DutchPower - Transformatoren ingewikkeld!?

1. DNV GL © 2017 SAFER, SMARTER, GREENERDNV GL © 2017
Transformer short-circuit withstand capability
Ruben Wiggers
23 March 2017
ENERGY
23 March 2017
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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 Inner
winding
Outer
winding
N2 x I2N1 x I1
B Leakage
flux
Fr
Fr
Fr = radial force
3

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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
5

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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 testBefore test

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SAFER, SMARTER, GREENER
www.dnvgl.com
Thank you for your kind attention!
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Ruben Wiggers
ruben.wiggers@dnvgl.com
Tel: +31 26 356 2641