Computer simulation of three-phase prestriking transients in cable-connected motor systems

Abstract

Steep-fronted voltage transients, generated by prestriking conditions in circuit-breakers and contactors when closing, produce severe inter-turn insulation stresses in machine windings. There is a need to calculate the nature and severity of these transients at the system design stage. A computer program simulating the production of prestriking transients is described. The simulation is based on a full three-phase system representation, including busbars, switching device, cable and motor winding. The complex interaction between the system and the switching device, and also the interaction between the three poles of the switching device, are fully taken into account. A solution method based on the Fourier transform and using a combination of voltage and current generators to simulate switching actions is used in the calculations. Validation of the accuracy of the simulation is made by a comparison of on-site test results with the computer simulation. It is shown that system conditions can be simulated with a reasonable degree of accuracy. Illustrations of the wide range of system variables, current and voltage, that can be determined are presented. Finally, to indicate the utility of the program two studies are made for a simple system controlled by a vacuum switching device. The first of these studies examines the effect of mechanical pole closure scatter. The results clearly indicate the large effect that this parameter has on the magnitude and severity of prestriking transients. The second study gives a comparison of series inductor and shunt resistor-capacitor schemes for motor winding prestriking transient protection. The results of this study indicate that although both schemes are useful in reducing the severity of prestriking transient voltages, the choice of which particular scheme to use, purely on a voltage severity basis, is not as clear as might be initially considered.