Transient synchronization stability of grid-forming and grid-following hybrid renewable energy grid-connected system considering current limitation in grid-forming control

Grid-forming converters (GFM) provide voltage and frequency support in renewable energy grid-connected systems. However, under grid disturbances, they may become locked in the current-limiting mode, making the stability characteristics of hybrid renewable energy grid-connected systems composed of GFM and grid-following converters (GFL) more complex than those without considering current-limiting. Therefore, it is essential to clarify the coupling mechanism between hybrid converters and the transient synchronization stability characteristics under faults. To characterize the transient stability and post-fault recovery process of hybrid renewable energy grid-connected systems under current-limiting state switching, this paper first establishes a transient mathematical model of the hybrid system considering the current-limiting switching process, and depicts the state switching boundary in the phase-angle plane. Then, based on the proposed model, the effects of the GFL injected current and capacity ratio on the transient stability of the GFM during faults are investigated. On this basis, for different saturated current angles, the differences in post-fault phase-angle trajectories and the positional relationship between the current-limiting equilibrium point and the switching boundary are analyzed. A method for determining the GFM saturated current angle is then proposed, which simultaneously considers first-swing stability and normal exit from the current-limiting state. Finally, a simulation model of the hybrid renewable energy grid-connected system is established to verify the correctness of the theoretical analysis.