Research on Frequency Coupling Analysis and Suppression Strategies for Three-Phase Grid-Connected Converters in Weak Grid

This paper proposes a symmetric control method to suppress the frequency coupling phenomenon in new energy grid-connected converters under weak grid conditions. First, a complex vector model of the main circuit for three-phase grid-connected converters was established using conventional small-signal modeling analysis combined with the complex vector approach. Building upon this foundation and comprehensively considering the phase-locked loop (PLL) and dual-loop voltage-current control, a second-order complex vector admittance model in the stationary coordinate frame was ultimately constructed. Based on this model, the impacts of factors such as the DC bus capacitor, the PLL, and the DC voltage loop on frequency coupling were analyzed. Subsequently, a symmetric control strategy targeting the dq-axis asymmetry in both the PLL and the DC voltage loop was proposed. Theoretical analysis demonstrates that this method can effectively suppress frequency coupling and enhance system stability. Finally, experimental validation demonstrates the correctness of the proposed symmetric control strategy.