Effects of Cooling Lubricants on the Partial Discharge Inception Voltage of Hairpin Winding Interphase Insulation in EV Traction Motors

The ongoing electrification of the transport sector is driving a continuous increase in the power and torque density of electrical machines: thermal management becomes a critical challenge, since more heat must be dissipated in confined volumes. Hence, conventional air-cooling solutions are no longer sufficient, leading to the adoption of dielectric oils-based cooling strategies. If oils provide significant thermal benefits and, generally, are not detrimental to insulation systems, their introduction can modify the dielectric environment surrounding the windings. As a result, electrical phenomena such as PD inception may be affected, potentially leading to changes in the Partial Discharge Inception Voltage (PDIV). This thesis focuses on the investigation of changes in PD in oil-cooled electrical machines, with particular emphasis on comparing the effects of different dielectric cooling lubricants on PDIV. The aim is to compare different dielectric fluids under representative operating conditions, principally different temperatures,considering the interphase insulation configuration of hairpin windings motors. Particular attention is devoted to assessing possible PDIV reductions with respect to air-insulated conditions. Besides quantifying PDIV variations, a goal is also to compare the relative performance of different dielectric oils and to investigate the physical and chemical mechanisms responsible for the measured variations; a physico-chemical characterization of the insulating fluids is carried out so to link the electrical behaviour to changes occurring within the oil during thermal exposure. Techniques such as TGA and FTIR are employed. These analyses are made to to investigate whether changes in oil composition and volatility may be associated to the observed PDIV variations. The outcome of the thesis is a comparison between dielectric fluids, providing insight into the interplay between oils and PD behaviour in oil-cooled electrical machines.