An HVDC system converts and transmits the electrical power generated by the wind turbines, at 66 kV AC, and transformed to say 132 kV AC by AC convertor stations, to the onshore substation through the export cables at say 320 kV DC. Equipment in the onshore substation converts the voltage back to say 400 kV AC for connection to the onshore transmission grid.

Who supplies them

GE Grid Solutions, Hitachi Energy and Siemens Energy.

Key facts

Key components of an HVDC system include:

  • HV switchgear sets to isolate and protect each array and export connection to the substation
  • Converters to convert AC to DC at a higher voltage for onward transmission
  • Earthing systems including lightning protection connecting electrical components and the substation structure, and
  • Cable trays, tracks, clamps, and supports to protect electrical items.

An HVDC transmission system, including the export cables and offshore and onshore substations, typically offers a lower lifetime cost (when also taking into account the lower electrical losses of an HVDC system over these distances) than the equivalent HVAC system for wind farms where the distance to the onshore substation is greater than about 80 to 100 km. The factors used in choosing between HVAC and HVDC are, however, complex.

HVDC systems use relatively new technology and systems that are custom designed for the transmission of high power, say over 750 MW, over long distances. HVDC systems currently only operate point-to-point and require the use of a matched pair of converters at each substation (one onshore and one offshore).

Cost reductions have been seen in recent years and are expected to continue, driven by new technology and the increase in the use of interconnectors, as well as by offshore wind.

What’s in it

Guide to a Floating Offshore Wind Farm