Short-circuit contribution of large induction motors

The relatively slow operation of short-circuit protection in the past allowed adequate time for the currents generated by induction motors to decay to an insignificant magnitude. Progress in protection techniques introduced rapid fault clearing and, in consequence of that, the fault-current contribution of induction motors can no longer be neglected. The paper describes a series of tests carried out on 6.6 kV power-station induction motors to determine their fault-current contribution in the event of short circuits on the supply system. Results are given in the form of graphs showing the variation of peak and asymmetrical breakingcurrent contributions as a function of time after the fault. Various factors influencing the initial inductionmotor fault contribution and its rate of decay are discussed and assessed.