Abstract
We evaluated the diagnosis of insulation performance in gas insulated switchgears (GIS). We measured the temporal change of partial discharge (PD) characteristics from initiation to breakdown in SF6 gas with the pressure of 0.4 MPa for ac voltage application to needle-plane electrodes. As a result, breakdown occurred about 150 minutes after the voltage application of 55 kVrms. We observed that tip radius of needle electrode was enlarged due to PD during the voltage application, resulting in the shift of corona-stabilized breakdown characteristics to the lower pressure side. At the final stage close to breakdown, positive PD pulses with relatively higher magnitude began to emerge in the phase region near the applied voltage peak. This event can be interpreted by the change of PD mechanism in SF6 gas from streamer corona to leader corona.
Moreover, we investigated the possibility of breakdown prediction. We try to seek a breakdown prediction parameter which characterize the change of PD form: the ratio RL of the maximum charge to the average charge near the peak in positive half cycle. This suggests that positive PD form may change as RL increase. As approaching to the breakdown, RL rose from 5 to 11 in the approximated curve for most cases. When RL was used as a breakdown prediction parameter, the breakdown time could be successfully predicted within the error of 20%.
