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REAL-TIME TRANSIENT TESTING AND PERFORMANCE OFTRANSFORMER DIFFERENTIAL RELAYS

NORMAN T.STRINGER, LARRY JAWHEAD, TIM WILKERSON, JASON BIGGS, G.D. ROCKEFELLER*,PE ., MEMBER MEMBER NON-MEMBER MEMBER LIFE FELLOWBasler Electric CompanyRoute 143, P. 0. Box 269

Highland, IL 62249Abstract-Differential relays a re used extensively t o

provide protection for power transformers. Several issuesmus t be considered in their application to ensureadequate protection. These include transform er magne-tizing inru sh current, relay burden and current tra ns-former performance. The use of low ratio c urrent tra ns-formers in high magnitude faul t applications canproduce distorted secondary waveforms to the relaywhose performance may be directly affected. There isalso a relationship between the magnetizing inrush andcurrent trans former performance which may affect theoperation of the differential relay. This paper discussesthese specific applications and the testing of a t rans -former differential relay under conditions of sa tura tedCTs using the Electro-Magnetic Transient Program(EMTP).

I. INTRODUCTIONThe phenomena of transformer magnetizing inru shcurrent h as been discussed in many papers t hroughoutthe years. [l-71 In 1958 the complexity and characteris-tics of single phase versus three phase banks werediscussed in detail. [l ] A brief review of magnetiz inginru sh follows to provide the basis of need for tr ansi ent

testing.Previous studi es have shown that inrush currents

contain harmonic components of the fundamenta lwaveform. [l-21 Conventional philosophy has been toutilize the second harmonic current t o provide inhibitedoperation of differential relays because of its dominance.Manufacturers have used various designs regardinghigher order harmonics. The use of second and thi rd, a swell as the inclusion of all harmonics, have been seenover the years. The relay considered in this paperutilized t he second and fifth harmonic curre nts toprovide restr aint on magnetizing inrush. The reasons forthis will be discussed later.

Magnetizing inrush phenomena in three phasetransformer banks s more complicated tha n in s inglephase banks. Because of the inherent phase angledifferences in the th ree phase cu rrents, one phase willexperience initial inrush. It i s also possible tha t moretha n one phase will experience inrush. The magnitude ofthe instantaneous phase inru sh currents can also differdepending on the electrical connections of th e tr ans -former windings.

Another item th at will affect the i nrush current isthe residual or rem anent lux remaining in t he

*Rockefeller Associates, Inc.96 Sylvan DriveMorris Plains, N J 07950

transformers core. When a transformer is de-energized,a remanent flux will typically remain in the core. Themagnitude of the rem anent flux depends o n severalfactors:

A. the point on the voltage waveform when th etransformer is de-energized

B. the type of electrical connections of the windingsC. the type of core material usedD. the power factor of the load a t the point ofinterruptionMagnetizing inrush has a close relationship with

the nonlinearity and hysteresis characteristics of thetransformers iron core.As a resu lt, a s the closing angleincreases from 0 degrees, the magnitude of the inrushcurrent decreases. Therefore, the maximum inrushcurrent is obtained when the closing angle is 0 degrees.

Figure 1shows the relationship between theexciting current an d the flux in the transformer core.The flux waveform will be sim ilar in shape to thewaveform applied a t the term inals of the transformer.In this case a 60 cycle sinusoidal waveform is used.The polarity and magnitude of the remanent flux willdepend on the point of de-energization.

0-7803-3008-0195$4.000 1995 IEEE 1142

Fig. 1 Exciting current and f lux in transformer coreWhen re-energized, the core flux will increase fromthe remanent flux point. If th e re-energization occurs atthe maximum negative value on the applied waveform,

the full peak-to-peak magnitude will be added to theexisting remanent flux. The resu lting excitation cur rentwill increase from zero. Because most modern transform-ers are operated nea r maximum flux density, increases inexciting current during inr ush conditions will mostprobably drive the transformer into saturation with


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resulting harmonic currents.

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11. RELAY TRANSIENT TESTINGThe influence of remanent flux to peak inrushcurrent is significant during the first few cycles, and

then begins to decrease quickly. It is relatively flat bythe 10th cycle. The polarity of the remanent flux is alsoa dominant factor of the inrush cu rrent. During the firstfew cycles, a positive polar ity remanent flux producessignificantly higher inrush current tha n does a negativepolarity flux.Another major component in the applica tion of

transformer protection i s th e current transformer. Inmany cases, low ratio toroidal CT's a re used i nswitchgear [3]. he fault duty a t these locations istypically very high. And because these low ratio CT's ar enormally of lower accuracy class ratings as well, theywill have a tendency to saturate.

A satura ted CT fails to deliver a t rue rep resenta-tion of the primary current. Saturated CT's will producesome amount of harmonic curren t - predominantly thirdharmonic. I n fact, early designs of bus differential relaysutilized harmonic restrain t to provide security againsttripping on external faults which caused severe satura-tion of the current transformers. 121However, an internalfault can cause CT saturat ion jus t as severe as anexternal fault.

Table 1 below ha s been exerted from Reference [21to show the harmonic content for the three conditions -internal fault with no CT saturation , external fault withCT saturation, and trans former magnetizing inrush.

FundamentalSecond HarmonicThird HarmonicFourth HarmonicFifth HarmonicSixth HarmonicSeventh Harmonic

Differential Current Due To:

Fault MagnetizingInrushl 145%38%

100%9%' 1%

4%

~ :;2%

~~126%0%100%4%32 %9%2%1Yo3yo

Table 1. Harmonic wave analysis of typical currentsappearing in differential relay circuits due to variouscausesTherefore, without proper relay design, magnetiz-

ing inrush and current transformer saturation can causefalse tripping and produce overvoltage from harmonicresonance. Herein lies the problem. A differential relaymust be able to distinguish between magnetizing inrushharmonics and those harmonics caused by a saturatedCT. Because of the relay's difficulty to distinguishbetween these differences in the time domain, it isnecessary to perform harmonic analysis of inrush currentin the frequency domain.

Transient testing is a critical aspect of the relaydesign verification process. It proves tha t th e relay willperform a s intended. Other types of testing, such as in-service testing, are marginal due to the infrequency ofsystem operations and the inability to control conditions.

Relay transient testing provides a means tosimulate real world conditions in a controlled environ-ment. It provides the ability to challenge the relay undertes t, without th e effects of in-service testing. The primepurpose of the transient testing presented in thi s paperwas to evaluate the performance of a transformerdifferentia l relay in the presence of severe deveredissimilar CT saturation.Also, setting criteria for thisrelay related to CT performance was examined.

There are several types of relay transien t testingavailable. Lab or bench testing provides ANSI standardfast transient, surge withstand capability, and radiofrequency interference testing. In addition, high powerlabs or power system simulators provide more real worldconditions; however, they are limited in control of systemconfiguration. Similar to high power labs is factorytransient testing which is accomplished using availableutili ty supply and associated equipment. This methodalso ha s limited control of system configurations.

To address th is limitation, Electro-MagneticTransient Program testing is available. These testssupplement transient factory tests, using a 120/208V,2000 Amp source. EMTP generated currents providedtests which confirmed the designs and provided informa-tion for setting criteria.

111. EMTP TRANSIENT TESTINGEMTP is a computer driven non-linear analysisprogram, simulating the real world as seen at the relayterminals. I t permits greate r control of system conditionssuch as L/R, fault inception angles, fault magnitudes,and so on. With EMTP, worst case scenarios can becreated with multiple cases.One of the major advantages of EMTP testing is

the great flexibility available in circuit configuration andsystem parameters. The abil ity to specify (and control)th e L/R ratio is an example. Analog elements tend tohave too low an L/R atio to match field conditionsadequately. The EMTP software allows the user totranscend these hardware limitations in a softwareenvironment. EMTP also allows the user to control theangle of transient inception. Other benefits of theprogram are th at the primary transient, easily generatedoff-line, is isolated from issues such a s the influence ofCT burden.

The downside of EMTP is that t he associatedprograms and documentation are not generally userfriendly. The te st facilities can also be expensive because

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there are only a few of these facilities available. Theadvantage of being able t o run several hundred widelydiverse test cases in a very shor t period of time greatlyoutweighs these disadvantages.

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W.FACTORY TRANSIENT TESTS

41

The first step in design verification test s wascompleted a t the factory. Inru sh test s used a 52kva, 3phase transformer connected to th e local utility system.External fault tes ts verified secur ity with no CT (C-infinity accuracy class) on one side and a l/1 C-10 ringCT on the other side of the transformer differentialcircuit. The characteristics without a CT simulated a"good" CT which would not reach sa turat ion . Thecharacteristic of the "bad" CT simulate s satura tion at aminimal input value. Connections for the se externalfault tests are shown in Figure 2. These "good ctmad ct"test s included dramatic differences in s atur atio n charac-teristics which are typical of many applications. Thefault inception angles were random, and the systemparameters were fixed.

Figure 2. Factory simulation of External Faultswith Severe Dissimilar CT Saturat ionOnce factory transient test ing is complete, and ahigh degree of confidence is achieved i n th e design, real-time transient testing is in order to confirm theseresults.

V.EMTP TESTING FACILITYThe facility used for these tes ts was funded by TheEmpire S tat e Electric Energy Research Company

(ESEERCO). It was designed an d operated by ElectricResearch and Management. ESEERCO is the researcharm of the NY utilities. The tes t facility is located inStat e College, PA, near the campus of Penn St ateUniversity.The ESEERCO facility can accept field recorded aswell as computer simulated da ta. The dat a for our tests

was all generated by off-line EMTP simulations. EMTP

generated data is very appropriate for general designconfirmation and the development of setting criter ia,since it can provide a broad spectrum of cases for a widevariety of applications.

The main elements of the EMTP facility are shownin Figure 3.Once a particular model has been pro-grammed, the parameter values are specified for eachcase. Each case is the n run off-line. The EMTP output isloaded into the test system and then converted, in real-time, to analog form. These analog signals ar e amplifiedand applied to the relay under test, with the resultingaction monitored.v Off Line- t eal Time4

Parameter-Selection

7-Figure 3. Testing Using EMTP-Simulated Data

The real-time elements ar e along the fa r wall infigure 4.The two computers in the foreground allowedsimultaneous generation of new EMTP cases while relaytesting of previous cases proceeded.

Figure 4. Test Facility Using EMTP SimulationsTests were performed using non-linear power

transformer and CT models. Power transformer rema-nence was included i n th e calculations. Consistent withthe low burden of solid state relays, relay burden was

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neglected in t he model.VI. TRANSFORMER DIFFERENTIAL EMTP TESTS

The relay under test uses conventional secondharmonic rest rain t philosophy, as well as an unre-strained function tha t provides an ou tput as a multipleof input TAP, and includes a sm all inverse delay toensure security. Fifth harmonic r estraint is included toensure security during overexcitation conditions. Therelay under test uses analog electronics with activefiltering and gapped core input CTs. It has no conven-tional operate winding, deriving the unbalance curren twithin the electronics. Figure 5shows a representativeblock diagram of the relay, including connections ofmonitoring points. Note that the fifth harmonic restr aintcircuit is not shown in the figure, as the function is notmonitored in these tes ts.

Figure 5. Tkansformer Differential Relaywith Oscillogram ConnectionsTransient testing of the relay had two major goals.

The primary goal was to ensu re proper relay operationduring extreme system conditions. Relay response toseverely dissimilar CT satur atio n was of particularconcern,as was response dur ing magnetizing inru shconditions. The o ther goal of the EMTP testing was toformulate and validate the setting criteria for therestraint slope characteristic and t he unrestrainedpickup setting. Figure 6shows the through currentrestraint characteristic of the transformer differentialrelay.

Figure 6. Transformer Differential Relay OperatingRegion

I I

Figure 7. Modelling and Connections forTransformer Differential Relay Testing

VII. ElVITP TRANSFORMER MODELLINGDetails of the transformer model used for the te sts

are shown in figure 7.The transformer was non-linearlymodeled, including hysteresis. The current transformermodels were also non-linear, with no remanence. Theeffect of remanence was accounted for by varying theeffectiveANSI accuracy class.

Series elemen ts of the model consisted of th eleakage resistance and inductance of the power trans-former, along with the source equivalent resis tance (Re)and inductance (Le). The CT burden included externalresistancesRg an d Rb in addition to winding resistanceand leakage inductance. For these tests, the leakageinductance was neglected (appropriate for C class CTs).

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The relay inductance was neglected since the relay ha snegligible burden. Computed current data (Ib and Ig)was loaded into the tes t facility (figure 6b), where it wasconverted into analog signals. The analog signals wereamplified and applied t o the relay under test. Systemperformance was monitored on a digital storage oscillo-graph.

Approximately 50 system conditions were tested,with over 200 individual test cases. Three representativecases are included for discussion.VIII. TRANSFORMER DIFFERENTIALRELAY -

SELECTED CASE STUDIESCase 46: External Fault wi th Full DC Offset

This case was a n external fault created by closingswitch S2 on Figure 7 with the voltage a t 0 degrees. Thisclosing angle provides maximum DC offset. The goodCT (transformer high side) was equivalent to a C-1500with a 1ohm burden. The bad CT was C-100, with 2ohms burden. The relay was set a t th e 2.0 A Tap (bothinputs ), with a slope of 15% (minimum sett ing) andunrest rained tripping at 12 times tap (24A). The unre-strained trip sett ing was intentionally low for this te stcase.

Oscillogram traces for this case are shown inFigure 8. Refer to Figure 5for relation of traces t o testsetup. DC offset was not too appare nt in Traces 1and 2,due to the attenuation effect of gapped cores in t he relayinput transformers. This att enuation is the reason forthe gapped core CTs. Note tha t t he first peak of trace 2(Ig) was clipped by the relays amplifier.

This case shows severe saturation of the bad CT (Ibtrace) and negligible saturat ion of the good CT (Ig trace).This caused a substantial unbalance current, as shownin Trace 4 (Iop).

Trace 7 (PU) shows that sufficient unbalancecurren t was developed to cause differential relay re -strained uni t pickup (i.e. in t he operating region ofFigure 6).Undesired tripping was prevented by theharmonic inhibit circuit of the relay as shown in Trace 8(HI).The time between relay pickup (PU trace) andharmonic rest rain t was compensated for by a coordina-tion delay in t he relay circuitry. The desired harmonicinhibit of the restrained unit was enabled by the highsecond harmonic content of the saturated CT output.Note that the relay was set at 15% slope characteristic,providing minimum through-current restra int.

Trace 6 (URO) shows that t he unrestrained outputof the relay did operate, causing an undesired relay trip.This undesired tripping was due t o the intentionally lowsetting of the unres trained unit. The recommendedset ting for these conditions would be 17 x TAP, instead of12 x TAP used. Repeat test s (not included in this paper)proved proper security with a set ting of 13x TAP.

Note tha t a s the bad CT recovered from sat urat ionat about 280 milliseconds, the error-current Iop (Trace 4)has substantially decreased, and the restrained pickup(Trace 7) resets as conditions enter the non-operate zonein Figure 6.

Case 212: Transformer Energization - Worst CaseThis case was simu lating transformer energization(closingS1 in Figure 7). Source voltage was set to 140%

of rated, t o produce an extremely large inrush condition.The transformer model was connected wye - groundedwye, with simultaneous phase energization. Phase Aclosing was at 0 degrees, with 49% transformer rema-nence on all three phases. The remanence polarity wasset to increase th e flux excursion on phase A. Maximumenergization cu rrent was on A phase. The good CT(trans former high side) was a n equivalent C-1500 with a1 o hm burden. The relay is se t at the 2.OA Tap (bothinputs), with a slope of 15% (minimum) and un restra inedtripping at 16 times t ap (32A).Zero sequence blockingand phase shifting required for the CT connections wasenabled through the internal CT compensation ability ofth e 3 phase relay used (rat her than using delta con-nected CTs). Figure 9 shows the oscillogram traces forthis case.

Traces 1 , 2 and 7 show the relay signals derivedfrom phases A, B and C, respectively. Trace 4 shows the Aphase cu rren t applied to the relay. Note that the relayclipped the waveform in the first 2 cycles (compare traces4 and 1).Substan tial unbalance or operating signal wasdeveloped,as shown in Trace 5 (OP).

For this case, the second harmonic unit properlyinhibited relay operation, as shown in Trace 8.Theunrestrained unit of the relay, shown in Trace 6, trippedundesirably.As with the previous case, this i s due to a nintentionally low set ting on the relay. With an unre -stra ined unit setting of 17 x TAP, the relay was secure.Note that this is a very extreme test case, due to the140% energization voltage.

The negative excursion of the A phase inpu t,beginning in the thi rd cycle, indicates satu ration of theC-1500 high side CT. A lower qual ity CT would havesatu rate d more severely, developing more negativeexcursion. With a delta t ransfo rmer winding (or deltaconnected CTs), the relay inp ut cu rrents may be pre-dominantly oscillatory.

Case 176: Internal Fault - Source SideThis case was an inte rnal faul t (S3 losing onFigure 7) applied at the transformer high side terminals.

The fau lt inception angle was 30 degrees. The good CT(transformer high side) was a n equivalent C-1500 with a1ohm burden. The bad CT was C-200,with 2 ohmsburden. The relay was set a t the 2.0 A Tap (both inputs ),with a slope of 15% (minimum) and unrestrainedtripping a t 17 times t ap (34A). Figure 10 shows theoscillogram traces for this case,

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The fault caused significant operate curre nt(Traces 4 and 51, and associated pickup of both therestrained and unrestrained units of th e relay. The C-1500 CT experienced severe DC saturat ion caused by theoffset current. This produced sufficient second harmoniccontent to operate the harmonic inhibit function (Trace8).The undesired inhibit las ted about 7 cycles, delayingthe relays restra ined output (Trace 3). However, therelay tripped by unrestrained outpu t (Trace 6) wellbefore the undesirable harmonic inh ibit reset. Theunrestrained trip setting of 17 x TAP had been verifiedsecure in th e previous tests.

Note that CTs experiencing symmetrical (AC)satura tion will produce sustained odd harmonics, but noeven harmonics. The 5 th harmonic inhib it of the relaywas set at 35% of fundamental, to be above expectedlevels during faults.

CONCLUSIONSReal-time transient testing of relays is an effective

alternative to high power test labs for protective relaydesign development and verification. The ability todefine system conditions in detail, off-line, permitsextensive evaluation, at a reasonable cost. Various whatif scenarios can be tested, covering many worst caseevents.

Transient testing of the trans former differentialrelay presented in this paper suppor ted the conclusionsreached in factory design verification tests and confirmedthe security of the relay design, and verified setti ngcriteria for slope and un restra ined pickup. he capabilityof the unrestrained unit to operate properly and t rip t hebreaker for a severely saturated CT, indicates thenecessity for this type of testing. With conventional typeunits , the harmonic inhibit function would have operatedand resulted in undue damage of the transformer.

Testing of this na ture provides the u ser theassurance that the relay will perform to the applicationsand t hat setting criteria, a s well as relay security anddependability have been verified. Several papers haveaddressed CT performance under sat ura ted conditions.However, to determine t he actua l response of th e relay,design verification testing mus t go one ste p further.Continued and expanded use of real-time transi enttesting in the evaluation of new relay designs, andreview and modification of existing relays is highlyrecommended.

REFERENCES1. W.K. Sonnemann, C.L. Wagner, G.D. Rockefeller,

Magnetizing Inrush Phenomena i n TransformerBanks, AIEE Transactions, Pa rt 111, Vol. 77,Oct. 1958, pp 884-92

2. James E. Waldron and Stanley E. Zocholl,Design Considerations for a New Solid StateTransformer Differential Relay with HarmonicRestraint, Western Protective Relay Confer-. ence, Oct. 1978.

3. The Relay Performance Considerations withLow-Ratio Current Transformers and High Fau ltCurrents Working Group of the IEEE PES PowerSystem Relaying Committee: C.W. Barnett ,et al., Relay Performance Considerations withLow-Ratio CTS and High Fault Current s,I&CPS 1993 Technical Conference, May 1993,pages 154-165.

4. G.D. Rockefeller, System Test Report - BaslerBE1-87T Transformer Differential Relay(Factory Tests) , Basler Publication 99-1244,July 1990.

5. G.D. Rockefeller, System Test Report - BaslerBE1-87T Transformer Differential Relay - TestsConducted November 1990 and Ju ly 1991(EMTP Tests), September, 1991.

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00530429 Real Time Measure Me Net