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

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