Partial discharges in VPI winding insulations in dependence on the impregnating resin

In order to quantify the differences in partial discharge activity between various vacuum pressure impregnated winding insulations for rotating machines, the evolution of partial discharges activity during the ageing of these insulations has been investigated. In order to gain insight into the differences between various insulating materials, specimens impregnated with two different resins were tested. The mean apparent charge, the maximum apparent charge, the quadratic rate and the skewness were extracted from the phase resolved partial discharge patterns and trended over ageing time. None of these parameters showed a sudden rise or change as a consequence of imminent insulation breakdown. So the imminent failure of an insulation needs not always to be indicated by partial discharge quantities. The pattern skewness and the quadratic rate can be used to provide threshold values in order to estimate the severity of internal defects. However, one must account for the impact of the material characteristics on the dominating failure mechanisms in order to interpret partial discharge measurements