Study on the relation between PD characteristics and degradation of PP films for power capacitors by utilizing artificially formed through holes

The authors have been investigating the deterioration characteristics of polypropylene (PP) films due to partial discharge (PD) in a closed void as a basic study of long-term deterioration of power capacitors. We reported that (1) the number of PD pulses decreased with time and then fluctuation, (2) through hole was observed only after the inception of fluctuation, and so on. In this work, we focused on the observation of PD properties of PP films with artificial through hole and comparison with conventional samples (without artificial through hole). As a result, (1) no discharge of the large charge band occurs, (2) polarity differences in the number of discharges and PD patterns occur, (3) the degraded range of PP film is narrower than that of the conventional sample, and (4) donut-shaped emission images are observed, which were not observed in the conventional sample. However, the fluctuation itself did not occur. The polarity difference is considered to be due to the presence of the through-hole, as compared to the conventional sample before and after the fluctuation. It was suggested that the presence of degradation products on the PP film surface and the gas atmosphere in the void also contributed to the fluctuation.