Impedance Reshaping Method of DFIG System Based on Compensating Rotor Current Dynamic to Eliminate PLL Influence

Abstract

The phase-locked loop (PLL) used in the doubly fed induction generator (DFIG) can cause frequency coupling phenomena, which will give negative resistance characteristics ofthe DFIG at low frequency, resulting in stability issues under weak grid operation. Based on the multi-input-multi-output (MIMO) impedance model of DFIG system, it is found that the frequency coupling phenomena is mainly introduced by the transfer function matrix related to rotor current dynamic. This paper presents an improved impedance reshaping method based on compensating rotor current dynamic to reduce the influence of PLL, in which the rotor current dynamic is compensated before being introduced to the PI controller. Thus, the frequency coupling effect can be almost eliminated and the stability of DFIG is improved a lot. Furthermore, a simplified compensation method is proposed,which can easily be implemented. Robustness analysis is performed to illustrate the availability of the proposed methods when the system operating conditions and parameters vary. Finally, simulations based on MATLAB/Simulink are also carried out, and the results validate the effectiveness of the proposed methods.