Effect of X-ray irradiation on partial discharge inception and extinction voltage characteristics of cast resin transformer insulation system

In general, it is hard for discharge to be initiated in a small void existing in solid insulating material because of insufficient initial electrons. For this reason, the discharge occurs after a long delay even at a critical electric field to initiate discharge. In this study, the initial electrons are supplied by X-ray irradiation to a cavity formed in epoxy resin (ER) sample. We have investigated PD characteristics of ER with spherical void. Additionally, we investigated the effect of X-ray irradiation on PD inception voltage (PDIV) and PD extinction voltage (PDEV) of the sample with delamination. It was found that the lowest and stable PDIV was brought by the X-ray irradiation for the samples with the cavity and delamination. Furthermore, we estimated quantitatively attenuation of X-ray dose by the structure such as winding, an iron core of a cast resin transformer. Discharge time lag was estimated from X-ray dose, and compared with that measured for the ER sample with the cavity. This report also deals with the effect of X-ray irradiation on PD generating characteristics in various artificial detects of epoxy insulation system simulating insulation of winding, delamination, and void.