Dynamic Current Limiting of Grid-forming Converters for Transient Synchronization Stability Enhancement

Affected by the current limitation, the operation of grid-forming (GFM) voltage source converters (VSCs) will be switched from a voltage source mode to a current source mode under grid faults. This mode switching may exacerbate the transient synchronization stability of GFM-VSCs. To enhance the transient synchronization stability under mode switching, a dynamic current limiting strategy is proposed in this paper. By establishing the Lyapunov energy function, the transient synchronization stability criterion of GFM-VSCs under mode switching is obtained and the effect of different saturated currents is investigated. It reveals that the transient synchronization stability of GFM-VSCs can be improved by appropriately adjusting saturated current phases when the saturated current amplitude is fixed. Based on the stability criterion, the transient stability region is clearly described and the optimal saturated current phase of GFM-VSCs is derived. The investigation illustrates that the transient synchronization stability of GFM-VSCs can be guaranteed by designing active and reactive current limits with the optimal saturated current phase. Therefore, a dynamic current limiting strategy based on the optimal active and reactive current distribution is proposed. With this strategy, GFM-VSCs can not only avoid being damaged by the overcurrent under faults but also keep transient synchronization stability after mode switching. The effectiveness of the proposed dynamic current limiting strategy is verified by simulations and hardware-in-the-loop experiments.