Stability Analysis of Multi-machine Parallel Grid-connected Systems with Grid-forming Converters in Weak Power Grids

Research on the stability of multi-machine parallel systems with grid-forming converters in weak grid environments was conducted. The fundamental characteristics of weak grids and their impact on the dynamic response of multi-machine parallel systems were systematically analyzed, revealing three instability mechanisms: Impedance-matching instability, power-distribution instability, and frequency-coupling instability. Corresponding instability criteria were established. By establishing a unified state-space model of virtual synchronous generators that accounts for frequency coupling effects, small-signal stability analysis and large-signal transient stability analysis were conducted. Quantitative criteria based on damping ratio and stability margin were proposed. Finally, MATLAB/Simulink simulations validated the theoretical analysis. Results indicate that as the short-circuit ratio decreases from 5.0 to 1.5, the system stability margin gradually diminishes. The introduction of grid-forming converters effectively enhances the equivalent short-circuit ratio, significantly improving the system′s dynamic performance.