Stability Analysis of Grid-Forming VSC Connected to Low Impedance Grid

The grid-forming converter has good stability in weak grids, but it may be unstable in stiff grids. In view of this, this paper first establishes the state space model of the grid-forming converter, and combines the characteristic root trajectory and the participation factor to analyze the dominant mode and stability trend of the system in different grid strengths and different control parameters. Then, the dynamic model of the grid-forming converter connected to the AC grid is established to characterize the dynamic characteristics of the internal potential-inertia, damping component and synchronous component, and the stability mechanism of the grid-forming converter connected to low impedance grid is revealed. The analysis results show that the terminal voltage control loop of the grid-forming converter will introduce significant additional negative damping components, which reduces the stability of the system. By increasing the bandwidth of the terminal voltage control loop, the negative additional damping can be reduced and the stability can be improved. At the same time, increasing the inertia coefficient or reducing the damping coefficient can also increase the damping component and improve the stability.