• Weak grid DFIG systems with GFL-SVG exhibit significant mid-frequency resonance risks. • Shunt capacitance and wind speed variations directly influence system resonance stability. • VSG single and dual-loop controls effectively reshape impedance to suppress resonance. • Dual-loop VSG control offers faster reactive power tracking and resonance damping. Under weak grid conditions, the static var generator controlled by a virtual synchronous generator exhibits enhanced stability and can mitigate mid-frequency resonance in doubly-fed induction generator wind farms, which commonly occurs under grid-following control. To comparatively analyze the characteristics and mechanisms of the static var generator in improving resonance stability under single-loop and dual-loop virtual synchronous generator control modes, this study first establishes sequential impedance models for the static var generator and doubly-fed induction generator under different control modes. Subsequently, the impedance analysis method is employed to investigate the generation mechanism and key influencing factors of mid-frequency resonance. Furthermore, the dynamic performance and resonance suppression effectiveness of the static var generator under the virtual synchronous generator single-loop and dual-loop control modes are compared. Finally, simulation verification is conducted using the StarSim-HIL platform. The impedance analysis reveals that the grid-following static var generator introduces a negative damping region in the mid-frequency band, causing the system resonance. In contrast, the virtual synchronous generator control effectively reshapes the static var generator output impedance, elevating the phase from negative to positive damping in the critical frequency range, thereby fundamentally suppressing the resonance. And the dual-loop virtual synchronous generator control the static var generator, leveraging its current inner loop, exhibits superior reactive power tracking capability, faster and smoother transient responses, and enhanced rapidity and effectiveness in suppressing mid-frequency resonance.
Chen et al. (Wed,) studied this question.