The deterioration and breakdown of dielectrics resulting from internal discharges

The deterioration of polythene and other dielectrics, when exposed to internal discharges, has been examined under controlled conditions. The discharge-inception voltage and the magnitude and energy of individual discharges have been determined, and the location and number of discharges in voids of different dimensions have been studied, by electrophotography. The discharge sequence is explained by the difference in the distribution of the residual charges on the positive and negative surfaces of a void after a discharge. Accelerated deterioration tests using a frequency of 150 kc/s show that, initially, discharges cause slow erosion at the surfaces of the void and the formation of a transparent film of resin which fluoresces under ultra-violet irradiation. About 10¿15 cm3 of polythene is eroded by each discharge, probably as a result of thermal degradation. The rate of deterioration increases rapidly with increasing voltage; at twice the discharge-inception voltage, the discharges concentrate and form several deep uncarbonized pits near the periphery of the void. When the pits attain a critical length, the mechanism of deterioration is believed to change; narrow semi-carbonized channels are found and, generally, breakdown follows immediately. Deterioration is greater when the void is adjacent to an electrode than it is when the void is enclosed in polythene. Tests show that, under equivalent conditions, polytetrafluorethylene and perspex are less resistant to discharges than is polythene.