All-DC offshore wind farms: when are they more cost-effective than AC designs?

The use of MVDC collector systems has been proposed as a way to reduce the levelised cost of energy (LCOE) of offshore wind farms. This study provides a quantitative assessment of the conditions required for such all-DC wind farms to be cost-effective. A comprehensive LCOE analysis of two AC and two all-DC wind farm designs is performed, with sensitivity studies on wind farm size, distance from shore, collector voltage, and component costs. The results show that for MVDC-based wind farms to be more cost-effective than equivalent HVDC wind farms, the DC/DC converter cost must be less than 90% of the cost of an equivalent MMC, with a cost reduction of 25% for the DC platform. Alternatively, if cost reductions of 30% can be achieved for the DC platform, then the DC/DC converter can be the same cost as an equivalent MMC. For all-MVDC wind farms without HVDC conversion stage to have the lowest LCOE, the collector voltage must be increased, preferably to ±100 kV or above. The all-MVDC configuration can also become cost-effective if a reduction of more than 50% in the cable installation cost can be achieved, for example through the simultaneous burial of multiple cables.