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Abstract -Although HVDC transmission systems have been available since mid-1950's, almost all installations worldwide are point-to-point systems. In the past, the lower reliability and higher costs of power electronic converters together with complex controls and need for fast telecommunication links may have prevented the construction of multi-terminal DC (MTDC) networks. The introduction of voltage-source converters for transmission purposes has renewed the interest in the development of supergrids for integration of remote renewable sources, such as offshore wind. The main focus of the present work is on the control and operation of MTDC networks for integration of offshore wind energy systems. After a brief introduction, the paper proposes a classification of MTDC networks. The most utilized control structures for VSC-HVDC are presented, since it is currently recognized as the best candidate for the development of supergrids, followed by a discussion of the merits and shortcomings of available DC voltage control methods. Subsequently, a novel control strategy -with distributed slack nodes -is proposed by means of a DC Optimal Power Flow. The distributed voltage control (DVC) strategy, is numerically illustrated by losses minimization in a MTDC network. Lastly, dynamic simulations are performed to demonstrate the benefits of the distributed voltage control strategy.

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This paper reports on a feasibility study of potential floating structures suitable for wind turbines in shallow seas (around 50 m). It describes the concepts, the evaluation and the selection process and includes ancillary issues, such as grid connection and O & M. Finally, it reports detailed analysis of the concept selected as most suitable in the circumstances, namely, a ‘triple-floater’ construction. A main conclusion is that although, in this case, this technology may not yet be ready for commercial application, the gap to economic viability is closing.