Hybrid Threshold Virtual Impedance for Fault Current Limiting in Grid-Forming Converters

This paper presents a hybrid threshold virtual impedance method for grid-forming (GFM) converters to effectively limit current under balanced phase jumps and short-circuit faults. To this end, a virtual impedance method based on voltage information (VIv) is proposed to improve the current limiting capability of the standard threshold virtual impedance method for phase jumps (e.g., due to fault clearing or open circuit faults). To leverage the strengths of both threshold virtual impedance and VIv methods, an integrated hybrid threshold virtual impedance method is proposed, which enables reliable current limiting capabilities for both phase jumps and short-circuit faults. Electromagnetic transient (EMT) simulations demonstrate the improved fault ride-through (FRT) performance of the hybrid threshold virtual impedance method compared to threshold virtual impedance or VIv methods.