Stability Control for Grid-Connected Inverters Based on Hybrid-Mode of Grid-Following and Grid-Forming

Grid-connected inverters (GCIs) operating in grid-following (GFL) mode may be unstable under weak grids with low short-circuit ratio (SCR). Improved GFL controls enhance the small-signal stability of GCIs under weak grids but result in a decrease in dynamic performance and remain unstable under very weak grids. While the small-signal stability of GCIs operating in grid-forming (GFM) mode is just the opposite: stable under weak or even very weak grids, unstable under strong grids. Both GFL and GFM modes are difficult to meet the stability requirements under large fluctuations in SCR. Therefore, this article proposes a hybrid-mode control strategy for GCIs that can adapt to large fluctuations in SCR. This control strategy weights the modulation signals output from GFL mode and GFM mode, and removes the phase-locked loop of GFL mode, improving the small-signal stability of GCIs under a wide range of SCR. The sequence impedance model of the hybrid-mode GCIs is established, and the small-signal stability is analyzed in this article. The experimental results verify the effectiveness of the proposed strategy.