Power Quality Program

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PE-384-PWRD-0-11-1997

Solving Customer Power Quality Problems Due to Voltage Magnification

R. A. Adams, Senior Member S. W. Middlekauff, Member

Duke Power CompanyCharlotte, NC 28201 USA

E. H. Camm, Member J. A. McGee

S&C Electric CompanyChicago, IL 60626 USA

Abstract: Reports by a Duke Power customer concerningequipment malfunctioning coincident with the switching of a100 kV shunt capacitor bank led to field measurements and EMTPsimulations to identify the possible sources of the problems and tofind an economical solution. Measurements and simulationrevealed the occurrence of voltage magnification at the customerssite due to the effects of three different distribution capacitor

banks on the feeder supplying the customer. A comparativeevaluation of commercially-available capacitor-switching

transient mitigation switching devices suggested the use of low-resistance pre-insertion inductors applied with circuit switchers toenergize the 100 kV capacitor bank. Field measurements of

capacitor-switching events after installation of pre-insertioninductors verified the elimination of voltage magnification andcustomer power quality problems.

Keywords: power quality, capacitor switching, switchingtransients.

1. INTRODUCTION

Voltage magnification occurring as a result of utilitycapacitor switching is now a well-know phenomenon. Inparticular, the effects of voltage magnification on

adjustable-speed drives (ASDs) are well documented [l-3].

As utility customer power quality awareness increases,there is an increasing need to reduce or eliminate the effects

of transients on customer electrical equipment. The devicesthat are commercially available to reduce or eliminate the

effects of voltage magnification include high-resistance orlow-resistance pre-insertion inductors used with circuitswitchers, controlled closing circuit breakers or vacuuminterrupters, and circuit breakers with pre-insertion

resistors. Bellei, et al presented a comparative evaluation

of capacitor-switching devices to prevent nuisance trippingof ASDs due to voltage magnification in [4].

PE-384-PWRD-O-1l-1997 A p a p e r recommended and

approved by the IEEE Transmission and Distribution

Committee of the IEEE Power Engineering Society for

publication in the IEEE Transactions on Power Delivery.

Manuscript submitted August 1. 1997; made available for

printing November 7, 1997.

Duke Power Company

Greenville, SC 29607 USA

II. CUSTOMER POWER QUALITY CONCERNSIDENTIFY VOLTAGE MAGNIFICATION

A commercial customer called Duke Power Companywith concern regarding the electrical service to theirfacility. The customer, a local TV station, wasexperiencing problems with high-voltage power supplies ona regular basis, resulting in loss of their audio signal, andpartial power to their video signal. The problemsexperienced were reportedly very prevalent in fall andspring, but less during the winter and summer. The feeder

supplying the customer had three installed distributioncapacitor banks. A large underground subdivision was alsoserved by the same feeder.

Duke engineers suspected there may be a correlationbetween the high-voltage power supply breaker trips andthe switching of the capacitors at the 100 kV tie station. To

investigate, a pair of BMI 4800 power quality monitors

were placed at the tie station, and at the electrical deliveryof the TV station. Later, for a higher speed capture of theevents, a Dranetz 658 was installed at the customerselectrical delivery. The customer was requested to keep adisturbance log of breaker trips, to attempt to correlate tosystem events.

The disturbance logs indicated that the problems of thecircuit breakers was coincident with the switching of a100 kV, 57.6 Mvar capacitor bank at a 100 kV substation

located approximately 14 miles from the customers site.The capacitor bank was being switched on average 3 to 4times a week, and resulted in tripping of circuit breakers at

the customers site in about 50% of the switching events.

Recognizing the possibility of voltage magnificationoccurring due to the presence of the distribution capacitors,voltage measurements were performed at the customers480 V bus with all but one (a fixed 600 kvar bank) of thethree distribution capacitor banks switched off. However,measurements indicated that the transient overvoltage was

still sufficiently large (about 1.8 per unit of nominal line-to-

line voltage). See Fig. 1.

1997 IEEE. Reprinted, with permission, from IEEE/PES

1998 Winter Meeting, February 1-5, 1998, Tampa, Florida USA

71l -T66


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Fig. 1. Field measurements of voltage magnification at customers 480 Vbus due to energizing a 100 kV, 57.6 Mvar, ungrounded-wye-connected

shunt capacitor bank at Duke Powers 100 kV substation. Phase-to-phasevoltages are shown. Only 600 kvar distribution capacitor bank is ON.

III: EMTP SIMULATION OF CAPACITORENERGIZING TRANSIENTS

An Electromagnetic Transients Program (EMTP)simulation model of the distribution system in the 100 kVsubstation area was developed to determine the effects ofthe three distribution capacitor banks and the capacitance ofcables of a large subdivision in the area. Fig. 2 is asimplified one-line diagram of the 100 kV substation andthe distribution system involved. The 450 kvar capacitorsare located about 4800 ft (0.91 mile) from the customersstep-down transformer, the 600 kvar capacitors about 3.26

miles, and the 900 kvar capacitors about 4.02 miles. Thelarge subdivision is located about 3.69 miles away from thecustomers step-down transformer.

The 3-phase EMTP simulation model included equivalentloads at the customer site, the subdivision, and the 100-kVsystem. The loads of the subdivision were modeled indetail, distributed along the lengths of equivalent cablemodels. This representation of the undergroundsubdivision would show the effects of cable capacitanceduring simulations. The transient damping effected by long100 kV transmission lines was represented at the 100 kVsubstation. The available 3-phase and phase-to-ground

short-circuit currents were used to determine the parametersof the equivalent source at the 100 kV substation. Thedistribution feeder and lateral to the customers site was

Fig. 2. System one-line diagram showing 100 kV substation anddistribution system to affected customers utilization voltage bus.

The EMTP equivalent circuit parameters were also usedto calculate the approximate frequencies of oscillation ofthe capacitor-switching transient at the 100 kV substationand at the customers 480 V bus. Fig. 3 shows theequivalent circuit for calculating the resonant frequencies atthe two locations.

Fig. 3. Equivalent circuit illustrating circuit parameters determiningtransient frequencies at the switched capacitor bank and at remote

capacitors.

The frequency of the oscillation at the 100 kV substationis based on an equivalent source inductance of about6.2 mH (i.e., about 24.7 kA 3-phase available short-circuitcurrent) and the capacitance of the 100 kV, 57.6 Mvarcapacitor bank of 15.3 uF. Frequencies of oscillation at the480 V bus were calculated for each individual connected

represented by sections of overhead line and cable models.All capacitor banks were represented as lumped equivalent

distribution capacitor bank to determine if the matching of

capacitances.the frequencies of the L-C circuit at the 100 kV substationand at the 480 V bus can be effectively changed to reducethe effects of voltage magnification. The resultantfrequencies are summarized in Table 1. Actual frequencieswill be slightly different from the calculated values due tothe effects of damping from system loads and losses. Whenmore than one distribution capacitor bank is connected, the

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frequency of oscillation at the 480 V bus is determined bythe interaction of the different L-C circuits.

TABLE I

APPROXIMATE FREQUENCIES OF OSCILLATION OF CAPACITOR

Lumped capacitance of subdivision cables.

It is clear from the tabulated frequencies in Table I thatthe frequencies of the L-C circuits at the two locations aresufficiently closely matched with each of the individualconnected distribution capacitors to result in voltagemagnification.

In order to validate the EMTP simulation model, theenergizing of the 100 kV, 57.6 Mvar capacitor bank wassimulated while only the 600 kvar distribution capacitorswere connected. The closing instants of the three poles ofthe circuit switcher (without pre-insertion inductors). wasselected to coincide with the closing instants which yieldedthe voltage measurements at the customers 480 V busshown in Fig. 1. This also served to confirm the apparentvoltage magnification caused by the 600 kvar distributioncapacitors. The resulting phase-to-phase voltages at the100 kV substation and at the customers 480 V bus areshown in Fig. 4. Note that the effects of voltagemagnification due to the 600 kvar capacitors are clearly

shown. The peak phase-to-phase transient overvoltage atthe 100 kV substation is only 215.4 kV (or 1.52 per unit),while the corresponding peak overvoltage at the customers480 V bus is about 1383 V (or 2.04 per unit). Thefrequency of the simulated transient overvoltage at the100 kV substation is about 565 Hz (compared to thecalculated frequency of 518 Hz), while the correspondingfrequency at the 480 V bus is about 627 Hz (compared tothe calculated frequency of 681 Hz). The magnitude of thesimulated transient overvoltage at the 480 V bus issomewhat higher than the measured overvoltage shown inFig. 1, but the simulated transient frequency agrees very

closely with the 625 Hz of the measured transient. Thedifference in transient overvoltage magnitude is due to theincreased damping in the field, particularly since the effectsof the harmonics at the customers site were not representedin the simulation model.

The EMTP simulation model was then used to determinethe severity of voltage magnification due to the threedistribution capacitor banks. Simulations were performedwithout any of the three distribution capacitor. banksconnected, and with each of the three capacitor banks

connected to the feeder individually, and also incombination with one or both of the other banks. For

comparison, the same closing instants of the three poles ofthe circuit switcher as during the field test were usedthroughout the simulations. The resulting peakovervoltages are summarized in Table II. As expected,voltage magnification is apparent with all differentcombinations of connected distribution capacitor banks.The most severe voltage magnification at the 480 V bus(1645 V or 2.42 per unit of nominal peak line-to-linevoltage) occurs with the 900 and 450 kvar distributioncapacitor banks switched on. See Fig. 5. Note also theslight magnification due to the capacitance of the largesubdivision cables when all three distribution capacitorbanks are switched off.

Fig. 4. Simulated phase-to-phase voltages at (a) 100 kV bus and at (b)customers 480 V bus when energizing the 100 kV, 57.6 Mvar capacitor

bank at Duke Powers 100 kV substation. Only 600 kvar distributioncapacitor bank is ON.

TABLE Il.PEAK TRANSIENT OVERVOLTAGES WHEN ENERGIZING THE

100 kV, 57.6 MVAR CAPACITOR BANK.

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Fig. 5. Simulated most severe phase-to-phase voltage at customers480 V bus when energizing the 100 kV, 57.6 Mvar capacitor bank with

the 900 and 450 kvar distribution capacitor banks switched ON.

IV. EMTP SIMULATION OF TRANSIENTMITIGATION USING PRE-INSERTION

INDUCTORS

Alternatives for minimizing the effects of voltagemagnification due to the distribution capacitor banksincluded applying either high-resistance or low-resistancepre-insertion inductors to energize the 100 kV, 57.6 Mvarcapacitor bank at the 100 kV substation. These were thepreferred transient mitigation options since the capacitorbank at the 100 kV substation was being switched with acircuit switcher to which pre-insertion inductors couldeasily be retrofitted.

Pre-insertion inductors furnish an impedance, which isfrequency dependent, in series with the bank capacitanceduring the initial energization of the capacitor bank. Thisimpedance reduces the collapse in bus voltage by the

amount of voltage developed across the inductor during theinrush of current into the bank. The pre-insertion inductoralso limits the magnitude of the initial inrush current. Sincethe impedance of the pre-insertion inductor is frequency-dependent, its value appears to be quite large during initialinrush current into the bank when the frequency is quitehigh. Thereafter, the effective impedance of the pre-insertion inductor is reduced when the steady state, 60 Hz,current value of the bank is obtained. The pre-insertion

inductor - like any other pre-insertion impedance - givesrise to a second transient when the inductor is bypassed(after the 60-Hz current is obtained). This transient,referred to as the bypass transient, is generally much

smaller than the initial transient, but can be largerdepending on the size of the bank and the impedance of theinductor. The pre-insertion inductor is typically comprisedof a number of close-coupled layers of stainless-steel (highresistance) wire wound to form a hollow glass-reinforcedtube; low-resistance aluminum wire is also sometimes used.The pre-insertion inductor is typically applied with a circuitswitcher having a high-speed disconnect blade, whichinserts the pre-insertion inductor for 7 to 12 cycles

(depending on system voltage) during closing, to energizethe bank. See Fig. 6.

Fig. 6: Pre-insertion inductors mounted on a Mark V circuit switcher atthe Duke Power 100 kV tie station.

High-resistance pre-insertion inductors are usuallyrecommended for mitigating capacitor-switching transientsdue to voltage magnification. However, for largercapacitor banks where the bypass transient may be a

concern, the use of low-resistance pre-insertion inductorsmay be more suitable. With high-resistance pre-insertioninductors, the bypass transient, which is proportional to thevoltage developed across the inductor due to the capacitorbank current flowing through it, increases as the capacitorbank size increases. For the 57.6 Mvar capacitor bank thepeak bypass transient magnitude when using 40 mH-81ohm high-resistance pre-insertion inductors would beapproximately 0.47 per unit of nominal peak phase-to-ground voltage. Realizing that this magnitude bypasstransient could result in magnified overvoltages at thecustomers 480 V bus which may be unacceptably high, theuse of 40 mH-5.5 ohm low-resistance pre-insertioninductors were considered instead. The low-resistance pre-insertion inductors are somewhat less effective than thehigh-resistance pre-insertion inductors during the initialinsertion transient since the resistance thereof is too low toeffectively damp the oscillatory transient. However, theinductance of the low-resistance pre-insertion inductors isas effective as that of the high-resistance pre-insertioninductors in reducing the initial collapse in the bus voltage,which reduces the excitation of the remote L-C circuit.Furthermore, since the impedance of the low-resistance pre-insertion inductors is small compared to that of the high-resistance pre-insertion inductors, the bypass transient is

always small compared to the insertion transient.

To determine the effectiveness of the low-resistance pre-insertion inductors, the performance of 40 mH-5.5 ohmpre-insertion inductors was determined using the EMTPsimulation model with all combinations of connecteddistribution capacitor banks. For comparison purposes, theclosing instant of the three poles of the circuit switcher wasconsidered to be the same as for the previously simulatedcases without pre-insertion inductors. Resulting peakphase-to-phase transient overvoltages are summarized in

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Table III. The case resulting in the most severe overvoltage

at the customers 480 V bus is illustrated in Fig. 7. Notethat the pre-insertion inductors practically eliminated thetransient overvoltages at the customers 480 V bus.

TABLE IIIPEAK TRANSIENT OVERVOLTAGES WHEN ENERGIZING THE

100 kV, 57.6 MVAR CAPACITOR BANK WITH A CIRCUIT

SWITCHER WITH LOW-RESISTANCE PRE-INSERTIONINDUCTORS.

Note: Since the 100 kV capacitor bank is ungrounded, the peakovervoltages at the two locations do not necessarily occur on the samephases due to the transient reduction by the pre-insertion inductors.

(b)Fig. 7. Simulated phase-to-phase voltages at (a) 100 kV substation and at(b) customers 480 V bus when energizing the 100 kV, 57.6 Mvar

capacitor bank with a circuit switcher with low-resistance pre-insertioninductors. Illustration of most severe overvoltage at 480 V bus. All

distribution capacitor banks are ON.

V. FIELD MEASUREMENTS OF TRANSIENT

MITIGATION USING PRE-INSERTION INDUCTORS

To verify the performance of the low-resistance pre-insertion inductors, field measurements of transientvoltages at the 100 kV substation and at the customers 480V bus were again performed. At the tie station, a 20 kHz

digital oscilloscope was used to capture switching events ata high sampling rate. A Dranetz 658 was again used tocapture the transients seen at the customers 480 Vdelivery.

Fig. 8. Measured phase-to-phase voltages at (a) 100 kV substation and at(b) customers 480 V bus when energizing the 100 kV, 57.6 Mvarcapacitor bank with a circuit switcher with low-resistance pre-insertioninductors. Note that the capacitor switching transient is hardly visibledue to the harmonic voltage distortion at the 480 V bus.

Fig. 8 shows the resulting waveform from the switchingof the capacitor bank at the 100 kV tie station. Fig. 8(a)

was measured using the 20 kHz oscilloscope at the tiestation. The transient, occurring at the trigger point(indicated by the arrow), is very subtle, a vastimprovement from before. Fig. 8(b) was measured usingthe Dranetz, at the customer 480 V electrical delivery. Thetransient is almost non-existent at that point, and is maskedfurther by the voltage distortion from the electronics of thetransmitter.


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VI. CONCLUSIONS

Voltage magnification occurring as a result of switchinga 100 kV, 57.6 MVAR capacitor bank on Duke PowerCompanys system caused problems with high-voltagepower supplies at a local TV station. Field measurementsand EMTP simulations resulted in identification of the

problem and suggested the use of low-resistance pre-

insertion inductors for transient mitigation.

Pre-insertion inductors offer a cost-effective solution forreducing capacitor switching transients, especially in

existing installations where circuit switchers are used.Capacitor switching transients often cause problems forcustomer equipment, however the transient does not need tobe completely eliminated. The pre-insertion inductorprovides a means for controlling the transient to an

acceptable level.

For most capacitor-switching transient mitigationapplications where voltage magnification is an issue, thehigh-resistance pre-insertion inductor is recommended.However, for larger capacitor bank sizes where the bypasstransient becomes significant, low-resistance pre-insertioninductors may provide better performance. This isparticularly true for applications where the sourceimpedance is low. EMTP simulation was used effectivelyto confirm this theory. Low-resistance pre-insertioninductors retrofitted on a circuit switcher worked asexpected, eliminating power quality problems at the TVstation.

VII. REFERENCES

[I] M. F. McGranaghan, et al, Impact of Utility Switched Capacitors onCustomer Systems: Part II - Adjustable Speed Drive Concerns,Presented at the 1991 IEEE-PES Winter Meeting, New York, NewYork, February 3 - 7, 1991.

[2]. J. A. Oliver, and R. A. Ferraro, The Myths of ASD Power Quality,in Proceedings of the Second International Conference on PowerQuality: End-Use Applications and Perspectives, September 28 - 30,1992, Atlanta, Georgia.

[3]. H. G. Murphy, Power Quality Issues with Adjustable-FrequencyDrives: Coping with Power Loss and Voltage Transients, PowerQuality Assurance Magazine, May/June 1993.

[4] T.A. Bellei, R.P. OLeary, and E. H. Camm, Evaluating Capacitor-Switching Devices for Preventing Nuisance Tripping of Adjustable-Speed Drives Due to Voltage Magnification, in IEEE Transactionson Power Delivery, Vol. II, NO. 3, July 1996.

VIII. BIOGRAPHIES

Ron A. Adams received his B.S. degree in Electrical Engineering fromClemson University in May, 1985. He has been employed with DukeEnergy for 12 years and has held various engineering positions withinSubstation Project Engineering, Industrial Marketing and System PowerQuality. Currently he holds the position of Subprocess Owner of Provide

Technical Services within the Electric Transmission Business unit. Ronis a past Chairman of the IEEE Power Engineering Society, CharlotteChapter and been active with the IEEE Working Group forRecommended Practices and Requirements for Harmonic Control inElectrical Power Systems (IEEE 519). Ron is registered as a professionalengineer in both North and South Carolina.

Stephen W. Middlekauff received his BSEE and MSEE from Clemson

University, Clemson, SC, in 1993 and 1996 respectively. He has beenemployed by Duke Power in the System Power Quality department since

graduation in May 1996. As a graduate student and Duke employee,Stephen served as a principle investigator on the Westinghouse DynamicVoltage Restorer (DVR) project. He is a member of IEEE and the PowerEngineering Society, and serves as a chapter chairman for the CustomPower Task Force Application Guide. His primary research interestsinclude power quality, custom power, and harmonics.

Ernst H. Camm received his BSc(Eng) degree in Electrical andElectronic Engineering from the University of Cape Town, South Africain 1984, and his MSEE degree from the Ohio State University in 1992.From 1984 to 1990, he held various positions in Plant and ProjectEngineering. He is currently a Project Engineer in the EngineeringServices Department at S&C Electric Company. Ernst has had extensiveinvolvement in capacitor switching transient analysis at S&C, includinganalysis in the development of optimally sized pre-insertion inductors forcapacitor switching transient mitigation. He is the author of and co-

presenter of S&Cs Seminar on Capacitor Switching Transients andTheir Impact on Your System. He is a member of the SwitchingTransients Task Force of the IEEEs Modeling and Analysis of SystemTransients Working Group and the Shunt Capacitor Application GuideWorking Group.

James A. McGee is a 1976 graduate of North Carolina State Universitywith a B.S. degree in Electrical Engineering. Since that time he has beenemployed by Duke Energy in a variety of assignments, includingprotective relay engineering and transmission systems operations.Currently, Jim serves as a power quality specialist. In 1981 he wasawarded an M.E. degree in Electrical Engineering from ClemsonUniversity. He is a registered Professional Engineer in North and SouthCarolina.

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