The characteristics of spread of residual charge in micro gap and PD break down

In this study, we have discussed the partial discharge (PD) degradation process of fatigue-failure-type in a micro gap under the surface discharge pattern of Polbuschel-type. Moreover a polypropylene (PP) film was used in micro gap and measured residual charge distribution on upper surface of PP film in micro gap. An artificial void model was made based on the Whitehead abc model. Ac voltage of 60 Hz at which PD degradation process of fatigue-failure-type under the surface discharge pattern of Polbuschel-type was dominant was applied to this model. As a result, the residual charge on upper surface of PP film in micro gap appeared a positive charge distribution and negative charge distribution at early time with applied voltage. After that the negative charge was remain under the high voltage electrode, on the other hand the positive charge was remain around the negative charge. After that the both residual charge were increase and the distribution was formed like a caldera in volcano. Then the PD break down was occurred in center of residual negative charge. On the other hand, PD degradation area on the upper surface of PP film in micro gap spreads circularly just under the high voltage electrode. After that the PP firm was break down in center of residual negative charge. Furthermore, the positive charge area is always wider the negative charge area. The result shows that the charge density of negative charges in the micro gap is large compared with that of positive charges and that space charge effect caused by the negative charges is dominant in the micro gap. Therefore, negative residual charges play an important role in PD break down of the micro gap.