Low-frequency Resonances in Grid-Forming Converters: Causes and Damping Control

Abstract: 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.