Modeling of the heat generation on stress grading coatings of motors fed by high speed drives

Stress grading (SG) coatings used in high voltage rotating machines are considerably affected when the machines are controlled by adjustable speed drives. A special case is found in high speed machines where an excessive heat generation can destroy the material of the SG coatings in a short time. In this type of machines the SG coatings are subject not only to voltage components of high frequency associated with the switching devices used by the drive but also to a component of the fundamental frequency above hundred Hertz. In this work, the heat generation on SG coatings under a 3-level PWM waveform is studied. The effect of fundamental frequency, rise time, and switching frequency on the resistive heat generation is computed by using a circuital model implemented on PSCAD/EMTDC, which takes into account the nonlinear behavior of the material. According to the results, with a PWM waveform, the average heat density increases with the fundamental frequency, but this increment is considerable smaller than the expected with a pure sinusoidal waveform. Also the simulation results show how a reduction of the average resistive heat in the stress grading coating can be expected as the rise time of the pulses is increased by means of a suitable dV/dt filter. However, the effect of changes on fundamental frequency or rise time of the pulses on the average heat is less significant compared to the changes on the switching frequency, which is the feature of more influence on the heat generation.