Nonstationary behaviour of partial discharge during discharge induced ageing of dielectrics

Changes in the stochastic behaviour of pulsating partial discharge with time have been observed when an alternating voltage is applied to point-dielectric gaps in which the dielectric is a cast epoxy resin either with, or without, Al₂O₃ filler. The changes in discharge behaviour are shown, with the help of a Monte-Carlo simulation, to be consistent with discharge induced increases in the surface conductivity of the epoxy. This `ageing´ effect is shown to be accelerated as the frequency of applied voltage is increased from 50 to 800 Hz. The implications of the results on accelerated ageing tests and definition of partial discharge `signatures´ for possible pattern recognition are discussed. The relationship between average partial discharge current and partial discharge pulse height distributions is also discussed