Multilevel converters for a 10 MW, 100 kV transformer-less offshore wind generator system

The nacelle weight reduction is a key design criterion for offshore wind turbines, when these are reaching towards 10 MW. To overcome the weight challenge, a transformer-less concept is under development. This concept employs a special permanent magnet synchronous generator (PMSG) with an innovative system for high insulation level. The generator supplies nine series connected converter modules, which results in a high voltage DC output of 100 kV, reducing the total weight of the system. The work presented here focuses on one of the 11.1 kV converter modules, and identifies the modular multilevel converter (MMC) as the best suitable candidate for the proposed system. The comparison criteria are qualitative: no direct series connection of IGBTs, converter efficiency, voltage quality and redundancy for increased reliability. Simulations in PSCAD/EMTDC are presented, with focus on redundancy, submodule voltage balancing, and implementation of the MMC in full system models. The results show that the MMC performs well in the full system, and is therefore considered as a viable converter for the proposed system.