Enhancement of Reactive Power Support Capability in Grid-Forming Converters Under Circle Limitation

Grid-forming (GFM) converters play a critical role in modern power systems, and their dynamic behavior during grid faults-particularly reactive power support-directly affects system stability. This paper identifies a shortcoming in the reactive power support capability of GFM converters when a circular current limiting strategy is activated during faults. To overcome this limitation, a power angle constraint method is proposed. Based on a virtual synchronous generator (VSG) model, the power-angle characteristics under the circular current limiting strategy are analyzed for different fault conditions, and analytical power expressions are derived for both constant voltage and constant current operation modes. An angle saturation control scheme is then introduced to limit the variation range of the power angle, effectively reducing the reactive power drop. Simulation results confirm that the proposed method not only improves reactive power support under various voltage sag conditions but also enhances active power recovery performance.