Effect of high frequency conducted disturbances on the interturn 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 the presence of micro voids inside a composite insulation is considered to be unavoidable. Since such micro cavities 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. On the basis of a numerical study carried out by using a multi conductor 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, in this paper, a new experimental approach to evaluate the effect of PD generated by inverter distorted voltages is proposed. PD measurements under 50 Hz -10 kHz quasi-square and sine wave voltage waveform are performed by means of an Hv capacitive bridge measurement system in order to reject external disturbances, like voltage overshoots, and they are acquired by ultra-wide band (UWB) acquisition system.