Impedance Network Modeling of GFL and GFM Converters System Considering AC/DC Couplings

With the rapid development of grid-forming (GFM) control technology, integrating GFM control into grid-following (GFL) control-dominated systems has been attached with great importance due to the enhancement of grid supporting capability. To keep the normal operation of such GFL/GFM hybrid system, its small-signal stability needs to be studied, while the current research mainly focuses on the GFL/GFM converters' ac-side dynamics and usually ignore their ac/dc couplings. Therefore, this paper proposes an impedance network modeling method for GFL/GFM multi-converters system, and then a nodal admittance matrix retaining both ac and dc nodes is obtained. With such model, system marginal unstable condition can be accurately determined. Further, the influence of the capacity ratio between GFL and GFM converter is discussed. Simulation results of a typical GFL/GFM hybrid system verify the theoretical analysis.