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BackgroundOffshore wind energy has a large potential, and power ratings in offshore wind turbines are reaching for 10 MW. When the turbine size increases, the nacelle weight reduction becomes a key element. A new module-based generator concept that omits both the gear box and the transformer, and by that creates a lightweight turbine solution, is under development in a PhD-project. The new concept utilizes a special permanent magnet synchronous generator with nine converter modules in series to obtain a 100 kV DC voltage output.The basis for this master thesis is the master specialization project from the fall of 2011, which presents the modular multilevel converter (MMC) as the most interesting candidate for the proposed concept. To verify the MMC topology as a realistic candidate, further studies are required. Expected outcome of the project:The work should continue the comparison of the MMC and a conventional twolevel converter, with respect to their suitability for the proposed concept. The list of criteria includes voltage quality, harmonic content, DC voltage ripple and filter demands. The simulations performed in the specialization projects were simplified and ideal. A natural delvelopment of these models should include control challenges revealed in the specialization project, i.e. voltage balancing control and redundancy. The MMC should also be implemented in the full scale simulation models from the PhD-project.The student will work with PhD-candidate Sverre S. Gjerde, who will be the main contact person. He started his PhD-work during fall 2009. Responsible supervisor will be professor Tore Undeland. The size of offshore wind generators is increasing, and the trend is moving towards full converter gear-less solutions with permanent magnet synchronous generators (PMSG). The nacelle weight reduction is a key design criterion for offshore wind turbines. To overcome the weight challenge, a transformer-less concept is under development. This concept employs a special PMSG with an innovative high insulation level between the groups of windings. 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. Conventional three phase 2-level voltage source converters, each with 11.1 kV output, are utilized in concept studies and simulations. However, other voltage source converter topologies are assumed to be more beneficial in terms of efficiency, voltage quality and reliability issues. This work compares multilevel converter topologies with regards to their suitability for the proposed concept.The result of an initial study is that the modular multilevel converter (MMC) is the most promising candidate. The MMC adds more components and complexity to an already intricate system, but gives benefits that are in line with many of the ideas behind the proposed concept. A modular structure grants the easiest expansion to a high number of levels, providing a high-quality voltage with less demand for filters to save both vo...

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