HIGH-POT TEST FOR POWER TRANSFORMER

[mutawa97]

If we have a power transformer of 3750/4687KVA and 69KV to 13.8KV/7.967KV, Is it acceptable to apply DC voltage to test the high voltage side (140KV-DC which equal to 100%) and if it is not acceptable explain what can I do? can we apply more DC voltage,less, or it will not be possible to do the test with a DC-Voltage.
finally, What is the perecntage of the voltage supplied to the high-voltage side, is it 100% or 75% as ANCI requiremnts?

 

[PaulT]

Mutawa97,

Are the tests you want to do part of factory testing,or testing on site, that is, precommissioning/commissioning, or customer specific testing,maybe diagnostic insulation checks?

The J&P Transformer Book by Martin J. Heathcote is very informative.

For factory winding dielectric testing, the insulation of HV and LV windings are subjected to the following tests,

a)induced overvoltage withstand
b)separate source voltage withstand
c)impulse withstand tests when required

For a 72.5kV System voltage a Power frequency test voltage of up to 140kV rms can be applied(BS171,part 3,IEC76-3).
However, refer to the particular standards for your country.

If repeat factory tests are required a reduced voltage to 75% of original values maybe applied.

DC maybe used provided the peak value of the rectified DC supply does not exceed the peak value of the AC test value.

 

[mutawakh]

Thank you pault,
This test is part of the factory test.
I understand the perecentageto use that 74% but I did not understand if the test with DC voltage enough, although it violate ANSI requirements.