — 4/0 MCM LIMITER FUSING banks of single-phase distribution transformers, and the — CHARACTERISTICS network protector was usually mounted on the wall of the network vault. The network protectors were cable- 1 connected to the secondary terminals of the transformer bank. This length of exposed cable in the vault gave an .5 undesirable degree of exposure to this portion of the system. Due to the nature of the network protector protective relaying, the most severe fault condition, from I 1 I 1111 1 1 1 I Ill II 1 1111 the standpoint of network-protector operation, exists 100 500 5000 50000 for a fault between the secondary terminals of the trans- 1000 10000 100000 formers and the network protector itself. Reducing the CURRENT IN AMPERES Fig. 6a

10000 I I I likillk ‘6. CABLE INSULATION DAMAGE CHARACTERISTICS 4 BALANCED CIRCUIT CONDITIONS. 3 )0,181/4‘ 3 PHASES 13 NEUTRAL IN DUCT. 2 NII1‘/4 , N. i600 MCM 1000 -I, 500 MCM 8 400 MCM 6 Il 4 k N. 350 MCM 3 ;.— 300 MCM 2 cr) —.-- 250 MCM Z 100 NIX .....__ 4/0 o 8 6 (I, 4 3 — 2 10 Alk z 8 600 VOLT LIMITER lb.'" FUSING CHARACTERISTICS 4 3 600 MCM 2 500 MCM 400 MCM 350 MCM '4..0.,41%....S44,44 ."441...... 300 MCM A 250 MCM '41%\..,...... s....„ .3 4/0 .2 T_ HIIIIII i II IIIIII III III .1.1,1W .I.I.1,1, ,1.11,1111 t11 111111111 111111111 I It Ill itt III It, 400 500 600 700 900 1000 2000 3000 4000 5000 7000 9000 800 6000 8000 10000 CURRENT IN AMPERES Fig. 6b Fig. 6—Limiter characteristics. Time to fuse versus current in amperes. a—Limiters used in 120/208-volt systems b—Limiters used in 265/460-volt systems

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 161/576 154 Secondary Network Systems fault exposure of this connection was a major factor in bringing about the development of the network unit with the network protector is mounted directly on the transformer. The electrical connection consists of bushings completely enclosed by a steel throat. The individual components of the network unit are more fully treated in the sections following.

5. Network Transformers In addition to providing the basic requirement of voltage transformation, the network transformer has been developed to meet the special and oftentimes severe requirements for underground service. Limita- tions of vault size and ventilation, and requirements of submersibility have all had a strong influence on the design of the network transformer. In most installations, three-phase transformers have replaced the earlier banks of single-phase units because of the decreased vault space required by three-phase units, and the relative ease of installation and connection. Single-phase transformers offer no advantage from the standpoint of service continuity, because the innerconnected secondary grid maintains the service at a transformer point even though that transformer is out of operation. How- ever, where existing single-phase transformers have Fig. 7—View showing a modern network unit mounted in impedances and voltage rating such that they can be an underground vault. paralleled with network units, they can be used in developing a network system. This often occurs when A recent design of network transformer is shown in an existing distribution system is being converted to a Fig. 8, incorporating a number of improvements over low-voltage secondary network. Single-phase trans- prior designs. Such improvements include reduction in formers sometimes are necessary because of space and overall size and weight, and new design of coolers. A weight limitations of elevators, hallways, doorways, serious problem with prior transformer designs has been manholes, and other means of access to the transformer the difficulty in properly cleaning and painting the tank vault. walls and cooling tubes. The new design overcomes this The publication containing industry standards for problem by providing radiating surfaces in the form of network transformers is entitled EEI-NEMA Standards fins or corrugations rather than tubes. A further step for Secondary Network Transformers'. One of the towards reducing corrosion has been the elimination of principal characteristics of network-transformer classi- many corners and crevices where dirt and moisture fication is the type of cooling. With respect to cooling, lodge. The reduction in size of the transformers permits network transformers are available in three general a reduction in vault volume and cost, or the replace- types. These will be discussed individually. Oil-Filled----The most commonly used network transformer is the oil-filled type. A typical installation is shown in Fig. 7. These units normally are constructed with cooling devices along both of the long sides of the transformer tank, and with high-voltage cable terminating facilities and a high-voltage switch on one end of the tank. A throat for direct connection of the network protector is provided on the other tank end. Auxiliary devices commonly employed are: a no-load tap-changer available for operation with a special wrench by removal of a pipe plug in the tank; temperature indicator; liquid level indicator; oil sample valve and filter press connections. Tanks are 'constructed of copper bearing steel plate and the cooling tubes or radiators are of a suitable thickness to resist corrosion. The vault-type transformer has the thinner tubes and headers and is recommended for ordinary conditions of submersibility and corrosion. The thicker tubes and headers are used on Fig. 8—Comparison in physical dimensions between conven- the subway type, where frequent submersion presents a tional and new (space-miser design) network transformers. greater than normal corrosion problem. Both units are 500 kva, 15-kv class primary.

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Table 3—Standards for Secondary Network Transformers

Standard Ratings for Three-Phase Transformers

Preferred Transformer High Voltage Standard Kva Ratings Nominal BIL Taps for Low-Voltage Rat- System Rating (Kv) ing of 216Y/125 Volts Voltage Above Below

4,160 (a) None None

4,160Y/2,400 (a)(b) None None 2,400/ 4160Y, 60 4,330 None None 300, 500, 750 4,330Y/2,500 (b) None None

4,800 5,000 60 None 4,875/ 4,750/4,625/4,500 300, 500, 750

7,200 (a) None 7,020/6,840/6,660/6,480 7,200 '75 7,500 None 7,313/7,126/6,939/6,752 300, 500, 750

11,500 95 None 11,213/10,926/10,639/10,352 300, 500, 750, 1,000

12,000 (a) None 11,700/11,400/11,100/10,800 12,000 95 12,500 None 12,190/11,875/11,565/11,250 300, 500, 750, 1,000

7,200/12,470Y 13,000Y/7,500 (b) 95 None 12,675/12,350/12,025/11,700 300, 500, 750, 1,000

13,200 (a) None 12,870/12,540/12,210/11,880 13,200 13,200Y/7,620 (a)(b) None 12,870/12,540/12,210/11,880 or 95 13,750 None 13,406/13,063/12,719/12,375 300, 500, 750, 1,000 7,620/13,200Y 13,750Y/7,940 (b) None 13,406/13,063/12,719/12,375

14,440 14,400 (a) 95 None 14,040/13,680/13,320/12,960 300, 500, 750, 1,000

22,900 (a) 24,100/23,500 22,300/21,700 23,000 150 24,000 25,200/24,600 23,400/22,800 500, 750, 1,000

(a) Preferred ratings which should be used when establishing new ne works. NOTE: All windings are delta connected unless otherwise indicated. (b) High-voltage and low-voltage neutrals are internally connected by Table reproduced from Reference 3. means of a removable link. ment of an existing unit with one of the new type hav- mersibility are not a problem. Fig. 9 shows a photo- ing a higher rating. graph of a ventilated dry-type network unit. These units Askarel-Filled—Oil-filled transformers have been used are directly air-cooled, with the core and coils enclosed in nearly all underground vault applications. But since in a sheet metal housing. Ventilating louvres provide an oil-filled transformer offers the potential hazard of subsequent fire in the event of tank structural failure, transformers are available with a non-flammable liquid ..fi0011111,1011111=1111M1111=811111=LfAMBeem.—A. - used in place of mineral oil. Askarel liquid-filled transformers are similar in construction and appearance to the oil-filled units. It is possible, in the field, with one manufacturer's unit, to substitute one liquid for the other in a given transformer by following a prescribed procedure. The Askarel liquid-filled network transformers do not completely eliminate the hazards of tank rupture, since an internal winding failure of sufficient severity can generate gas at a sufficiently high rate to cause tank rupture. Because of this, it is desirable to provide Askarel-filled units with a mechanical pressure relief device. The chief advantage of the non-flammable liquid is in eliminating the potential fire hazards of a large volume of transformer oil. Ventilated Dry Type—It is sometimes possible to obtain space for locating network transformers in a building basement or other dry vault location. The ventilated dry-type network transformer finds applica- Fig. 9—Ventilated dry-type network transformer in a net- tion in these locations where dirt accumulation and sub- work unit.

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 163/576 156 Secondary Network Systems for circulation of air over the core and coils. Solid insula- its primary feeder circuit when a network unit must be tion in these transformers consists of such materials as taken out of service for any length of time. The first porcelain, mica glass, or asbestos in combination with a step is the opening of the primary-feeder circuit breaker. sufficient amount of organic materials and binders, gen- With the network transformer disconnected, the feeder erally in form of a thermosetting varnish, to impart the can be re-energized and the other units on it put back in necessary mechanical strength to the insulation struc- service. This procedure is particularly advantageous ture without a detrimental effect to fire-resistant quali- where the primary feeder supplies other radial loads in ties. Combinations of these materials are defined in addition to the secondary network. Should a network American Standards C-57.1 as Class B insulation. The transformer failure occur, or if for some other reason a dry-type transformers are designed for an average transformer must be taken out of service, the affected winding temperature of 80°C. Fans are not required for unit can be isolated without a long-time outage of the normal cooling but may be used to provide increased radial loads supplied from the particular primary- capacity above the self-cooled rating. The use of dry- feeder circuit. The grounding position in the conven- type network transformers has largely been in building tional disconnecting-grounding switch provides a means vault locations. For these locations, such units offer for grounding of points on the high-voltage primary important advantages of maximum safety and mini- feeder circuit. These readily available grounds assure mum installation and maintenance costs. safety to personnel during work done on primary Sealed Dry Type—Another type of network trans- feeders. Another purpose served by the disconnecting former in use is also a dry-type and is made suitable for switch is to allow high potential dielectric testing of the submersible service by enclosing the core and coils in a primary feeder cables. This can be done with the net- hermetically sealed tank. Insulation used in these units work transformers connected to the primary cable in is designated in American Standard C-57.1 as Class H the case of delta-connected transformer primary wind- and consists of such materials as glass tape, glass sili- ings, properly insulated. Star-connected primary wind- cone, plate glass, and porcelain. A small amount of high ings make it practically impossible to test the feeder temperature silicone varnish is used to impregnate the cables without disconnecting the transformer from the insulation of the windings and furnish mechanical cables. In certain large network installations, the dis- strength. The core and coils of the submersible dry-type connecting feature of the primary switch is not re- unit are enclosed by a steel tank which is purged with quired, and only a grounding switch is. applied. dry nitrogen and sealed under a small positive pressure Regardless of type, the primary switch is a manually of this gas. These transformers completely eliminate the operated device equipped with the necessary interlocks hazards of fire or explosion, and the cost of equipment to prevent improper operation. The most commonly and labor for maintaining oil or Askarel. Since sealed used type of primary disconnecting switch is not de- dry-type transformer initial cost is more than that of signed to open load current or transformer exciting cur- either oil or Askarel units, the additional cost must be rent, and is therefore interlocked electrically so that the evaluated in terms of increased safety and decreased switch cannot be moved unless the transformer is de- maintenance. Sealed dry type units are somewhat energized. This type of switch requires that the primary larger in size than equivalent ratings of liquid-filled feeder circuit be de-energized before the switch can be units; the larger size is necessary because the major insu- operated. The three-position disconnecting and ground- lation is air and greater electrical clearances are required. ing switch is designed so that the closed or transformer Another characteristic of dry type transformers, both position is between the open position and the ground ventilated and sealed, which must be considered in some position. This requires that the switch be in the trans- applications is their lower dielectric strength. Since former position just prior to going to the ground posi- these units depend on air for dielectric strength, their tion, which allows the interlock to function to prevent impulse insulation level is substantially lower than cor- grounding a live primary feeder should the feeder be responding liquid-filled units. This factor becomes im- energized. The switch is designed with a forced pause to portant only when dry-type transformers are applied to allow the interlock to pick up. circuits exposed to lightning surges. Though nearly all It is sometimes desirable to be able to disconnect a underground networks are free from such exposure, network transformer from an energized primary feeder. when it is needed, adequate protection can be provided This procedure is desirable where the primary feeders with low ratio, rotating-machine type lightning ar- are not exclusive to the network. For this purpose, a resters installed in the primary leads of the transformer. primary disconnecting switch capable of opening trans- Fig. 10 shows a comparison of approximate weights, former exciting current is used, and is interlocked with dimensions and other application information for the the associated network protector to assure that load is various types of network transformers. removed from the transformer before operating the High-Voltage Switch—An integral part of nearly all switch. network units is the three-position switch provided in the primary leads for disconnecting the primary circuit 6. Network Protectors from the transformer. This standard switch is provided Functions—The network protector is an electrically with a position for grounding the primary circuit. In operated low-voltage air circuit breaker with self-con- small or medium size network systems, it is desirable to tained relays for controlling its operation. The network be able to disconnect an individual network unit from protector performs three basic functions in the system.

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Conventional Design Conventional Design Approximate Dimen- n sions in Inches Net L.11::•;), Weight High tector Submers- Voltage Kva ible Pounds * Class Rat- ing A• CEPH Protector B G D. Oil int eeern-

:11%—V 150 600 31 70 46 85 103 61 5400 6500 0 11 225 800 36 73 90 117 6700 7800 — — 48 — 66 300 1200 68 37 65 74 91 11 7700 9000 5 Kv — through 500 1600 75 52 92 119 70 840 9500 15 Kv 44 600 2000 77 53 94 121 71 8700 10500 750 2500 77 45 80 58 104 136 77 11100 12700 65 1000 3000 79 52 89 62 11 145 82 13800 16000 150 600 74 50 89 107 65 7100 8800 68 38 65 225 800 7800 9600 77 52 94 121 70 300 1200 73 60 8700 10300 39 25 Kv 500 1600 78 64 95 122 72 10800 12500 79 65 600 2000 81 56 98 125 74 11501 13500 44 overall dimension with I. v. 750 2500 87 82 60 107 131 84 12700 14800 G 70 protector door open 90° 1000 3000 89 53 91 66 115 147 90 14200 16400 300 1200 73 44 60 87 54 104 131 72 9500 11900 500 1600 48 90 58 107 134 76 11700 14000 79 65 33 Kv. 600 2000 91 60 108 135 78 12500 15000 50 750 2500 87 95 68 119 151 92 14200 16700 Space Miser Design 70 1000 3000 89 53 98 70 122 154 94 16800 19400 Space Miser Design Approximate Dimen- sions in Inches Net High Pro Weignt Voltage Kva tector Submersible Pounds÷ Class Rat- Protector ing A4 C E F H Iner- B G D Oil teen- 300 1200 54 38 68 38 87 111 52 595 6900 44 5 Kv 500 1600 56 43 71 41 90 114 59 7450 8450 through 15 Kv 750 2500 65 44 77 48 104 132 72 9900 11150 46 1000 3000 73 47 85 67 112 140 81 13300 15400 300 1200 37 75 44 94 118 61 720 8200 64 500 1600 42 78 46 97 121 63 8750 10050 25 Kv 56 750 2500 71 44 83 53 110 138 77 11450 13050 1000 3000 81 48 91 63 118 146 87 1490D 17300 NOTE: Above dimensions and weights are based on transformers with an impedance of 5% and a low voltage rating of 216 Y/125 or 480 Y/277 volts. B '"A" dimension does not include relief device. If relief device is required, overall dimension with I.v.. add 3 inches from "A" dimension. G '"A" dimension covers oil-filled units only. Add 3 inches for Inerteen- protector door open.90° filled units and oil-filled units using mechanical relief device. $"D" dimension is height over stud on protector. If other type terminal is used, use "H" dimension and add height from center line of protector throat to top of terminal for correct "D" dimension. Fig. 10—Network-transformer physical characteristics. 'Weight of protector is not included. Weights listed cover transformers of vault-type construction. For weights of subway-type construction, use the (a)—Oil filled following approximate percentage addition: Oil-5.5%. Inerteen-5.0%.

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One Triple Conductor F or Three Single Conductor Switch Operating Wiping Sleeve. Mechanism

Network Protector

Removable Ponels G • "6 is Overall Dimension With L.V. Protector Door Open 90° Fig. 10—(b)—Ventilated dry-type

Dimensions and Weights Recom- Approximate Dimensions—Inches Net High Kva mended With Submersible With Dust-Proof Weight Voltage Ratin Protector Network Protector Network Protector Class g Rating A C E F —Pounds --Amperes B D G B D G 150 600 75 42 81 86 103 72 120 104 64 120 4000 225 800 75 42 81 95 114 72 140 116 64 140 5000 300 1200 75 42 81 95 114 77 140 116 64 140 5500 15 Kv 114 77 140 116 64 140 6700 and 450 1600 75 42 81 95 114 140 116 64 140 7000 Below 500 1600 75 42 81 95 77 600 2000 80 48 86 104 124 90 149 126 76 149 8500

750 2500 80 48 86 104 132 90 160 132 76 157 9100 1000 3000 80 48 86 104 132 90 160 132 76 157 11100

Above dimensions and weights are based on standard transformers with an impedance of 5 percent and a low voltage of 216Y/125. 450 and 600 kva sizes are special.

The first and most important function is to provide The second function of most network protectors is automatic isolation of faults in the primary feeders or provided by the ability of the reverse-power relay to trip network transformers. This basic tripping function is the protector on relatively small values of reverse power provided by the three-phase power directional relay such as required to magnetize the transformer. This called the master relay. This relay trips the protector sensitive tripping assures proper network protector op- when power flows in a reverse direction from the ener- eration for phase-to-ground fault conditions in a uni- gized secondary grid back towards a fault in the trans- grounded three-wire primary feeder supplying delta- former or primary feeder. A faulty primary cable or connected network transformers. It is also commonly transformer is thus removed from the system automati- used to provide a ready means of checking for proper cally by tripping the substation breaker in the affected network-protector operation. By individually opening primary feeder, and by tripping the network protectors each primary feeder breaker at the substation supplying associated with that feeder. By this means, the network the network, an operator can conveniently check the system makes use of its multiple paths of supply, pro- network protectors for proper tripping. The voltmeter vided by the several primary circuits and by the com- on the load side of each primary feeder breaker will indi- mon secondary grid, to give a high degree of service cate to the operator whether or not all network protec- reliability. tors on that feeder have opened.

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0 f 0 0 I ©

-4- B overall dimension with protector door open 90.

Fig. 10—(c)—Sealed dry-type

Recom- Approximate Dimensions: Inches High mended Net Kva With Submersible With Dust Proof Voltage Protector Weight: Rating A C E F Network Protector Network Protector Class Rating: Pounds amperes B D G B D G

300 1200 81 47 76 99 117 88 138 119 80 138 11200

500 1600 86 48 76 102 120 93 141 122 85 141 12000

600 2000 ...... 15 kv and 750 2500 95 49 76 108 132 109 160 132 95 156 13250 Below 1000 3000 99 51 76 114 138 113 166 138 99 162 17500

1500m 2500 113 52 76 117 141 111 174 141 97 170 23140

2000 3000 119 52 76 120 144 111 177 141 97 173 24640

Above dimensions and weights are based on standard transformers with a °For a low voltage of 480Y/277. low voltage of 216Y/125. Size 600 kva is special and dimensions can be fur- nished upon request.

A third function of the network protector is to auto- network relays prevent the protector from closing automatically close when certain predetermined system con- matically under crossed-phase conditions. ditions are met: When a primary feeder associated with Network-Protector Standards—The National Electrical a given network protector is energized, if the voltage on Manufacturers Association (NEMA) Standards for Net- the transformer side of the open network protector is work Protectors consist of five major parts: Definitions; slightly higher in magnitude, and is in phase with, or General Standards; Rating Standards; Manufacturing leading, the voltage on the secondary grid side of the Standards; Application Standards. protector, the relays cause the protector to automatical- These standards discuss such topics as types of en- ly close and connect the transformer to the grid. The closure, insulation classifications, service conditions, result of the closing function of the network relays is voltage and current ratings, relay operating characteris- such that the protector will close only when voltage tics and adjustments, circuit-breaker interrupting rat- conditions are such as to cause any resultant power flow ings, network protector fuse characteristics, dimensions, to be towards the load. In addition to the operating con- etc. Tables 4 through 10, which are taken from these venience of being able to restore a network feeder to NEMA Standards, are representative of the information service merely by closing the substation breaker, the contained therein. These standards cover what are

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Table 4—Network Protectors Table 5—Network Protectors

Classification of Insulating Materials Voltage and Current Ratings The temperature limits on which the rating of network pro- For applications at altitudes greater than 3300 feet (1000 tectors is based are largely determined by the character of the meters), the standard voltage and continuous-current ratings insulating materials used. of network protectors shall be multiplied by the following cor- For the purpose of establishing temperature limits, insulat- rection factors : ing materials shall be classified as follows: Altitude Correction Factors Class of Feet Meters Voltage Current Insulation Description of Material 3300 1000 1.00 1.00 Class A insulation consists of (1) cotton, silk, A 4000 1200 0.98 0.996 paper and similar organic materials when either 5000 1500 0.95 0.99 impregnated* or immersed in a liquid dielectric; 10000 3300 0.80 0.96 (2) molded and laminated materials with cellulose filler, phenolic resins and other resins of similar NEMA Standard 5-17-1953. properties; (3) films and sheets of cellulose acetate and other cellulose derivatives of similar properties; and (4) varnishes (enamel) as applied to con- protector. These are the master relay, phasing relay, ductors. and de-sensitizing relay. B Class B insulation consists of mica, asbestos, Master Relay—The master relay is a three-phase, fiberglass and similar inorganic materials in built- power-directional, induction relay which provides the up form with organic binding substances. A small basic functions of directional tripping and correct-volt- proportion of Class A materials may be used for age closing for the network-protector circuit breaker. structural purposes onlyf. The master relay is pictured in Fig. 11. C Class C insulation consists entirely of mica, por- Adjustments There are only two adjustments to be celain, glass, quartz and similar inorganic ma- made on the Westinghouse Type CN-33 relay, namely, terials. the overvoltage adjustment and the reverse-current ad- H Class H insulation consists of (1) mica, asbestos, justment. When the relay is completely deenergized, the fiberglass and similar inorganic materials in built- moving contact is held firmly against the stationary up form with binding substances composed of sili- closing contact by means of a spiral spring around the cone compounds or materials with equivalent prop- moving element shaft. By adjusting spring tension, a erties and (2) silicone compounds in rubbery or continuous overvoltage adjustment, having a range of resinous forms or materials with equivalent proper- approximately 1.0 to 5 volts 75° leading, or .5 to 2.0 volts ties. A minute proportion of Class A materials may in phase with the network voltage, is obtained. Three be had only where essential for structural purposes flat springs, placed side by side, are carried on the mov- during manufactures. ing contacts. In conjunction with an adjustable thumb O Class 0 insulation consists of cotton, silk, paper screw stop which can be located in any one of three tap- and similar organic materials when neither impreg- ped holes in its mounting block so that it will deflect nated* nor immersed in a liquid dielectric. one, two, or all three of the springs, these springs pro- l'An insulation is considered to be "impregnated" when a suitable substance replaces the air between its fibers, even if this substance does not, completely vide a continuous range of in-phase reverse current ad- fill the spaces between the insulated conductors. Thu impregnating substances, justment from about 0.1 to 10 per cent of the protector in order to be considered suitable, must have good insulating properties• must entirely cover the fibers and render them adherent to each -other and rating in amperes. to the conductor; must not produce interstices within itself es a consequence of evaporation of the solvent or through any other cause; must not Sow during Closing Characteristics Figs. 13, 14, and 15 show the the operation of the machine at full working load or at the temperature limit specified; and must not unduly deteriorate tinder prolonged action of heat. operating characteristics of the Type CN-33 network tTho electrical and mechanical properties of the insulated winding must not be impaired by application of the temperature permitted for Clam B master relay. Curve No. 1 of Fig. 13 shows the closing material. (The word 'impair" is hero used in the sense Of causing any change which could disqualify the insulating material for continuous service.) The characteristics of the relay. Lines drawn to it from the temperature endurance of different Class B insulation assemblies varies over origin at various angles with the network voltage repre- a considerable range in accordance with the percentage of Class A materiide employed and the degree of dependence placed on the organic binder for sent, in both magnitude and phase position, the trans- maintaining the structural integrity of the insulation. rl'he electrical and mechanical properties of the insulated winding must not be impaired by the application of the temperature permitted for Class H material. (The word "impaired" is here used in the senile of causing any change which would disqualify the insulating material for continuous service.) Table 6—Network Protectors NEMA Standard 5-17-1953. Voltage Ratings termed "heavy-duty" and "medium-duty" network pro- The standard voltage ratings shall be: tectors. The "light-duty" unit is discussed below in con- Alternating-current nection with overhead network systems. Standards for 600-volt class network protectors are not yet available. Maximum Design Voltage, Voltage Rating, Volts Volts 7. Network Relaying 125/216 or 240 250 The planning, design, and operating features of the 480 500 a-c secondary network system are made possible by the 575 600 particular type of relays which are a part of the network NEMA Standard 5-17-1953.

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Closeup crossection showing gasket re- tainer and highly compressible tubu- lar gasket

Connection Plate, Ammeter Plate, or Type BN Relay Type CNJ Phasing Relay

Type CN-33 Master Relay

Engagement Tongue For Operating Handle (a)—Heavy-duty type network protector. Transformer mounted submersible enclosure. View showing door open and unit rolled out on rails. Circuit breaker is located behind relay panel.

Terminal Screw —Molded Tqrminal' Plate Terminal Screw Molded Terminal Bloch Moving Contacts Stationary Tripping Contact Current and Phasing Mounting Hole _Stationary Closing Coil and Iron Assemb Contact Damping Magnet Overvoltage Adjusting Pinion Drum Current and Phasing Coil Spiral Spring Adjuster Assembly Mounting Screws Trent Bearing Plots Mounting Plate Reverse Current Springs with Insulating Cover Rear Electing Plots — Captive Cover Screw Glass Cover averse Current Stationary Slop ------Adjusting Strew Damping Magnet

Moving Element Slop

(b)—Network master relay. Front view with glass cover (c)—Network master relay. Rear view of relay removed removed. from base. Fig. 11—Network protector and network master relay. former voltages which will produce a torque in the relay will produce a relay torque in the tripping direction just sufficient to cause its closing contacts to make. The which prevents the closing contacts from making and closing contacts will also make and connect the trans- the network protector will remain open. The Curve No. former to the network if the transformer voltage termi- 1-A in the same figure shows a small section of the clos- nates above the closing curve. Any transformer voltage ing curve plotted to a much larger scale, so as to show which does not terminate on or above the closing curve the characteristics of the relay for the values of phasing voltage at which it normally operates. Lines drawn from Table 7—Network Protectors the origin to this curve represent in magnitude and Dielectric Test Voltage phase position the phasing voltage, that is, the voltage The dielectric test voltage shall be an alternating voltage across the open contacts of the network protector neces- having a crest value equal to times the following test volt- sary to produce a torque in the relay just sufficient to age. A sine wave shape is recommended. make its closing contacts. The upper end or line potential end of the network voltage vector is at the origin in A-c Rms Voltage Rating A-c this case. The network voltage vector cannot be shown of Network Protector, Dielectric Test Voltage, in its true relationship to this curve, because of the Volts Volts* large scale to which the curve is plotted. It will be noted 125/216 or"240 1500 by referring to Curve No. 1-A that the relay will just 480 2200 close its closing contacts with approximately 0.8 volt 575 2200 across the phasing circuit in phase with the network *EXCEPTION—The closing motor shall be tested at 900 volts. When the voltage. When the phasing voltage leads the network network protector is tested in the field, the dielectric teat voltage shall be not greater than 75 per cent of the foregoing values. voltage by 75°, about 2.0 volts are required to close the

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Table 9—Network Protectors Standard Ratings for 125/216- or 240-volt A-c Network Protectors*

Continuous Maximum Dielectric .Interrupt- Short-time Current Design Test Volt- ing Rating, Rating, Rating, Voltage, age, Except Rms Rms Rms Except Relay Closing Amperes Amperes Amperes and Control Motor

It 2f 3f 4t 5f

300 250 1500 10000 10000 600 250 1500 10000 10000 1200 250 1500 20000 20000 1600 250 1500 30000 30000 2000 250 1500 35000 35000 2500 250 1500 45000 45000 3000 250 1500 60000 60000

*Standards have not yet been developed for 480- and 575-volt network protectors. tFor a further explanation of the column headings, see the following standards: Column 1—see 803.1-3.09. Column 2—see SG3.1-3.03. Column 3—see SG3.1-3.04. Column 4—see 8G3.1-3.15. Column 5—see SG3.1-3.17. NEMA Standard 5-17-1953.

Opening Characteristics—The opening characteristics Fig. 12—Installing a modern network unit for the secondary of the type CN-33 relay are shown by Curve No. 2 in network system in a large city. Fig. 13. Lines drawn from the origin to Curve No. 2 represent in magnitude and phase position the line cur- closing contacts. This voltage at 75° leading, however, rents which will produce a torque in the relay just suffi- means only a small angle between network and trans- cient to cause its tripping contacts to make. The trip- former voltages. This can readily be appreciated by ping contacts will also make and disconnect the trans- observing that 10 volts across the phasing circuit former from the network if the line current terminates leading the network voltage by 90° will throw the net- below the opening curve. If, however, the line current work and transformer voltages less than 5° out of phase. does not cross the opening curve but terminates above

Table 8—Network Protectors Table 10—Network Protectors Limits of Observable Temperature Rise Continuous-current Rating The temperature rise, above the ambient temperature of each A network protector shall have a continuous-current rating at least as great as the maximum load current which it will of the various parts of the network protector, when subjected to temperature tests in accordance with these standards, shall be required to carry. The following table should be used as a guide in applying not exceed the values given in the following table. All tempera- 125/216-volt network protectors to network transformers*. ture determinations shall be made by the thermometer method. Associated Network Transformer Limit of Network Protector Current Rating Temperature Rise, Rated Rated Part Degrees C Per Cent of Full-load 3-phase Secondary Rms Trans- Current Current and potential coils and Kva , Voltag, e Amperes former Rms other insulated parts Volts Class 0 Insulation 60 Full Load Amperes Class A Insulation 65 300 150 75 200 125/216 Class B Insulation 85 600 150 150 400 125/216 Class C Insulation No Limit Specified 1200 150 300 800 125/216 Class H Insulation 140 1600 120 500 1340 125/216 All other parts except thermal 2000 150 500 1340 125/216 elements, heaters, fuses, fuse 2500 125 750 2000 125/216 terminals and resistors 100 3000 150 750 2000 125/216 NOTE—Due to the oxidation of contact surfaces. there is an inherent 3000 112 1000 2680 125/216 decrease in the amount of current which can be carried by contact* in air. The continuous current rating, is, therefore, based on the assumption that main- *Tables have not yet been developed for 240-, 480- or 575-volt network tenance will be sufficient to keep the temperature rise within ecified limits. protectors. NMAE Standard 5-17-1953. NEMA Standard 5-17-1953.

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it, the relay will close its closing contacts and maintain necessary to prevent pumping, that is, periodic closing them closed as long as the line current amounts to one and tripping of the protector without any change in or two per cent of the protector rating. The curves load and voltage conditions on the system, other than shown in Fig. 14 represent a small section of the opening those produced by the operation of the protector. An curve just discussed, plotted to much larger scales in adj ustment is provided to change the phase angle char- order to show the operation of the relay on small current acteristics of the relay by means of changing taps on a values, such as the magnetizing currents of network resistor connected in series in the potential circuit and transformers. The minimum magnetizing current of mounted in the case of the relay. This phase angle ad- a 300-kva transformer bank will be about 12 amperes justment will not be used often, but will be found very per phase at 120 volts, and will lag the network voltage, convenient when needed and is obtained with practical- reversed, between 60 and 70 degrees. A network protec- ly no complication of the relay. Only one other adjust- tor rated at 1200 amperes would be used with such a ment is provided in the phasing relay. This is the over- bank, and it will be seen by referring to the opening voltage adjustment, which is similar to that used on the curve in Fig. 14 that the relay will operate satisfactorily master relay. to trip the network protector when exciting current only The curves in Fig. 16 show the normal operating is flowing. characteristics of the Westinghouse Type CNJ relay. On systems where the voltage of the primary feeders The relay may be adjusted to have closing characteris- is 11,000 volts or above, the charging current of the high tics similar to any one of the four curves shown (No. 6, tension feeder cables must be considered. When the sta- No. 7, No. 8, or No. 9). The network voltage, which is tion breaker is open, this charging current will flow the voltage from ground to phase "a" on the network side through the network transformer bank. In such cases, of the protector, is shown with the line potential end of therefore, the current on which the relay must operate is the vector at the origin. This voltage vector could not be not the magnetizing current of the transformer bank shown in its entirety because of the large scale used. alone, but the vector sum of the magnetizing current Lines drawn from the origin to one of the curves repre- and that part of the feeder charging current which flows sent in both magnitude and phase position the phasing through the associated protector. When the charging voltages which will produce a torque in the relay just current predominates over the magnetizing current, the sufficient to close its contacts. Any phasing voltage current on which the relay must operate is a leading re- which does not terminate on or to the left of the curve in versal rather than a lagging reversal. The relay will op- the zone marked "close" will produce a relay torque to erate satisfactorily on leading reversals or on lagging maintain the relay contacts open. It will be noted that reversals, provided the leading reverse current does not the relay will keep its contacts closed when the phasing exceed approximately 250 per cent of the rating of the voltage is reduced to zero if a closing adjustment is used protector, even if the current is almost 90 per cent out similar to that used when these curves were taken. The of phase with the network voltage reversed. curves may be shifted parallel to themselves however, Tripping Characteristics under Short-Circuit Conditions— either to the right or left by means of the spring adjust- The curves in Fig. 15 show the tripping characteristics er, if this is found to be desirable. The relay is connected of the Type CN-33 relay on current values up to 800 per in the factory to have a characteristic similar to that cent of the protector rating, such as are encountered shown as Curve No. 8. To meet system requirements, under short-circuit conditions. The bend in the curve is any of the closing characteristics shown by Curves 6, 7, caused by the saturation of the current transformers 8 or 9 can be obtained by placing a terminal screw in used with the relay. This bend in the opening curve at any one of the terminals 6, 7, 8, or 9 located at the bot- the higher values of current improves the contact action tom of the relay; e.g., if the terminal screw is placed in of the relay under certain short-circuit conditions. It terminal 8, the relay will have closing characteristics as will be noted that this curve is taken with normal volt- shown by Curve No. 8. ages, that is, 125 volts on the potential coils of the relay. Co-ordinated Operation of Master and Phasing Relays— However, curves taken with small values of voltages on The operation of the Type CNJ relay in conjunction the relay potential coils are essentially the same shape. with the Type CN-33 relay can best be explained by re- Phasing Relay—The phasing relay is a single-phase in- ferring to the curves in Fig. 17, which illustrate the duction drum relay having two operating circuits a closing characteristics of both the CNJ and CN-33 re- potential circuit and a phasing circuit. It is equipped lay; and Curve No. 8, which illustrates the closing curve with single-pole, single-throw contacts which are held of the CNJ relay. The area which lies in the "closing" closed by means of a spiral spring when the relay is zone common to both of these two curves is shaded. completely de-energized, just as are the closing contacts Thus a phasing voltage, such as El, which terminates in of the master relay. The contacts of the phasing relay this shaded area, will cause the CNJ relay to make its are connected in series with the closing contacts of the contacts and the CN-33 relay to make its closing con- master relay. Therefore, in order to close the protector, tacts, thus causing the network protector to close. The both relays must close their closing contacts. The phase current which will flow through the protector when it angle characteristics of the phasing relay are such as to closes will lag the phasing voltage across the open pro- prevent the network protector from closing when the tector by an angle approximately equal to the imped- phasing voltage and the voltage of its associated trans- ance angle of the system, and for a particular system former lag the network voltage appreciably. This is this current may be as shown by the vector I,. By not-

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 173/576 166 Secondary Network Systems

20' 10' 350' 340' 330" 330' 340" 350. a 10' 20' 30.

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Fig. 17—Combined closing characteristics of the types CN-33 and CNJ network relays. ing the position of I„ with respect to the network volt- and must be closed at the same time in order to allow age and referring to Curve No. 2 in Fig. 13, it will be the network protetcor to close, it will be seen that the seen that such a current will keep the CN-33 relay clos- CNJ relay prevents pumping due to phasing voltage ing contacts closed and thus the operation of the net- which appreciably lags the network voltage. It may be work protector will be stable. A phasing voltage such as similarly shown that the closing characteristics of the E., however, if the protector were manually closed, CN-33 relay prevent pumping, when the phasing volt- could cause a current I. to flow through the protector; age leads the network voltage by more than 900. It and by referring again to Curve No. 2 on Fig. 13, it will should be noted that the closing curve of the CN-33 re- be seen that this current would cause the CN-33 relay lay is such as to prevent the protector from closing un- to make its tripping contacts. The phasing voltage E„ der crossed-phase conditions, while the CNJ relay used lying on the closing side of the Curve No. 1-A, causes alone would allow the protector to close under certain the CN-33 relay to make its closing contacts. Thus, if crossed-phase conditions. the CN-33 relay alone controlled the network protector, Under certain conditions, a fairly large and very low the protector would pump under this condition. The power factor load may be carried by adjacent network CNJ relay will not close its contacts, however, when protectors and cause the phasing voltage E. to exist acted upon by a phasing voltage such as E.; and since across the protector under consideration. It will be noted the contacts of the two relays are connected in series that this phasing voltage, E., falls on the opening side

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 174/576 Secondary Network Systems 167

of Curve No. 8, so that under this condition the phasing gized. After a time delay of one to five minutes, deter- relay would prevent the protector from closing. If it is mined by the adjustment of the thermal element, the desirable to have the network protector close so that timing contacts close and complete the circuit to the its associated transformer can assist in carrying the trip coil. The protector then trips, if the master trip load, Curve No. 7 may be used for the CNJ relay, so as contacts are still closed. If the master contacts open at to allow the protector to close if such a change in charac- any time during the time delay introduced by the teristics will not cause pumping. It is to take care of thermal element, the timing is interrupted and the such more or less special cases that a tapped resistor is thermal element resets. Thus, for momentary reversals provided in the phasing relay to change its closing of power through the protector, tripping is delayed, so characteristics. that many operations of the protector are avoided. De-Sensitizing Relay—In very infrequent cases, when If the reversed current in any one of all of the poles regenerative loads, such as a bank of elevators, are sup- of the protector exceeds the pick-up setting of the cor- plied from the network, the usual sensitive tripping of responding instantaneous overcurrent elements in the the network protector relays result in too frequent op- desensitizing relay, the circuit to the trip coil will be eration of the protectors. Even greater problems than completed as soon as the master tripping contacts close. regenerative elevators are caused by radial loads on pri- This provides fast operation of the protector for high mary feeders. During light-load periods, a heavy fluc- values of reverse current which occur under fault con- tuating load such as motor starting, on one of the feed- ditions. The overcurrent elements can be set to pick up ers, can result in a reversal of power flow through the for currents above 100 to 200 per cent of the rated network protectors on that feeder. current of the protector. Because of these possibilities, network protectors are Network-Protector Fuses—The network protector is made with provision for the addition of a third relay. equipped with a set of fuses, one in each phase, elec- Without this relay, frequent reversals of power flow trically located between the circuit breaker and the would cause network-protector "pumping"; this would terminals for connection to the system secondary- result in excessive maintenance of the protectors. voltage grid. The primary function of the network- The desensitizing relay consists essentially of a ther- protector fuses is to provide back-up protection for mal timing element and three overcurrent elements. the protector. The fuses should promptly and definitely The contacts of the timing element and the contacts of open the circuit, if the protector fails to trip on a fault the three overcurrent elements are all in parallel. All causing a reversal of power, such as a fault in a trans- elements of the desensitizing relay are mounted in a sin- former, on a primary feeder, or in the secondary con- gle, quick-detachable case with plug-type terminals nections between the protector and its associated similar to the master and phasing relays. When the transformer. The blowing time of the fuses should not relay is removed, it must be replaced with an auxiliary be so short, however, that the network master relay panel that completes the current and control circuits. does not have ample time to operate and trip the pro- The instantaneous trip elements are small solenoid- tector when it is functioning normally. Also, the blow- operated contactors that can be adjusted by varying the ing time of the fuses should be long enough to allow position of their contacts and cores to operate at ap- network faults sufficient time to burn clear, or be proximately 100 to 200 per cent of the protector rating. cleared by limiters if they are used, before the fuses The timing element consists of a bimetallic actuating blow. Thus the overall tripping time of the network- spring and heating transformer. Its motion is opposed protector circuit breaker under fault conditions, and by a second piece of bimetal that supports a stationary the time required to clear secondary faults, are the contact and provides ambient temperature compensa- factors establishing the lower time limit for the current- tion. Operating time is varied by limiting the travel of time curves of the fuses. the moving contact between adjustable top and bottom The upper time limit for the current-time curves of stationary contacts. The entire operating cycle employs the fuses is governed by the following factors: It is both heating and cooling times of the bimetallic ele- desirable to have the fuses protect the network trans- ment, so as to reduce errors that would otherwise result former against dangerous temperatures due to high from the cumulative heating effects of successive oper- values of current over as wide a range of overload and ations. This is accomplished by an auxiliary relay. The fault currents as possible. Fuse blowing time should timing element introduces a time delay in the tripping also be coordinated with the tripping time of the asso- operation of the protector for low values of reversed ciated primary-feeder circuit breaker under fault con- current, such as may occur because of fluctuating loads. ditions on the secondary grid, or on another feeder. The ovefcurrent elements permit tripping of the pro- The ideal coordination of secondary-network protector without intentional time delay for high values of tective equipment is illustrated in Fig 18. Because of reverse current, such as will occur for faults in the net- practical design limitations of cable insulation and work transformer .,or its associated primary feeder. limiters, such ideal protection cannot be achieved, and The paralleled contacts of the timing element and typical protective coordination is illustrated in Fig. 19. the overcurrent elements are in series with the master relay tripping contacts in the trip coil circuit. When 8. Primary Feeders the master relay tripping contacts are closed by a small All secondary network systems utilize multiple pri- value of reversed current, the timing element is ener- mary feeders. The system is so designed through em-

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 175/576 168 Secondary Network Systems

10,000 1 i CLASS L260 INSULATION 5000 -4/0 CON— STATION BREAKER DUCTOR 3000 NETWORK PROTECTOR 2000 FUSE PRIMARY FEEDER STATION 1000 BREAKER

500 DS 300 ON

EC 200 IN S 100 ME IDEAL LIMITER TI 4/0 CONDUCTOR 50

20 (0 NETWORK PROTECTOR BREAKER

I0 NETWORK PROTECTOR FUSE

3 00 0 0 0 0 O 0 0 0 0 00 0 00 0 O 0 0 vin 0 0 0 0 0 0 8 0 LIMITERS C%1 re) In O ci O O o \ In 0 o CURRENT IN AMPERES

Fig. 18—Ideal coordination of network-system overcurrent protective devices.

ployment of network units, and design of secondary circuits, that service to the load is uninterrupted despite the de-energization of one, or in some systems, two primary feeders. Discussion in previous sections has dealt with the secondary mains and network units, and has lent itself to treatment of these two system components sepa- STATION BREAKER rately. However, primary feeders can best be discussed only from the standpoint of the system as a whole. In 500 the secondary-network system, the design of primary cn z 300 feeders is interrelated with network units and second- 0 CLASS L 260 • 200 — NSULATION aries. An important factor in this interrelationship is In 4/0 CONDUCTOR the "application factor." z w 100 Application factor is defined as the ratio of installed X network-transformer capacity to load. This factor is an 17- 50 extremely important parameter of network design, since the total capacity of network transformers re- 30 N ETWORK quired to serve a given load will be directly proportional 20 PROTECTOR FUSE to the application factor. This factor can be considered as the reciprocal of the more commonly used ratio of 10 peak load to installed transformer capacity, where the CL ASS L260 INSULATION IIII peak load is the design load for the network. The 5 4/0 LIMITER method employed for obtaining the application factors

3 1 1 1 used in secondary-network design is as follows: 400 1000 3000 10,000 50,000 200 000 Consider a conventional hypothetical network system 500 2000 5000 20,000 100,000 serving equal loads. Let these loads be represented by CURRENT IN AMPERES one unit of load at each junction of the secondary grid, Fig. 19—Practicably attainable coordination of network- which in turn is assumed to consist of square meshes system overcurrent protective devices. making up a square-shaped network. Network trans-

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 176/576 Secondary Network Systems 169 formers of a given rating are located with one at each TRANSFORMER SECONOARY MAIN junction. Each network transformer has an impedance /ccfr ZT, and the impedance of each section of secondary is UNIT ZM. Such a system, with six meshes, or seven trans- LOAD formers, in both directions, is shown in Fig. 20. Any given number of primary feeders (up to 49, i.e., one for each transformer) can serve such a system. Some possible primary-feeder arrays are given in Table 11-A.

Table 11-A Primary-Feeder Arrays

Two-Feeder Array 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 Three-Feeder Array 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 Fig. 20—Network used for D-C Calculating Board study to 2 3 1 2 3 1 2 obtain network-transformer loadings shown in Fig. 21. 3 1 2 3 1 2 3 1 2 3 1 2 3 1 culating board, and the loadings of network trans- Five-Feeder Array formers throughout the system were tabulated for the 1 2 3 4 5 1 2 condition of one feeder out of service, as the number of 3 4 5 1 2 3 4 primary feeders and ZM /ZT were varied. 5 1 2 3 4 5 1 2 3 4 5 1 2 3 As an example of the procedure for conducting this 4 5 1 2 3 4 5 study, consider the 5-feeder array. Removal of feeder 1 2 3 4 5 1 2 No. 1 from service leaves the array of in-service feeders 3 4 5 1 2 3 4 shown in Table 11-B. Ten-Feeder Array 1 2 3 4 5 6 7 Table 11-B 8 9 10 1 2 3 4 Five-Feeder Array Omitting Feeder 5 6 7 8 9 10 1 No. 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 2 3 4 5 2 6 7 8 9 10 1 2 3 4 6 2 3 4 3 4 5 6 7 8 9 5 2 3 4 5 2 3 4 6 2 3 4 5 2 3 4 5 In these arrays, each number identifies the particular 2 3 4 5 2 feeder serving the network unit located at that position 3 4 5 2 3 4 in the system of Fig. 20. Note that the interlacing of two, three, five, or ten primary feeders in each array is such that when one feeder to the network is out of serv- In setting up the 5-feeder, 49-transformer, 49-load ice, the increased loading on the network units served system on the calculating board, the following condi- by the remaining feeders is distributed as equally as tions were satisfied: possible. 1. All loads draw 1 pu current, both beforg and after The application factor is based on the loss of one a feeder outage. primary feeder. Thus the factor is a function of the 2. All sections of secondary main have equal im- following: the number of primary feeders; ZM and ZT, pedance. or the more commonly used term Z M/ZT ; and the extent to which load is not uniformly distributed among the From observing the primary-feeder array with feeder network units remaining in service when one primary No. 1 out of service, shown in Table 11-B, it will be noted feeder, out of the total number being considered, is out that the unsymmetrical pattern of locations for the of service. To investigate the relationship between these transformers served from the feeder out of service will factors, the system of Fig. 20 was set up on a d-c cal- result in unequal loading of the transformers remaining

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 177/576 170 Secondary Network Systems in service. The distribution curves of Fig. 21 represent conditions are more numerous and more severe the results of this statistical survey of transformer as ZM /ZT increases. loadings under first contingency, i.e., with one feeder out of service. The philosophy of secondary-network system design In Fig. 21, the per cent of all the network units in is that all load is to be served without service inter- the system that are loaded within the per cent load ruption, or undue damage to the system, under the intervals indicated are shown for two, three, five, or contingency of one primary feeder out of service. But ten feeders, and for values of ZM /ZT of 0.25, 1.0, and since a primary-feeder outage is a random occurrence 4.0. Study of these network-transformer-loading dis- which may involve any one of the feeders to the network, sufficient network-transformer capacity must be tribution curves under first contingency verifies several important effects: provided so that even the extremely loaded transformers of Fig. 20 are not loaded beyond some maximum allow- 1. As the number of feeders to the network increases, able loading under first contingency. Thus the maximum the dispersion or "spread" in transformer per cent per cent loadings in the loading distribution "histo- loading diminishes under first contingency. grams" of Fig. 21 permit determination of the applica- 2. As the ZM /ZT ratio decreases, and the network tion factor required to keep network transformers from units can be considered to be more nearly bussed, being loaded to more than a given value, in a system dispersion in transformer loading also decreases. having a specified ZM /ZT ratio and number of primary 3. Relatively few transformers are loaded either ex- feeders. Specification of normal loading (i.e., loading tremely high or extremely low, relative to the before a feeder outage) determines the kva represented majority of loading values, but extreme loading by the per cent loading values of Fig. 21.

2 PRIMARY FEEDERS 3 FEEDERS 5 FEEDERS 10 FEEDERS 100 ZM/ZT = 0.25 0.25 0.2 0.25 80 7 60 .7/ 40

20 v0

cf) 0

z D100 ZM /Z T = 1.0 1 1.0 1.0 1.0 CC O• 80

•z 60

_J et 40 u. 0 20 z V 0

w• 100 a. ZM /Z T = 4.0 4.0 4.0 4.0 80

0 100 140 180 220 100 140 180 100 140 180 100 140 PER CENT OF NORMAL LOADING

Fig. 21—Statistical distribution of network-transformer loadings under first contingency, i.e., one primary feeder out of service.

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Figure 22 is a composite family of curves obtained loop or grid. In practical operation, the reliability of from Fig. 21, and shows the transformer application service from the secondary network is limited by the factor versus Zm/ZT for different values of feeders to reliability of supply to the network feeders; hence, the the network. This figure is the basis for determining power supply to the network should be made to coin- the amount of network-transformer capacity required cide with the reliability of the network itself. to serve a given amount of load in a system having a stipulated number of feeders and Zu/ZT ratio. The primary feeders to a secondary network system III. PLANNING A CONVENTIONAL SECONDARY may originate at a distribution substation, at a bulk NETWORK power substation, or at a generating plant. It is desir- Although recent progress in the development of new able that where possible all primary feeders to a single types of secondary-network systems has introduced network originate at the same substation. Voltage such concepts as spot networks, the spot-network-radial magnitude and phase angle differences between feeders system, the use of higher utilization voltages, and dual supplying the same network are more likely to occur utilization-voltage systems, a large number of load with feeders supplied from different sources. Such volt- areas still fall within the range of application of the age differences, if appreciable, result in improper load conventional secondary network. division among network units, effectively reducing The conventional low-voltage a-c secondary network transformer capacity for supplying network load. system is generally of the distributed type; i.e., single- During light-load periods on a network supplied from transformer network units feed into the gridwork of two or more electrically-remote bulk sources, power secondary mains. Multiple-transformer vaults are com- flow may actually be in a reverse direction in some of the monplace, however, and in systems where secondary feeders. Many unnecessary network-protector opera- mains are short and heavily conductored, such main tions may occur with consequent increased maintenance circuits are really busses. In the conventional types of requirements on the protectors. systems, primary-feeder voltages are in the 5-kv and It may be necessary, in the case of separate supply 15-kv classes, with the latter predominating. A relative- substations, to plan the network on the basis of one ly small number of systems utilize primary-feeder volt- substation being out of service, instead of only one ages above 15 kv. Network transformers are rated 300, primary feeder. The reserve network transformer ca- 500, 750, and 1000-kva three-phase, with low-voltage pacity required in such a case becomes very costly, rating of 216Y/125 volts. This transformer low-voltage and generally it is better to provide a firm source of rating applies to systems where the nominal utilization supply at each substation supplying the network voltage at the load center is 120/208 volts. feeders. This can be done by using multiple-trans- Most secondary networks are begun in existing load former substations supplied from a sub-transmission areas, although exceptions are encountered in shopping centers and in supply to commercial buildings. Existing loads in downtown commercial areas of towns and cities, where a secondary-network system is contem-

2 . plated, are usually supplied by a radial, or banked- EDR secondary system which may be in part, or entirely, overhead or underground. The same load area may contain two different sets of distribution transformers and . secondaries for single-phase lighting and three-phase TOR . power supply. Usually load density governs whether the existing N FAC . system is overhead or underground. In smaller cities EDR where downtown or commercial-area loads are in the CATIO I . - form of one- or two-story buildings with stores and . offices, the existing system prior to secondary-network installation is generally overhead. Load densities in ER APPL . such areas are the order of 150 to 300 kva per square EDR RM block, or 15,000 to 30,000 kva per square mile. Primary FO . feeders and secondaries are often carried in alleyways, . -.-- with distribution transformers mounted on poles or TRANS 1 EDR platforms. In larger towns and cities where commercial . establishments resemble those of the big city, an underground system, possibly of the radial type, exists at the . time of conversion to a secondary network. In the following discussion, a step-by-step method of O 2 3 4 5 planning a conventional secondary-network system will Zm/ZT RATIO be given. No two network systems are exactly alike, Fig. 22—Network-transformer application factors used as and the example accompanying the discussion is in- a basis for network studies. tended for illustration purposes only.

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and conductor type and size determine whether voltage 9. Required Basic Information regulation in the primaries will be satisfactory. The The following information should be available before current-carrying capacity of all feeders should be such starting to plan a secondary network: that the network load can be carried with any one Locations and Sizes of Loads—The location and size of feeder out of service. the various loads should be known, especially for the Power Source—The location and capacity of the avail- large loads in the area. Location and rating of existing able source of power for the network primary feeders transformers can be used as a basis of estimating load must be known. It is desirable that all the primary distribution, but actual loads are more satisfactory. feeders originate at one bulk-power source having suf- Diversity among loads should be considered. Methods ficient capacity for the whole network load. Inasmuch for estimating and measuring existing loads are given as the primary feeders supply network transformers in Chapter 2. with secondary windings paralleled through the low- Figure 23 shows a map of the downtown area which voltage network grid, problems involving the circula- will serve as an example throughout this discussion. tion of power between remote bulk sources, via their The loads indicated are based on kva peak diversified respective primary feeders and the secondary network, demands. In general, there would be a larger number of can be eliminated if the network power supply is a individual metered customers than the number of loads single source. shown. Figure 23 represents the result of first combin- Existing Facilities—Knowledge of the locations and ing loads into lumped sums for ease of load represen- sizes of existing facilities is useful for establishing cir- tation, and then applying a reasonable multiplier to cuit routings and transformer locations in the proposed account for some load growth which would occur be- network, and for adapting the existing facilities to the tween the determination of existing loads, and the com- new plan. The network system can frequently make pletion of the network installation. Taking into account use of existing secondary conductors, primary feeders, network-system construction time, and economic spac- transformers, pole lines, ducts, and vaults to facilitate ing of capacity additions, a load anticipated to exist in conversion from the existing system. two or three years is a reasonable starting value. Circuit Routing—Preferred and available routings for The sum of the kva loads shown in Fig. 23 is 3000 secondary and primary circuits should be known. kva. Since the area within the network-service bound- Standards—Choice of ratings for equipment, or cir- ary is approximately 0.1 square mile, the load density cuit conductor types and sizes may be limited because is 30,000 kva per square mile. of the need to adhere to standard practices. Such infor- Primary Feeders—The number, voltage, size, length, mation should be given to the network planner. and carrying capacity of the primary feeders available In the illustrative example, assume that the network to supply the network must be known. The number of system is to be underground. The extent to which ex- feeders has a direct bearing on the necessary reserve isting facilities can be employed in-the network depends transformer capacity in the network. Feeder voltage on how much of the existing system is underground.

I L THIRD AVENUE — — —

15 32 10 68 59 133 73 228 237 87 188 54 55 69 42 37 12 21 28

FOURTH AVENUE 1— W cc w EET $-• 48 133 207 i cc 175 51 190 40 115 26 25-4j 45 15 58 5 NETWORK AREA 0 Ill STR BOUNDARY = = I- TH I-

FIFTH AVENUE

0 0 0 0 0 0 0 0 19 47 115 59 29 49 32 20 23 24 CV In Nr

FEET

SIXTH AVENUE— — —

Fig. 23—Load area for proposed secondary network system, showing locations and values of kva peak diversified demand.

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OP this quantity is reduced, load division among trans- 10. Preliminary Design formers improves. A secondary network system consists of a number of Under ideal conditions it would be necessary to have functional components—primary feeders, network units, about twice as much transformer capacity as total load secondaries—operating in a coordinated fashion. This in a network served by two primary feeders, so that the coordination must be designed into the system. Con- network units served by one feeder could carry the sequently, each component cannot be designed inde- total load when the other feeder is out of service. Table pendently of the others. Using the illustrative example, 12 gives both the ideal and practically attainable ratio a procedure for preliminary design involves the follow- of peak load to total transformer capacity in networks ing considerations: supplied by 2, 3, 4, 5, or 6 feeders. (The reciprocal of Arrangement of Secondary Mains—Given the loads the ratio of load to capacity is the application factor. and street layout of Fig. 23, and taking into account Refer to Fig. 22 for a more comprehensive description existing facilities (neglected in the example), the sec- of the factors affecting this ratio.) Increasing the num- ondary mains can be routed as shown by Fig. 24. ber of feeders improves the ideal ratio rapidly for low Further Lumping of Loads—After the arrangement of values of total number of feeders, but slowly for net- secondary mains is accomplished, loads may have to works supplied by a large number of feeders. This is be further combined, as shown in Fig. 24, for two prin- especially true for the ratio that usually can be attained. cipal reasons: loads at adjacent corners of streets can The ideal ratio can be realized only if the transformers be considered as connected to the same electrical point can be loaded exactly to their capacities, and if the load in the network grid (a junction in most cases); and divides uniformly among the units in service at any middle-of-the-block loads may have to be further com- time. bined to reduce the total number of loads to a value which can be accommodated on a d-c calculating board, Table 12—Relationship Between Transformer Capacity the application of which will be discussed below. and Peak Load in a Secondary-Network System Number and Rating of Transformers—As in most other Ratio of Peak Load aspects of secondary-network design, there is no exact to Transformer Capacity formula to apply at this step. Note that the number of Number of Feeders secondary-mains junctions is ten. It is always desirable Ideal Usually Attainable to install one or perhaps more network units at a sec- 2 0.50 0.40 ondary-main junction. This permits better load sharing 3 0.67 0.54 between transformers, and generally lower current and 4 0.75 0.58 voltage drop in secondary circuits. Exceptions are often 5 0.80 0.60 encountered where large loads are located in the middle 6 0.83 0.61 of blocks, and in these cases one or more transformers must be located at such points. The example includes A possible arrangement of 500-kva network trans- no such case. formers to serve a total load of 3000 kva is shown in In order to avoid excessive loading of network units Fig. 25. Eleven transformers are used. Assuming that when one primary feeder is out of service, the total the maximum-allowable load on each transformer is network-unit capacity required must exceed the peak 500 kva, the ideally attainable ratio of peak load to load. The necessary installed capacity depends on the installed network-transformer capacity is number of feeders and on how well the load divides 3000 kva among network units, particularly when a feeder is out (11)(500 kva) — 0.544 of service. This latter criterion can be gaged by the average ratio of two impedances: the impedance of the O section of secondary mains between transformers, and FEEDER NO:•-®l 0 transformer impedance. This ratio is called Zm /ZT. As

57 127 7 7 7 7 124 79 61 57 127 206 465 87 242 124 79 61 0

7 48 133 382 141 70 77 48 133 382 51 230 141 70 77

66 174 29 81 67 174 29 81 87 66 Fig. 25—Network-transformer locations and primary- Fig. 24—Secondary-mains arrangement, and lumped loads. feeder interlacing.

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In the network unit, the network transformer is com- Table 13—Current Carving Capacity of Single- bined, with a network protector having a current rating Conductor 600 Volt Cables in a Duct Bank greater than the kva-rating of the transformer. In many Based on not exceeding a copper temperature of about 77°C systems, loading of network transformers above name- with an earth temperature of 20°C when installed in a duct plate kva-rating is permitted under certain operating containing one additional loaded three-phase circuit. Load fac- contingencies. tor assumed to be 75%. Primary-Feeder Considerations—Because of the previ- Cables Carrying Cables Carrying ously stated interrelated nature of primary feeders, net- Number Per Phase Capacity Number Per Phase Capacity work units, and secondary mains, it is desirable to and Size Amperes and Size Amperes temporarily leave network-transformer design factors at this point to examine the effect of primary-feeder 1-1/0 213 1— 250 MCM 346 considerations. 2-1/0 394 2— 250 MCM 652 Figure 25 indicates the use of eleven 500-kva network 3-1/0 576 3— 250 MCM 921 4-1/0 718 4— 250 MCM 1179 transformers serving 3000 kva of load. Table 12 shows 5-1/0 845 5— 250 MCM 1390 that three primary feeders are a practicable number to 6-1/0 960 6— 250 MCM 1580 employ. With three feeders in service, each would carry 1-2/0 243 1— 300 MCM 382 1000 kva at peak load. With one feeder out, each of the 2-2/0 450 2— 300 MCM 720 remaining two would carry 1500 kva. This magnitude 3-2/0 651 3— 300 MCM 1020 of loading indicates that a primary-feeder voltage at the 4-2/0 815 4— 300 MCM 1264 4-kv level is suitable. But other factors may favor the 6-2/0 960 5— 300 MCM 1480 use of a higher primary-feeder voltage in the 15-kv 6-2/0 1090 6— 300 MCM 1660 class. 1-3/0 276 1— 350 MCM 418 Three primary feeders can be interlaced within the 2-3/0 518 2— 350 MCM 786 network area so that adjacent network transformers are 3-3/0 741 3— 350 MCM 1107 served from different feeders, as indicated in Fig. 25. 4-3/0 935 4— 350 MCM 1417 Note that two transformers are located at the relatively 5-3/0 1090 5— 350 MCM 1655 large 465-kva load. Also note that primary feeder No. 1 6-3/0 1254 6— 350 MCM 1860 serves three network units, while feeder Nos. 2 and 3 1-4/0 315 1— 400 MCM 451 2-4/0 594 2— 400 MCM 846 each serves four units. 3-4/0 843 3— 400 MCM 1194 Network-Protector Considerations—The network unit 4-4/0 1074 4— 400 MCM 1492 consists of both a network transformer and network 5-4/0 1265 5— 400 MCM 1745 protector. The network protector is a maximum-rated 6-4/0 1440 6— 400 MCM 1960 device; i.e., it has no allowable overload rating, but the 1— 500 MCM 614 1-1250 MCM 845 usual rating employed with a 500-kva 216 Y/125 volt 2— 500 MCM 960 2-1250 MCM 1562 transformer is 1600 amperes. The current equivalent to 3— 500 MCM 1350 3-1250 MCM 2175 500 kva at this voltage is 1340 amperes. Depending on 4— 500 MCM 1700 4-1250 MCM 2765 the duration of network peak load, ambient tempera- 5— 500 MCM 1985 5-1250 MCM 3220 ture, and ventilation of network-unit vaults, the net- 6— 500 MCM 2230 6-1250 MCM 3660_ work transformers may be loaded above their ratings, 1— 600 MCM 568 1-1500 MCM 926 but no higher than network-protector rating. Short- 2— 600 MCM 1058 2-1500 MCM 1710 time overloads on network transformers may then 3— 600 MCM 1488 3-1500 MCM 2373 reach 120 per cent of nameplate rating. In connection 4— 600 MCM 1874 4-1500 MCM 2890 5— 600 MCM 2190 5-1500 MCM 3330 with this practice, however, possible loss of life should 6— 600 MCM 2480 6-1500 MCM 3630 be taken into consideration where transformer loadings 1— 750 MCM 641 1-1750 MCM 991 are in excess of nameplate rating. 2— 750 MCM 1192 2-1750 MCM 1822 Secondary Mains—Choice of three No. 4/0 AWG cop- 3— 750 MCM 1674 3-1750 MCM 2529 per conductors per phase is made from Table 13. The 4— 750 MCM 2080 4-1750 MCM 3120 ratio of secondary-mains to transformer current-carry- 6— 750 MCM 2420 5-1750 MCM 3690 ing capacity is then 843/1390 = 0.6. A rule of thumb 6— 760 MCM 2680 6-1750 MCM 3910 often used is that secondary mains should be able to 1-1000 MCM 747 1-2000 MCM 1053 carry one-half to two-thirds of the rated capacity of 2-1000 MCM 1384 2-2000 MCM 1934 the predominant size of transformer. In the case of 3-1000 MCM 1938 3-2000 MCM 2679 this example, eventual load growth will necessitate the 4-1000 MCM —2450 4-2000 MCM 3320 installation of additional network units at the second- 5-1000 MCM 2860 5-2000 MCM 3840 ary-mains junctions. This practice results in the avail- 6-1000 MCM 3150 6-2000 MCM 4180 ability of transformer capacity at mains junctions even with a primary feeder out of service; also, a relatively sated sections. Although loads are lighter along the lighter burden is placed on secondaries with respect to network-area periphery, these loads are supplied from load transfer when one feeder is out of service. Three only one or two directions, and perhaps only one direc- No. 4/0 conductors per phase are employed in the tion when some transformers do not carry any load dur- fringe sections of mains as well as in the centrally lo- ing a primary-feeder outage.

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of 4.34 ohms. Choice of multiplier depends on the 11. Use of the D-C Network Calculator ohmic values of the board rheostats, and the The d-c network calculator is an analog computer. accuracy with which they can be set, particularly Although there are many different designs, they all at low values. operate on the same basic principles. On the d-c calcu- 3. Since 4.34 ohms resistance on the d-c board is lating board, the secondary-network system circuit, equivalent to five per cent impedance on a 500- transformer, and load impedances are represented by kva base, then 100 per cent impedance on that rheostats. These rheostats are equipped with readily base is accessible terminals, and are connected together by MO means of flexible, removable leads of negligible resist- (4.34)(--5—)= 86.8 ohms ance. The network of rheostats in the d-c calculating Hence, a load of 500 kva can be represented by a board simulates the circuit meshes of the actual net- resistance of 86.8 ohms. A tabulation is then work system. Each rheostat can be adjusted so that its made of the kva values of all loads in Fig. 24 with resistance, in ohms, is a multiple of the impedance of the corresponding board-ohm representation. A the particular system element it represents. 50-kva load would have an initial board-rheostat In the actual network system, load impedances, in setting of 868 ohms. ohms, are 50 or more times greater than network- transformer or secondary-mains branch impedances. 4. In the event that initial choice of system-to-board Consequently, some d-c boards are equipped with two ohms multiplier does not yield values of resistance sizes of rheostat, one size to represent circuit imped- within the range of the board rheostats, it is some- ances, and a size of perhaps 100-times greater in maxi- times feasible to combine rheostats in series and mum resistance to represent loads. When only one size of parallel to obtain the correct ohmic values. If too rheostat is employed for all the board elements, it must many such resistance combinations are required, have a range and accuracy to permit setting ohmic a new system-to-board multiplier must be chosen. values over a one hundred-to-one range, or more. 5. A suitable value for d-c board voltage is chosen. Many years ago, when the d-o calculating board was This voltage is impressed between the positive first adapted to the study of a-c secondary-network and negative busses. Assume 20.8 volts for illus- systems, considerable investigation was carried out to tration purposes. The multiplier to convert board determine whether the d-c analog permitted a sufficient- amperes to system amperes is obtained from the ly accurate representation of an a-c system. Results of formula these extensive investigations showed that for the purposes of obtaining current distribution and kva-flow in I system _ systemXR board ) a secondary-network system, for both load and short- I board E board Z system circuit studies, the d-c calculating board proved to be )(1000` a surprisingly accurate tool. = (1) (10)4 System Representation on the D-C Board Wiring up or (20.8 1 "plugging" the d-c calculating board to represent the network system is a simple procedure in which pro- Hence one milliampere current on the d-c board ficiency can be quickly acquired. Input terminals of is equivalent to ten amperes in the actual system. rheostats representing transformers are connected to D-C calculating board voltage must not be set the positive bus on the board, and output terminals of so high that rheostats carry current in excess of all rheostats representing loads are connected to the their current or wattage ratings. The most heavi- board negative bus. After the d-c calculating board is ly-loaded load rheostat carries a current approx- correctly wired, the rheostats must be adjusted to ap- imately equal to board voltage divided by the propriate values. This step can be carried out in many value of resistance representing the largest kva different ways, and depends upon the particular type load. Paralleled resistors should be taken into of board employed. One possible method is as follows: account in checking rheostat loadings. 1. Calculate the ohms impedance of a typical net,- 6. Adjustment of load rheostats is usually required work transformer, from its known per cent im- at this point, so that d-c board load currents are pedance. For a 500-kva transformer having five equivalent to the desired values of load current per cent impedance, or kva. These adjustments allow for voltage drops in the secondary-grid circuits. Sometimes it is (% Z)(10)(kv)2 Z ohms = desirable to read kva flows as well as currents, in kva which case another multiplier is determined which (5) (10) (.208)2 will permit reading kva on the d-c board am- — 0.00434 ohms 500 meter. 2. Select a suitable multiplier for converting system Load-Flow Studies—The d-c calculating board repre- ohms to board ohms. For example, if the multi- sentation of the system is complete when the loads are plier is 1000, the rheostat representing each 500- drawing the correct currents, and all circuit and trans- kva network transformer would be set to a value former rheostats are correctly set. The next step is

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176 Secondary Network Systems

commonly called a load study, and consists of obtain- circuit branches, three or more conductors per phase ing the flow of currents in the network under various are prescribed for this reason, and others. conditions of system operation. The two types of op- The load study is made to determine the best com- erating conditions studied are: all primary feeders in bination of network-transformer ratings and locations, service; and one primary feeder out of service. Figures with secondary-mains routings, conductor size, and 26-a, b, c, and d show the results of these board studies, number-per-phase. Changes in any one or more of in which Fig. 26-a applies for all three primary feeders these design and cost factors can be made quickly by in service, and the subsequent figures show load-current rheostat and circuit changes on the d-c calculating distribution with Primary Feeder No. 1, 2, or 3, re- board. Figures 26 to 29 show the results of such a spectively, out of service. cut-and-try process of system design. In instances where a particular section of secondary Short-Circuit Studies—A d-c calculating board is also main carries current well below thermal capability, employed for short-circuit studies. The purpose of the some attention might be given to a reduction in the short-circuit study is to determine whether faults in sec- number of conductors-per-phase employed in these ondary-mains conductors will be self-clearing, or whether sections. In no case, however, should only one con- limiters are required to assure secondary-fault clearing. ductor-per-phase be employed, as a fault would dis- In systems that are entirely limitered, the short-circuit continue the flow of three-phase power in that section. study serves to check the coordination of system over- Sometimes it is feasible to use a smaller conductor size current-protective devices, e.g., limiters with other in some mains sections, but standardization of conduc- limiters, and limiters with network-protector fuses. tor size is desirable. After a long period of network Loads may be disconnected, or left on the network operation, occurrences of faults in secondary conductors while the short-circuit study is made. Generally fault over the years may result in the opening of all con- currents will be so high in the neighborhood of a fault, ductors associated with one phase in some network and secondary voltage drops so great, that load con- branches, so that to minimize the risk of single-phasing tributions to system currents are negligible under fault

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325 1901 1851 3501 12 50 0 860 800 ( ) (d) (-1/ V 175 • 225 195 r•5 (85 185 r_— 25 55 fr/f 175 475 BO 210 185 185 480 80 225 165 Fig. 26—Load-flow results obtained from D-C Calculating-Board study. Currents in system amperes. (a) All primary feeders in service (b) Primary feeder No. 1 out of service (c) Primary feeder No. 2 out of service (d) Primary feeder No. 3 out of service

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Secondary Network Systems 177 conditions. The particular electrical point in the system 6080 26500 27000 5100 1200 where a fault is to be assumed is connected directly to IA C7750/ 1450 the negative bus of the d-c board. If it is desired, fault impedance can be introduced. 71500 Theoretically, the number of different points in the 1350 1 79001 12001 secondary system where a fault can occur is infinite. 2200 4650 1600 Fault-current distribution in the network would be :.5e( 850 . ...V (4' different for every fault. It is necessary, however, for any fault to be cleared. Larger systems have sufficient network-transformer capacity to assure the conductor 3001 melting and vaporization necessary for self-clearing of (a) faults. Smaller networks may not be able to supply sufficient currents to secondary cable faults to assure self-clearing. In some cases, choice of a larger number of smaller-size conductors per phase is desirable. In 13100 other cases, limiters are employed at both ends of single-conductor cable sections where self-clearing of 550 1100 1100 1550 700 faults is not assured. Xt de r-V5(1. The limiter is a special type of fuse with a blowing • 20q • •• 200 • ..11_3 0 characteristic designed to protect the cable in which it is placed from sustaining insulation damage caused by overload currents. Limiters are discussed more fully 1700 22001 7550 20501 7600 4001 1700 elsewhere in this chapter. (7 2009 r 13750 13600 F moo The first case in the d-c calculating board short- / ! circuit study is a single-phase fault placed at the electrical point on the secondary grid where maximum 27350 A fault current occurs. Figure 27-a shows the current 19501 1950 19501 1950 flow in the example network for this maximum-current (b) single-phase fault located at the secondary-mains junction in the two-transformer vault. The current flowing 7%' to a single-phase fault on the secondary of a transformer Fig. 27—Results of D-C Calculating Board short-circuit on the periphery of the network is also shown for study for faults at various locations on the secondary grid. contrast. Fault-current flow for a fault location chosen Currents in amperes. at random is shown in Fig. 27-b. Location for the a) Maximum and minimum single-phase fault currents maximum fault current can usually be found by in- on secondary grid spection of network-transformer and secondary-mains b) Fault selected at random arrangement. To determine whether a particular fault is self- clearing, Table 2 is consulted. This table shows the That is, the side of the fault where current is higher minimum current, as determined by laboratory tests, will clear first. This opens the circuit to the fault from which is required to burn clear a faulted single-con- that side, changing the current distribution in the ductor cable. The ampere values shown are the mini- network and resulting in a higher current on the other mum currents required to flow into the fault from side of the fault in the faulted conductor, thereby speed- each side of the fault. The values for underground ing the clearing of the fault. circuits are for either lead-covered or non-leaded cables. Use of More Than One Network-Transformer Rating— Values for overhead circuits are for weatherproof con- As the example discussed here illustrates, the secondary ductors on racks. network loads may be different in kva size over a wide To investigate the effects of faults on a single con- range. Certain "spot loads," although few in number, ductor rather than on all the conductors, or on a bus may account for a sizeable percentage of total system associated with one phase, the d-c calculating board load. In the system design of Figs. 25 to 27, large spot wiring must be expanded to include circuits repre- loads are recognized by the installation of two network senting individual single-conductor cables in the vicinity units at such locations. During the loss of a primary of the fault. Fig. 28 is an example of this type of feeder, at least one of the units would always remain study, showing a part of the network, with a half in service, regardless of the feeder affected. Although section of secondary main represented by three single this practice is highly desirable, choice of a relatively conductors per phase. A zero-impedance fault is moved small rating of network unit means that through time, along one of the 4/0 AWG conductors to observe the as network load grows, many more vaults would contain change in current distribution as fault location changes. multiple-transformer installations than would be the The burning-clear of the conductor at the point of case if a larger size had been initially chosen. fault, or the blowing of limiters located at the end Fig. 29 shows a system design which modifies terminals of the conductor section occurs sequentially. network unit ratings in the vicinity of the heaviest

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(a) ampere network protectors. Hence, the transformers are loaded to 120 per cent of nameplate rating when the network protector carries its rated current. Note that in Fig. 29(c), with primary feeder No. 2 out of service, one of the 500-kva network transformers carries 1400 amperes, which is slightly in excess of its kva rating, but well below the 1600-ampere rating of its associated network protector.

12. Load and Network System Growth The true test of any plan is its success in meeting (b) 415 0/ 63501 22 5 0:y 2000 future requirements. Although the a-c secondary net- 1050j work is a highly flexible system for meeting future load 1100 6100 growth, the magnitudes and locations of future loads are uncertain. The nature of load growth in downtown 7100 1(3100 45000 commercial areas has been characterized in recent years 8001 6050 by two trends: a general overall increase in load density; / 1 and a more marked increase in the development of 48100 relatively large "spot" loads. The first trend is due to the general improvement in store and office lighting and in the use of electrical appliances such as window air (c) conditioners, electric water fountains, etc. The second trend is the more significant from the system-planning standpoint, and its cause can be traced to the construction and renovation of department stores and office buildings. These newer buildings are equipped with central air conditioning, and in general have a higher level of lighting and electrical-equipment usage. At the time of initial network-system installation, 5200 some large spot loads may already exist, as illustrated by the example of Fig. 23. Such loads may already represent relatively modern completely air-conditioned stores (d) or offices. The per cent increase in magnitude of these initial spot loads would consequently be less than increases in initial light-density locations where new or renovated stores and buildings are likely to develop. Study of network system requirements for increased load should take into account the most accurate information available as to forecasted load developments. 17450 In the event that no such specific information is available, all initial network loads can be multiplied by the same percentage to obtain a new load pattern to study network-expansion requirements. (e) For the purpose of illustrating system planning for load growth, Fig. 31 shows each of the loads in the original 3000-kva of system of Fig. 24 doubled, so that total load is 6000 kva. The number of 500-kva network units has been increased from 11 to 19. The new ratio of peak load to installed network-transformer capacity is 6000 63 8300 (19) (500) — Fig. 28—Results of study of different fault location along a section of secondary main, and change in fault current flow A fourth primary feeder is added. Table 12 indicates when one side of the fault has burned clear. that the usually-attainable ratio is 0.58. Therefore the design of Fig. 31 may not be quite satisfactory from this standpoint, but further study will check this point. load on the system. This modification employs two The addition of network units at the locations of single-unit vaults containing 750-kva network trans- large spot loads improves load division among network formers, replacing three 500-kva units originally located units when a primary feeder is out of service. When load on the basis of two units in one vault and one in another. is doubled and network transformers are added in or The 750-kva transformers are combined with 2500- adjacent to existing vaults, secondary mains are

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0 1160 0 790 1515 8357 635.. 1290 960

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Y. 500 KVA TRANSFORMER

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160 480 80 205 175 185 475 80 205 185 Fig. 29—Load-flow study of system employing two kva-ratings of network transformers. a) All primary feeders in service b) Primary feeder No. 1 out of service c) Primary feeder No. 2 out of service d) Primary feeder No. 3 out of service

generally required to carry higher currents under normal Fig. 32 shows the expanded system using a combi- conditions. But only in a very few cases do secondary nation of 500-kva and 750-kva network units serving mains carry substantially higher currents with a primary 6000 kva of load. Use of the 750-kvs, units permits a feeder out of service. reduction in the total number of network units in the system. However, there are four primary feeders, and one of the four 750-kva units must be associated with a 122 primary feeder out of service. The load on the in-service 114 254 unit in the vault with one unit out can be expected to be excessive, as shown in Fig. 32 (b). Consequently for this system, the use of one 750-kva network unit to replace two 500-kva units in certain locations is not permissible.

96 266 764 282 140 154 13. Primary-Feeder Outage Probability In the preceding example, network design is based on "single contingency," i.e., the outage of one primary feeder. In smaller network systems, probability of the coincident outage of two primary feeders is remote, 132 348 162 134 generally less likely than the loss of the bulk-power Fig. 30—Network-area loads doubled, for the study of source. Consequently, design based on single contin- future system required to handle load growth. gency is satisfactory. However, as the network system

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180 Secondary Network Systems

increases in size, primary feeders are added, and as the feeder outages may be caused by cable faults; splice, number of feeders increases, the likelihood that two will pothead, and junction failures; network-transformer be concurrently out of service also increases. faults; and miscellaneous construction work accidents The probability of occurrence of coincident primary- resulting in physical damage to circuits or equipment. feeder outages can be expressed in terms of several fac- The curves of Fig. 33 show the effect of scheduled tors related to network design and operation. These factors are: 0 0 1. Total number of primary feeders. °830 830® 970 479700 CX) 1010 101.Z 995 2. Total mileage of primary-feeder circuit. , / , 3. Number of accidental feeder outages per year. 320 :.) r• 730 410 •-•• f• 100 375 .ivc• 350 345 r- 340 4. Scheduled feeder outage time per year. 7 7 /- // 7 5. Time duration of a feeder outage. 320 680 1140 2580 480 1340 695 430 340 401 1751 ® 401 ®® not 0 00 89.50 / Scheduled feeder outages are usually for the purpose 730 895 895 885 885 685 76),/ of testing, maintenance, or network extension. Conse- r• 425 310 44 /"....CL1145 •,,,., r• 320 455 xr: quently, the time of occurrence of such outages is known, J / /7 •-.\ 135 / / / / and their duration can be estimated. 270 735 2120 280 1280 775 390 430 An accidental feeder outage may occur at any time, 0 501 101 and its time of occurrence cannot be predicted. Nor is 750 750 it possible to know in advance the identity of feeders ( a ) which will suffer unanticipated outages. The prognosti- r• 120 40 cation of such outages can be expressed only in terms of their probable frequency of occurrence. Accidental a) All primary feeders in service

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/ 7.1— AX 370 935 160 440 370 365 925 160 435 370

d) Primary feeder No. 3 out of service e) Primary feeder No. 4 out of service

Fig. 31—Load-flow results obtained from D-C Calculating Board study of system serving future load.

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Secondary Network Systems 181

100 0 09 815 815 575 157P 65 9 95

r 660 480 r•- 210 270 21 r; 360 330 • 349 \/ —'' 1 / / / / / / Cfl CC 320 seo i 1140 2580 480 1340 690 440 4 340 301 0 351 0 0 1101 w 0 ® 651 >- 725.../ v20s.. 1303e ‘67 az Ns...7 10 F 3; 425 315 -;) 145 135 325 455 7./ O / /

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0 0 NUM 7;56„, (a) I.0 NO. OF ACCIDENTAL20 120 40 AGE FEEDER OUTAGES PER YEAR 370 960 160 450 370 AVER

750 KVA TRANSFORMER

500 KVA TRANSFORMER 0.1 0.001 0.01 0 1 1.0 YEARS PER YEAR I- 1 4 4 I I 1 I 1 1 1 1 I I 4 o o 0 00 0 0 0 00 0 0 0 00 ®0 0 0 .0 cv g-w) 0 0 0 00 0 0 0 00 CO 1050 v0 2650( 1110 i 1040 M a ir) 0 0 000 C4 M1•10 11 x: f: r; HOURS PER YEAR SCHEDULED FEEDER OUTAGE TIME / r Fig. 33—Average number of years between the occurrence 0 ® of an accidental primary-feeder outage during a scheduled 16109 00080 ( 0/ v 1600 feeder outage, i.e., a "second contingency." ••••• J<:*) •

O\ 1250 OO 1060" (b)

Fig. 32—Future system employing two network-transformer kva ratings. a) All primary feeders in service b) Primary feeder No. 1 out of service

feeder outage time on the probability that an accidental outage to a second feeder will occur during a scheduled feeder outage. As an example to illustrate how Fig. 33 can be used in secondary-network design and evaluation, consider a secondary network system in which primary feeders will be out of service for scheduled reasons, one at a time, for a total of 200 hours per year. Five accidental feeder outages per year are anticipated. What is the likelihood of an accidental feeder outage occurring during a scheduled outage, thereby resulting in a "second contingency"? In Fig. 33, for a total scheduled feeder outage time of 200 hours per year, and five accidental outages per year, the average frequency for the occurrence of an accidental outage during a scheduled outage is nine years. Fig. 34—Network-unit installation in an underground vault.

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Transformers—Transformers in an overhead network IV. SECONDARY NETWORKS IN OVERHEAD- are mounted on poles or platforms. The smaller units, SYSTEM AREAS e.g., 150 kva, can be pole mounted. Larger units, e.g., There is an increasing trend in the application of the 300 kva, are mounted on platform structures. The plat- secondary-network system in commercial areas of towns form may bridge an alley between two poles. Steel and cities where the distribution system is overhead. structures sometimes are used. In some cases the plat- The inherent flexibility of the secondary network makes form is built on a single steel column. In this arrange- it possible to place any or all system components either ment, the platform can be parallel with the alley and overhead or underground. After initiation of a network, may simplify the overhead structures in the alley, the usual evolution of the system follows a pattern of especially if the primary feeders are underground. conversion to underground-system construction, but The transformers may be either standard network both overhead and underground extension of the sys- transformers or banks of single-phase distribution tem is possible. transformers. Existing distribution transformers in an An overhead network system can be practically and overhead area can be used to form a three-phase bank economically applied in areas of load density ranging in the network when the ratings and transformer im- from approximately 12,000 to 100,000 kva per square pedances fit into the network plan. New units should be mile; but as load density increases, it becomes feasible three-phase transformers of either the distribution type to place a larger portion of the system underground. or standard network type. The latter are particularly Even in areas of the heaviest densities, fringes of the advantageous in the heavier load density areas, where network load area are often served from overhead conversion to an underground system is more likely to secondaries and network units. occur in the future. The standard network transformer is suitable for either underground or outdoor service. The three-phase network transformer is a long narrow 14. General Description of System Components unit that is especially adapted to platform mounting. The components of an overhead secondary-network A three-phase unit is generally lighter in weight and system are designed and operated on principles similar smaller in dimensions than a bank of single-phase trans- to those applying to an underground system. However, formers having the same rating. The three-phase trans- certain modifications to the classical network are ad- former simplifies the problem of connecting the trans- vantageously applied to overhead networks. former into the system. Secondary Mains—The secondary mains in an over- Some form of disconnecting means usually is used on head network usually are open-wire circuits on racks. the high-voltage side of the network transformer. The Weatherproof conductors are sometimes used. The simplest arrangement is to use cutouts with solid blades principal difference between overhead and underground in place of fuses. The load should be taken off the trans- secondary mains is that the overhead secondaries usu- former by opening the protector before opening the dis- ally consist of a single conductor per phase. connects. On small units, hot-line clamps can be used The conductor size required for the secondary mains under favorable conditions, but there is considerable depends on the transformer rating. Where the secondary hazard of improper operation and they are not recom- mains form a grid, the conductor size should be capable mended. Hot-line clamps are adequate when used in of carrying about sixty per cent of the full load current conjunction with a transformer-mounted high-voltage of the largest transformer used in the network. If the switch that can open the exciting current of the trans- secondary mains form a ring or loop, the conductor former and disconnect the transformer windings before carrying capacity should be about 80 per cent of the the hot-line clamps are removed. Fuses should not be full load current of the largest transformer in the net- used in the primary leads of transformers in a network work. A loop-secondary system will, in general, re- system, because of the hazard of opening the fuses when quire a secondary conductor of higher current-carrying a secondary fault occurs. Such opening of fuses can capacity than a grid system. interrupt the operation of the network and cause a Clearing secondary faults on an overhead secondary complete outage. network depends on burning off the fault just as in a Network Protectors—The network protector usually is radial system, except that generally there is more fault mounted on the pole below the transformer when small current available to burn off faults in the network sys- transformers are mounted on the pole. The low-voltage tem. The secondary current for burning off faults on leads from the transformer to the protector and from the overhead secondaries is shown in Table 2. The current protector to the secondary mains sometimes are carried required to burn clear faults in overhead secondary net- in conduit to minimize the congestion on the pole. For work mains is less than that required in underground the larger units that are mounted on platforms, the pro- mains because of the tension in the overhead open-wire tector is mounted on the platform of pole, near the conductors. The spacing between conductors supported transformer, or on the transformer. The preferable on racks reduces the likelihood of secondary faults. method is to use a standard network transformer with a Limiters can be used to minimize the hazard of burning transformer mounted protector. This uses the minimum down conductors. The limiters should be connected of space and eliminates the leads between the trans- around strain insulators so that the limiters are not former and the protector. Weatherproof network pro- stressed by the conductor tension. tectors are available in the medium-duty and light-duty

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 190/576 Secondary Network Systems 183 ranges. In the heavy-duty range, the submersible protector is used for outdoor service. S . 4, \\• Q . Primary Feeders—Open-wire primary feeders can be 900 WA' CiWr A used to supply a network in an overhead area, but every >-cr 800 etv 0 precaution should be taken to avoid running more than < vt• 4. z z one feeder circuit along a given route. The reason is that 700 irt 44, when two open-wire feeders follow the same route, there O o v-A11111- is the hazard that both circuits may be interrupted by 1-(7, o hjo 600 ">4?Al zw o the same cause. Such an outage is likely to cause a 0 o 500 Vptv Pr \ ..... (t.' • I complete system outage. jr,9:, ,,,\ o pc g„114 re.A Aerial cable is preferable for overhead primary feeder -I le -- 400 , oldi circuits supplying a network, in order to minimize the V p 0 Z 300 . cr AIF 0”0- -0 13-• hazard of simultaneous outage of feeders following the < CO o(n dir s'''• same route. The use of aerial cable also improves the Ce 200 appearance of the system and permits simpler pole-top a_Lai structures. Where the substation supplying the primary 100 feeders is some distance from the network area, open-wire z 0 circuits following different routes can be used up to the 0 10 20 30 40 50 60 70 80 90 100 periphery of the network area, and aerial-cable circuits can be employed in the network area itself. This reduces LOAD DENSITY IN THOUSANDS the amount of aerial cable required. OF KVA PER SQUARE MILE Fig. 35—Load density relationships between kva per square 15. Overhead Networks in High- and Medium-Density mile, kva per block, and kva per 1000 feet of secondary Areas main. This information is useful in overhead network-system The commercial areas of many small cities and large planning. towns are typical high-load density areas that often are supplied by overhead systems. Generally, these areas rangement of the secondary mains may be justified by comprise several blocks along one or two main streets, the use of smaller secondary conductor sizes, and im- with alleys parallel to the streets. Where alleys are proved division of load among the transformers. available, the distribution system usually is built in The amount of load involved in most of these heavy them. Many of the outlying shopping districts in bigger load areas requires the capacity of three or more pri- cities have conditions similar to those in the com- mary feeder circuits. Sometimes more than three pri- mercial areas of small cities and towns. Loads of several mary feeders can be justified by the savings in trans: hundred kva per block are common in these areas. The former capacity, even though the total load may require relation between kva per block and kva per square mile only three feeders. Since the primary feeders usually is indicated in Fig. 35. A load density of 100 kva per must be carried along the same route in the alley where block corresponds to 15, 000 to 20, 000 kva per square the network units are located, aerial cable is highly mile. Many underground secondary network systems desirable to reduce congestion and the hazard of more serve load densities no greater than these values. than one circuit being involved in a fault. Distributed Network—A secondary ring arrangement of Spot Networks—Spot networks offer some advantages secondary network mains often fits nicely into an area in heavy-load overhead areas, particularly where alleys where high-density loads are served from overhead cir- are not continuous and it is difficult to establish con- cuits in alleys paralleling a main street. Such an arrange- tinuous secondary mains to form a ring or grid. A spot ment is shown in Fig. 36. Cross ties between the sides of network consists of two or more network units supplying the secondary ring usually must be underground because a common bus from which services are tapped. Where a of restrictions on crossing a main street with these cir- spot network can be located in the middle of a block, cuits. Unless cross ties are used at frequent intervals, the services can go directly to the various loads in the they provide little help in equalizing transformer load- block. There are no secondary mains from one block to ings. The benefit derived from cross ties seldom justifies their use, except at the ends of the area to close the ring. Usually the secondary mains can be extended to the boundary of the heavy-load area where a cross tie to complete the secondary ring can be carried overhead without detracting from the appearance of the area or presenting an undesirable obstruction. Where the area comprises several main streets and the corresponding alleys, two or more secondary rings can be used. These rings may or may not be interconnected, depending on the ease with which interconnecting cross T tT flit t) T i) ties may be made. If cross ties can be made easily, the Fig. 36—One-line diagram of a typical overhead second- increased amount of secondary conductor in a grid ar- ary network system, utilizing three primary feeders.

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another across streets. Various load values can be ac- head network arrangement or conversion to an under- commodated by using two or three units in the spot ground network system. network and perhaps two transformer ratings. For example, a spot network using two 225-kva units can sup- Table 14—Light-Duty Secondary Networks ply up to about 300 kva of load, and a three-unit spot Correlation of Transformer Ratings and Secondary Mains network using 225-kva transformers can supply about and Transformer Secondary Leads 600 kva. Similarly, three 500-kva units can supply up to about 1333 kva of load. Thus, the use of 225-kva and Transformer Rating 500-kva units in two- and three-unit spot networks can 75 Kva 150 Kva accommodate a range of loads up to about 1333 kva. Recommended conductor Each unit in a spot network should be connected to a size for overhead open- #1 250 MCM separate feeder. wire loop secondary mains. The spot network better utilizes transformer capacity than does the distributed network, where the trans- Conductor size which may be used with risk in formers are separated by secondary ties. The reason is #2 #4/0 that with virtually zero impedance of secondary between overhead open-wire loop transformers, the load at each spot is equally divided secondary mains. among the transformers in that spot network regardless Recommended aerial cable of whether all feeders are in operation or one feeder is size for overhead loop #2/0 350 MCM out of service. For this reason, it is practical to load the secondary mains. spot network transformers to a somewhat higher percentage of their rating than can be done safely in the Recommended conductor distributed network. size of aerial cable to be Many network systems have been started by installing used between transformer one or more spot networks to supply concentrated, new, and protector and be- or important loads, such as in new buildings. These tween protector and 2-350 MCM spots are interconnected with secondary tie mains to secondary loop. These 350 MCM cond. per form a single distributed network as additional loads sizes will require phase adapters for connecting are changed over to network service when their im- the conductors to the portance increases, or when additional new loads occur transformer terminal between adjacent spots. In overhead areas where there is stud. likelihood of early conversion to underground, the spot networks can be installed in building vaults that later Secondary Mains—The loop arrangement of the three- are connected into the underground system. phase, four-wire 120/208-volt secondary mains is likely to fit most of the areas in the small cities and towns 16. The Light-Duty Overhead Secondary Network where the network is adaptable. The loop form of System secondary mains as shown in Fig. 37 usually is run in Towns and small cities having a population of around alleys paralleling a main street along the same route as two or three thousand are likely to have several blocks existing radial secondary mains. In some areas the ar- in the business district where the light-duty overhead rangement of the streets and blocks may make it dif- network system can provide the advantages of a net- ficult to complete a loop of secondary mains. A line ar- work. Where the load in these areas is between about 60 rangement can be used in these cases with only a little kva and 300 kva per block, or between about 125 kva sacrifice of the advantages of a network system. In the and 600 kva per 1000 feet of secondary main, the light- end sections there probably will be somewhat worse duty network can distribute power with a high degree of voltage flicker conditions and perhaps somewhat higher reliability and useful flexibility for load growth. voltage drop than would be the case if a loop could be The light-duty network system resembles a conven- completed. The line arrangement has the advantages of tional network in certain respects, but there are im- avoiding some street crossings and using less secondary- portant differences which will be described. The light- main conductor. duty system will have its longest usefulness when it is Transformers and Protectors—The network unit, con- applied in an area where the load density is near the sisting of -a three-phase transformer or three-phase bank lower limit of the above range. A 75-kva network unit and the light-duty CM-88 network protector, is usually can supply loads at a density of 60 kva per square block mounted directly on a pole or on a platform structure with one transformer for each block. Then by adding along or across-an alley. An installation of a pole-mounted network transformers between the original units, and network unit is shown in Fig. 38. It is generally satis- eventually changing to 150-kva units and larger factory to mount equipment directly on a pole up to a secondaries, an increased load four times the original total weight of 2000 to 2500 pounds. The weight of the load can be accommodated. This represents a period of 75-kva network unit is within this limitation. Under 15 to 25 years in most cities. In those places where the favorable circumstances the 150-kva unit may be load density is near the top of the range, it is better to mounted directly on a pole. However, the 150-kva unit consider larger conventional network units in an over- usually is too heavy for pole-mounting and requires a

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 192/576 Secondary Network Systems 185 platform structure. Experience and practice will usually the area, the choice of transformer ratings and spacing, indicate the proper manner of installation. the estimation of the division of load among the units, The transformer part of the network unit may be the choice of secondary main conductor size, and the composed of either a three-phase transformer or three switching and protection at the primary terminals of the single-phase transformers in a bank. Existing trans- transformers. formers of proper rating can be used. Usually it is de- Determination of the Loads—The amount and location sirable that new transformers be three phase. The cost of the loads in the area are seldom known accurately. of the transformers installed on the pole is about the One way of estimating the kva demands is based on same for the three-phase transformers as for three the kilowatthour usage at each service. To use the kilo- single-phase transformers of equivalent rating. How- watthour usage in this way, it is necessary to know or ever, the appearance of the installation will be better assume a load factor and power factor. Then kva de- with the three-phase unit, and it will be easier to make mand is given by the following formula: the connections on both the primary and secondary sides. KWHR Primary Feeders—The operation of the light-duty KVA= network system requires at least two primary feeder (no. hours) (Load Factor) (P.F.) circuits to supply the network units.. As illustrated in The load factor for business-district loads is influenced Fig. 37, each feeder is connected to alternate trans- by seasonal conditions, but ranges between 30 per cent formers along the secondary mains. In most small cities and 40 per cent on the average. A load factor of 40 per and towns, there are two feeders in or adjacent to the cent means that the peak kw demand is 2.5 times the business district. If only one of these feeders already average demand. However, the peak demand must be supplies the loads in the business district, it will be expressed in terms of a time-interval of measurement, necessary to take a short lateral tap off the other feeder such as a "l5-minute" or "30-minute" demand, to be circuit to establish the second primary feeder in the truly correct. business district. Usually these primary feeders will be Most frequently the only load information available overhead, open-wire, radial circuits. is the location and rating of each distribution trans- Within the network area, the two feeders probably former in the area. If this information is tempered by will be on the same structures and follow the same good judgment based on the location of known heavy routes in several places. For this reason it is preferable loads and the areas actually served by the various to use aerial cable for these sections of primary feeder transformers, it can serve very well for planning a net- circuits. The use of the aerial cable reduces the hazard work installation. It is satisfactory to assume that of a fault in one circuit being communicated to the loads will be balanced three-phase loads when they other circuit where both feeders are on the same struc- are connected to a three-phase, four-wire secondary ture and follow the same route. Furthermore, the aerial main. Any information as to the severity of loading on cable is less susceptible to the hazards of storms, falling the individual transformers should be taken into ac- objects, and fire-fighting procedures. The aerial cable count when this method of estimating loads is used. If makes a neater installation than does an overhead, information is available, the primary-feeder loading open-wire, three-phase circuit. corresponding to the transformers in the area being The open-wire, overhead circuits are entirely suitable considered can be used to temper the values of load for those sections of the primary feeders where they do estimated for individual transformers. Diversity among not follow the same routes and where there are no the loads should not be taken into account when the severe obstructions to open-wire construction. loads are estimated on the basis of existing transformer ratings, since much of the diversity is already accounted 17. Planning the Light-Duty Network System for in the load on the transformers. The general procedure in planning the light-duty The loads on which the arrangement of the light- network system involves consideration of the loads in duty network application is based should include some allowance for future growth. While the network system can be easily expanded and capacity can be increased conveniently, there should be enough spare capacity to allow for load growth. The network should be designed to supply at least a fourth more load than exists in the area when the system is installed. Nor- mally this allows for three to five years of growth before additional capacity is required. Transformer Ratings and Spacings—Voltage drop on the secondary main between transformers should not be more than three per cent. This voltage drop, plus the drop through the transformer at normal peak load, will amount to about five per cent or six per cent in CONSUMER SERVICES the worst places. This generally will allow a maximum Fig. 37—One-line diagram showing general arrangement spacing between transformers of from 600 to 800 feet. of the light-duty, overhead secondary network system. Choice of transformer size depends on the density of

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 193/576 186 Secondary Network Systems load along the secondary main and the spacing between to the associated transformer rating. Since this emer- transformers. The transformer rating should be low gency situation is not likely to occur very frequently, enough to permit locating the transformers no more it may be a reasonable risk to use conductors one size than 600 to 800 feet apart in most of the sections. smaller than that indicated above. Longer spacings may be permitted where it is desirable Generally there should be no overcurrent protection to complete a loop through a sparsely loaded area. A in the secondary mains. Faults in overhead open-wire reasonable first approach is to choose the transformer circuits usually can be burned clear. Even in radial size that provides for an average spacing along the sec- secondary mains, the faults that occur are generally ondary main of about 400 or 500 feet. burned clear. Since the secondary mains in the network The transformers should be located along the sec- system are fed by several transformers, the available ondary main at about equal intervals of load. There- short-circuit current is correspondingly greater, making fore, after one transformer has been located on the basis it easier to burn off faults from the secondary mains. In of pertinent factors in the particular case, the other long sections of secondary mains, and where it is es- transformers can be located, as a first approximation, pecially desirable to avoid having conductors burn by dividing the load along the loop into groups equal down, limiters can be installed in the ends of the con- to the average load per transformer. Frequently there ductors in each section of the secondary main. will be evident reasons for modifying these locations to Network-Unit Rating—The network unit, comprising some extent. The locations determined in this manner the transformer and the network protector, should be often will be entirely satisfactory if there is a reasonable mounted so that the leads between the protector and margin between the ratings of the individual network the transformer are relatively short and well protected. units and the average load per unit. If a network is A convenient way of mounting the transformer and the installed on this basis, load checks will determine which protector directly on the pole is shown in Fig. '38. The transformers are heavily loaded and where additional transformer is mounted on the pole between the primary transformer capacity will be required as the load circuits and secondary circuits. The network protector increases. is mounted below the transformer and preferably below Load Division—In the initial planning, the division of the secondary main. Mounting the protector below the load among the units can be checked approximately secondary mains makes it convenient for a lineman to by a rather simple procedure which locates the zero- work on the protector without climbing into or through current balance point between transformers on the the circuits. Where the weight of the bigger network secondary loop. This balance point is determined by units makes it impractical to mount the transformer the load-distance products of all loads in a section—a and protector directly on the pole, a platform is used, load-distance product being the kva of a load times the and the protector is mounted in such a way that the distance in feet to the transformer supplying it. The cover can be dropped down off of the protector for in- sum of the load-distance products on one side of the spection or maintenance work. balance point equals the sum of the load-distance The ratings for the transformers in a light-duty products on the other side. The load on any trans- network system are 75 and 150 kva. Other ratings former is the sum of the load between the balance points can be used, but generally one or the other of these on either side. two ratings should give a satisfactory arrangement. Secondary Mains—The simplest and most practical There are two corresponding ratings of light-duty net- arrangement of the secondary mains is the use of open work protectors, 75 and 150 kva. Either of the protectors wire on racks. For reasonable spacing between trans- will work satisfactorily with transformers smaller than former locations along the secondary main, the open- the corresponding rating. wire construction will give satisfactory voltage condi- The nominal voltage level for the light-duty sections. Although aerial cable provides better appearance, ondary mains is 120/208 volts. In order to maintain a larger size conductor is required to obtain the same this voltage under normal load conditions, it is de- carrying capacity. The open wire has the advantage of sirable to operate the transformers at somewhat higher being easy to splice and tap. This becomes very im- voltage levels. For this reason the preferable secondary portant because of the large number of service connec- rating for transformers is 216Y/125 volts. This permits tions that must be made to the secondary mains. As operating the no-load voltage levels at about 123 or indicated in Table 14, the secondary main conductor 124 volts and maintaining nearly 120 volts at most should be #1 copper or its equivalent when 75-kva of the services during normal full-load periods. This transformers are used, or 250 mem when 150-kva also reduces the possibility that 220-volt motors will transformers are used. These conductor sizes have a not operate satisfactorily on 120/208-volt systems. capacity about equal to the rated capacity of one of the Existing single-phase transformers can be used in associated transformer sizes. In emergency situations the network system along with three-phase trans- when one of two primary feeders supplying the network formers if the three-phase banks thus formed have the is out of operation, it may be necessary for the secondary same capacity as the rest of the network transformers. main to carry away from the transformer location, in It may be possible in some cases to use existing trans- each direction, a load about equal to the rating of the formers with 120-volt low-voltage rating along with transformer. For that reason it is desirable for the sec- new transformers having 125-volt low-voltage rating, ondary main to have a capacity approximately equal providing that the no-load secondary voltage of the

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ally, existing transformers will have impedances closer to three per cent than to six per cent. Where existing transformers are available and are used with new three-phase transformers, it is likely that the impedances will not be exactly equal. There seldom will be any problem involved in operating transformers whose impedances are considerably different. Where the ratio of impedances of different transformers is less than about 1.3, satisfactory operation should result. The only exception is that the impedances should be fairly evenly matched when it is necessary, because of high concentration of loads, to locate two transformers adjacent to each other at the same location. If two network transformers have to be located less than 150 feet apart, transformer impedances should be equal. Transformer Connections—The three-phase distribution transformers or banks of single-phase transformers used in the light-duty overhead network system must have their primary windings connected in wye or delta, as determined by the arrangement of the substation or other source supplying the network. When the network primary feeders are supplied from a wye-connected grounded-neutral source, the distribution transformer primary windings must be connected in wye with their neutrals tied to the primary feeder neutral and grounded. When the primary feeder source is delta-connected, ungrounded, the transformer primaries should be delta- connected. In any event, the transformer secondaries should be wye-connected to provide a common supply for both single-phase and three-phase loads. Wye-Connected Grounded Neutral Source—Wye-connected and grounded transformer primary windings are required to permit tripping the CM-88 network protectors for a line-to-ground fault on a primary feeder. This is essential when the primary feeders are supplied from a grounded neutral source and the primary-feeder breakers at the source substation are relayed to trip on feeder ground faults. If the distribution transformer primaries are delta-connected, presenting an open circuit to line-to-ground fault current, the only reverse current the network protectors would experience during a line-to-ground primary feeder fault is the reverse magnetizing current of the transformers occurring after the primary feeder breaker has opened. The light-duty Fig. 38—A light-duty overhead network-unit installation. (Type CM-88) network protectors used in the light- duty network are relayed insensitively; i.e., they require a reverse current of somewhat more than their normal rating to trip them. Thus these protectors do not recog- different banks can be matched within about one per nize the reverse magnetizing current of the trans- cent. Where it is desirable to operate transformers on formers. In order to isolate a line-to-ground primary this basis, the transformers with different secondary feeder fault, reverse fault current is needed to trip the voltages should be separated by a reasonable amount protectors and is allowed to flow by using wye-con- of secondary main, preferably not less than about 150 nected, grounded-neutral, transformer primaries. feet. The reverse power relaying of the CM-88 protectors The impedances of the transformers used in the is made insensitive to avoid unnecessary protector network system should be between three and six per operations. Such operations would occur frequently dur- cent. The impedances in the low end of this range are ing light load on the network because of loads outside favorable to better voltage conditions and higher short- the network area which are supplied from the same circuit currents that make it easier to burn secondary feeders that supply the network. Differences in these faults clear. Higher impedances tend to improve the radially supplied loads, and consequent voltage magni- distribution of load among the different units. Gener- tude and phase angle differences between feeders, would

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 195/576 188 Secondary Network Systems result in current reversals of sufficient magnitude to and the development of new types of utilization equip- trip some of the protectors if they were relayed sensi- ment. Serious consideration had to be given to the tively. Operations of this nature are not likely in development of new basic concepts or possible changes medium- and heavy-duty networks where the primary in power supply to the new commercial-area loads. feeders supply only network load. Over the thirty years' use of the 120/208-volt net- Delta-Connected, Ungrounded Source—When the network, and particularly in the last decade of that period, work primary-feeder source is delta-connected and un- there had been not only a large increase of load, but grounded, isolation of a primary feeder ground by also a shift in the character of the load. By way of tripping of feeder breaker and network protectors is not illustration, Fig. 39 shows graphically the rapid in- required. With such a source, the distribution trans- crease in multi-transformer installations and the former primaries should be delta-connected. This is progressively increasing proportion of the total load particularly true when three-phase distribution trans- represented by them in an area below 60th Street in the formers are used, as these transformers, being core form, Manhattan Borough of New York City. This trend has can serve as a source of unwanted ground fault current been experienced in most cities, and is expected to con- to a primary feeder ground if their primary windings are tinue with new building construction and modernization wye connected. of old buildings. Transformer Overcurrent Protection—Individual trans- In 1954 a group of papers6 was presented before the former primary overcurrent protection is neither neces- AIEE, dealing with the general subject of commercial- sary nor desirable. Generally, a transformer fault is area distribution systems to serve modern loads. These recognized by the primary-feeder breaker. Primary papers introduced a new fundamental plan of power overcurrent protection at the individual transformers distribution at 265/460 volts, which is readily applica- may jeopardize the service continuity of the network. ble to existing metropolitan power systems employing Cutouts, with blades instead of fuses, in the transformer the conventional 120/208-volt secondary network sys- leads may be desirable as a disconnecting means for tem. In conjunction with a higher utilization voltage, isolating the network units for inspection and mainte- higher primary-feeder voltages are fully compatible. nance. Hot-line clamps can be used for this purpose if The philosophy behind such increases in voltage level they are disconnected carefully in the manner dictated is to achieve, for both utility and consumer, the econ- by the transformer winding connection. In any case the omies associated with reaching as close to the load network protector should be opened before the trans- center as possible with as high a voltage as practicable. former primary leads are disconnected. Studies of network system types and voltages for Primary Feeders—The primary feeders supplying the large systems, such as those encountered in New York network should have the current-carrying capacity to City, concentrated on designs which are based on supply the entire load with one feeder out of service. "second contingency," which means that with two Voltage drop is not a serious factor in the design of the primary feeders and their associated network trans- primary feeders where the supply for these circuits formers out of service, continuity of supply to the load usually is close to the network area. In such cases the is maintained. Although this basis of system design is primary-feeder circuit breaker at the source substation correct for the largest network systems, it is the excep- will provide satisfactory overcurrent protection without tion rather than the rule for the majority of a-c second- intervening sectionalizing in the primary feeder circuits. ary networks. If there is need for additional overcurrent protection to Both statistical surveys and theoretical studies", lb isolate the lateral supply to the network system from indicate that, for networks of medium and small size the main primary feeder circuit, this should be done compared with the largest existing systems, "first- with a three-pole recloser or circuit breaker. Single-pole contingency" design provides for a reliable and econ- devices, such as fuses or single-pole reclosers, may open omical system. This design results in a network system in only one phase and leave a single-phase condition capable of supplying the load in the event of the loss of that will prevent the proper operation of the network one primary feeder and its associated network units. protectors. 18. Comparison of Secondary Voltages The relative cost of serving a uniform peak load V. MODERN TRENDS IN COMMERCIAL-AREA density of 10 to 240 megavolt-amperes per square mile DISTRIBUTION by a conventional network at 208 and 460 volts is shown The decade of the 1950's has marked the thirtieth in Fig. 40. The costs are for first-contingency design. anniversary of the automatic, low-voltage, a-c second- The costs for primary feeders are not included, but ary network system. During this thirty-year period, the system design is based on five primary feeders. In this well-known 120/208-volt system has satisfied, in an cost comparison, load density alone is a suitable para- outstanding manner, the requirements of electric power meter, because primary-feeder costs are not included distribution to growing loads in commercial areas, with and no limitation on primary-feeder capacity exists. surprisingly little change in basic system principles. However, the number of primary feeders influences the However, the post World-War II years witnessed the cost, because the choice of the number of feeders affects introduction on a vast scale of air conditioning for large many other factors in secondary-network system de- buildings, an improvement in standards of illumination, sign. While the number of feeders affects total cost, the

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• •• • • • •• • • • S. • • • •••.%'• .11:•‘4%* s 7~3 . • • •• st .1"• • ••\:\s‘ • t • • •• • •• • • • 4..

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712 MVA 1600 MVA 160 2376 MVA

150

DISTRIBUTED NETWORKS 67 % 40% 140 33 %

130 SPOT NETWORKS 33 •16 60% 67% 120

120 110

110 110 100 100 100 90 1958 90 90 80

80 ATIONS ATIONS ATIONS 80 L 70 L L

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MAX MW DEMAND MAX MW DEMAND MAX MW DEMAND Fig. 39—Growth of spot loads in Manhattan. relative cost of 208-volt and 460-volt systems is practi- portant part in both system engineering and cost. The cally the same for a different number of primary feeders, primary supply to a secondary network system is in- when considering the same number of feeders for either fluenced by three principal factors. These are the num- system. The spot-network system costs shown in Fig. 40 ber of primary feeders, the feeder voltage, and the are discussed later. distance of the bulk-power source from the network area. 19. System Planning Including Primary Supply Studies have been made of the cost of serving a uni- In planning the distribution system to an extensive form peak load density with a conventional-type, i.e., commercial area, the primary-feeder supply is an im- distributed network, including the cost of primary

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2.0 1.0 P U. COST PER KVA OF NETWORK near to the edge of the area. A primary-feeder voltage UNITS 6 SECONDARIES FOR SERVING above 13 kv is economical for serving either a 208- or 1.8 50 MVA PER SQUARE MILE WITH "- 120/208 VOLT NETWORK, AT 13.2 KV 460-volt secondary network, particularly where the 1.6 PRIMARY FEEDER VOLTAGE bulk power source is an appreciable distance from the edge of the network area and long express feeders are 1.4 required. The combination of a higher secondary volt,- age and a higher primary-feeder voltage becomes in- 1.2 creasingly favorable as load density, network load, and T PER KVA 1.0 distance of the bulk power source from the edge of the COS 120/208 VOLT NETWORK

E network area become greater. 0.8 rano mum The curves of Fig. 41 are generalized results which TIV A 0.6 show trends, and do not necessarily provide exact values

REL 265/460 VOLT NETWORK for application to a particular case. For example, it was 0.4 MOON assumed in the studies that a 34.5-kv bulk source was 265/460 VOLT SPOT NETWORKS available. Consequently, savings that might result due 0.2 to the elimination of a transformation from 34.5 kv sub-

0 transmission voltage to 4-kv or 13-kv primary-feeder 50 100 150 200 250 voltage are not apparent. The curves of Fig. 41 indicate LOAD DENSITY-MVA PER SO MI. trends which show that a particular case should be Fig. 40—Effect of secondary voltage on the cost of general studied carefully, including consideration of both higher and spot-network systems. Primary-feeder costs are not secondary and primary voltages. included. 20. Primary-Feeder Costs The primary-feeder circuits serving a secondary- network system can be considered as two components: feeders. The following factors were considered as vari- the express feeders carried from the bulk power source ables: to the periphery of the network area; and the feeders 1. 460-volt secondary voltage compared to 208-volt interlaced within the network area itself. Primary secondary voltage. feeders within the network area may in turn be classi- 2. Primary-feeder voltage of 4, 13, and 34 kv. fied into two components: mains and laterals. 3. Primary-feeder distance including express feed- The first step in arriving at a method for costing ers from the bulk power source to the edge of the primary feeders is to obtain a relationship between the network area. length of primary-feeder circuit serving a network and 4. Number of primary feeders. the design parameters of the network system. Length 5. Load density. of circuit is the final objective of such an analysis, since 6. Total network load served. it can be assumed that feeder cost is directly related to the length of circuit. Differentiation is necessary, be- Optimized secondary-network costs were obtained for tween installed cost per foot of express feeders, mains both 208- and 460-volt networks for different numbers within the network area, and laterals, however, in order of primary feeders, including costs of mains and laterals to make costs more realistic. but excluding the costs of express primary feeders. An The variables determining primary-feeder cost are: available bulk power source for supplying •the primary 1. Distance of bulk power source from the network. feeders was assumed to be located at distances of 0.1, 1, 2. Number of feeders. and 5 miles from the periphery of the network area. 3. Feeder voltage. Express feeder costs were obtained and combined with 4. Area of network. the cost of the system within the network area. Three 5. Number of network units within the network primary-feeder voltages of 4, 13, and 34 kv were con- area. sidered. The size of the network systems costed on this basis was a function of primary-feeder voltage and Express feeder length is determined on the basis number of feeders. The results of the studies are shown shown in Fig. 42. Where the network area load is too in Fig. 41. large to be served by the maximum allowable number of A study of the curves of Fig. 41 permits a number of primary feeders at the desired voltage, two or more observations, the most important of which are sum- networks are formed, each served by the appropriate marized here. It is uneconomical to serve either a 208- or number of feeders required to carry the load, under first 460-volt secondary network with a primary-feeder volt- contingency. Fig. 42 shows a specific case of four age of 4 kv for practically any load density, unless the separate network areas, each supplied by a maximum- network load is small and the bulk power source is at allowable number of ten feeders. It is assumed that all the edge of the network area. A primary-feeder voltage four networks are served from one bulk power source as of 13 kv is in general an economic voltage for serving shown. Hence, express primaries which serve the most a wide range of load densities and network loads, par- distant networks must traverse the systems closest to ticularly if this voltage is available at a bulk power source the bulk power source.

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Secondary Network Systems 191

25,000 KVA PER SO MI. 50,000 KVA PER SO MI. 200,000 KVA PER SO MI. 2.6 1 I 1 1 1 I I I 4 KV 4 KV/I BULK SOURCE DISTANCE QI MILE BULK SOURCE DISTANCE 0.1 MILE 2.4 4 2.2 / 120/208 VOLTS / 120/208 VOLTS ----- 265/460 VOLTS / - - - 265/460 VOLTS 6 2.0 a. 345 KV 4 K 4 KV/ 1.8 7 4 -1 1 1.6 /:______.-.....--'----...... „.._13 KV 34 KV / r 1.4 • 7' 8 13 KV )/ 34 KV 4 KV 4 KV...

ti 1.0 `- - 34 KV 34 KV -.1 --- ! - - -- 13 KV .8 120/208 VOLTS ------rS KV -----265/460 VOLTS .6 1 1 t aL BULK SOURCE DISTANCE 0,1 MILE .4 I

2.6 1 4 101 4 KV' BULK SOURCE DISTANCE 1 MILE 4 KV BULK SOURCE DISTANCE I MILE 2.4 I 4 KV/ 2.2 120/208 VOLTS if 120 208 VOLTS 265/460 VOLTS / -- 265/460 VOLTS • 2.0 -- /1/ / / fc2 1.8 7. 31-----ICV 4 13 KV / a. 1.6 -. -..., t j / 34 KV 4 KV 1- 1.4 7" / L__-- 4 Ky, .7* O ..../...."7.___ /!------. 13 KV - _ _...... / 34 1.2 KV --- -...... -- ..., 13 KV _ -- -- ..- ...- _... .7-• 34 v C- 1.0 _ -_ __-______11.1.(v-,. kl 73 V 13 KV • .8 ___--- 120/208 VOLTS - - - -- 265/460 VOLTS .6 13 V BULK SOURCE DISTANCE 1 MILE 1 1 .4 I 1

2.6 4 %V 4 KV 4 KV/ T BULK SOURCE DISTANCE 5 MILES 2.4 ./ / 4 KV 1 / ...* •4 2.2 / 2 120/208 VOLTS 4 KV 34 KV ..--... 265/460 VOLTS t 2.0 ....b.%.,...„....._ „. iy 13 KV ' 4 K> 1.8 Z .... 4 34 KV ..7 ..! g 1.6 ------7:-.0*--;-:-: --- a .... 34 KV 13 KV_ 13 KV -_... _d-2-,-. f. 1,4 34 KV 13 Ktr cr -.. .."- - 34 KV S3 KV 1.2 13 KV 34 kV I ti I.0 120/208 VOLTS 34....-4.- - - - 1 13 KV - -- - - 265/460 VOLTS ------[ - - --- 120/206 VOLTS 265/460 VOLTS .6 BULK SOURCE DISTANCE 5 MILES BULK SOURCE 'DISTANCE ' S MILES I 1 I .4 1 1 1 1 ...... - ., 40 BO 120 160 200 0 40 80 120 160 200 NETWORK LOAD -MVA NETWORK LOAD-MVA NETWORK LOAD-MVA 1 4 KV t 4 KV 4 IN 2 10 40 2 , 10. , 40 2 10. 40 I , 1 a 13 KV 13 KV 13 KV 2 5 10 20 30 25, ,. , 10, 20 30 2 5 10. 20 30 1 L I 34 KV 34 KV 34 KV 2 3 4 5 10 2 3 4 5 10 2 3 4 5 10 NUMBER OF PRIMARY FEEDERS NUMBER OF PRIMARY FEEDERS NUMBER OF PRIMARY FEEDERS

Fig. 41-Effect on network system cost of load density, network load, primary-feeder voltage, secondary voltage, and distance of bulk power source from network area. 1.0 pu cost is the same as that of Fig. 40.

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0.6 I 1 11111 I I I 1 I I I 1 1.0 P U. IS THE DOLLARS PER KVA COST \ / ONE OF SERVING A DENSITY OF 50 MVA PER 2 SQUARE MILE WITH A 132 KV/208 VOLT SECONDARY TO NETWORK 0.5 CONVENTIONAL NETWORK SYSTEM, BUT NOT NETWORK UNITS INCLUDING PRIMARY FEEDER COST

/ I

NI. 4. 6 KV 3

2 5 FEEDERS

10 13.2 KV i O J NII 2 0.1 co id rc 10 w w ..----414.5 KV ..„. 4 a_ 1- io x 11) w o 0 TEN EXPRESS 3 4 5 10 20 30 40 50 100 200 FEEDERS NETWORK LOAD —MVA I BULK POWER Fig. 44—Express primary-feeder cost compared with SOURCE secondary-network system cost not including primary feeders. Fig. 42—Method of serving two or more secondary networks from a bulk-power source where the maximum number of primary feeders to a network is ten. Costing the interlaced feeders within a network area Fig. 43 shows the allowable size of networks versus is a much more complex problem than that of obtaining the number of primary feeders at three different volt- reasonable express feeder costs. Consider the 49-trans- ages. Fig. 44 shows the cost of express primary feeders, former, 5-feeder system shown in Fig. 20. Suppose that plotted on a relative basis, with 1.0 pu cost correspond- five main feeders are considered to be carried fully across ing to the 1.0 pu cost of Fig. 40. the network area. Laterals are carried perpendicular to the feeder mains, and serve the network units so as to

140 give the interlacing pattern indicated by the five- feeder array in Table II-A. 130 Using primary-feeder patterns similar to that of Fig. 45 as the basis for a mathematical derivation, total 120 length in miles of primary-feeder circuit in a network 110 area can be expressed as a function of the following three variables: network area in square miles; number 100 of feeders; and number of network units in the area. The results of such a study are given in Fig. 46, where 90 34.5 KV the relative total mileage of interlaced primary-feeder

-MVA BO circuits (mains plus laterals) in a network system is

ZE plotted versus the number of network units in the 70

-SI system. An interesting feature of these curves is that

RK they are normalized so that a hypothetical, one-feeder

O 60 system is used as the reference for determining primary 50 circuit length. This permits very simple calculation of NETW 13.2 KV primary length in the following manner: 40 Given a map of the array of network units to be served

30 by interlaced primary feeders, find the length of a single, hypothetical feeder which will reach all the network 20 units via the established routes for primary feeders. 4.16 KV Next go to Fig. 46 and read the relative-mileage number 10 for the given number of network units and the number o of feeders to be considered. Then multiply the length o 2 3 4 5 6 7 8 9 10 of the hypothetical single feeder by the applicable NUMBER OF PRIMARY FEEDERS relative-mileage number to get the total mileage of Fig. 43—Effect of number of primary feeders and primary- primary-feeder circuits required to serve the array of feeder voltage on network size. units shown on the map.

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30 6 40 17) 2r2-) 177 421 8

5 lc° ar?..) FEEDERS

57)

5?) 1) _2) id c EEDERS ARY Y F 4 d a

5 12) 2.12-) AR F PRIM O 2) '1

T 4 /17 3 EAGE IL M MBER OF PRIM E

2 NU

52) LATIV RE

I —

3) O 4 16 36 64 100 144 196 256 324 400 NUMBER OF NETWORK UNITS SERVED

\\ 4 IV 6 1014 16 110 20 I 2 3 4 5 0 2 SQUARE ROOT OF NUMBER Fig. 45—Primary-feeder interlacing in a conventional Fig. 46—Effect of number of primary feeders and number secondary network system used in comparative cost studies. of network units in system on the total mileage of primary- feeder circuit required within the network area. Primary circuit relative costs for express feeders at the three voltages considered are shown in Fig. 44, voltage is illustrated in Fig. 48, which depicts a 3-feeder which has already been discussed. For interlaced mains system serving the same load area at either 120/208 and the laterals within the network area, Table 15 lists volts or 265/460 volts. In costing the primary feeders the relative per-foot cost of mains and laterals at the for systems at either secondary voltage, this effect is three voltages. These costs include the duct space automatically taken into account with the use of Fig. 46. required to carry the primary cables. In Fig. 47, it is seen that optimum-cost network

systems at 120/208 volts and 265/460 volts do not 20 1000 utilize the same sizes of network unit. The higher-volt- 1 age system can utilize larger-size transformers, and 18 900 ET \ -- E consequently can serve the same load area with a fewer 16 \ r' ------800 number of units. Fig. 47 shows that at 265/460 volts, \...... , -- RATING network transformers in an optimum-cost combination 14 700

V PACING—F VOLTS of transformers and secondaries may be spaced approxi- / \ 265/460 mately 1% times the spacing required in the 120/208- 12 Ns. 600 S `w\PACING

` MER

volt system serving the same load area. The reduction N.. R 10 -...... , 500 in primary-feeder circuit length that can be realized SPACING ...... FO because of fewer transformers for the higher secondary -- 400

8 ANS 120/208 VOLTS TR 6 300 RATING Table 15 ORK 4 200 PU* Cost of Primary-Feeder Circuits NETW for 1,000 Feet of 3-Conductor Cable 2 100

4.16 Kv 13.2 Kv 34.5 Kv 0 0 50 100 150 200 250 Main feeder 0 333 0 444 0 667 LOAD DENSITY—KVA PER SQ MI.

Lateral 0 222 0 333 0 444 Fig. 47—Comparison of network-transformer ratings and *Referto Fig. 40. spacings in 120/208-volt, and 265/460-volt systems.

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f")

1?1

212-) 31?) I?) t?' T f ) 1?) ??) L L J NETWORK 120/208 VOLT 265/460 VOLT NETWORK

1 2 3 I 2 3 PRIMARY FEEDERS PRIMARY FEEDERS a) 120/208 volts; b) 265/460 volts.

Fig. 48—Illustrating the reduction in primary-feeder circuit length possible with the use of larger-size network units in a higher secondary-voltage system.

economically at the higher secondary voltage, with first 21. Spot Networks contingency as a design basis, compared to second- Studies have been made for serving concentrated contingency design. These results do not include costs loads with spot networks designed on a first-contingency of primary feeders, although for a particular case the basis. Fig. 49 shows the cost to serve a spot load with comparison can be made easily, including primary- 500, 750, and 1,000 transformers at either 208 or 460 feeder costs, knowing the specific conditions. volts, 13-kv primary voltage. Costs for 2,000-kva Transformer Loading—Both the load-carrying capa- transformers apply only for 460 volts. The minimum bility and economics of spot-network application are re- points of these curves are plotted in Fig. 50 to show lated to the individual network-transformer and net- comparison of spot-network costs at 208 volts and 460 work-protector ratings employed. For example, to serve volts. Spot loads in the order of 3,000 kva or higher can a given spot load, the question may arise whether it be more economically served at the higher secondary would be better to use a fewer number of higher-rated voltage. A smaller magnitude of spot load can be served network units as compared with a larger number of

0.8 2 2 TRANSFORMERS 2 TRANSFORMERS 460 VOLTS ONLY 0.7 1

A 3 KV 0.6

PER 0.5

T 2 3 TRANSFORMERS OA 4 3 0.3 4

0.2 RELATIVE COS 500 KVA 750 KVA 1000 KVA 4 0.1 2000 KVA TRANSFORMERS 00 2 0 2301234012 3 4 5 6 7 8 SPOT NETWORK CAPABILITY -MVA

Fig. 49—Spot-network transformer relative cost Transformers with a kva rating of 500, 750 and 1000 kva are rated 208 volts or 480 volts; 2000 kva transformers rated 480 volts only.

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0.6 served should not be greater than 4100 kva. A 0.1 per cent loss-of-life will result each time that one of the three units is taken out of service for a two-hour period preceded by a period of 91 per cent normal loading on 12 265/460 VOLTS all the units. The transformer load of two-hours duration giving a 0/208\%...... "VOLTS.G—" 0.1, per cent loss-of-life is approximately the same as the loading that would give 0.5 per cent loss-of-life if it were 265/460 VOLTS in effect for four hours. Network-Unit Costs—Fig. 52 shows the relative costs of 0 I 2 3 4 5 6 7 8 equipment only, i.e., network transformers and protec- SPOT NET CAPABILITY —MVA tors, versus spot-network load for units of different rating. Low-voltage rating is 480 volts. Costs do not in- Fig. 50—Lower envelope of cost curves of Fig. 49. clude installation, nor any of the equipment costs required to connect the units together to form the spot network. The maximum-allowable loading for a feeder- lower-rated units. In making this choice, allowable outage contingency condition is based on a value which overload (i.e., above nameplate rating) of equipment would give 0.1 per cent loss-of-life after two hours, must be considered. based on the ASA Guide. The network protector is a maximum-rated device and From Fig. 52 it can be seen that 750-kva units should therefore has no allowable overload rating above its be used only for small spot-network loads, up to 2000 nameplate current rating. But the network transformer kva. If the ultimate spot load will require more than may be loaded above its nameplate-kva for certain three 750-kva units, units of larger rating should initial- lengths of time provided that possible loss-of-life of the ly be used. When the initial spot load is slightly greater transformer is taken into account. than 1000 kva and is likely to grow to 3000 or 4000 kva, Transformer overloading in the event of a first con- 1000-kva units should be used; when the initial load is in tingency (i.e., the outage to one of the primary feeders the 1500-3000-kva range and is likely to grow to 6000 serving the spot network) incurs less risk than trans- kva, then 1500-kva units are the best choice; and finally, former overloading for a first contingency in a conven- when the initial load is 4000 kva or greater, 2000-kva tional network. The reason is that in-service trans- units should be used. formers in a spot network are equally loaded, both under normal and first-contingency conditions. Adher- 22. Primary-Feeder Supply to Spot Networks ence to the ASA Guide for Loading Transformers' in In previous discussion it was demonstrated, with the calculating allowable transformer overload as a function aid of Fig. 48, how the permissible use of larger-size of loss of life should give conservative results, and network transformers with a higher secondary voltage loads that are quite safe. results in a reduction in circuit length of primary feed- Fig. 51 gives curves that are useful in spot-network ers, and hence a reduction in primary-feeder cost. A design evaluation. These curves relate the number of further reduction in primary-feeder circuit cost is pos- network transformers required; the normal and con- sible by the use of 265/460-volt spot networks. tingency loadings; and the permissible load magnitude Fig. 53 shows the load area of Fig. 48 served by a 3- and duration for 0.1 per cent loss-of-life—for any spot- feeder spot-network system. It has been shown that in network load served by designs made up from any one general the optimum network transformer size for use in of four popular transformer ratings. The spot-network spot networks is larger than the optimum size for use in load is assumed to be the peak kva load. the general network. Therefore, the 16 network units of The curves may be used in the following manner: A Fig. 48-b are replaced with four 3-transformer spot net- 4000-kva load using 3-1500-kva transformers has a works for the purpose of illustrating this effect. Tho, normal transformer loading of 89 per cent and a first- total length of primary-feeder circuit is less for the spot- contingency loading of 133 per cent (see. Fig. 51-c). network system of Fig. 53 than for the general 460- Also on the curves is the permissible time duration volt network system of Fig. 48-b. of this overload for 0.1 per cent loss-of-life as determined from the ASA Guide with a 30°C ambient temperature. 23. Mutual Support of Adjacent Multi-Transformer This value for the example is approximately hours. Installations The loss-of-life curves were determined using the In practice, an a-c secondary network has both single- transformer normal loading for the load preceding the and multiple-transformer installations at various loca- overload condition. Curves for two, four and eight tions on the secondary grid. The locations of the multi- hours are shown. The point where a particular loss-of- transformer installations are dictated by the heavy spot life curve crosses the dotted contingency load curve gives load requirements. The spot networks thus formed are the maximum size spot-network load that should be essentially self-sufficient, neither contributing much served from the number of transformers in the spot net- power to the network nor receiving much power from it work. For example, for a two-hour overload period and in the event of a primary-feeder outage. using three 1500-kva network units, the maximum load It is extremely difficult to obtain generalized informa-

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196 Secondary Network Systems

750 KVA UNITS 1000 KVA UNITS TIME FOR 0.1% TIME FOR 0.1% LOSS OF LIFE LOSS OF LIFE 150 - 150 - \ I I / / I 2 1 2 HaR2s ; I HRS. I PROTECTORSi1750 1 140 - 140 *OF UNITS I *OF UNITS RATING i IN SPOT I I IN SPOT -1600° I I 1 4 / I 1 / 130 - 130 - DOTTED CURVES I I 1 / 4 ,ONE UNIT OUT 4 ...... LLil I i 5 / HRS. HRS. / 5,OF SERVICE I I 120 - I I / / DOTTED CURVES 120 - I I / / ING-% /6 I I I /-'—' ONE UNIT OUT 8 i 1 / / AD 8 I / OF SERVICE HRS.---4 I / / / 110 -FIR S'S• .-- 1 / I I / 110 - I I / / / I I I RMER LO 100 - 100 - FO S

RAN 90 90 - T *OF UNITS *OF UNITS IN SPOT IN SPOT RK O 80 80 - ETW N

70 - 70 SOLID CURVES SOLID CURVES NORMAL LOADING NORMAL LOADING

60 60

50 I 50 0 2 3 4 5 0 2 3 4 5 6

SPOT-NETWORK LOAD SPOT- NETWORK LOAD MVA M VA a b

1500 KVA UNITS 2000 KVA UNITS TIME FOR 0.1 % TIME FOR 0.1% LOSS OF LIFE LOSS OF LIFE *OF UNITS -275°- 150 0 *OF UNITS 150 - a / IN SPOT 2 HRS 3500- ;2 /51 IN SPOT / 3 140 140 -2500° - 2 HRS 4 HRS I 130 130 a \ /4 - 4 HRS 4=_re 2250- / E 120 - 8 HRS/ 120 —3000° / DOTTED CURVES / / DOTTED CURVES _2000E-1 ---#.--__rt-- ONE UNIT OUT - \ 8 H01§-1 4—ONE UNIT OUT ▪ 110 - OF SERVICE 110 cr / / / OF SERVICE L.1 100 \ I 100 PROTECTOR PROTECTOR RATING - RATING ) 90 - 4•••...! OF UNITS 90 z IN SPOT ••,*00F UNITS cc IN SPOT 80 1- 80 - 5

cc 70 o 70 SOLID CURVES SOLID CURVES w- 60 NORMAL LOADING 60 NORMAL LOADING z 50 50 1 1 1 1 0 I 2 3 4 5 6 0 1 2 3 4 5 6 8 SPOT-NETWORK LOAD SPOT-NETWORK LOAD MVA M VA d

Fig. 51—Permissible spot-network transformer per cent loading under first contingency for different numbers of network units in spot network.

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15 secondary tie, but otherwise isolated from the rest of the 14 - 750 KVA secondary mains system that would serve the surround- --- 1000 KVA 5-1500 KVA UNITS ing areas. The use of such a secondary tie results in a 13 -•- 1500 KVA 2000 KVA firm capability for the two spots combined, which is 12 r444-2000_ greater than the sum of their individual firm capabilities. J In considering the economics of tieing together spot networks with secondaries, there is a maximum separa- I ARS 10 tion beyond which it is not economical to add more 9 secondary-tie capacity in an attempt to increase the DOLL firm capability of the two installations combined. This

OF 8

S distance is variable with the size and number of trans- D 7 2-2000 formers comprising the spot networks, and with the SAN 6 secondary voltage. OU In Fig. 54, the upper curves show the additional firm

-TH -11-C. 5 - 2-1500 capacity attained through the use of a secondary tie be- j...-- 2-1000 KVA UNITS TEN 4 tween two spot networks, each consisting of three 500- --2-750 kva transformers. The lower curves show the cost of providing the incremental capability afforded by the tie EACH LARGE STEP A TRANSFORMER compared with the cost of providing an additional 500- IS ADDED. EACH SMALL STEP A NEW RATING PROTECTOR IS USED kva transformer at each spot. It can be seen that it is economically feasible to tie such spot networks together 0 2 3 4 5 6 7 at 460 volts up to a spacing of about 500 feet. At 208 SPOT NETWORK LOAD - MVA volts it is not economical to tie the spots together if the spacing exceeds approximately 225 feet. Fig. 52—Spot-network costs versus load. Cost includes transformers and network protectors only. Installation cost is not included. Transformers rated 13 KV/480/277Y volts. tion on power transfer via secondary ties between multi- transformer installations. So many variables are in- TWO 3-500 KVA SPOT NETWORKS volved that generalized results can not be confidently 4000 applied to specific cases. One special case that has a general solution is that of two multi-transformer spot- 460 VOLTS network installations connected to each other by a 0 3000 0 208 VOLT Z CAPABILITY OF TWO SEPARATE SPOTS I-

O. O.CO 2000 0 14.1 X t CC uj 1000 0 I- F

0.4 208 VOLTS

460 VOLTS A - - - - 0.3 COST OF A 4-500 KVA TRANSFORMER SPOT

PER KV NETWORK AT EACH LOCATION

ST 0.2 CO VE ATI 0.1 REL L _ —J 1.0 PER UNIT COST SAME AS FIG. 40 265/460 VOLT SPOT NETWORKS 0 _J I 2 3 200 400 600 600 1000 1200 PRIMARY FEEDERS SPACING BETWEEN SPOT NETWORKS-FEET

Fig. 53—Illustrating the reduction in primary-feeder circuit Fig. 54—Relative economics of 208-volt and 460-volt length possible with the use of spot networks. Load area is interconnected spot networks, for a specific transformer the same as Fig. 48. rating.

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1000 24. The Spot-Network/Radial System The spot-network/radial system is made up of a number of spot networks that are designed to be independ- 600 ent of each other in supplying their respective loads.

FEET Each spot network, with the secondaries carried from it,

- resembles the system pattern pictured in Fig. 56. 600 Secondary voltage is 265/460 volts. Fig. 54-a shows a ATION 4-transformer spot network, but in general two or more transformers may be considered. Each network unit in SEPAR the spot network is supplied by a different primary 400 feeder. Fig. 56-a shows what is defined as a 1% block OMIC reach for 265/460-volt secondaries. Wherever the secondaries form a grid, it is assumed that junctions of ECON 200 the grid can be made only at street corners. In studying the requirements for 265/460-volt secondary-mains capability, distribution of load is based on Fig. 56-b, where the locations of equal-size 0100 200 300 400 500 600 loads within one square block are shown. Contiguous SECONDARY VOLTAGE -VOLTS square blocks have loads placed similarly. In many Fig. 55—Economic separation of interconnected spot net- instances it is possible to serve two or more services works versus system secondary voltage. from the same tap-off point, where loads on both sides of the street are served from the same point on the On the basis of first-contingency outage, i.e., one secondary. transformer out of service at one of the spot networks, it In the study of the spot-network/radial system, is found that the general zone of economic separation is different patterns for radial secondaries and variable 180 to 320 feet at 208 volts, and 360 to 640 feet at 460 load densities were considered to find the optimized volts. These results are illustrated in Fig. 55, which lowest-cost system conforming to this type of design. shows the effect of voltage on the economic ability of These design relationships are shown in Fig. 57. The adjacent spot networks to transfer load through a spot-network/radial system cost curve is duplicated in secondary tie between them. To obtain Fig. 55, the Fig. 40 (265/460 volt spot networks) to show a direct secondary voltage of the interconnected spot networks comparison with conventional network-system cost. was varied over a considerable range to show the effect These comparative costs show that: of voltage on the extent of power transfer for such an 1. The spot-network/radial system is generally arrangement of inter-connected spot networks. Power more economical than the general network for load can be economically transferred a greater distance at densities of approximately 50,000 kva per square mile a higher secondary voltage. and higher. First-contingency design does not change

f f (b)

(a)

Fig. So—Arrangement of spot-network/radial system; a) layout for a 1 1/2 block reach; b) Customer's service taps may be taken off at any point along the radial secondaries.

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1200 ...< 6-5 The Relative Feasibility of 460-Volt or 208-Volt Serv-

1100 1.1 ice in Commercial Buildings, H. G. Barnett, R. A. Zimmerman, H. E. Lokay. 1000 1.0 6-6 Service Voltage Spread and Its Effect on Utilization

900 0.9 Equipment, H. G. Barnett, R. F. Lawrence. 6-7 Network Application at 277/480 Volts for A Medium 5a 800 0.8 o Density Area, E. Rothfus, J. R. Oberholtzer, W. R. SPOT -NETWO MN FIRM CA PAD$TY 7 rr, 7005 0.7 -..81 Folck. • 1" 7. Economics of Higher Primary and Secondary Voltages for 6 6010.6 5 Commercial Areas, R. F. Lawrence, D. N. Reps, AIEE C3¢ 300¢ 0 5 Transactions, Vol. 74, Pt. III, October, 1955, pp. 1034-46. A01 g - The Limiter—Its Basic Functions in Network Distribution • 4 400 OA _, 8. Vol. 74, Pt. III, COSTS Systems, C. P. Xenis, AIEE Transactions, 1 300 0.3 October, 1955, pp. 913-15, 944-50. Am t 200 0.2 9. Limiters—Their Design Characteristics and Application,

I too 0.1 Fred Heller, Irving Matthysse, AIEE Transactions, Vol. 74, Pt. III, October, 1955, pp. 924-30, 944-50. O 0 0 40 eo 120 160 200 240 280 320 10. Overload Classifications for Secondary Network Cables, LOAD DENSITY-WA PER SO MI. R. C. Graham, AI EE Transactions, Vol. 74, Pt. III, Octo- Fig. 57—Spot-network/radial system economics. ber, 1955, pp. 916-19, 944-50. 11. Time-Current Damage Characteristics, Cable in Duct, L. F. Porter, AIEE Transactions, Vol. 74, Pt. III, October, 1955, pp. 919-921, 944-50. the economics of spot-network/radial distribution 12. Characteristic Properties of Secondary Network Cables, S. appreciably from second-contingency design. J. Rosch, AIEE Transactions, Vol. 74, Pt. III, October, 1955, pp. 939-50. 2. The combination of a higher primary-feeder 13. Co-ordination of Secondary Network Protection, E. L. Lein- voltage and the 460-volt spot-network/radial system bach, A. S. Brooks, AI EE Transactions, Vol. 74, Pt. III, can be expected to show savings greater than those October, 1955, pp. 922-4, 944-50. indicated for higher-voltage primary supply to a 460- 14. The Probability of Coincident Primary-Feeder Outages in Secondary Networks, D. N. Reps, AIEE Transactions, volt general network. To serve a large network area, Vol. 73, Pt. III, December, 1954, pp. 1467-1478. the use of a smaller number of larger-size transform- 15. Coincident-Outage Probability in Secondary-Network Vaults, ers arranged in multiple-unit spot networks results D. N. Reps, AI EE Transactions, Vol. 74, Pt. III, August, in fewer locations to which primary supply must be 1955, pp. 580-586. brought, and hence a reduction in total mileage of primary-feeder circuit. Articles on Overhead Secondary Networks 16. Combined Overhead and Underground Network of the De- troit Edison Company, Harold Cole, Edison Electric Institute REFERENCES Bulletin, May, 1939, pp. 247-252. Aerial Cable Secondary Network, T. D. Thomas, Electrical Industry Standards and Reference Works 17. World, July ; 1949, pp. 59-61. 1. A.C. Network Operations, 1953-1955, Publication No. 57-8, 18. Network Equipment Goes Overhead for Economy, Harold A Report. of the Transmission and Distribution Committee, Cole, Electric Light and Power, June, 1946, pp. 84-87. 19_ Postpone Costly Underground with Aerial Cable Secondary Edison Electric Institute. Ibid., 1950-52, 1947-49, etc. Network, George R. Parker, W. A. Campbell, F. J. Raker, 2. Underground Systems Reference Book (Book), Edison Elec- Electrical World, August 14, 1950, pp. 76-79. tric Institute, 1957. This entire book is an excellent refer- 20. Light Duty Overhead Networks, II. G. Barnett, H. B. ence work. See especially Chap. 5, "Underground Sec- Thacker, R. A. Zimmerman, Consulting Engineer, August ondary Networks". 1953, pp. 35-38. 3. EEI-NEMA Standards for Secondary Network Transformers, 21. Wapakoneta-1st Small Town to Get Overhead Network, EEI Publication No. 57-7, NEMA Publication No. TR4- Electrical World, March 22, 1954. 1957, Edison Electric Institute, National Electrical Manu- facturers Association. Earlier Published Papers and Articles 4. NEMA Standards for Network Protectors, NEMA Publi- cation No. SG3.1-1954, National Electrical Manufacturers 22. Underground Alternating Current Network Distribution for Association, June, 1954. Central Station Systems, A. H. Kehoe, AIEE Transactions, 5. ASA Guide for Loading Oil-Immersed Distribution and Pow- Vol. 43, June, 1924, pp. 844-53, (discussion pp. 869-74). Electric er Transformers, American Standards Association, Publica- 23. Evolution of the A.C. Network System, H. Richter, tion No. C57.32, 1948. Journal, Vol. XXII, July, 1925, pp. 320-36. 24. The Automatic Network Relay, J. S. Parsons, Electric Jour- nal, Vol. XXII, July, 1925, pp. 339-44. Recent Published Technical Papers 25. Operating Requirements of the Automatic Network Relay, 6. "Symposium on Higher Distribution Voltage for Metropoli- W. R. Bullard, AI EE Transactions, Vol. 45, November, tan Areas". AIEE Transactions, Vol. 73, Pt. II, November, 1926, pp. 1203-1211 (discussion pp. 1220-27). 1954, pp. 306-38; Pt. December, 1954, pp. 1508-42. 26. Low-Voltage A.C. Networks, D. K. Blake, General Electric Also AIEE Special Publication 8-66, 1954. Review, Vol. 31, February, March, April, May, August, Sep- 6-1 A New Approach to the Problem of Higher Distribu- tember and November, 1928, pp. 82-84, 140-43, 186-90, tion Voltages, A. M. de Belie, S. B. Grissom. 245-48, 440-43, 480-82, and 600-604, and Vol. 32, March 6-2 Economics of Various Secondary Voltages for Com- 1929, pp. 170-73. mercial Areas, T. C. Duncan, J. P. Neubauer, J. M. 27. Vertical Distribution in the World's Tallest Structure, J. A. Comly, R. F. Lawrence, Miles Maxwell. Electrical World, Vol. 97, February 14, 1931, pp. 6-3 Distribution Equipment. Used on 265/460-Volt Net- Walsh, works and Its Operating Features, L. Brieger, C. P. 328-34. Xenia, A. J. Mason, J. De Leine. 28. Overhead Secondary Networks Offer Real Economy, J. S. 6-4 Secondary Network Equipment for 250- to 600-Volt Parsons, L. M. Olmsted, Electrical World, Vol. 99, May 7, Systems, R. L. Schwab, R. W. Stohr. 1932, pp. 808-13.

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29. Spot Networks Reduce Cost of Duplicate Service, J. S. 34. Vaults for AC Secondary Networks, J. S. Parsons, Electrical Parsons, S. B. Griscom, Electric Journal, Vol. 29, Novem- World, March 23, April 20, and May 18, 1940, pp. 886-88, ber, 1932, pp. 503-505, and 530. 995, 1201-1203, 1277-78, 1512-13 and 1572-73. 30. Distribution System Within Rockefeller Center, Electric 35. Secondary Networks to Serve Industrial Plants, C. A. Journal, Vol. 31, May, 1934, pp. 180-83. Powell, H. G. Barnett, AIEE Transactions, Vol. 60, 1941, 31. Small Underground Networks, J. A. Pulsford, Electric Jour- pp. 154-56, (discussion pp. 698-700). nal, Vol. 34, March, 1937, pp. 111-14. 36. New Applications for Secondary Networks, J. S. Parsons, 32. Planning a Distribution System, J. F. Fairman, Electric Westinghouse Engineer, Vol. 1, May, 1941, pp. 24-27. Journal, Vol. 35, June, 1938, pp. 236-39. 37. Secondary Network Planning, H. G. Barnett, Electrical 33. Operation of AC Low Voltage Network from Two Voltage World, Vol. 116, August 9, August 23 and September 6, 1941, Sources, W. B. Kenyon, EEI Bulletin, Vol. 7, March, pp. 422-23, 425, 575-76, 579, and 718-19. 1939, pp. 115-18.

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B. L. LLOYD

Transformers with a rating of 3 to 500 kva and all net- ferent types, along with an indication of their general work transformers are classified as distribution trans- area of application. formers.") Transformers larger than 500 kva are classed Conventional Transformers—The "conventional" trans- as power transformers and are discussed in Reference 7. former is so designated because it has no integral light- Instrument transformers are discussed in Chapter 11. ning, fault, or overload protective devices supplied as a The transformer theory discussed in Reference 7 is part of the transformer. These protective devices must equally applicable to distribution transformers and will be separately purchased and mounted. A cutaway view not be repeated here. This chapter is concerned primar- of a conventional transformer is shown in Fig. 1. Single- ily with the application of distribution transformers and phase transformers 333 kva and below, 14.4 kv and be- covers standards, design and construction, and loading low can be pole-mounted. Three-phase transformers guides which are important in application problems. rated 225 kva and below with a primary voltage of 13.8 Typical electrical characteristics, transformer connec- kv and below can be pole-mounted. The methods of tions, and equivalent circuits are given to facilitate cal- pole-mounting are shown in Fig. 2. culation of voltage regulation and short-circuit currents. Pole-mounted conventional transformers are used primarily for supplying residential, small industrial, and I. TYPES OF DISTRIBUTION TRANSFORMERS small commercial customers. However, transformers of higher voltage ratings for special applications, such as 1. Methods of Cooling providing auxiliary power at substation locations, are One means of classification to differentiate between also available for pole mounting. The larger size distri- the various types of distribution transformers is the bution transformers falling outside the voltage and kva cooling and insulating medium employed. In the broad- limits stated above are mounted on a platform or in a est classification, distribution transformers will be either small substation. dry-type or liquid-filled. Completely Self-Protecting Transformers (CSP)—Com- Dry-type distribution transformers are air-cooled and pletely self-protecting transformers are self-protecting air-insulated. They are most commonly used in indus- from lightning or line surges, overloads, and short cir- trial, commercial, and institutional applications where cuits. Lightning protection of the primary winding is oil might present a safety hazard. Voltage ratings as accomplished by lightning arresters mounted directly on high as 15 kv are available for ratings ranging from 3 to the transformer tank. Each external primary bushing 500 kva. which is not permanently grounded has its own light- Liquid-filled transformers can be further segregated ning arrester. Lightning protection of the low-voltage into oil-filled and Inerteen-filled types. Inerteen is a secondary winding is obtained with gaps across each non-flammable askerel. The oil-filled types are usually secondary bushing. No secondary arresters are required, used for pole-top installations, in distribution substa- because the normal secondary voltage is not sufficient to tions, and in users' outdoor substations. Inerteen-filled sustain a gap breakdown which is initiated by a light- transformers can be used in locations where extreme fire ning stroke. The arresters on the primary are usually of hazards exist. the expulsion type. Virtually all distribution transformers are presently Overload protection is accomplished by circuit break- self-cooled. Forced-oil and forced-air cooling are not ers inside the transformer tank. The breakers are co- used for the ratings of transformers in the distribution ordinated thermally with each individual rating to follow class. In the past few years a number of transformers closely the true copper temperature of the coils. This have been built using other cooling media, such as the is accomplished by having the secondary current flow gas-insulated, vapor cooled transformer. At the present through a bi-metal of the breaker and also by placing time, however, these new cooling media have not been the bi-metal in the same path of oil flow used to cool extensively applied to distribution transformers. the windings. Excessive temperature of the bi-metal (hence, of the coils) causes the breakers to trip to the 2. Transformers for Overhead Installations open position. Most CSP transformers have a red warn- Transformers for overhead (as contrasted with under- ing light built into the external breaker operating han- ground) installations are divided into three general dle to indicate overheating of the windings. The signal types: conventional, completely self-protecting (CSP), light comes on when the transformer winding tempera- and completely self-protecting transformers for banking ture reaches the ASA temperature limit for Class A in- (CSPB). Following is a description of these three dif- sulation. If the warning light goes unheeded and the 201 L Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 209/576 202 Distribution Transformers

vaSO (A) (B) (C) DIRECT POLE ONE CROSSARM TWO CROSSARMS Fig. 2—Methods of pole-mounting distribution transformers.

All standard CSP distribution transformers can be pole-mounted. Completely Self-Protecting Transformers for Secondary Banking (CSPB)—CSPB distribution transformers are designed for banked secondary service. This consists of connecting in parallel the secondaries of a number of distribution transformers, all of which are supplied from the same primary feeder. The purpose is to connect transformer capacity so that temporary overloads are distributed among three or more transformers, to reduce voltage fluctuation due to sudden load changes, and to prevent customer outage if a transformer fails. Banking may take the form of an open or closed secondary loop. Lightning and short-circuit protection for the CSPB transformer is similar to that described for the CSP. Fig. 1—Cutaway view of a conventional distribution transformer. winding temperature increases to a dangerous level, the breaker will trip. Following a tripping, the breaker can be reset with the external operating handle. For most CSP transformers the handle can be set to an "emergency" position, which recalibrates the breaker trip setting and permits carrying increased load until the transformer can be changed out. Internal protective links between the primary winding and the primary bushings isolate the transformer in event of an internal fault. The purpose of the links is to protect the remainder of the primary system from a faulted transformer and also to minimize the possibility of having an oil fire develop in a faulted transformer. A cutaway view of a CSP transformer is shown in Fig. 3. Single-phase CSP transformers are available for a range of primary voltages from 2400 volts to 14,400/ 24,940 volts grd. Y. The associated kva range is 5 to 167 kva, with certain of the higher and lower ratings not available for some combinations of primary and secondary voltages. The secondary voltage is 120/240 or 240/480 volts. Three-phase CSP transformers are available for a primary voltage range of 2400 volts to 13,800 volts, and a kva range of 9 to 150 kva. Again, some of the higher and lower kva ratings are not manufactured for some primary and secondary voltage combinations. In general, the secondary voltage will be 208 Y/120, 240, or Fig. 3—Cutaway view of a single-phase CSP distribution 480 volts. transformer.

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However, CSPB transformers are supplied with two sets systems to feed residential and industrial, appliance, and of circuit breakers, the second set being used to section- single-phase motor loads. In a bank of three, distribu- alize the secondary (see Fig. 4(a)). Hence, it is possible tion transformers will supply three-phase loads. Class on the secondary to sectionalize a faulted phase or over- B-1 transformers are suitable for operation on three- loaded line section without cascading and without re- phase, 4-wire systems with neutral grounded at the sub- moving any transformer capacity from the bank. Single- station, or on solidly grounded systems. Class B-2 phase CSPB transformers are designed for single-phase transformers are for use with solidly grounded systems application only, and three-phase CSPB transformers only. are designed for three-phase application only. It can be seen from Fig. 5 that a classification accord- Single-phase CSPB transformers are available in ing to bushing arrangements reflects to a large degree ratings of 10, 15, 25, and 37% kva. Three-phase CSPB prior determination of such factors as operating volt- transformers are available in ratings of 30, 45, and 75 age, system design, and the extent of grounding required. kva. A cutaway view of a 75 kva, three-phase CSPB transformer is shown in Fig. 4(b). 3. Transformers for Underground Installations Bushing Arrangement—A further consideration per- The overhead installation of distribution transformers taining to the installation of pole type transformers is sometimes proves objectionable in certain congested the various bushing arrangements available to the user. areas and at locations where good appearance is impor- CSP transformers are divided into two general classifi- tant. These situations often justify the use of distribu- cations: sidewall or cover mounted high voltage bush- tion transformers which are especially designed for un- ings. The single-phase, CSP, class A distribution trans- derground installation. Underground transformers can former is suitable for either delta or wye distribution be divided into three types: subway, low cost residential, and network transformers. Each of these transformers meets the needs of a particular type of distribution system. PROTECTIVE LINK Subway Transformers—Subway transformers are gen- TRANSFORMER A erally mounted in underground vaults or where occa- CONTACTS ONTACTS sional submersion in water can occur. There are two types of subway transformers: conventional and CP BIMETAL BIMETAL (current protected). The CP subway units provide in- SIGNAL UGHT 3 SIGNAL LIGHT

CUSTOMER SERVICES SECONDARY MAINS 1,13. ;or r(tr

`TRANSFORMER B TRANSFORMER D

•

SECONDARY MAINS

CUSTOMER SERVICES

-.-PRIMARY FEEDER

LT, —TRANSFORMER C

Fig. 4a—One-line diagram of typical CSPB installation. Fig. 4b—Cutaway view of a three-phase CSPB transformer.

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204 Distribution Transformers

BUSHING ARRANGEMENT - COMPONENTS OF VARIOUS CSP TRANSFORMER RATINGS

H. V. WINDING H. V. LOW VOLTAGE - MOUNTED H.V. BUSHINGS MOUNTING SIDEWALL INSULATION BUSHINGS BUSHINGS

HIGH- VOLTAGE il * A 0 lit 2 ,A-A-A-LA-): 0 @ CI il 5000 full AND BELOW kii • * B-I iiiii iiiiiii ...... A.....A...A...... A..., 2 -- @SO@ full

COVER-MOUNTED H.V. BUSHINGS MOUNTING N.V. WINDING H.V. LOW VOLTAGE INSULATION BUSHINGS BUSHINGS HIGH-VOLTAGE * - 7200/12470 Y A k.-A...A.-&._A-A...0 2 TO @ CD @ 14400 DELTA full

._ ,. 12470grd Y/7200 * It --- B-2 Q- 13200 grd Y/7600 i L.A....A.A.1::::, 111, 1 (I ) 0 24940 grd W14400 • ---- -7 graded (2)

* These are Westinghouse class designations . (I) Also class B -3. Electrically identical to B -2, but has two support lugs, equally spaced either side of the bushing , for two position mounting . (2) Midpoint on low voltage winding connected to ground by means of jumper from center bushing to tank body .

Fig. Typical CSP distribution transformer bushing arrangement.

ternally mounted low voltage breakers and high-voltage Standard housings are available for single-phase protective links. The CP transformers have the same transformers through 167 kva, 14.4 kv, and for three- protective features as the CSP transformers except that phase transformers through 112% kva, 4800 volts or 45 no lightning protection is provided. Lightning protection kva, 14.4 kv. The restricted circulation of air due to the is not usually required for cable-fed systems. enclosure makes it necessary to de-rate the semi-buried A three-phase conventional subway transformer is transformers by 10 to 15 per cent.2 shown in Fig. 6. The range of preferred kva ratings is 15 Network Transformers—The network transformer is to 167 kva single-phase, and 15 to 150 kva three-phase. used to supply power to a low-voltage secondary net- The range of primary voltages is 2400 volts to 14,400 work system. Mounted integrally on the transformer volts. are the primary disconnecting and grounding switch Low-Cost Residential—Transformers used in low-cost and the network protector. The primary switch facili- residential underground systems are the same conven- tates maintenance of the transformer and incoming tional transformers normally used for overhead installa- feeder. The network protector provides protection tions. The transformers must be enclosed in some type of against primary feeder and transformer faults, as well surface-mounted housing or in a semi-buried housing as protection against overloads. The purpose and opera- such as shown in Fig. 7. The transformers can be either tion of these auxiliary devices are amplified in Chap- the conventional type or CSP transformers. ter 5.

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Fig. 6—Cutaway view of three-phase subway transformer. Fig. 7—Conventional transformer in a semi-buried housing for low-cost residential underground service.

Network transformers come in three general types: transformers are designed is stated in NEMA Standards liquid-filled, ventilated dry-type, and sealed dry-type. for Transformers, TR1-1954, section TR1-2.036.' All three types are available in ratings of 150 to 2000 "A. Transformers shall be capable of withstanding kva. Liquid filled units are available up to 2500 kva without injury the mechanical and the thermal stresses ratings, but for all types above 1000 kva, the secondary caused by short circuits on the external terminals of voltage is 480 volts or higher. The liquid-filled type is any winding or windings, with rated voltages main- the most commonly used and is used for almost all util- tained across the terminals of all other windings in- ity secondary network systems in downtown areas. They are usually installed in underground vaults, which may be subject to occasional immersion in water. Ventilated dry-type units are used where oil is considered a fire hazard and where contamination in the air presents no problems for the open ventilating system. These units are often used for supplying a network system in an industrial plant or a commercial building. The sealed dry-type unit can be used where the transformer is subject to occasional submersion, or in areas where it is exposed to an extremely corrosive or an explosive atmosphere. A cutaway installation view of a liquid-filled network transformer is shown in Fig. 8.

II. DISTRIBUTION TRANSFORMER STANDARDS

4. Temperature and Short-Circuit Standards1 .3 The temperature standards used for establishing the rating of distribution transformers are the same as those previously discussed for power transformers in Refer- ence 7 and will not be repeated here. The short-circuit capability for which distribution Fig. 8—Cutaway installation view of a network transformer.

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 213/576 206 Distribution Transformers tended for connection to sources of energy, provided on the use of the dash (—), the slant (/), and the that: cross ( X ) .4 1. The magnitude of the RMS symmetrical current The dash is used to separate the voltage rating or in any winding of the transformer does not ex- ratings of separate windings. The slant is used to sepa- ceed 25 times the base current of the winding ... rate voltages to be applied to or to be obtained from the the initial current is assumed to be completely same winding by (a) arrangement of external circuits in displaced from zero when determining the me- which the transformer is connected, such as a single- chanical stresses. phase winding suitable for connection in a delta bank or a wye bank, or a winding that can be externally con- 2. The duration of the short circuit is limited to the nected in a two-wire or a three-wire circuit; or (b) in- following time periods. Intermediate values may ternal reconnection of windings such as a three-phase be determined by interpolation. When used on winding arranged for connection either in delta or wye circuits having reclosing features, transformers by having all leads brought to a suitable terminal board. shall be capable of withstanding successive short The cross is used to designate separate voltages which circuits without cooling to normal operating can be obtained by reconnecting the coils of a winding temperatures between successive occurrences of in series or multiple combinations, but where the wind- the short circuit, provided the accumulated ing is not suitable for three-wire operation from the duration of short circuit does not exceed the series connection. time given in the table." For example, a transformer designated 7200/12470Y —240 X 480 identifies a transformer whose primary RMS Symmetrical Current in Time Period, Any Winding Seconds windings are designed to be connected for operation at either 7200 volts delta or 12470 volts wye (the unit con- 25 times base current 2 20 times base current 3 nected from phase to neutral), and a secondary winding designed for either 240-volt multiple connection or for a 16.6 times base current 4 14.3 or less times base current 5 480-volt series connection, but not for 240/480-volt three-wire connection. Rules for designating the voltage The first provision states that a transformer will not ratings of single-phase and three-phase transformers necessarily be self-protecting against internal mechan- are summarized in Tables 1 and 2. ical damage if an external short circuit results in more Transformers can be operated at rated kva with a than 25 times rated current in the windings. A trans- secondary voltage of 5 per cent above rated voltage. A former will be self-protecting if the impedance is 4 per transformer can withstand no-load operation at a volt- cent or greater on the transformer base and if only one age of 10 per cent above rated voltage. winding is connected to a source which can contribute short-circuit current. This limitation of a maximum of 6. Terminal Markings and Polarity 25 times base current seldom proves of importance in The terminals of a transformer are the points to which applying power transformers, because the reactance of external electric circuits are connected. ASA Standards power transformers is usually considerably higher than C6.1-1956, "Terminal Markings for Electric Appa- 4 per cent. ratus," specify the markings which are applied to the Distribution transformers often have an impedance terminals of distribution transformers.' below 4 per cent and sometimes less than 2 per cent (see The windings of a two-winding transformer are dis- Table 10). These transformers are not guaranteed to be tinguished from one another by the designation High self protecting against short-circuit current exceeding 25 Voltage (HV or H) and Low Voltage (LV or X). Trans- times normal current. Occurrence of such currents is formers with more than two windings have the windings unlikely because of the limiting effect of system im- designated as H, X, Y, and Z. The highest voltage wind- pedance and fault impedance. In rare cases, however, ing is designated as HV or H and the other windings in special precautions to limit fault current may be order of decreasing voltage as X, Y, and Z. If two or desirable. more windings have the same voltage and different kva ratings, the higher kva winding receives the prior letter 5. Designation of Voltage Ratings of Windings designation of the two or more letters available, accord- Distribution transformers are usually quite flexible ing to the sequence by voltage as previously explained. in the primary voltages to which they can be connected External terminals are distinguished from one another and in the secondary voltages which can be obtained. by marking each terminal with a capital letter followed For example, a particular transformer might be suitable by a subscript number, for example, Hi, H2, X1, X2, etc. for connection between phases of a primary supply or A neutral terminal of a three-phase transformer is from phase to ground on a wye system with the same marked with the proper letter followed by the subscript winding voltage, and two or more voltages may be pos- 0. A neutral terminal common to two or more windings sible on the secondary side. A complete description of is marked by the combination of the proper winding the voltage designation of the primary and secondary letters, each followed by the subscript 0, for example, windings is rather long and space-consuming, and a Ho, Xo. If a transformer has a two-terminal winding standard shorthand method of designating the voltage with one terminal grounded, the subscript 2 terminal ratings has evolved. This shorthand designation is based is to be the grounded terminal.

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Table I—Designation of Voltage Rating for Single-Phase Table 2—Designation of Voltage Rating for Three-Phase Distribution Transformers. Distribution Transformers.

Designation Designation Designation of Designation of of Low- of Low- High-Voltage High-Voltage Voltage Description Voltage Winding Winding Description Winding Winding (Example) (Example) (Example) (Example)

E— For line-to-line connection on E or E delta Delta-connected winding (12000) system of E volts line-to-line. (2400 or 2400 suitable for connection to delta) system of E volts line-to-line- E/V-3-EY— For line-to-line connection on (2400/4160Y) system of E volts line-to-line, E Y Wye-connected winding with- or for line-to-neutral connec- (4160 Y) out neutral available for con- tion on system of -VIE volts nection on system of E volts line-to-line. line-to-line.

VTE grd. Y/E— Single bushing transformer -Vg E Y/E Wye-connected winding with (12470 grd.Y/7200) for line-to-ground connection (12470 Y/7200) neutral available for connec- on effectively grounded sys- tion to E/Va E Y system. tem with V3E volts line-to- line. V5 e Y/e Wye-connected winding with (208 Y/120) neutral available for connec- E/1/3E grd. Y For line-to-line connection on tion to e/-VT e Y system. (14400/24940 a system of E volts line-to- grd. Y) line, or line-to-neutral con- e X 2 e Two-section winding suitable nection on an effectively (240 X 480) for multiple connection for grounded system of -V3E e delta or series connection volts line-to-line. for 2e delta.

E/V3EY Two-section primary wind- e or e delta Delta connected winding for X 2E/2-VEY— ing reconnectable for line-to- (480 or 480 connection to system of e (2400/4160Y line connection on system of delta) volts line-to-line. X 4800/8320Y) E or 2E volts line-to-line: Can be connected line-to- neutral on system of -V3E or resent increasing potential difference between that 2-V3E volts line-to-line by terminal and the lowest numbered terminal. If a reconnection. winding is divided into two or more parts for series- multiple connections and the terminals of these parts e/2e Two-section secondary wind- are brought out of the case, the terminals of each portion (120/240) ing which can be connected of the winding are given consecutive numbers. in parallel for output voltage e, in series for output voltage The H1 terminal is located as the right-hand terminal 2e, or in series for 3-wire of the high-voltage group as seen when facing the service for e/2e output volt- highest voltage side of the case, and the other H termi- age. nals are brought out in numerical order from right to left. When the high-voltage winding has only one e X 2e Two-section secondary wind- terminal brought out, that terminal is designated H1. (240 X 480) ing which can be connected Numbering of the terminals of the H winding and in parallel for output voltage terminals of the X winding is such that when H1 and e, in series for output voltage X1 are connected together and voltage applied to the 2e, but not for 3-wire service. transformer, the voltage between the highest numbered H terminal and the highest numbered terminal is less 2e/e Mid-tapped secondary wind- X (240/120) ing suitable for 2-wire service than the voltage of the H winding. The same relation- at voltage 2e, or for 3-wire ship exists between each pair of windings when more service. Cannot be recon- than two windings are used. nected for 2-wire service at Polarity—When terminals are marked in accordance voltage e. with the above rules, the polarity of a transformer is subtractive when H1 and Xi are adjacent, and additive when H1 is diagonally located with respect to X1. The Single-Phase Transformers—The terminals of any wind- same rule applies between the H winding and any other ing brought out of the transformer are numbered 1, 2, winding if the transformer has more than two windings. 3, 4, etc., the lowest and highest numbers marking the Additive polarity is standard for all single-phase full winding and the intermediate numbers marking transformers in sizes 200 kva and smaller, having high- fractions of windings or taps. Increasing numbers rep- voltage windings rated 8,660 volts and below. All other

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208 Distribution Transformers single-phase transformers are connected for subtractive When tap terminals are brought out of the case, they polarity. are marked with the proper letter followed by the figures The terminal markings for some of the more usual 4, 7 . . ., for one phase, 5, 8 . . ., for another phase, and single-phase connections are shown in Fig. 9. 6, 9 . . ., for the third phase. For a delta connection, the Three-Phase Transformers—The three terminals for order of numbering tap terminals is 4, 7 . ., from ter- each winding which connect to the full phase windings minal 1 toward terminal 2; 5, 8 . . ., from terminal 2 are marked HI, H2, Hs, X1, X2, X3, Yl, Y2, Y3, etc. If toward 3; and 6, 9 . . ., from terminal 3 toward terminal the phase sequence of voltage on the H winding is Hr, 1. For a wye connection, the order of numbering tap H2, H3, it will be X,, X2, X3 and Y,, Y2, Y3, etc., on the terminals is 4, 7 . . ., from terminal 1 toward neutral; other windings. The markings of the phase terminals 5, 8 . . ., from terminal 2 toward neutral; 6, 9 . . ., from are made in accordance with one of the groups shown terminal 3 toward neutral. This is illustrated in Fig. 10(f). in Fig. 10. When so marked, there will be zero degree The H1 terminal is brought out as the right-hand ter- phase displacement between corresponding numbers on minal of the high-voltage group as seen when facing the the various windings for the connections shown in Fig. highest voltage side of the case. The H2 and H3 terminals 10(a), (b), and (c). For the wye-delta connection in are brought out so that the three terminals are arranged Fig. 10(d), (e), and (f), the voltage of a high-voltage in numerical order reading from right to left. The Ho terminal leads the voltage of the correspondingly num- terminal, if present, is located to the right of the 1/1 bered low-voltage terminal by a time angle of 30 de- terminal as seen when facing the highest voltage side grees. When more than one low-voltage winding is used, of the case. the angular displacement between the H winding and The numbered X terminals are arranged in numerical each of the other low-voltage windings is established in order from left to right when facing the low-voltage side the same manner. of the case. Other low-voltage winding terminals, if present, are numbered in the same manner as the X SUBTRACTIVE ADDITIVE SUBTRACTIVE winding terminals. H, H 2 H, H 2 H, H2 H3 H4 7. Distribution Transformer Insulation Classes' The insulation class of a distribution transformer defines the dielectric tests which the unit can withstand.

The insulation class is usually specified as a number in X, Xz X2 X, X2 X3 4 kv. The number corresponds to the maximum rated (a) SIMPLE H AND X WINDINGS WITHOUT ( b)SIMPLE H AND X TAPS voltage between terminals for phase to phase connection WINDINGS WITH TAPS of the highest rated voltage which falls within that par- Hz Hz H2 ticular insulation class. For example, a transformer for connection in either wye or delta on a 69-kv system would be in the 69-kv insulation class. There is an exception to the rule stated above for designation of voltage class.' Single-phase transformers with voltage ratings of 8.66 kv and below are insulated X, Xs X2 X4 X4 X2 X3 X, X1 X2 X4 Xs 6 X6 for the test voltages corresponding to the wye connec- (C)SERIES MULTIPLE X WINDING WITH- (d)SERIES MULTIPLE tion; hence, the insulation class for these transformers OUT TAPS WINDING WITH TAPS applied in a delta connection are one class higher than necessary for their voltage rating. Thus, a single phase H, H z transformer with a 2400-volt winding has adequate insulation to be applied to a 4160-volt, wye connected system. Dielectric Test—Designation of insulation class defines the standard dielectric tests which the transformer must be able to withstand. These dielectric tests consist of an X i X 2 Xs Xz X5 impulse test, an applied-potential test, and an induced- (e)THREE-WIRE SERIES CONNECTION MWO-WIRE MULTI- WITH NEUTRAL BROUGHT OUT PLE CONNECTION potential test. These dielectric test voltages for the various insulation classes are shown in Table 3 for liquid- H, H2 H, H immersed distribution transformers and in Table 4 for dry-type transformers. The dielectric tests are similar to those described in Reference 7 for power transformers, with two exceptions: (1) the magnitude of the impulse test waves for distribution transformers in the 15-kv class and below are lower X, Xz X, Xz than the impulse waves used to test power transformers; (g)TWO-WIRE SERIES CONNECTION (h)TWO-WIRE MULTI- PLE CONNECTION (2) distribution transformers in the 15-kv class and below have no established standard of front-of-wave im- Fig. 9—Terminal markings for single-phase transformers. pulse tests.

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H3 H2

H2 X2 H2

DDHi H3 X1 X3

Xi X2 Xo X1 X2 X3

(a) DELTA-DELTA CONNECTION (d) DELTA-WYE CONNECTION

H2 X2

Ho X0

...""/".... HI H3 X1 X3

X 3 Xi X2 X3

(b) WYE - WYE CONNECTION (e) WYE - DELTA CONNECTION

H2 H2 X2 H5 X5 -X2 HoXo H XO X e

X 1 3 X9 Hi H3 H9 H6 X6 X3

X I X3 XI X3 X5 X7 X9 HOXO ( f) DELTA- WYE CONNECTION WITH TAPS IF NEUTRAL TERMINAL IS BROUGHT OUT (c) AUTOTRANSFORMER CONNECTION

Fig. 10—Terminal markings for three-phase transformers.

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Table 3-Insulation Levels and Dielectric Tests for Oil-Immersed Distribution Transformers'

Rated Voltage Between Terminals Impulse Tests

Single Phase Three Phase Low For Wye For Delta For Wye Frequency Full Chopped Minimum Nominal Basic Connection Connection or Delta Test** Wave Wave Time to System Insulation on 3-Phase on 3-Phase Connection, Voltage Crest Crest Flashover Voltage, Level (BIL) System, System, KV KV KV KV KV KV KV KV sec.

1.2 30 0 to 0.69 0 to 0.69* 0 to 1.2 10 30 36 1.0 2.5 45 - - 2.5 15 45 54 1.25 6.0 60 2.89 2.89* 5.0 19 60 69 1.5 8.66 75 5.0 5.0* 8.66 26 75 88 1.6 15 95 8.66 15 15 34 95 110 1.8 18 125 14.4 - - 40 125 145 1.9 25 150 14.4 25 25 50 150 175 3.0 34.5 200 19.9 34.5 34.5 70 200 230 3.0 46 250 26.6 46 46 95 250 290 3.0 69 350 39.8 69 69 140 350 400 3.0

*Single-phase transformers rated 8.66 kv and below are insulated fo the wye **This is an applied potential test for fully insulated windings. For graded connection so that a single line o apparatus serves bath the wye and delta insulation, applied potential is in accordance with the insulating claaa of the connection. Hence the test voltages for such delta-connected apparatus are neutral end. Induced potential test employe twice normal voltage developed one step higher than required for their voltage rating. in winding, but is subject to certain limitations. See ASA C57.12-1956, Section 12-06.230.

Bushing Insulation Levels-Distribution transformer standard taps and a standardized list of accessories bushings must withstand a full wave impulse, which in which can be added to the so-called "standard" trans- general is the same as the transformer BIL for the same former. There is a strong incentive to use the standard-. voltage class. In addition, bushings must withstand one ized transformer, because this transformer is lower in minute, 60-cycle overpotential tests, and bushings in price than a non-standard transformer. the 15-kv class and above have standard specified creep- The EEI-NEMA tables of standardized ratings of age distances over the porcelain. The EEI-NEMA bush- single-phase and three-phase transformers for overhead ing standards for distribution transformers are listed in installation are given in Tables 7 and 8. Table 5.4 Preferred ratings have been established for trans- Insulation Class of Transformer Neutrals-Transformers formers other than the overhead type. The preferred designed for wye connection only with the neutral ratings for these other types of distribution transformers brought out may have a lower insulation level at the are listed in Table 9. neutral than at the line end. Table 6 lists the minimum insulation class at the transformer neutral. III. ELECTRICAL CHARACTERISTICS

8. Preferred Voltage and KVA Ratings 9. Transformer Equivalent Circuits° EEI-NEMA standards have been established for A schematic representation of a transformer is shown ratings of distribution transformers for overhead instal- in Fig. 11(a). The primary winding consists of a number lation.' These standardized transformers also include of turns of insulated wire around an iron core. The

Table 4-Insulation Levels and Dielectric Tests for Dry-Type Transformers'

Rated Voltage Between Terminals

Single Phase Three Phase

For Wye For Delta For Wye 'Low Frequency Impulse Nominal Connection Connection or Delta Test** Test*** System on 3-Phase on 3-Phase Connection Voltage Voltage, System, System, KV KV KV KV KV KV

1.2 0 to 0.69 0 to 0.69* 0-1.2 4 10 2.5 - - 2.5 10 20 5.0 2.89 2.89* 5.0 12 25 8.66 5.0 5.0* 8.66 19 35 15 8.66 15 15 31 50 *Single-phase tranefo mere rated 8.66 kv and below are insulated for the neutral end. Induced potential test employs twice normal voltage developed wye connection so that a s'ngle line of apparatus serves both the wye and delta in winding, but is subject to certain limitations. See ASA C57.12-1956. section connection. Hence the teat voltages for such delta-connected apparatus are 12-06.230. one step higher than required for their voltage rating. ***These are present-day values for both full-wave and chopped-wave teats. **This is an applied potential test for fully insulated windings. For graded No standard impulse tests have been established. insulation, applied potential is in accordance with the insulating class of the

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N TURNS I TURN Table 5—Single- and Three-Phase Transformer Bushings3

Basic 60-Cycle 60-Cycle Bushing Impulse Creepage Dry One Wet 10 Voltage Level, Dry Distance, Minute Second Rating, Withstand, Inches* Withstand, Withstand, KV KV KV KV

ES 1.2 30 10 6 X2 6 60 21 20 8.66 75 27 24 15 95 10/±/ 35 30 23 150 17 70 60 34.5 200 26M 95 95 46 250 35 120 120 (a) SCHEMATIC REPRESENTATION 69 350 48M 175 175 SINGLE-PHASE TRANSFORMER BUSHINGS ONLY: 16.34 95 35 30 Rp+j Xp R s+jXs 18 125 16/±1/42 I 36 HI i *Creepage distances are minimum values where no tolerance is specified. IP IS tion transformer, the primary winding usually has a higher voltage rating than the secondary winding. IDEAL The primary winding develops a flux Om in the iron, TRANSFORMER TES which also links the secondary winding. The change of flux linkages due to the sinusoidal variation of voltage applied to the primary winding induces a voltage in the secondary winding, which is inversely proportional to the ratio of turns on the primary and secondary windings (N). Connection of a load to the secondary termi- b) COMPLETE EQUIVALENT CIRCUIT nals will cause a current I. to flow in the secondary winding and a corresponding current Ip to flow in the primary winding. If the transformer were an ideal transformer, that is, XI having infinite permeability in the iron and zero iron losses, zero resistance in the windings, and no winding leakage flux which does not link the other winding, the primary and secondary quantities would be related exactly as follows:

Xa (1) (c) SIMPLIFIED EQUIVALENT CIRCUIT NEGLECTING SHUNT BRANCH (2)

Table 6—Minimum Insulation Class at Neutral of Oil- R j X Immersed Distribution Transformers'

Minimum Insulation Class, KV Grounded Through Winding Insulation Grounded Solidly Ground Fault Class, KV or Through Current Neutralizer or Transformer Isolated but Impulse Protected (d)SIMPLIFIED EQUIVALENT CIRCUIT 1.2 1.2 1.2 EXPRESSED IN PER UNIT SYSTEM 2.5 2 . 5 2.5 Fig. 11—Equivalent circuits for simple transformer. 5.0 5.0 5.0 8.66 8.66 8.66 secondary winding (X1-X2) similarly consists of a num- 15 8.66 8.66 ber of turns around the same iron core. The primary 25 8.66 15 winding is connected to the source of power supply, and 34.5 8.66 25 the secondary winding is connected to the secondary 46 15 34.5 15 46 system which feeds the load. For a two-winding distribu- 69

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Table 7-EEI-NEMA Standard Ratings for Single-Phase Distribution Transformers of the Conventional Type4

Transformer Risk-Voltage Standard Kva Ratings for Low-Voltage Ratings of (a) Preferred Nominal Tape 2,400 2,520 7,200 or Voestem Rating BIL 120/240 240/120 240/480 240x480 600 or or 6,900 7,560 or (KV) Above Below (b) (c) (b) (d) 4,800 5,040 i 7.980 480 4800) 30 None 2-5% 3-100 ------600 600(1) 30 None 2-5% 3-100 ------None None 3-50 ------None 4-234% 10-167 ------2,400 or 2,400 60 - - 10-100 - 25 - - - - 2,400/4,160Y 2-234% 2-234% - - - - 100 ------167 - 167 - - - - 250-500 - - 250-500 333-500 - - - - None 4-234% 3-100 ------2,400/4.160Y 4,160 75 2-234% 2-234% - - 10-100 ------None None 3-167 ------4,800 - - 10-100 - 25 - - - - or 4,800 75 - - - - 50 - - - - 4.800/8.320Y 2-2 36 % 2-2 34% - - - 100 ------167 - 167 - - - - 250-500 - - 250-500 333-500 - - - - 2,400 or 2,400/4,160Y or 4,800 or 2,400x4,800 75 None None 3-100 ------4,800/8,320Y None None 3-60 ------None 4-2 34 % 3-167 ------10-100 - 25 - - - - 7,200 95 _- 7,20b2;3102,&Y - - - 50 50 - - - 2-234% 2-234% - - - 100 100 - - - - - 167 - 167 - - - - 250-500 - - 250-500 333-500 ------167-500 333-500 - - None None 3-25 ------3-50 ------7,200/12.470Y 12,470Ord2/7,200(e) 95 None 4-2 34 % 3-25 ------3-50 ------3-167 - 10-100 - 25 ------50 - - - - 7,620/13,200Y 7.620 95 2-234 % 2-234 % - - - 100 - - - - -_- 167 - 167 - - - - 250-500 - - 250-500 333-500 - - - - None None 3-5 ------_ • 3-5 ------7,620/13,200Y 13,200GrdY/7,1320(e) 95 2-2 34% 2-234 % 3-25 ------3-50 ------None 4-234% 5-167 - 10-167 ------250-500 ------10-100 - 25 - - - - 12.000 12,000 95 - - - - 2-234% 2-234% - - - - 100 100 - - - - - • 167 - 167 - - - - 250-500 - - 250-500 333-500 ------167-500 333-500 - - None 4-234% 3734-167 ------13,200 13,200 95 167 - 167 - - - 2-235% 2-2 34 % 250-500- - - 250-500 333-500 _7_ ------1e7-500 3337500 - - 187 - 167 - 167 - - - - • 14,400 13,800 95 14,400/14,100 13,500/13,200 250-500 - - 250-500 333-500 ------167-500 333-500 - -- 10-100 - 10-100 - 25 - - - - 13,200 or 14,400 14,400(f) 95 None 13.800/13,200 - - - - 50 50 - - - 12,870/12.540(g) - - - - 100 100 - - - 25 - 25 - 100 - - - - 100 - 50 167 167 - - - - 23,000 22,900 150 24,100/23,500 22 300/21,700 - - 100 333-500 333-500 ------100-167 - - 167------333-600 333-500 333-500 333-500 25 - 25 167 167 - - - - 100 - 50 333-500 333-500 - - - - 34,500 34,400 200 36,200/35,300 33.500/32,600 - - 100 ------107 167 - - - - - 333-500 333-500 333-500 333-500 50 - 50 167 167 - - - - 46,000 43,800 250 46,200/45,000 42,600/41.400 100 - 100 333-500 333-500 ------167 - 167 - - - - - 333-500 333-500 333-500 333-500 100 - 100 167 - - - - - 69,000 67,000 350 70,600/68,800 65,200/63,400 - - - 333-500 ------167 167 - - - - - 333-500 333-500 333-500 333-500

a) STANDARD KVA RATINGS ARE 3, 5, 10. 15, 25, 3734, 50, 75, 100, 167, 250. (d) Low-voltage rating 240s480 v or high-voltage rating 2,400x4,800 v is suitable for 333, 500. aeries or multiple serviee but not for three-wire service. Kva ratings separated by a dash(-) indicate that all Intervening standard ratings One high-voltage bushing only. are lueluded e) Trans formere in Da class are designed to cover in one rating the general voltage range 13,000-10,000 v. (b) Low-voltage rating of 120/240 v or 240/480 v Is suitable for series, multiple, or (g) The lowest voltage ehall be reduced kw. rating. All others shall be at rated kva. three-wire service. (h) Suitable only wnen system ground conditions permit the use of 18 kv arresters. (o) Low-voltage rating 240/120 v Indicates that the transformer shall be suitable for ) Not equipped with tap changers. aeries or three-wire operation but not for multiple operation at 120 v. (k) Transformers of the following ratings will also be available: Basic Tranefonner Impulse High-Voltage Taps Standard Kva Ratings for Low-Voltage Ratings of: Figure HV Rating Level Above Below 120/240 240/480 240x480 Reference 5-25 116 50 26 16,340 95 17,200/16,770 15,910/15,480 100 167 86 333-500 87 13,800/13,200 3-25 83 24,940GrdY/14,400(e)(h) 125 None 12,870/12,540 (g) 3-50 84 13,800/13,200 86 14,400/24,940OrdY(h) 125 None 12,870/12.540 (g) 3-50 10-50

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Table 8—EEI-NEMA Standard Ratings for Three-Phase Distribution Transformers of the Conventional Type(41

Transformer High Voltage Standard KVA Ratings for Low-Voltage Ratings of (a) Preferred Nominal 12,470Y/ etem Tape 240(b) 240x480 2,400 or B/L or 7,200 or Voltage Rating (KV1 208Y/120 (b) 240:480 4.160Y/2.400 13 200Y/ Above Below 480 or 4,800 ,62f) None None 9-75 9-45 75 - - - 2,400 2,400 46 None 4-234% 11234-150 - - - - - 225-500 - - - - - 2-2 34 % 2-234 % - 112 34-150 - - - - 225-500 - - None None 9-75 - - - - - 4,160Y/2,400 60 None 4-2 34% 112 34-150 - - - - - 225-500 - - - - - None None - 9-45 75 - - - 4,160Y 60 2-234% 2-234% - 11234-150 - - - - 225-500 _ - 2.400/4.1601 1-5% 1-5% - 9-45 75 - - - 4,180 60 None 2-5% 9-75 - - - - - None 4-2 34 % 11234-150 - - - - - 225-500 - - - - - None None 9-75 9-46 75 - - - None 4-234% 150 .-- - - 4,800 4,800 66 300-500 _7_ - E 2-2 34% 22 A% - - 150 - - - - - 300-500 - - None None 9-75 - - - - _ 8,320Y/4,800 75 150 - - - - - None 4-234% 300-500 - - - - - 4,800/8,320Y None None -- 9-45 75 - - - 8,320Y 76 2-234% 2-234% - - 150 - - - - - 3007500 - - None 2-5% 9-75 - - - - - None 1-2 A% - - - 2 0 7.200 75 " A-1837, 0 - - _T_ - 1-5% 1-5% - 9-45 75 - - _ 2-234% 2-214% - - 11234-150 - - - - - 225-500 - - None 2-5% 15-75 - - - - - None 4-2 34 % 112 34-150 - - - - - 225-500 - - - - - 12,000 12,000 95 1-5% 1-5% - 15-45 75 ------11234-150 150 2-234% 2-234% - - - 225-500 ------300-500 - None 2-5% 15-75 - - - - - 12,470Y/7,200 95 None 4-234% 112 34-1 50 - - - - - 225-500 - - - - - 7,200/12,470Y 1-5% 1-5% - 15-45 75 - - - 12 470Y 95 2-234% 2-234% - - 11234-150 - - - - - 225-500 - - 1-5% 1-5% 15-75 - - - - - 13,200Y/7,620 95 2-234% 2-234% 11234-150 - - - - - 225-500 - - - - - 7,620/13,200Y 1-5% 1-5% - 15-45 75 - - - 13,200Y 95 2-234% 2-234% - - 11234-150 - - - - - 225-5- 00 - - None 4-234% 11234-150 - - .- - - 225-500 - - - - - 13,200 13,200 95 2-2 A% 2-234% - - 112A-150-- 150 - - - 225-600 - - - - - 300-50- 0 - 14,400 13,200 15-75 15-45 75 - - - 14,400 13.800 95 11234-150 - 11234-150 - 150 - 14,400/14,100 13,500/13,200 225-500 - - 225-500 ------300-500 - - - - 150 - - 23,000 22,900 150 24,100/23,500 22,300/21,700 - - - 300-600 _ _ - - 1-50 150 - - - - 300-500 300-500 - _ 34,500 34,400 200 36,200/35,300 33,500/32,800 - - 30°-6°°-- 3007500 300-500 -- 300-500 46,000 43,800 250 46,200/45,000 42,600/41,400 - _7_ - 300-500- 300-500 - - _ 500 _ 69,000 87.000 350 70,600/68.800 65.200/63.400 - - - sTo 500 a) STANDARD KVA RATINGS ARE: 9, 5, 30, 45, 75, 11234. 150, 225. 300, 500 . (b) A 120-volt reduced kva tap Is provided. See Par. T2.212. Eva ratings separated by a dash (-) indicate that all Intervening standard (c) All transformers are !elm connected unless otherwise specified. For complete ratings are Included. three-phase nomenclature see introduction to Part T of these Standards.

However, an actual transformer does not exhibit of the current required to Magnetize the core and to these ideal characteristics. The complete equivalent supply power losses due to hysteresis and eddy currents circuit for an actual transformer is shown in Fig. 11(b). in the iron. The shunt elements are shown as variable The series elements represent the resistance drop and elements, since magnetizing current and iron-loss current the apparent drop due to leakage flux in each of the are not linearly related to the applied voltage magnitude. windings. The shunt branch, Zm, represents the effect For most application work involving voltage-drop

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214 Distribution Transformers

Table 9—Preferred Ratings for Distribution Transformers Other Than the Conventional Type

Network CSP Transformers Subway Transformers CSPB Transformers Transformers 1-Phase 3-Phase 1-Phase 3-Phase 1-Phase 3-Phase (3-Phase) 3 kva 9 kva 15 kva 15 kva 10 kva 30 kva 300 kva 5 kva 15 kva 25 kva 45 kva 15 kva 45 kva 500 kva 10 kva 30 kva 50 kva 75 kva 25 kva 75 kva 750 kva 15 kva 45 kva 100 kva 150 kva 37A kva 112% kva 1000 kva 25 kva 75 kva 167 kva 50 kva 1500 kva* 37% kva 112% kva 2000 kva* 50 kva 150 kva 2500 kva* 75 kva 100 kva

*Thew ratings available only with eecondary of 480 volte and above.

and short-circuit calculations, sufficiently accurate re- not specify the transformer reactance, but he accepts sults can be obtained by neglecting the shunt branch. the reactance inherent in the standardized design of the This permits the transformer to be represented by a particular manufacturer. Each manufacturer has pub- single impedance in series with an ideal transformer lished values of impedance for his particular design. If having the same turns ratio as the actual transformer, the purchaser specifies a non-standard transformer, as shown in Fig. 11(c). If the per unit system is em- there will be a range of impedances within which the ployed, the representation is made even simpler by purchaser can specify the particular value of impedance eliminating the ideal transformer as is shown in Fig. desired. 11(d). The per cent impedance and resistance of certain Westinghouse standardized transformers (for both over- 10. Transformer Impedances head and underground installation) are shown in Table The impedance usually given for a transformer is the 10. Table 11 shows the range of impedances which can equivalent series impedance shown in Figs. 11(c) and be specified for non-standard pole-type transformers (d). The impedance is most conveniently expressed in without additional price increase. The non-standard per cent on the transformer kva base. The per cent transformer will cost more than the standardized trans- impedance of a transformer can be found by short cir- former, but there will be no specific added charge due to cuiting one winding and applying sufficient voltage to impedance alone, if the impedance is within range speci- the other winding to circulate rated current through the fied in Table 11. Impedances somewhat above or below transformer. The voltage required to circulate rated the limits cited in Table 11 can be obtained at additional current, expressed as a percentage of the rated voltage cost. Typical values of impedance for network trans- of the winding to which the voltage is applied, is numer- formers are shown in Table 13(b). ically equal to the per cent impedance of the trans- The impedances listed in Table 10 are only typical former. Per cent impedance can be converted to ohms estimating values for primary windings in the voltage on either the high- or low-voltage side by use of the classes shown. They will vary with the secondary voltage following formula: ratings and will vary among manufacturers. These 10 (kv..d) values can be used for general studies, and for particular Z(in ohms) = Z(in per cent) X 2 (3) studies where more accurate information is not available kva. t..1 from the manufacturer. where kva..,,,,d is the rated kva of the transformer and 11. Transformer Losses and Exciting Current6 hrated is the rated voltage of the winding on the side of the transformer where the ohms are to be expressed. In addition to impedance drop, other effects cause an For power transformers the reactance is usually much actual transformer to deviate from the ideal transform- larger than the resistance, and the impedance is often er. Two such effects which are important from an appli- considered to be entirely reactive with the resistance cation standpoint are the power losses in the trans- neglected. However, in distribution transformers, par- former and the no-load exciting current. ticularly the smaller kva sizes, the reactance is much Load (Winding) Losses—Transformer load loss or wind- smaller and may in fact be less than the retistance of the ing loss is defined as that power loss which occurs when transformer. Hence, for smaller distribution transform- the transformer is loaded, but which does not exist at ers, it is usually necessary to consider both the resistance no-load. Fig. 11(b) shows that the load losses are almost and reactance. entirely due to the resistances of the primary and sec- In discussing typical reactances for distribution ondary windings (R, and Re), since the losses in Z. are transformers, distinction must be made between stand- almost independent of load current. The load losses, ardized and non-standard transformers. For the EEI- then, are directly proportional to the square of the load NEMA standardized transformers, the purchaser does current.

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Distribution Transformers 215

Table 10-Typical Impedances of Distribution Transformers of Standardized Design

VOLTAGE RATING OF PRIMARY WINDING 24.9/14.4 2.4 KV 4.8 KV 7.2 KV 12 KV 23 KV 34.5 KV 46 KV 69 KV KVA Grd Y Rating - %R %Z %R %Z %R %Z %R %Z %R %Z %R %Z %R %Z %R %Z %R %Z 5 1.9 2.3 2.1 2.4 2.1 2.6 2.3 2.7 2.8 4.0 - 10 1.4 1.9 1.6 2.0 1.9 2.3 1.9 2.1 2.3 3.0 - 15 1.4 1.8 1.6 1.9 1.7 2.1 1.7 2.0 2.1 2.6 - - - - . 25 1.3 1.8 1.5 1.8 1.6 2.2 1.5 1.9 1.9 2.0 2.0 5.2 2.2 5.2 - - M 50 1.2 2.1 1.3 1.8 5.7 - - 2.2 1.3 2.2 1.3 2.3 1.9 1.7 1.7 5.2 1.7 5.2 °z 100 1.1 2.0 1.2 1.9 1.2 2.0 1.2 2.2 - 1.4 5.2 1.5 5.2 1.5 5.7 1.4 6.5 10 333 1.1 4.8 1.1 4.8 1.0 5.0 1.0 5.0 - 1.0 5.2 1.1 5.2 1.1 5.7 1.1 6.5 500 1.0 4.8 1.0 4.8 1.0 5.0 1.0 5.0 - 0.9 5.2 1.0 5.2 1.0 5.7 1.0 6.5

9 2.0 2.4 _ _ q 2.2 2.4 2.5 2.5 - - - 15 1.9 2.5 2.1 2.5 2.2 2.6 2.4 2.8 - - - A 30 1.6 2.4 1.8 2.5 1.9 2.6 2.1 3.1 _ - 75 _ w 1.5 3.2 1.6 3.2 1.6 2.9 1.6 3.3 - - - - 150 1.2 4.2 1.4 4.3 1.3 3.5 1.4 4.3 - 1.6 5.2 - - - - - 300 1.3 E-4 4.8 1.3 4.8 1.3 5.0 1.3 5.0 - 1.3 5.2 1.4 5.2 1.4 5.7 - - 500 1.2 4.8 1.2 4.8 1.1 5,0 1.1 5.0 - 1.2 5.2 1.2 5.2 1.3 5.7 1.2 6.5 February 1958

Computation of load losses is straight-forward, and of the transformer. The units in Equations (4) and (5) the procedures and formulas used will depend on the are consistent; that is, if the full load loss is known in form in which the base loss characteristics are known. watts, the calculated loss will also be in watts. Conver- Manufacturers usually furnish loss data as watts load sion between watts loss and per cent loss can be made by loss or per cent load loss (based on transformer kva use of the formula rating) with rated load current in the windings. If the watts load loss per cent loss= (6) data is known in this form, the load loss at other than 10 kvarat ed rated conditions can be calculated by the formulas 2 The loss characteristics may also be known in terms Load loss= ( I- load) -rated (load loss at rated load) (4) of either ohms resistance referred to the high voltage or low voltage side, or in per cent. Note that resistance in (kva load kvrat ed 2 X, (load loss at rated load) (5) ,per cent is the same as load loss at full load, and Equa- kva fa/a rat ed ctual tions (4) and (5) can be used directly. If the resistance Where the transformer load is known in kva rather than is known in ohms referred to either winding, the follow- in amperes, Equation (5) accounts for the possibility ing loss formulas can be derived from Fig. 11(b) by neg- that a transformer may be applied to a system whose lecting the shunt branch Zm such that I,= NI P. actual voltage differs somewhat from the rated voltage Load loss= /p2(Rpl-N2 R.) (7) = /82 (Rs+ Rp/N2) (8) Table 1 1-Range of Per Cent Impedances for Non-standard Pole-type Distribution Transformers Obtainable Without where the parenthetical term in Equation (7) represents Additional Price Increase the resistance referred to the high voltage winding and that in Equation (8) represents the resistance referred VOLTAGE RATING OF PRIMARY WINDING KVA to the low voltage winding. Rating 480 to 4.16 to 7.2 to 7.2 to 12 to Typical values of load losses for standardized trans- 4800 V. 4.8 KV 7.62 KV 8.32 KV 14 KV formers are given in Tables 12 and 13. The columns in Table 12, for example, list the total losses and the no- ani=1 5 2.2-2.7 2.3-2.8 2.5-3.0 - 2.5-3.0 -4 10 load losses. The value of load loss can be determined by 131 1.9-2.7 2.1-2.9 2.1-2.9 - 2.3-2.9 subtracting the no-load losses from the total losses. The P. 25 2_1-2.8 2.3-3.0 2.2-2.9 - 2.1-2.9 N 50 2.2-2.8 2.1-3.0 losses in Tables 12 and 13 apply only to standardized en 2.2-3.7 - 2.4-3.0 z 100 2.3-3.3 2.3-3.5 2.9-4.2 - 3.0-4.2 transformers; transformers with special impedances, 65 167 3.1-4.2 3.0-4.4 3.1-4.5 - 3.2-4.6 frequency, tap range, etc. may exhibit differences in ---- both total loss and in the ratio between load loss and r4C0 9 2.3-3.0 2.2-3.0 - no-load loss. For these special transformers the specific -c 2.2-3.3 - al 15 2.2-3.0 2.3-3.0 - 2.2-3.3 2.5-3.3 losses should be determined from the manufacturer. PA 30 1.9-3.3 2.1-3.3 m -- 2.1--3.3 2.1-3.3 The load losses are a linear function of the resistance m 75 2.1-3.6 2.3--3.6,. M -- 2.2-3.4 2.1-3.4 of the windings. Since the winding resistance varies with Fi 150 2.2-4.2 2.1-4.8 -- 2.1-4.8 2.2-4.8 temperature, the load loss similarly varies with temper-

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Table 12-Typical Values of No-Load Loss and Total Loss in Per Cent for Distribution Transformers of Standardized Design (At 75°C)

VOLTAGE RATING OF PRIMARY WINDING

24.9 Grd Y/ 7.2 KV 12 KV 46 KV 69 KV 2.4 KV 4.8 KV 14.4 KV 23 KV 34.5 KV KVA Rating No ,.,,, , No ,...„ No No , No ,..,,,, , No , No No No 1 1 Total Total Load Load .I. o t a A Load o t al Load Total Load Total Load otai Load Total Load Load Total Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss 5 .72 2.60 .77 2.84 .86 3.00 .90 3.24 1.02 3.82 ------wm 10 .57 2.00 .57 2.20 .67 2.50 .68 2.60 .84 3.10 ------.60 2.30 .4m 15 .52 1.95 .51 2.08 .60 2.30 .70 2.80 ------P. 25 .43 1.74 .43 1.88 .52 2,02 .52 2.04 .60 2.48 .80 2.92 .88 3.18 ------N .45 1.78 2.70 -- -- o 50 .35 1.58 .35 1.68 .43 1.77 .58 2.06 .64 2.45 .72 2.54 .84 z 100 .32 1.45 .32 1.51 .37 1.54 .40 1.55 -- -- .52 1.97 .57 2.10 .66 2.23 .83 2.35 65 333 .34 1.42 .34 1.46 .34 1.36 .34 1.42 -- -- .37 1.41 .40 1.50 .44 1.59 .47 1.64 500 .29 1.25 .29 1.30 ,29 1.25 .29 1.27 -- -- .33 1.27 .35 1.34 .38 1.38 .41 1.44

9 1.00 2.94 1.00 3.17 1.00 3.47 ------w w 15 1.80 2.66 .80 2.94 .96 3.14 1.04 3.46 ------a 30 .62 2.27 .62 2.46 .75 2.65 .79 2.84 ------m 75 .46 1.96 .46 2.08 .59 2.14 .63 2.20 ------M 150 .37 1.58 .37 1.72 .52 1.86 .54 1.92 -- -- .71 2.33 ------F, 300 .36 1.63 .36 1.69 .46 1.77 .48 1.78 -- -- .58 1.93 .64 2.02 .75 2.15 - -- 500 .36 1.52 .36 1.54 .44 1.57 .45 1.57 -- - .51 1.74 .55 1.75 .62 1.91 .74 1.96 February, 1958 ature of the windings. The loss as given by manufac- equivalent circuit in Fig. 10(b) indicates the presence of turers is usually based on winding temperatures of 75°C. a shunt branch Z,„. This shunt branch represents the If the average winding temperature differs from 75°C, current drawn from the supply system when one wind- the resistance of the winding can be determined at the ing is excited and no load is connected to the other new temperature t by the use of the following formula: winding (or windings). This no-load or exciting current t+234.4 . consists of a magnetizing component and a loss com- Resistance at t= (Resistance at 75°C) (9) ponent. 309.4 The magnetizing component is that current required where t is in degrees Centigrade. The load losses then to produce sufficient flux in the core, so that the applied change linearly with the change in resistance. voltage is equalled by the induced voltage in the wind- Exciting Current and No-Load Losses-The transformer ing. If there were no saturation of the iron, the mag-

Table 13-Typical Values for No-Load Loss and Total Loss in Per Cent for Network Transformers (Liquid Filled) of Standardized Design

a.) LOSSES

5.010/ 8.66 KV 15 KV 25 KV 34.5KV KVA Rating No Load Total No Load Total No Load Total No Load Total No Load Total Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss 300 .417 1.23 .417 1.23 .417 1.23 ------500 .354 1.16 .354 1.16 .354 1.16 .390 1.26 .436 1.38 750 .318 1.16 .318 1.16 .318 1.16 .354 1.17 .387 1.25 1000 .300* 0.99* .300 0.99 .300 0.99 .330 1.08 .365 1.15 1500* -- .273 0.98 .273 0.98 .287 1.03 .307 1.09 2000* -- -- .260 0,93 .260 0.93 .275 0.97 .295 1.00 2500* -- -- .238 0.92 .238 0.92 .248 0.96 .256 0.99

*Not available at 216/120 Volt.

b.) FOR KVA RATINGS SHOWN ABOVE TYPICAL IMPEDANCES IN PER CENT ARE:

KVA Rating 5.0 KV 8.66 KV 15 KV 25 KV 34.5 KV Below 1000 5% 5% 5% 5% 5%

Above 1000 - 7% 7% 7% 7% February 1958

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 224/576 Pr current areshowninTable14forstandardizeddistribu- standardized distributiontransformersaregivenin Table 12. tion transformerswhenexcitedatratedvoltage.Since E PI and losscomponents.Typicalvaluesoftotalexciting rZ rated voltageisnearthe"knee"ofsaturationcurve, ponents ofexcitingcurrentvaryarediscussedinsome detail inReference7.Typicalvaluesofno-loadlossfor losses arerepresentedbythevariableresistiveportion Fig. 11(b)isshownasavariabletorepresentthenon- proportional tothesquareofappliedvoltage. dielectric losses,andthecopperlosscausedbyexcit- ing current.Usuallyonlytheironlosses,thatis,hysteresis andeddycurrentlosses,areimportant.These of linear characteristicsrequiredformagnetizingthepar- fundamental sinusoidalcurrentbutalsoarangeofodd- tially saturatedcore. distorted inwaveshape,andwillconsistofnotonlya saturation curve.Hencethemagnetizingcurrentwillbe former willnormallyoperatenearthe"knee"of ordered harmonics.TheshuntreactorportionofX.in netizing currentwouldbesinusoidalinshape,phase of ironbekeptassmallpracticable,andthetrans- degrees. However,economicsdictatethattheamount with theflux,andwouldlagappliedvoltageby90 DI -4 14 al gii r pi p .9 I % OperatingVoltage Total excitingcurrentisthesumofmagnetizing The mannerinwhichthemagnetizingandlosscom- The no-loadlossesofatransformeraretheironlosses, Rating Table 14-TypicalPublishedValuesofExcitingCurrentinPerCentatRatedVoltageforDistributionTransformers Z. KVA 150 100 300 333 500 10 15 15 30 50 25 75 5 9 in Fig.11(b),becausethelossesarenotexactly FOR OPERATIONABOVERATEDVOLTAGE,MULTIPLYVALUES ANDINTABLE 105% 105% 110 110 4.8 KVand Below 1.5 1.5 1.7 3.8 3.2 2.0 2.1 2.0 2.0 2.1 2.7 3.0 6.0 7.0 7.2 KV 1.5 1.6 2.0 2.0 2.1 2.2 3.0 3.5 3.0 3.8 6.0 2.0 2.6 7.0 No-Load Loss 1.5 1.30 1.15 2.4 12 KV Correction Factors Gridco, Inc.v. Varentec, Inc.IPR2017-01135 1.5 1.6 3.5 3.3 4.5 2.3 2.3 2.0 2.0 2.0 2.6 3.0 6.0 - VOLTAGE RATINGOFPRIMARYWINDING

Distribution Transformers Standardized Design 24.9 Grd.Y/14.4KV Exciting Current GRIDCO 1004 Part 2 of 5 - 225/576 2.3 1.5 4.6 2.2 1.7 2.0 2.6 3.0 4.0 ------Fig. 12-Nomographfordeterminingefficiencyofdistri- load andlosspointsreadefficiencyatrequired bution transformers.Laystraightedgeatgivenpercentno- LOAD VALUES--m- -4 where straightedgecrossesthatscale. 23 KV 2.6 2.0 2.0 3.5 2.0 2.5 3.5 ------EFFICIENCY IN PERC ENT 984 9 3 927E 95i 96 91t' 97 99-_= and 225KVAAbove,3-phase. and 150KVABelow,3-phase.. For 250KVAandAbove,1-phase; For 167KVAandBelow,1-phase; L. -7 3 f - 3 ] 3 4. f 3 - 4 2 1

944 95- 96:. 93": 9e7 992 97 923 91=== E- L' 7. 4 ; 5 7 7 • = 7, - I - 12

4 34.5 KV I FULL. 92 9 5'3 9e3 94-: 931 96 99' 971 9I 2.8 2.0 2.0 2.0 BY FOLLOWINGFACTORS 3.5 2.5 - _ ------4 95-fs 93;' 92-1 94 eel 99 91-J -; 4 1 - 7 • . - 2: 4

3 46 KV 3.1 3.0 2.0 2.0 3.5 - .- - - - - _ - - 901 96 89 94 93- 98 92 97 99 95 91 2 1 84 89 Be 86 90 94 83 96 85 87 93 99 98 95 97 92 91 February 1958

-1 69 KV 4 1 .3 4-i .5 .1 •: 2: 9 2.0 2.0 3.7 3.0 ------_ 217 PER CENT NO L OAD L OSS A T FULL LO AD If

218 Distribution Transformers

exciting current increases very rapidly if the applied constant. Per cent regulation can be calculated at any voltage substantially exceeds rated voltage. Table 14(b) load and any power factor by the following formula: gives a list of multiplying factors which should be applied to no-load loss and total exciting current if the applied voltage exceeds the voltage rating by 5 or 10 % Regulation = [prd-qx±(Px — gr)21 200 per cent. (12) The losses and exciting currents in Tables 12, 13, and X operating current 14 apply to standard transformers of Westinghouse derated current sign only. Transformers with special insulation, im- where: pedance, tap range, frequency, etc., will in general ex- rated load losses in watts r = per cent resistance — hibit different values from those tabulated. 10 X rated kva Efficiency—The efficiency of a transformer, expressed z = per cent impedance in per unit, is the ratio of power output to power input; x = per cent reactance = Vz2 —r2 output losses 0= power factor angle of load (positive when current Efficiency= . ut —1 . ut (10) inp inp lags voltage) Total losses are the sum of the no-load losses and load p=cos losses. Fig. 12 presents a chart to facilitate determina- q=sin tion of transformer efficiency at various loads. Loss Ratio and Product—Maximum operating efficiency 13. Transformer Tap Selection for a transformer occurs when the no-load losses are The charts shown in Figs. 13 and 14 are useful in equal to the load losses. Since the no-load losses are determining the range of high voltage taps, the selection constant and the load losses are variable, this maximum of transformer ratings and associated taps, and the de- efficiency condition will occur at only one particular termination of tap settings. The charts are divided into load. two parts. The left half is for considering the voltage conditions which are part of the actual distribution sys- L= I Fe 1 (11) tem. The diagonal lines appearing across the voltage Cu -V R grid lines represent the turns ratio between the high where: voltage and low voltage windings. The right hand por- L = per unit kva load at which transformer oper- tion of the charts lists the standard high voltage taps ates most efficiently available. They are arranged in vertical columns and are Cu = load losses at rated load, watts related to their respective low voltage and kva ratings Fe = no-load losses, watts which appear at the top of each column. Cu Use of the charts requires knowing the distribution R= loss ratio = Fe and utilization voltages. Two conditions are considered when applying Figs. 13 and 14: (1) full load on the trans- A variation of the loss ratio will change the per unit load former and (2) light load on the transformer. Allowance at which maximum efficiency occurs. must be made for the voltage drop of the transformer. For power transformers a purchaser has some latitude It is evident that the equivalent low voltage on a loaded in specifying the desired loss ratio; however, this is not transformer must be in excess of the actual or desired economical for distribution transformers, because the voltage by the amount equal to the transformer drop. cost involved in designing and constructing a distribu- Two examples for use of the charts follow. tion transformer with a loss ratio different from the mass produced design would outweigh any potential Example 1. (use Fig. 14) savings due to loss reduction. Determine High Voltage Rating and Tap Setting The loss ratio for standardized distribution trans- A. Full load on transformer—determine tap formers generally falls in the range of 2.5 to 5, depending Actual high voltage = AOV = 7200 v. upon the size, manufacturer, and voltage rating of the Desired low voltage =L VR =117 v. transformer. In general, the pole-type transformers tend Transformer voltage drop = 3 v. to fall in the middle or upper part of this band, while the larger platform-mounted units tend to fall in the middle 1. Start at (A) on the AOV scale and draw a line or lower part of the band. The limits of the band and the across the graph until it intersects a similar line distribution of units within the band vary somewhat drawn vertically from (B) on the LVR scale. among manufacturers. (This is 117 v.+3 v.). This intersection determines the ratio of transformation (RT) which 12. Regulation is 60/1. The full load regulation of a transformer is the change 2. Follow this (RT) line into the right half of the in secondary voltage, expressed in per cent of rated chart to "box" (C). The box is 7200 under the secondary voltage, which occurs when the rated kva column designated 12470 grd. Y/7200 for high output at a specified power factor is reduced to zero, voltage rating, kva rating from 3 to 50, low with the primary impressed terminal voltage maintained voltage 120/240.

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 226/576 Distribution Transformers 219

DISTRIBUTION TRANSFORMER APPLICATION CHART

2000-6000 VOLTS

TRANSFORMER HIGH VOLTAGE RATINGS These rating. are Included in Column 2 of Tabla Si I. the &lath report. 2400 4160 4800

STANDAR?) EVA RATINGS 8 The« atendard rattan ape iselodad fa Table Cl of the sixth report for voltages 480 CO 8 yolk uld bele.. KVA ratios. for voitages above 410 not included although war applkation madly be obtained br soaddaring the volts.. .uhiplea ea- rd sZb 120 volts. O 441 10

TRANSFORMER LOWVOLTAGEVO RA TINGS There eating, are Enoludad in Tabl of th• firth report for low 0olIsce rating. •ct .12 400 rolls sad below. For notaktelatu • end additional explanatory notes relating to fathota re4Te15-01 should be mad to Tat:Abet and its explanatory footnotes. O 0 sr

.5.040 awry SC.40

1402041410 4010 / 1'

40 4400 000 eat.* 44100 00

so'

V) O

A 35 ( ..001 FIIIP-ffillir"/%7Ader" E IN 1W .1440 TAG OL V TING RA OPE

0z TUAL C A

1310 2510 2510 1400 2400 2440

1400 160 2400 2010 W 2240 2340 1340 1340 •• 12•0 2240

The taps shown In the above "boxes" are those included In the sixth report as standard for their respective voltage and RYA ratings. For transformers having other voltage ratings, their appropriate RT Haul can be drawn on the charts and their corresponding voltage taps given In boxes located as above.

1 I 0 I I 5 120 125 130 LOW VOLTAGE REQUIRED (L.VR)

Fig. I3—Distribution transformer application chart, EEI-NEMA standard (4),

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 227/576 220 Distribution Transformers

DISTRIBUTION TRANSFORMER APPLICATION CHART

6000-9000 VOLTS

2470GrtY 13/00GriLY TRANSFORMER HIGH VOLTAGE RATINGS 7200 7620 These ratings are Included In Column 2 of Table Si in the sixth report. /7200 7620 STANDARD KVA RATINGS 8 8 These standard ratings are included In Table 51 of the sixth report for 0 8 8 8ti8 voltages 480 volts and below. KVA ratings for voltages above 480 volts are 'h -• , .1-1 Ln. — ail ,e) not included although their application can readily be obtained by constd- . 0. I ' l b ' ,-.. i sting the voltages as multiples of 120 volts. e, rn un 0 rgm— el er) mi. in 0 erz. rn ox CO ',TRANSFORMER LOW VOLTAGE RATINGS Qa •=. C5 4D C:. These ratings are Included In Table Si of the sixth report for low voltage -.3• Nr r•J e0 co X kil 0 ...... ,. ratings 480 Veils and below. For nomenclature and additional explanatory SCI "4 — •cl- "cl• notes relating to these ratings, reference should be made to Table Si and '. s'.... ''..,. '...... „ Its explanatory footnotes on r‘f CM •rt ' o al al lil 8

niI Infd i I I bit L /a

1414141 / , ®TB®i 000000

0V) • 11

it o 211113a"SI IN I

C /dmilm zir

U El 111.10.111 II ®D LI ill

E IN KV ( dAe. AOM. F

a I

TAG . . ' natl 0 U

O VOL riO f..Idr 1e r 411"4110r -Pr ..4110. r. Iw i I]

TING _

-..• .... _ a

> 5S — tax PERA

O ellI AWAY" iffill•

i -,

4 / ' CTUAL 0 A

A _ r roirF//0..101117:10dilr1'' ' ' zi,,, 50

RT) 1f r 11 TION ( 11 ORMA . NSF 4I 6 11 t

RA The tape shown In the above "boxer are those Included In the I sixth report as standard for their respective voltage and KVA F T 11

I_ ratings. For transformers having other voltage ratings, their

O appropriate RT lines can be drawn on the charts and their

IO corresponding voltage taps given In boxes located as above.

/ RAT 110 E1I5 G 120 125 K 130 LOW VOLTAGE REQUIRED (LVR) Fig. 14—Distribution transformer application chart, EEI-NEMA standard (4).

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 228/576 Distribution Transformers 221

3. This tap condition (no tap) is selected because and size of bushings and terminals, accessories, etc. If it appears in the column with the kva and low there is any deviation from EEI-NEMA standards for voltage rating desired. distribution transformers, a non-standard transformer B. Light load on transformer-determine low voltage exists. The price of a non-standard transformer is about Actual high voltage= 6840 v. (7200-5%) 6 per cent greater than for the corresponding standard Transformer Tap = 7200 transformer, along with a specific price increase for the Transformer voltage drop =1 v. non-standard feature specified. The cost per kva for standard single-phase and three- AOV scale, draw a line 1. Starting at (D) on the phase conventional transformers in various voltage across the graph until it intersects the (RT) classes is shown in Fig. 15. Fig. 16 shows the cost per kva line of 60/1. of standard 2400-volt CSP transformers. Fig. 17 gives 2. This intersection corresponds to (E) on the the cost per kva of conventional 2400-volt subway LVR scale. Point (E) is 113 volts at the low transformers. All the costs in Figs. 15, 16, and 17 are voltage terminals of the transformer, remem- expressed in per unit, with the base being the cost per bering that the transformer voltage drop is 1 volt. 5.0 Example 2. (use Fig. 14) CONVENTIONAL Determine the H.V. operating limits for standard trans- 4.0 SINGLE-PHASE formers rated 7620-120/240 volts, 25 kva. The operating range must be selected so that the utilization volt- 3.0 ages will not exceed 125 volts, nor will not be less than

115 volts. KVA 2.0

A. Full load on transformer-determine lowest AOV PER 14.4 KV Transformer Tap = 7239 v. 12 KV Desired low voltage =115 v. 2.5 KV Transformer voltage drop = 2 v. T COST 5.0 KV NI 8.66 KV 1. Start with the lowest tap (F) and follow (RT) 1.0 line 60.3/1 until it intersects the vertical line .9 PER U .8 from (G) on the LVR scale. (Point (G) is .7 115 v.+2 v.). .6 From this point of intersection, read across to 2. .5 (H) on the AOV scale. 7070 v. is the lowest operating voltage which will satisfy the speci- .4 5 10 15 25 37.5 75 167 333 fied condition. 50 100 250 500 B. Light load on transformer-determine highest TRANSFORMER RATING IN KVA AOV 5.0 Transformer Tap = 8001 v. CONVENTIONAL 4.0 THREE- PHASE Desired low voltage = 125 v. Transformer voltage drop = 2 v. 3.0 1. Now using highest tap, start at (J) and follow the (RT) line 66.7/1 until it intersects the ver- 2.5 KV tical line from (K) on the LVR scale. (Point (K) 2.0 I a_ 5 KV 12.5 KV is 125 v.+2 v.). 8.66 KV 15 KV ---, From this point of intersection read across to 0 2. U (L) on the AOV scale. 8740 v. is the highest N\ N I NI 2 operating voltage which will satisfy the speci- 1.0 cc fied condition. w .9 a. 3. The operating limits for the high voltage side of .8 the transformer are 7070 v. and 8470 v., which .7 correspond to the specified low voltage limits of .6 115 v. and 125 v. .5

.4 IV. DISTRIBUTION TRANSFORMER PRICES 9 15 30 45 75 112.5 225 500 150 300 14. Prices of Standardized Transformers TRANSFORMER RATING IN KVA

A standard distribution transformer is designed in Fig. 15-Comparative prices of standard distribution trans- strict accordance with EEI-NEMA standards and is formers. 1.0 per unit is cost/kva for 25-kva, 2400-volt, completely defined as to kva, voltages, taps, location single-phase conventional transformer.

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 229/576 simple two-windingtransformershowninFig.18(a) ing branch."' equivalent circuitconsistingofsimpleimpedancesand which flowduringfaultconditions.Determinationof for determiningtheperformanceoftransformer. age regulation,theshuntmagnetizingbranchshownin age andmutualfluxeswouldbequitedifficultwhenthe Consequently, therepresentationofatransformerbyan single-phase transformer. ideal transformershasbecomeacommonlyusedmethod phase transformerarethevoltageregulationunder with the with atwo-wireprimaryandsecondary.This following equivalentcircuitswillneglectthemagnetiz- type oftransformercanberepresentedbytheequiva- transformer isconnectedintoacomplexpowersystem. these characteristicsbyuseofsuchquantitiesasleak- Fig. 11canusuallybeneglected.Consequentlyallthe various loadconditionsandtheshort-circuitcurrents of theactualtransformer.Ifit provesmoreconvenient, kva ofastandardconventional25-kva,2400-volt, characteristics whichdefinetheperformanceofasingle- lent circuitshowninFig.18(a).Theimpedance the impedanceinohmsasmeasuredfi•om is cost/kvafor25-kva,2400-volt,single-phaseconventional distribution transformerswith2400-voltrating.1.0perunit N Fig. 16—ComparativepricesofstandardCSPpole-type 15. SimpleTwo-WindingTransformers 222 V. SINGLE-PHASETRANSFORMERCONNECTIONS For thecalculationofshort-circuitcurrentsandvolt- From theapplicationstandpoint,mostimportant The simplestformofsingle-phasetransformeristhe of theidealtransformeris sameastheturnsratio

PER UNIT COS T PER K VA 4.0 2.0 3.0 5.0 I.0 .8 .4 .5 .7 .6 .9 X SINGLE PHASE winding short-circuited,and the turnsratio 3 5 AND EQUIVALENTS TRANSFORMER RATINGINKVA 9 15255075 transformers. 37.5 SE Gridco, Inc.v. Varentec, Inc.IPR2017-01135 100 CSP 24 167 333 00 Distribution Transformers - H VO winding LT ZIIX GRIDCO 1004 Part 2 of 5 - 230/576 is secondary windingswhichcanbeconnectedinseriesor secondary windingsforthree-wireserviceorseries- is changedinthefollowingmanner: the equivalenttransformerimpedancecanbeinserted in the for theseriesconnectionwillbefourtimesimped- The impedanceZ,,E,istheinohmsas formers with2400-voltrating. 1.0perunitiscost/kvafor 25-kva, 2400-volt,single-phase conventionaltransformer. the impedanceviewedfrom multiple (parallel).Theconnectioncanbe multiple connection,buttheturnsratioofideal ance usedforthemultipleconnection. viewed fromthe represented bytheequivalentinFig.18(a),where circuit showninFig.18(a)canstillbeused.Thevalue circuited. in halfofthesecondarywinding.Theimpedance If thetransformerimpedanceistobeinsertedin the secondaryleadsshort-circuitedtogether. turns ratio transformer hasasingleprimarywindingandtwo transformer isonehalfthatforthemultipleconnection. nected inseriesfortwo-wireoperation,theequivalent of Fig. 17—Comparativepricesof standardsubwaytrans- X 16. 17. One commonlyencounteredtypeofdistribution Many distributiontransformershavemid-tapped If thetwohalvesofsecondarywindingarccon- leads ratherthanthe Zgx a 6 a_ 9 z U) U Series-Multiple Secondaries Three-Wire Secondaries 6.0 4.0 5.0 Lo .7 .8 .4 .6 .5 X for thisconnectionisthesameasthat leads ratherthanthe N 3 is theratioofprimaryturnsto THREE PHASE 5 TRANSFORMER RATINGINKVA X SINGLE PHASE winding withthe 9 152537575 ZgEf =" H 1 —C leads, theimpedanceused 1 r2 H Z1IX 50 100250500 leads, iftheimpedance H TRANSFORMERS SUBWAY 2400-VOLT winding withallof H winding short- 167 333 Zllx (13) is Distribution Transformers 223 CONNECTION EQUIVALENT CIRCUIT If possible, the values of ZHX,.3 and Z -1-2 should be determined from the manufacturer. In general, the value Ni ZHX ZEix,.3 will be available either from the nameplate or HI X I from published information, but the value of ZFIXL _2 is not so readily available. In the absence of more accurate data for distribution transformers, the following approx- H2 X2 XZ imation can be used. If ZED, (0) SIMPLE TWO-WIRE 33 is defined as (rt+ixt), SECONDARY 1.5rt+j1.2xt (14)

18. Booster Transformer

(ZHX1.2ZH X 1.3) 2:1 A booster transformer provides a fixed buck or boost voltage to the primary of the distribution system. This XI is accomplished by connecting the secondary winding in series with the primary winding as shown in Fig. 18(c). (2ZHX1.3-ZHX1.2 The secondary winding is insulated to have the same H I X 2 Hi BIL as the primary winding. The booster transformer acts exactly the same as an autotransformer and has the same electrical equivalent circuit. Depending upon the x3 —X3 definition, the transformer may be considered to have a H2 b THREE-WIRE (ZHX 1..2-ZHX1.3)i secondary, consisting of the winding between X1 and X2 SECONDARY N or between Xi and X3. The most straightforward manner of determining the equivalent impedance is to determine ZIC,OI, which is the impedance in ohms viewed from terminals X1 and X2 with terminals H1 and H2 short circuited. As shown in xI Fig. 18(c), this impedance is inserted in the X leads of an n:(nti) ideal transformer having the turns ratio n:(n + 1), $41 where n is the ratio of turns between H1 and H2 to the n x2 turns between X1 and X2. In the term (n+ 1), the plus sign is used if the open circuit voltage from X1 to X3 is H2 X3 H 2 higher than the voltage from H1 to H2; otherwise the ( c)BOOSTER (SEE NOTE I.) minus sign is used. TRANSFORMER The amount of load which can be carried by a booster NOTE 'I' IDENTIFY WHEN + OR transformer is considerably greater than the actual Fig. 18—Equivalent circuits for common single-phase trans- winding kva of the primary and secondary windings. former connections. The through kva (U0) which can be carried by a booster transformer with a primary or secondary winding kva multiple secondary windings which are suitable for of Up is series connection for three-wire service. The diagram for Uc = Up(n ± 1) (15) this type of transformer is shown in Fig. 18(b). The mid-tapped lead is usually, although not necessarily, where the plus sign is chosen if the secondary is con- tied directly to ground. nected to add to the primary voltage, and the minus The equivalent circuit for this type of connection is sign if the secondary is connected to subtract from the somewhat complicated, because the loads on the two primary voltage. halves of the secondary winding are not necessarily Determination of the equivalent circuit in per unit is equal. The impedance from the H winding to half the sometimes confusing because the through kva base dif- X winding differs from the impedance from the H wind- fers from the winding ratings, and because of possible ing to the total X winding. In the equivalent circuit, it is misinterpretation of the term "secondary." To avoid necessary that a partial impedance appear in the pri- this confusion, it will usually prove desirable to first ob- mary leads and another impedance in the X1 and Xs tain the equivalent circuit in ohms as shown in Fig. leads. 18(c). Once this has been done, any convenient arbi- The impedance of a transformer with three-wire trary base voltage and kva can be selected and the im- secondary is most conveniently expressed in terms of pedance ZXL .211 can then be expressed in per unit on the impedances viewed from the H winding, first with these bases. Short-cut formulas are available for direct one half of the secondary short circuited, then with the conversion of per unit winding impedances to the per total secondary winding short circuited. These two unit equivalent circuit; however, until considerable pro- impedances are designated Znxi., and Zax,_, respec- ficiency in the use of these formulas has been obtained, tively. The impedances to be inserted in the equivalent it will prove less confusing to go through the interme- circuit, in terms of the two impedances defined above, diate procedure of determining the equivalent circuit in are shown in Fig. 18(b). ohms.

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fected by the turns ratio, but only by the relative 19. Parallel Operation of Single-Phase Transformers impedance. Single-phase transformers can be successfully oper- Equation (18) gives the secondary voltage in terms of ated in parallel if the turns ratios are equal or nearly so, the transformer constants, the primary voltage, and the and if the ratio of ohmic impedances of the transformers load current. The first term in Equation (18) is the is approximately equal to the inverse ratio of kva rat- Thevenin's equivalent voltage as viewed from the sec- ings. Fig. 19 shows two transformers whose primaries ondary terminals. The parenthetical portion of the sec- are connected to the same source and whose secondaries ond term is the Thevenin's equivalent impedance as are connected in parallel. viewed from the secondary terminals with the primary terminals short circuited. 12 - - Equations (16) and (17) illustrate why the turns N I N2 ratios must be almost equal and why the ratio of impedances must be almost inversely proportional to the kva ratings. If the turns ratios are appreciably different, the circulating current can be sufficiently high to necessitate considerable derating of the bank, or in the extreme case could even result in overloading of the transformers without any secondary output to an external system load. For example, if the turns ratios differ by 5 per cent and the transformers have equal impedances of 2% per cent, the circulating current alone will load the transformers to their rated kva. Fig. 19—Parallel operation of two-winding sing e-phase If the ratio of impedances is not inversely proportion- transformers or balanced three-phase bank with balanced al to the transformer ratings, one transformer will take loads. more than its proportional share of the load current. When that transformer is fully loaded, the other trans- Writing voltage drops around the secondary loop former will be carrying less than its rated load. Conse- yields the following relations, where the quantities are quently, the entire bank would be derated to prevent the defined as shown on Fig. 19. first transformer from being overloaded. The impedances in Equations (16) through (18) are Z2 phasor rather than scaler impedances. If the impedances ri- V P(I r N are properly proportioned in magnitude but are differ- z,±z2 z,+z„ ent in phase angle, the transformers will split the load l 1 ) component proportionately in magnitude, but the trans- vp (Ar,Ni- 12= Z' IL+ former currents will not be in phase with the load cur- zi+z, z,+z, rent. Hence, the sum of the kva loadings on the two (Z2 Z i transformers will be greater than the output kva of the ill IV) Z2 bank. This out-of-phase component of load current due vs-vp IL i-FZ2 + Z2) to different phase angles of the impedances can be viewed as an additional circulating current between the Examination of these equations shows that the trans- two transformers. former currents are made up of two components: a load Equations (16) through (18) can be used to determine component and a circulating component. The second whether single-phase transformers can be successfully terms in Equations (16) and (17) are equal in magnitude paralleled. The equations can also be applied to three- and opposite in sign, and represent a circulating current phase transformers if the transformer banks, the applied leaving one secondary winding and entering the other voltage, and the load current are balanced among secondary winding. This circulating component does phases, and if the impedances, voltages, and currents not contribute to the load current IL. The numerator of are expressed on a phase-to-neutral basis. this term indicates that the driving voltage for the circulating current is the difference in voltages induced in the two secondary windings, and the denominator shows VI. CONNECTIONS AND EQUIVALENT CIRCUITS that the impedance which limits this circulating current FOR BALANCED THREE-PHASE BANKS is the sum of the leakage impedances of the two trans- A balanced transformer bank is made up of a three- formers in series. The magnitude of the circulating cur- phase transformer or three similar single-phase trans- rent is independent of the load current drawn from the formers connected for three-phase supply. The windings transformer bank if Vp is constant. can be connected in either a delta (from line to line) The second component of transformer current is a configuration or a wye (from line to neutral) configura- fraction of the total load output of the bank. The load tion. The three types of balanced connections (delta- current splits between the two transformers in inverse delta, wye-wye, and wye-delta) are shown in Fig. 20. proportion to the transformer impedances. The division These transformers are capable of supplying three of load current between the two transformers is not af- phase-loads, single-phase loads connected between lines,

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 232/576 POSITIVE SEQUENCE NEGATIVE SEQUENCE ZERO SEQUENCE

ZI N:I Z I N:I Al at A2 Ga Ao—G ..._..00

0 H M S

C C N 1 ni N2 112 GO 90

A >0 Z1 P Z1t Al =I al A2 ME az Ao---• s--a0 B b E R

U N (A) 1 N1 nt N2 n2 Go 90 T

Z t N:1 Z t Nil Z N:I Al al A2 az AO =I 00

0 H M S 3Z I I 3Z 9 N1 ni Nz n2 GO 90

ZG Z1t Z1 t ZIt + 3 ZIG +3 ZIQ p A 1 al A 2 I 0 2 AO 1 00 E R

(B) U N N1 n1 N 2 nO GO 90 I T

Z t N:10 3°• Z t Noe 130• z t Al al A2 02 A0—F-1 '6-00

0 H M S 3ZGQ C C N1 ni N2 "2 GO 90 N 1:16130' )30° A Z1 Zit 1:16 Z1t 30. At GI A2 02 A0-0 lit •-----oo Z B b 0 7 P E R (C) u 34 cl N I N "I N2 T 1 112 GO 90

EBA NOTES: (1) For wye-delta connection shown, phase A leads phase a by 30° N is no-load voltage ratio with balanced applied voltages. If transformer is connected so phase A leads phase a bye degrees, Eba the phase shift transformer in positive sequence diagram has phase shift of Ho from wyc to delta and negative sequence has +ie. 74 is transformer impedance in ohms on ABC windings. (2) Zero sequence circuits for 20(B) applies when magnetising branch is negligible, i.e. for three single-phase transformers or for shell- 74 is transformer impedance in per unit. form (or similar) three phase transformers. Fig. 20—Equivalent circuits for balanced three-phase transformer connections.

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 233/576 226 Distribution Transformers and single-phase loads from line to neutral or ground for necessarily negligibly small compared to the zero-se- certain of the wye connections if excessive shift of the quence leakage impedance. The reason for this is illus- neutral does not occur. trated in Fig. 21. The impedance of either a single-phase or three- Fig. 21(a) and (b) show the flux paths in a core-form phase transformer will generally be given in per cent or (or three-legged) transformer with either positive-se- per unit on the rated kva base of the transformer. In set- quence or zero-sequence applied voltages. For positive- ting up the equivalent circuits in Fig. 20, the per cent sequence voltages, the flux produced in the phase A and impedance of the three-phase transformer or of a single- phase B legs finds a convenient return path through the phase transformer in a balanced three-phase bank can phase C leg because the flux in the phase C leg is the be used directly in the per unit equivalent circuits. For negative of those produced in the phases A and B legs. example, if Fig. 20(a) is a three-phase transformer with However, when zero-sequence voltages are applied, a per cent impedance of 5 per cent, the value of Zt in the fluxes produced in the three legs are equal in magni- per unit positive-sequence equivalent is 0.05 per unit. tude and in phase (rather than 120 degrees out of If the windings were connected in wye as shown in Fig. phase). This flux 300 must then return through the air, 20(b), the value of Z; is still 0.05 per unit. These per cent the tank, and various structural support members. This impedances can be converted to ohms by use of Equa- high reluctance magnetic path results in a much lower tion (3), where kvr.ced is the rated line-to-line kv of the exciting impedance branch than was the case for posi- transformer bank and kvantted is the rated three-phase tive-sequence voltages. For the small three-phase distri- kva of the bank. bution transformers, zero-sequence exciting impedance In Fig. 20, where the equivalent circuits are expressed may be as low as 15 to 30 per cent; for the larger trans- in ohms, the voltages to be used are system voltages formers, the impedance will be in the order of 30 to 75 from line to neutral, the currents are phase currents per cent. Consequently the zero-sequence exciting im- rather than delta currents, and the impedances are pedance may have to be taken into account for three- those which appear or can be considered to appear in the phase core-form transformers with certain types of phase leads (rather than within the delta). When single- winding connections. phase transformers are to be connected in delta and the Fig. 21(d) shows the zero-sequence flux paths for a ohms of the transformer is known (rather than per cent three-phase transformer with shell-form construction. impedance), the ohms impedance must be divided by 3 The outside core legs provide a path for the zero-se- before inserting as Zt in the equivalent circuits. quence flux. The exciting impedance to zero-sequence The quantity N is the ratio of line-to-line voltages on voltages will not be greatly different from that for posi- the primary and secondary with no load on the trans- tive-sequence voltages for the shell-form construction. former. For wye-delta transformers, this ratio N differs Note that shell-form construction is not used for distri- from the winding ratio by bution transformers, although certain transformers may The equivalent circuits in Fig. 20 are given in terms of have three separate cores in one tank. the positive-, negative-, and zero-sequence equivalent circuits. The use of sequence equivalents and the deter- 21. Delta-Delta Connection mination of short-circuit currents and voltage regula- Fig. 20(a) shows the delta-delta connection. The tion are described in Reference 8. equivalent circuits are given for positive-, negative-, and zero-sequence in ohms and on a per unit base. The 20. Zero-Sequence Impedance of Three-Phase Banks equivalents in ohms utilize ideal isolating transformers The equivalent impedance of a transformer is usu- having a turns ratio N, which is equal to the ratio of ally determined by short-circuiting one of the windings open-circuit voltage from A to B to that from a to b. If and measuring the impedance as viewed from the ter- the terminals are so numbered that the voltage on terminals of the other winding. For determining positive- minal a is in phase with that on terminal A, the isolating sequence impedance (which is equal to negative-se- transformer has no phase shift. If the terminals are re- quence impedance), the applied voltages for the imped- lettered such that phase a leads phase A by 120 degrees ance measurement are balanced positive-sequence volt- or 240 degrees, the isolating transformers will then have ages. For determination of zero-sequence equivalent a phase shift such that the phasor ratio is N :0120' or impedance, the three line terminals are connected to- N :€;240-, respectively, in the positive-sequence circuit. gether and a single-phase voltage is applied between The shift in isolating transformers in the negative-se- terminals and ground. The impedance determined for quence equivalent circuit is the conjugate of the shift in either positive- or zero-sequence is approximately the the positive sequence, that is, —120° or —240°. In the sum of primary and secondary leakage impedances, zero sequence, no phase shift is required. paralleled by the exciting impedance. For positive-sequence applied voltages, the exciting 22. Wye-Wye Connection impedance is very high compared to the leakage im- The wye-wye connection with each neutral grounded pedance, and the exciting impedance branch is normally through an impedance is shown in Fig. 20(b). The iso- neglected in setting up the positive-sequence equivalent lating transformers are shown for phase a in phase with circuit. However, when a zero-sequence (or single-phase) phase A. If the terminals are so numbered that phase a voltage is applied to all three terminals of a wye-con- leads phase A by 120 or 240 degrees, the isolating trans- nected winding, the magnetizing impedance is not formers have a phasor ratio N :1020° or N :Wm°, respec-

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C OtA a2 al ael 227 tively, in the positive-sequence circuit. The shift in isolating transformers in the negative-sequence equivalent circuit is the conjugate of the shift in the positive sequence, that is, —120° or —240°. In the zero-sequence circuit, no phase shift is required. The simple series impedance representation in the zero-sequence equivalent circuit shows that, except for magnetizing current, any zero-sequence current which enters one winding must find a path to flow in the zero- sequence circuit external to the other winding. The grounded wye-wye connection will not pass zero-sequence current unless both external connected circuits have sources of ground current. (a) CORE FORM- POSITIVE SEQUENCE If the neutrals are either solidly grounded or are ungrounded, this can be represented in the equivalent circuits by letting the proper neutral impedance equal zero or infinity, respectively. The equivalent zero-sequence circuit for the wye-wye connection utilizes an equivalent transformer impedance which is equal to that used in the positive-sequence 3+o equivalent. As explained earlier, this will be the case if the bank consists of three single-phase transformers or if the transformer is shell-form. If the transformer is core-form and it is desired to include the exciting branch, the manner in which this branch is included is shown in Fig. 22(a). The transformer leakage reactance is split into halves on the assumption that half the per cent leakage drop occurs in the transformer primary and half in the secondary. Splitting leakage impedance equally (b) CORE FORM - ZERO SEQUENCE between primary and secondary windings is very approx- A B 2 ti a et Ca el imate, but this will not usually cause substantial error, because the magnetizing impedance is so much larger ---0 2 than the leakage reactance. The exciting branch is then connected between these leakage impedances and the transformer neutral. e's e-1 Three-phase grounded wye-wye transformers with 4)B - core-form construction can be subject to excessive heating of the tank under certain unusual fault conditions 1/4-, 1/4.1 which result in a sustained application of zero-sequence voltage to the windings. For example, consider the case in Fig. 22(b), in which one of the primary wires has broken and fallen to ground. The break in the primary wire prevents any current flowing in the phase A winding from the primary. The short circuit on the phase A winding does not cause large currents to flow; hence, no auto- (c) SHELL. FORM- POSITIVE SEQUENCE matic protective devices such as reclosers or fuses will A e0 f0 <>.:0 clear the primary fault. However, the shorted winding will effectively prevent flux in the phase A leg of the ts? core and the flux in phases B and C legs must return through the air or the transformer tank as shown in Fig. 22(c). Under these conditions, the heating in the tank wall can be considerable and may result in damage to t*-1 1-1 r'N /".411 — the transformer. Certain other rare fault combinations, 1-t>fo in addition to the one described, can cause this tank \-1 heating phenomenon, where sustained zero-sequence flux exists without the flow of currents of fault magni-

Fig. 21—Positive- and zero-sequence flux paths for core- and shell-form transformers. (d) SHELL FORM-ZERO SEQUENCE

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Zt Zt spondingly lettered low-voltage terminal-to-neutral voltage by 30 degrees in time phase. However, many sys- Ao tems today use other than this 30-degree preferred con- Zmo nection. 3Z G This phase shift is accounted for by the use of phase- Go shifting isolating transformers in the equivalent cir- (a) ZERO SEQUENCE EQUIVALENT FOR FIG. 21 cuits. The phase shift in the positive-sequence corre- INCLUDING MAGNETIZING BRANCH sponds to the angle between the vector voltage from A to neutral and that from a to neutral. The phase shift in the negative-sequence equivalent circuit is the negative of that in the positive-sequence circuit. Fig. 20(c) shows the phase shifts for the 30-degree connection where the wye-connected winding is the high-voltage winding. The zero-sequence equivalent circuit requires no phase-shifting transformers. The transformer zero-sequence leakage impedance is shown to be the same as that for the positive sequence. This assumption is valid for shell-form transformers and for banks made of three single-phase transformers. For a three-phase core-form transformer, the equivalent impedance in the zero-se- A--• b quence circuit will be 80 to 90 per cent of the positive- sequence impedance because of the lower impedance in (b) WYE -WYE CONNECTION WITH BROKEN the shunt branch. AND GROUNDED SUPPLY LEAD The delta primary to wye secondary is also a commonly used connection. The equivalent circuit in Fig. 20(c) can also be used for this connection by use of a2, and ao as the primary terminals, and by selection of lA B the proper turns ratio and phase shift.

olDA. 0 me toc9 VII. UNBALANCED TRANSFORMER CONNECTIONS c AND EQUIVALENTS There are a large number of unbalanced transformer I connections, usually made up of single-phase transform- r ers, which have been used for supplying three-phase loads, single-phase loads, and combination three- phase and single-phase loads. These unbalanced connec- r tions are much more frequently encountered on distribution circuits than on transmission or subtransmission circuits. They are most frequently used where the three- phase load is small, or where the largest part of the load is either single- or three-phase, but a small amount of 0 b C the other type load must also be served. Space does not permit considering all the unbalanced (C) FLUX PATHS FOR CONDITION SHOWN IN ( b) connections which have been employed, but some of the Fig. 22—Illustration of tank heating caused by certain fault more frequently encountered connections will be dis- combinations near wye-wye core-form transformers. cussed. The equivalent circuits will be given in forms which will permit solution by either conventional loop equations or by the use of symmetrical components. In the sequence connections, the unbalanced bank will re- tude. Sensitive, automatic, circuit-protecting devices sult in one or more unbalances within the transformer can be employed in the circuit to relieve this condition. representation itself. Unbalanced faults in the vicinity of these banks necessitate the solution of sequence 23. Wye-Delta Connection equivalents with multiple unbalances. Solution of these The wye-delta connection and its equivalent circuits cases sometimes becomes quite involved, and in many are shown in Fig. 20(c). In thii connection there will al- cases solution of the loop equations will prove simpler ways' be a phase displacement between the primary and than the use of the sequence equivalents. A choice of the secondary terminals. The preferred NEMA and ASA simpler method must be decided upon for each case. standard connection today is that which results in the The following equivalent circuits are derived in ohms voltage on a high-voltage terminal leading the corre- rather than the per unit system. For these unbalanced

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 236/576 Distribution Transformers 229 connections, considerable confusion can arise as to the former in series with two equal impedances in the B and kva and voltage bases which are appropriate for the C leads and zero impedance in the A lead. By the connec- bank impedances, and the more straightforward ap- tion of the sequence diagrams as given in Reference 8 proach of developing the equivalent in terms of volts for this series unbalance, the transformer equivalent in and ohms is less likely to result in an error in determin- Fig. 23(b) is derived. ing the proper impedances to use in the equivalents. If If the two transformers are not identical, the equiva- it is desired to work the problem in the per unit system, lent circuit becomes somewhat more complex and neces- it is recommended that the equivalent first be set up in sitates representing two series unbalances in the se- ohms, and the impedances in the final equivalent can be quence equivalent. This is illustrated in Section 27. converted to a per unit base. 25. Open-Wye Connection 24. Open-Delta Connection The "open-wye" connection is ,shown by the solid The "open-delta" connection is usually formed by lines in Fig. 24(a) for two identical transformers. This connecting two single-phase transformers as shown by circuit can be represented as shown in Fig. 24(a), where the solid lines in Fig. 23(a), where the two transformers the wye primary and the open-delta secondary have are identical. This connection is similar to the conven- been completed in dashed lines by addition of a third tional delta-delta connection except that one of the transformer, but the primary lead to that transformer transformers has been removed. The bank can be repre- is open-circuited. The transformer bank can now be sented as shown in Fig. 23(a), where each transformer is considered to be balanced, with the only unbalance represented as a leakage impedance Zt in series with an being the open circuit in the phase A primary lead. Use ideal transformer having the same turns ratio as the of the connections in Reference 8 yields the sequence actual transformer. circuits and connections shown in Fig. 24(b). Since ideal transformers have no loss or regulation, If the two transformers are different in size, a second the voltages on the secondary of the ideal transformers are identical to those on the primary except for the turns EBA ratio. Consequently, the open-delta connection of ideal .N transformers can be closed by addition of a third ideal Eba transformer shown in dotted lines, without altering any voltages or currents. Addition of the third ideal transformer reduces the equivalent circuit to a balanced three-phase ideal trans-

- ••• '••-••••• Zt 0 (a) OPEN-WYE CONNECTION.

b N:0306 al

(a) OPEN-DELTA CONNECTION NI nI

N: lei 3°. al

t z

(b)SEOUENCE CONNECTIONS (b) SEQUENCE CONNECTIONS. Fig. 23—Equivalent circuits for open-delta connection of two Fig. 24—Equivalent circuits for open-wye connection of two identical transformers. identical transformers.

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230 Distribution Transformers

series unbalance exists because the impedance in the EBA = N phase C lead differs from that in the B and A leads. This is illustrated in Section 27.

26. Three-Transformer Banks with Unequal Size Transformers Three-phase banks are sometimes made up with one transformer appreciably larger than the other two, so that single-phase loads can be tapped off the secondary of that transformer. Fig. 25(a) shows the connections for a delta-delta bank with one transformer different (a) DELTA-DELTA CONNECTION in size from the other two. The three-phase equivalent, using ideal transformers and transformer leakage im- ZAZB ZA+2Zo N;I pedances in ohms, are shown in Fig. 25(a). Note that A1 --• 01 the impedances ZA and ZB are the impedances of the single-phase transformer in ohms as viewed from the high-voltage side. Fig. 25(b) shows the connection of al sequence equivalents for this transformer configuration. ZAZB Fig. 25(c) shows the connections for a wye-delta ZA+24 N:I bank where one transformer is different in size from A2 • az the other two. The equivalent circuit in ohms is shown in Fig. 25(c), and the connection of the sequence circuits is shown in Fig. 25(d). nz For wye-wye banks with one transformer differing from the other two, the sequence equivalents are similar to those in Fig. 25(d), except that the isolating Ao-4, • 2 transformers have no phase shift and the secondary of 3tZg - AZB)Z the ideal transformer in the zero-sequence circuit is con- ZA+2ZB nected to ao and go rather than being shorted. The Go go turns ratio in the zero-sequence circuit is N :1. (b) SEQUENCE CONNECTIONS FOR DELTA -DELTA

27. Duplex Transformer A duplex transformer consists of two single-phase cores and coils, usually with unequal kva ratings, mounted in one tank. The two primary windings are connected in open-wye or open-delta, and the low voltage is connected for 240 volts open-delta with 120-volt mid-tap in one phase of the larger winding. The one transformer then provides both single- and three-phase service. The equivalent circuit for the duplex transformer connection is shown in Fig. 26. For the more usual duplex (c) WYE-DELTA CONNECTION arrangement, where one winding is larger in kva rating _J30° than the other, this configuration involves three dis- z e PVIC tinct unbalances so far as the sequence connections are at concerned. First, the transformer which would close the bank is missing, which results in one series unbal- n ance; second, the existing transformers have different impedances, which results in a second series unbalance; j30° and finally the one low-voltage winding is grounded at Z B N IC the mid-point, which results in a shunt unbalance. A at Hence, the equivalent sequence connections will be quite complex and it will usually be simpler to use the r-11 nt equivalents given in Fig. 26 for calculation purposes.

Z B AO

- ZAZB)) Fig. 25—Equivalent circuits for delta-delta and wye-delta 3 3Zo[ connections with one transformer different in size from the other two. Voltage ratios of all three transformers are Go the same. (dl SEQUENCE CONNECTIONS FOR WYE-DELTA

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Distribution Transformers 231

ZCH a

b C B B

(a) WINDING ARRANGEMENT ( b ) 30 DEGREE CONNECTION

A a. A 0 (a) OPEN-WYE CONNECTION 3/. II,

C 1 b 12I•. Ba 7L c C b B I. La (c) IN PHASE CONNECTION (d)SECONDARY ZERO SEQUENCE CURRENT FLOW Fig. 27—T-T connected three-phase transformer.

phase voltage supply. The primary winding is normally ungrounded. The tapped point on the secondary teaser winding will usually be grounded. The principal advantage of this transformer is the resulting smaller tank, core, and coils as compared to a conventional three-phase transformer of the same rating. By proper distribution of impedances, the T-T transformer causes little or no unbalanced voltage drop (b) OPEN-DELTA CONNECTION with balanced load. Also, this design has considerable Fig. 26—Equivalent circuits for duplex transformers for solution by loop currents. ZCH and Za determined by test, manu- 250 facturers' data, or by use of Equation (14). I I c SP BREAKER TR AT 25°C AMBIENT D Fig. 26 shows that the impedances for the phase whose 200 mid-tapped secondary is grounded must be split be- ...... tween the primary and secondary windings. In the absence of specific information from the manufacturer, 150 the procedures in Section 17 and the typical split in PHASE LOA 55.AYERAGE % — 25115 KVA impedances described in Equation (14) can be used to COPPER RISE REE- - - , 1 -- 25 al5Kva determine the amount of impedance which should be -- TH . 100 t inserted in both the primary and secondary equivalent T \\ % i circuits. 1 Since duplex transformers provide for simultaneous I I single- and three-phase loading, the amount of single- PERCEN 50 i i phase load which can be carried without excessive 1 1 1, winding temperature is dependent upon the amount of I three phase load which is being carried simultaneously. 0 1 Some typical curves showing the relationship between 50 100 150 200 250 permissible single-phase loading and three-phase loading are shown in Fig. 28. PERCENT SINGLE-PHASE LOAD

28. T-T Connected Transformer Fig. 28—Permissible combined three-phase and single- phase loads on 25- and 5-kva, and on 25- and 15-kva duplex Another form of unbalanced transformer is the T-T transformers. Loads at same power factor. 100 per cent connected three-phase transformer shown in Fig. 27. single-phase load is 25 kva. 100 per cent three-phase load This transformer uses essentially two single-phase cores is 8.66 kva and 26 kva for 25- and 5-kva and 25- and 15- with windings connected as shown to provide a three- kva transformers respectively.

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232 Distribution Transformers

(A) TRANSFORMER CONNECTION

ZH+N2ZL 3 o f

2N 1

1e16°.

ZH I-N2ZL 3 2

11 2 2Nz 2 060* Z L 4 nl

A0 •

go ttl j a l O Z L G0 4 e go

(B) SEQUENCE DIAGRAMS

Fig. 29—Equivalent circuits for delta-delta connection with mid-tapped secondaries. Dotted lines show connection for a' grounded.

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 240/576 Distribution Transformers 233 flexibility of connection, in that transformers for either and currents at all points in the circuits can then be in-phase connection or 30-degree connection can be written in terms of these arbitrary quantities. Voltage obtained. drops around the loops starting with A1, Az, and a'1 can The T-T connection provides a low impedance path be written in terms of these arbitrary quantities as for the flow of secondary zero-sequence currents, with given below: no transfer to the primary windings (Fig. 27d). The . (1_ e j120° 1-€ i61 c, 4E.,s120.+J 2() - =.277e-i 30 (19) primary can be left ungrounded, and secondary phase- 3 2 4 to-ground loads can still be served with normal regu- 20 (1 --i1200 1 j600 lation. In this respect, the T-T transformer acts much j 3 i. e _-Le )±4Eye-D.200 = 0 (20) like a delta-wye transformer. 2 .4 The complete equivalent circuit for the T-T trans- .3 former is quite complex. However, the windings are so 3E. -J-/, +3E, =0 (21) 4 arranged that there is practically no coupling between the various sequence circuits. Consequently, for prac- Note that the phase-shifting transformers rotate the tical calculations, when the primary is ungrounded and voltage and current vectors by the same amount. It secondary grounded, the equivalent circuit for the wye- should be noted that if impedances are to be reflected delta transformer can be used. For the in-phase connec- through a phase-shifting transformer, only the magni- tion, the phase shift in the ideal transformer disappears. tude of the turns ratio is used in reflecting the impedance. The phase shift is ignored in reflecting impedances 29. Delta Secondary with Mid-Tap Grounded from one side to the other. A closed delta connection with a secondary mid-tap The above three equations involve three unknown grounded is illustrated in Fig. 29. This type of connec- quantities (I,,, Ex, Er), which can then be solved for tion is used for supplying simultaneous single-phase and numerical quantities. This has been done and the re- three-phase secondary loads. If the three transformers sulting voltages and currents have been labeled on are identical in size, this connection involves only a Fig. 30(a). single unbalance in the sequence equivalent connec- Note that for this example in which system impedance tions. This unbalance is the result of the mid-point of was neglected, calculation using loop equations would one of the secondary windings being grounded. be simpler than using the sequence equivalents. This configuration can be viewed as a three-winding transformer, consisting of a primary (ABC), and two secondaries (abc and a'b'c'). The equivalent circuit VIII. CONSIDERATIONS IN CHOICE OF for the three-winding transformer is shown in Fig. 29(b). CONNECTIONS FOR THREE-PHASE BANKS The split of impedances between the primary and sec- Out of the wide variety of possible transformer con- ondary windings can be obtained from the procedures nections for three-phase banks, there are usually only described in Section 17. Grounding of the mid-point a' one or two connections which are best for any particular can be represented by connecting terminals a'o to n'2, application. The purpose of this section is to present a'2 to n'i, and a'1 to g'o. The extreme left and extreme some of the considerations in choosing the best connec- right-hand terminals are then the terminals for the tions. equivalent transformer representation, including the ground on the a' terminal in Fig. 29(a). 30. Phase Shift or Voltage Requirements Example—Since many of the sequence equivalent If the transformer is to be connected in parallel with connections require the use of isolating transformers to an existing bank or an existing system, the necessary represent the unbalanced connection, it may be helpful phase position of the secondary voltage with respect to to work out an example illustrating how a problem is the primary will already be specified. This existing solved in symmetrical components using these isolat- phase relationship may eliminate consideration of some ing transformers. The delta secondary with mid-tap of the possible transformer connections. For example, if grounded will serve as a typical rather complex connec- the secondary voltage lags the primary voltage by 30 tion which will illustrate the use of these transformers. degrees, this would eliminate the possibility of parallel- Fig. 30 shows a delta-delta connection with mid-tap ing the existing system with either a wye-wye or delta- c' grounded on the secondary. Typical transformer im- delta bank where the 30° phase relationship cannot be pedances are assumed. Balanced voltages are applied to obtained. the primary winding, and the secondary phase b lead is For wye-wye or delta-delta connections, phase shifts short-circuited to ground. The first set of isolating trans- of 60, 120, 180, 240, and 360 degrees can be obtained by formers in Fig. 30 are for the bank; the second set are appropriate connection and lettering of the secondary for locating the two faults on the b and c' phases, rather terminals. For wye-delta banks, phase shifts of 30, 90, than the a and a' phases. The sequence circuits are 150, 210, 270, and 330 degrees can be obtained by ap- connected in series at the point of fault as indicated in propriate connection and lettering of secondary ter- Reference 8. Assume arbitrary voltages 2 Ex in the posi- minals. tive sequence diagram at the fault point, 2 Ey in the The desired magnitude of secondary voltage also negative sequence diagram at the fault point, and cur- often dictates the secondary connection. For example, if rent /"= leaving terminal al at the fault point. Voltages the load requires 240 volts three-phase supply and 120

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75.6/120°

75.6120" b

( A) TRANSFORMER CONNECTIONS AND FAULT CURRENTS

j120° Ix 01 1 20T •

: C- 1120° 2E

j l20° i e ••E *

0120° 60•‘ I xe-P2°. I X 2 4 )

A i 1120•• 1 20 C 2 Ey • 4E ye-1120° Az

••j 120° 1:e -E N c-1120* e160\ I ( 2 Ix I x 0 2 m••••••

I: t - 2 (Ex+Ey) • go

0 1:1 -2(Ex + Ey) • I x Ix go

(B) SEQUENCE CONNECTIONS

Fig. 30—Solution for line-to-ground fault on secondary of 480-240 volt delta-delta transformer with secondary mid-tap grounded. Impedances are shown on diagram.

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volts grounded for lighting, a transformer with a sec- I 3RD ondary voltage rating of 120/240 volts must be connected in delta on the secondary side with either a mid- tap or one corner grounded.

31. Wye-Wye Connections The positive-sequence equivalents for wye-wye transformers are essentially the same as for the delta-delta connections and the wye-delta connections, except for a difference in phase shift for the latter type. However, A a the difference in the zero-sequence circuit gives rise to Fig. 31—Suppression of third harmonic currents in un- a number of problems for the wye-wye connection that grounded wye transformer banks without delta winding. do not apply for the other types. Third Harmonic Exciting Currents—When a sinusoidal voltage is impressed across a transformer winding with tral on the wye side would then be sinusoidal. The cur- the other winding open-circuited, the transformer draws rents in the delta would not be high in magnitude, but a magnetizing current sufficient to induce a flux in the are only the normal third harmonic currents required to core to generate a back emf equal to the applied voltage produce a sinusoidal flux wave. except for a very small leakage drop. If the transformer The above discussion applies principally to the case did not saturate, the magnetizing current would be in which the three-phase bank is made up of three single- sinusoidal in shape and directly in phase with the flux phase transformers, or a shell-form three-phase transfor- wave. However, economics dictate that transformers mer in which the zero-sequence magnetizing reactance be operated near the "knee" of the saturation curve is of the same order as the positive-sequence magnetiz- where some degree of saturation exists. This non-linear ing reactance. As explained previously, for core-form magnetizing impedance results in a magnetizing current transformers the magnetizing impedance to zero-se- which has an appreciable content of odd harmonics. quence voltages is much less than for positive-sequence The predominant harmonic is the third, which can be voltages. This effect can be approximately represented as high as 50 to 60 per cent of the fundamental exciting by considering a high impedance delta tertiary to exist current. The magnitude of the harmonic content de- in this type of transformer. This impedance may be creases as the order of the harmonic increases. These from 15 to 100 per cent, depending upon transformer harmonics can be considered to flow in the positive-, size and type. This fictitious delta greatly decreases the negative-, and zero-sequence circuits. The triple har- third harmonic component of flux and voltage to neutral. monics (third, ninth, fifteenth . . . ) flow in the zero- Voltage Regulation With Single-Phase Loads—Certain sequence paths; the fifth, eleventh, seventeenth . . . types of wye-wye connections produce very large volt- flow in the negative-sequence circuit; and the seventh, age regulation for secondary loads taken off from line to thirteenth, nineteenth . . . flow in the positive-sequence neutral. This is illustrated in Fig. 32 for a transformer circuit. with an ungrounded primary and a grounded secondary Certain of the wye-wye connections suppress the flow with a load taken off from line to ground. of all the triple harmonic currents. The suppression of For balanced three-phase loads on the bank, the reg- these currents results in a distorted flux wave in the ulation is determined by current flowing through the iron, where fluxes at the triple harmonic frequencies are leakage impedance of the transformer. However, for a superimposed upon the fundamental flux wave. The single-phase load from line to neutral, the current must third harmonic is by far the most troublesome, because flow through the leakage impedance plus the zero-se- it is larger in magnitude than the other triple harmonic quence magnetizing impedance. For three-phase core- components. form transformers, the regulation will be high for single- The suppression of triple harmonic currents is illus- phase loads, but may not be intolerable. If the bank is trated in Fig. 31, which shows an ungrounded wye-wye made up of a shell-form transformer or three single- bank made up of three single-phase transformers. Third phase transformers, the regulation will be intolerable. harmonic currents are zero-sequence in character and If the primary is connected in closed delta or if a must be equal and in phase in all three line leads. Since closed delta tertiary winding exists, the magnetizing the primary neutral is isolated from ground, there is no impedance will be paralleled by the leakage impedance path for the third harmonic currents to flow. As a result, from the secondary to the delta, and the regulation will the voltage from A to N will contain a large third har- not be appreciably worse than for three-phase loads. monic component. This voltage may be 50 to 60 per Neutral Inversion—The neutral point of an ungrounded cent of the fundamental voltage, with a resulting rms three-phase system is usually held at or very near voltage of 15 per cent or more above the normal line- ground potential by the capacitance of the three phase to-neutral 60-cycle voltage. wires to ground. If the capacitances of the three phases If the primary, secondary, or a tertiary winding is are nearly equal, there will be little displacement of connected in closed delta, this delta provides a low im- the neutral from ground. If three transformers are con- pedance path for the circulation of third harmonic cur- nected in grounded wye without a delta winding, under rents. The flux wave and the resulting voltages to neu- certain conditions the neutral can be appreciably dis-

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236 Distribution Transformers

Wye-Wye Connection With Isolated Neutral—When a wye-wye bank with isolated neutral is made up of three single-phase units or one three-phase shell-form transformer, the third harmonic component of exciting current required to produce a sinusoidal flux wave is sup- pressed by the connection. As explained previously, this suppression results in a third harmonic flux and (a) TRANSFORMER CONNECTION WITH induced voltage-to-neutral of about 50 per cent of rated SECONDARY LOAD TO NEUTRAL voltage-to-neutral. This third harmonic voltage appears between each phase lead and the bank neutral, but does not appear between phase leads. The rms voltage across each winding POS. Z t will be about 115 per cent, but the peak voltage will be almost 150 per cent of normal. I. I If the supply system is well grounded, almost all the z NEG. , third harmonic voltage will appear between ground and 3ZL the transformer neutral; very little third harmonic voltage will appear from line to ground. If the supply system is ungrounded, part of the third harmonic voltage will appear from line to ground and part from bank ZERO ma neutral to ground, depending upon the relative capacitances to ground of the transformer windings and the ( b) SEQUENCE CONNECTIONS line leads. Another difficulty with this connection is that the Fig. 32—Regulation with single-phase load to neutral on bank neutral is fixed by the exciting currents of the indi- wye-wye bank with primary neutral isolated. vidual transformers, and the bank neutral will not necessarily coincide with the system neutral. placed and can even go outside the normal vector tri- The regulation for single-phase secondary loads from angle of line voltages. This latter condition is known as line to neutral is very large, since the load current must neutral inversion. be drawn through the magnetizing impedance. This condition is illustrated in Fig. 33, where the three TRANSFORMER lines have equal capacitance to ground. Now assume MAGNETIZING that a wye-connected transformer is added with neutral REACTANCES grounded and with the three magnetizing reactances a having the values shown in Fig. 33(a). Fig. 33(b) shows j Z.6T UNGROUNDED the resulting phase voltages with respect to ground. The SUPPLY neutral has been shifted completely outside the vector triangle. "13 To accomplish this neutral inversion, it was necessary to assume that the magnetizing reactances were of unequal values; that the lower value of reactance was less a than the capacitive reactances of the line; and that the higher value of magnetizing reactance was greater than b • the capacitive reactance. This difference in magnetizing (A) IMPEDANCE DIAGRAM reactances can occur if the transformers, making up the bank are not identical. Even with identical transformers, this condition can exist if some switching operation or temporary fault results in over-excitation and saturation of one or more windings. Saturation will appreciably alter the effective magnetizing impedance of that phase. This phenomenon of neutral inversion or neutral instability is seldom encountered on transmission, subtransmission, or distribution systems. This is partly due to the fact that grounded wye-wye transformers are C seldom used on ungrounded systems and also because the charging current for these typical systems is appreciably greater than transformer magnetizing current. This condition is often important in applying grounded (6) VECTOR DIAGRAM

wye-wye potential transformers for ground fault detec- Fig. 33—Illustration of neutral inversion due to wye-ground- tion. The condition can be avoided by appropriately ed transformer (without delta winding) on ungrounded sys- loading the secondary windings.' tem.

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For this system arrangement, difficulties with third bilized, and the regulation for single-phase secondary harmonic voltages and regulation for single-phase loads loads is normal. The third harmonic exciting current are somewhat alleviated if the bank consists of a three- must still be supplied to the transformer over the supply phase core-form transformer. The relatively low magne- lines. This sometimes causes telephone interference tizing impedance in the zero-sequence equivalent per- along the primary lines, particularly if the neutral wire mits carrying a single-phase load of about 10 to 15 per is not carried along and the third harmonic currents cent of total three-phase rating with regulation which must return through the earth. The amount of third is usually tolerable. Suppression of the third harmonic harmonic voltage in each phase is greatly reduced below exciting current is equivalent to applying the negative that for the case in which the supply neutral is isolated. of this exciting current through the transformer wind- The amount of the reduction is determined by the ings. Because of the high reluctance in the third har- impedances of the lines, supply, and ground. monic flux paths, the third harmonic voltage will be If the transformer is a three-phase core-form trans- only 3 to 5 per cent of normal 60-cycle voltage. The former, the fictitious delta reduces the likelihood of effect of this low reactance in the zero-sequence equiva- telephone interference due to the triple harmonics. lent can be represented as an artificial tertiary winding Whether the fictitious delta provides a substantial connected in delta. This artificial delta stabilizes the reduction depends upon the relative impedances of the bank neutral at the supply system neutral, supplies the delta and of the supply system. required third harmonic exciting current, and supplies the zero-sequence currents which flow to loads or faults 32. Delta-Delta Banks from line to neutral. Delta-delta banks present no problems of system Wye-Wye Connection With Neutral Grounded and Sup- neutral displacement, neutral inversion, or difficulties ply System Neutral Isolated—Grounding the bank primary due to the triple harmonics. Zero-sequence currents neutral will establish the neutral at ground potential cannot flow through a bank with this connection. Indi- and will cause the third harmonic voltages to appear be- vidual transformers should have approximately the tween line and ground. If the capacitance from supply same voltage ratios and impedances if loads are ex- leads to ground is very small compared to the magne- pected to divide proportionately among the windings. tizing reactances, the magnitude of third harmonic If the voltage ratios are different, circulating currents voltage on the supply leads will be about equal to the within the delta will result. third harmonic voltage at the neutral for the un- With a bank of identical units and with balanced grounded case. sinusoidal voltages applied, the exciting currents in the If the line capacitive reactance is much smaller than lines will have no triple harmonic components. These the magnetizing reactance, third harmonic currents will components circulate in the primary and secondary flow through this capacitance and approach the condi- windings, but do not flow to the external lines. Un- tions for a grounded supply system. In the range where balanced voltages applied to a symmetrical bank will capacitive reactance is approximately equal to mag- not produce circulating currents inside the delta netizing reactance, neutral instability or neutral inver- windings. sion can occur, along with the resulting hazards to Single-phase loads for power service can be con- connected equipment. nected between lines on the secondary side. Single- Another difficulty arising from grounding the trans- phase lighting loads and other loads requiring grounded former neutral is that a line-to-ground fault on the service can be obtained from one of the secondary lines, primary (or line-to-neutral short circuit on the second- with either one corner of the secondary delta or the ary) results in a full neutral shift, with line-to-line mid-point of one secondary winding connected to voltage impressed on the two remaining phases. Assum- ground. ing that the transformers are rated for normal line-to- The delta-delta connection is probably the most fool- neutral voltage, this results in an applied voltage of proof three-phase connection which can be applied. 1.732 times normal, and the exciting current can be Problems associated with the flow of ground currents several times full load current. This can cause severe and harmonic components of magnetizing currents are damage to the transformer unless the fault is removed. virtually eliminated. Usually the most important con- However, the fault current will be low (even for three- siderations are loading on the individual units, regula- phase core-form transformers with fictitious delta) be- tion, and unbalanced voltages when the units are not cause the supply system is not grounded. Precautions identical or when the secondary load is not balanced. should be taken to assure that these faults are promptly Open-Delta Connection—Two transformers can be detected and cleared, or (in the case of potential trans- connected in open delta as shown in Fig. 23 for supply- formers for ground fault detection) transformers with ing three-phase loads. Single-phase loads can also be rated voltage equal to line-to-line voltage should be supplied either between secondary phases or from used. phase to ground, with either one terminal or the mid- Wye-Wye Connection With Neutral Grounded and Sup- point of one winding being grounded. ply System Neutral Grounded—When the transformer The use of the open-delta connection requires that the primary neutral and the supply system are both well three primary phase leads be carried to the transformer grounded, most of the difficulties cited previously disap- bank; hence, there is no saving in the primary wiring pear. The system neutral and bank neutral are sta- over that which would be required if a closed delta were

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used. The open-delta connection, using two single- supplying three-phase motors. For example, the nega- phase transformers, is most often used in the following tive-sequence impedance of three-phase induction cases: motors may be only 10 to 20 per cent of the positive- sequence impedance. A given magnitude of negative- 1. For emergency conditions where one transformer sequence voltage would then cause 5 to 10 times as in the bank has failed. Removal of the failed trans- much negative-sequence current as if the same voltage former (by disconnecting both primary and second- were applied in the positive sequence. Further, the ap- ary leads) will permit the load on the bank to con- plication of negative-sequence voltage to a rotating tinue being supplied by the open-delta connection if machine results in a flux wave within the machine the remaining transformers are not excessively over- which rotates in a direction opposite to that of the loaded. If the bank consists of a three-phase trans- motor rotation. The linking of this flux with the rotor former, the open-delta connection can be used if the can cause much more serious heating effects than a windings on the faulted phase are short circuited in comparable current in the positive sequence. the case of a shell-form transformer, or completely Since three-phase motors are not rated to withstand removed in the case of a three-legged core-form any specified amount of negative-sequence current, the transformer. voltage unbalance should be kept as small as possible. 2. The open-delta connection is sometimes used for Excessive unbalance would necessitate derating the supplying three-phase loads where the magnitude of motor. In general, the maximum secondary phase the load is expected to increase substantially in the voltage should not exceed the minimum secondary future. The open delta presents an easy means of in- phase voltage by more than 5 to 6 per cent. Certain creasing the bank capacity by merely closing the motors will have even more stringent restrictions. delta with the addition of a third transformer, rather than changing out an entire bank or adding a parallel 33. Wye-Delta Banks bank. The wye-delta connection eliminates many of the 3. The open-delta connection using two different problems discussed for the wye-wye connections. The size transformers is often used for simultaneously delta offers a path for circulation of third harmonic supplying a large single-phase load and a small three- currents; hence, practically no third harmonic currents phase load. The larger transformer will be used for appear in the supply lines. The closed delta practically supplying all of the single-phase load and its pro- locks the bank neutral to the system neutral, so that portionate share of the three-phase load. This type problems of neutral shift seldom occur. Of course, the of application can involve the use of two separate delta can appear as either the primary or the secondary transformers or a single duplex transformer which has winding, and the delta serves to stabilize the neutral on been described previously. the wye side of the transformer, but not on the delta side. For supplying balanced three-phase loads, the output If the wye is grounded, the wye is a source of ground kva is only 86.6 per cent of the kva load actually im- current. Consequently, any ground fault on the con- posed on the two transformers in the bank. For exam- nected system (whether or not there are additional ple, assume that the output current from the bank is 1.0 ground sources) will result in the flow of short-circuit per unit and the rated secondary voltage to neutral is current through the transformer. Means must be pro- 1.0 per unit. The total output kva, is then 3.0 per unit. vided to clear these short circuits from the system or to However, each transformer will have a load current of remove the grounded transformer to prevent damage 1.0 per unit and the voltage across the transformer to the transformer. This normally presents no particular winding is 1.732. The kva of the two transformers will problem, since the fault is usually removed by a fuse, then be 3.464. breaker, etc. The open-delta connection does require that the The grounded wye-delta bank also has a tendency to third harmonic component of exciting current be sup- balance other single-phase loads connected line-to- plied from the supply system. However, this current neutral on the wye system. A single-phase load far out will circulate through the positive- and negative-sequence on the system may cause circulating current in the paths rather than through the ground. Hence, the diffi- adjacent wye rather than being supplied directly from culties with telephone interference are much less the source. This can produce overloading of a small formidable than with the wye-connected transformers bank near a large single-phase load. discussed earlier. A small third harmonic voltage will Care should be exercised in applying small grounded exist in the phases, with the magnitude dependent upon wye-delta transformers to an otherwise ungrounded the impedance in the supply. system which has a high short-circuit capacity for The regulation in the three phases is different even phase faults. Line-to-ground faults on the system will for balanced secondary loads. In general, this results in be limited by the zero-sequence impedance of the secondary voltages which are slightly different in transformer and will he of fault current magnitude as magnitude and are not exactly 120 degrees apart. This far as the transformer is concerned. However, this cur- unbalance results in the development of a negative- rent might be relatively small as far as total system sequence voltage on the secondary terminals, and this load current is concerned. Hence, devices which protect negative-sequence voltage can be quite objectionable for feeders on the supply system may not clear a ground

lir Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 246/576 Distribution Transformers 239 fault, and the short-circuit current can damage the well grounded. This latter case is electrically identical to transformer in a short period of time. This situation can that for the open wye connection, which will be dis- be corrected in a number of ways, such as application of cussed later. Single-phasing or considerably unbalanced protective relays which recognize ground faults, insert- secondary voltages can occur with a number of configu- ing impedance between the transformer neutral and rations, depending upon where the breaks or fuses are ground, fusing of the transformer, etc. assumed to exist, the degree of grounding, the presence The wye-delta connection inherently has a phase of parallel banks, etc. All of these combinations are too shift between the primary and secondary windings. The numerous to cite here, but the possibility and the con- transformers are usually connected in such a way that sequences of single phasing should be considered in the voltage from phase A to neutral on the high-voltage selecting the transformer connection and in applying side leads the voltage from phase a to neutral on the fuses and other single-phase protective devices. low voltage side by a time angle of 30 degrees. However, Open-Wye Connection—If the primary system is a connections other than this are encountered. four-wire system, a three-phase secondary supply can be When the bank is made up of three single-phase units, obtained by connecting two transformers in open wye as the positive-, negative-, and zero-sequence impedances shown in Fig. 24. The reasons for applying the open-wye are equal. For three-phase shell-form transformers, the connection are much the same as those listed for the zero-sequence impedance is almost identical to the open-delta connection. positive-sequence impedance. For three-phase core-form The open-wye connection has certain advantages over transformers, the zero-sequence impedance is generally the open-delta connection. The principal advantage is 10 to 20 per cent less than the positive-sequence im- that the open-wye connection permits use of transform- pedance because of the relatively low impedance in the ers with a single primary bushing and with graded in- magnetizing branch in the zero-sequence equivalent. sulation. This results in a lower cost for the transformers For faults near a grounded wye-delta transformer, it than would exist if the open-delta connection were used. should be noted that the ground fault current is some= The open-wye connection does not require that all three times higher than the phase fault current. For example, phase wires be carried to the bank. However, a neutral consider a short circuit on the grounded wye-connected wire will usually be carried along so that there may be secondary of a transformer whose primary is connected no net saving in primary copper compared to the open- in delta. The ratio of ground fault current to three- delta. Whether a saving will result depends upon the phase fault current is particular case and is considerably influenced by whether Z1 other single-phase or three-phase banks are located on the feeder. 1/3(Z1d-Z2+Zo). The open-wye connection also has disadvantages com- The zero-sequence impedance is only the impedance of pared to the open-delta. The third harmonic currents the transformer bank, whereas the positive-and negative- and load currents must return through the primary sequence impedances include the bank plus the supply ground circuits. There is potentially a greater source for lines plus the system impedance. Since Zo is less than telephone interference than existed for the open delta. = Z2, a line-to-ground short circuit on the secondary The regulation for the open-wye bank may also be will cause higher fault currents than would a three- somewhat greater than for the open-delta bank. This is phase fault. because load current returns through the primary A grounded wye-delta bank can be used as a ground- ground circuits, and in general, the zero-sequence im- ing transformer on an otherwise ungrounded system. If pedance of the supply lines will be higher than the posi- it is desired to limit the current to a greater extent than tive-sequence impedance which would be encountered in does the transformer alone, additional impedance can be the open-delta connection. introduced between the bank neutral and ground (If The considerations for supplying simultaneous single- used as a grounding transformer, an impedance might be phase and three-phase loads, and the problems of unbal- placed in the secondary delta). anced secondary voltages are essentially the same as A delta primary, wye secondary connection is suit- have been described for the open-delta connection. able for four-wire secondary service, with normal regula- The use of the open-wye connection may also permit tion for single-phase loads to neutral. use of transformers with a lower insulation level than if If the primary is connected in ungrounded wye, open- an open-delta connection were used. For example, in a ing one of the supply leads will result in single-phase 14.4/24.9 kv grounded system, transformers in the 18-kv voltage on the secondary. The voltage between one pair insulation class can be used in the open-wye connection, of secondary terminals will collapse to zero, while that but transformers in the 24-kv class would be used in between other pairs will drop to 86.6 per cent of the open-delta. This will result in lower transformer costs. normal value. This single-phasing condition would be intolerable for three-phase motors connected to the 34. CSP Transformer Connections secondary. Most of the common single-phase CSP transformer This single-phasing condition with an open primary connections are shown in Fig. 34. Connections 1 through wire can also exist with the bank neutral grounded and 8 are considered satisfactory, while connections 9 and 10 the supply system ungrounded. The condition will not are not recommended. exist if both the bank neutral and the supply system are With connection 9, loss of transformer A results in

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 247/576 240 Distribution Transformers line-to-line voltage being impressed upon the series com- The objection to connection 10 is that loss of any bination of the two groups of single-phase loads normally transformer is equivalent to opening one primary lead. supplied by transformer A. Since the impedances of the This results in single-phase, line-to-line voltage being two groups of single-phase loads are usually different, applied to the remaining two primaries in series. The the result is an over-voltage condition for one group of secondary would then be supplying single-phase, three- single-phase loads and a corresponding undervoltage on wire service to connected three-phase loads and one-half the second group. Use of a balance coil as in connection normal or zero voltage to single-phase loads. If a pri- 4 is an acceptable remedy. mary neutral wire is added, the connection becomes

SINGLE-PHASE CSP TRANSFORMER CONNECTIONS RECOMMENDED CONNECTIONS

I. DELTA -WYE 2. DELTA-DELTA

3. OPEN DELTA 4. BALANCE COIL

6. DELTA- DELTA {TWO CSP'S 5. OPEN WYE - OPEN DELTA ONE CSPB

4L\

ONE CSP 7. OPEN DELTA r I ONE CSPB 8. WYE WITH NEUTRAL; DELTA

NOT RECOMMENDED

9. DELTA 10. ISOLATED - NEUTRAL WYE - DELTA

ANY PRIMARY

Fig. 34—Recommended connections of single-phase CSP transformers in three-phase banks.

Gridco, Inc. v. Varentec, Inc. IPR2017-01135 GRIDCO 1004 Part 2 of 5 - 248/576 Distribution Transformers 241 identical to that shown in Part 8 of Fig. 34 and is satis- Table 15-Permissible Daily Short-Time Transformer Load- factory. Grounding the primary neutral and the source ing Based on Normal Life Expectancy 17) neutral, but not adding a neutral wire, would not completely correct the situation. The ground path would be Period Maximum Load in Per Unit of Transformer Rating* of In- of comparatively high impedance and would also increase Average** Initial Load in Per Unit of the possibility of telephone interference. creased Loading, Transformer Rating Hours 0.90 0.70 0.50 IX. LOADING OF DISTRIBUTION TRANSFORMERS 0.5 1.59 1.77 1.89 Distribution transformers are assigned a kva rating •Figures are for OA, OW cooling only 1.0 1.40 1.54 1.60 "Use either average load for two hours which, according to Standards, the transformer can 2.0 1.24 1.33 1.37 previous to overload period, or average load for 24 hours (less the overload carry continuously without exceeding an average copper 4.0 1.12 1.17 1.19 period), whichever is greater. temperature rise of 55°C.3 This nameplate rating also 8.0 1.06 1.08 1.08 serves a useful commercial purpose by specifying the kva at which guaranteed losses and regulation must be met. In service, however, a distribution transformer is tial curve having a time constant in the order of a few rarely loaded continuously at its rated kva, but usually hours. The temperature differential between copper and goes through a daily load cycle characterized by a short- oil increases to its ultimate value much more quickly, time peak load. Cyclic loads on a transformer which is but the total copper temperature is held down by the oil. rated on the basis of continuous load presents a problem The magnitude and duration of the overload which can in applying transformers to utilize the full loading capa- be carried without exceeding 95 degree hot-spot tem- bility without unduly shortening the life. Tables 15 and perature depends upon ambient temperature, initial 16 indicate two approaches to the problem of short time loading, loss ratio, etc. Procedures for calculating the overloads on distribution transformers. The load in- temperature transients are given in Reference 7 and will creases based on capacity factor or on short time over- not be repeated here. loads cannot be applied simultaneously. It is necessary The second factor which aids in carrying peak over- to choose one or the other. loads is the thermal aging characteristics of insulation The primary consideration of loading which deter- used in distribution transformers. Temperatures con- mines the life of a particular transformer is the deterio- siderably above 95°C can be carried for short periods of ration of insulation during its service life. The rate of time without decrease in normal life expectancy, if this deterioration is greatly influenced by the temperature to condition is offset by extended operation at tempera- which the insulation is subjected. For class A insulation tures below 95°C. In other words, the elevated tempera- used in liquid-immersed transformers, present standards tures do not cause failure of insulation but only increase call for hot-spot temperature to be limited to 95°C for the rate at which deterioration occurs. normal life expectancy.° The desirability of loading equipment on the basis of One solution to the cyclic loading problem would be winding (hence, insulation) temperature has long been to limit the peak load to the transformer nameplate recognized, and ASA has developed a guide for loading rating. However, this would result in uneconomical use oil-immersed distribution and power transformers. This of the transformer load capability. There are two pri- guide is discussed in detail in Reference 7. However, ex- mary characteristics of the transformer which permit tensive research work, particularly during the last short-time peak overloads to be carried without de- decade, has strongly indicated that the loading guides creasing the expected life. for distribution transformers are unduly conservative The first of these is the relatively long thermal time and that the criterion established for insulation deterio- constant of the transformer. While the load on the ration (tensile strength of the insulation) is not a com- transformer can increase very rapidly, the oil tempera- pletely realistio indication of loss of transformer ture increases much more gradually along an exponen- life." 12

Table 16-Permissible Short-Time Transformer Loading Based on Reduced Life Expectancy (OA, OW)7

Following 50 Per Cent or Less of Rated KVA I Following 100 Per Cent of Rated KVA Period of Probable Sacrifice In Per Cent Of Normal Life Caused By Each Overload Increased Loading, 1 1 1.00 0.10 0.25 0.50 I 1.00 1I 0.10 1 0.25 0.50 Hours Maximum Load In Per Unit Of Transformer Rating

0.6 2.00 2.00 2.00 2.00 1.75 1.92 2.00 2.00 1.0 1.76 1.91 2.00 2.00 1.54 1.69 1.81 1.92 2.0 1.50 1.62 1.72 1.82 1.35 1.48 1.68 1.68 4.0 1.27 1.38 1.46 1.53 1.20 1.32 1.40 1.48 8.0 1.13 1.21 1.30 1.37 1.11 1.20 1.28 1.35 24.0 1.05 1.10 1.15 1.23 1.05 1.09 1.15 1.23

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This section will discuss some of these recent research I00 activities and their effects on increasing permissible loading of distribution transformers. 50 20 35. Aging of Insulation' ) SCALE Many attempts have been made to develop a rela- I0 tionship between some physical property of the insulat- 41.•

ing material which could be used as a measure of the re- 5 13

maining insulation life, and the magnitude and time .4 duration of the temperature to which the insulation is C.) 2 exposed. Such a relationship would be valuable for de- 100 1 termining the loss of insulation life due to certain overload cycles, and for developing general rules as to the cr 50 0.5 magnitude and duration of overload which can be per- EEKS (SCALE 20 0.2 mitted. SCALE A An empirical time-temperature relationship of this 10 0.1 1 E IN W nature was first suggested by Montsinger,"3> using M tensile strength as a measure of remaining insulation w 0.05

2 TI life. According to his findings, the required heating time CD z 2 0.02 ING to reach a certain value of tensile strength is cut about in CD AG half with each 8°C increase in temperature. Another re- 1 0.01 lationship between these quantities was introduced by Dakin" and Malmlow" on the basis of a chemical 0.5 0.005 reaction law, and is usually referred to as the "chemical rate rule". This rule gives straight, parallel time-tem- 0.2 0.002 perature lines for constant values of remaining tensile 0I 0.001 strength if a reciprocal absolute temperature scale is 20 40 60 80 100 140 180 240 used. A typical aging curve of this type is shown in Fig. TEMPERATURE IN DEGREES CENTIGRADE 35." The "chemical rate rule" will be used in the fol- (RECIPROCAL ABSOLUTE TEMPERATURE SCALE) lowing discussion. Fig. 35—Aging time of manila paper in oil, based on tensile For analyzing overload cycles, the data in Fig. 35 can strength". Curve 1—Time to reach 80 per cent of residual be expressed in a more convenient form. For this pur- tensile strength. Curve 2—Time to reach 20 per cent of pose, the quantity a is defined as the rate of aging at any residual tensile strength. given temperature relative to the rate of aging at some reference temperature. Since the time-temperature lines in Fig. 35 are parallel, the rate of aging is independent of the terminal value of tensile strength, and the lines in 36. Analysis of Overload Cycles" Fig. 35 become a single rate curve. The analysis of a given loading on a transformer to A curve showing aging rate as a function of tempera- determine the effect upon life requires the following pro- ture is given in Fig. 36. This curve, which is based on the cedures: "chemical rate rule", would not be coincident with that 1. Determination of the winding temperatures based on the "8 degree rule" nor with curves based upon throughout the cycle, either by test or by calcula- properties other than tensile strength. There are other tion as explained in Reference 7. limitations in the use of such data, which should be 2. Translation of the temperature data into corre- borne in mind when applying them: sponding aging data by means of experimentally 1. A given aging factor characteristic can be used in determined aging curves for the insulation in- analyzing the performance of completed apparatus volved. only when the apparatus insulation and its envi- 3. Comparison of the aging thus obtained with that ronment are closely comparable to those used in to be expected at some reference loading. obtaining the data from which the aging factors were derived. To illustrate these procedures, assume that a heavy overload run has been made on a transformer, producing 2. The relative thermal endurance of two types of the temperature cycle shown in Fig. 37. The hot-spot insulation cannot be determined by comparing temperature can be determined either by test or by cal- aging factors, even when the factors are based culation, using the procedures discussed in Reference 7. upon the same physical property of the material, The aging rates corresponding to the various hot-spot unless the rate of aging at the selected reference temperatures of the cycle, as obtained from Fig. 36 or temperature is known for both insulations. from similar curves, are also plotted as a function of In spite of these limitations, suitable aging factor data time. The area under the curve of aging rate versus time can be very useful in determining the aging effects of is an indication of the total deterioration at the hot-spot. overload cycles. Since the aging rates in Figs. 36 and 37 are relative

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10P00 io9 for a number of cycles is the arithmetic sum of the equivalent aging times for each load cycle. 5,000 SCALE A These procedures permit evaluation of aging relative id 2,000 to the aging which occurs at some reference temperature.

C) However, they do not permit determination of the ab- c.) Ipoo los solute aging or life expectancy of the transformer under the specified load conditions. Determination of life ex- 500 pectancy requires a definition of the end point of insula- • C tion life. There has been a great deal of discussion, and 200 95 also a great diversity of opinion, as to what constitutes T I00 !or this end point. For example, the ASA Guide for Loading, which necessarily was intended to be conservative, indi- 50 cates that 100 per cent of the life will be expended at a VALUE A residual tensile strength of about 80 per cent. Other in- 20

T OF vestigators have used 10 and 20 per cent of initial

NI 10 los strength in their illustrative examples. Some more recent test data suggest that much lower values, perhaps less ER U 5 than 0.1 per cent, may be reached before insulation fails SCALE B/ to pass standard tests for new equipment." IN P 2 A curve of life expectancy of insulation as a function

TORS los of sustained hot-spot temperature, based upon the ASA criterion, is shown as curve A in Fig. 38.10 05 Field experience and recent laboratory tests have def- NG FAC initely indicated that the ASA Guide is overly conserv- AGI 0.2 ative in the loadings permitted on distribution trans- 0I 104 formers. A more recent test procedure (rather than de- 50 80 100 140 180 240 300 350 termination of tensile strength) has been evolved which offers promise of more realistic determination 'of distri- T=TEMPERATURE IN DEGREES CENTIGRADE bution transformer life. This type of program is known (RECIPROCAL ABSOLUTE TEMPERATURE SCALE) as "functional life testing". Fig. 36—Aging factor of insulating material (manila paper) in oil based on tensile strength. 400 5 380 360 rather than absolute rates, the area under the aging TEMPERATURE 340 curve is the deterioration relative to that which would E ----AGING FACTOR have occurred at the reference temperature. The sever- 320 RAD ity of a given overload cycle can most conveniently be G 300 TI 4( expressed in terms of an equivalent aging time if the N 280 hot-spot were maintained at some constant reference CE 260 temperature. The equivalent aging time at any desired EES 240 3

reference temperature can be determined by use of the GR following formula: E 220

f a dt IN D 200 teT= (22) aT RE 180 TU where teT is equivalent aging time at reference tempera- A 160 1. ture T, aT is the relative aging rate at temperature T, ER 140 and the integral is the area under the aging-time curve

TEMP 120 for the actual overload cycle. 100 For example, the overload cycle shown on Fig. 37 ING 80 causes the same aging that would be experienced by IND continuous operation at the maximum hot-spot tem- W 60 perature of 262°C for a period of four minutes, or by con- 40 /1' tinuous operation at 95°C for a period of 38.2 years. 20 Use of the "chemical rate law" indicates that the 0 0 effects of successive load cycles are cumulative. Hence, 0 10 20 30 40 50 for a series of load cycles, the resulting equivalent aging time can be determined by use of Equation (22), where TIME IN MINUTES the integral is the area under all the cycles considered. Fig. 37—Hot-spot temperature and aging transients from Stated differently, the resulting equivalent aging time tests on small distribution transformers.

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100 expectancy corresponding to continuous operation at a hot-spot temperature of 122°C.12 By comparison the 50 ASA Guide shows this life expectancy at less than 95°C. (10) Some more recent tests have indicated a life 20 expectancy substantially greater than that represented by Curve B. I0 The improvement in overload capability indicated in Fig. 38 has not been solely due to improved ability to 5 analyze aging characteristics. Advances in thermal de- I sign and the use of better insulating materials have con- EARS tributed greatly to this improved life expectancy. 2 The ability to load transformers at higher temperatures without decreasing life expectancy is borne out by CY IN Y experience in the field on transformers in actual service. (16) TAN B The subject of functional life testing and life expect- .5 PEC ancy has now received official recognition by AIEE and

EX is now being actively considered by the Sub-Group on

FE .2 Thermal Evaluation of Oil-Immersed Distribution and I L Power Transformers. 38. Loading of CSP Transformers .05 CSP transformers are thermally protected by circuit breakers in series with the secondary windings. The of .02 I breakers closely follow the true copper temperature ‘ the coils. .01 ‘I When the winding temperature reaches the ASA tem- 80 100 140 180 240 300 350 perature limit for class A insulation, a red signal light SUSTAINED HOT SPOT TEMPERATURE IN C comes on. If the signal light goes unheeded and the load RECIPROCAL ABSOLUTE TEMPERATURE SCALE and temperature continue to rise, the breaker will trip Fig. 38—Life expectancy for distribution transformers for before temperatures resulting in excessive loss of life are various sustained hot-spot temperatures. Curve A—ASA reached. The area between the signal light operation loading guide. Curve B—Reference 10. and the breaker trip is available to carry overloads until the transformer can be changed out. A handle is provided on the outside of the transformer tank by which 37. Functional Life Testing12 the breaker trip setting can be increased to provide Functional life testing involves operating complete additional load-carrying ability in emergencies. structures or models of the transformer under accele- Over the years since the development of the CSP rated aging conditions, which are somewhat representa- transformer, several changes have been made in the tive of actual service, until failure occurs on an end- point test which is presumed to indicate unfitness for further service. This form of testing permits comparison 500 of the life expectancy of various structures and also permits a forecast of the service life expectancy of a given structure. Functional life testing has the advantage of G 400 evaluating the life of insulation as it is applied in the NEW EMERGENCY TRIP

ATIN NEW BREAKER TRIP transformer. R FORMER BREAKER TRIP F 300 Selection of a loading cycle to obtain accelerated aging SIGNAL LIGHT represents a compromise between the desire to approxi- T O mate actual service conditions and the necessity of CEN accumulating results within a reasonably short period of R 200 time. The end-point tests should include short-circuit PE tests and overpotential tests to evaluate both the AD IN mechanical and electrical life of the insulation. No basic O 100 life expectancy test has yet been agreed upon by the in- L dustry as a whole. The most complete suggested test routine is given in Reference 12. Functional life tests which have been performed on a 0 2 3 4 5 6 large number of distribution transformers indicate much TIME IN HOURS greater life expectancy than that predicted by the ASA Fig. 39—Load-time curves of type LR breaker used in 25 Loading Guide. Curve B of Fig. 38 shows the results of kva, 2400-120/240 volt CSP transformer. 35°C ambient, one set of tests, which indicate at least a 20-year life following 75 per cent load.

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"normal" and "emergency" trip settings of circuit r= fixed charge rate on capital investment in breakers in CSP transformers. The changes reflected per-unit increased overload capability, which was obtained by The above formula assumes that the cost of transmit- the use of materials with greater thermal endurance and ting exciting vars from the shunt capacitors to the dis- by improved design methods. Results of functional life tribution transformers is negligible. tests permitted making the changes with confidence that service life of the transformer would still be satisfactory. 40. Core Loss This confidence has been confirmed by extensive,, super- Distribution transformer core loss (no-load loss) con- vised field experience. stitutes a load on the system, and it should be evaluated Fig. 39 shows typical load-time curves for breaker trip in the same manner as that of supplying any other load and signal light operation, and indicates the effect of a on the system. This is normally done in terms of a recent change in trip settings. It will be noted that the "demand" charge, which expresses the annual cost of signal light operation was not changed. The former the system investment required to supply the core loss signal light characteristic was retained for two reasons: energy, and an "energy" charge which expresses the cost 1) To provide increased time for transformer change- of fuel and labor to generate the required energy. out between signal operation and breaker trip, The demand charge component, per unit of loss, will and so reduce the chance of service outage. be the product of the system investment per kva of load 2) To provide warning of a load level beyond which and the fixed charge rate on this investment. The uneconomical operation and voltage complaints energy component, per unit of loss, will be the product are probable. of the incremental cost of energy generation per kwh, and the number of hours per year that the transformer X. DISTRIBUTION TRANSFORMER EVALUATION is energized. Thus, the total annual cost of core loss is: Cpl = (Sr+ C kwhh)kwei (24) Many different opinions for evaluating distribution where: transformers exist throughout the industry. No attempt C.1= annual cost of core loss in dollars will be made to evaluate the various methods used, nor S = system investment per kva of load will a complete method be given in this section. How- r = fixed charge rate on capital investment in ever, almost every method includes either individually per unit or collectively an evaluation of losses and exciting cur- Ckwh= incremental cost of energy generation per kwh rent; hence, these items are discussed below. Other items h =number of hours per year the transformer is such as life expectancy, fixed charges, maintenance, and energized cost of revenue lost because of regulation, which may kuki = transformer core loss in kilowatts or may not (based on individual preference) be part of an evaluation study, are not discussed here. A method 41. Copper Loss of transformer evaluation which has gained considerable Copper loss, like core loss, is a load on the system, and acceptance and contains a number of the above items its cost may be considered as composed of a demand is outlined in Reference 17. and energy component. However, there are fundamental 39. Exciting Current differences between core and copper loss, which must be recognized when each are evaluated. Exciting current has been evaluated in a number of The copper loss in each transformer varies with the ways, most of which depend on the manner in which the load on that transformer. Therefore, the lost energy over exciting vars are supplied to the transformer. Since there a period of time is less than if the loss were continuous is a growing tendency to supply vars in the distribution at the peak value, in the ratio of the average loss to the system from shunt capacitors located out in the distri- peak loss. This ratio is called the "loss factor". The de- bution system, it gives good reason to evaluate the ex- mand charge component, per unit of loss, will be the citing vars in terms of the annual cost of primary shunt product of the system investment per kva of load and capacitors required to supply the vars. the rate of fixed charge on the investment. This com- The annual cost of exciting current is equal to the ponent is the same as when evaluating core loss. The product of the per-unit exciting current of the distribu- energy charge component, per unit of loss, will be the tion transformer under study; the kva nameplate rating product of the incremental cost of energy generation per of the transformer; the installed cost per kvar of primary kwh, the number of hours per year, and the loss factor. shunt capacitors; and the fixed charge rate on capital Since copper loss is maximum at peak load, whereas investment : core loss is constant and often considered as a "base C.= (I..) (kva) (Cc) (r) (23) load", it is often necessary to use a higher unit cost per where: kwh for generation of copper losses. The total annual C.,, = the annual cost of exciting current in dollars cost of copper loss is then: I. = per-unit exciting current of the transformer C.= (Sr+ C p_kwhhf)P2love. (25) kva = the kva nameplate rating of the transformer where: C.= installed cost per kvar of primary shunt capa- S = system investment per kva of load in dollars citors per kva

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r = rate of fixed charge in per unit Sumner, R. A. Zimmerman. Westinghouse Engineer, vol. 15, no. 5, September, 1955, pp. 156-159. Cp-kwh = incremental cost at peak of energy genera- 3. ASA Standards for Distribution and Power Transformers. tion per kwh ASA C57 Series. 4. EEI-NEMA Standards for Distribution Transformers, Over- h= number of hours per year head Type—Sixth Report. EEI Pub. No. 55-14, NEMA Pub. f =loss factor in per unit No. TR2-1956, March, 1956. 5. American Standard Terminal Markings for Electric Appa- P =peak load on the transformer in per unit ratus. ASA C6.1-1956, May 11, 1956. 6. Electrical Transmission and Distribution Reference Book Irw.,,= transformer copper loss at rated load and kw (book), Central Station Engineers. Westinghouse Electric If copper losses of a group of distribution transform- Corporation, East Pittsburgh, Pa., 1950. 7. Ibid., Chapter 5, "Power Transformers and Reactors." ers were being evaluated, the fact that the peak loads 8. Ibid., Chapter 2, "Symmetrical Components." on the individual transformers do not occur simultane- 9. Criteria for Neutral Stability of Wye-Grounded Primary ously must be considered. Hence, the peak system de- Broken-Delta Secondary Transformer Circuits, H. S. Shott, mand due to the copper losses will not be eqiial to the H. A. Peterson. AIEE Transactions, vol. 60, 1941, pp. 997- 1002. sum of the individual transformer peak losses, but it will 10. ASA Guide for Loading Oil-Immersed Distribution and be the peak loss times a "coincidence factor". This is the Power Transformers. ASA C57.92, January, 1956. ratio of the actual system peak demand to the sum of 11. Life Expectancy of Oil-Immersed Insulation Structures, W. the individual peak demands. For this case, Equation A. Sumner, G. M. Stein, A. M. Lockie. AIEE Transactions, (25) would be as follows: vol. 72, pt. III, 1953, pp. 924-930. 12. Functional Life-Expectancy Tests for Liquid Filled Trans- C.,i= (Srfo-FC,k,,hhf)nP2(kwe.) (25a) formers, A. M. Lockie. AIEE Conference Paper, Winter where: General Meeting, New York, N. Y., 1955. 13. Loading Transformers by Temperature, V. M. Montsinger. f„= coincidence factor in per unit AIEE Transactions, vol. 58, 1939, pp. 776-792. n =number of distribution transformers 14. Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon, T. W. Dakin. AIEE Transactions, vol. 67, 1948, pp. 113-122. 15. Thermal Aging Properties of Cellulose Insulating Materials, XI. REFERENCES G. Malmlow. ACTA Polytechnica, Electrical Engineering Series, vol. 2, 1948, Stockholm, Sweden. 1. NEMA Standards for Transformers. NEMA Pub. No. TR1- 16. Field Tests. 1954, May, 1954. 17. Economic Loading of Distribution Transformers, A. M. 2. Low-Cost Underground Residential Distribution, W. A. Lockie, H. W. Book, Electric Light and Power, April 15, 1958.

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