Charge decay monitoring on polymer films aged by exposure to surface partial discharges

Specimens of Polyethylene Terephthalate (PET) and Polyimide (PI) films, were exposed to the action of surface partial discharges (PDs). They were positioned in a suitable test cell designed so that the PDs were concentrated in a small area. The tests were performed at pre-set relative humidity (RH%) conditions, applying a PWM-like voltage waveform. First the time to breakdown (TBD) values relevant to each test condition were obtained; then, several specimens were aged stopping the tests before the TBD was reached so to obtain specimens aged at different TBD percentages. In order to obtain specific information about the above described aged film areas both kinds of films, PET and PI, were explored by means of charge decay measurements. The films were corona charged, the surface voltage distribution was measured and its time decay was monitored. The obtained results are presented and discussed with particular attention to the comparison between the behaviour of the two polymer films