Study on flashover characteristics of insulators in SF6 under combined voltage of DC and lightning impulse

Insulators are under residual DC voltage after opening of disconnectors. When disconnectors reclose, overvoltage will be superimposed on the pre-existing DC voltage. As a result, insulators in GIS will be under combined voltage of DC and impulse. In this paper, flashover characteristics of insulators in SF6are studied under combined voltage of DC and lightning impulse. The results show that for non-contaminated insulators, pre-stressed DC voltage doesn't affect the flashover voltage if it is in the same polarity with the impulse voltage. When they are of the opposite polarity, flashover voltage decreases with increasing DC voltage. In the presence of metallic particles or powder around insulator, pre-stressed DC voltage will cause adhesion of the contaminants on the insulator surface. For the contaminated insulators, pre-stressed DC voltage will have big influence on the flashover voltage of insulators depending on its polarity. The results are analyzed from the perspective of surface charge accumulation. In addition, since combined voltage is found to be more sensitive to detect some insulation defects in SF6, combined voltage test could be considered to serve as a supplement of field test to verify the insulation performance of GIS.