Effects of photoionization and overvoltage on simultaneous discharges

Simultaneous discharge, characterized by single current pulse corresponding with more than one discharge spot in one PD (partial discharge), was often observed in the partial discharge sequences, especially for the first discharge after voltage application. As it tends to be of larger magnitude than other PDs, to clarify its mechanism is important for understanding PD. In this paper, a combination measurement system of surface charge distribution and PD current was developed to identify simultaneous discharge in the samples with double-spheres plate electrode. Besides, a single square voltage pulse was applied so that only one discharge took place at a sphere electrode and the effect of residual charges can be excluded. Based on these, we investigated the effects of photoionization and overvoltage on the probability of simultaneous discharges. Photoionization effect was considered by adjusting the distance and adding a barrier between two sphere electrodes. When two spheres were separated by a glass slide, the simultaneous discharge probability was smaller than that without one. Besides, as the gap distance between two spheres increased, the simultaneous discharge probability diminished. These indicated that photoionization contributed to simultaneous discharges. Furthermore, simultaneous discharge probability became higher as the overvoltage increased, in both cases with and without glass slide.