Insulation characteristics of epoxy insulator with internal crack-shaped micro-defects - fundamental study on breakdown mechanism

The degradation characteristic of solid insulators is considered to be one of the key factors to perform a risk assessment of highly aged gas insulated switchgear (GIS). The present study experimentally obtained the breakdown voltage-time (V-t) characteristics of epoxy insulator with an internal microscopic crack-shaped defect. This defect model was designed and produced so that an apparent partial discharge (PD) might be 1 pC in an actual 550 kV-GIS spacer. During the experiment, the change in the PD magnitude over time and the PD waveform were measured to clarify the PD characteristics until breakdown occurred. Consequently, in terms of the change in the PD magnitude over time, it was likely that the lower the applied electric field (EF), the larger the ratio of the time until the PD magnitude drastically increased (tree induction time) to the total time until breakdown occurred. When 6 kVrms/mm, or the lowest EF in the present test, was applied, two distinctively different trends were observed; one where the PD stably continued and breakdown occurred within several to several tens of hours, and the other where the PD was suspended in mid-course and it was longer than 100 hours until breakdown occurred. Subsequently, the breakdown EF under long-term operation was estimated by extrapolating the V-t characteristics, whereupon a risk under long-term operation was recognized when a micro-defect of 1 pC was present in actual GIS; even though such defect was extremely unlikely. According to the observation results of the PD magnitude, however, the tree induction time was longer at a low EF of the operating EF level and the declining gradient of the V-t characteristics was considered lower. For a rigorous evaluation of breakdown risk under long-term operation, therefore, it was considered that evaluation via simple extrapolation of the V-t characteristics by straight line was not sufficient and the insulation characteristics under a low EF must be obtained.