Short circuit current analysis of DFIG with crowbar under unsymmetrical grid fault

In this paper, the fault characteristic and its formation mechanism of double-fed induction generator (DFIG) are studied, from the point of view of power system relay protection. A new method of analyzing and calculating fault current of DFIG is presented, which combines the advantages of physical process analysis and frequency domain analysis. According to the physical process analysis, the frequency domain equation of DFIG is divided into several equations about zero state and zero input response. By separately solving the several equations, formulas for calculation of the fault current are deduced. These formulas are not only applicable to calculation of short circuit current under symmetrical fault but also unsymmetrical fault. Based on this method, the fault current characteristics of DFIG under symmetrical and unsymmetrical fault are compared. Comparison result shows that AC component of fault current of DFIG under symmetrical fault soon decays end. But AC component under unsymmetrical fault is stable at the end. These new characteristics have an effect on the applicability of the traditional relay protection, and provide a theoretical basis for new methods of relay protection. At the end, simulation results show validity of the proposed formulas under symmetrical and unsymmetrical fault.