Investigating and addressing synchronous instabilities in inverter-based resources within microgrids

With the rapid integration of renewable energy into the power grid, power oscillations between inverter-based resources (IBRs) have become a significant issue. To address this problem, it is necessary to identify the unstable poles of the grid and adjust the contributing parameters. However, in microgrids (MGs) with IBRs, the system frequency can vary significantly from the nominal value due to power-frequency droop characteristics. The system complexity increases further when inverter control and line dynamics are incorporated. Therefore, conventional eigenvalue analysis methods, which often assume a fixed system frequency or use a simplified model, may not be effective. In this study, a comprehensive mathematical model, including transmission lines and inverter controls of an MG consisting only of grid-forming (GFM) and grid-following (GFL) inverters, was developed. Parameters with high eigenvalue sensitivity in each inverter control were identified as sources of power oscillation between IBRs. In addition, a mutual interference loop composed of these parameters was identified to clarify the mechanism of mutual interference among IBRs. The study also explored the possibility of suppressing power oscillations among IBRs by preferentially adjusting parameters in IBR systems, and the effect was confirmed through numerical analysis and experiments. The proposed method can be applied to the analysis and addressing of actual events, such as the power fluctuation event that occurred on Kauai Island, Hawaii.