Oil Gap dependence of partial discharge starting point in the oil/pressboard composite insulation system

This paper deals with the impulse PD (partial discharge) characteristics in oil/pressboard composite insulation systems. We examined the PD starting point in test samples with the oil gap length varied from 1.6 mm to 9.6 mm, which simulated the oil duct in an oil-immersed transformer. Consequently, we found that when the oil gap length was below 4.8 mm, the PD starting point shifted from the oil gap to creepage of a spacer when the oil gap length increased. On the other hand, when the oil gap length was over 4.8 mm, the PD starting point shifted from creepage of a spacer to the oil gap as the oil gap length increased. These results of experiments can be explained by considering the volume effect of the transformer oil and assuming that the fibrous protrusions on the surface of the pressboard had an influence.