Transient Stability Modeling of Droop-Controlled Grid-Forming Inverters with Fault Current Limiting

Grid-forming (GFM) inverter control techniques are attracting increased attention for interconnecting inverter-based resources (IBRs) to the electric grid because of their demonstrated performance advantages over existing grid-following (GFL) IBRs in weak system scenarios and low-inertia power systems. GFL inverters require a stiff voltage and frequency source for their operation. In contrast, GFM inverters can independently control both voltage and frequency. However, due to their inherent behavior as voltage sources, GFM inverters do not naturally possess the ability to limit fault current during grid transients such as short circuit faults. In this work, a virtual impedance-based fault current limiting approach is presented for droop-controlled grid-forming inverters. This approach has been tested and validated using electromagnetic transient simulations. A transient stability model of the GFM inverter has been developed and used to study the performance of the IEEE 2 Area system with GFM inverters displacing traditional generators. The results of this study demonstrate that GFM inverters can successfully restrain fault currents within allowed limits while retaining their original stability characteristics.