Synchronization Instability Mechanism and Damping Enhancement Control for DFIG-Based Wind Turbine During Grid Faults

Doubly fed induction generator (DFIG)-based wind turbines (WTs) are prone to lose synchronization with the power grid during weak grid faults, and the grid synchronization instability phenomenon features the frequency oscillations or divergence of phase-locked loop (PLL). However, the instability mechanism of PLL caused by other parts dynamics of DFIG-basedWTs remains unclear. Thus, this article investigates how the PLL dynamics are affected by the current control loops of the grid-side converter (GSC) and rotor-side converter (RSC) in terms of grid synchronization. Firstly, a small-signal model is developed, where the transfer function of PLL is cascaded with another transfer function that represents the dynamics of other control loops in the open-loop transfer function of this model. Then, the instability mechanism is elaborated by analogy with the Heffron-Phillips model of synchronous generator, which indicates that the current control loops of GSC and RSC deteriorate the synchronization stability by providing negative damping. Based on it, the GSC-based stabilizer for damping enhancement is proposed for DFIG-basedWTto improve synchronization stability, which takes full advantages of the GSC capacity potential. Finally, the theoretical analysis and proposed method are validated by experiments.