Abstract
For high voltage induction motors of large ratings, new driving systems such as inverters using Pulse Width Moduration (PWM) are becoming popular in industrial plants using mill motors and traction motors. Understanding of the partial discharge characteristics under inverter voltage stress is very important to achieve reliable insulation design for these apparatus. A model sample with a single void in insulation and IEC (b) electrode system for surface discharge on insulation were used. Partial discharge inception voltages under ac voltage stress, unipolar rectified ac voltage stress and unipolar rectangular step pulses were investigated by varying the applied voltage. As a result, it was found that the zero-to-peak value of partial discharge inception voltage under unipolar voltage stress was 1.5 to 2 times higher than that under ac voltage stress. This reason can be attributed to the behaviour of the residual voltage after discharge at the void since the occurrence of the partial discharge inception voltage under unipolar stress is considered to be higher than under ac stress.
