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Modeling of heat generated on stress grading coatings of motors fed by multilevel drives

Stress grading (SG) coatings used in high voltage rotating machines are considerably affected when the machines are controlled by adjustable speed drives. In some applications the SG coatings are subjected 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 100 Hz. In this work, the heat generated on SG coatings under multi-level PWM waveforms is studied. The effect of the fundamental frequency, rise time, switching frequency and number of voltage levels, on the resistive heat generation is computed by using a circuit model implemented in 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 considerably smaller than that 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 the fundamental frequency or rise time of the pulses on the average heat is less significant compared to the changes in the switching frequency, one of the features of more influence on the heat generation. An increment in the number of levels of the PWM waveform produces a significant reduction in the average heat. With a 5 level configuration the resistive heating becomes similar in magnitude and distribution to the one produced by a pure sinusoidal voltage waveform.