Protection functionsTransformer differentialANSI code 87THigh set pointPercentage-based curveA non-restrained differential current set point willThe percentage-based curve prises a number of segments,which are definedensure fast tripping in the event of significant faultas follows:currents.This threshold must be set to a value higherthan that of the inrush current.2 straight lines crossing zero and with adjustable slopes(ld/It and Id/It2)the slope change point.The curve must be set so that it can protect itself against current-sensormeasurement errors and transformation errors,which can be attributed to the tapchanger.Furthemmore,the protection function must be made immune to powershunts on auxiliary windings.Setting zone forslope change point3min.Tripping zone(lds max.)0.3(lds min.)Slope change pointSelf-adaptive restraintThe self-adaptive restraint is particularly suitable for transfommers,where:where linr is the peak tripping currentIn is the rated peak currentThis neutral work restraint ensures stability in the event of an external fault byanalyzing the second-and fifth-harmonic factors,the differential currents and thethrough currents.It ensures stability:in the event of the transformer closingin the event of an asymmetrical fault outside the zone,which saturates the currentin the event of the transformer being operated on avoltage supply,which is toohigh (overfluxing).Detecting the presence of hammonics and taking into account the through anddifferential currents,the restraint automatically increases the low set point and thepercentage-based slopesIt is also more sensitive than the high set point.It is,therefore,not essential to usethe high set point when this restraint is active.Furthemmore,as the restraintintegrates the stabilization slope for high through currents,which can saturate thecurrent sensors,slope ld/It2 does not have to be activatedConventional restraintThe conventional restraint prises a second-hammonic set point for each phaseand a fifth-harmonic set point for each phaseThe second-harmonic set point ensures that the protection function will not pick upin the event of the transfommer closing or the current sensors being saturated.The restraint can be global (cross-blocking:all three phases are restrained as soonas the harmonic in one phase exceeds the set point)or phase-specific(nocross-blocking:only the phase with a harmonic exceeding the set point is restrained).Cross-blocking is remended for transformers used in three-phase mode.The fifth-hammonic set point ensures that the protection function will not pick up in theevent of the transfommer being connected to a voltage supply,which is too high.The restraint can be global (i.e.,all three phases are restrained)or phase-specific(only the phase with a hammonic exceeding the set point is restrained).Restraintwithout cross-blocking is remended for normal operation.15830May2005SEPED303001EN June 2005Protection functionsTransformer differentialANSI code 87TRestraint on closingsufficient to activate hammonic restraints.An additional restraint can be activated:when the through current exceeds an adjustable set point Isinrby an internal variable,P87T_1_118,controlled by logic equations or LogipamThis restraint is applied to the percentage-based differential elements for anadjustable time period T.It is not applied to the high set point.Max.(It1,It2,It3)>IsinRestraint ofclosingRestraint on CT lossCTloss can distort the differential current and cause nuisance tripping.This restraintdetects a measurement dropping to zero abnormally by analyzing sampleddifferential and through currents.Dimensioning phase-current sensorsInRwThe primary rated current of the current transformers is govemed by the followinginrule:87TThe dimensioning of the current sensors for windings 1 and 2 is determined by theand 2.In is the primary rated current of the CT.I'nin is the secondary rated current of the CTRCTRCT is the internal resistance of the CT.Rw is the resistance of the wiring and the CT loadScenario 1:linr <6.7/2InOnly the following current transformers can be selected:■type5P20 with a rated burden VAcT≥Rw.lin2or defined by a kneepoint voltage Vk(RCT Rw).20.inOnly the following cumrent transformers can be selected:■type5 Pwith:口a rated burden VAcT≥Rw.in2or defined by a kneepoint voltage Vk(RCT+Rw).3.-2.In'SEPED303001EN-June 200530May2005159Protection functionsTransformer differentialANSI code 87TCharacteristicsSettingsLow set point ldsSetting range30%to 100%of In1Resolution1%Drop-out/pick-up ratio93.5%5%Percentage-based characteristic Id/ItSetting range15%to50%Resolution1%Drop-out/pick-up ratio93.5%5%Percentage-based characteristic Id/It2Setting rangeResolutionDrop-out/pick-up ratiog3.5%5%3Slope change pointSetting rangeResolution0.1 In1Drop-out/pick-up ratio93.5%5%Test modeSetting rangeActive/Not activeAdvanced settingsSelection of restraintRestraint on CT lossSetting rangeActive/Not activeRestraint on closingSetting rangeActive/Not activeMagizationSetting rangecurrent set pointAccuracy()5%ResolutionDrop-outpick-up ratio90%5%0r0.5%n1Time delaySetting range0to3008Accuracy)%or -10 ms to +25 msReso lutionHigh set point ldmaxSetting rangeConventional restraint3to 18 In1Self-adaptive restraintNone,3 to 18 In1Resolution1%Drop-out/pick-up ratio93.5%5%Second-harmonic set point for conventional restraintSetting rangeNone,5 to 40%ResolutionDrop-out/pick-up ratio90%5%Second-harmonic restraint for conventional restraintSetting rangePhase-specific/GlobalFifth-harmonic set point for conventional restraintSetting rangeNone,5 to 40%Resolution1%Drop-out/pick-up ratio90%士5%Fifth-harmonic restraint for conventional restraintSetting rangePhase-specific/GlobalCharacteristic timesOperating time high set pointOperating time percentage-based curve45 ms at 2 ldReset time45 ms at2 ldInputsDesignationSyntaxEquations LogipamProtection inhibition■Restraint on closingP87T-1-118OutputsDesignationSyntaxEquations Logipam MatrixProtection outputHigh set point■■Percentage-based thresholdCT loss(1)Under reference conditions (IEC 60255-6).Test mode■■16030May2005SEPED303001EN-June 2005Protection functionsTransformer differentialANSI code 87TExample 14 MVA,Dyn11,20 KV/1 kV transformer,the peak closing current is:inr =5inThe transformer operates normally at its rated load,but will tolerate operation at upto 120%of its rated power.Un1 20 kVn1=116ASensor sele ctionThe rated current of the windings is:S=4 MVA4MVA4 MVA=2.3kAThe current sensors can support an overload of 120 %In2 2.3 kAln>116AX12=139.2Aand'n>2.3kAX1.2=2.76kAThe main currents of the current sensors must also meet the following requirements:I'nSo,for this transformer:11.6 As In s 290 A and 230A s I'n s 5.75 kA3Taking these two restrictions into account,the values selected are thosestandardized by the IEC:In 150 A and In =3 kAnr1=5X2x116A=820A1nr2=5X2x2.3kA=16.3kAThese tripping currents must be pared with the rated current of the currentsensors in order to select the accuracy limit factor.820AThe accuracy limit factor is,therefore,20,with a rated burden of:VAcT≥Rw.in2.The following sensors are selected:for winding 1:150 A/1 A,5P20 where VACT1for winding 2:3 kA/1 A,5P20 where VACT2.Setting the percentage-based curve and the maximum thresholdAs this transformer does not feature a tap changer or an auxiliary winding,thetripping threshold is,therefore,set to a minimum value(lds =30%)and the slope toAs the ratio between the closing current and the rated current is less than 8/2,theself-adaptive hammonic restraint is selected.The second slope on the percentage-based curve and the maximum threshold are not necessary and are not,therefore,used.SEPED303001EN-June 200530May2005161Protection functionsTransformer differentialCode ANSI 87TExample 2in2.5 MVA,Dyn11,20.8 kV/420 V transformer,the peak closing current is:The transfommer features a tap changer with a tap range of 15%of the rated voltageof winding 2.Un1=20.8kWSensor selectionS=2.5 MVAThe rated current of the windings is:Dyn11Changer±15%2.5 MVA3.4 kAThanks to the tap changer,the current sensors can support an overload of 115%:h>69AX1.15=79.4Aand'n>3.4kAX1.15=3.91kAThe main currents of the current sensors must also meet the following requirements.36.9 A s In s 173 Aand 340 As I'n s 8.5 kATaking these two restrictions into account,the values selected are thosestandardized by the IEC:In 100 A and I'n =4 kAThe tripping current is Tinr =9.6 in,so,for both winding 1 and winding 2:These tripping currents must be pared with the rated current of the currentsensors in order to select the accuracy limit factor:937A=72x100A=6.6<67and46.2kA=8.2>6.7The accuracy limit factor is,therefore,20 for the sensors in winding 1and equal46.2kA24.5 for winding 2.The closest standard value,i.e.,30,is selected.for winding 1:100 A/1A,5P20for winding 2:4kA/1A,5P30.Setting the percentage-based curve and the maximum thresholdThis transformer features a tap changer.The continuous differential current due tothe voltage variation of the tap changer is:where x is the maximum variation of the tap changer.In this example,x =0.15The differential current due to the change in the transformation ratio is:ldchanger015Type 5P sensors with a maximum measurement error tolerance of 10%are used.The measurement accuracy of the relay is +1%for lds and Id/It.The minimum setting is,therefore:Assuming a margin of approximately 5%,the minimum setting is,therefore:lds=17.6+10+1+534%lds and the ld/It slope are set to 34 %The ratio between the closing current and the rated current is 9.6.As this ratio is greater than 8//2,the conventional harmonic restraint is selectedThe second slope on the percentage-based curve is set to 70%,starting at 6 In1 inorder to ensure sufficient stability of the protection fault in the event of external faultsThe high set point is set to a value higher than that of the closing current with thefollowing margin:linr=2x9.6ln1=13.6ln1The conventional harmonic restraint is set with:a second-harmonic set point equal to 20 %with cross-blockinga fifth-harmonic set point equal to 25 %without cross-blocking.16230May2005SEPED303001EN June 2005