A Double-PLL-Based Impedance Reshaping Strategy for DFIG System Under Grid Frequency Deviation

The stable operation of doubly fed induction generator (DFIG) systems under weak-grid and off-nominal-frequency conditions is important for reliable wind-power integration. However, the phase-locked loop (PLL) dynamics can degrade DFIG impedance and damping, while conventional single-PLL reshaping may suffer from compensation drift under grid-frequency deviations and often relies on a high-pass filter. Therefore, this paper proposes an integrated Double-PLL-Based impedance-reshaping strategy for DFIG systems. Firstly, a complete DFIG admittance model incorporating the rotor-side converter, grid-side converter, DC link, and PLL dynamics is established to identify the critical coupling channel responsible for the adverse impedance characteristics. Secondly, a supplementary rotor-current compensation path is constructed to directly reshape the adverse impedance characteristics and improve system damping. Thirdly, the relative phase angle between the main and auxiliary PLLs is used to generate a frequency-adaptive compensation signal, thereby avoiding continuous compensation drift under persistent frequency deviations and reducing reliance on a dedicated high-pass filter. In this complete strategy, the compensation path directly performs impedance reshaping, while the Double-PLL-Based implementation provides frequency adaptation. Finally, generalized Nyquist analysis and MATLAB/Simulink simulations demonstrate improved impedance matching and oscillation suppression under the considered weak-grid and continuous-frequency-deviation conditions, while hardware-in-the-loop (HIL) experiments corroborate the robustness under PLL-parameter variations.