Simulation of the effect of armor coating conductivity on the stress grading coating performance under PWM multilevel waveforms

The high electric field present along the stress grading (SG) coatings in coil ends of medium voltage rotating machines fed by adjustable speed drives (ASD) can lead to surface discharge and an increased resistive heating. Previous works had shown that the analysis of SG coatings for coil ends in motors working with ASD requires consideration of the effect of the conductive armor coating. Usually the design of the conductive armor coating (CAT) and the SG coatings is done separately; however, it is important to understand the combined performance of both coatings, especially under pulse width modulated (PWM) waveforms. With this purpose, transient finite element method (FEM) was used to analyze how changes in the conductivity of the CAT modify the electric field and resistive heat density along both coatings for multilevel PWM waveforms. According with the results, the increment of the CAT conductivity reduces the maximum heat and maximum electric field in the CAT; however, a change in the properties of the SG coating can be required to avoid transferring the problem to this coating, either for three or five levels PWM waveforms.