Power Supporting Control of Grid-Forming Converters Under Grid Voltage Sags for Transient Stability Enhancement and Overcurrent Limitation

There is an increasing demand for grid-forming controlled voltage source converters (GFM-VSCs). However, to maintain its voltage source characteristics, the GFM-VSC under grid voltage sags may face conflicts between transient stability enhancement and overcurrent limitation. In this article, the power-supporting control method is proposed for GFM-VSC to solve this conflict by tuning the active power and voltage magnitude reference. The active power reference is introduced as an extra variable, therefore, the power angle during grid fault and the voltage magnitude can be tuned at the same time to increase the design of freedom. First, the feasible region for power angle and voltage magnitude is shaped considering the constraints in transient stability and current limiting. Under the constraint of the feasible region, three optimal control modes are designed to determine the power and voltage reference, concluding the maximum active power, maximum reactive power control, and optimal transient control modes. Finally, the performance of the proposed control method is verified by experimental results. Compared with existing fault ride-through methods, the proposed method could achieve higher power-supporting capability while maintaining the voltage source characteristics, which makes full utilization of power angle and voltage boundaries.