Fast Frequency Response Analysis for Grid- Following and Grid-Forming Controlled BESS Considering Voltage Coupling Effect

The fast frequency response (FFR) provided by the battery energy storage system (BESS) is essential for maintaining the frequency stability of a power system. However, the voltage coupling effect (VCE) may negatively affect the FFR of the BESS in weak grids. The VCE, particularly when comparing grid-following (GFL) and grid-forming (GFM) control strategies, remains inadequately explored. This paper proposes a novel FFR modeling approach for the GFL & GFM BESS considering the VCE. The impacts of BESS converter parameters and system characteristics on the VCE are investigated. Comparative analyses between GFL & GFM control are performed to quantify the VCE differences and identify critical parameters affecting the VCE on the FFR. The proposed models and analyses are validated through electromagnetic transient simulations and control-hardware-in-the-loop (CHIL) experiments. It is revealed that the VCE emerges from the instantaneous voltage drop during step disturbances, leading to reduced active power support and delayed frequency recovery. The VCE increases significantly as the short circuit ratio (SCR) approaches 2, but can be mitigated through increased voltage droop coefficients or, in GFM control, through adjusting virtual impedance. With appropriately selected parameters, the GFM BESS demonstrates superior performance in weak grids with reduced capacity requirements compared to GFL BESS.