Virtual Impedance-Based Droop Control Method for Grid-Forming Multilevel Converters with Enhanced High-Frequency Stability in Strong Grid

Grid-forming (GFM) multilevel converters (MCs) are usually applied in weak grids, and their stability characteristics have been extensively investigated. However, grid strength is determined by multiple factors, and they may change from time to time. Meanwhile, the adaptability of GFM-MCs in strong grids has not yet been discussed. Therefore, to improve the high-frequency stability of GFM-MCs in strong grids, this paper proposes an improved droop control-based virtual impedance optimization scheme for GFM-MCs. Firstly, the impedance model of GFM-MCs is established and its frequency response is analyzed. Then, the internal causes of negative-damping characteristics for GFM-MCs in strong grids are revealed through Bode diagram analysis of output impedances. Afterwards, through introducing virtual impedance with strong suppression effects in high-frequency band, the high-frequency oscillations can be avoided in GFM-MCs. Finally, simulation results under grids of different strengths indicate that the high-frequency stability of GFM-MCs can be effectively enhanced in strong grids.