Dynamic Responses of DFIG Fault Currents Under Constant AC Exitation Condition

In wind power system, the fault ride-through capability of doubly fed induction generators (DFIG) has become the focus of study. The analysis and calculation of fault currents are of important directive significance for implementing DFIG fault ride-through. This paper analyzes dynamic responses of DFIG stator and rotor currents when three phase short circuit occurs at stator terminal, and also proposes a new approach, which transfers DFIG into an induction machine with rotor excitation and an idle squirrel-cage induction machine, to calculate the fault currents. The rotor excited induction machine and idle squirrel-cage machine respectively represent the particular solution and general solution of DFIG differential equations in physical meaning. Furthermore, main components of short-circuit currents as well as their relations are presented. The stator and rotor currents contain omega_s component and somega_s component respectively, which relate to steady state, and both comprise DC component and (1-s)omega_s component, which relate to transient state. The maximum rotor and stator short-circuit currents as well as the times when they appear are derived in this paper. Finally all above results are validated in Matlab/Simulink.