Power transformers

One of the important type tests is the dynamic ability to withstand short-circuits.

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Rene Smeets René Smeets
Service Area Leader

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KEMA Laboratories
Power transformers HPL Arnhem
Power transformers are vital components in the power supply network. Once commissioned, they are expected to function reliably for at least three decades, without interruption. This means that utilities have to get their power transformers comprehensively type tested and certified before installing them in the network. In order to help ensure this requirement, IEC and ANSI standards have specified the various type and routine tests to be performed based on the collective experience of experts working in asset management of power networks, leading manufacturers and test laboratories around the world. One of the important type tests is the dynamic ability to withstand short-circuits.
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  • Keywords: KEMA Laboratories, Manufacturers, Utilities
Test potential for power transformers
Figure 1 - Present and future test potential for power transformers versus rated power, voltage and test method
Initial power transformer failure rates 1996-2015
Figure 2 - Initial power transformer failure rate for various ranges of MVA rating in the period 1996-2015

Expansion of KEMA High-Power Laboratory in Arnhem, the Netherlands
In recent years, there has been a significant increase in requests for short-circuit testing of power transformers. Also with the growth of power networks and manufacturing capabilities, the MVA and voltage rating of transformers has been increasing. In turn, the demand for testing much larger transformers is also growing.

The present capacity of KEMA High-Power Laboratory, Arnhem (8400 MVA, 16.67 to 60 Hz) is adequate for testing large power transformers with ratings up to 550 kV and approximately 330 MVA single-phase.  In order to cater to testing of higher ratings up to 800 kV, it was decided that an expansion of the short-circuit capacity in the Arnhem laboratory was needed.

This decision has led to an investment of over EUR 70 million for the expansion project. Two more short-circuit generators and four additional short-circuit transformers are being installed as part of this project. After the expansion, the maximum available short-circuit power will be 13,000 MVA in the test cells, which makes it possible to test 800 kV single-phase transformers up to 500 MVA (depending of the short-circuit impedance of the transformer). Also transformers with rating 550 kV and up to 500 MVA can be tested without limitations.

Many of the short-circuit tests on mock-up models of large power transformers can be replaced by full scale testing. In figure 1 below is shown the technical feasibility before and after the expansion.

Test experience shows that many transformers fail to pass the short-circuit test requirements as stipulated in the IEC standard. Test failure statistics collected by KEMA Laboratories in the past 20 years on large (≥ 25 MVA) power transformers, shows that from the 297 tests with 258 different power transformers, 23% of the tests initially result in a failure. From the 67 tests that did not lead to success, 39 re-tests followed, usually leading to a success. The failure rate of the transformers with ratings above 200 MVA was observed to be noticeably higher.

Failure to pass short-circuit tests is mainly by an unacceptable increase of short-circuit reactance, but several other, directly apparent failure modes are frequently observed, such as oil spill, internal flash-over, damage to bushing, leads, et cettera.

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