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System Voltage Regulation CHAPTER 7 SYSTEM VOLTAGE REGULATION H. E. LOKAY The primary objective of system voltage control is to system are discussed, as well as the characteristics of economically provide to each power user voltage that each application. The equipment is discussed by de- conforms to the voltage design limitations of the utiliza- scribing its method of operation and how it affects an tion equipment. Almost all utilization equipment is de- application. signed for use at a particular, definite terminal voltage: I. DEFINITIONS the nameplate voltage. It is economically impossible to provide each and every consumer on a distribution In discussing system voltage control, certain terminol- system with a constant utilization voltage correspond- ogy is naturally used. Following are common terms and ing to the nameplate voltage of the utilization devices. definitions used throughout this chapter. Other terms Voltage drop exists in each part of the power system that refer only to a particular section of this chapter are from the source to the consumer's service drop. Voltage defined in that particular section. drop also occurs in his interior wiring. Voltage drop Voltage drop—"Voltage drop (in a supply system) is is proportional to the magnitude and phase angle the difference between the voltage at the transmitting of the load current flowing through the entire power and receiving ends of a feeder, main, or service."' system. This essentially means that the consumer elec- The voltage drop is not necessarily the impedance trically closest to the source would receive a higher drop (IZ) of a feeder, main or service, but the difference voltage than the consumer most remote from the source. in absolute magnitude of the sending and receiving end Each of the consumers has practically the same utili- voltage. The IZ drop when added vectorily to the re- zation devices; therefore it is necessary to provide each ceiving end voltage equals the sending end voltage. with almost equal supply voltage. A compromise has Maximum voltage—"Maximum voltage is the great- been required between the allowable deviation from est five-minute average or mean voltage."2 utilization equipment nameplate voltage that is sup- Minimum voltage—"Minimum voltage is the least plied by the power system and the deviation above and five-minute average or mean voltage."2 below the nameplate voltage at which satisfactory Voltage spread—"Voltage spread is the difference equipment performance can still be obtained. If too between maximum and minimum voltages."3 broad limits were maintained by the power companies, With regard to power systems, voltage spread is the the cost for appliances and equipment would be high, difference between maximum and minimum voltage because they would have to be designed to operate which exist in a single voltage class under steady-state satisfactorily at any voltage within the limits. If the conditions. Voltage spread does not generally include voltage limits maintained were too narrow, the cost of the momentary voltage changes resulting from motor providing power would become too high. The com- starting—frequently called voltage dip. Voltage spreads promise that has resulted has proved satisfactory over at certain locations within a power system are discussed the past years, although it must be re-evaluated from further in Section 1. time to time as new utilization equipment or appliances Nominal voltage—"The nominal voltage of a circuit are made available. The widespread acceptance and or system is a nominal value assigned to a circuit or high saturation of television has resulted in a check of system of a given voltage class, for the purpose of con- the permissible limits previously held. The operating venient designation."2 voltage limits as determined by the compromise are The nominal voltage is intended to be a common listed in Table 1. Further discussion of the voltage designation for all systems whose operating voltages lie limits in the tables is in Sections 2-4. within the same general class. The value lies, in gen- System voltage regulation is essentially no more than eral, a little above the midpoint of the band of voltages maintaining the voltage at the consumer's service en- which fall within the same general class. trance within permissible limits by the use of voltage Rated voltage—"The rated voltage is the voltage at control equipment at strategic locations within the which operating and performance characteristics of system. This chapter discusses the application of this equipment are referred."' voltage control equipment within the distribution sys- The rated voltage of equipment is normally the name- tem. Application of voltage control equipment (regu- plate voltage • and the voltage at which optimum per- lating transformers, synchronous condensers, etc.) ap- formance would be realized. plied in the transmission system either for voltage Service voltage—"Service voltage is the voltage control or load flow is not included. The various measured at the terminals of the service entrance methods of controlling voltage in the distribution equipment."2 247 Gridco, Inc. v. Varentec, Inc. IPR2017-01134 GRIDCO 1004 Part 3 of 5 - 255/576 248 System Voltage Regulation The terminals of the service entrance equipment is 11. UTILIZATION VOLTAGES essentially the last point of the distribution system. The service entrance equipment is normally owned by the Utilization voltages—the voltage as measured at the consumer. This service point is not necessarily the kwhr terminals of the utilization equipment or at the con- meter socket, although for rural distribution systems it venience outlets—have a voltage spread which is inherent is often considered as that. to distribution system operation. A voltage band occurs Utilization voltage—"The utilization voltage is the at each point of utilization. The width of the band and voltage measured at the terminals of the machine or the location of the band with respect to the base voltage device."2 depends upon the location of the consumer with respect It is the voltage at any convenience outlet where an to the physical layout of the distribution system. Before appliance or device would be connected, or the voltage going into the acceptable voltage variations at the point at the terminals of large permanently located equip- of utilization for proper equipment operation and how ment. The utilization voltage is not to be confused with the variations affect equipment performance, the con- the service voltage, as it is a voltage less than the service cept of voltage spread should be clearly understood. voltage by the amount of the consumer's interior wiring voltage drop to the point of utilization. 1. Voltage Spread Base voltage—"Base voltage is a reference value Voltage spread—the difference between maximum and which is a common denominator to the nominal voltage minimum voltage at a particular point in the distribu- ratings of transmission and distribution lines, transmis- tion system—will vary in magnitude depending upon sion and distribution equipment, and utilization equip- the particular location within the system where the ment."4 spread is measured. Not only will it vary in magnitude, For example, the base voltage of a subtransmission but the relation of spread with reference to the base line having a nominal voltage rating of 34.5 kv is 115 value will vary depending on the point of measurement. volts; the base voltage of a distribution line having a An illustration of the voltage spreads occurring at the nominal voltage rating of 4.8 kv is 120 volts. A survey3 utilization point is shown in Fig. 1. Consumer A, which of the existing operating power companies showed that is the first consumer served by the feeder, has a voltage distribution lines and associated equipment having a spread of just one volt when going from light-load nominal voltage rating of from 2.4 kv to 14.4 kv have a (123v.) to heavy-load (122v.) conditions. Consumer B, 120-volt base while subtransmission or transmission which is the last consumer served by the distribution lines and associated equipment having a nominal volt- feeder, has a voltage spread of seven volts: 111 volts at age rating of from 23 kv to 230 kv have a 115-volt base. Voltage regulation—Voltage regulation is the per cent sues/yawl BUS VOLTAGE voltage drop of a line with reference to the receiving rC REGULATOR PRIMARY FEEDER end voltage. W:1841M 100(14 HE'D where, Per cent regulation — SECONDARY 4 l: 4 1Er1 SERVICE DISTRIBUTION FEEDER ONE-LINE DIAGRAM E. = sending end voltage. 13 PRIMARY = receiving end voltage. 12 DISTRIBUTION 171ANSPOWER Voltage drop in any system component of a distribu- O ISO $225V FIRST tion system is often referred to as per cent voltage drop. CUSTOMER Normally the per cent value would be the same as cal- ,J$ 14 SERVICE B culated using Equation (1), but when referring to per HOUSE WIRING g cent voltage drop of the various system components in I ID III V distribution systems, they are all referred to the same VOLTAGE PROFILE AT LAST HEAVY LOAD CONDITIONS CUSTOMER base. Systems which have a base voltage of 120 volts (ILLUSTRATIVE ONLY) use this value as the per cent reference, and systems with a base voltage of 115 volts use that value. For example, a distribution feeder with a three per cent voltage drop would have an absolute voltage drop of 3.6 volts (3 X120/100) on a 120-volt base. If the feeder voltage was actually 2400 volts, the absolute voltage drop would be 72 volts (3.6 X 2400/120). Equation (1) would produce the same results only if the receiving end VOLTAGE PROFILE AT voltage is the base voltage. That is, if 'Ed is 120 volts LIGHT LOAD CONDITIONS for the 3.6 volts drop or 2400 volts for the 72 volts drop.
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