Synchronization Stability Analysis Based on Dual-Sequence Dynamic Coupling Model for Grid-Following Inverters Under Asymmetrical Grid Faults

To deal with asymmetrical grid faults, grid-following inverters are required to output positive- and negative-sequence currents to regulate the voltage at the point of common coupling. Owing to the negative-sequence current injection, the stability mechanism of the dual-sequence synchronizations becomes more complicated under asymmetrical scenarios. In this article, a dual-sequence dynamic coupling model, which takes into account both positive- and negative-sequence current couplings as well as power angle difference, is proposed. Based on this model, not only the existence conditions of positive- and negative-sequence equilibrium points are identified, but also the transient process stability conditions of the operating points are determined, resulting in a complete transient stability judgment foundation during asymmetrical grid faults. It is pointed out that couplings between the positive- and negative-sequence synchronization processes bring about more transient synchronization instability. Simulation and experimental results verify that the proposed synchronization stability analysis based on the dual-sequence dynamic coupling model has higher accuracy for inverters under asymmetrical grid faults compared with the previous methods.