Impedance‐Based Characterisation of Grid‐Forming and Grid‐Following Converter Interactions in Multi‐VSC Systems

ABSTRACT This paper studies stability in converter‐dominated AC networks using an impedance‐based approach, with emphasis on interactions between grid‐forming (GFM) and grid‐following (GFL) converters. The reduction of synchronous machines has increased the influence of converter control dynamics, creating new stability risks in weak grids with low short‐circuit strength. GFL converters rely on phase‐locked loop synchronisation, which links their behaviour directly to grid impedance and may introduce negative damping and resonance. In contrast, GFM converters operate as controlled voltage sources and regulate frequency and voltage through power‐synchronisation mechanisms, providing inherent stabilising effects. A frequency‐dependent impedance model is developed for both converter types. System behaviour is evaluated using impedance ratio (IR) and eigenvalue impedance ratio (EIR) methods to identify resonance frequencies, dominant modes and damping margins. The framework is applied to two‐ and three‐converter systems, including hybrid GFL‐GFM‐GFL configurations, to examine the impact of control type and placement. Results show that GFM converters maintain stable impedance characteristics, with impedance ratios remaining within stable regions and phase responses less sensitive to grid strength. This behaviour suppresses negative resistance interactions and control‐induced oscillations. In comparison, PV‐controlled GFL converters exhibit pronounced resonances and reduced stability margins, while PQ‐controlled GFL converters provide improved damping. Central placement of a GFM unit further enhances stability by uniformly damping oscillatory modes across the network. Time‐domain simulations confirm the analytical predictions, demonstrating fast oscillation decay and robust transient performance. The proposed approach offers clear stability criteria and practical guidance for the design and operation of weak, renewable‐dominated power systems.