Synchronization Stability Analysis of Grid-Tied Virtual Synchronous Generators During Asymmetrical Grid Faults

Virtual synchronous generators (VSGs) are highly compatible with the grid, as they emulate the inertia and frequency response of synchronous generators (SGs), making them a promising solution for future power systems. However, under asymmetrical faults, both positive- and negative-sequence synchronization stability become critical concerns, unlike in SGs, which maintain only positive-sequence stability. This is because VSG is recommended to generate negative-sequence current during such faults. Additionally, virtual impedance (VI) control is a widely adopted current-limiting strategy. However, the dual-sequence synchronization stability of VSGs employing VI control remains unexplored. This paper investigates the root causes of loss of synchronization (LOS) in such VSGs. Considering the effect of sequence coupling, a dual-sequence synchronization model of the VSG is developed, where the negative-sequence circuit is represented as an impedance within the positive-sequence model. Based on this model, a phase portrait approach is employed to analyze LOS both in the absence and presence of equilibrium points (EPs). The dominant factors for different types of LOS are analyzed, including those related to positive-sequence VI, negative-sequence current injection, and the control parameters of positive- and negative-sequence. The model and the analysis are verified by the PSCAD simulations.