Relation between Concentration of Partial Discharge and Surface Degradation and Breakdown

Partial discharge (PD) in a void reflects degradation of insulating material and, therefore, is utilized for insulation diagnosis. Although it is well known that PD pattern changes with voltage application, it is not clear how the pattern change is related to material degradation or how it is related to final breakdown. We simultaneously observed PD characteristics and light emission from partial discharge and the results were compared with those of surface observation. Both in an open void where atmospheric condition does not change and in a closed void, PD activity become concentrated in a small spot after certain time of voltage application. This seems to be caused by field concentration due to deposit of degradation products. In an open void, the intensity of each PD pulse is strong but the time to final breakdown is long. This is explained by the fact that the PD spot does not remain in the same place but constantly moves around, leading to smaller local energy input. On the other hand in a closed void, though the intensity of each PD pulse is small, the time before final breakdown is relatively short. This is explained by the experimental result that the PD spot in this case remains in the same place and the local energy input seems large. Finally, by terminating voltage application just before the final breakdown it is confirmed that the breakdown starts at the spot of PD concentration where the deposit exists.