State Transfer Induced Transient Synchronization Instability of GFM-VSC: Analysis and Improvement

The operation mode of grid-forming voltage source converters (GFM-VSCs) may switch between voltage source mode (VSM) and current source mode (CSM) under some situations such as grid faults, owing to the current limitation control. During the mode-switching process, there is state transfer from the final state of the last mode to the initial state of the next mode, which impacts the transient synchronization stability (TSS) of GFM-VSCs. This paper primarily focuses on analyzing and improving the TSS of GFM-VSCs by considering the effect of state transfer. A novel transient instability mechanism is revealed through the existence analysis of equilibrium points. It clarifies that the state transfer may cause the operating trajectory during faults to bypass the stable equilibrium point in CSM before diverging to the next cycle, thereby resulting in transient synchronization instability. Besides, to further analyze the TSS of mode-switched VSCs considering the dynamics during faults, multiple Lyapunov functions are adopted to derive the TSS criteria and boundaries. It has been identified that lowering the minimum critical current and adjusting the saturated current phase in accordance with virtual power angle (VPA) dynamics can enhance the TSS. Therefore, a VPA feedback-based current limiting strategy is proposed to safeguard GFM-VSCs against overcurrent and ensure the TSS. The validity of the new transient instability mechanism and the efficacy of the proposed strategy are confirmed through simulations of a GFM-VSC connected to an IEEE 39-bus power grid and hardware-in-the-loop experiments.