Impulse voltage

[smms]

Maybe someone can help me:
We measured the impulse voltage on high voltage coils. The result was, that the impulse voltage level is higher if the test is made in some steps.
E.g. one coil withstands about 50kV, if the test started with an impulse voltage of 30kV, then with 32kV, 34kV, ....
If we start with e.g. 45kV, the coil fails.
Is there any explanation for this?

 

[aolalde]

The impulse test should be designed based on the turn to turn insulation.

If repetitive tests are done, the dielectric stressing of the insulation could develop internal carbon tracking that make the insulation fail when a second test is done.

In my opinion only one test should be done at the voltage level compatible to the coil design and after it is fully cured. Consult Std IEEE-522 and the coil manufacturer recommendation.