Experiences of unexpected breakdown of current transformers during power frequency withstand test

Power frequency withstand test is one of the routine tests which must be performed on the high voltage current transformers in the factory. Special attentions are paid to the measured Partial Discharge (PD) value during increasing voltage and also during application of the specified voltage. The test is stopped if the measured PD value exceeds a specified limit, which is recognized on the basis of experiences. This interruption makes an opportunity to remove the possible CT defects (such as oil degassing) with lower cost, before a sudden occurrence of breakdown in the CT. But in some tests, insulation breakdown occurred in the CT, though no considerable PD value had been detected prior to the breakdown. So, more cost is imposed on the manufacturer to repair and replace the damaged components. The possible major causes of this problem and their corresponding solutions are analyzed in this paper.