Stability assessment and reactive power control optimization of hybrid grid-following and grid-forming system

The increasing integration of power electronic converters in modern power systems has significantly accentuated stability concerns. Among these, the synchronization stability challenges of grid-following (GFL) converters under weak grid conditions are particularly prominent. It has been demonstrated that the introduction of grid-forming (GFM) converters can effectively mitigate these stability issues. However, a theoretical understanding of how various GFM control strategies impact GFL stability is still incomplete, making the small-signal stability assessment of hybrid GFL-GFM systems a significant challenge. Moreover, the reactive power control (RPc) of GFM converters plays a crucial role in maintaining system stability. Research on how RPC affects its support capability remains unexplored. To address these issues, this paper proposes a stability assessment method for GFL-GFM hybrid systems. By employing modal decoupling and matrix perturbation methods, a decoupled low-order system is developed for characterizing the stability of the original system, thereby reducing the analytical complexity and offering physical insights into the supporting capabilities of GFM converters. Furthermore, an enhanced RPC structure for GFM converters is proposed to improve their grid-support capability within different frequency ranges. The effectiveness of the proposed method is validated on the IEEE 39-Node system.