Analysis of Whole-System VSG Control Angle Strategy and Dynamic Characteristics of DFIG Under Weak Grid

The grid-connection stability of doubly-fed induction generator (DFIG) has become an increasingly prominent issue in high-penetration renewable energy systems. Conventional phase-locked loop (PLL)-based grid-side control methods may exacerbate the system's negative impedance characteristics, leading to stability deterioration. To address this issue, a PLL-free grid-forming control strategy for DFIG is proposed, building upon conventional grid-forming control methods. A small signal model of the DFIG system is established to analyze the impact of PLL-free control on the system's impedance characteristics. Subsequently, impedance analysis and simulation comparisons are conducted for two different grid-forming control structures: fully virtual synchronous generator (VSG) control and grid-side PLL-based VSG control. Simulation results demonstrate that the proposed PLL-free grid-forming control effectively mitigates negative impedance characteristics and significantly enhances the inertia response capability of the wind turbine system.