Effect of voltage distortion on stress grading coatings working at high fundamental frequency

Stress grading 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 in the stress grading coatings can destroy the material in a short time. The coatings are subject not only to voltage of the fundamental frequency above hundred Hertz, but also to voltage of a higher frequency distortion associated with the switching devices used in the drive. In this work, the performance of stress grading coatings under the combined effect of a high fundamental frequency and voltage distortion is studied. By using a simplified model, which takes into account the nonlinear behavior of the material, the resistive heat for different voltage distortion conditions is computed. The combined high fundamental frequency and voltage distortion significantly increases the heat generation in the stress grading system.