AC Breakdown of Epoxy Resins by Partial Discharges in Voids

The subject of this paper is the effect of partial discharges in a void on the performance of epoxy resin insulation. Theoretical and experimental investigations are reported that seek a relationship between measurable electrical quantities and a possible breakdown of the insulation. All experiments were done under controlled conditions with samples of quartz-power filled and unfilled epoxy resin containing an artificial void of known cylindical shape and dimensions. The test specimen and the measuring circuits are described. Theoretical considerations of the equivalent circuit of the test circuit indicate that the charge fed into the specimen per pulse and the discharge power are independent of the external circuit. The electric charge and power fed into the specimen are appropriate fundamental quantities for the assessment of the pulse-type process. The theory was confirmed by measurements. It was found that the power consumed by the internal partial discharges up to a breakdown could be measured by two independent methods. Experimental results of long-time aging investigations with alternating current voltage are reported. Partial discharges in voids in epoxy resin insulation subjected to long-term voltage stressing can lead to breakdown of the insulation. The period of time of voltage application to a test specimen can be divided into two characteristic intervals, which were of different duration in the filled and unfilled resins. During the first interval, all important corona quantities (i. e., corona charge, discharge current, power and radio interference voltage) remain either constant or decrease.