Recent offshore wind farms have increased their capacity and distance from the shore. To transmit the bulk power to the shore, some projects employ high voltage dc (HVDC) transmissions. Such wind farm has a local ac grid in their site to collect power from each generator, and then the voltage is increased by bulky and heavy step-up transformers. In order to simplify and to reduce the weight of the offshore facilities, dc collection grids have been also considered in recent years. However, a technical issue is the development of high voltage and high power converters to boost the dc voltage for the HVDC transmissions. This paper proposes a novel dc boost conversion system for interfacing the wind farms and the HVDC transmissions. The proposed boost conversion system consists of multiple isolated dc-dc converter modules. The modules are connected in parallel in the wind farm side and in series in the HVDC transmission side in order to realize high power and high step-up ratio. A control method is also developed for maintaining the voltage of the offshore power system and keeping equal voltage sharing among the series connected modules. Computer simulations are carried out to verify the basic operation characteristics of the proposed system.
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