HowStep-VoltageRegulatorsOperate

McGraw-Edison® 32-step Voltage Regulator Auto-Booster® 4-step Voltage Regulator

How Step Voltage Regulators Operate

Figure 1.Transformer with 10:1 turns ratio.

Figure 2.Step-up autotransformer.

Figure 3.Step-down autotransformer.

Figure 4.Connection of 32-step and 4-stepregulators.

To measure the quality of electric power, con-sumers evaluate service continuity and volt-age regulation. Heavy air-conditioning andheating loads plus the ever increasingdemand for electric energy can tax electricsystems beyond acceptable limits. Since allelectrical equipment is designed for use with-in narrow limits, poor voltage conditions canresult in undesirable and unacceptableequipment performance such as distorted tvreception, flickering lights and/or burned outmotors.

Step-voltage regulators on utility distribu-tion systems deliver dependable voltage lev-els, to meet customer demands for improvedvoltage control.

OPERATION THEORYA voltage regulator holds line voltage withinpredetermined limits and assures the properoperation of lights, appliances, and motors.To understand how a regulator operates, onemust first understand how a two-windingtransformer operates.

Figure 1 is a basic diagram of a trans-former with a 10:1 turns ratio. If the primarywinding has 1000 volts applied, the sec-ondary winding will have an output of 100volts. These two independent windings canbe connected so that their voltages may aidor oppose one another. A voltmeter connect-ed across the output terminals will measureeither the sum of the two voltages or the dif-ference between them. The transformerbecomes an auto-transformer with the abilityto raise (Figure 2) or lower (Figure 3) the pri-mary or system voltage by 10%. Since thelow-voltage winding is operated at the systemvoltage level, adequate insulation to groundmust be incorporated.

Figure 5.McGraw-Edison® 32-step voltage regulator.

Figure 6.Auto-Booster® 4-step voltage regulator.

Cooper Power Systems offers the conven-tional McGraw-Edison® 32-step voltage reg-ulator, and the Auto-Booster® four-step regu-lator.The basic operation is the same for both32-step regulators (which use a reactor forswitching) and the 4-step regulator (whichuses resistors for switching.

There are two types of step voltage regu-lator construction which are defined by theAmerican National Standards Institute(ANSI). These are ANSI type “A” and “B.”These are also known as “straight” (type A)and “inverted” (type B) designs.

The internal electrical construction ofthese regulators differ; however, the externalconstruction, connection to the utility electri-cal system, and use is the same.

All Auto-Booster® 4 step voltage regula-tors are “type B” construction. For simplicitythe following will discuss only that type.

Type B 32-step and 4-step regulators areconnected as shown in Figure 4. This

connection places the low-voltage winding onthe source side of the high-voltage winding.An interrelationship between the two wind-ings still exists; however, it is the reverse ofthat shown in Figures 2 and 3. The voltageincrease or decrease takes place ahead ofthe high-voltage winding; therefore, the volt-age measured across the high-voltage wind-ing will be either 900 or 1100 volts. Whenreferring to a regulator, the low-voltage wind-ing is called the series winding; the high-volt-age winding is called the shunt winding(Figures 5 and 6).

For greater regulator versatility, taps maybe added to the series winding. A controlwinding completes the transition, sensing theload voltage and supplying this intelligence tothe control of an automatic tap changer.

To Bridge or Not to BridgeThe Auto-Booster voltage regulator stepswith no bridging position, while the McGraw-Edison 32-step regulators include the bridg-ing position. Bridging positions decrease voltage steps, improving system voltage lessnoticeably. A center-tapped reactor connectstwo movable contacts that are spaced so thatone is always on a stationary contact.

Figure 7 shows the two movable contactson the same stationary contact. With bothmovable contacts at the same position, thecenter tap of the bridging reactor is at thesame potential.

Figure 8 shows a bridging position withone movable contact on tap 2, the other ontap 3. The bridging reactor limits the circulat-ing current caused by the two contacts beingat different positions. This difference causesthe voltage change to be half the 1 1/4% tapvoltage of the series winding at the centertap.

Some regulators, depending on the rating,use an equalizer winding to improve tapchanger contact life. An equalizer winding isa 5/8% winding on the same magnetic circuitas the shunt and series windings. The equal-izer winding is connected into the power cir-cuit so that the reactor is excited at 5/8% ofline voltage on both even and odd positions.Figure 9 shows an equalizer winding incor-porated into the main coil of a regulator.

Without an equalizer winding, the reactoron a regulator is not energized on even posi-tions and energized at 1 1/4% of line voltageon odd positions. When an equalizer windingis used, the reactor is designed so the circu-lating current is maintained at 50% of rated

load current when 5/8% of line voltage isapplied. Thus, both the interrupted voltageand the interrupted kVA have been halved,reducing the tap changer interruption duty.

The Auto-Booster 4-step voltage regulatortaps directly from one tap position to the nextwithout a stop at the bridging position (Figure10). For the 10% voltage regulator shown,every time a tap change occurs, a 2 1/2%voltage change is made. The 6% voltage reg-ulator provides a 1 1/2% voltage change.Although the bridging position is not availableas a steady-state condition, circuit continuityduring switching can only be obtained if thebridging position is used during the transitionperiod. Bridging resistors prevent excessivelyhigh circulating currents from flowing duringthe switching interval. Bridging resistors havebeen used extensively in Europe. The mainadvantages are simplicity and small size.

Line-Drop Compensation (LDC)A major difference between the conventional32-step voltage regulator and the 4-step volt-age regulator is the incorporation of line-dropcompensation (LDC). This control feature—available with the 32-step voltage regulatoronly—allows a constant voltage to be main-tained at a load center remote from the regu-lator.

A resistive and a reactive element in thesensing circuit of the control is set to simulatethe resistance and reactance of the line fromthe regulator to the load center. A sampling ofthe load current current and regulated sys-tem voltage is applied to the resistive andreactive elements. A voltage drop proportion-al to the load on the actual system isobtained across the elements. Because theresistive and reactive elements are in serieswith the voltage sensing circuit and connect-ed across the load side of the regulator, theadditional drop in the LDC circuit reduces thevoltage sensed by the sensing circuit. Thisvoltage reduction causes the control to callfor additional regulator raise operations. Theamount of voltage increase at the regulator isa direct function of the load. This circuit isshown in Figure 11.

The general criteria for setting LDC arebased on whether the regulator location isremote from the load center and whether theload varies greatly during a 24-hour period.

It is difficult to establish exact distance orload variations that would dictate the use ofLDC; however, it is important to understandthe distribution system, and then meter thevoltage variation at the load center. An alter-native approach is to place the regulator at ornear the load center.

Auto-Booster® 4-step voltage regulatorsshould be applied on laterals or circuitswhere frequent switching is not required.They are not recommended for use in sub-stations. Use on laterals is recommendedwhen 32-step voltage regulators are used onmain feeders and no special coordination isrequired. The life of the tap changer dependslargely on the frequency of switching and thedegree of loading.

Figure 7.Two movable contacts on the samestationary contact; center tap ofthe reactor is at the same potential.

Figure 9.An equalizer winding incorporatedinto the main coil of a regulator.

Figure 10.The Auto-Booster® 4-step voltageregulator goes directly from onetap position to the next, without astop at the bridging position.

Figure 8.Two movable contacts on differenttaps; voltage change is one-half the1 1/4% tap voltage of the serieswinding because of its center tap.

Figure 11.A typical line-drop compensation(LDC) circuit.

TYPES OF CIRCUITSThe following types of circuits can be regulated;• A single-phase circuit (Figure 12);• One phase of a three-phase wye or delta

circuit (Figure 13);• A three-phase, three-wire wye or delta

circuit (Figures 14 and 15);*• A three-phase, four-wire, multigrounded

wye circuit (Figure 16).

* Because of possible neutral shift withresulting overstressing of insulation andcontinual hunting, three regulators cannotbe connected in ungrounded wye on a three-phase, three-wire circuit.

Wye-connected regulators (Figure 17) workindependently; delta-connected units areinterrelated. Voltage improvement dependson the range of regulation and the voltage onone phase of the system.

Figure 18 shows an open-delta connec-tion and the interrelationship between phas-es. A voltage improvement of 10% in thephase with the regulator connected causes a5% voltage improvement in the phase withouta regulator. When both regulators provide a5% voltage improvement to the third phase,all three phases are regulated to 10%.

Figure 12.Regulating a single-phase circuit.

Figure 13.Regulating one phase of a three-phase, three-wire circuit.

Figure 14.Regulating a three-phase, three-wire wye or delta circuit with tworegulators.

Figure 15.Regulating a three-phase, three-wire wye or delta circuit with three regulators.

Figure 16.Regulating a three-phase, four-wire, multigrounded wye circuit with threeregulators.

Figure 17.Wye-connected regulators operateindependently.

Figure 18.Open-delta connection has interre-lated phases.

The closed-delta connection (Figure 19)causes a 10% voltage improvement in the con-nected, phase and a 5% improvement in theadjacent phase. As all three phases have regu-lators connected (Figure 20), the overall effect isto increase the range of regulation to 15%.

DETERMINING REQUIRED REGULATOR TYPE AND SIZEThe circuit determines the type of regulatorrequired. The circuit voltage and kVA ratings andthe required amount of voltage correction deter-mine the regulator size.1. Example

To regulate a three-phase, four-wire, multi-grounded wye circuit with a system voltage of7620/13200 wye and a 1250 kVA connectedload that requires a 10% voltage correction:A. Multigrounded wye circuit indicates

grounded-wye regulator connection.B. Calculation to determine the size of the

units:Rated load in amps

= three-phase kVA x 1000line-to-line volts x 1.732

= 1250 x 100013200 x 1.732

= 54.6 amps

C. Range in kV:

= range x line-to-neutral kV

=0.1 x7.62

= 0.762 kV

D. Regulator rated kVA

= load amps x range in kV

= 54.6 x 0.762

= 41.7 kVA

With the selection of a standard rating, 32-step regulators rated 57.2 kVA, 7620 volts, 75amps, plus or minus 10% regulation meet theapplication requirements. Three regulatorsconnected line to neutral are required.

2. ExampleTo regulate a three-phase, three-wire circuitwith a system voltage of 13800 volts and aconnected load of 1250 kVA that requires a10% voltage correction:A. Three-wire wye or delta circuit.B. Calculation to determine the size of theunits:

Rated load in amps

= three-phase kVA x 1000 line-line volts x 1.732

= 1250 x 100013800 x 1.732

= 52.3 amps

C. Range in kV

range x line-line kV

= 0.1 x13.8

= 1.38 kV

D. Regulator rated kVA

= load amps x range in kV= 52.3 x 1.38= 72.2 kVA

Standard rating, 32-step regulators rated138 kva, 13800 volts, 100 amps, plus or minus10% regulation meet this requirement. Two reg-ulators connected line to line in open-delta con-nection are required.

REDUCED RANGE OF REGULATIONIn many applications, the full 10% regulation isnot required. The range of regulation may bereduced by simply adjusting the tap-changerlimit switches in the position indicator. In thisway, increased current capacity is available anda smaller regulator can be used, with corre-sponding lower costs.

The increased current becomes availablebecause the losses for the combination ofreduced range of regulation and increased cur-rent capacity are not higher than the losses atfull range of regulation and 100% load.Increased current in the series winding is offsetby reduced current in the shunt winding. Also, ata reduced range of regulation, a part of theseries winding is unused. This unused section iseffective in dissipating the heat which resultsfrom the I2R losses in the current-carrying partof the series winding.

CAPACITOR BANKS IN THE SYSTEMA capacitor bank on a distribution system mayaffect regulator settings. Capacitors generallycorrect for voltage back to the source; regulatorscorrect for voltage on the load side. The follow-ing considerations can be made:1. If the capacitor bank is on the source side of

the regulator, the regulator sees the resultsof the capacitor bank; therefore, no changesare required in the control setting.

2. If the capacitor bank is connected beyondthe load center, the regulator and load cen-ter are affected equally by the bank; there-fore, no changes are required in the controlsetting.

3. The capacitor bank may be connectedbetween the regulator and the load center.

NOTE: This application is not recommend-ed.

In this case, the voltage level or compensationsettings must be adjusted.

A. For a fixed capacitor bank, increase thevoltage level setting by the amount of volt-age increase at the regulator over that atthe load.

B. For a switched bank, increase the voltage-level setting by one-half the actualincrease produced by the capacitor bank.

Figure 19.Closed-delta connection regulators.

Figure 20.One regulator of closed-delta con-nectin shows range of regulatorincreased to ± 15%.

Bulletin 77006 February 1993Supersedes October 1991File Ref: Catalog 225© 1993 Cooper Power Systems, Inc.

McGraw-Edison® and Auto-Booster® are registered trademarks of Cooper Industries.

P.O. Box 1640 Waukesha, WI 53187