Coordinated current-limiting and transient stability control of grid-forming VSGs under grid-voltage sags

Abstract

When power systems experience large disturbances such as grid-voltage sags, virtual synchronous generators (VSGs) can suffer from power-angle instability and output overcurrent. Existing current-limiting strategies often cannot restrict fault currents while preserving converter transient stability and grid-forming support capability. To address this challenge, this paper analyzes the transient instability mechanism and current-limiting characteristics of VSGs and proposes a transient stability control strategy that coordinates steady-state current regulation and instantaneous-overcurrent suppression. The proposed strategy maintains power-angle stability by improving the active-power loop; reconstructs the reactive-power loop through output-current closed-loop integral control to achieve accurate steady-state fault-current limiting while preserving voltage-source characteristics; and uses adaptive virtual impedance to suppress instantaneous overcurrent at fault inception and clearance. The proposed control scheme is validated comprehensively through simulations and hardware-in-the-loop verification.