ref no 1 design and evaluation of a circulating current differential relay test system

8/7/2019 Ref No 1 Design and Evaluation of a Circulating Current Differential Relay Test System

1/7

IEEE Transactions on Power Delivery, Vol. 13, No. 2, April 1998 427Design and Evaluation of a Circulating CurrentDifferential Relay Test System

P.A. Crossley, H.Y. LiUMIST, Manchester, UKManchester Centre for Electrical Energy

Abstract:- This paper describes how an EMTP basedsimulator can be used in conjunction with a relay test set totype-test a protection relay operating within a circulatingcurrent differential protection scheme. The combination of asimulator and a test-set is designed to replace a high powersynthetic test plant that has been used for more than 30 yearsfor type and approval testing of differential protection relays.The paper will include results that describe the operatingperformance of the new test system, on both internal andexternal faults, and these will be compared with experimentalresults obtained from an identical set of tests on the synthetictest plant.Keywords:- Circulating current differential protection,current transformer behaviour, digital modelling of currenttransformers, differential protection test system, synthetic testplant.

1. INTRODUCTIONA current differential protection relay derives an operatingsignal that is based on the vector summation of the currentsignals measured by current transformers located at theextremities of the protected zone. If this signal, referred to asthe differential signal, exceeds an operating threshold therelay concludes that a fault exists within its protected zone.Depending on the type of differential protection, the relay willthen either trip all the circuit breakers that are required toisolate the zone fi-om the network, or will trip a single breakerand rely on other relays within the protection scheme toisolate the zone. All differential protection schemes aredesigned to detect and clear, in an acceptable time period, allshort-circuit faults that occur within its zone ofprotection, butremain stable, or inoperative, during all load or external faultconditions. These objectives can be difficult to achieve, sincethe electro-magnetic Current Transformers (CTs) may benon-linear during a fault. Th e non-linearity is associated withthe current required to excite or magnetise the CT core. Theexcitation current depends on the magnitude of the fluxdensity within the core and the B-H characteristics of the corePE-853-PWRD-2-06-1997 A paper recommended and approved bythe \E Power System Relaying Committee of the I EEE PowerEngineering Society for publication in the IEE E Transactions on PowerDelivery. Manuscript submitted December 27, 1996; made availablefor printing June 11, 1997.

A.D. ParkerStafford, UKGEC ALSTHOM T&D

material. When considering a protection CT, the flux densityrequired to drive the normal load current and low values offault current through the relays connected to the CT isnormally well within the linear region of the B-H curve.Consequently, the excitation current is small and thesecondary current is a scaled down replica of the primarycurrent. However, if the burden connected to a CT issignificant and the fault current is high and contains a dcoffset, then unless a large over-dimensioned CT has beenselected, the flux in the core will enter the saturated region ofthe B-H curve. During the period when the flux is within thisregion, the excitation current will be high and the secondarycurrent will be severely distorted.Saturation of a CT resulting from a high current fault locatedwithin the protected zone is unlikely to affect the operation ofthe differential protection on this fault. The magnitude of thedifferential current will be reduced but it will still exceed theoperating threshold by a large margin. The main problemcaused by an internal fault is the effect on the fbture stabilityof the protection of the remanent flux left in the CT after theinternal fault has been cleared. Since the load current isunlikely to reduce the remanent flux to a low value, the fluxin the core will continue to cycle around the remanent value,using a minor B-H loop. This will continue until the nexthigh current fault occurs. Then, depending on the Point-On-Wave (POW) at which the fault occurs, the remanent flux inthe core may either increase, driving the CT into deepersaturation, or decrease, reducing or eliminating the level ofsaturation. The former may cause a major problem for adifferential protection relay, if the second fault is locatedoutside the protected zone and one of the current transformershas been left by a previous fault with a high value ofremanent flux, whilst the flux in the other CT or CTs is low.The differential current, which should be zero for an externalfault, will now be non zero and may exceed the operatingthreshold of the relay. In practise, differential relays maintaintheir stability, even when a CT is saturated, by using a biasedoperating characteristic or a stabilising resistor or acombination of both, The limitation of these solutions is thatany increase in the stability of the protection results in adecrease in the sensitivity.When designing a new type of relay for use within adifferential protection scheme, it is necessary to confirm thatthe protection when used with the recommended CTs, canachieve a realistic level of stability on external faults, whilstmaintaining an acceptable level of sensitivity on internal

088.5-8977/98/$10.00 0 1997 IEEE


2/7


3/7


4/7

430the second and third fault cycles. This is because the results infigure 3 were obtained for a fault at a POW of 270, afterthree previous faults all at POWs of 90'; the results in figure4 were obtained for a fault at a POW of 270' after fourprevious bult, the last at 270' and the others at 90' .Theresults in figure 3 show how the remanence in the CTchanges from a positive value to a negative value. The resultsin figure 4 shows how the remanence increases in thenegative direction for repeated faults at the same POW. Whencomparing the current in shunt-1 in figure 3 with that infigure 4, it can seen that in figure 4 saturation starts almostimmediately after the fault occurs, whilst in figure 3saturation is delayed for approximately 1 cycle. Whencomparing the current in shunt-2 in figure 3 with that infigure 4, it can again be seen that in figure 4 saturation startsimmediately after the fault occurs, whilst in figure 3saturation is delayed for approximately 2 cycles. As expected,this confirms that the POW at which the fault occurs affectsthe output from a CT.It can be seen in figure 3, that for the first 2 cycles after faultoccurrence the differential current is high since CT1 issaturated and CT2 is unsaturated. Me r 2 cycles CT1 andCT2 are both saturated, their currents are similar butopposite, and consequently the magnitude of the differentialcurrent is small.The results in figure 4 were obtained when the 5th fault wasat a POW of 270', the same POW as the 4th fault.Consequently the level of remanence in the CTs is small. Theexperimental results clearly demonstrate the effects on theremanence of multiple faults applied at different POWs.3.2 EMTP Relay Test System Evaluations and ValidationsThe circulating current differential protection relay testsystem shown in figure 1was implemented using the EMTPsimulator. The CTs were modelled using the CT modelsavailable in the EMTP auxiliary program HYSDAT. Thisrequires the utilisation of the hysteresis and V-I curves [4].Based on the V-I curves of the CTs used in the synthetic testplant, a CT knee point voltage of 160V and an excitationcurrent of 0.56A were selected as the inputs to the HYSDATprogram. The hysteresis loop generated by the HYSDATprogram was incorporated within theCT model.Themodel was used to simulate the output m m t s &om CT1and CT2 and the differential current under the same internaland external fault conditions as investigated using the STP.The resulting currents, I1and I2 at RI and R2were stored andsubsequently played back via a relay test set. The output fromthe amplifiers in the test set were electrically connectedtogether and then connected to the relay, i.e. the differentialcurrent is injected into the relay input circuit. The simulatedtest system was used to evaluate low and high impedancedifferential protection scheme. Typical examples of th edifferential currents observed with both schemes are shown insections 3.2.1 and 3.2.2 respectively.

3.2.1 Low Impedance SchemeThe parameters for the low impedance scheme of relay testingare the same as in section 3.1, i.e. the power system X/R is40, the stabilising resistor is 1.7!2, the fault is A-N and thesteady state fault current is 10x1,. Figures 5 to 7 show boththe simulated and experimental differential currents obtainedduring a sequence of three consecutive faults, the 1st was aninternal fault with a steady state cwrent magnitude of 10Aand full positive dc offset (POW = 90), the 2nd and 3rd wereexternal faults with a steady state current magnitude of 10Aand full positive dc ofiet.

F a u l t P k i n t of Wave a t i S O aI

Time. 40mS/d iv

Fig.5 Comparison between simulated and experimentaldifferential currents (1st internal fault shots at POW of90')

Faul t eo in t of [ W a v e a$ QOa\ hi

SoL+3 Line:+ E x p e - e n t a l F e s u l t~ a & ine:- SimzLL&tion reSuL t

Time, 40mS/divFig.6 Comparison between simulated and experimentaldifferential currents (2nd consecutive external faultshots at POW of 90')In figures 5 to 7, the solid lines are the differential currentsobtained using the STP (experimental results), and the dashlines are the differential current obtained using EMTP(simulation results). Figure 5 shows the differential currentduring the first internal fault shot (90'). In this case, onlyCT1, was driven into saturation. When comparing the solidline (experiment) with dash line (simulation), it can be seen


5/7

that the simulation produces a larger differential current thanthe experiment during the first cycle but a smaller differentialcurrent on all subsequent cycles. When the currents from CT1and CT2 are examined, it was found that the exciting currentin the simulated CT2 was higher than the equivalent currentobserved with the STP. This can be explained by consideringthe hysteresis loop of a CT in the STP as compared to theEMTP CT. The EMTP CT has a lower gradient in theunsaturated region than the STP CT. Consequently, for thesame exciting voltage more current will be diverted throughthe exciting branch in the EMTP CT than that in the STP CT.

S O L ~ O ? ~ine:- $xperirnjentaL .rdsuLtDash il


6/7

432selection of the results obtained are included in this sub-section and the conditions under which the tests wereperformed are summarised in Table 1. In all cases theremanence in both CTs was zero prior to the start of each twofault test sequence. In all cases both faults were applied at thesame POW and at the same location. For each simulatedexample, the currents flowing out of CTl (signal-1) and CT2(signal-2) and the relay differential current signals weremeasured and plotted on a single graph. The results for thetest cases are discussed as follows:-Table 1 Relay testing conditionsI Test. I Fault I P-0-W I WR I If

Key:- Ext. = External A-phase-earth faultInt. = Internal A phase-earth faultIf= fault current (multiples of nominal current)&ab = stabilising resistanceCT turns ratio of 1 10,CT lead resistor of RI=l.OR, R2=0.0RRelay burden=O 3a.

Test 1:- an external fault is applied to a power system with anX/R of 10 and the relay stabilising resistance is 0.0 ohms.Figure 10shows that during the 1st fault there are two cyclesdelay before CT1 starts to saturate and the differentialcurrent, 1-U becomes finite. During the 2nd fault, I ismuch great& due to the large remanent flux existing in theCTs after the 1st fault.

. . . . . . . . . .......l&t IQ* p&- i o& I End shot berjodsw ................................, I : : j S w l j o f j ~ w i : o n : :. . . . . .I . .. . . .

- - " " " ..... .................. ,_... \. ... ...............1.. .-. "....". : . Y . . .. . .. . . : : : : : : : '1. . . . . . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . ............................................. . . . . .I I I I .. I , . . . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . . . . . .

T i m e , 4 0 m s / d i vFig.10 Test 1:- Currents associated with two consecutiveexternal faults at POWs of 90'. (WR=IO, &ab =OQ,1=CT1 output, 2=CT2 output, 3=differential current)

Test 2:- same conditions as test 1 except WR is 30. Figure 11 shows that due to this change, and its effect on the saturationof CT 1, the magnitude of the differential current is increased.

. . .I I & tshot pekiod : 2ndj elsot keriodlC lG3E 2

1dr+-El

. . - . : : : : : ; ~ 1 - ' 1............. ............. ................,........... G .....,......2 1. .___ ;. . .. . . . . . . . .. . . . . . . .. . . . . . . ..............3 . . . . . . . . . . .. . . . . . . . . . .

Time. 4Oms/div

Fig. 11 Test 2:- Currents associated with two consecutive A-Nexternal faults at POWSof 90'. (X/R=30, ab =on)Test 3:- same condition as test 1 except the stabilisingresistor is 2.8 ohms. Figure 12 shows that due to this change,the magnitude of the differential current is reduced.

Time, 4COme/d ivFig.12 Test 3:- Currents associated with two consecutive A-Nexternal faults at POWS of 90' (X/R=30, ab =2.80)

Time. .I.Oms/divFig.13 Test 4:- Currents associaited with two consecutive A-Nexternal faults (X/R=30, b = 1 O O f i )


7/7

433Test 4:- same condition as test 2, except the stabilisingresistor is 100ohms (i.e. high impedance scheme). Figure 13shows that the differential Current has been reduced to a lowvalue as compared to the results in test 1. This demonstratesthat on an external fault a high impedance schemesignificantly reduces the magnitude of the differential current

ACKNOWLEDGEMENTn e uthors would like to acknowledge the support of GECLSTHOM T&D and Manchester centreor ~l~~i~.lEnergy,UMIST.

Test 5:- describes the performance of a differential protectionscheme during an internal fault. The system parameters aresimilar to those in test 3, except that the faults are internaland their POWs are 90. Figure 14 shows that for an internalfault the magnitude of the differential current is significantlylarger than that observed with the external fault in test 3.Note:- the former is plotted at 40Aldiv, the latter at 2OAldiv..

. . .

. . .. . ............... . .

. . . . . . . . . . .. . . . . . . . . . .. . ...............3

. . . . . . . . . .Time. + O m e/ d I v

REFIZRENCES[l] A. K.S. Chaudhary, K-S Tam and A. G. Phadke,Protection System Representation in the ElectromagneticTransients Program EEE Trans. on Power Delivery,Vol. 9, No. 2, April 1994.[2] M. Kezunovic, L.J. Kojovic and all, ExperimentalEvaluation of EMTP-Based Current Transformer Modelsfor Protective Relay Transient Study, IEEE Transactionson Power Delivery, Vo1.9, No.1, January 1994, pp. 405-412.[3] H.Y. Li, E.P. Southern, P.A. Crossley S. Potts, S.D.A.

Pickering, B.R J.Caunce and G.C. Weller A New Typeof Differential Feeder Protection Relay using GlobalPositioning System for Data Synchronisation, IEEEPower Summer Meeting (SM 381-4 PWRD), denver,1996.[4] M. Monseu, and all, Evaluation of Characteristics andPerformance of Power System Protection Relays andProtection Systems, CIGRE-SC34-WGO4. January 1985.[5 ] Omicron Electronics, 1995, CMC Version 2.5 UserFig.14 Test 5:-Currents associated with two consecutive A-N mnuar.internal faults (X/R=IO, R =2.8 Q, POW=27O0)

BIOGRAPHY4. CONCLUSIONThis paper has described how an EMTP based simulator canbe used in conjunction with a relay test set to evaluate theoperating performance of a relay operating within acirculating current differential protection scheme. In additionto modelling the effect of a fault on the primary powersystem, the simulator also represents the effect on thesecondary current signals of directly connecting multiplecurrent transformers into a single input on the relay. Theaccuracy of the simulator was validated using results obtainedfrom a synthetic test plant. Close agreement between themeasured current signals and the simulated current signalswere observed in all tests. All the results presented in thispaper are based on CTs with a 1: 0turn ratio and knee pointvoltage of 160V. Tests have also been performed on the STPusing CT s with 130 turns ratio and a knee point voltage of240V. The results from these tests have been compared withwith 1:SO turns ratio. The results described in this paper andother results obtained using 1230 turns ratio CTs support theconclusion that EMTP can be used to implement a circulatingcurrent differential protection test system.

those obtained &om BMTP test system using the same CT

P.A. Crossley is a lecturer in Electrical Engineering atUMIST. He graduated with a BSc degree from UMIST in1977 and a PhD degree from the University of Cambridge in1983. During the period 1977-1990, he worked for GECALSTHOM Protection and Control on the design andapplication of digital protection relays. He is a member of theIEE and IEEEH.Y. Li received the BEng and MSc degrees ffom HIT andShanghai University of Technology, China, in 1982 and 1985respectively. He attended the University of Bath, UK, in 1990,and received the PhD degree in 1994. He is currently aresearch associate in Electrical Engineering at UMIST, UK.A.D. Parker received the BEng degree fi-om Liverpool in1972. After early training in the aircraft industry, he joinedGEC ALSTHOM T&D Protection and Control in 1978. He iscurrently a Senior Engineer in Relay DevelopmentDepartment and is a member of the IEE.

ref no 1 design and evaluation of a circulating current differential relay test system

Documents