Analytical stability boundary assessment and coordinated enhancement for hybrid GFM-GFL converters considering transient mode switching

To address the challenges of high-penetration renewables, hybrid systems utilizing Grid-forming voltage source converters (GFM-VSCs) and Grid-following voltage source converters (GFL-VSCs) have attracted significant attention. However, transient mode switching induced by current limiting in the GFM-VSC significantly reduces the stability margin of the hybrid systems. Addressing the challenge of accurately quantifying stability boundaries under switching-induced dynamics and coupling effects, this paper establishes the equivalent transient mathematical analysis model of the hybrid system. Subsequently, a systematic quantitative analysis method is proposed, which enables the accurate calculation of the critical clearing angle and critical clearing time by explicitly incorporating the transient mode switching of the GFM-VSC and the dynamic coupling effects. Based on the quantitative evaluation, the stability boundaries under varying control parameters and fault conditions are characterized. Furthermore, the interaction mechanism between the GFL-VSC and the GFM-VSC is unveiled, specifically how the dynamic variation of the virtual power angle difference and the current injection of the GFL-VSC impact the mode switching of the GFM-VSC. Finally, a transient synchronous stabilization enhancement strategy is proposed to facilitate the reversion of the GFM-VSC to its normal operating mode following a fault, with its effectiveness verified through simulations.