State-of-the-art HVDC systems contain a transformer-input stage. In case of MMC-based HVDC grid-connection systems in offshore applications, the enormous and undivisible weight of the transformers is a major concern in terms of platform construction and maintenance operations. Additionally, environmental issues have to be assessed as critical. Thus, a transformerless HVDC would be an eminent advantage. In this paper, a transformerless MMC-based HVDC-system layout is presented taking into account the necessary boost operation of the 155-kV level due to the missing transformers. A basic comparison in the scope of dimensioning and systemic issues is given for transformerless and transformer-based HVDC systems. Furthermore, simulation results concerning the operability of a transformerless HVDC system are presented.
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