Influence of conductive particle contaminationon the insulation system of rotating electrical machines with direct oil cooling

An integration of the electrical machine and the gearbox is attracting particular attention for the design of modern electric and hybrid drive trains, since it saves overall space and subsequently increases the power density. Another benefit of a high level of integration is that it enables a combined application of oils as both cooling fluid for the electrical machine and as lubrication fluid for the transmission system. In this way, the power density of the integrated drive train can be further increased. During the oil cycling, conductive contaminations may be introduced and subsequently have an influence on the function of the insulation system of the electrical machine. In the present work, the influences of the cooling oil and its conductive contaminations, conductive particles as well as their combination with humidity, on the electrical and dielectric properties of the insulation system are studied. The results show that by application of the cooling oil, the partial discharge inception voltage (PDIV) of the winding insulation increases significantly so that an electrical breakdown is prone to happen before a partial discharge (PD) occurs. With increasing particle contamination, the PDIV of the insulation system decreases significantly, while the capacitance increases. Besides, conductive particles and humidity decrease the surface resistance and surface breakdown voltage of the insulation papers significantly. The results indicate that the conductive particle contaminations can play an important role for the electrical degradation of the insulation system.