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

It is well known that it is hard for discharge to occur in a small void in solid insulating material, because of lack of initial electrons. For this reason, the discharge occurs after a long delay even at a critical electric field sufficient to initiate discharge. To resolve the problem, supplying initial electron by X-ray irradiation to various defective samples has been attempted over the years. However there are not many studies that tried to apply X-ray irradiation to actual equipment. In this study, the initial electrons are supplied by X-ray irradiation from various directions to void formed in an actual cast resin transformer (CRTr). We have investigated PD characteristics of CRTr. Additionally, we investigate the effect of X-ray irradiation on PD inception voltage (PDIV) and PD extinction voltage (PDEV) of the CRTr, and difference of PDIV and PDEV in various irradiation directions. An attempted is also made to identify the void location using X-ray irradiation. It was found that the X-ray irradiation gave the lowest PDIV and stable PD activities in the transformer with void and allowed to identify the location. Furthermore, the effect of the X-ray irradiation on the PD phenomena is discussed by estimating quantitatively attenuation of X-ray dose with considering the structure such as winding of the transformer.