Diagnostic Technique for Electrical Tree by Current Integration Method

The degradation of insulating property of polymeric materials was evaluated by the new diagnostic technique of current integration system, hereafter referred to as the “Q(t)-meter”. The Q(t)-meter measures the change with time in integrated charge accumulated by a capacitor inserted between a DC high-voltage power supply and a sample of the insulation material under study. Q(t)-meter is suitable for the monitoring of time dependence of leakage current of the insulating sample under DC voltage. To demonstrate the performance of this new device, we evaluated the influence of generation and propagation of “electrical tree” in epoxy resin sample on DC leakage current. The electrical tree is a typical electrical degradation of polymeric materials. Hence, nondestructive diagnostic methods for electrical tree are expected. In this study, the electrical tree was generated in the epoxy resin by AC high voltage between two needle electrodes. The interelectrode distance was 2 mm. In the Q(t) measurement, the sampling frequency, total sampling time, DC applied time, and test voltages were 2 s, 600 s, 900 s, 250-1,000 V, respectively. We conducted Q(t) measurement at three stages of electrical treeing - before tree generation, after tree generation, and after tree propagation. It was found that the leakage current increased with increase in the tree length. A remarkable trend of steep rise and fall in the Q(t) curve was also observed. Experimental results obtained by the Q(t)-meter were analyzed assuming an “RC circuit model”.