Current-limiting Inductors Used In

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CURRENT-LIMITING INDUCTORS USED IN

CAPACITOR BANK APPLICATIONS AND THEIR IMPACT

ON FAULT CURRENT INTERRUPTION

Terrance A. Bellei

Ernst H. Camm

S&C Electric Company

Chicago, Illinois

Gene Ransom

ComEdChicago, Illinois

180-T69





Abs t rac t

—

Utilities often use in-line current-limiting inductors for capacitor bank installationsto limit the severity of outrush currents from the

bank(s) into close-in line or bus faults. Because of the very high inherent frequency of the current-limiting inductors used in these applications,circuit breakers used for protecting the capacitorbank bus can be subjected to very severe transientrecovery voltages after the interruption of faultslimited by the inductors. The impact of current-limiting inductors on circuit breaker interruptingperformance was demonstrated by a recentincident involving a phase-to-ground fault in one of four 138-kV, 57.6-Mvar capacitor banks at ComEd’sSilver Lake Substation. A 138-kV, 2000-A circuitbreaker used for protecting two capacitor bankswas unable to interrupt the fault current because

of the severe transient recovery voltage producedby a 1-ohm current-limiting inductor in the circuit.This paper describes the details of the analysisinvolved in identifying the sequence of events thatled to the circuit breaker being unable to interruptsuccessfully and highlights the impact of current-limiting inductors on fault current interruption incapacitor bank applications.

I. INTRODUCTION

Current-limiting inductors are often connected in serieswith shunt capacitor banks to limit the severity of outrushcurrents into close-in bus faults. High-magnitude and

high-frequency outrush currents that would otherwiseoccur can cause damaging overvoltages when line circuitbreakers reignite and subsequently interrupt at high-frequency current zeros [1]. The rate of rise of thetransient recovery voltage (TRV) which a line circuitbreaker will be subjected to during a close-in fault isconsiderably lower than normal when a capacitor bank ispresent, while the peak of the TRV is higher. The lowerrate of rise of the TRV will enable the line circuit breakerto interrupt with a very short arcing time, therebyincreasing the possibility of reignitions in the circuitbreaker if the dielectric withstand capability of thecontact gap is exceeded after interruption with a smallcontact gap.

The size of the current-limiting inductor is generallyselected to ensure that the product of the peak outrushcurrent and the frequency is less than 2

10

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for general-purpose circuit breakers [2, 3]. In back-to-back capacitorbank applications, a single bank of three-phase current-limiting inductors is usually used to limit outrush currentfrom two or more shunt capacitor banks.

While the current-limiting inductor reduces thseverity of the outrush current during close-in bus faultsit also presents a severe TRV to the circuit breake

protecting the capacitor bank(s) when a fault occurs ithe capacitor bank(s) or between the inductor and thcapacitor bank. This is due to the very high i nheren

frequency of the inductors, which results in a very highfrequency oscillation on the load side of the circuibreaker when it attempts to interrupt the fault current.

This paper describes the details of the analysiinvolved in identifying the sequence of events that led to circuit breaker being unable to successfully interrupduring a phase-to-ground fault in a 138-kV, 57.6-Mvacapacitor bank in ComEd’s Silver Lake Substation.

II. BACKGROUND

In September 1999 extensive damage occurred to a138-kV, 57.6-Mvar grounded-wye capacitor bank inComEd’s Silver Lake Substation. A 2000-ampere circuibreaker, used to protect this bank and a second 57.6-Mvacapacitor bank in parallel with it, was unable to interrupthe associated fault current, causing the bus circubreakers to open to clear the fault. A 1-ohm currentlimiting inductor is connected on the load side of thcircuit breaker to limit outrush currents from the twocapacitor banks. Two additional 138-kV capacitor bankare connected in a similar circuit arrangement on a secon138-kV bus. Each of the capacitor banks is switchedwith a Circuit-Switcher equipped with 40-mH–5.5-ohmpre-insertion inductors. See Figure 1.

Figure 1. Simplified one-line diagram of 138-kV circuit inComEd’s Silver Lake Substation.

Current-Limiting Inductors Used in Capacitor Bank Applications and

Their Impact on Fault Current Interruption



Initial reports on the incident indicated that the eventoccurred shortly after the capacitor bank in which thefault occurred (bank #4) was de-energized. Anothercapacitor bank (bank #2) was also de-energized a shorttime prior to the incident. At the time of the incident thenormally open bus tie circuit breaker was closed.

Initial review of oscillograms of bus voltages andcircuit breaker phase currents suggested that the A phaseof the Circuit-Switcher used to switch the bank may nothave interrupted successfully during de-energization of the bank. If this happened the high-speed disconnectblade, which opens shortly after the interrupter openswould have caused the pre-insertion inductor to beinserted into the circuit. If a fault occurred in thecapacitor bank during that stage of the blade opening, thepre-insertion inductor would have limited the initial faultcurrent substantially and failed thermally shortlyafterwards. At first glance, this theory appeared to be also

supported by the damage observed on the high-speeddisconnect blade and the pre-insertion inductor in thefaulted phase and the initial oscillograms of circuitbreaker phase currents. See Figure 2.

Figure 2. Recorded phase currents through 2000-A circuitbreaker during incident at Silver Lake Substation.

III. ANALYSIS OF RELAY EVENT RECORDING

The initial analysis of the oscillograms of the SEL-251Crelay event recording indicated that a phase-to-groundfault occurred in the A phase of the capacitor bank. Thefault current was initially limited to about 3.1 kA RMS bythe pre-insertion inductor, and increased to about 21 kARMS after thermal breakdown of the pre-insertion

inductor. It was also evident from the oscillograms thatthe circuit breaker failed to interrupt the fault current asthe relay initiated a trip about 4 cycles after the start of the event recording.

Closer examination of the event summary reportassociated with the relay event recording indicated thatthe RMS current values of the unfaulted phases wereactually increasing after the event was triggered. This wascontrary to what was expected as the unfaulted phasecurrents were expected to decrease to zero after currentinterruption if the event occurred during the opening of the Circuit-Switcher. This led to a reconstruction of theunfaulted current waveforms based on the relay’s eventrecording. The resulting waveforms indicated that theunfaulted phase currents were actually increasing fromRMS values representative of the RMS phase currents withone bank on-line to that of two banks on-line. See Figure 3. There is some evidence of increased current in the B phaseof the circuit breaker, which may have been caused bydamage to some of the B phase capacitor units resultingfrom the damage in the A phase capacitor units.

Figure 3. Recorded B- and C-phase currents through

2000-A circuit breaker during incident at Silver Lake Sub-station.

The reconstructed current waveforms of the unfaultedphase currents indicated that the event actually occurredduring closing of the Circuit-Switcher used to switch thecapacitor bank. Based on the reported switching events atthe time of the incident it was concluded that the fault inthe capacitor bank might have been initiated by closinginto a bank with trapped charge voltage shortly afteropening the Circuit-Switcher. This possibility was furtherinvestigated by developing an Electromagnetic TransientsProgram (EMTP) simulation circuit model of theinstallation and simulating the closing of the Circuit-

Switcher into a capacitor bank with trapped chargevoltage. Figures 4 though 6 show the simulated capacitorbank and 138-kV bus voltages, and the circuit breakerphase currents when closing the Circuit-Switcher into a

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bank with trapped charge voltage. The 138-kV busvoltages and circuit breaker phase currents showed aclose correlation to that obtained from the relay eventrecordings. The peak overvoltage at the capacitor bankbus is approximately -284 kV on A phase (i.e., 2.52 per unitof normal peak phase-to-ground voltage). The associatedhigh-voltage stress on the top rack of capacitor units mayhave been sufficient to cause a flashover of the fuse linksprotecting the capacitor units. The final fault current

value is higher than measured, most likely due todifferences in the actual available fault current during theincident and that specified for the simulation.

Figure 4. Simulated capacitor bank phase-to-groundvoltages during energizing with trapped charge voltage onthe capacitor bank. A full-blown phase-to-ground faultoccurs in A phase at 16 ms. The pre-insertion inductor isshorted out after thermal breakdown at 50 ms. The B and Cphases of the 2000-A circuit breaker interrupt after 80 ms.

Figure 5. Simulated 138-kV bus phase-to-ground voltagesduring energizing with trapped charge voltage on thecapacitor bank. A full-blown phase-to-ground fault occursin A phase at 16 ms. The pre-insertion inductor is shorted

out after thermal breakdown at 50 ms. The B and C phasesof the 2000-A circuit breaker interrupt after 80 ms.

Figure 6. 2000-A circuit breaker phase currents duringenergizing with trapped charge voltage on the capacitobank. A full-blown phase-to-ground fault occurs in A phaseat 16 ms. The pre-insertion inductor is shorted out aftethermal breakdown at 50 ms. The B and C phases of the2000-A circuit breaker interrupt after 80 ms.

IV. CIRCUIT BREAKER TRV ANALYSIS

While the analysis of the relay event recording gave an

indication of the initial sequence of events, the cause othe circuit breaker’s failure to interrupt was still noknown. Because of the presence of the 1-ohm currentlimiting inductor in the circuit, the effect of the inherenfrequency of the inductor on the TRV was the suspectecause leading to further analysis. The effectivecapacitance of current-limiting inductors, consisting othe terminal-to-ground and terminal-to-terminacapacitances, are typically quite small (usually tens to few hundreds of picofarads). Consequently, theseinductors have a very high i nherent

frequency, whichcould be on the order of tens to hundreds of kilohertz. If phase-to-ground fault occurs on the load-side terminals oa current-limiting inductor, a voltage determined by th

magnitude of the fault current and the impedance of thinductor develops across the inductor. When a sourceside circuit breaker in series with the inductor interruptthe fault current, the voltage on the load side of the circubreaker will rapidly oscillate to zero. The voltage on thesource side of the circuit breaker will return to itnominal power-frequency value through a low-frequencoscillation, which is dictated by the effective sourceinductance and effective system capacitance. The highfrequency oscillation on the load side of the circuibreaker gives rise to a very severe TRV across thecontacts of the circuit breaker. The frequency of thioscillation is determined by the inductance of the reactoand the equivalent phase-to-ground capacitance on th

load-side terminals of the circuit breaker. The load-sidecapacitance includes the effective capacitance of thinductor, the capacitance of the length of bus between thinductor and circuit breaker, and the phase-to-groundcapacitance of the circuit breaker.



The manufacturer of the current-limiting inductors wascontacted to obtain the actual capacitance of theinductors installed at Silver Lake Substation. Thecapacitance of the current-limiting inductors wasspecified to be 81.7 pF phase-to-ground and 16 pFterminal-to-terminal, yielding an effective capacitance of 97.7 pF. This capacitance, combined with the capacitanceof the buswork between the circuit breaker and thecurrent-limiting inductor, as well as the equivalent

capacitance of the circuit breaker, yielded a total effectivecapacitance on the load side of the 2000-A circuit breakerof 249 pF. The inductance of the current-limiting inductoris approximately 2.65 mH, which yields a frequency of oscillation on the load side of the circuit breaker of approximately 196 kHz.

The EMTP simulation circuit model was used todetermine the peak TRV across the circuit breakercontacts when interrupting the phase-to-ground faultcurrent. The resulting TRV is shown in Figure 7. Theinitial TRV has a peak value of approximately 86.8 kV anda time-to-peak value of approximately 2.55 microseconds,giving an average rate of rise of recovery voltage of

approximately 34 kV per microsecond. For a 63-kA circuitbreaker the preferred ratings of rated transient recoveryvoltage specified in ANSI C37.06 [3] include a peak voltageof 255 kV, a time-to-peak of 310 microseconds, and aninitial rate of rise of recovery voltage of 1.8 kV permicrosecond at rated interrupting current. For a 45%fault(i.e., for a fault limited to 28.2 kA by the 1-ohm current-limiting inductor), the preferred TRV parameters includea peak voltage of 281 kV, a time-to-peak of 89microseconds, and an initial rate of rise of recoveryvoltage of 1.8 kV per microsecond.

Figure 7. Simulated initial TRV across A-phase contacts of2000-A circuit breaker when interrupting capacitor bank

phase-to-ground fault current.

The TRV parameters during interruption of faultcurrent in this application greatly exceeded the ANSI-specified TRV time-to-peak and rate of rise values. This is

the most likely reason why the circuit breaker was unableto interrupt the fault current involved during the incidentat Silver Lake Substation.

Review of the special circumstances of the faultinvolved in this incident by the circuit breakermanufacturer also led to the conclusion that the evolvingnature of the fault may have contributed to the inability of the circuit breaker to clear the fault. The circuit breaker

uses an arc-assist interrupter unit, which develops apressure proportional to the magnitude of the fault currentto assist with extinguishing the arc during fault currentinterruption. The gradual increase in the magnitude of thefault current, initially limited by the current-limitinginductor and pre-insertion inductor, resulted in insufficientpressure in the interrupter to successfully interrupt thecurrent.

In order to limit the severity of the initial TRV acrossthe circuit breaker contacts, the use of a capacitorconnected in parallel with the current-limiting inductorwas considered. The capacitance value must be selectedsuch that the voltage acceleration associated with theinitial TRV is less than that resulting from the ANSI-

specified rate of rise of recovery voltage of 1.8 kV permicrosecond. This translates into a limiting voltageacceleration of 175 volts per microsecond

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. For an initialpeak TRV of 86.8 kV, the frequency on the load side of thecircuit breaker must be limited to less than approximately10 kHz in order not to exceed the specified rate of rise of recovery voltage. This would require a capacitance of atleast 94 nF across the terminals of the current-limitinginductors in each phase. A 100 nF capacitor per phase wasrecommended to be used for this purpose.

V. CONCLUSIONS

Current-limiting inductors applied to reduce the severityof outrush currents from capacitor banks during close-inbus faults can impact the interrupting capability of circuitbreakers used for protecting the capacitor banks. Thevery high inherent frequency of the current-limitinginductors can cause very severe transient recoveryvoltages to occur across the opening contacts of thecircuit breaker during interruption of faults that involvethe current-limiting inductors.

The impact of current-limiting inductors on faultcurrent interruption was demonstrated by the incidentdescribed in this paper. Care should be exercised in theapplication of current-limiting inductors that areconnected directly on the load side of the circuit breakerused for protecting the capacitor bank(s). If the inherent

frequency of the current-limiting inductor will result in arate of rise of recovery voltage across the contacts of thecircuit breaker that would exceed that specified in thestandards, additional capacitance must be connected onthe load side of the circuit breaker.

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VI. REFERENCES

[1] L. van der Sluis, A.L.J . J ansen, “Clearing Faults NearShunt Capacitor Banks,” Presented at the1990 IEEE-PES Wi nter Meeti ng, Atlanta

,Georgi a

, February 4 - 8, 1990.

[2] Brian C. Furumasu, Robert M. Hasibar, Design andInstallation of 500-kV Back-to-Back Shunt CapacitorBanks,” in I EEE Tran sacti ons on Power Deli ver y

, Vol. 7,No. 2, April 1992.

[3] ANSI C37.06-1997, “AC High-Voltage Circuit BreakersRated on a Symmetrical Current Basis — PreferredRatings and Related Required Capabilities.”

Terrance A. Bellei

received his BSEE from MarquetteUniversity in Milwaukee, Wisconsin. He has been withS&C Electric Company since 1974 and has held variouspositions in R&D and the Power Systems ServicesDivision. He is currently Manager, Engineering Services,in the Power Systems Services Division.

Within IEEE, he has served as a member of the High

Voltage Fuses Subcommittee, Working Group on ExternalFuses for Shunt Capacitors, Working Group on Full-RangeCurrent-Limiting Fuses, and as the secretary of theWorking Group on Revision of Fuse Standards. He hasserved as Chairman of a Task Force formed by theWorking Group on Revision of Fuse Standards. He wasalso a former representative of S&C in the North AmericaShort-Circuit Testing Liaison (STLNA).

Ernst H. Camm

received his BSc (Eng) degree inElectrical and Electronic Engineering from theUniversity of Cape Town, South Africa in 1984 and hisMSEE degree from the Ohio State University in 1992.From 1984 to 1990, he held various positions in Plant andProject Engineering at S&C Electric Company. He iscurrently a Senior Engineer in the Engineering ServicesDepartment at S&C Electric Company.

Ernst has had extensive involvement in capacitorswitching transient and power quality analysis at S&C,including analysis in the development of optimally sizedpre-insertion inductors for capacitor switching transientmitigation. He is the author of and co-presenter of S&C’s“Seminar on Capacitor Switching Transients and TheirImpact on Your System.” He is a member of the Switching Transients Task Force of the IEEE’s Modeling andAnalysis of System Transients Working Group and theShunt Capacitor Application Guide Working Group.

Gene Ransom

received his BS degree in Electrical andElectronic Engineering from the Illinois Institute of Technology. He has been with ComEd since 1972 and hasheld various positions in Transmission, Distribution, andSubstation engineering and construction. He is currentlySubstation Engineering Manager for ComEd.

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Current-limiting Inductors Used In