Experimental research on creepage discharge between oil-impregnated pressboard layers

Partial discharges are considered to be an important role in the deterioration process of the oil-impregnated pressboard insulation system. It was found that the PDs are favor to produce tree like carbonized trace inside the oil-impregnated pressboard. High voltage experiments were carried out on the needle-plate configuration to research the change of the PDs, the formation and propagation of the carbonized trees. The oil-impregnated pressboards were parallel with the electric field, and the needle was inserted the pressboard so that the trees can propagate between the layers of the pressboards. The PDs and the tree phenomena were continuously record until the pressboard was breakdown under certain AC voltage. Then the characteristic parameters of the PDs and the length of the trees were contrast to find out their correlation. The white mark was the outside appearance of the carbonized trees inside the pressboard. The propagation of the carbonized trees appeared as the expending white mark, which can be observed and recorded easily. Two kinds of PDs were found, one type of PDs has high and increasing charge quantity and numbers, the other has low and decreasing charge quantity and numbers. Before and after the appearance of the white mark, the PRDP of the PDs has changed. The repetition rate of the PDs and their total charge quantity in one minute have linear relationship with the length of the white mark, except the accelerative stage near the breakdown. Thus, they can be used to indicate the propagation of the carbonized trees. But there is exception that the PDs are very small but the carbonized tree develops quickly.