Strategic sizing of grid-forming BESS for reliable wind farm integration in weak grids

Renewable energy sources (RESs) are growing rapidly due to their environmental benefits, energy security, and cost-effectiveness. However, RESs are integrated into electricity networks via inverter-based technologies that lack inherent reactive power support, potentially weakening grid stability. Traditional solutions to this limitation involve the use of synchronous condensers, which are costly and have long deployment times, prompting interest in battery energy storage systems (BESS) equipped with grid-forming inverters (GFMI) as a flexible alternative. This study investigates the optimal sizing of GFMI-BESS to reliably support the integration of large-scale wind farms into weak grids (SCR = 1.3, X/R = 3), focusing on minimizing capacity without compromising stability. Using detailed PSCAD/EMTDC simulations, the dynamic performance of a 100 MW wind farm (Type-4) with various GFMI-BESS capacities (5–25 MVA) is evaluated under steady-state conditions, active power step changes, symmetrical and asymmetrical faults and frequency disturbances.Results demonstrate that, under the benchmark weak-grid condition studied (SCR = 1.3, X/R = 3.0), a minimum GFMI-BESS size of 15 MVA is required to maintain voltage stability and fault ride-through capability. This value should be interpreted as a case-specific lower bound, with the methodology readily extendable to other temporal and spatial grid conditions.