Grid-Forming Inverter Control Strategy with Improved Fault Ride Through Capability

In recent years, with the energy share of inverter-based distributed generations steadily increasing in distribution power systems, grid-forming inverters (GFIs) have gained a great many interests due to their superiorities in grid stability enhancement. This paper shows that under grid-level faults, conventional virtual synchronous generator (VSG) based GFIs are likely to experience grid-synchronization mechanism failures, therefore suffer from severe instabilities, low frequency fluctuations and current distortions. A new VSG-based grid-forming control strategy is proposed in this paper for an improved performance under grid-level faults while preserving the inverter grid-forming feature. By adaptively changing the virtual moment of inertia and using positive-sequence virtual impedance calculation, the proposed control can synchronize the GFI to the grid side and eliminate any low-frequency fluctuations under fault conditions, even under extremely severe faults and unbalanced faults; thus, the GFI's fault ride through capability is significantly enhanced. The GFI fault behaviors under the proposed control are also assessed under different fault scenarios. PSCAD simulation results based on a 10-bus test feeder interfaced with a utility-scale photovoltaic inverter are provided to validate the proposed control strategy.