Partial Discharge Evolution of Hairpin Winding Insulation during Multi-Stress Aging under Various Square Wave Voltages and Environmental Conditions

With the rapid development of wide bandgap devices and electric vehicles (EVs), electric machines with hairpin windings have gained extensive attention due to the superior slot-filling factor and relatively higher power density. As EVs face a harsher environment, such as high elevation where air pressure is relatively low, the hairpin winding insulation, subjected to multi-stresses including high dV/dt square wave voltages, extreme temperatures, and low air pressures, could trigger partial discharge (PD) that may compromise insulation. However, critical issues such as the PD evolution mechanism under diverse operational conditions and the fundamental mechanism of insulation failure have not been systematically investigated yet. This paper focuses on PD evolution during multi-stress aging of hairpin winding, which could be affected by PD characteristics. A down-mixing ultra-high frequency PD detection method is applied. PD characteristics such as partial discharge inception voltage (PDIV) and phase-resolved partial discharge (PRPD) are analyzed under square wave voltages with various rise times and frequencies at different temperatures and air pressures. Accelerated aging experiments are performed and a lifetime model is proposed considering the effect of square wave voltage parameters and environmental conditions. With the decrease in air pressure and square wave voltage rising time, and the increase in temperature and square wave frequency, the lifetime decreases. The aging process comprises a development stage, characterized by gradually decreased PD magnitudes, and a pre-breakdown stage, which appears near the end of the lifetime and displays a slight increase in PD magnitude.