Stability analysis of DFIG controlled by hybrid synchronization mode and its improved control strategy under weak grid

In order to improve the stability of doubly-fed induction generator (DFIG)-based wind turbines under weak grid, previous works have proposed the power synchronization control strategy in the rotor side converter (RSC). However, the potential instability risk of dc-link voltage control in the grid side converter (GSC) has been neglected. Here, DFIG with power synchronization and phase-locked synchronization for RSC and GSC respectively, is called as hybrid synchronization mode (HSM)-controlled DFIGURE and, the full-order state space model of HSM-controlled DFIG are established. Eigenvalue and participation factor analysis shows that the HSM-controlled DFIG is unstable under extremely weak grid due to the interaction of phase-locked loop (PLL) and current-loop in GSC. Furthermore, dominant pole analysis indicates that the HSM-controlled DFIG exhibits weak damping at the AC side in strong grid. To enhance the stability of HSM-controlled DFIG under extremely weak grid, an improved hybrid synchronization control strategy is proposed. The improved control strategy utilizes active power and dc-link voltage to achieve PLL-free operation of DFIGURE. The effects of the parameters in dc-link voltage synchronous loop on control bandwidth, system stability and dynamic coupling are analysed. Finally, the validity of the theoretical analysis is investigated with experiments based on 2MW control-hardware-in-loop platform.