Derivation of the Stress Voltage Waveform Enhancement Factor for Partial Discharge Risk Assessment in Transportation Motor Windings

Due to the increasing DC bus voltage, electrical machines (EMs) for transportation applications are exposed to a great partial discharge (PD) risk nowadays. Thus, PD risk assessment is essential for fulfilling the reliability requirement. PD inception voltage (PDIV) measurement under sinusoidal voltage is sometimes preferred because it demands an easy experiment setup, both in terms of the voltage source and PD detection system. However, the obtained results may lead to an underestimation of PD risks since, in actual applications, EMs are fed through inverters and pulsewidth modulation (PWM) voltage waveforms are applied. In this article, the difference between PDIV results obtained under sinusoidal and pulse voltage signals is quantified, and different environmental conditions are investigated, considering the potential transportation electrification applications. Finally, based on the experimental results, a stress voltage waveform enhancement factor (EF) is proposed and precisely determined. The study reveals that this factor is not constant under varying environmental conditions, with a maximum value of 1.18 observed at 220C and 200 mbar. Furthermore, the influence of the statistical processing method on the derived factors is analyzed, showing a maximum variation of approximately 10% when different Weibull percentiles are employed.