A Dynamic Equivalent Method for Doubly-fed Induction Generator Wind Farm Based on the Thevenin Equivalent Circuit

According to the third-order simplified dynamic model of doubly-fed induction generator(DFIG),an equivalent method of DFIG wind farm based on the Thevenin circuit(EMBTC)is proposed.Firstly,based on the characteristics of mechanical power and rotor speed curve of wind turbine,a faster power flow calculation of power system with wind farm is carried on to determine the initial states of DFIGs.Then,off-line calculation is made to find the upper and down limits of injected wind speed when dynamical characteristics of transient electromotive force change evidently.The DFIGs in the wind farm are clustered according to the dynamic similarities.Secondly,the transient electromotive force buses of DFIGs in one group are aggregated to an equivalent bus through ideal transformers with complex ratios.Parameters of the equivalent DFIG are identified with a similar method of synchronous generators.According to the energy conservation law and a presumed mechanical power and rotor speed characteristics of the equivalent DFIG,the equivalent wind speed and the rotor speed are obtained.Finally,network reduction is accomplished by the elimination of phase shifting transformers,which determines the electrical connections between equivalent DFIG and the point of common coupling.Simulation results of an example wind farm with six DFIGs show that one-DFIG equivalent model has a high steady accuracy while the dynamical accuracy is relatively low.The dynamic precision can be effectively improved when three-DFIG equivalent model are utilized.The EMBTC is suitable for wind farms with complex connections and various wind speeds.