Pit observation and partial discharge measurement before tree initiation

In this research, an experimental study has been performed to investigate the generation of a pit before tree initiation. Since a pit has influence on subsequent degradation, a detailed examination is required. Degradation experiments are conducted using a PMMA block, which has a needle-shaped void at the tip of the needle electrode. The observation of pit form and measurement of partial discharge are performed using a digital microscope of 100 magnification. The discharge magnitude at several phase angle areas of applied voltage was measured using an original measurement system. As a result, in discharge measurement, the discharge pulse becomes a swarming pulsive micro discharge (S.P.M.D.) before the pit degradation process. When S.P.M.D. was not seen, there was no generation of a pit. This showed that there was a close relation between S.P.M.D and pit generation. In form observation, it was seen that a tree immediately grew up to about 100 μm after its generation, even when the growth of the pit was very slow, and also finally it was less than 100 μm.