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

The now worldwide use of inverter driven high voltage motors utilizing pulse width modulated (PWM) inverters makes higher efficiency and controllability possible. One downside with this technique, however, is that the electrical stress has been observed to age the high voltage insulation system more rapidly. One of the most important factors behind these observed prematurely failures, is due to the presence of partial discharges (PDs) in the insulation system. This paper present observed changes in the PD characteristics within a high voltage mica- epoxy based motor insulation when exposed to a three-level inverter voltage shape with controllable rise time. This with the main purpose to lower the electrical stress level and reduce the risk of failure. To evaluate the insulation performance, measurements of PD characteristics are presented for the different cases. Experimental results presented in this work show that the average maximum PD magnitude is indeed reduced in the investigated motor insulation when applying this filtering approach. It is also noted that the extinction voltage can be increased to a similar level as for 50 Hz sinusoidal wave shapes. It can, based on these observations, be concluded that adapting the risetimes of a limited part of the complete waveform can lower electrical stress and likely reduce the risk of motor failure. The observations and measurements discusses both drawbacks with this method as well as advantages. This with the overall objective of reaching the same insulation performance as when applying longer rise times on all voltage flanks. To highlight the influence of the different wave shapes, examples illustrating how the PD behavior changes for different cases are investigated. This with the aim to enhance the understanding of the PD performance for this important type of motor insulation system