Partial discharge characteristics of interturn insulation used for inverter-fed traction motor under bipolar impulses

Inverter-fed traction motors, which are employed for electric-multiple-units (EMUs), are subjected to Pulse Width Modulated (PWM) impulse voltage. Impulse voltage has fast rise/fall time and high repetition frequency. The failure mechanism of insulation under impulses is different from that of standard 50 Hz sinusoidal condition. Partial discharge (PD) characteristics (discharge number, mean discharge quantity and phase resolved partial discharge (PRPD) pattern) with different frequencies and different rise times of impulse voltage were investigated. The results reveal that discharge number and mean discharge quantity increase, while PRPD pattern moves to the zero of impulse voltage with decrease of rise time. Discharge number and PRPD pattern follow that trend with increase of frequency, the value of mean discharge quantity is similar. Memory effect of the deposited space charges on PD was studied. PRPD pattern moves to the zero of bipolar impulse voltage due to the role of space charges, with increase of frequency and decrease of rise time. It is revealed that available discharge time is prolonged and overvoltage across the cavity increases with augment of frequency, which results in discharge number increasing. Under the condition of shortening the rise time, with the same statistical time lag t (L) , Delta V would increase if dV/dt is higher, which results in enhancement of discharge quantity. Discharge number increases with reduction of rise time, because the decreased multiple of t (d) is lower than the increased multiple of dV/dt. Study of PD characteristics and role of space charges is helpful to the exploration of the destructive mechanism of insulating materials under bipolar impulse.