Phase to ground insulation in low voltage machines: lifetime evaluation under enhanced thermal stress

Turn to turn insulation breakdowns are often identified as a primary cause of stator-related failures in rotating electrical machines. However, it is not the interturn short circuit in itself which causes the machine outage, but rather the localised increment in temperature in proximity of the affected turns. If the fault is not promptly detected, the latter can quickly trigger a chain reaction, which eventually degenerates in the puncturing of the interphase or the ground-wall insulation systems. If a machine is designed with features for fault-tolerance, then, a physical and galvanic separation is guaranteed among different phases. Thus, a temperature increment caused by an interturn breakdown is most likely to affect the phase to ground insulation. This paper presents the results of an experimental investigation, whose aim is relating the winding hot-spot temperature with the time to failure caused by a phase to ground short circuit.