A DFIG impedance reshaping method based on dynamic compensation of rotor current to enhance the stability in the mid-frequency band

Although the research on doubly-fed induction generator (DFIG) has gained popularity, the phase-locked loop (PLL) used in the DFIG system leads to frequency coupling with negative resistive characteristics in the mid-frequency band, which reduces the stability of the grid-connected DFIG system and consequently leads to system oscillations. Meanwhile, because of the rise in the share of new energy generation and the long-distance transmission characteristics of wind power systems, the AC grid short circuit ratio (SCR) decreases, which exacerbates the occurrence of system instability. On this basis, based on dynamic compensation of rotor current, an impedance remodeling method for DFIG is proposed to achieve stable operation of the DFIG system in the mid-frequency band. First, an impedance model incorporating the DFIG generator characteristics, rotor-side converter (RSC), and PLL control is established. Second, to analyze the generation of frequency coupling, the multiple-input multiple-output (MIMO) impedance model of the DFIG is developed. In addition, the equivalent positive and negative impedance models are built to analyze the dominant elements of the DFIG system's frequency characteristics, which provide the basic prerequisites for the design of a subsequent impedance reshaping strategy. Moreover, a DFIG impedance reshaping method is proposed to enhance the stability margin in the mid-frequency band when the system operates under a weak grid by compensating the rotor current. Furthermore, the effectiveness of the control strategy under the changing operating conditions of the system is also theoretically analyzed. Finally, the proposed impedance remodeling method is validated by simulation results and experimental results.