Construction of a Critical Short-Circuit Ratio Index for Systems with Grid-Forming Energy Storage and Influencing Factors

[Objective] Aiming at the stability issues arising from weak grid conditions （short-circuit ratio < 1.5） and the large-scale centralized integration of renewable energy in desert， Gobi， and barren areas， this paper proposes a critical multiple renewable energy station short-circuit ratio calculation and discrimination method that reflects the role of grid-forming energy storage. [Methods] Based on the analysis of the multiple renewable energy station short-circuit ratio， key parameters such as virtual impedance， energy storage capacity， and connection reactance are introduced to establish a modified formula. By combining eigenvalue analysis of the extended Jacobian matrix， a real-time calculation method for the critical short-circuit ratio and stability criterion is proposed， thereby unifying the short-circuit ratio index with static voltage stability theory. Furthermore， a 3-machine 9-bus system simulation model is constructed on the electromagnetic transient simulation platform PSCAD/EMTDC. [Results] After connecting grid-forming energy storage， the system’s short-circuit ratio level is significantly improved. The critical short-circuit ratio increases from 1.07 to 1.52， the wind power penetration limit rises from 50% to 54%， and the voltage sag amplitude is reduced， indicating enhanced support capability. Further simulation analysis reveals that rationally configuring the virtual impedance can improve the voltage stability margin， increasing the energy storage capacity can enhance system strength， whereas increasing the connection reactance weakens the coupling effect and reduces the critical short-circuit ratio. [Conclusions] The proposed method can accurately determine the critical stable state of the system and quantify the sensitivity of grid-forming energy storage parameters to grid strength. It addresses the deficiency of current short-circuit ratio indicators in reflecting the role of grid-forming energy storage.