PD evolution of conventional and corona resistant enamels

The stator motor windings of low voltage rotating electrical machines are usually insulated by organic enamel. When the motors are supplied by power electronic converters, the stator insulation system is subjected to different voltage waveforms such as PWM. Due to reflection phenomena, overvoltage can be associated to pulsed waveforms and partial discharge (PD) activity can incept leading to the insulating system failure in short time. In this work the PD evolution is monitored both for conventional and corona resistant enamels subjected to different voltage waveforms at different amplitudes. Suitable twisted pair specimens insulated by the considered enamels have been prepared and tested. Aging tests have been performed, monitoring the PD activity until the enamel breakdown, which is assumed as the end-point criterion. Pulsed and sinusoidal voltage waveforms have been applied having different amplitudes. In order to monitor the degradation phenomena, the partial discharge activity has been observed by means of a phase resolved partial discharge analyzer. In order to evidence the behavior of the considered enamels the obtained life curves have been compared; the trends of partial discharge activities have been evidenced and have been related with the degradation phenomena.