Breakdown of insulation by partial discharges. Influence of supply frequency on discharge characteristics

The paper considers the validity of frequency-accelerated discharge tests for sold insulation when discharge measurements, dielectric deterioration and sample life are related to the service 50Hz conditions. The work includes details of a high-resolution discharge detector developed for use over the frequency range 50¿1250Hz. Experiments on low-loss organic materials, e.g. polythene, showed that, under certain experimental and environmental conditions, fundamental changes in the discharge-pulse-amplitude distribution occurred with changes in the test frequency. These were related to the manner in which the dielectric surface discharged as a number of single events over each voltage cycle. There was no change in the mechanism of failure over the frequency range 50¿1250Hz, and `lifetime ¿ 1/frequency¿ was a reasonable approximation. Trends in the progression of discharge measurements were not dependent on the test frequency. The greatest change in the discharge-pulse-amplitude distribution with test frequency occurred in circulating air at relatively high flow rates. This environment gave the most consistent result in terms of sample lifetime in cycles of applied voltage.