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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 200016Application of Transformer Ground DifferentialProtection RelaysPeter E. Sutherland, Senior Member, IEEEAbstractTransformer ground differential protection relays(device 87G) have been used to protect the windings of resis-tance-grounded transformers. A number of strategies have beenutilized with electromechanical relays in the past. With the adventof the multifunction digital protective relay, these strategies canbe adapted to continue this form of protection.Index TermsDifferential relaying, ground-fault protection,protective relaying, transformer protection.I. INTRODUCTIONINDUSTRIAL power distribution system substation trans-formers often utilize resistance-grounded wye secondarywindings for medium-voltage power distribution. The pur-pose of this is to limit damage due to ground-fault currents,while providing sufficient fault current for the operation ofground-fault relaying. The relaying utilized for the protectionagainst ground faults in the system may not provide sufficientprotection of the transformer winding against internal faultsbecause the backup ground overcurrent relay in the transformerneutral-to-ground connection must be set to coordinate withdownstream relays. In order to protect the winding itself,special relays are utilized. These include the following: ground differential protection with a time-overcurrentrelay; ground differential protection with a percentage differen-tial relay; ground differential protection with a percentage differen-tial relay designed for this application; product-type ground directional overcurrent relay; restricted earth-fault relays.Ground differential protection is now provided in multifunc-tion digital relays. Transformer protection relays may includethis feature using one of the schemes used with component re-lays. If a feeder protection relay is used on the secondary, insome cases, this may have a ground directional feature that canbe utilized for ground differential protection.Paper ICPSD 9915, presented at the 1999 IEEE/IAS Industrial and Com-mercial Power Systems Technical Conference, Sparks, NV, May 27, and ap-proved for publication in the IEEE TRANSACTIONS ONINDUSTRYAPPLICATIONSby the Power Systems Protection Committee of the IEEE Industry ApplicationsSociety. Manuscript submitted for review May 6, 1999 and released for publi-cation July 24, 1999.The author is with Power Systems Energy Consulting, GE Energy Services,Schenectady, NY 12345 USA (e-mail: peter.sutherland).Publisher Item Identifier S 0093-9994(00)00038-4.Fig. 1.Time overcurrent relay connected as a differential ground relay.II. GROUNDDIFFERENTIALSCHEMESThe schemes discussed here have been implemented withcomponent-type relays, where one relay performs each func-tion.A. Ground Differential Protection with a Time-OvercurrentRelayThe simplest method of ground differential relaying is toconnect a time-overcurrent relay between the residual point ofthe phase CTs and a neutralground CT (Fig. 1) 2. Becausethe CT ratios are usually not equal, an auxiliary matching CTis required. This application will require a neutralgroundCT with a high saturation voltage. There have been problemswith the misoperation of this scheme using electromechanicalrelays. However, the application of low-burden electronicrelays makes this scheme much more secure.The sensitivity required depends upon the portion of thewinding to be protected. Assuming that the voltage is induceduniformly across the windings, a relay which is set at 5% of themaximumground-faultcurrentwillprotect95%ofthewinding.This would require a sensitivity of 20 A for a 400-A groundingresistor. The design issue is to select a relayCT combinationthat will be sufficiently sensitive to cover the winding yet beinsensitive to external faults. If there is unequal remnant flux inthe phase CTs, a ground-fault current may be sensed when an00939994/00$10.00 2000 IEEE17IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 2000TABLE IPERFORMANCE OF87GWITHVARIOUSRELAYCT COMBINATIONSexternal phasephase fault occurs. For this reason, a relay witha restraint winding is normally preferred for this application.An external ground fault, in the worst case, will be the full400 A, and may cause saturation of the CT on the resistor. (Theline-side CTs will have a higher ratio and can be assumed notto saturate.) Table I shows the calculation of the required min-imum CT voltages where the leakage current is less than therelay tap setting. CT saturation calculation methods are given in1. When electromechanical relays are used, many commonlyused auxiliary CTs will not meet this requirement. It is neces-sarythatahigh-accuracyCTbeselected.However,theuseofanelectronic relay effectively eliminates this problem. With stan-dard relays, a pickup setting of 0.5 or 1.0 A would be used.The electronic relay used in this application may have a typ-ical burden of less than 0.1, regardless of setting. Thus, therewould be no potential problems of false operation on externalfaults due to CT saturation. Electronic overcurrent relays havenot, however, been used very frequently in this application. Asingle-phase unit would be necessary. A packaged three-phaserelaywouldneedanadditionalground-faultfunctionwithasep-arate current input.B. Ground Differential Protection with a PercentageDifferential RelayThis will replace the time-overcurrent relay (87G) of Fig. 1with a percentage differential relay (Fig. 2). With the auxiliaryCT in use, both windings may be set on the same tap.The purpose of the percentage differential relay is to permitsensitive operation for low fault currents while preventingmisoperation due to CT saturation on high-current externalfaults. The CT leakage current at the maximum external faultcurrent (here, 400 A) is calculated. It is then divided by therelay current and compared to the slope of the relay. IfCTRSlopewhereis the CT excitation current, then the relay will notoperate for external faults.The sensitivity of the percentage differential relay will typi-cally be about 30%40% of tap setting. The tap setting shouldbe selected so that 95% coverage of the winding is achieved.Single-phasepercentagedifferentialrelayshavelongbeen available in electromechanical versions. Electronic per-centage differential relays are typically available in packagedFig. 2.Percentage differential relay connected as a differential ground relay.three-phase units designed for the protection of a particularpiece of equipment. This scheme could be implemented byadding it to such a multifunction relay.C. Ground Differential Protection with a PercentageDifferential Relay Designed for This ApplicationThis type of induction disk relay has its operating coil in theneutralground CT circuit, and three restraint coils, one in eachphase of the circuit breaker CT circuits (Fig. 3). The restraintcoils produce a torque in proportion to the square of their cur-rent.Thismeansthattherelayisnondirectional,andshouldonlybe used when there are no other sources of ground-fault currentthanthetransformerbeingprotected.Therestraintcoilsproducetorque in a direction that opposes the closing of the relay con-tacts.This type of relay was made in electromechanical form, butno electronic version has been produced.D. Product-Type Ground Directional Overcurrent Relay 4The product-type relay is an induction disk unit originally in-tended as a directional overcurrent relay for the protection oftransmission lines against ground faults 3, 4. The electro-mechanical product type relay itself is of interest mainly be-cause of its extensive use in existing installations. No electronicversion of this relay has been produced.It has two coils, which are placed in the neutral and residualcircuits. The operating torque is proportional to the product ofthe two currents and the cosine of the phase angle betweenthem. When the two currents are in the same direction, con-tact closing torque is produced. Fig. 4(a) shows the relay con-nectedasadirectionalrelay,whileFig.4(b)showstherelaycon-nected as a differential relay. It should be noted that the schemein 3 is slightly different from that shown in the figures, but hasthe same general effect. When used for protection of the trans-former winding, the direction of maximum torque is into thetransformer. The construction of this now-obsolete relay typeSUTHERLAND: APPLICATION OF TRANSFORMER GROUND DIFFERENTIAL PROTECTION RELAYS18Fig. 3.Transformer ground differential relay.resulted in constraints on the relationship of operating and re-straining current which do not exist with newer technology.The burden of the relay coils could result in saturation of theneutral CT for the maximum ground-fault current. This wouldreducethecurrentintotherelayfromthissource.Foranexternalfault, a current will be produced in an opposing direction by thephase CTs and auxiliary CT. If the current from the auxiliaryCT is not reduced by saturation, while that from the ground CTis, then a net reverse current will flow into the operating coil.This will result in a contact opening torque. If no CT saturationwere involved, then the operating coil would have a net currentof zero, and no torque would be produced. Thus, CT saturationadds to the reliability of the scheme.E. Restricted Earth-Fault RelaysThe restricted earth-fault relay 2, 5 is a high-impedancedifferential relay scheme (Fig. 5), which is widely used in Eu-rope.Itisessentiallysimilartothehigh-impedancebusdifferen-tialrelay.AllCTsmusthavethesameratio.Thevoltagesettingof the relay is higher than the voltage that would be seen if oneof theCTssaturated for anexternalfault.The relayisprotectedby a surge arrester against damage from the high voltages thatwill be seen for faults within its zone of protection.Forthisexample,with1200/5CTsasabove,thevoltageseenon CT saturation for an external fault iswhereisthemaximumthroughfaultcurrent,istheCTratiowith resistance, and lead resistance. If the phase-faultcurrent is assumed to be 6500 AV(a)(b)Fig. 4.(a) Product-type relay connected as a directional ground relay. (b)Product-type relay connected as a differential ground relay.The relay is set by means of an adjustable resistor in series withthe operating element. The sensitivity of the relay isAwhereis relay operating current at its setting (this data pro-videdbyrelaymanufacturer),is thenumberof CTsandisthe CT leakage current at the setting voltage (determined fromCT saturation curve). In this example, 95% of the winding isprotected.Because the restricted earth-fault relay is designed to operatewith saturated CTs, no external time delay is required.III. MULTIFUNCTIONMICROPROCESSORRELAYSA. Ground Differential Protection with a PercentageDifferential Relay FunctionNew transformer protection schemes are usually built usingmultifunction microprocessor relays. Some relays of this type19IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 2000Fig. 5.Restricted earth-fault relay.incorporate a ground differential function. This may be imple-mented as a percentage differential relay with inputs from eachof the phase CTs and the ground CT. The relay connections areas shown in Fig. 6. The block diagram of the protective functionis shown in Fig. 7.For a 3750-kVA 4160-V transformer, the relay settings areas follows. The current to be detected for 95% coverage of thewinding with a 400-A resistor iscoverageAThe full-load phase current is 520 A. The ground differentialcurrentis the difference between the ground currentandthe residual currentThe residual currentcan be found by looking at the in-crease in per-unit primary currentdue to thefault. If the relayistoprotect95%ofthewinding,thisisasmalleffect,whichcanusually be ignored. For example, if there is 20 A flowing into aninternal ground fault, the secondary load current in that phase isnot decreased by 20 A. In fact, it is increased by 1 A. This canbe seen from Fig. 8, where the secondary winding is consideredas an autotransformer. Letbe the current in the faulted sec-tion,be the fault current, andbe the current in the rest ofthe winding due to the fault. Then,by Kirchhoffs law, and because of the turns ratio,can becompared to the full-voltage resistor currentFig. 6.Multifunction microprocessor transformer protection relay.These equations can be solved forwithA,A,A, andis. Then,the differential current isAThe relay will be set to pickup at this value.The slope is set so that the relay will trip for the minimumdifferential current at the maximum line current (full-load cur-rent) :SlopeIf the 1-A correction had been neglected, the result wouldhavebeen essentiallythesame.In ordertopreventfalse trippingdue to CT errors, the minimum slope setting should be about4%. This will decrease the sensitivity slightly under full-loadconditions. Because the slope is calculated based upon the max-imum phase current rather than the current into the differentialcircuit, the percentage values appear to be lower than with thetraditional differential relay, although the effect is similar.The final setting is for time delay. This will be set to allowforinstantaneous relays downstream to clear high current faults.B. Ground Differential Protection with a DirectionalOvercurrent Relay FunctionAnothermultifunctionmicroprocessorrelaythatmaybeseenonatransformeristhefeederprotectionrelay.Manysuchrelayscontain a ground directional function, which may be configuredin a similar manner to the product-type relay discussed above.In order for this to be possible, the types of polarization avail-able should be examined, and the CT input connections. Therelay must have a separate ground CT input which provides theSUTHERLAND: APPLICATION OF TRANSFORMER GROUND DIFFERENTIAL PROTECTION RELAYS20Fig. 7.Multifunction relay ground differential block diagram 6.Fig. 8.Transformer secondary winding ground-fault currents.operating current for this function. Residual current should notbe used as the operating current.For relays with current polarization, and a separate currentpolarizing input, there should also be a separate ground CTinput.If thisisthecase,thenthe relaycanbeconnectedwith theoperating coil in the differential circuit and the polarizing coilin the ground circuit, as with the product-type relay Fig. 4(b).The relay coils should be connected with the polarization suchthat the relay will trip for a ground fault within the differentialzone.Voltage polarization may also be used. The relay would thenbe a directional relay looking into the transformer, rather than atrue differential relay.Dual polarization uses both current and voltage polarization.Becausecurrentpolarizationwouldalsobeavailable,thereisnoreason to use dual polarization.IV. COMPARISON WITHOTHERFORMS OFPROTECTIONThe choice of whether to apply ground differential in addi-tion to transformer differential protection (87) and ground over-current protection (51G) will depend upon the size and impor-tance of the transformer being protected. The usual guideline isto start ground differential protection with 10-MVA and largertransformers 7.With ground overcurrent protection (51G) on a resistance-grounded system, pickup is normally set at 25% of the resistorrating, which results in 75% coverage of the winding. GroundTABLE IISUMMARY OFGROUNDDIFFERENTIALPROTECTIONdifferential protection typically protects 95% of the winding, anincrease of 20%.Transformer differential protection with harmonic restraint(87) is recommended on units rated 5 MVA and above 7. Theprimary current due to the fault will be quite small for faultsnear the neutral point, because the grounding resistor is seen onthe primary side through the square of the turns ratio. A faultat 5% of the winding will increase the primary side current by0.25% of what it would be for a full winding fault. With ourprevious example of 3750 kVA at 138004160 V, with a 400-Aresistor, the primary current for a phaseground fault would be21IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 200069.6 A. With a 300/5 CT, the relay would see 1.16 A. A fault at5% of the winding would result in a relay current of 0.0029 A.For a typical differential relay sensitivity of 0.20 A, the deepestlocation where a ground fault could be detected would be atwindingCTRwhereis the relay sensitivity, CTR is the CT ratio,andis the primary current for maximum secondarylineground-fault current. For the example givenwindingofthewinding.Thus,thephasedifferentialrelayinthisexampleprotects 58% of the winding against ground faults. A grounddifferential relay that protects 95% of the winding protects 37%more of the winding than the phase differential relay.The destructive force caused by a fault is a function of thesquare of the current, which is controlled by the grounding re-sistor, not the size of the transformer. The closer the fault is tothe neutral point, the lower is the likelihood that it will do anydamage.Becauseofthelowervoltage,thelikelihoodofthefaultoccurring is also smaller. The effect of such a fault is to short afew turns through a resistor, causing a very small unbalance inthe output from the transformer (see calculations above).With smaller transformers, the damage caused by a groundfault that is not detectable by the phase differential protectionmay be quite small. A ground differential relay could trip atransformer for a fault that the transformer could have survived.V. CONCLUSIONSIn the past, ground differential relaying schemes were im-plemented with electromechanical relays. These require carefulevaluation in terms of sensitivity, effect of CT saturation, cost,and complexity. The various types of protection considered aresummarized in Table II. Many of these schemes are no longerused in new designs, as electronic relays have come into gen-eral use. Those schemes which can be implemented using asingle-phase overcurrent, differential, or restricted earth-faultrelay can be constructed with electronic relays. The specializedtypes of transformer ground differential relays have not beenreplaced with electronic equivalents. Transformer protection isbecoming the province of packaged multifunction relays, andthis is where ground differential protection will be found. Ei-ther transformer- or feeder-type relays can be utilized, if theycontain the necessary functions.While harmonic-restraint phase differential relays have beenapplied on transformers of 5 MVA or greater, transformerground differential relays have been applied for transformersof over 10 MVA 7. In cases where a greater degree ofprotection is desired, these relays have been applied on smallertransformers. With the introduction of multifunction electronicrelays, the issue of the cost of adding this protection decreasessignificantly, becoming only a small adder for the extra relayfunction. An evaluation must then be made of the degree ofprotection required for the size transformer being considered.The added protection provided by ground differential prot