Considerations in design of an offshore network

The vast potential of offshore wind energy and the challenge of connecting it into onshore AC systems from possibly very distant locations is attracting interest in offshore networks based on HVDC technology using voltage source converters (VSC). However, while a number of visions have been expressed for how an offshore grid in, for example, the North Sea in Europe might look, investors are yet to commit to the development of offshore grids as such and a number of design and economic questions should be considered. The paper discusses various engineering, regulatory and economic issues associated with offshore networks. In particular, it compares AC and HVDC options for simple connections, considers the benefits of coordinated offshore network development and asks whether, in an offshore context, conventions on secure operation that are customary in design and operation of onshore transmission networks are still relevant. It is argued that, except where a loss of infeed issue would be caused for an onshore grid to which an offshore DC grid is connected, the lack of demand offshore means that rules can re-evaluated. This leads into a discussion of the need for DC circuit breakers and reliable fault detection and selection on a multi-terminal DC grid and options for the control of a DC grid. Finally, a number of institutional issues are described including the role of offshore transission owners and sharing of the costs and benefits of an offshore network.