Optimization Method of Topology for Hybrid Modular Multilevel Converter in Offshore Wind Turbines

The hybrid modular multilevel converter (MMC) consisting of half- and full-bridge submodules (SMs), with DC fault ride-through capability, have become a promising candidate for offshore DC wind turbines. However, previous methods only focus on the MMC with fixed number of SMs, without considering the impact of power loss on cost. Therefore, the optimal topology could not be proposed at the initial design stage. This paper presents optimization method of topology for hybrid MMC with reliability and cost objectives, in which the reliability is defined to be not only reliable under normal operation, but also able to ride through DC fault even with faulted SMs. Firstly, considering active redundancy configuration, the reliability and cost model with power loss are established. Then, taking a 10-MW and 10-kV hybrid MMC in offshore wind turbines for example, the influences of original and redundant SMs on reliability and cost are analyzed. Finally, a multi-objective optimization model that regards higher reliability and lower cost is built, and the compromise is selected from the Pareto-optimal solutions according to fuzzy membership function, which derives the optimal topology of hybrid MMC.