A Low-frequency AC Transmission System Based on FBSM-9AMMC for Large-scale and Long-distance Offshore Wind Farm Integration

This paper proposes a low-frequency AC (LFAC) transmission system for large-scale and long-distance offshore wind farms (OWFs) integration based on a nine-arm modular multilevel converter utilizing full-bridge submodules (FBSM-9A-MMC). Compared with the widely adopted modular multilevel matric converter (M3C) in LFAC transmission, the proposed 9A-MMC offers several advantages despite having an equal number of arms. Specifically, it requires fewer submodules (SMs), which directly contributes to reduced system costs and smaller physical dimensions. Furthermore, the inherent lower coupling degree between the low-frequency and fundamental-frequency sides in the 9A-MMC results in more explicit circulating current paths, enabling a more straightforward control scheme compared to the M3C. In this paper, the topologies of $9 \mathrm{~A}-\mathrm{MMC}$ and M3C are firstly presented together for a direct comparative analysis. Subsequently, their equivalent circuits and fundamental voltage and current equations are derived and discussed. A detailed hierarchical control strategy for the $9 \mathrm{~A}-\mathrm{MMC}$ is then elaborated. Finally, the simulation results validate the feasibility of the proposed the LFAC transmission system using FBSM-9A-MMC for large-scale and long-distance OWF integration.