Interactions and stability analysis of grid-forming and grid-following converters in close proximity: an impedance-based approach

This paper addresses the stability challenges of integrating multiple converters into AC networks, focusing on potential instabilities from interactions between converter control loops and network components. The increasing penetration of renewable energy necessitates a transition from grid-following (GFL) to grid-forming (GFM) control, particularly under weak-grid conditions. Using impedance-based models, this study investigates the impact of remote grid-forming converters on the stability of local converters. Innovative methods, such as eigenvalue and impedance ratio (IR) analyses, evaluate system stability through frequency-dependent impedance changes. By representing the contributions of grid-forming and grid-following converters through impedance ratio (IR) value calculations, this methodology identifies impedance variations and potential instabilities in grid-tied converters, offering a roadmap for optimizing converter design and network parameters. The study also compares the impedance and stability of remote converters under grid-forming control with power synchronization and grid-following with Phase Locked Loop (PLL) (PV or PQ control modes) under varying frequency disturbances. The proposed methods and behaviours of grid-forming converters in network interactions are validated through time-domain simulations and analytical analysis, providing robust strategies for the stable integration of remote converters into AC networks.