Controlling Over Voltages Produced by Shunt Capacitor Bank Switching

8/2/2019 Controlling Over Voltages Produced by Shunt Capacitor Bank Switching

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International Conference on Large High Voltage Electric Systems112, boulevard Haussmann - 75008 Paris

1988 Session - 28th August - 3rd September

EVALUATIONOF METHODS FORCONTROLLING THE OVERVOLTAGESPRODUCED BY THE ENERGEATIONOF A SHUNT CAPACITOR BANK

R. P. OLEARY, R. H. HARNERS&C Electric Company(United States)

ABSTRACTA computer study was performed to evaluate theswitching-surge voltages associated with the energiza-tion of shunt capacitor banks. Various methods ofcontrolling these switching-surge voltages are studiedand a comparative analysis is made. The methodsstudied are the use of fixed inductors, pre-insertion(closing) resistors and inductors, and synchronousclosing of the switching device. The investigationemphasizes overvoltages generated at a remote, radiallyfed transformer terminal, rather than the overvoltagesproduced at the local substation bus. The influence ofvarious system parameters on the overvoltages areevaluated. A pre-insertion inductor is shown to haveadvantages over other control methods.KEY WORDS:Switching - vervoltage - hunt capacitor banks -closing impedance - nductors - esistors- ynchro-nous closing.1 O INTRODUCTIONFixed inductors or pre-insertion (closing) resistors havebeen applied by the electric utility industry for manyyears to control switching transients resulting from theenergization of shunt capacitor banks. These methodshave also been used to control interference in instru-mentation circuits resulting from the inrush currentsassociated with the energization of back-to-backcapacitor banks. The nature of these inrush currentsand the overvoltages associated with capacitor switchingoperations has been studied in the literature (1, 2, 3,4). The overvoltages produced at a remote radially fedtransformer terminal have recently been shown to beof concern (5 , 6) . As a result, it is believed that theswitching-surge voltage-withstand capabilities of powertransformers may be inadequate. The emphasis withinindustry standards has been to ensure an adequatetransformer phase-to-ground switching-surge voltage-withstand capability. Phase-to-phase switching-surgevoltage-withstand values are not defined in present* S & C E l e c t r i c Company, 6601 N . Ridge Blvd, Chicago

standards at voltages of 230 kV and below, and perhapsare not adequate at the higher voltages. There has beenan increasing interest within the industry to determineunder what conditions severe overvoltages might begenerated and what means can be used to control theseovervoltages.

It is the intent of this paper to study variousswitching-surge overvoltage-control methods. Theavailable literature does suggest that the use of pre-insertion resistors is an effective means of controllingthese overvoltages (5 , 6) . Another method suggested isthe synchronous closing of the switching device contacts(7, 8, 9). Pre-insertion resistors have been used on bothhigh-voltage switches and circuit breakers. A numberof utilities have used fixed inductors, but the objectivein these instances has primarily been to control inrushcurrents associated with energizing back-to-backcapacitor banks. Typically the value of these fixedinductors has been on the order of hundreds of mic-rohenries. It has not been recognized that the inductormay also provide a more effective control of switching-surge overvoltages.Extensive computer studies have been performed toillustrate the comparative performance of the availablecontrol methods. The conditions producing the worst-case overvoltages are examined initially, followed by anexpanded study to determine the influence of systemconfigurations and various system parameters. Theadvantages and disadvantages of the various control

methods are reviewed and, finally, the design alterna-tives of pre-insertion impedances are summarized.2.0 GENERAL SYSTEM STUDYThe intent of this study is to compare, on a worst-casebasis, the overvoltages experienced when energizing acapacitor bank with and without a controlling means.The worst-case method was chosen to most clearlypresent and promote a fundamental understanding ofthe overvoltage phenomena. In that worst-case linelengths will not, in all probability, be encountered inI l l 6 0 6 2 6 , United S t a t e s .

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the field; the switching voltages that will actually appearwill very likely be less than those suggested by thisstudy. A simplified computer model, Figure 1, wasdeveloped using only the elements necessary to dem-onstrate the overvoltage phenomena. The capacitor-bank size, length of transmission line, and available faultcurrent, as well as the closing sequence of the switch,were all selected to maximize the switching overvoltagesat the remote terminal. Phase-to-ground surge arresterswere not modeled. Phase-to-ground surge arrestersapplied at the terminal of a transformer could limitphase-to-ground overvoltages to 2.2 per unit. Even withthe use of phase-to-ground surge arresters, phase-to-phase voltage could be as high as 4.4 per unit (5).

(20 kA Available)CapadtorBUSTransmission RemoteBus

Source ImpedanceASource(138 kV)

Swltchf1 7 5 A RCapacitorBank

rl-L

Figure 1. 138 kV system model used lor computer study 01overvoltages producedby capacltor-bank energizations.

Upon energizing an isolated capacitor bank (a singlecapacitor bank without other energized capacitor bankson the bus), moderate inrush-current transients andsevere voltage transients can be generated. Whenenergizing a capacitor bank with other energizedcapacitor banks on the bus (back-to-back switching),current transients can be more severe; yet, the voltagetransients are mitigated to some extent by the supportgiven the bus voltage by the energized capacitor banks.Since this study is concerned with the generation ofovervoltages, t he only cases considered are thoseinvolving an isolated capacitor bank being energized.When pre-insertion impedances are used to controlinrush currents, there are two transient periods. Thefirst occurs when the capacitor bank is initiallyenergized through the pre-insertion impedance. Thesecond occurs when the pre-insertion impedance is

removed from the circuit. The voltage transientsassociated with the initial energization of a capacitorbank through a pre-insertion impedance are muchgreater than the voltage transients associated with theshorting out of the pre-insertion impedance. The firstvoltage transient is driven by full system voltage, whilethe second transient voltage is driven only by the voltagedrop across the pre-insertion impedance, typically onthe order of 10% to 40% of full system voltage. Thecurrent transients associated with both the initialenergization of the capacitor bank and the removal ofthe pre-insertion impedance can be significant.

If the capacitor bank neutral is grounded in agrounded supply system, all three phases can actindependently. Thus, simultaneous closing of twophases should be highly unlikely. It has been observed,however, that with closely coupled phases on a groundedsystem, the transients experienced when one phase isenergized may induce a second phase to prestrike,thereby creating a simultaneous closing of two phases.If the capacitor bank or the supply system isungrounded, a simultaneous energization of the first twophases will occur. Under the assumption that the firsttwo phases close simultaneously, the overvoltagesassociated with the simultaneously closing phases arethe same for a grounded and an ungrounded capacitorbank. The computer studies for uncontrolled energiza-tion and for energization through pre-insertion impe-dances are applicable to grounded or ungrounded banks.For an energization with synchronous closing, theoptimal time for energizing the second phase differs forgrounded and ungrounded banks; therefore, bothconditions are studied.

Upon energizing a capacitor bank, the voltage at thecapacitor-bank bus will undergo a transient oscillation,having a frequency generally on the order of a fewhundred Hertz. The frequency of this transient oscil-lation is determined primarily by the system inductanceresonating with the capacitance of the bank. Overvol-tages at the remote transformer location are maximizedwhen the transmission line has a length such that theround-trip travel time for a switching surge is equal tothe time to peak of the transient oscillation. In the caseof the computer model illustrated, the critical lengthof transmission line is approximately 142 km.Th e phase-to-ground overvoltage at the remoteterminal will be maximized when the bank is energizedslightly before peak system voltage to allow the peakof transient overvoltage to occur at the peak of thesupply-system-frequency voltage. In the model, B phase

closes just before peak phase-to-ground voltage onB phase.The switching sequence which results in maximumphase-to-phase overvoltages at the remote terminalrequires two phases to close simultaneously at nearlyequal and opposite voltages (slightly before the peakof the phase-to-phase voltage between those phases).Simultaneous closing of two phases with oppositepolarity ensures that the transient overvoltages occurat the remote terminal simultaneously with oppositepolarities, thereby maximizing the voltage difference.In the model, A and C phases close simultaneously withequal, bu t opposite, phase-to-ground voltages.When analyzing the case of an uncontrolled ener-

gization, an energization through a pre-insertionresistance, or a synchronized closing, the peak overvol-tages are not significantly affected by the system load.This is primarily because the peak overvoltages in thesecases are associated with very steeply rising waveforms.The fast-changing transients cannot interact with loadswhich are located beyond the leakage impedances oftransformers. When dealing with a fixed inductor ora pre-insertion inductor, however, the effect of loadsbecomes significant. Transients generated when ener-gizing a capacitor bank through an inductor generallyhave moderately rising ramp voltages instead of fast-

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rising step voltages. There is a significant time-to-peakvalue of these ramp voltages such that some interactionoccurs with the loads connected to the system. Whencomparing pre-insertion inductors to pre-insertionresistors, it is therefore important to appropriatelymodel system load, particularly at the remote terminal.

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2.1 UNCONTROLLED ENERGIZATION(REFERENCE CASE)

Y -x Remote Buso Capacitor BusI I I I I , , , I , I 1 I , I I . l l l l l l l l l l -

2.1.1 Phase-to-ground overvoltagesThe phase-to-ground voltages at the capacitor-bank busduring an uncontrolled energization of the capacitorbank are shown in Figure 2. Energization of A and Cphases occurs simultaneously with equal and oppositevoltages at approximately 18 milliseconds on the timescale. Energization of B phase occurs approximately 3milliseconds later, just before the peak of B-phase line-to-ground voltage.

Time (ms)Figure 2. Voltages at the capacitor-bank bus during an uncon-trolled energization.

At the instant of capacitor-bank energization, the busvoltage abruptly falls to zero, since the capacitor bankinstantaneously appears as a very low impedance. Thisabrupt change of voltage injects a step-voltage wave intothe transmission lines connected to the capacitor-bankbus. A negative step-voltage wave is transmitted on Aphase, while a positive step-voltage wave is transmittedon C phase. After the initial drop to zero voltage, thephase voltages recover in a transient oscillatory fashion.The frequency of this transient is determined by thesource inductance and the capacitance of the bank.Damping of this transient is due primarily to the surgeimpedance of the transmission lines connected to thecapacitor-bank bus. Some addit ional damping maycome from system load connected at the capacitor-bankbus. Because this transient is under-damped, thetransient voltage overshoots the source voltage. In thiscase, C phase overshoots more than A phase becauseC-phase source voltage is increasing during the initialtransient, whereas A-phase source voltage is decreasingduring the initial transient.

Figure 3 shows an expansion of C-phase voltage atthe capacitor-bank bus during the time just before andafter capacitor-bank energization. The voltage on Cphase at the remote bus is also plotted. A-phase voltageresponds in a similar fashion, having an oppositepolarity and a somewhat lower magnitude of overvol-tage. The collapse in bus voltage on the C-phase remote

bus occurs approximately 480 microseconds after thecollapse of voltage at the capacitor-bank bus; this timedelay is the travel time of the step-voltage wave alongthe 142 km line to the remote bus. Note that the voltageat the remote bus does not simply collapse to zero, butrather swings to the opposite polarity of the bus voltageprior to the collapse. The steeply rising step wave seesthe remote bus and transformer as a very high surgeimpedance. Initially, therefore, the reflection coeffi-cient a t the remote bus is very nearly plus LO.A secondreflected wave of lesser magnitude but of the samepolarity as the incident wave is thus generated. The stepchange in voltage at the remote bus is the sum of theincident reflected waves, which is very nearly twice themagnitude of the initia l wave. The step overvoltage willdecay exponentially with a time constant determinedby the X/R of the load at the remote bus and the surgeimpedance of the line.

The second wave is transmitted back to the capacitor-bank bus. Since the surge impedance of the capacitorbank is very small and therefore looks like a short-circuit to such a steeply rising wave, the reflectioncoefficient is nearly minus 1.0. Thus, a third wave isgenerated and transmitted to the remote bus. This thirdwave is of opposite polarity to the original step wave.When the third wave reaches the remote bus, a fourthwave of approximately equal magnitude is generated.The third and fourth waves are the same polarity asthe transient voltage on C phase. The first and secondwaves have been attenuated due to the load at the remotebus. The net result is that the third and fourth wavesadd to the transient oscillation, thereby yielding anovervoltage to ground of 3. 5 per unit [1.0 per unit =( fi 4 6 ) rated system voltage].

2.1.2 Phase-to-phase overvoltageA-phase experiences a transient voltage similar to thatof C-phase, except for slightly lower magnitudes andan opposite polarity. A-phase line-to-ground voltageapproaches 3.0 per unit, but of opposite polarity toC-phase line-to-ground voltage. C-phase to A-phasevoltage therefore approaches 6.5 per unit, as shown inFigure 4.

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4 , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- 8 86 17 18 19 20 21 22 23Time (ms)

Figure 4. C-phase-A-phase voltage at the remote bus during anuncontrolled energizatlon.2.2 ENERGIZATION THROUGH APRE-INSERTION RESISTOR2.2.1 Phase-to-ground overvoltageFor this study , a 40-ohm pre-insertion resistor isinserted during the energization of the capacitor bank.In this case, bus voltage at the capacitor bank does notcollapse to zero. The extent to which the bus voltagecollapses depends upon the ratio of the resistance ofthe pre-insertion resistor to the resultant surge impe-dance of the transmission lines connected to thecapacitor-bank bus. Thre e transmission lines of approx-imately 380ohms each are connected in the bus, yieldingan effective surge impedance of 125 ohms. The capac-itor-bank bus voltage drops to a value determined bythe ratio of 40 ohms to the sum of 125 ohms plus 40ohms, or approximately 25% of the system voltage atthe time of energization. This reduction in the collapseof bus voltage manifests itself as a reduction in the step-voltage wave injected into the system. As shown inFigure 5, the pre-insertion resistor results in thecapacitor-bank transient oscillation being nearlycritically damped, so that very little overswing of thecapac itor-bank voltage occurs.

1.51 .oP' 0.5

9 -0.5-1 .o

hYg 0.08

- 1 . 5 j- 2 . 0 ? . r ~ . l r # ~ 9. . . . . . . . . . . . . . .

0 5 lb llfi io 2'5 i oTime (ms)

Figure 5. Voltages at the capacitor-bank bus when energizingthrough a 40 ohm pre-insertion resistor.During the transient period, there are small discon-tinuities in the bus voltage at the capacitor bank. Thesediscontinuities occur because traveling waves returningfrom the remote bus will see the 4O-ohm pre-insertionresistor rather than the very low surge impedance ofthe capacitor bank. The reflection coefficient at thisbus is approximately minus 0.8, resulting in less-than-perfect cancellation of the incident wave.With the pre-insertion resistor, the initial collapseof voltage at the remote bus, Figure 6 , transmits a step

wave of approximately 75% of the magnitude of thereference case, Figure 3, for an uncontrolled energi-zation. This results in a voltage doubling at the remotebus of a smaller wave and therefore the extent to whichthe voltage collapses to an opposite polarity is muchsmaller. As in the case of an uncontrolled energization,a second wave is generated and transmitted toward thecapacitor-bank bus. Upon reaching this bus, the voltagewave is reduced somewhat in magnitude due to theeffect of the $0-ohm pre-insertion resistor, and it is alsochanged in polarity and transmitted as a third wavetoward the remote bus. The third wave is again doubledupon reaching the remote bus. It is important to note,however, that with the pre-insertion resistor the initialstep wave has been reduced due to the effect of theresistor, and subsequently the third wave has also beenreduced. Further, the transient oscillatory voltage hasbeen considerably reduced due to the damping effectof the resistor. The result is that the peak C-phase line-to-ground voltage is reduced to 2.2 per unit, approx-imately 63% of the voltage experienced with an uncon-trolled energization.

2~:s -2-3j x Remote Bus 1-4j o Copocitor us

1 ~ ' " 1 ~ ~ ~ ' I ~ ~ ' ' I " ~ ' I ~ ' " I ' ~ ~16 17 18 19 20 21 22 23Time (ms)

Figure 6. C-phase line-to-ground voltages when energizingthrough a 40 ohm pre-insertion resistor.2.2.2 Phase-to-phase overvoltageThe effect of the pre-insertion resistor, as shown inFigure 7, has reduced the phase-to-phase overvoltageby approximately 38%, or from 6. 5 per unit to 4.0 perunit.

-"j- a ' . a r a . . . . . . . . . . . . . . . . . . . . . . , . I16 1b 1$ 2b 2'1 d2 23Time (ms)

Figure 7. C-phase-A-phase voltage at the remote bus whenenergizing through a 40 ohm pre-Insertion resistor.

2.3 ENERGIZATION THROUGH APRE-INSERTION INDUCTOR2.3.1 Phase-to-ground overvoltageFor this analysis, the energization of the capacitor bank

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occurs through a 10-mH pre-insertion inductor. As isthe case with the pre-insertion resistor, the extent towhich voltage collapses at the capacitor-bank bus isreduced, largely due to the surge impedance of the pre-insertion inductor, Figure 8. In fact, because theinductor has a very high surge impedance relative tothe surge impedance of the lines connected .to the bus,there is no abrupt step change in bus voltage. Rather,the voltage initially decays exponentially as determinedby an LR circuit comprised of the inductance of thepre-insertion inductor and the surge impedance of thelines connected to the capacitor-bank bus. As in thereference case, the capacitor-bank bus voltage recoversin an oscillatory fashion. Because the initial drop ofvoltage at the capacitor-bank bus is significantly lowerthan that of the reference case, the magnitude of thetransient oscillation voltage is reduced. Since the pre-insertion inductor has a relatively low resistance, thetransient oscillation is not significantly damped as itis with the pre-insertion resistor.

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1.51 .o2 0.5-vg) 0.0

90 -0.5

-1.0-1.5

-x Remote Bumo Capacitor BusI ~ ' " " ~ ' l ' ' ' ~ I ' ~ ' ' I ~ ~ ~ ~ I ~ ~ ~ ~

- 2 . 0 2 10 5 10 15 20 25 30Time (ms)

Figure 8. Voltages at the capacitor-bank bus when energizingthrough a 10 mH pre-insertion inductor.

As shown in Figure 9, the capacitor-bank bus voltagedoes not collapse abruptly, but falls at a moderate rate.Because the bus voltage falls at a moderate rate, a rampvoltage wave is transmitted down the line which, uponreaching the remote bus, does not double. At this lowerrate-of-change of voltage, the transformer at the remotebus acts like a high, but not infinite, impedance. Thus,the second voltage wave generated at the remote busis of a lower magnitude than that for either an uncon-trolled energization or for use of a pre-insertion resistor.Consequently, upon returning to the capacitor-bank

bus, the third voltage wave generated is of a substan-tially lower magnitude. The result is that C-phase line-to-ground voltage at the remote bus is approximately54 % of that for an uncontrolled energization, ascompared to 63%for the pre-insertion resistor.2.3.2 Phase-to-phase overvoltageWith the pre-insertion inductor, the phase-to-phaseovervoltage, Figure 10, between C and A phases isapproximately 3 .3 per unit or 51% of that for uncon-trolled energization, as compared to 62%for the pre-insertion resistor.

24-4

16 17 18 19 20 21 22 23Time (ms)

Figure 10. C-phase-A-phase voltage at the remote bus whenenergizing through a 10 mH pre-insertion inductor.

2.3.3 Consideration of rates of change of voltageat the transformer terminalThere is a significant benefit to the fact that theovervoltages produced by the use of a pre-insertioninductor are characterized by a ramp function ratherthan a step function as produced by an uncontrolledenergization or energization through a pre-insertionresistor. It has been suggested (10) that low-magnitude,steep-rising wave forms, appropriately timed, may causedamaging internal resonances in transformers. Also, fora steeply rising transient voltage wave, the voltagedistribution across a transformer winding wi l l beinitially determined by stray capacitances rather thanthe inductance of the winding, creating stress concen-trations in the first several turns of the winding (11).Even without high peak overvoltages, rapid changes involtage, with their associated stress concentrations, maybe harmful to transformers. The lower rate of changeof voltage produced by the pre-insertion inductor willallow the voltage to be distributed more evenly acrossthe initial turns of winding of the transformer.2.4 SYNCHRONOUS CLOSINGSynchronous energization of a capacitor bank can bean extremely effective means of controlling overvoltages(7, 8, 9). To accomplish synchronous closing at or neara voltage zero, thereby avoiding high prestrike voltages,i t is necessary to apply a switching device whichmaintains a dielectric strength sufficient to withstandsystem voltage until its contacts touch.Also, a highlyconsistent operating mechanism is required. Such idealclosing characteristics may be difficult to attain withpresent-day high-voltage switches and circuit breakers.Consequently, it has been suggested (9) that, if theswitching device has a closing consistency within kmilliseconds, overvoltage magnitudes will be limited toacceptable values.

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There is a fundamental difference between groundedand ungrounded systems when applying controlledclosing. In a grounded system, closing of each phaseshould occ ur at a phase-to-ground voltage zero,assuming an uncharged capacitor bank is being ener-gized. In an ungrounded system, energization of the firstphase can occur at random. The second phase shouldbe closed when the phase-to-phase voltage between thesecond phase and the first phase is zero, which occurswhen both phase-to-ground voltages are of the samepolarity and have a magnitude of one-half per unit. T hethird phase is then closed when its phase-to-groundvoltage is zero. The optimum time for energizing acapacitor bank, therefore, is different for an ungroundedsystem than for a grounded system. The overvoltageswhich result from errors in closing, i.e., not closing atthe ideal time, are also fundamentally different forgrounded systems and ungrounded systems.2.4.1 Phase-to-ground overvoltage-grounded banksAn ideal closing, as shown in Figure 11, occurs whenthe phase-to-ground voltages are zero for each phase.Under this condition, there is a slight distortion of thephase voltages at the capacitor-bank bus due to a modesttransient current; however, overvoltages are minimaland the rate of change of voltage is also very low.Overvoltages at the remote station are therefore alsominimal and consequently have not been illustrated.

Figure 12 is similar to Figure 11, except the closingtimes of each phase have been delayed by 2 ms,representing a maximum closing tolerance for con-2*.51o2 0.5nv

8 0.090 -0.5

-1 .o-1.5

0 5 10 15 20 25 30Time (ma)

Figure 11. Voltages at the capacitor-bank bus during an ener-gitation of a grounded bank by means of ideal synchronousclosing.

2 . 0 ~.51 o2 0.5

3 -0.5-1.0-1.5

nY8 0.09

0 5 10 15 20 25 30Time (ms)

trolled closing. The voltages shown are similar to thosedepicted in Figure 2, for an uncontrolled energization;however, there are two major differences. The peakovervoltages shown in Figure 12 are significantly less.Also, there has been no attempt to simulate simultane-ous closing on two phases, which could occur shouldone phase close early and another phase close late withinthe 2 ms tolerance.The peak phase-to-ground voltage at the remote bus,Figure 13, has been reduced from 3.5 per unit to 2.8

per unit, because energization occurred at less thanpeak phase-to-ground voltage.21

5 0ng -1s9 -2v

-3-4 16 17 18 19 20 21 22 23

Time (ms)Figure 13. C-phase line-lo-ground voltages during an energiza-tlon of a grounded bank with closing shifted 2 ms fromsynchronous.

2.4.2 Phase-to-phase overvoltage- roundedbanksThe peak overvoltage for this case is 3.9 per unit,Figure 14, versus 6.5 per unit for an uncontrolledenergization. Most of the reduction in phase-to-phasevoltage at the remote terminal has occurred becauseof the non-simultaneous closing of the two phases.

4 , . . , ~ , . , . ~ , , , , ~ . , , . ~J 4

3 0:nv -g -2 -$ -4-0 .e -

-8 16 17 18 19 20 21 22 23Time (ms)

Figure 14. C-phase-A-phase voltage at the remote bus duringan energizationof a grounded bank with closing shifted2 ms fromsynchronous.

Figure 12. Voltages at the capacitor-bank bus during an ener-gitation of 8 grounded bank with closing shifted 2 ms fromsynchronous.

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2.4.3 Phase-to-ground overvoltage-ungrounded banksIn Figure 15, A phase has been closed at a time ofapproximately 5 ms. C phase closes at approximately16 ms, or 2 ms after the ideal time for synchronousclosing. B phase closes at approximately 20 ms, or 2 msafter its voltage zero. For these assumptions, A phasecloses nearly at peak voltage, but unlike the conditionof an uncontrolled energization with a grounded bank,the collapse in A-phase voltage occurs only to the pointmidway between A-phase and C-phase voltages.

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Time (ms)Figure 15 . Voltages at the capacitor-bank bus during an ener-gization 01 an ungrounded bank with closing shifted 2 ms fromsynchronous.

A-phase line-to-ground voltages have been plotted inFigure 16because, for the particular switching sequencechosen, A-phase line-to-ground voltages are the highest.Peak overvoltage-to-ground of A phase at the remotebus is 2.7 per unit . Figure 16 closely resembles Figure3 of the reference case, except for the polarity differenceand a slightly smaller transient magnitude.

h3 2av$ 13 0e

-1 x Remote Bu so Capacitor Bu s

1 6 17 18 19 20 21 22 23lime (ms)

Figure 16. A-phase line-to-ground voltages during an energiza-lion of an ungrounded bank with closing shifted 2 ms fromsynchronous.

2.4.4 Phase-to-phase overvoltage-ungrounded banksFor an ungrounded bank, simultaneous energization oftwo phases is assured. Because of the simultaneousenergization of A and C phases, the step-voltage wavescan add at the remote terminal. The net result is a peakphase-to-phase overvoltage of 5.0 per unit, Figure 17,compared to 3.9 per unit for the sequential closing ofa grounded bank, as illustrated in Figure 14.

16 17 18 19 20 21 22 23Time (ms)

Flgure 17. C-phase-A-phase voltage at the remote bus duringan energization of an ungrounded bank with closing shifted 2 msfrom synchronous.

A closing tolerance of 2 ms, as used in this study,is at the limit of that suggested within the literature(9). It is likely that systems can be developed whichon the average will have less than a 2-ms error. Suchsystems will produce average overvoltages less thanthose described by this study; however, these systemsmay not have the capability of consistently controllingovervoltages to as reliable an extent as the use of pre-insertion impedances.

The phase-to-phase overvoltages for all cases studied,as shown in Figures 4, 7, 10, and 17, are consolidatedin Figure 18 for a convenient comparison.

16 17 18 19 20 21Time (ms)

Figure 18. Comparison of phase-to-phase overvoltages deve-loped at a remote radially led transformer using various controlmeans, during the energization of a shunt capacitor bank.

3.0 EXTENSION OF RESULTS TO OTHERSYSTEM CONFIGURATIONSThe simplified model shown in Figure 1 was chosenfor illustrative purposes to establish in principle theperformance of pre-insertion impedances and synchro-nous closing compared to using no controlling means.This circuit was selected to produce worst-case over-voltages. Other system configurations will, of course,produce different results. As an aid in evaluating a givensystem for the production of overvoltages, a furtherstudy was made to illustrate the influence of a varietyof system parameters.

Figure 19 shows the relationship between length ofline to the remote bus and peak phase-to-phaseovervoltage. The criterion for highest overvoltages ismet by a transmission line length such that the round-trip travel time on that transmission line will equal the

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time-to-peak of the transient oscillation produced by therecovery of capacitor-bank bus voltage. This ensuresthat the third wave is generated at the capacitor-bankbus at the peak of the transient oscillation and con-sequently will arrive at the remote bus simultaneouslywith the peak of the transient oscillation. If thetransmission line were half of the length used in thismodel, then four travel times would have elapsed beforethe peak. of the transient oscillation occurs at thecapacitor-bank bus. Because it is the fifth wave whichtravels to the remote bus along with the peak of thetransient oscillation and not the third wave, the polarityof the step wave is opposite to that of the transientoscillation and therefore subtracts from it. Thus, a71-km transmission line represents a relative minimumof peak overvoltage versus line length. At a line lengthof 47 km, the seventh wave is generated at the peakof the transient oscillation and is of a polarity to addto the transient peak. Therefore, at 47 km, a relativemaximum of peak overvoltage versus line length occurs.The pattern of relative minima and maxima theoret-ically repeats as the line length is reduced; however,with the extra reflections and the damping which occursa t each reflection, the magnitudes of the maxima arereduced as the line length is reduced. In the limit, asthe transmission line length becomes very short, duringthe time to the peak of the transient oscillation voltage,many reflections can occur and due to the dampingat each reflection, the step or ramp part of the transientcan be damped to zero. In this situation, use of theinductor wil l result i n a greater net-peak low-frequencytransient voltage than if a pre-insertion resistor is used,due to the fact that the transient oscillation is notsignificantly damped by the inductor. As an example,refer to Figure 20 in which the transmission line lengthis reduced to 6.4 km. With pre-insertion impedancesthe step waves have been attenuated to near zero bythe time the transient oscillation reaches a peak. Thepre-insertion inductor circuit generates 2.3 per unitovervoltage versus 2.0 per unit for the pre-insertionresistor circuit. Note, however, that the initial stepchange in voltage for the resistor is substantially greaterthan that for the inductor. This rapid change ofapproximately 2.6 per unit voltage, associated with theresistor, may be harmful to the transformer insulation.Note, also, that for an uncontrolled energization, thepeak overvoltage is still relatively high at 3.6 per unit,suggesting that even for this relatively mild case, acontrol means would be desirable.

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1 +a With Pro-inwrtion Inductorl ' " ' l ' ' ~ ~ " ~ ' " ~ ~ ~ ' l ~ ' ' ~ l ' ~ ' '0 50 100 150 200 250 300

Line Length (krn)Figure 19. Effect of line length to remote bus on the phase-to-phase overvoltage at the remote bus for the system of Figure 1.

0o Energized Through a 10 mH inductorEnergized Through o 40 Ohm Red&or -

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. . . . . . . . . . . . . . . . . . . . . . .18.0 18.5 19.0 19.5 20.0Time (rns)Figure 20. Phase-to-phase voltages at the remote bus with a line

length of 6.4 km to remote bus for the system of Figure 1.

w Uncontrolled Energizotione-+ Wlth Pro-ineartion Reaidora-a Wlth Pro-ineartion Inductor

--

I I I I

Figures 21 and 22 show the relationship of peakphase-to-phase overvoltage to capacitor-bank size, andsystem available fault current, respectively.A smaller capacitor bank or higher available faultcurrent at the capacitor-bank bus would result in ahigher frequency of the transient oscillation such thatthe time-to-peak of this transient would be shorter. Thisshorter time-to-peak would require a shorter transmis-sion line to fit the maximum overvoltage criterion of

two travel times equaling the time to reach peaktransient overvoltage. Referring to Figure 21, for anuncontrolled energization, there is a relative minimumfor a capacitor-bank size of approximately 20 MVAR.- 7

7 '1 v-v Uncontrolled Energizotion1 M W ~ hro-insertion Resistor1 e-e With Pro-insertion Inductor0 20 40 60 80 100 120 140 160 180 200

Capacitor Bank Size (MVAR)l ' l ' l ' l ' l ' l ' l ' l ' l ~ l '

Figure 21 . Effect of capacitor bank size on phase-to-phaseovervoltage at the remote bus for the system of Figure 1.

8 I I I I

The reduction in capacitor-bank size has increased thefrequency of the transient oscillation to the point where

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it peaks in one-half the time that would elapse witha 75-MVAR capacitor bank. In this case, the third step-voltage wave is generated at a relative minimum of thetransient oscillation and is therefore out of phase withthat voltage. The same phenomenon can be seen inFigure 22, where a relative minimum of the phase-to-phase overvoltage without pre-insertion impedance willoccur at approximately an available fault current of80 kA.The effect of additional transmission lines, Figure 23,at the capacitor-bank bus was investigated by utilizingmodels having no additional lines (only the line to theremote bus), one, two, or six additional lines. Only verymoderate changes in the phase-to-phase overvoltage areexperienced when adjusting this parameter of the model,particularly when using pre-insertion impedance. Theeffect of adding lines is to change the parallel-dampingcharacteristic of the resonant LC circui t. When addinga line, the effective damping is increased, but notenough to effect a large change in the peak of thetransient oscillation.

-e. 2.0-I .* 1.0-o.o

8 1 r%

-v-v Uncontrolled Energlzation-++ With Pro-lnsertlon Resistor -M With Pro-lnsertlon InductorI - ' ' 1 ' I ' -

P-v Uncontrolled Energlzotlon0-0 With Pro-insertion Inductor

0 1 2 3 4 5 6Number of Lines ot th e Copacitor Bus

Figure 23. Effect of the number of transmission lines connectedto the capacltor bank bus on the phase-to-phase overvoltage atthe remote bus for the system of Figure 1.

Finally, the effect of system load is shown inFigure 24. The overvoltages at the transformer termi-nal, when using a pre-insertion resistor as the controlmeans, are not significantly affected by system load.The load does not significantly add to the dampingobtained from the resistor. When using a low-lossdevice, such as a pre-insertion inductor, or when usingno control means, adding load has a substantial effecton the damping of the transient oscillation.8 . 0 - ~ . I I I . I . , I I

p 5.08 3.04*o*i

4.0 APPLICATION CONSIDERATIONS FORTo varying degrees of efficiency, all control methodswill limit peak switching-surge voltages, both at thelocal capacitor-bank station and at a remote, radiallyfed station. Table I summarizes, on a relative basis,various aspects of the application of these methods. Thecosts of implementing a control means appear to be ofthe same order of magnitude, except for fixed inductors.Fixed inductors generally must be designed to carrynormal load current, to have a system BIL rating, andto withstand system short-time currents. As a result,the inductors are physically large, relatively expensive,and may require costly mounting structures. Further-more, the fixed inductor losses add to the cost ofutilizing the inductor. Because the pre-insertioninductor is only inserted for a few cycles, the normalcurrent, short-time current, and full BIL ratings arenot required. As a result, for a given inductance, a pre-insertion inductor may be made much lighter andsmaller than a fixed inductor. Because the pre-insertioninductor is so small and lightweight, it can be mounteddirectly on the switching device, thereby obviating theneed for a mounting structure and possibly additionalspace in the substation. Figures 25 and 26 illustrate theinstallation of pre-insertion inductors on SF6 high-voltage switches.

Pre-insertion resistors have the advantage of yieldingthe lowest phase-to-ground voltages at the capacitor-bank station, with moderate phase-to-ground and phase-to-phase voltages at remote, radially fed stations. Thereare two principal disadvantages associated with pre-insertion resistors. The resistors are not effective inreducing the very high rate of change of voltageassociated with the energization of a capacitor bank.Additionally, pre-insertion resistors may have thermal-capability limitations. Typically, pre-insertion resistorsabsorb large amounts of energy with each energizationof a capacitor bank. Depending upon the type ofswitching device utilized, the energy capability of theresistor can limit the size of the capacitor bank whichcan be switched, as well as the frequency of switchingoperations.Synchronous closing has the potential of being anideal means of controlling overvoltages and inrushcurrents associated with energizing capacitor banks.With the constraints of practical switching devices, an

ideal synchronized closing of capacitor banks is unat-tainable. Performance of a controlled closing system,operating within an accuracy attainable with present-day technology, will allow a degree of control ofovervoltages similar to that obtainable with a pre-insertion resistor, without the thermal disadvantage ofthe pre-insertion resistor. The main disadvantage of acontrolled closing scheme may be its complexity. Sincethe ideal time to close is different for each of the threephases, the switching device must have three independ-ent closing mechanisms or an accurate mechanical delaybetween the three poles. The scheme also relies onaccurately monitoring voltages, both at the capacitor-bank bus and on the capacitor bank. The requirementsfor timing electronics and a highly reliable switchingdevice and operating mechanism are formidable. This

VARIOUS OVERVOLTAGE-CONTROL MEANS

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relatively complex systems approach, compared to arelatively simple pre-insertion impedance, suggests that the reliablility of controlled closing schemes may be lessthan that for a pre-insertion impedance control means.

Energlzation OvervoltageLocal Bu s Remote Bus Rate 01 Change ~ InstallationRequirements@ontrol Means Phase-Ground Phase-Phase of VoltagePUO PUO (Rise Time)@

None High (1.6) High (6.5) Very High ( 1 ) -

EstimatedRellablllty@RelatlveEstlmatedCor@- -

I Fixed Inductor 1 Moderate ( 1 4) 1 Low (3.3) 1 Low (100) I Difficult I Very High I Good to Excellent IIoderate Good to ExcellentIinimalIow (100)Ioderate (1.4) Low (3.3)re-InsertionInductor Ioderate Good to ExcellentIinimalIow (1 1 )re-InsertionResistorClosing@ (1 0-1.4) (1.7-5 0) (1 1 000) Moderate Fair to Goodontrolled None to High Low to Very High

@ Relarive phase-to-ground or phase-phaae overvoltages and per unirvalues in parenthese5 based on worst-case computer studies( 1 P tI = phase-ground peak voltage) .

@@cxperience.

Estimare of ease of installation of control means.Esrimare of costs and reliability based on literature and field

0 elative rare of change of overvoltage and an order of magnitude 0 Range o fin synchronous closing.ar e for ideal synchronous

to a 2-ms shiftof risc times i n microseconds within parentheses.

Pre-insertion .

Pre-insertion

Figure 25. A single 10 mH pre-insertion inductor mounted oneach pole of a 115 kV sF6 Circuit-Switcher. Figure 26. Two 10 mH pre-insertion inductors act in series ona 230 kV sF6 Circuit-Switcher (single-pole illustrated).

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5.0 DESIGN CONSIDERATIONS OFAs indicated in paragraph 2.0, in utilizing pre-insertionimpedances, there are two transient periods. Again, thefirst transient is associated with the initial energizationof the capacitor bank through the pre-insertion impe-dance, while the second transient occurs when theswitch which bypasses the impedance closes.

Forty-ohm pre-insertion resistors utilized in thecomputer study have been in service many years onhigh-voltage switches used to switch shunt capacitorbanks. The 40-ohm value for a 138-kV system voltagewas chosen to provide optimum control of inrushcur ren ts in back-to-back switching. A lower resistancewill cause an increase in the inrush current dur ing thefirst transient period referred to above. An increase inresistance beyond 40 ohms will increase the inrushcurrent dur ing the second transient period. Forty ohmsis an optimal value, yielding approximately the sameinrush current for both transient periods. Should theemphasis for the use of a pre-insertion resistor be placedsolely on the control of overvoltages, disregardinginrush currents, a higher resistance value would bemore appropriate.

An inductance of 10 millihenries was chosen for thepre-insertion inductor, such that it would yield approx-imately the same initial inrush current as that for a40-ohm pre-insertion resistor. Because the impedanceof the pre-insertion inductor is significantly lower atsupply system frequency, the inrush currents expe-rienced during the second transient period will besignificantly lower than the initial inrush currents.There is a possibility that the pre-insertion inductorcould be further optimized for both inrush-currentcontrol and overvoltage control. Further reductions ininrush currents, as well as overvoltages, could beobtained with a higher inductance pre-insertioninductor; however, higher inductances needed tooptimize inrush current and overvoltage control maynot be economically feasible.

PRE-INSERTION IMPEDANCES

6.0 CONCLUSIONSThe overvoltages produced at a remote, radially fedtransformer terminal, as a result of energizing a shuntcapacitor bank, are shown to be of a magnitude thatpossibly may exceed the phase-to-phase switching-surgevoltage-withstand capability of the connected trans-former. Various means of controlling these overvoltagesare evaluated and a relative comparison is made. Thevarious control means evaluated are pre-insertion(closing) resistors and inductors, fixed inductors, anda controlled closing of the switching device contacts.A worst-case computer study is utilized for a com-parative analysis and indicates that, without a control-ling means, the phase-to-phase switching overvoltagemay be as high as 6.5 per unit. Under the same worst-case conditions, a pre-insertion resistor will reduce thisovervoltage to 4.0 per unit; a fixed inductor or pre-insertion inductor will reduce this overvoltage to 3.3per unit; and, a synchronized closing of the switchingdevice contacts has a theoretical potential of reducingthis voltage to 1.7 per unit. For practical controlledclosing systems, however, depending upon the natureof the switching device and the ability to reliably control

the mechanical and electrical closing sequence, theovervoltages may be significant.Perhaps equally as important as the absolute mag-nitude of the switching overvoltage at the remotetransformer terminal is the rise time or rate of changeof voltage seen by the transformer. The rise times ofthe switching surge overvoltages, as produced with nocontrol means, with a pre-insert ion resistor, or with apractical controlled closing means, is on the order ofmicroseconds. The rise time for either a fixed inductor

or a pre-insertion inductor is on the order of 100microseconds. A slower rise time or longer time-to-peakvoltage will clearly impose a less severe duty on thetransformer, since the voltage will be more equallydistributed along the winding of the transformer, ascompared to that which would occur for a transienthaving a very short rise time.This study suggests that either a pre-insertioninductor or a synchronized closing of the switching-device contacts would be preferred as a means ofmitigating switching overvoltages. The use of a pre-insertion inductor may be the preferred method,particularly from the standpoint that not only are thepeak overvoltages modest, but the rise time of the

transients generated are considerably less severe thanthat which might be generated by using other controlmeans.

6.0 REFERENCES1. H. M. Pflanz, G. N . Lester - Control of Overvol-tages on Energizing Capacitor Banks, IEEE Transac-tions on Power Apparatus & Systems, Volume PAS-92,pages 907-915, May/June, 1973.2. E. W . Boehne, S . S. Low - Shunt CapacitorEnergization with Vacuum Interrupters--A PossibleSource of Overvoltage, IEEE Transactions on Power

Apparatus & Systems, Volume PAS-88, pages 1424-1443, September, 1969.3. Sue S . Mikhail, Mark F. McGranaghan - valua-tion of Switching Concerns Associated with 345 kVShunt Capacitor Applications, IEEE Paper 85 SM401-5, presented at the IEEE Power EngineeringSociety Meeting, July, 1985.4. K. L. Spurling, A. E . Poitras, M. F. McGra-naghan, J . H. Shaw - Analysis and Operating Expe-rience for Back-to-Back 115 kV Capacitor Banks, IEEEPaper No. 86 TD 602-7 presented at the IEEE PowerEngineering Society Transmission & Distribution

Conference & Exposition, September, 1986.5. Robert A. Jones, Hoke S . Fortson, Jr. - Consid-eration of Phase-to-Phase Surges in the Application ofCapacitor Banks, IEEE Transactions Power Delivery,Vol. PWRD-1, No. 3, pages 240-244, July, 1986.6. L. Lishchyna, R. H. Brierley - Phase-to-PhaseSwitching Swges Due to Capacitor Energization,Canadian Electrical Association, 1986 Transactions ofEngineering & Operations, V25, Part 4, Paper NO.P86-SP- 148.

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7. E. Maury - Synchronous Closing of 525 kV and765 kV Circuit Breakers: A Means of Reducing Switch-ing Surges on Unloaded Lines, CIGRE #143,1966.10. R. S. Bayless, J. D. Selman, D. E. Truax, W. E.Reid - Capacitor Switching and Transformer Tran-sients, IEEE Paper 86 SM 419-6, presented at the IEEEPower Engineering Society Meeting, July, 1986.11. IEEE Guide for Transformer Impulse Tests, IEEENo. 93, June 1968/ANSI (37.37.

8. J. H. Brunke, G. G. Schockelt- SynchronousEnergization of Shunt Capacitors at230 kV, IEEE PaperA78 148-9, presented at the IEEE Power EngineeringSociety Meeting, January, 1978.9. R. W. Alexander - ynchronous Closing Controlfor Shunt Capacitors, IEEE Transactions on Power

Apparatus & Systems, Vol. PAS-104, pages 2619-2626,February, 1985.

Excerpt from International on Large High1988 Session Voltage Elecfric Sysferns

IMPRIMERII LOUIS JEAN - OSWZ GAP- 2-

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Controlling Over Voltages Produced by Shunt Capacitor Bank Switching