Abstract
The extinction of the partial discharge (PD) pulse has been frequently observed in an epoxy specimen with a closed void during voltage endurance tests using a CIGRE Method-II electrode. The individual discharge pulse becomes so small that the discharges cannot be detected by conventional pulse detection methods. Such discharges are known as "swarming pulsive microdischarges" (SPMD). In this paper, the SPMD characteristics are investigated by varying the frequency of the applied voltage from 0.1Hz to 240Hz. As a result, the SPMD are found to occur less frequently as the frequency is lowered. To seek this cause, one-shot voltages were repeatedly applied at constant intervals. By varying the interval from 0.02s to 100s, the PD of electrically aged specimen was measured. It was revealed that the discharge magnitude increases as the interval becomes longer. Onset of SPMD was found to depend on the preceding discharge. The results explain the reason why swarming is harder occur at lower frequency with longer discharge interval. In the conventional pulse detection, hence, it is useful to employ the very low frequency for diagnosis of machine insulation containing enclosed voids.
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