Change in PD pattern with aging

The changes in the PD /spl Phi/-q-n pattern and the PD current shape due to the degradation of insulating materials have been investigated with a fast digital oscilloscope and a PD measurement system. In order to verify the effect of gas change, the volume of gas in a void was observed with the /spl Phi/-q-n pattern. It showed that the change in the maximum PD magnitude is related to the change in gas volume according to ageing time. The PD /spl Phi/-q-n pattern could be classified into the five stages. At stage 3 for example, which had a "rabbit-like pattern", the current shape of the first PD in each half-cycle had a large magnitude of the order of 100 mA and a narrow width. At stage 5, which had a "turtle-like pattern", the current shape had a slow rise-time and a slow decay-time. Such changes in PD characteristics will be applicable to the diagnosis of insulation degradation.