DC withstand test with partial discharge measurement of convertor transformers: Problems and suggestions for improvement

To pre-discover the insulating defects and avoid possible insulation failures, DC withstand voltage test with partial discharge (PD) measurements are routinely conducted before the operation of a convertor transformer. However, a "downward trend" of PDs under DC voltage studied in this paper indicates the limitation of the current test. According to the study, under DC voltage both PD magnitude and PD repetition rate decrease with voltage applying time. At U _pdiv =29 kV, the average charge is 105 pC in the first 10 min and 33 pC in the last 10 min. Whereas for 3U _pdiv =87 kV, the value are 2700 pC and 1561 pC separately. On the other hand, at U _pdiv , PD numbers decrease from 12 times in the first 10 min to 5 times in the last 10 min. At 3U _pdiv , the numbers are 102 times and 83 times. When PD magnitude is higher than 1400 pC, a whitish area appears at the surface of the pressboard, accompanied by the decrease of DC withstand voltage after 2 hours' PD exposure. The results raise the question that the current test measures PDs over 2000 pC in the last 30 minutes but may introduce PDs of higher magnitude at the early stage, which will damage the dielectrics. Based on further analysis, an improved test procedure is suggested.