Advanced Per-Phase Controller for Fault Ride-Through of Unbalanced Islanded Microgrids

The integration of Inverter-Interfaced Distributed Generators (IIDGs) into islanded microgrids presents significant fault ride-through (FRT) challenges, particularly under asymmetrical fault conditions. Key issues include limiting per-phase fault currents, protecting healthy phases, and restoring voltage balance after fault clearance. Existing approaches that rely on fixed or symmetrical virtual impedance often lack the flexibility to address dynamic unbalanced scenarios and fail to ensure post-fault voltage stability. To address these limitations, this paper proposes an adaptive unsymmetrical virtual impedance fault current limiter (UVIFCL), which dynamically regulates each phase's fault current by modifying its voltage reference. The UVIFCL is governed by a voltage-dependent droop function, enabling adaptive response to fault severity and damping of post-fault oscillations. A frequency-freezing technique is also incorporated to enhance transient stability during faults. The controller is implemented in the natural (abc) frame to allow phase-specific control under both symmetrical and asymmetrical conditions. The proposed strategy is validated using PSCAD simulations on a 4-bus unbalanced islanded microgrid (UBIMG), the IEEE 34-bus distribution system, and a real-time simulation of a 6-bus UBIMG using OPAL-RT. Results confirm the controller ability to limit inverter output currents, preserve voltage in healthy phases, and improve dynamic system performance, offering a robust and communication-free FRT solution for UBIMGs.