Resilient Operation of Network of Grid-forming Inverters During Symmetrical Faults

Due to their better performance in maintaining the voltage and frequency stability, grid forming inverters (GFMIs) are promising solutions for the future grid with high penetration of inverter-based resources. However, limiting the output current of the GFMIs during faults while retaining them in a stable operating condition is challenging specially when multiple GFMIs are connected to the grid. Although the virtual impedance can decrease the value of the current during the fault, it is designed based on the presumed operating condition which may change due to the load/generation uncertainties. Limiting the current by using the virtual impedance can decrease the fault current to some extent at the onset of fault for the GFMIs. However, this makes the GFMIs vulnerable to instability due to the further consecutive decrease in their output voltages caused by the interactions between the GFMIs during the fault. This paper proposes a method to decrease the interactions between the GFMIs, more specifically virtual synchronous generators (VSGs), during the fault which lessens the consecutive decrease of the output voltage of the VSGs. The proposed approach modifies the nominal power of the VSGs in real-time during the fault according to their frequency trajectory. Multiple presented case studies simulation result shows the impact of the proposed approach in improving the resiliency of the VSGs during the symmetrical faults comparing to the state-of-the-art virtual impedance method.