Transient stability analysis of grid-forming VSC based on hybrid synchronization control under asymmetrical AC faults

The application of grid-forming control (GFMC) for converters exhibits superior active support characteristics and weak grid adaptability, which makes it a hot spot in academia and industry. However, the transient stability of GFMC during grid faults remains a critical challenge limiting its widespread deployment. While existing studies predominantly address the transient stability of GFMC under symmetrical faults, there is a notable lack of in-depth research on its performance under asymmetrical AC faults. This paper focuses on the latest grid-forming hybrid synchronization control (HSC), investigating its transient stability under asymmetrical AC faults. Firstly, a dual-sequence dynamic model of HSC is developed, accounting for the coupling of positive-and negative-sequence currents. Based on this model, the conditions for the existence of positive-and negative-sequence equilibrium points are qualitatively analyzed. Moreover, the transient stability boundary of HSC, characterized by the positive-and negative-sequence limit currents, is quantitatively determined using phase portrait. Additionally, the impact of control parameters on transient stability is examined. Finally, the accuracy of the transient stability analysis under asymmetrical AC faults are verified based on the hardware-in-the-loop (HIL) experiments.