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 categorized into: 1) synchronous machine (SM)-inspired methods, which emulate SM control at various abstraction levels and 2) virtual oscillator control (VOC) methods, based on oscillator 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 power angle $\delta $ to (−90°, 90°), limiting converter utilization and reducing grid support during contingencies. This article 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 transient stability in ultra-weak grids, i.e., with a short circuit ratio (SCR) ${\approx }1$ across diverse line X/R ratios. The strategy estimates $\delta $ using only local measurements while maintaining a constant output voltage. By regulating active power to control $\delta $ , it enables GFM operation with SCR $\lt 1$ across the full four-quadrant power angle range during high-impedance faults. Activation/deactivation algorithms are developed, offering an implementable solution from concept to realization. Experimental validation confirms enhanced controllability for dVOC and VSG systems. Controller hardware-in-the-loop (CHIL) results demonstrate the method’s superiority over existing solutions in transient stability and voltage support, while showing robustness to X/R ratio measurement errors. The strategy fully exploits GFM converter controllability, delivering superior performance for dVOC and VSG-based systems.