Numerical Simulation of Overvoltage Generated at Switching on Medium-voltage Motors with the aid of Different Circuit Breakers

So far voltage surges at motor switching-on have not been the subject of thorough investigation and discussion, for they were assumed to be of safe levels. Lately a number of works appeared stating that vacuum circuit breakers (VCB) can produce voltage surges critical for motor insulation, unlike circuit breakers with oil or SF6 arcing medium. To examine the described phenomenon a SimPowerSystems model has been developed. It allows simulating transient processes in cable-motor system at switching-on, under variation of the following parameters: motor power, cable type and length, surge arrester (SA) type and location, CB type and poles closing time scatter. The VCB model used considers dielectric strength reduction with time and high-frequency current interruption. The model was successfully verified using reference data. However, simulation results have not confirmed particular danger of vacuum compared to other interrupting media with respect to switching overvoltage.