The contribution of discharge area variation to partial discharge patterns in disc-voids

Abstract

Experimental ac partial discharge (PD) patterns are presented for a disc-void (area greater than length) with metal surfaces, one metal and one insulating surface, and with both surfaces insulating. These patterns indicate that the portion of the surface over which the charge is deposited (discharge area) plays an important role in producing the fluctuations in discharge magnitude commonly observed for such voids. A simulation model for PD patterns in voids is presented, which expressly includes the effects of the charge distribution left on the void surface by consecutive PDs. This model defines two factors that control the PD propagation. These are: a minimum field required to maintain the discharge within an existing surface path Ein and a minimum peripheral field required to extend the discharge path Ep. There is also one other factor, the occurrence probability that controls the incidence of the PD. It is shown that the model gives a variation in PD magnitude throughout the active region of phase without any stochastic factors, i.e. when only variations in the discharge areas are allowed for, but that in order to reproduce the typical (turtle-like) PD patterns observed for the disc-void with insulating interfaces the occurrence probability and surface conductivity have to be included. The current model is compared to previous models and the relationship of the model factors to the change of PD patterns with ageing is briefly discussed.