IEEE-A Primer on Capacitor Bank Protection

1174 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 4, JULY/AUGUST 2001

A Primer on Capacitor Bank ProtectionMartin Bishop, Senior Member, IEEE, Tim Day, Senior Member, IEEE, and Arvind Chaudhary, Senior Member, IEEE

Abstract—Capacitor banks are applied in power systems to pro-vide reactive power. The reactive power results in lower current inlines upstream of the bank improving system voltage and powerfactor and reducing line losses. Capacitor banks can be configuredas filters for harmonic reduction. The protection systems for capac-itor banks include fuses, surge arresters, and protective relays. Thispaper will focus on protective relaying philosophies of groundedand ungrounded Y-connected shunt capacitor banks, which arecommonly applied on industrial and utility power systems.

Index Terms—Capacitor bank overcurrent protection schemes,capacitor bank protective relaying, capacitor bank unbalance pro-tection.

I. INTRODUCTION

CAPACITOR banks are normally constructed using indi-vidual capacitor units connected in series and parallel to

obtain the required voltage and Mvar ratings. Individual ca-pacitor cans are constructed using series and parallel capacitorunits, called packs.

The purpose of an unbalance protection scheme is the re-moval of a capacitor bank from the system in the event of a fuseoperation in a fused bank, or a pack failure in a fuseless bank.This will prevent damaging overvoltages from being impressedacross the remaining capacitor units in the group where the oper-ation occurs, thereby protecting against a situation which can beimmediately harmful to the capacitor units or associated equip-ment.

There are many methods available for detecting unbalances incapacitor banks; however, there is no practical method that willprovide protection under all possible conditions. The cost of afoolproof detection scheme would be economically prohibitiveand physically complicated. For example, all unbalance detec-tion schemes assume that unbalanced phase impedances will re-sult from the outage of one or more capacitor units. It is possible,of course, to lose units in such a manner that balanced phaseimpedances result while overvoltage conditions exist within thebank. Experience and the small probability that such a conditionwould occur have indicated that designing a protection schemefor such a condition is typically not required.

Paper PID 01–22, presented at the 2000 IEEE Petroleum and ChemicalIndustry Technical Conference, San Antonio, TX, September 10–14, and ap-proved for publication in the IEEE TRANSACTIONS ONINDUSTRYAPPLICATIONS

by the Petroleum and Chemical Industry Committee of the IEEE IndustryApplications Society. Manuscript submitted for review September 15, 2000and released for publication May 17, 2001.

M. Bishop and T. Day are with the Systems Engineering Group, CooperPower Systems, Franksville, WI 53126 USA (e-mail: [email protected]).

A. Chaudhary is with the Relay and Integrated Systems Group, Cooper PowerSystems, Franksville, WI 53126 USA.

Publisher Item Identifier S 0093-9994(01)06307-1.

Generally, all unbalance detection schemes are set up tosignal an alarm upon an initial failure or failures in a bank.Upon subsequent critical failures where damaging overvoltagesare produced, the bank would be removed from the line.Typical detection schemes associated with grounded-wyeand ungrounded-wye banks are discussed in this paper. Sincedelta-connected banks are so seldom used and ungrounded-wyebanks serve the same purpose, delta configurations will not beevaluated.

II. CAPACITOR BANK CONNECTION

There are certain advantages and disadvantages associatedwith grounded- versus ungrounded-wye capacitor banks,regardless of the unbalance detection scheme used.

The advantages of the grounded-wye arrangement comparedto the ungrounded wye are as follows.

1) The initial cost of the bank is lower, as the neutral doesnot have to be insulated from ground at full system basicimpulse insulation level (BIL), as in the case with floatingneutral arrangements.

2) Capacitor switch transient recovery voltages are reducedsince the neutral is grounded and the bank is switched asthree single-phase sections.

The disadvantages of the grounded-wye arrangement are thefollowing.

1) The grounded neutral may cause telephone interference.2) It provides a low-impedance fault path to ground. For

this reason, grounded-wye banks are not applied to un-grounded systems.

3) System fault current flows through a failed unit (singleseries group).

4) There are high-frequency inrush currents into substationground grid.

III. D ETECTINGBANK PROBLEMSUSING UNBALANCE

To help sensitize the reader to the problem of respondingto unbalances in capacitor banks, a numerical example is pre-sented. Consider an externally fused, grounded-wye bank con-sisting of the series/parallel arrangement of capacitor units asshown in Fig. 1.

The bank manufacturer will determine the particular se-ries/parallel arrangement based upon tradeoffs between voltagerating, var requirements, and overall economics. The examplefor this discussion shows four series groups per phase and eightcapacitors per group. Assume that a single unit in the lowergroup of B Phase fails followed by operation of its associatedfuse. Fig. 2 shows an equivalent circuit of the faulted phaseafter fuse operation has removed the failed capacitor fromservice.

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BISHOPet al.: A PRIMER ON CAPACITOR BANK PROTECTION 1175

Fig. 1. Example capacitor bank configuration.

Fig. 2. Equivalent circuit after single-fuse operation.

The problem at hand is to quantify the voltage stress seen by theremaining capacitors in the bank. The lower group of Phase Bwill now have greater reactance than a healthy group resulting inan altered voltage divider. Since the remaining capacitors will beexposed to different electrical stresses, it is of value to calculatethe highest steady-state capacitor voltage following the fuse op-eration. If the phase-ground voltage is assumed as 1.0 per unit,the prefault capacitor voltage anywhere in the bank will be 0.25per unit for this example.

However, after operation of the fuse, may be found usingstraightforward circuit analysis techniques

per unit

This voltage increase of 10.3% is developed acrossthe other remaining seven capacitors in the series groupwhere the failure occurred. [1] provides the mathematicalrelationship to determine the percent voltage increase fordifferent bank configurations and number of failed units.The voltage increase calculated for this example is abovethe 110% steady-state voltage rating for capacitor units [2].Therefore, a protection scheme must be applied to detectthe overvoltage that now exists and trip off the bank beforeother capacitors are stressed to the damage point.

A commonly measured signal to reveal the state of unbal-ance is found at the bank’s neutral. In this grounded-wye ex-ample, the unbalance caused by the fuse operation will result inneutral current flow: the greater the bank imbalance—and in-ternal voltage stress—the greater the neutral current. (Note thatfusing issues are not covered here; see [3] for discussions.) Inungrounded banks, the neutral-ground voltage provides a sim-ilar indication. Reference [1] details the expressions for pre-dicting this quantity

percent of nominal

TABLE IEXPECTEDNEUTRAL UNBALANCE FOR 110% INTERNAL VOLTAGE

wherenumber of series groups;number of units in each group;number of units removed.

Substitution into the expression with the values for this ex-ample yields the expected neutral current

of nominal phase current

A neutral CT and definite-time overcurrent relay would suf-fice to generate a trip signal for the bank’s breaker. Table Ishows the value of the neutral quantity (percent of nominal bankvoltage appearing at the neutral for ungrounded banks) for dif-ferent numbers of series groups. Values in the table are calcu-lated under the assumption that an unbalance sufficient to resultin an internal 110% overvoltage has occurred.

Note that for banks with more series groups—to accom-modate higher system voltages—the external neutral signalbecomes smaller, and more difficult to detect, for like valuesof internal voltage stress.

Correct application of an unbalance protection scheme de-pends upon proper understanding of system conditions that con-found reliable measurement of the indicating quantity, in thiscase the neutral current. This neutral current (or voltage if un-grounded) is influenced by unbalances in the system bus volt-ages as well as can failures. Even though the bank may be per-fectly healthy, a system voltage unbalance may result in a falsetrip by causing sufficient neutral current flow, or , if a bona fidebank problem does exist, it may be masked by the phasor re-lationship of the unbalanced bus voltages combining to reducethe magnitude of neutral current calculated above. A robust pro-tection scheme should provide means for compensating for theeffects of system voltage unbalance.

Assuming a healthy bank (no blown fuses) and balanced busvoltages, a neutral current may still be present due to manu-facturing tolerances of the individual capacitors. That is, notall phases have the same capacitive reactance. This is usuallya secondary concern in modern banks but is more problematicin banks with many series groups where the measured signalis small. A robust protection scheme should provide means forcompensating for this inherent imbalance due to manufacturingtolerances.

The neutral current detection scheme mentioned above hasan additional shortcoming: although unlikely, if one unit fails ineach of the three phases, overvoltages will exist within the bank.However, since all phases have the same reactance, there is no



bank unbalance and, hence, no neutral current. The protectionschemes presented in the next section will address such limita-tions.

IV. SINGLE-SIGNAL UNBALANCE DETECTION

The following discussions refer to both single and split-wyebanks. Assuming a given system voltage and bank size, thesingle-wye bank affords the following advantages over splitwye: each series group contains more units which yields areduced voltage stress when any unit fails, and the bank mayrequire less substation area and connection material. Con-versely, the split-wye bank has fewer parallel units per seriesgroup thus reducing the parallel stored energy that can reducefuse interrupting duty and capacitor rupture probabilities.

A. Grounded-Wye-Connected Capacitor Banks

For a grounded-wye bank or each wye of a splitgrounded-wye bank, the allowable number of units thatcan be removed from one series group, given a maximumon the remaining units, can be calculated using the followingformula:

If is fractional, use the nextlowerwhole number. The relayis then set to signal the alarm upon failure ofunits. To deter-mine the neutral-to-ground current flow and relay setting uponloss of units for the protection schemes shown in Figs. 3 and4, use the following formula:

Amperes

The relay would further be set to trip the bank upon loss ofunits. The neutral-to-ground current flow and relay setting

can be determined using in place of .The percentage of overvoltage for any number of units re-

moved from a series group can be determined by the followingformula:

whereapplied line-to-neutral voltage;rated voltage of capacitor units;voltage on remaining units in group with units re-maining;neutral-to-ground current flow;rated current of one phase;number of series groups per phase;number of parallel units in one series group;number of units removed from one series group.

The advantage of the unbalance protection scheme shown inFig. 3 using the neutral-to-ground current sensing is its relativelow cost.

The disadvantages of this scheme are as follows:

Fig. 3. Unbalance protection scheme using neutral-to-ground current sensing.

Fig. 4. Neutral-to-ground current sensing in a double-wye-connected bank.

1) sensitive to system unbalance (unless compensated withreal-time input of zero sequence system voltage) whichmay prevent application on large banks;

2) sensitive to triple harmonics and will generally require afilter circuit or a special relay;

3) no indication of involved phase;4) masks balanced failures.In the scheme shown in Fig. 4, a split-wye-connected bank

uses two current transformers and one overcurrent relay. Unbal-ance current flowing in the ground connections due to systemvoltage unbalance will circulate in the CT secondary circuitand will not flow through the relay; internal unbalances willresult in relay operating current. The advantages of the protec-tion scheme for double-wye-connected capacitor banks shownin Fig. 4 are as follows:

1) scheme not sensitive to system unbalance; and thus, it issensitive in detecting capacitor unit outages even on verylarge multiseries group capacitor banks;

2) not affected by harmonic currents;3) relatively inexpensive protection scheme;4) possible to compensate for inherent capacitor tolerances.The disadvantages of the Fig. 4 scheme are as follows:

1) no indication of involved phase;2) masks balanced failures (although relatively unlikely).A voltage unbalance sensing relay scheme can be applied on a

grounded-wye-connected bank using three voltage transformers(VTs) as shown in Fig. 5. (In place of the VTs, it may be eco-nomical to use capacitive coupled potential devices, resistive di-viders, or a combination of these.) This can be created using alarge-capacitance unit in the series group at the ground poten-tial and low-voltage VTs. The zero-sequence voltage is presentduring unbalance conditions in the bank. The advantage of thescheme shown in Fig. 5 is that the scheme can be adjusted toaccount for constant system unbalances.

The disadvantages of the scheme shown in Fig. 5 are as fol-lows:

1) if system unbalance changes, the detection circuit settingwill also have to be adjusted;



Fig. 5. Zero-sequence voltage-sensing method for a singlegrounded-wye-connected bank.

Fig. 6. Differential voltage sensing for a single grounded-wye-connected bankmethod.

2) sensitive to triplen harmonics and will generally requirea filter circuit or a special relay;

3) relatively expensive protection scheme;4) No indication of involved phase;5) masks balanced failures (although relatively unlikely).The protection scheme shown in Fig. 6 uses voltage inputs

from VTs connected to the bus and also connected into thebank. This allows the protective device to develop a differentialvoltage signal. The scheme self-compensates for system voltageunbalances. The advantages of the protection scheme shown inFig. 6 are as follows:

1) scheme not sensitive to system unbalance and, thus, it isextremely sensitive in accurately detecting capacitor unitoutages even on very large multiseries group capacitorbank;

2) indicates involved phase and possibly which portion ofthe phase;

3) possible to compensate for inherent capacitor tolerances.

The disadvantages of the scheme shown in Fig. 6 are as fol-lows:

1) relatively expensive protection scheme due to the quantityand ratings of the VTs;

2) masks balanced failures;3) subject to blocking in case of loss of potential from the

bus VTs.

B. Ungrounded-Wye-Connected Capacitor Banks

For thesingle-wyearrangement of Fig. 7, and each wye ofFig. 9, the allowable number of units that can be removed from

Fig. 7. Unbalance protection scheme for an ungrounded-wye-connectedcapacitor bank using neutral voltage sensing.

one series group, given a maximum on the remaining unitscan be calculated using the following formula:

If is fractional, use the nextlowerwhole number. The relayis then set to signal the alarm upon failure ofunits. To deter-mine the neutral shift voltage and relay setting upon loss of unitsfor the protection scheme shown in Fig. 7, use the following for-mulas:

V

The relay would further be set to trip the bank upon loss ofunits. The neutral shift voltage and relay setting can be

determined by the same formulas using in place of .Similar equations can be derived to determine the relay settingsof for the protection scheme shown in Fig. 9.

For the protection schemes shown in Fig. 7 and each wyeof the protection scheme shown in Fig. 9, the percentage over-voltage for any number of units removed from a series groupcan be determined by using the following formula:

whereneutral shift voltage;applied line-to-neutral voltage;rated voltage of capacitor units;volts on remaining units units removed;number of series groups per phase;number of parallel units in one series group;number of units removes from one series group.

For thesplit-wyearrangement of Fig. 8, the allowable numberof units that can be removed from one series group, given amaximum on the remaining units, can be calculated withthe following formula:

If is fractional, use the nextlowerwhole number. The relayis then set to signal the alarm upon failure ofunits. To deter-



mine the neutral current flow and relay setting upon loss ofunits, the following formula may be used:

Amperes

The relay would further be set to trip the bank upon loss ofunits. The neutral current flow and relay setting can be

determined by this same formula using in place of .The percentage of overvoltage for any number of units re-

moved from a series group can be determined using the fol-lowing formula:

whereapplied line-to-neutral voltage;rated voltage of capacitor units;volts on remaining units units removed;current between neutrals of two banks;rated current of one unit;number of series groups per phase;number of parallel units in one series group;number of units removed from one series group.

Fig. 7 shows a connection diagram for a neutralvoltage sensing unbalance protection scheme for an un-grounded-wye-connected capacitor bank. This scheme maybe used to protect banks of relatively modest size. A VT isconnected between the center point of the wye connection andthe substation ground. Another type of voltage transducer,e.g., coupling capacitor voltage transformer (CCVT), mightbe a more economical application in some high-voltage ap-plications. If all capacitor units are in service and the systemvoltage is balanced, then the center of the wye should be veryclose to ground potential. An overvoltage relay senses voltagethat appears between the center point of the wye connectionand ground indicating an unbalance in the bank due to acapacitor that is out of service, or due to system unbalance. Theadvantages and disadvantages of the scheme shown in Fig. 7are the same as those for the scheme shown in Fig. 3.

Fig. 8 shows the connections for a neutral current sensingunbalance protection scheme for an ungrounded-split-wye con-nected capacitor bank. This is a common and simple connectionto negate the effects of system voltage unbalance. A CT is con-nected between the center point of the wye connection of each ofthe two wye-connected banks. If all capacitor units are in serviceonly small values of current flows in the connection. An over-current relay senses current that flows between the center pointof the wye connection of each bank indicating an unbalance inthe bank due to a capacitor that is out of service. The advantagesand disadvantages of the scheme shown in Fig. 8 are the sameas those for the scheme shown in Fig. 4.

Fig. 9 shows a variation of the split-wye scheme of Fig. 8 ex-cept voltage versus current sensing is employed. The operationprinciples of the two schemes are similar as are the advantagesand disadvantages. Since the VT measures small signals rela-tive to its rating, the CT scheme generally displays greater sen-sitivity.

Fig. 8. Unbalance scheme for an ungrounded-split-wye-connected capacitorbank using neutral CT.

Fig. 9. Neutral voltage unbalance scheme for an ungrounded-split-wyecapacitor bank.

Fig. 10. Impedance-based operation characteristic.

V. IMPEDANCE-BASED UNBALANCE DETECTION

An impedance-based unbalance method was developed inwhich the bus voltage and capacitor bank phase currents areused to determine the actual impedance of each phase of thecapacitor bank. An offset mho characteristic is used. In thecase of an ungrounded-wye bank, a fourth voltage, that of thecapacitor bank neutral to ground is also required to properlydetermine the impedance [4].

The capacitor bank’s nominal impedance is calculated fromits nameplate ratings. This establishes a normal operating pointon the axis, given by :center. See Fig. 10. This calcu-lation is accomplished using the following formula:

(Ohms, secondary)

At time of relay commissioning, provisions are made to ad-just these ideal values to null out any influences of manufacturertolerances in the capacitors and the voltage and current trans-ducers. Typically, an alarm level is computed whereby the relayindicates that some initial capacitor failure has occurred, but the



Fig. 11. Per-string impedance unbalance method for fuseless capacitor banks.

steady-state overvoltages in the capacitor bank are within thecapacitor withstand capabilities on a continuous basis. A triplevel corresponds to the number of capacitor failures that resultin a steady-state overvoltage in excess of the continuous ratingof the capacitors, typically 110% of rated.

Advantages: The advantages of the impedance method com-pared to the methods previously discussed are as follows.

1) It is inherently immune to the masking caused by “bal-anced” failures.

2) It is inherently immune to effects of system voltage un-balance.

3) Faulted phase discrimination is inherent to the scheme.4) For grounded banks where phase CTs will be applied for

overcurrent protection, the use of impedance methods en-sures that no other sensing devices are required, reducingthe cost of the capacitor bank.

Disadvantages:

1) Since impedance is a mathematical combination ofvoltage and current, the error associated with the mea-surement of each is compounded. In the implementedsystem, an overall uncertainty less than 0.5% was real-ized; this is suitable accuracy for a very large populationof banks. Grounded banks permit much less expen-sive/higher accuracy CTs to be utilized if located in theneutral end of the bank.

2) Phase CTs are required if not being used for phase over-current protection.

3) It is subject to blocking in case of loss of potential fromthe bus VT’s.

4) It may require special provisions to account forimpedance changes over temperature.

Advantage with Fuseless Capacitor Banks: The impedancemethod is very appealing for fuseless capacitor bank applica-tions. In the event of a protective relay alarm or trip in a fuse-less bank, it is difficult to determine which capacitor unit orunits contain failed elements as there is no external indicationof failure. Because each phase of a fuseless bank is divided intoparallel strings of capacitors Fig. 11, it is possible to place a CTin each string of each phase. This provides the ability to discrim-inate not only which phase is faulted, but which string or stringscontain faulted units.

VI. CONCLUSIONS

This paper has presented an overview of capacitor bank pro-tection schemes, discussing the variety of techniques that can

be applied to detect unbalances due to failure of individual ca-pacitor units in the bank. The problem of measuring externalquantities to determine the internal voltage distribution withinthe bank was described. Equations were presented to determinethe resulting unbalance as cans are removed from service forboth grounded banks and ungrounded banks.

Protection technologies have been developed by the industryto overcome some of the difficulties in sensing problems withincapacitor banks. The capabilities of microprocessor based re-lays allow the protection engineer to implement sophisticatedprotection logic at a reasonable cost. A new technique usingimpedance monitoring devices was also presented.

REFERENCES

[1] IEEE Guide for the Protection of Shunt Capacitor Banks, IEEE C37.99-2000.

[2] IEEE Standard for Shunt Power Capacitors, IEEE Standard 18-1992.[3] M. T. Bishop, S. R. Mendis, J. C. McCall, and W. M. Hurst, “Capacitor

overcurrent protection for industrial distribution systems,” presented atthe IEEE Petroleum and Chemical Industry Tech. Conf., Vancouver, BC,Canada, September 11–14, 1994, Paper PCIC-94-33.

[4] J. McCall, T. R. Day, S. Wu, and T. Newton, “New techniques for ca-pacitor bank protection and control,” presented at the Western ProtectiveRelay Conference, Spokane, WA, Oct. 1999.

Martin Bishop (S’79–M’79–SM’92) received theB.S.E.P.E. and M.S.E.P.E. degrees from RensselaerPolytechnic Institute, Troy, NY.

He is the Supervisor of the Reliability Improve-ment Studies Section, Systems Engineering Group,Cooper Power Systems, Franksville, WI. Thesection is responsible for studies related to powersystems reliability, including overcurrent protectionsystem applications and the impact on power quality.The section is also responsible for applications ofthe V-PRO II program, and the Distrely program

for distribution reliability analysis. He is also an instructor in the CooperPower Systems’ Overcurrent Protection Workshop, the Distribution AnalysisWorkshop, and the Transformer Application and Protection Workshop.

Tim Day (M’88–SM’00) received the M.S.E.E. de-gree from Washington State University, Pullman, in1991.

He is a Senior Power Systems Engineer inthe Systems Engineering Group, Cooper PowerSystems, Franksville, WI. His present professionalendeavors include modeling and analysis of elec-trical power systems in order to assess and optimizeprotection schemes. He enhances existing protectivealgorithms and develops customized schemes for theEdisonPro line of relays and incorporates Cooper

Power Systems’ simulator to verify all scheme modifications.

Arvind Chaudhary (S’85–M’85–SM’94) receivedthe B.S.E.E. degree from the Indian Institute ofScience, Bangalore, India, the M.S.E.E. degree fromNorth Carolina State University, Raleigh, and thePh.D. degree with a concentration in electric powerengineering from Virginia Polytechnic Institute andState University, Blacksburg.

He is a Staff Engineer with the Protective Relaysand Integrated Systems Group, Cooper Power Sys-tems, Franksville, WI. He is responsible for relay ap-plications for the Cooper lines of relays and relay set-

tings for power system equipment, including capacitor banks. His previous ex-perience has included Sargent & Lundy consulting engineers (1991–1998) andBharat Heavy Electricals Limited, India (1979–1983).


Documents

IEEE-A Primer on Capacitor Bank Protection