Calculation of system-fault currents due to induction motors

Abstract

The paper makes recommendations for a standard method of determining the contribution made by induction motors to the fault currents in a power system. The need for such a method arises from the increasing number of large induction motors now being installed, and from the more rapid operating times of present-day switchgear. The basis of the proposals is that, for this purpose, an induction motor can be treated in the same way as a synchronous machine, using mainly the `subtransient reactance' of the induction motor. The recommendations are supported by a detailed theory of induction-motor transients, in which accurate formulas are derived for the current following a sudden short circuit. The `deep-bar effect' usually present in large squirrel-cage induction motors is dealt with in a similar manner to that adopted internationally for the `solid-pole effect' in synchronous machines, where the effect is theoretically simulated by two corresponding time constants appear in the formulas. An operational impedance function, similar to that of a synchronous machine, is used to obtain the transient parameters. The method is a simple one, and is particularly useful for determining switchgear ratings in a mixed system containing both synchronous and induction machines.