Overload mitigation for grid-forming inverters in islanded microgrids with synchronous generators

The prevalence of inverter based resources (IBRs) in microgrid applications continues to increase. However, energy storage remains costly and so parallel operation of IBRs and synchronous generators (SGs) is expected for microgrids requiring sustained, independent operation. Grid-forming (GFM) control for inverters can improve frequency stability in these low inertia environments, but only if poor transient load sharing does not cause the GFM IBRs to exceed current or power limits as demonstrated in recent literature. In this paper, an overload mitigation strategy for GFM IBRs to address the poor transient load sharing problem is proposed. The approach includes several key aspects. First, closed loop control of inverter currents without integrating a phase locked loop is used. Second, the logic of the mitigation strategy is designed to be non-intrusive so that proportional load sharing through GFM operation resumes following the event. Last, the overload mitigation strategy is validated in several simulations with both switching device and average value models of inverters using RTDS. The simulation results demonstrate successful mitigation of overloads and seamless integration of the strategy in-spite of the high level of penetration of IBRs in the study system.