Impact Analysis of SCR on Different Modes Dominated by Heterogeneous Synchronous Control of Converters

With the large-scale increase in phase-locked loop (PLL) synchronous grid-connected converters in the power system, the system grid strength or short-circuit ratio (SCR) is relatively reduced, resulting in frequent oscillation problems dominated by PLL in the system. The external characteristics of grid-forming converters contribute to the grid strength but are limited by the black-box nature of the converter dynamic model. In addition, SCR is generally used as an evaluation index of grid strength in engineering applications. However, existing research has not yet fully revealed the mechanism of grid strength in heterogeneous converter grid-connected systems. Therefore, this paper first establishes the frequency domain admittance and impedance models corresponding to the grid-following converter and the grid-forming converter respectively, based on the duality of the PLL control and virtual synchronous control. Then, by analyzing the small disturbance stability of a single-converter grid-connected system, the influence of SCR on different dominant modes of heterogeneous synchronous control converter is clarified. Finally, the analysis results were simulated and verified in MATLAB/Simulink.