Reduction of Electric Stress in High Voltage Motor Insulation by Adjusting Individual PWM Flanks

The widespread use of inverter driven high voltage motors enables higher efficiency and controllability in application. The electrical stress within motor insulation has however been observed to increase considerably when employing such solutions. Employing ultra-short rise times is one of the most important factors behind increased stress often resulting in partial Discharges (PDs) and early failures. This paper present observations of the measured electrical stress exposure within a high voltage mica- epoxy based motor insulation systems when exposed to a three-level inverter waveform. The tests waveshape employ a controllable rise time and voltage magnitude aiming to reduce the electrical stress. To study this method, a suitable diagnostic method (PD detection) is employed to evaluate the motor insulation stress while exposed to rapidly changing voltages. To quantify the stress, the PD amplitude and number of PDs were measured at 2 kVpp. Measurements illustrates for example how the average maximum PD exposure is reduced as the controllable filtering method is applied. Also, the extinction voltage changes towards what is observed for sinusoidal exposure. Thus, the presented approach show that filtering of selected voltage flanks can be sufficient for reducing electrical stress considerably. This can increase the insulation life of the motor. The presented results shows both limitations and recommendations on how the waveform should be adapted and controlled. To highlight the influence of the different test cases, examples illustrating the difference in PD behavior for the different cases are included. This with the aim to enhance the understanding of the insulation performance for this important type of motor insulation system.