Transient potential measurement on stress grading under multi-level PWM

In this paper, the influence of lower-frequency voltage components in PWM voltage on the performance of stress grading system of motor insulation is investigated by observing the light emission of partial discharge and by measuring potential distribution under 2- and 3-level PWM and sinusoidal ac voltages. Computations by finite element method are conducted to determine electrical field on coil surface. According to the results, the potential gradient under PWM voltage is largest during the first pulse after polarity reversal, as space charge of reversed polarity accumulates at the interface between corona armor tape (CAT) and stress grading tape (SGT) due to a fundamental frequency component in PWM voltage. Therefore, the intensity of light emission during the first pulse is the largest. From the computed results, it has become cleared the maximum field at the interface between the CAT and the SGT becomes higher under PWM voltage than that under rectangular ac voltage whose jump voltage is the same due to such charge accumulation.