Symmetrical and unsymmetrical short-circuit current of squirrel-cage and doubly-fed induction generators

Abstract

A general method for calculating the short-circuit currents of a squirrel-cage (SCIG) and a doubly fed induction generator (DFIG) using crowbar protection is presented. The method results in an approximate analytic expression that allows calculating the SCIG and DFIG short-circuit currents as a function of time for symmetrical and unsymmetrical faults in the power system. The presented theoretical analysis highlights some important differences between the short-circuit behavior of the SCIG and the DFIG with high crowbar resistance. A comparison with time simulations of the same model shows that the proposed approach leads to results whose accuracy is comparable to those obtained through dynamic simulations and may be sufficient to replace the use of simulations in many contexts, e.g. calculation of maximum current, calculation of its DC and AC components and short-circuit calculations for protection relay settings.