transformer testing

[jay80]

can anybody guide on applicability of lightning impulse test on hermetically sealed oil filled power transformer meant for indoor installation with plugin termination for hv connections.
regards
jay

 

[DTR2011]

IEEE c57.12 would be a good place to start.

 

[prc]

YOu have not indicated the voltage class of the HV plug in connections.Today these are available up to 220 kV Voltage. To conduct impulse or any other dielectric tests at factory, you have to buy the adaptors with the plug in connectors so that you can apply voltage from adaptor through plug in connector to the winding.
The standards relevant are
Applicability of test - IEC 60076-1(2011)/ C57.12.00-2006
Dielectric tests,General - IEC 60076-3(2013)/C57.12.90-2010
Impulse Testing -IEC 60076-4(2002)/C57.98-2000

 

[jay80]

Thanks a lot prc & Dtr2011 for the above replies... however my question is more in regards to applicability of impulse testing on indoor transformer. The insulation strength is already getting tested by applied voltages test and induced voltages at rated frequency and twice the frequency respectively... I feel these test are more applicable in case of indoor distribution transformer fed by cable rather than impulse test.. which is more applicable for transformers connected to overhead lines or with exposed bushings..

regards

 

[prc]

Indoor transformers may not be subjected to direct lightning strokes. But it will see switching surges and line surges through cable (though attenuated )So impulse test is relevant for indoor transformers too.

 

[jay80]

That's right PRC, however I feel if they are going to be subjected to switching surges.. then more relevant is switching impulse test rather than Lightning impulse test.
Thanks..

 

[ausphil]

Is your complete system, from generation to load, completely underground?

If not (which is the most likely answer), then there is always a chance of a lightning strike on an overhead conductor, that will transfer its way into the cables that feed your transformer, hence it is completely valid to test an indoor transformer with a lightning impulse waveshape. The voltage may be slightly under the operating point for spark gaps or surge arrestors at the start of the cable, but high enough to cause a problem in the insulation that you don't know about, because you didn't test it. And the rate of rise for a lightning impulse is much faster than that of a switching impulse, which can be very onerous, depending on your insulation structure.

You may well be performing an interturn test and a bulk insulation to earth and the other windings test, but that is a 50 or 60 Hz test, which is fine for a 50 or 60 Hz applied voltage. But during the life of a transformer, the insulation will not only see this power frequency, but also lightning shape and switching shape impulses, which are affected greatly by the capacitance of the insulation structure. This may produce overvoltages in certain areas of the insulation, which will not be overstressed at power frequency, and may cause failures that you don't expect because you assumed that they are not relevant.

Ausphil