Numerical simulation of PD activity in a spherical cavity embedded in the stator winding insulation of an inverter-fed induction motor

As shown in recent papers, partial discharge activity has a fundamental role in the premature failure of electrical insulation of machine stator windings fed by PWM inverters. The existence of partial discharges can be easily explained if we take into account that the presence of microvoids inside a composite insulation is essentially unavoidable. Since such microcavities are filled with a gas whose permittivity is generally several times lower than the permittivity of the surrounding dielectric, the electric field in such occlusions can be much higher than the local expected average stress. The aim of the present paper is to study the influence of the different voltage waveshapes impinging on the motor on the partial discharge activity in a spherical cavity embedded in the stator winding electrical insulation. The numerical study is carried out by using a multiconductor transmission line model for the evaluation of the potential distribution inside the machine and a PD simulation model for the evaluation of the possible ensuing PD activity.