Investigation of the failure mechanism of insulation subjected to repetitive fast voltage surges

Electrical rotating machines are subjected to nonsinusoidal wave shapes when supplied from high frequency inverters. PWM (pulse width modulated) voltage sources with steep wave fronts can be destructive to the insulation and criteria normally used under sinusoidal conditions cannot necessarily be transposed to this new case. In order to determine the influence of this electrical stress, laboratory experiments were carried out to determine whether such repetitive voltage surges can precipitate premature failure of high voltage motor windings. A fast voltage surge generator and laboratory cells have been built, to study partial discharge development and material degradation according to voltage magnitude, polarity, rise time and surge repetition rate. Partial discharges (PD) are strongly dependent on the voltage waveform, related to the statistical time lag which is necessary before PD inception. Physical and chemical analysis and surface potential decay measurements show that the ageing process is related to the voltage waveform (frequency, shape factor, etc.). It is suggested that under PWM operation, insulation ageing mechanisms are predominantly controlled through an electrostatic phenomenon, different from the 50/60 Hz sinusoidal case