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LV-system:Y

LV-system:X HV-system:X

HV-inlet

HV-system:Y

Tap disks

Tap disks

upper yoke

lower yoke

core

Fig.: 1 Cross section of windings

Short Circuit Test on a 440 MVA, 400 kVGSU-Transformer

byGerald Leber

Elin Transformatoren GmbH Weiz, Austria

Abstract

The intention of this paper is to provide someinformation and experience about Elins recentshort circuit test on a 440 MVA, 400 kV threephase generator step up transformer, performedand certified by KEMA High Power Laboratoryin the Netherlands. Design considerations aboutthe radial stress calculation, specific challengesas the optimization of axial stresses and theperformed tests are described.

Design considerationsAt first, a survey of the specific technical dataand a sectional sketch of the tested transformershall be shown. (Table 1 and Fig. 1)

Rated power

Rated voltage

440 MVA

400 kV 2x2.5% // 2x13.2 kV

Short circuit impedance

System X+Y

System X

System Y

17.9 % based on 440MVA

16.8 % based on 220 MVA

16.7 % based on 220 MVA

Vector group YNd1d1

Rated frequency 60 Hz

Table 1

The inner winding (LV) is a double layer helicalwinding with spacers. Each system consists oftwo radial epoxy bonded CTC layers. The highvoltage winding is a disk winding with twoaxial blocks separated at the high voltage inlet.Each high voltage system can be regulated by 4tap disks located approximately at the center ofeach block.For the mechanical stress calculation, the mostsevere fault type has to be considered. Due tothe low short circuit impedance of system Xresp. Y, the highest short circuit currents andtherefore the maximum radial forces inwindings occur during a three phase fault at theterminals of system X or Y.The determination of the mechanical forces,during short circuit condition in the windingarea, is a result of an axisymmetric magneticfield calculation based on FEM-Methods.Fig. 2 shows the calculated maximum radial andaxial forces on the disks of LV-system Ynumbered from the top end of the winding to thelower end.

Fig. 2

A view on the compressive radial forces at thetwo layers shows, that the outer layer is morestressed at short circuit. Therefore thecompressive hoop stress d at the CTC of theouter layer is of interest.

cu

rad

cu QF

QDp

=

=p

d22

where;d compressive hoop stress [N/mm]p distributed radial force at disk [N/m]

Radial and axial winding forces at LV-system Y

-250

-150

-50

50

150

0 10 20 30 40 50 60 70 80

LV-disk No.

Frad (outer layer)

Frad (inner layer)

Fax (outer layer)

Fax (inner layer)

2/5

D medium diameter of conductor [m]Qcu cross section of conductor [mm]In this case the maximum radial force at disknumber 40 (Fig. 2) with 230 kN has to beselected to gain the distributed maximum radialforce of 61.5 kN/m for a medium layer diameterof 1.19 m. For a CTC 45 x (6.1x1.68) mm, thecalculated compressive hoop stress is 81N/mm. The compressive hoop stress is in anycase a characteristic value for the valuation ofradial forces. According to ETG standard, themaximum permissible compressive hoop stressis 60 % of the Rp0.2 value of the copper material.The Rp0.2 value of this CTC is 150 N/mm anddefined as the proof stress with a permanentelongation of 0.2 %.In addition, two investigations should be doneto check the stability of the winding againstbuckling.In the first case Eulers stability formula forlower modes of buckling is used, whichdetermines the critical load per unit length:

3

2 )4(2D

nIEp radcrit

-=

where;pcrit critical value of distributed radial force

on conductor per unit length [N/m]D medium diameter of conductor [m]E modulus of elasticity of conductor

material (incremental value, dependenton proof stress of copper quality) [N/m]

n number of radial supports along thecircumference

Irad Second moment of area of the conductorcross section [m4]

The above calculated radial short circuit force p,distributed along the circumference of the disk,must be below the value of pcrit .In the second case the maximum radial bendingstress in the conductor is determined. The CTCof the outer layer is regarded as a supportedbeam structure with the distributed load pbetween the radial supports. The stress in theconductor is a result of the bending moment onthis beam structure and the cross section of theconductor. The formula for the stress in theconductor is:

radWdp

=12

2

maxs

where;smax maximum stress in the conductor [N/m]p distributed radial force at conductor

[N/m]d distance between axial spacers [m]Wrad Section modulus of the conductor cross

section for radial stress [m3]

The maximum permissible stress in theconductor is 80 % of the Rp0.2 value of thecopper material.For epoxy bonded CTCs special assumptionsabout the effective values for Irad and Wrad mustbe made for both buckling criteria. Theseeffective values for Irad and Wrad are higherthan the values obtained for totally independentbending of the individual strands but lower thanthe values for a single solid conductor with anequivalent cross section. The calculatedeffective values depend not only on thegeometric data of the single strands, but also ondifferent influences such as transpositions, oiltemperature or the lap shear stress capability ofepoxy glued strands. The methods used incalculation are based on the results ofexperimental research performed by ETG andASTA.In addition to the design considerationsregarding the radial compressive forces, theaxial forces on windings are as well important.This kind of transformer is a typical example foraxial forces hard to get under control.

Fig. 3

As shown in Fig. 3 the axial forces on each HVwinding disk have a completely differentdistribution for different tap positions along the

Axial winding forces at HV-system Y

-300

-200

-100

0

100

200

300

4 8 12 16 20 24 28 32 36 40 44 48 52 56HV-disk No.

tap-pos.:-2

tap-pos.:+2

3/5

Tap-pos: -2 Tap-pos: +2

winding height. At the maximum tap positionthe radial forces at the disks are uniformlydistributed from the upper winding end to thelower end at the HV inlet. Four tap disks(No.29-32) are out of service at the minimumtap position. This disconnection of disks leadsto a radial component of the magnetic stray fluxin the tapped area. Thus the system Y issubdivided in two axial winding blocks asgraphically illustrated in the following fieldplotfor the extreme tap positions.

The axial forces produced in each individualwinding disk due to the radial flux componenthave a cumulative effect on the radial windingstampings. This cumulated axial load is ofinterest for the review of the stress on theconductor. The stressed area of the conductor isthe surface of the bare copper conductors inradial direction, covered by stampings. Fig. 4shows this stress on the HV winding stampingsfor the extreme tap positions. The knowledge ofthe compressive stress distribution along thewinding length is of fundamental importance,especially in case of different types of con-

ductors within the winding. The reason for theselection of different types of conductors alongthe winding length is basically a result of eddyloss optimization and transient voltagedistribution and is less influenced by shortcircuit considerations. Therefore the moststressed conductor group must be determined.

Fig. 4

Considering this short circuited HV winding,the maximum accumulated force appears in theaxial center of the winding height. However,this is not the most stressed area, because of thehigher number of turns in the tap disks, made ofsingle conductors. The highest stress on stamp-ings and therefore on the covered conductorarea appears in the CTC disks above and belowthe tap disks (HV disk No.28 & 33).The most likely damage in case of overstress isin any way tilting of conductors. The maximumpermissible axial stress for different types ofwindings and conductors is based on practicalexperience. At ETG, each winding must under-go a static pressure test after completion in thewinding shop. In our experience, the mostpractical method to make sure that tilting ofconductors under short circuit condition can beavoided is a routine pressure test, where an axialforce on the winding is applied, to produce astress on the conductors equal or higher than themaximum calculated short circuit stress.In addition to the design considerationsregarding the axial stress inside the winding, theaxial winding end forces to the upper and loweryoke are as well of interest.For conventional transformers with commonwinding arrangements the location of the axial

Compressive stress on HV-winding stampings

0

5

10

15

20

25

4 8 12 16 20 24 28 32 36 40 44 48 52 56

HV-disk No.

tap-pos.:+2

tap-pos.:-2

4/5

winding center is selected for all windings in anequal distance from the lower yoke. Such anaxial winding arrangement prevents axialwinding end forces under the assumption thatthe winding heights can be adjusted ascalculated.This kind of axial arrangement would not bepracticable for this type of transformer with twosystems. The reasons are the non uniformdistribution of axial forces (Fig.3) on the onehand, and on the other hand the different typesof short circuit faults, which should be takeninto consideration.

Fig. 5

The axial winding end force to the upper yoke isshown in Fig.5 as a function of the axial turncenter displacement for LV and HV system Y.(The displacement LV to HV for system X isthe same and has the same influence on theupper yoke force). The upper yoke forces havean antagonistic trend for the most likely andsevere fault cases. These faults would be at theterminals of low voltage system Y (resp. X) orat the high voltage terminals. The optimaldisplacement for this different fault cases can befound at the point of intersection. In spite of allefforts, a maximum yoke force (440 kN)appears at the optimum axial displacement ofwindings. The minimun clamping force of thewindings is by all means the upper limit for theyoke forces.In addition to the theoretical investigationsabout axial displacements it must be ensured,that the calculated winding lengths and optimalaxial center displacements can be achieved

during manufacturing with minimum tolerance.Therefore the axial height of the winding isadjusted during the pre-stabilization processunder nominal clamping pressure. Theadjustment of length can be performed byremoving as many adjustment spacers asnecessary, which are provided in addition to thenominal spacers only for this purpose. Anaccuracy of 0/+2 mm can be achieved by thisprocedure. Due to this length adjustment and theapplication of a common clamping system forall windings of one leg, the calculated relativedisplacement of axial centers can be ensured.

Performed tests

For testing of power transformers, two testprocedures with respect to the application ofshort circuit can be used(in order of preference):1) Closing a breaker at the faulted terminal to

apply a short circuit to the previouslyenergized transformer (=post-set shortcircuit).

2) Closing a breaker at the source terminal toapply energy to the previously shortcircuited transformer (= pre-set short circuit)

For this large power transformer, the firstmethod could not be used, because oflimitations regarding the voltage source and therequired power supply of the short circuitgenerators of the high power laboratory.A three phase short circuit with three phaseenergy source should be preferred for threephase transformers. In this case, due to the highshort circuit power requirements of this threephase transformer, a single phase test wasperformed with the pre-set short circuit method.One LV system (X or Y) was three phase pre-set short circuited and a single phase voltagewas supplied between one HV line terminal andthe other two line terminals connected together.The single phase voltage during the test has tobe equal v3/2 times the rated voltage of thetransformer between phases. During the testwith a duration of 0.25 seconds one phase istested with full short circuit current and the twoparallel connected phases are tested with thehalf short circuit current. Therefore this kind ofsingle phase test of a three phase transformer isalso called 1.5-phase test method. Attentionshould be paid to the inrush current at the high

Upper yoke force

0

200

400

600

800

1000

13 15 17 19 21

Axial turn center displacement LV to HV [mm]

3 phase fault at LVsystem Y

3 phase fault at HV

5/5

voltage winding. The additional inrush currentincreased the calculated short circuit currentabout 8 to 10 percent for the first few cycles.This effect, which only occurs at the pre-setshort circuit method, should be considered forthe design calculations of the high voltagewinding.Prior to the short circuit test the transformer wassubjected to a routine test.Table 2 shows the short circuit tests on this 440MVA transformer performed on the two LVsystems (X and Y) at different tap positions.According to the test standard (ANSI/IEEE)each phase of the transformer shall be subjectedto a total of 6 tests. Two of these tests on eachphase shall be performed with asymmetricalcurrent.

Number of tests at HV phase and tap positionTestedLV

System H1 H2 H3Tap-pos.: +24 x Isymmetr. 2 x Iasymmetr.

(+8 setup tests)

- -

-

Tap-pos.: 04 x Isymmetr. 2 x Iasymmetr.

(+5 setup tests)

-X

- -

Tap-pos.: -24 x Isymmetr. 2 x Iasymmetr.

(+4 setup tests)

-

Tap-pos.: +24 x Isymmetr.2 x Iasymmetr.

(+4 setup tests)

-

- -

Tap-pos.: 04 x Isymmetr.2 x Iasymmetr.

(+5 setup tests)Y

Tap-pos.: -24 x Isymmetr. 2 x Iasymmetr.

(+6 setup tests)

- -

Table 2

The total number of tests (68) was about 2 timeshigher than the number of tests according to thestandard (2*3*6=36). The reason for thisincreased number of tests are setup tests for thesynchronization of the test circuit on the onehand and tests with negative voltage polarity toprevent saturation effects at the transformers ofthe test laboratory on the other hand.The impedance of each phase and system at anytap position was measured before and after theshort circuit tests. The maximum differenceafter the short circuit test series was 0.11 %.

According to the standard the allowablevariation is 2 %.Two additional criteria, a visual inspection androutine tests including standard dielectric tests,have to be met satisfactorily to obtain a type testcertificate of short circuit performance.After the short circuit test the active part of thetransformer was untanked at manufacturersplace and the visual inspection was witnessedby a representative of the high power labor-atory. Steel tank, core, coils, bus ducts and tapchanger were inspected for any movements anddamages. After the visual inspection, whichgave no indication that there had been anychange in mechanical condition of the testedtransformer, all routine tests including impulsetests were performed at the full specificationlevel.The tested transformer fulfilled the requirementsaccording the relevant standard and a type testcertificate of short circuit performance wasissued by the authorized high power laboratory.