Thermal failure of stator winding insulation and useful life assessment by impedance spectroscopy

This paper presents a novel application of impedance spectroscopy as a means of inspection for stator winding insulation degradation identification. It proposes, for the first time, a tool for health monitoring and failure identification during thermal degradation effects in the insulation systems of stator coils from a concentrated stator winding. By periodical ex-situ inspection of these coils, the thermal ageing data of the insulation are profiled until the point of failure. The study maps this evolution of the thermal degradation pattern via novel interpretation of the differential-mode impedance frequency responses, using well known techniques such as the Bode and Nyquist plots. This grants further insight into the characteristics shown in the ageing profiles over time and frequency. These thermal failure profiles enable the identification of critical points that can act as pre-emptive indicators of coil health and failure inception. This is a key finding as it renders the capability to provide an adequate lifespan tool for electrical machine concentrated windings and coil assemblies in terms of remaining useful life and failure interpretation. The insulation system examined in this study is from the coils of the concentrated stator winding of a 2MW permanent magnet generator used in an aerospace application.