Ruben Wiggers - Transformer short-circuit withstand capability

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Transformer short-circuit withstand capability

Ruben Wiggers

23 March 2017

ENERGY

23 March 20171

Kema Laboratories’ experience 1996 – 2017

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Position paper KEMA Laboratories 2016

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▪ Survey of transformer failure rates in service

▪ Root cause analysis of failure

▪ Effects of short-circuit

▪ Electrodynamical forces

▪ Methods to verify short-circuit withstand capability

▪ Design review

- Critical analysis

▪ Short-circuit testing

- Test methods

- New possibilities at KEMA Laboratories

- Failure rates in testing

- Observed failure modes

▪ Conclusions

CONTENTS

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Radial and axial internal forces

B

B

Br

Ba

FaFa

Fa = axial force

Core Innerwinding

Outerwinding

N2 x I2N1 x I1

B Leakage flux

Fr Fr

Fr = radial force

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Free buckling by radial forces

Circular winding

Compressive force

External bulge Bertagnolli

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Tilting due to axial forces

Normal position

F

FTilted conductors

Bertagnolli

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Short-circuit testing at KEMA Laboratories – lab extension

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Preparing an 800 kV transformer for a short-circuit test

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Test statistics 1996 - 2016

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In 70 cases a transformer showed a problem due to short-

circuit stresses that became immediately apparent at the test

site. Mostly, this problem was an unacceptable increase of

short-circuit reactance due to the short-circuit stress, but a

range of other, immediately evident problems also occurred.

During the past 21 years, in total 320 times a test access for a

transformer larger than 25 MVA (278 transformers from which

42 are re-tested) has been counted.

In 230 cases, the transformer showed no problem at the

test-site.

In 42 transformers from the latter group had been re-tested

after modification in the factory and most did not show a

problem at the test-site at the re-test.

In 7 cases, transformers after having not having experienced

any problem at the test-site, did not pass the routine tests

and/or visual inspection after the tests.22% of tests resulted in an initial failure

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Visible failure mode: oil spill

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Broken bushing

Oil spills

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Before test – after test

After test Before test

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Thank you for your kind attention!

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Ruben Wiggers

ruben.wiggers@dnvgl.com

Tel: +31 26 356 2641