Frequency-Coupled Impedance Model of Grid-forming Converters

Grid-forming (GFM) converters are increasingly used in power systems for its advantage of self-synchronization without phase-locked loop. To analyze its small-signal stability, impedance model is required to be established. In this paper, frequency-coupled impedance model (FCIM) of the GFM converter is mathematically derived using harmonic linearization method. The developed FCIM takes full consideration of the power synchronization control, virtual control, and inner current control, and thus can provides sufficient accuracy for the evaluation of oscillatory stability. Further, the measured FCIM is obtained through perturbation injection method in the electromagnetic simulation software of PSCAD/EMTDC, and is compared with the theoretical FCIM to verify the consistence between them.