The effect of surface charge decay on the variation of partial discharge location

The accumulation of surface charges generated by a discharge at the solid-gas interface in a void is an important factor that can affect partial discharge (PD) characteristics. In this paper, with the help of the surface charge measurement system based on Pockels effect, the phenomenon of surface charge decay was observed by comparing its dynamic distribution during a PD sequence under variable voltage cycle. Moreover, the decay time of surface charge was measured. It is found that the decay rate of positive surface charge was much faster than that of negative one. The effect of surface charge decay on variation of partial discharge location was investigated when the voltage with variable cycle was applied. Two terms, the coincidence rate of hetero-discharge location (CRHEL) and the coincidence rate of homo-discharge location (CRHOL) were introduced to describe this effect. As the applied voltage cycle increased, CRHEL before and after voltage zero-crossing point decreased. Besides, CRHEL from a void between two dielectric surfaces (called VTD electrode) was larger than that from a void between a metal surface and a dielectric surface (called VMD electrode). Regarding VMD electrode, CRHEL for the case with negative discharges occurring in the former half-cycle was larger than that with positive discharges occurring in the former halfcycle. However, there was almost no difference about CRHEL for these two cases when VTD electrode was used. After the applied voltage cycle exceeded 500 ms, the coincidence phenomenon of homo-discharge location during negative half-cycle was observed in the usage of VMD electrode, and CRHOL became larger with voltage cycle increasing. As for VTD electrode, there was no coincidence of homo-discharge location. These results were attributed to the charge decay and the difference of discharge initiation position.

1 Introduction

Although the discharge time lag had been assumed to be the most significant parameter causing the stochastic character of partial discharge (PD) [1], [2], surface charges at the interface between solid dielectric and gas, resulting from the previous discharges, play an important role in the characters of subsequent partial discharges (PDs) as well, which is confirmed by a large number of experimental and simulation results [3]–[5]. In detail, the surface charges can affect the electric field distribution in a void and hence the PD parameters [6]. Moreover, the production of free electrons which can initiate discharges may be also affected by residual charges [1]. However, compared with the discharge time lag, the effect of surface charge on PD characters has not attracted enough attention.