Transient Modeling of Hybrid Grid-Following/Grid-Forming Systems and Analysis of the Impact of Line Impedance Differences on Stability

The hybrid grid-following (GFL) and grid-forming converter (GFM) system represents an effective solution for enabling secure long-distance power transmission from renewable energy sources. The hybrid system’s diverse synchronization mechanisms, heterogeneous control frameworks, and intricate control parameters reduce the efficiency of stability analysis under fault conditions, while the impact of line impedance variations remains mechanistically unclear. Therefore, this paper proposes a unified transient model for hybrid grid-following/grid-forming systems and conducts a comprehensive stability assessment. First, a transient model is constructed based on the grid-following phase-locked loop (PLL) and grid-forming power loop, explicitly characterizing the interactions between converters. Secondly, the impact of line impedance variations from the grid-following converter, grid-forming converter, and ideal grid on the mutual damping, power transfer, and transient characteristics of the hybrid system was analyzed. Finally, simulation verification and systematic analysis are performed to elucidate the transient-stability mechanisms under variations in the magnitude of line impedance across three typical branch configurations.