Mid-Frequency Oscillation Analysis and Suppression in Grid-Following Renewable Energy and Gridforming Energy Storage Hybrid Systems

With the increasing penetration of renewable energy (RE) into the power grid, a large number of power electronic converters with different control strategies are being integrated into the grid. As two common control strategies for power electronic converters, grid-following (GFL) and gridforming (GFM) have drawn widespread attention due to the issue of wide-band frequency oscillations during grid connection. In this paper, an impedance model of the hybrid system consisting of GFL RE and GFM energy storage (ES) is established, and equivalent Bode diagram method of Nyquist criterion is used to analyze the influence of the DC voltage level of RE on the system stability. The research reveals that an increase in the DC voltage level of RE may lead to mid-frequency oscillations in the system. Based on this finding, two impedance reshaping methods are proposed in this paper to suppress these oscillations.