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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 SF, gas with the pressure of 0.4 MPa for ac voltage application to needle-plane lectrodes. 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 ue to PD during the voltage application, resulting in the shift of corona-stabilized breakdowncharacteristics 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 SF, 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 PDform: the ratio RL of the maximumcharge to the average chargenear 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, thebreakdown time could be successfully predicted within the error of 20%.

Breakdown Prediction Technique in GIS Based on Transition of Partial Discharge Characteristics