Capacitive Grading of 13.8 kV Form-Wound Motor Coil Ends for Pulse Width Modulated Drive Operation

The use of solid state drives for medium voltage motors has been growing significantly. Voltage source inverter (VSI) is one of the most popular classes of these motor drives that along with pulse width modulation (PWM) create many capabilities for motor speed and torque control. On the other side, these drives have adverse effects on the insulation system, particularly on the stress grading (SG) tape on form-wound coils. Enhancement of electrical and thermal stresses on the SG tape leads to rapid degradation of the groundwall insulation. The main objective of this paper is to introduce an effective (SG) system for these motors. The results of laboratory tests and computer simulations for a capacitive SG scheme based on conductive foils are presented and discussed. The results indicate that this method can effectively control both the electrical and thermal stresses at the endwinding region under both power frequency and repetitive fast rise time pulses from PWM drives.