Transformer Testing - Routine Tests, Type Tests and Special Tests (Part-2)

Test Sequence

These tests should be performed in a specific sequence to achieve the best results. Although the IEC 60076 standards for power transformers do not lay down a comprehensive test sequence, this often leads to long discussions between customers and manufacturers.

IEC Standards

IEC 60076-3:2013, clause 7.2.3

“The tests shall be performed in the sequence given below:

The tests shall be performed in the sequence given below:

a) Lightning impulse tests (LI, LIC, LIN, LIMT);

b) Switching impulse (SI);

c) Applied voltage test (AV);

d) Line terminal AC withstand test (LTAC);

e) Induced voltage withstand test (IVW);

f) Induced voltage test with partial discharge measurement (IVPD).

NOTE:? This is a comprehensive list, not all these tests will be applicable to a particular transformer.

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By agreement between the manufacturer and purchaser, the switching impulse test may be performed before the lightning impulse test.”

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IEC 60076-1:2000, clause 10.5

“In deciding the place of the no-load test in the complete test sequence, it should be born in mind that no-load measurements performed before impulse tests and/or temperature rise tests are, in general, representative of the average loss level over long time in service. Measurements after other tests sometimes show higher values caused by spitting between laminate edges during impulse test, etc. Such measurements may be less representative of losses in service.”

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This test sequence is a recommendation and not obligatory.

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IEEE C57.12.90-2015, Clause 4.3

However, IEEE C57.12.90-2015 - IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers stipulates the following:

Clause 4.3 states “the resistance, polarity, phase relation, ratio, no-load loss, and excitation current, impedance, and load loss tests (and temperature-rise tests, when applicable) should precede dielectric tests. Using this sequence, the beginning tests involve voltages and currents, which are usually reduced as compared with rated values, thus tending to minimize problematic effects to the transformer.”

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Clause 10.1.5.1

Clause 10.1.5.1 specifies the sequence for dielectric tests as follows:

“Impulse tests, when required, shall precede the low-frequency tests. The dielectric tests should be performed in the preferred sequence as follows: a) Lightning impulse tests (on all terminals) b) Switching impulse tests c) Applied voltage test. d) Induced voltage test.

By agreement between the manufacturer and purchaser, switching impulse tests (when required) may be performed before lightning impulse tests (as also indicated in IEC 60076-3). The induced voltage test shall be the last dielectric test performed.”

ABB Recommendation

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Taking into account all relevant IEC and IEEE regulations and recommendations, and based on their own experience (authors), the proposed test sequence is as follows:

·???????? Ratio, polarity and phase displacement

·???????? Resistance measurement

·???????? No-load test (followed, if specified, by the sound level test)

·???????? Load loss and impedance

·???????? Zero-sequence impedance test (if specified)

·???????? Dielectric tests:

o?? Switching impulse (when required)

o?? Lightning impulse test (when required)

o?? Separate source AC voltage test

o?? Induced voltage test including partial discharge test

The test sequence of the tests preceding the dielectric test can be slightly changed due test field loading or other operational reasons.

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