Degradation-informed coil insulation modelling by impedance frequency response analysis

The highly demanding operation of electrical machines in aerospace ultimately leads to the degradation of the electrical insulation system within the machine stator windings over time. This paper presents a novel hybrid methodology for high-frequency modelling of the insulation system in the concentrated stator winding of a 2 MW aerospace generator. A degradation-informed coil model is presented for purposes of characterisation and monitoring of the coil insulation system by the impedance responses using Bode plots in frequency response analysis (FRA), notably electrical impedance spectroscopy. The proposed approach utilises a finite element model which is informed by measurements of dielectric properties on unaged and thermally aged insulation material specimen and deployed in tandem with an electrical equivalent circuit representation. An analytical tool built in Matlab is used to quantify the results taken from finite element modelling and implemented into LTSpice for circuit considerations, so that the model considers mutual coupling effects of inductances and high-frequency effects such as proximity and skin effect. Dielectric properties measurements allow to inform this model to account for degradation effects and how these reflect in the impedance frequency spectrum. This novel methodology allows extraction of the impedance profile with respect to the material changes within the simulated and experimentally measured unaged and degraded insulation system. The theoretical results acquired by the hybrid co-simulations are validated experimentally using thermally aged coil samples.