Magnetic Saturation at Short-Circuit Tests onPower Transformers

A.L.J. Janssen, L.H. te Paske, W.A. van der Linden, R.P.P. SmeetsKEMA High-Power Laboratory

Arnhem, the Netherlands. e-mail: hpl@kema.nl

Abstract: The post-set and pre-set short-circuit test methodsforpower transformers are discussed, K&VIIb testing expediencewithspecial attention to the pre-set method, inrushcurrents andprernagnetization, are highlighted, By means of pre-magneti-i:ationinrushcurrents can be avoided to a large extend.The saturation of the supply side transformers leads to adistortionof the supplyside voltage, thuspreventing saturationoflhe test objects. The dl~ferentsaturationmodes of the transformerunder test are shortly mentioned in relation to the dynamic~:tresseson the windings.

Keywords: Power transformers, short-circuit tests, pre-set shortcircuit, inrush currents, pre-magnetization, magnetic saturation

1. INTRODUCTION

In order to test the ability of large power transformers towithstand short-circuit currents, two methods of applyingthe short-circuit are defined in the Standards: the post-setshort-circuit and the pre-set short-circuit method.

A. Post-set short-circuit

With the post-set method the transformer is energized atone side with the other side open. After the inrush currentshave disappeared the short-circuit is switched on at theother side.Before short-circuiting, the source voltage is applied at theterminals of the transformer, so that the source voltage hasto be limited in relation to the rated transformer voltage.To the IEEE :Standards [1] a maximum source voltage of[10% related to the rated tap voltage of the transformerunder test is allowed and to the IEC Standards [4] 115/0.

13.Pre-set short-circuit

With the pre-set method one side of the transformer isshort-circuitecl before the application of the voltage. Thesource voltage may differ to a large extend from thetransformer voltage, as long as the specified short-circuitcurrent flows. For testing, the required short circuit poweris much lower than the power for testing in post-setcondition. [6].The post-set short circuit method is mostly applied in teststations that are supplied from the grid. There is enoughpower available, but the main drawback is the difficulty toachieve the correct voltage, especially for the maximumtap position. The main drawback of test stations with own

generators is the limit of the available power. At KEMAHigh Power Laboratory, being the largest short-circuitlaboratory in the world with 4 generators and a short-circuit power of about 10.000 MVA 3-phase and5.800 MVA l-phase (at 60 Hz), the pre-set short-circuitmethod is quite often applied, mostly in the 1,5 phasemethod. Hereby, the phase under test is connected in seriesto the other two parallel connected phases in a Y-con-nection (for a schematic outline, see fig. A 1 in Appendix).Note that the power required fi-om the test station is muchsmaller than the short-circuit power in the network (20.000MVA @ 245 kV to 83.500 @ 800 kV).

The fact that the inrush current is superimposed to theshort-circuit current is inherent to the pre-set method. But,due to the short-circuited windings, the inrush current isnot comparable with a normal inrush current or the inrushcurrent of the post-set method. The effects of the inrushcurrents during pre-set short-circuits are discussed in thisreport.

II INRUSH CURRENTS

Both manufacturers and testing authorities worry about thenon-linear effects of the magnetic flux and the residualmagnetism, resulting in magnetizing currents and inrushcurrents.Under no-load conditions, the energization of a trans-former leads to a sinusoidal magnetic flux, that is flowingcompletely in the magnetic core: pm(ij. For a particulartransformer with a given magnetic characteristic, the fluxand therefore the inrush current is depending on theamplitude of the voltage applied, on the phase angle of thevoltage applied and on the residual flux (@r):

V(t) = U sin(o t +u)(jm(t) = @ [1- COS(Ot+cx)]+@r

With the post-set testing method the applied voltage can beas large as 11So/O of the rated tapping voltage, thus gener-ating inrush currents, which are far larger than those undernormal service conditions. A very accurate and veryreliable controlled switching (synchronous switching) ofthe HV-circuit-breaker, to energize the transformer atvoltage maximum rather than at voltage zero, is aprerequisite for the post-set testing condition [8], [9].

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Opposite to the no-load situation, the energization of ashort circuited transformer gives a large component of themagnetic flux flowing outside the magnetic core,The magnetic flux consists of three components:@m the part flowing completely through the core and

forming the coupling between the windings@ow the part of the leakage flux that flows through the

gap between the windings and returns through themagnetic core

@ol the part of the leakage flux that flows through thegap between the windings and does not return viathe magnetic core, but via oil or tank.

Supposing that the applied voltage is identical to that of theno-load energization, the at-component of @m undershort-circuit conditions is much smaller than under no-loadconditions (40 to 600/0), caused by the voltage drop acrossthe leakage impedance. As a part of the leakage flux(@om) returns via the magnetic core, that part of the corehas to conduct @m + @rim. But, even then the ac com-ponent of @ZU+ CPgmis smaller than the ac component of@m under no-load conditions. So, with the same state ofresidual flux and the same voltage angle at energization ashort-circuited transformer gives less saturation effects,only local saturation (see chapter 6) and lower inrushcurrents.

Compared to the random energization under normalservice conditions, the above is an advantage for the pre-set testing method. In addition, the applied voltage is 100/0in comparison to the 110 to 115/0 used with the post-setmethod. The main drawback of the pre-set testing methodis that the voltage angle has to be the most onerous one: a100% offset of the flux is inherent to the requirement oftesting with a.full asymmetrical short-circuit current.

With the post-set method a magnetic flux is already flow-ing through the core, so that at the initiation of the short-circuit (at voltage zero), the flux has the maximum ampli-tude belonging to the terminal voltage (that can be as highas 115%0).Ccmsequently, after short-circuiting the flux willfollow smoothly a COSfunction, superimposed to -1 15%.Again a vet-y accurate and reliable closing of the high-vohage circuit-breaker, now applied at the short-circuitside, is required.

Post-seta. Initiation of magnetization (most onerous moment):V(t) =1. 15i7 sin(m t)(p(t) = 1.15@ [l-cos((o t)]+mr =>q)Peak =2.3@+@,with U and @ as 1 pu.

b. Initiation c}fmagnetization (optimal moment):v(t) = 1.15U Cos((llt)T(t) =l.15@sin(of)+@r $pea/c 1.15@+@r

c. At initiation of short-circuit:V(t) =1. 15U sin(o t)(f(f) = 1.15@cos(ol t)

d. After initiation of short-circuit:V(t) = U sin((ir t)(p(f) = -o.15m Ocos(co f)

Pre-setV(t) = U sin(~ t)Q(t) =@@COS(13t)+@r >~peak =2@+@7

L+

1 4 pre.magnltlzingshot + fulltest I

0

Fig. 1 Pre-magnetization of magnetic core

III. PRE-MAGNETIZATION

The extra magnetizing current forced by the residual flux,that results fi-om the former test, can be influenced bycontrolling the direction or polarity of the residual flux.KEMA takes advantage of the residual or remanent flux(remanence) by deliberately pre-magnetizing the magneticcore in the opposite polarity, see fig. 1.When necessary, the correct pre-magnetization is forcedby one or two short 70~o to SOY.tests preceding the succ-essive 10OO/.tests. Now the remanent flux will be subtract-ed instead of added to the transient flux. In this way,during the 100V0test, the saturation effects are limited.Such intermediate tests are regarded as harmless for thetransformer under test as the forces and stresses are lessthan 50% compared to the full tests. See [11].

IV CONTROLLING THE RESIDUAL FLUX

A possibility to estimate the residual flux is the appearanceof inrush currents. Inrush currents appear when the mag-netic core gets saturated, meaning that the residual fluxafter switching off is at maximum or close to its maximumpossible value. An example of a test series is shown in fig.2.Starting with the residual flux from the former test, thevoltage excitation (which is a sine function in order toachieve the full asymmetric short-circuit current) leads toan ac component in the flux and a de-component:

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I I

k. Irl1s2

l!!L_-....I test 06

A T ------------ ______, ._,

I test 05 I1

Ust

,---------------

Irl

1s2

Fig. 2: Test results with various degrees of saturation (see text). Upper traces (Ust): voltage (20 kV/div); middle traces (Irl): source side current (20kA/div); lower traces (1s2):current at short circuited side (20 kA/div). Time: 20 ms/div.

FresldudstartTEST 00excitation +0,5

FresidualstartTEST 04

excitation +1.0

- start

-resldua(

SATURATIDb(

TEST 01excitation -0,7

max.resldua

.~

start

SATURATION

- start

- residual

TEST 02excrhtlon -1,0

Flax,resldua~ start

- residual

- start

TEST 03excitation +1,0

star7

SATURATID~

~esldua(

TEST 05 TEST 06 TEST 07excitation +1.0 excrhitlon +1.0 excltmtlon -1.0

Fig, 3: Schematic representation of variation of remanent magnetization in the course of the tests, shown in fig. 2.

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V(t) = U sin(m t)

(p(t) =CD, +CD-alcos(cot)

After a few cycles the de-component of the short-circuitcurrent has disappeared to a large extend. (But, like for theinrush current, the time constants for the remanent flux andthe de-component of the flux are much larger and itsdecrease is therefore neglected.) The short-circuit currentwill be interrupted at a current zero, meaning that cos(cot)is zero. The new residual flux is then ~r + ~.This is illustrated in fig. 3, where the evolvement of theresidual flux during succeeding tests (fig.2) is given.Ailer saturation one or two full tests are possible beforeagain saturation (but of opposite polarity) is reached (tests03 and 04). The residual flux cannot be larger than themaximum given in the BH-curve, as shown at tests 05 and06.

P Irl.-,, I

int(Ust)

Im

Fig. 4: Test result with high saturationUpper oscillogram: upper trace (Ust): voltage (20 kV/div); middle trace(Irl): source side current (20 kA/div); lower trace (1s2): current at short-

circuited side (20 kA/div).Lower oscillogram: upper trace: integrated voltage Ust (Int(Ust)) (flux)

(33 Vs/div); lower trace (Ire) magnetizing current 10 kA/div.Time: 20 ms/div,

The magnetization current can be determined by subtract-ing the secondary current multiplied with the transformerratio, from the primary current. In figure 4 the inrush cur-rent belonging to test 02 is shown. Also the flux is shownas the integral of Ust, but the starting point (i.e. the residualflLlxof the former test) is missing.

An alternative way to force the transformer into saturationis by means of a low voltage de-source. Such a method ispresently under investigation.

V SATURATION OF THE SOURCE

The pre-magnetization procedure explained in the formerchapters helps also to prevent saturation of the step-uptransformers of the High-Power Laboratory, used tosupply the transformer under test. However, another aspecthas to be mentioned.

Saturation of the supply side transformers gives inrush cur-rents at the generator side, but at the load side the current isdecreased. In fig. 5 the load side current of the saturatedtransformers of the High-Power Laboratory can be seen.This current, flowing into the transformer under test,shows no inrush peaks at all and is flattened in comparisonto the source side currents (Irl ) in the figs. 2 and 4. Thismeans that saturation of the source leads to lower stresseson the test object.

To evaluate the short-circuit tests, the total short-circuitcurrent is measured, including components that can be des-cribed as magnetization currents. The peak value of thecurrent measured is compared with the specified currentpeak in the Standards.In case of saturation in the transformers at KEMAs testplant, it maybe a difficult task to achieve the required peakvalues of the short-circuit current. Caretid estimation ofthe residual flux and adequate pre-magnetization togetherwith a full utilization of the capabilities of the High-PowerLaboratory are necessary.

VI SATURATION EFFECTS OF THE TRANSFORMER

The purpose of short-circuit tests is to verify the ability ofa certain design to withstand large currents. The localstresses in a winding are determined by the current flowingthrough that winding and the magnetic induction at thatlocation. The magnetic field direction and the magneticfield strength are influenced by saturated parts of the mag-netic core.The leakage flux will partly follow the magnetic core andstimulate saturation. The pattern of the leakage flux is de-pending on the winding that is short-circuited (inner orouter winding) and the tapping windings (and tap used).Split windings give even more complicated patterns.

In the gap between the primary and secondary windingsthe leakage fluxes of the two windings flow in the samedirection. The leakage flux of the inner winding returnsmainly through the limb. That of the outer winding flowspartly through the yokes, partly through other iron parts ofthe transformer and partly through the free space in oil.

The returning leakage flux of the short-circuited windingwill flow opposite to the main magnetic flux, so reducing

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the saturation, if any. The returning leakage flux of thesupplying winding will flow in the same direction, in thisway stimulating the saturation in either the yoke (supply tothe outer winding) or the limb (supply to the inner wind-ing). Short-circuiting the outer winding and supplying theinner winding is therefore regarded as giving larger

impedances, different currents, different electro-dynamicstresses.

7. Saturation of the test plant transformers has the effect toreduce the inrush current in advantage to the transformerunder test.

magnetizing currents than the other way around.REFERENCES

I

Ust

1s2

Fig. 5: Test result with saturation of the source. Upper trace (Ust):voltage (20 kV/div); lower (1s2):current at short-circuited side (1O

VII. CONCLUSIONS

1. At short-circuit tests, switching at or around voltagezero is a must to achieve the proper peak values of theshort-circuit currents. But the voltage drop across theleakage impedance reduces the flux in the magnetic core incomparison to the no-load energization.

2. Based on the direction of the flow of the leakage fluxthrough the magnetic core, it is, generally speaking, prefer-red to supply the outer winding and to short-circuit theinner winding in order to reduce its contribution to localsaturation.

3. An accurate and reliable controlled switching is a meansto reduce the inrush currents during the energization oftransformers subjected to the short-circuit test under post-set conditions where voltages of 110/0 to 115/0 areapplied.

4. With the pre-set testing method, the problem of theremanent magnetism can be turned into an advantage bycontrolling it in a way that the residual flux is subtractedinstead of added.

5. The polarity of the short-circuit current at the instant ofmaking has to be choosen properly. In case necessary,intermediate premagnetization shots may be required toensure a considerable residual flux.

6. One has to be aware that the saturation effects are dif-ferent for no-load and short-circuit energization. More-over, the (local) saturation is depending on supply to theinner or the outer winding, on the tap position, on theconfiguration of the windings and on the magnetic historyof the core. The (local) saturation leads to different

[1] IEEE Std C57. 12.00-1993: IEEE Standard General Requirementsfor Liquid-Immersed Distribution, Power, and Regulating Transform-ers

[2] IEEE Std C57. 12.90-1993, Prut I: IEEE Standard Test Code forLiquid-Immersed Distribution Power, and Regulating Transformers[3] IEEE Std C57. 12.90-1993, Part II: IEEE Guide for Shofi-CircuitTesting of Distribution and Power Transformers[4] IEC Standard 60076-5 (2000): Power transformers - Part 5: Abili-ty to withstand short circuit[5] G. Leber, Investigation of Inrush Currents during a Short CircuitTest on a 440 MVA, 400 kV GSU-Transformer, CIGRE SC12 Sess-ion 2000, report 12-104[6] A.L.J. Janssen, L.H. te Paske, Short-circuit Testing Experiencewith Large Power TransformersCIGRE SC12 Session 2000, report12-105[7] G.Bertagnolli, Short-circuit Duty of Power Transformers Bookprinted on behalf of ABB Trasformatori,Legano (Milrrno), Italy[8] J.H.Brunke, K.J.Frohlich, Elimination of Transformer InrushCurrents by Controlled Switching, Part I - Theoretical ConsiderationsIEEE-PES, preprint PE-019PRD (09-2000)[9] J.H.Brunke, K.J.Frohlich, Elimination of Transformer InrushCurrents by Controlled Switching Part 11 - Application and Per-formance Considerations IEEE-PES preprint PE-120PRD (09-2000)[10] G,Bertagnolli, Effects of Short-circuit Currents in PowerTransformers Key Note speech at the 8th International Symposium onShort-Circuit Currents in Power Systems, Brussels 1998[11] A.L.J. Janssen, L.H, te Paske, R,P,P, Smeets, A,G.A, Lathouwers,Short-Circuit Testing: a Crucial Instrument for the Design Asessmentof Large Power TransformersCEPSI 2000, Conference Proceedingspp. 344-351[12] N.V.C. Sastry, H. Gupta, A.L.J, Janssen, A.G,A, Lathouwers,Considerations about Short-Circuit Tests on EHV Power Trans-formers, 9th Symposium on Short-circuit Currents, Cracow (Poland)2000, paper 2.7[13] N.V.C. Sastry, H.Gupta, Short Circuit Tests on EHV Trans-formers 9th Symposium on Short-circuit Currents, Cracow (Poland)2000, paper 2.6[14] G,Bertagnolli. et al., Power Transmission Reliability - Technicaland Economic Issues Relating to the Short-circuit Performance ofPower Transformers CIGRE SC12 Session 2000, report 12-107[15] J.Foldi, D,BLrub4, G. Bertagnolli, R.Maggi, P,Riffon, RecentAchievements in Performing Short-circuit Withstand Tests on LargePower Transformers in Canada CIGRE SC12 Session 2000, report12-201

B1OGRAPHIES

Anton Janssen received his M.Sc.-de-gree in electrical engineering from theEindhoven University of Technology in1977, After a short period in theconsultancy & contracting business, hejoined PNEM, a power company in thesouthern part of the Netherlands, andbecame manager of the department ofTransmission, Iateron the department ofProjects (Transmission and Co-generat-ion). In 1993 he joined KEMA and

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became general manager of KEMA High-Power Laboratory. Mr.Janssen is a member of CIGRE (SC13, Special Reporter, CIGREWG13.06 and 13.08) and IEEE (IEC/IEEE 17A/WG23).

Henk te Paske was born in Winterswijk,the Netherlands in 1953. He received theB.SC. degree for electrical engineeringfrom the Technical College of Enschedein 1977. He joined KEMAs High-PowerLab, in 1979 as a test engineer and issince 1987 employed as a senior test-engineer. He has been actively involvedin testing, quality surveillance, on sitetesting and failure analysis. He is a mem-ber of the Dutch standardization com-mittees NEC 17A/C and NEC.

Wim A. van der Linden was born inEscharen, the Netherlands in 1941. Hereceived the B.SC. degree for electricaleng. from the Technical College of s-Hertogenbosch in 1963. He joinedKEMAs High-Power Lab. in 1965 as atest engineer and is since 1972 employ-ed as a senior test-engineer. He has beenactively involved in the design andconstruction of new test equipment inKEMA High Power Lab. He is a mem-ber of the IEEE Working Group on syn-

thetic testing of generator circuit-breakers.

Ren6 Peter Paul Smeets received theM.SC.degree in physics from the Eind-hoven Univ. of Technology in 1981. Heobtained a Ph.D. degree for researchwork on vacuum arcs. Until 1995, hewas an assistant professor at EindhovenUniversity. During 1991 he workedwith Toshiba Corporations HeavyApparatus Engineering Laboratory inJapan. In 1995, he joined KEMA. Atpresent, he manages the R&D activitiesof KEMAs High Power Laboratory. He

[3.04, WG13.12 the Current Zero lrb>IEEE and CIGRE

TEST-CIRCUIT DIAGRAM

EPOFITNo. m,l CIRCIJITMo.

CIRCUIT COMPONENTS MEASUREMENTS

G = Generator TO = Test Object U = Voltege MeasurementMB = Master Breaker I = Current MeasurementMS= Make SwitchPT = Power Transformer

R = ResistorC = CapacitorL = Inductance

di/dt lRn

Transformer under testI I ~- - .,

11 I

E

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: : ?

~::.,u: ;~;

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Phase R under test v 7 v =

-,----- -1-YNdl 1 transformer in single-phase test circuit 1.5 test method KEMA~

Fig. Al Three-phase transformer under test in a single-phase test circuit (1.5 phase method)

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