AC or DC? Economics of grid connection design for offshore wind farms

The design of collection and shorelink transmission systems for offshore wind farms raises unique technical and economic challenges. Study work and field experience by Econnect and others shows that conventional onshore wind farm design practices cannot be relied on to optimise the life-cycle economics of an offshore generation site. Assumptions about cost components and project risks must be revised to reflect the realities of the offshore operating environment. Advances in HVDC technology using voltage source converters (VSC) have increased the attractiveness of DC transmission for offshore connections. Numerous technical and commercial advantages arise from the use of DC cables and power electronics, but these are offset by the high cost of the converter infrastructure, exacerbated by the additional costs of offshore platforms. The economics of HVAC versus HVDC for offshore connections thus requires careful assessment. Similar care is required when evaluating novel network architectures such as offshore grid schemes, the costs and benefits of which are both significant. In this paper a comprehensive economic evaluation methodology is described which accounts for emerging design options such as AC/DC transmission and partial redundancy. This has been applied by Econnect to the evaluation of 6 GW of UK Round 2 offshore projects.