A novel comprehensive method to enhance stability of multi-VSG grids

This paper introduces a novel approach to enhance the stability of islanded grids with multiple virtual synchronous generators (VSGs) in-operation. The proposed method mainly relies on the concept of "center of inertia" (COI) response of the system. We discuss that an islanded grid with 'n' connected VSGs, can experience up to 'n-1' oscillation frequencies due to swing power between VSGs, while the COI-frequency only indicates the major disturbances. The main idea of this method is to control the input power of VSGs to suppress the power fluctuations and reduce oscillations of VSGs relative to COI-frequency. For each VSG, a controller which includes three proportional, integral and derivative terms is used to match the VSG speed with COI-frequency in transient conditions. The state-space analysis shows that these parameters provide a high degree of freedom for adjusting the grid stability-indices to desired values. In addition, the method is able to effectively eliminate the adverse effect of the high impedance of VSGs to achieve desired performances.