Grid-following and grid-forming current fusion control for battery energy storage converters under different grid conditions

Abstract

Battery energy storage system (BESS) converters are increasingly required to provide fast frequency support in low-inertia power systems under varying grid strength and limited energy headroom. Fixed grid-following (GFL) or grid-forming (GFM) control cannot readily coordinate power tracking, frequency support, and energy availability. This paper proposes a grid-stress-aware current-reference fusion strategy. A grid-stress index based on short-circuit ratio (SCR) and equivalent inertia is combined with direction-dependent state-of-charge (SOC) headroom to determine an adaptive fusion coefficient. The GFL and GFM current references are generated under a common droop-based synchronization angle and fused before a unified current-limiting and modulation path. Reduced-order models clarify key control effects, while local stability analysis confirms asymptotic stability of the reduced-order model. RT-Lab studies show that, under SCR = 2 , H = 4 s , and SOC = 0.6, the proposed strategy achieves a 49.866-Hz frequency nadir while avoiding the 1.20-p.u. hard current limit. At SOC = 0.4, the support-energy requirement is reduced by approximately 13.4%. An illustrative 24-h scheduling study further indicates a trade-off between support reservation and daily revenue.