Multi-Phase Permanent Magnet Synchronous Generator Variable Speed Constant Frequency Offshore Wind System Based on Modular Multilevel Converter

Large scale offshore DC wind farms are developing rapidly in recent years, which leads to large demands of highpower generator sets, high voltage converters and DC grid technologies. In wind energy system, multi-phase permanent magnet synchronous generator (PMSG) used is superior than traditional three phase generator due to its lower torque ripple, high efficiency, high reliability and high-power generation. As its power conversion equipment, modular multilevel converter (MMC) has shown well application performances in high-power and high-voltage wind systems. In this paper, MMC is applied as the AC/DC converter to six-phase PMSG variable speed constant frequency offshore wind system. Six-phase PMSG-MMC model is built based on the vector space decomposition theory. An optimal electromagnetic torque reference control method, combining the principle of the maximum power point tracing, ensures the minimum torque ripple and minimum harmonics. Additionally, a voltage averaging and voltage balancing method of sub-modular is adopted to suppress voltage fluctuation. The correctness and effectiveness of the proposed topology is verified in simulation results.