A transient stability enhancement method for Grid-Forming converters enabling operation across the four-quadrant range of Power Angle

Grid-forming (GFM) converters are a promising solution for integrating converter-based resources into power grids and can be broadly categorized into (i) Synchronous Machine (SM)-inspired methods, which emulate SM control at different abstraction levels, and (ii) Virtual Oscillator Control (VOC) methods, based on oscillator network synchronization principles. Both behave as voltage sources, making them prone to transient angle instability under severe disturbances, despite differences in their phase synchronization dynamics. Existing transient stability enhancement methods for GFM converters restrict operation of power angle  δ within the range [-90, 90°], limiting converter utilization and hence reducing its ability to support grid during contingencies. This paper analyzes the large-signal synchronization stability of two GFM methods—dispatchable VOC (dVOC) and Virtual Synchronous Generator (VSG)—and proposes a mode-adaptive power-angle control strategy to enhance their transient stability in ultra-weak grids, i.e., with a Short Circuit Ratio (SCR) ≈ 1 across varying X/R ratios. Assuming constant grid voltage and known line X/R ratio, the strategy estimates δ using only local measurements while maintaining a constant voltage amplitude at converter's output. By regulating active power to control δ, it enables GFM converters to operate with SCR lower than unity across the four-quadrant range of power angle during high-impedance faults. Experimental validation confirms enhanced converter power controllability for both dVOC and VSG systems. Controller Hardware-in-the-Loop (CHIL) results demonstrate the proposed solution's superiority over existing methods in terms of transient stability and voltage support, while exhibiting robustness against line X/R ratio measurement errors. Therefore, the proposed solution has shown improved voltage support and better utilization of the full controllability of converter output power for both dVOC and VSG-based systems.