Analysis of Transient Characteristics and Fault Ride-through Control of Hybrid Grid-tied Converters with Grid-following and Grid-forming

When grid-following (GFL) and grid-forming (GFM) converters are interconnected with the grid, they are mutually coupled through the grid impedance. During grid faults, the transient behavior of these converters becomes intricate due to the presence of coupling terms. Instabilities in one converter can propagate to the other, posing challenges to fault ride-through capability. Firstly, a mathematical model was presented for a hybrid grid-tied system comprising two converters, investigated the conditions for equilibrium points, and employed phase-plane analysis to elucidate the mutual influence mechanism during faults. Then, a method to adjust the power angle of GFM according to the voltage drop was proposed and used in GFL to improve its phase-locked loop. At the same time, considering the influence of GFL current injection, the fault current of GFM was limited, so as to realize the hybrid fault ride-through control. The proposed approach ensures that GFM converters contribute reactive current support to the grid during faults, while GFL converters adhere to grid guidelines for current injection, thereby preventing overcurrent and phase angle instability in both converters. Finally, simulation results validated the correctness and effectiveness of the proposed control strategy.