Temporal development of partial discharge and its application to breakdown prediction in SF6 gas

We measured the temporal change of partial discharge (PD) characteristics leading to breakdown in SF, gas for ac voltage application. At the final stage close to the breakdown, positive PD pulses with relatively high magnitude began to take place in the phase region near the applied voltage peak. This event was interpreted in terms of the change of PD type in SF6 gas from streamer to leader. Optical observation also revealed that PD type transition occurred. With the results considered, the mechanisms of PD were discussed. Moreover, we discussed the possibility of breakdown prediction in GIs. An attempt was made to find a breakdown prediction parameter which characterized the change of PD type: the ratio RL of maximum charge to the average charge of PD pulses appearing in the phase region near the peak in positive half cycle. It was found that R, allowed to predict the time to breakdown successfully within the error of 4 - 20 %.