Optimal Protection Coordination of Microgrids Powered by Synchronverters During Unbalanced Faults

Distributed energy resources (DERs) (i.e., wind turbines, photovoltaic systems, gas microturbines, etc.) integrated into conventional distributed networks result in active distribution networks (ADNs), which can be evolved into microgrids. Microgrids can operate either in grid-connected or islanded modes. Providing clean energy, reducing fuel use, and increasing grid resiliency and sustainability are among the advantages of microgrids. However, the advantages offered would be in jeopardy if not adequately protected. Short circuit currents are among the most significant issues in microgrids. DERs are integrated into AC networks through power electronic components. Power electronic devices provide power without enough inertia to the system. The lack of enough inertia leads to system instabilities. Synchronverters are inverters that mimic the behavior of synchronous generators (SGs) and provide virtual inertia to the grid. However, synchronverters generate high inrush currents during faults. In this work, adaptive virtual impedance fault current limiters (VI-FCLs) are employed in the synchronverter controllers to limit their fault currents and protect inverter switches from overcurrents. The incorporation of synchronverters with VI-FCLs should be considered in the protection scheme. The optimal protection coordination (OPC) scheme is presented to determine the minimum operating time for all relays while assuring protection coordination requirements. This protection scheme is then evaluated on a Canadian 9-bus system. Simulation results verify the efficacy of the proposed control scheme to maintain the protection coordination among all relays under different operating scenarios.