  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.
