Optimising the AC grid connection of remote offshore wind farms

For the past decade, there has been a general perception in the offshore wind industry that VSC-HVDC technology is the only option for offshore transmission distances longer than a threshold of approximately 80 km. However, the realisation of VSC-HVDC (Voltage source converter, High-voltage direct current) projects has shown much increased costs and delay in the delivery of offshore HVDC substations.

This development calls for revisiting the options offered by HVAC offshore transmission. In 2014, Ecofys investigated technologies and costs of the grid connection of offshore wind farms for the Offshore Wind Accelerator (OWA), a joint industry project involving nine developers who represent 71% of the UK’s licensed capacity: DONG Energy, E.ON, Mainstream Renewable Power, RWE Innogy, Scottish Power Renewables, SSE Renewables, Statkraft, Statoil and Vattenfall. The main objective of our work was to study the technical feasibility of innovative options for alternating current (AC) power transmission (LFAC, 50 Hz AC transmission with a number of different reactive compensation schemes) for far offshore wind farms, i.e. for power transmission distances of up to more than 200 km. The resulting study includes:

  • key issues and opportunities of AC transmission;
  • technological concepts applicable to longer transmission distances;
  • a comparative assessment of risks, barriers, benefits and costs for the application of each of these technologies in a range of scenarios; as well as
  • conclusions and recommendations for the application of AC transmission technology.


Edwin Haesen
Energy Systems and Markets
Huygen van Steen
Energy Systems and Markets
Offshore wind energy  (c) Ecofys