Partial discharge behavior on PI film with microgap under steep-front step voltage

Recent power devices such as IGBT generate switching surges with steep-front waveform in sub-microsecond. The highly repetitive surges may lead to unexpected insulation ageing of inverter-fed low-voltage motor windings, which have been designed without consideration of partial discharges (PD). Since the distance between adjacent magnet wires in a stator slot distributes in mum to mm, so-called micro-discharge of very short gap with steep-front surges have been investigated recently. In order to evaluate PD ageing of polymer material under steep-front surges, we started the fundamental study of PD inception condition under step voltage with rise-time of nanosecond. Using rod-plate electrodes with 20-50 mum air gaps, time delay and charge buildup were measured simultaneously. It was found that charge builds up with PD instantly and decreases very slowly in comparison with repetitive pulse intervals of PWM. The effect of the surface charge on PD inception of the following surge is discussed.