Analysis of broadband oscillation mechanisms in grid-forming and grid-following converters based on virtual synchronous generator

Aiming at the problem of increased complexity in broadband oscillations caused by the introduction of virtual synchronous generator (VSG) control to grid-forming and grid-following grid-connected converters interacting with inductive weak grids, positive and negative sequence impedance of such converters is established under small perturbations in phase angle and voltage using harmonic linearization. The impact of various control links on the broadband impedance characteristics associated with different control strategies is then analysed. It is observed that the grid-forming converter equipped with VSG demonstrates lower impedance magnitude and inductive behaviour in the medium-to-high frequency range, thus emulating the external behaviour of a synchronous generator more accurately. Furthermore, the analysis based on the maximum peak Nyquist stability criterion reveals that voltage control and current-type VSG lack phase margin, leading to potential oscillation risks within their respective dominant frequency bands. Finally, the validity of the broadband oscillation mechanism analysis is confirmed through hardware-in-loop experiments and impedance sweep analyses.