Considerations on Evaluation Methods for Reliable Stress Grading Systems of Converter-fed High Voltage Rotating Electrical Machines

SiC-based nonlinear resistive grading materials have been used for end turn stress grading of form-wound rotating electrical machines, such as high voltage motors and turbine generators, over a long period. Although these materials work well under power frequency, the electric field and local heating become problematic under PWM repetitive pulses for the stress grading systems of converter-fed rotating machines. In order to select the appropriate stress grading materials and to optimize the insulation design of end turn structure for PWM waveforms, accurate knowledge of the nonlinear conduction characteristics of stress grading materials as a function of temperature and electric field is required. In this paper, the authors analyze sophisticated sample configurations for accurate measurement of nonlinear conductivity characteristics of the materials based on transient nonlinear FEM computations.