Abstract
When partial discharge (PD) was measured with the conventional pulse method, it sometimes apparently disappeared. However, it turned out that the PD did not actually disappear, but only became a large number of very small partial discharges known as swarming pulsive microdischarges (SPMD). After this silent stage, detectable PD appeared again, leading to final breakdown. Since SPMD are missed in the conventional PD pulse detection method, it is necessary to pay careful attention to SPMD in terms of insulation diagnosis. We developed a personal-computer-aided partial discharge analyzing system (CAP DAS) for SPMD measurement. Based on the measurement with this system, a new PD parameter, the SPMD ratio, was introduced for quantitative analysis of SPMD. Using the developed system and the new parameter, partial discharge characteristics of the modified CIGRE Method-II (CM-II) specimen were studied to clarify the factors affecting the occurrence of SPMD. It became clear that void surface roughness caused by the PD degradation is found to be one of main factors. Further the amplitude and the frequency of the applied voltage also affect the occurrence of SPMD.
