Step response prediction for traction VSI drive with emphasis on signalling compatibility

An analytical method is described for predicting the transient response parameters when a railway traction voltage source inverter (VSI) switches from a pulsewidth-modulated (PWM) to a quasi-square-wave scheme. Typically, the minimum pulsewidth limit is about 150-200 mu s, and the corresponding input current transient can cause conductive interference in power frequency track circuits. The analytical predictions are confirmed by time-domain simulation of the complete drive system. It is also shown that although the system is of sixth order, the transient response is dominated by a pair of roots close to the imaginary axis, thus approximating the system to second order.