Partial Discharges in Electrical Insulation under Combined Alternating and Impulse Stresses

A study has been made of the partial discharge characteristics of artificial cavities in polyethylene under combined 60-Hz and negative 1.2/50-μs impulse stresses, the amplitude of the altemating stress being below discharge inception value. Samples of cross-linked and low-density polyethylenes were tested. The discharge sequence was derived from the results of previous tests, which used only impulse stresses. It showed that the ac discharges, initiated by an impulse, could continue indefinitely under certain conditions. The experimental results do not confirm this but show that the ac discharges always extinguish within 10 seconds after the application of the impulse. The number of ac discharges, although independent of the amplitude of the impulse, increases with increasing altemating stress but decreases as the sample ages. The effect of the angle of the cycle at which the impulse was applied on the combined discharge inception stress is also described.