Deterioration characteristics of insulation subjected to partial discharge in SF6 gas

The gas insulated transformer using SF/sub 6/ gas as the insulating medium has been developed and rapid increase in Japan recently. Different type insulating materials which are not used in a conventional oil-filled transformer are used in a gas-insulated transformer. The criteria of charge quantity at the partial discharge test currently performed with the transformer are based on the characteristics of conventional oil-filled transformer. The deterioration characteristics of the new material used in a gas-insulated transformers are not well known to be equivalent to those of a conventional insulation materials. In this paper, the insulation materials applied in a gas-insulated transformer were exposed to AC partial discharge, and deterioration characteristics which is estimated with a failing impulse breakdown voltage were investigated. As a result, harmful partial discharge level for gas-insulated transformers is expected to be about 10000 pC.