DC-link Dynamics Analysis and Optimal Control Strategy for Grid-forming DFIG-based Wind Turbines in Weak Grid

When the doubly-fed induction generator (DFIG) based wind turbines are connected to the weak AC power grid, the grid-forming (GFM) control strategy without the phase-locked loop for both the rotor-side converter (RSC) and the grid-side converter (GSC) can improve the grid-connection stability of the power generation system. However, for the DC-link port, the anti-disturbance ability of the DC-link voltage of GSC under GFM control is reduced, and the dynamic characteristics of the DC-link voltage cannot be ignored when the unit output power fluctuates. Therefore, this paper establishes a small-signal model for the grid-forming DFIG-based wind turbines considering the dynamic coupling characteristics of the AC and DC sides. The stability of the DC-link voltage control with different power grid strengths and wind turbine rotor speeds is studied based on the system model. According to the coupled transfer function matrix, the coupling influences of the stator power on the DC-link voltage in different conditions are analyzed. The AC/DC coupling path of stator power to the DC-link voltage is analyzed by using the small-signal model, and the decoupling optimization strategies are designed. Theoretical analysis shows that the proposed strategy can reduce the coupling influence of active power loading on DC-link voltage, and optimize the dynamic characteristics of the DC side. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified on the hardware-in-loop experiment platform. © 2024 Automation of Electric Power Systems Press. All rights reserved.