On the Effective Fault-Ride Through Operation of Grid-Forming Inverters Under Asymmetrical Faults in Low-Inertia AC Microgrids

Grid-forming inverter-based resources (GFM-IBRs) are playing a key role in realizing 100 % renewable energy-based power grid due to their inherent advantages, such as virtual inertia emulation, frequency regulation, and voltage support. However, the fault-ride through (FRT) and stable operation in post-fault conditions, particularly under asymmetrical faults, is a major challenge. The unbalance between phases induces negative sequence current, and if not effectively controlled, it leads to double-line frequency oscillations in output of GFM-IBRs. Moreover, the limited fault-current capability of GFM-IBRs necessitates an overcurrent limiting mechanism to protect the power-electronic based switching devices. Thus, this paper presents a novel control framework that ensures robust operation of GFM-IBRs during asymmetrical and in post-fault conditions. The proposed control scheme incorporates a fault detection module (FDM) to identify the start and end of asymmetrical faults, and a modified virtual impedance-based overcurrent limiter that suppresses the negative sequence current and restricts the fault current within safe threshold. The effectiveness of the proposed control for GFM-IBRs is validated through various cases and benchmarked against the conventional current saturation-based overcurrent limiting approach.