  Grid-forming voltage-source converter (GFM-VSC) may experience low-frequency
resonances, such as synchronous resonance (SR) and sub-synchronous resonance
(SSR), in the output power. This paper offers a comprehensive study on the root
causes of low-frequency resonances with GFM-VSC systems and the damping control
methods. The typical GFM control structures are introduced first, along with a
mapping between the resonances and control loops. Then, the causes of SR and
SSR are discussed, highlighting the impacts of control interactions on the
resonances. Further, the recent advancements in stabilizing control methods for
SR and SSR are critically reviewed with experimental tests of a GFM-VSC under
different grid conditions.
