Quantitative Parameter Conditions for Stability and Coupling in GFM–GFL Converter Hybrid Systems From a Small-Signal Synchronous Perspective

With the development of renewable energy sources, power systems are gradually evolving into a system comprising both grid-forming (GFM) and grid-following (GFL) converters. However, the dynamic interactions between the two types of converters remain unclear, particularly in the presence of low-inertia GFM converters, due to the fundamental differences in their synchronization mechanisms. To address this gap, this paper develops a small-signal synchronous stability model for power systems containing GFM and GFL converters, which considers network line dynamics. Based on matrix perturbation, we reveal that GFM and GFL subsystems can be effectively decoupled when GFL converters operate near unity power factor or when GFM converters possess sufficiently large inertia or damping, and provide lower bound of control parameters ensuring decoupling. Under the decoupling condition, we propose decentralized and analytical parameter-based stability criteria which have clear physical interpretations: the positive damping of converters compensates for the negative damping of the network. In the case of coupling, we also propose decentralized stability criteria based on the small phase theorem. The effectiveness of the theoretical analysis is validated through simulations in MATLAB/Simulink.