Transient Synchronization Stability and Enhanced-LVRT Scheme for Grid-Forming Converters Considering Current Limitation and Nonlinear Damping Simultaneously

Due to the fragile overcurrent capability, it is necessary to embed the current limitation into the control system of the grid-forming (GFM) converter. In such regard, the GFM converter undergoes the transient mode switching during low voltage ride-through (LVRT), which reshapes the power angle characteristics. Previous studies have proven that the introduction of current limitation poses challenges to transient synchronization stability. However, the impact of the unfavorable nonlinear damping, and the current limitation mode in which the current reference vector does not coincide with the coordinate axes are usually ignored. This paper reveals that two current limitation modes will appear during entering and exiting current saturation under severe grid faults, both for the d-axis priority and q-axis priority current limitation algorithm. And the modes of each fault stage during LVRT are clarified. Furthermore, the synchronization characteristics of the GFM converter under severe faults are investigated by power angle trajectories considering both current limitation and nonlinear damping. Based on intuitive physical images, a CCT calculation method is proposed to quantitatively analyze the influence of control parameters, namely active power reference and droop coefficient, on transient synchronization stability. To improve the resynchronization ability of the GFM converter after faults, an enhanced-LVRT scheme is proposed by applying the redesigned current limitation strategy. Thanks to the introduction of the power angle feedback into the current limitation, this scheme is adaptive to various large disturbance events. Finally, simulations and experiments are conducted to validate the theoretical analysis.