Impedance-based Stability Research Of DFIG System

Wind energy is clean and huge reveres in the earth.Development of wind energy generation is an important issue in the field of renewable energy generation.The doubly-fed induction generator(DFIG)is the one of the most used type of wind power generators in the world.Under the influence of interaction between grid condition and control system,problems of stability will occur in the process of grid-connected power generation.Therefore,the impedance-based method has been adopted in this paper to analyze the stability issues of DFIG connected with different grid conditions.The main research content of this paper is: modeling the equivalent input admittance of DFIG in the form of second-order matrix and complex vector;analyzing DFIG stability under weak-grid connection based on general Nyquist criterion(GNC);suppressing sub-synchronous resonance(SSR)phenomenon by reshaping characteristic of DFIG admittance under series-compensated grid condition.This paper focuses on the stability analysis of DFIG-grid connection system using impedance-based method.The DFIG equivalent input admittance considering the dynamics of stator and rotor flux,the rotor-side converter(RSC)current controller,phase-lock loop(PLL),active-power controller and alternating-voltage magnitude controller has been modeled.Frequency scanning method has been used to measure DFIG equivalent input admittance value to verify the modeling results.Impedance model of weak grid and series-compensated grid have been modeled.Stability of system could be analyzed using the deduced model based on GNC.As the DFIG equivalent input admittance established in d-q frame is in second-order form,GNC,which is aimed at the multi-input multi-output(MIMO)systems must be used.The process is complicated.The equivalent input admittance of DFIG including PLL dynamics in complex-vector form has been established in this paper.Stability issues of DFIG-grid connected system could be analyzed based on Nyquist criterion,which simplifies the process of analysis.Under series-compensated gird condition,SSR phenomenon will occur in the DFIG system.To suppress SSR problems,the method of reshaping the characteristic of DFIG equivalent input admittance is adopted in this paper.A band-stop filter will get work in the control system during the process of SSR.The proposed method is convenient to implement and requires no extra equipments.The validity of theoretical analysis is verified by experimental results.