Frequency dependent time domain models for calculating switching transients in PWM drives

This paper presents frequency dependent time domain models for both the induction motor and the supply cable in order to simulate the switching transients from PWM drive systems. The practical limit of using travelling wave theory to explain the variation of the transient overvoltage of the PWM pulse due to the change of the cable length is shown. The impedances in the conventional equivalent circuit of the induction motor are modified. This is based on Foster´s reactance, a concept from network synthesis theory, to obtain the required frequency dependence. The cable is divided into small sections so that its frequency dependent characteristic due to the skin effect can be included in the time domain model. The models are implemented using the EMTDC power system analysis package. The new model gives highly satisfactory results for calculating the amplitude of the transient overvoltage which occurs when the motor is supplied by a PWM drive via a cable