The cables deliver the power output from the wind turbines to the transmission network.
What it costs
About £140 million for a 450 MW floating offshore wind farm.
Who supplies them
Hellenic Cables, JDR Cable Systems, LS Cable & System, Nexans, NKT, Prysmian, Sumitomo Electric and TKF.
There are other cable manufacturers based in China and Japan, but they have yet to be used widely for UK projects.
Offshore wind farms use array cables to deliver power from the wind turbines to the offshore substation, and export cables to deliver power from the offshore substation to the onshore substation. Subsea cables are used for the array cables and the offshore section of the export cable. Onshore cables are used for the export cable section between the shore and the onshore substation.
A standard subsea cable used in offshore wind is made up of a stranded, profiled conductor with a combination of sealing layers, insulation, fillers, and protective armouring. Subsea AC cables have three cores (one for each phase). Onshore AC cables have single cores and are laid in groups of three. DC cables (land and subsea) have single cores (two, one positive and one negative, for each circuit).
There are three main insulated power core design types:
- Dry, with an extruded lead sheath over the insulation
- Semi-wet, with a polyethylene sheath over a non-fully impervious metallic screen, and
- Wet design, without a sheath over a non-fully impervious metallic screen.
Wet designs have the advantage of being lighter and more flexible. Currently, cables with voltages above 66 kV are only available as dry designs.
The terms for voltage ratings are not formally defined by the industry. Low voltage (LV) typically refers to cables rated up to 11 kV, medium voltage (MV) typically refers to cables rated up to 66 kV, high voltage (HV) typically refers to cables rated up to 220 kV and extra high voltage (EHV) typically refers to cables rated higher than 220 kV.
HV and EHV cables are generally associated with transmission networks and export cables, whereas MV is associated with array cables. The wind turbines generate at LV with a transformer at the base of the tower stepping up exported power to MV.
Cables have a specified minimum bend radius. Failure to maintain this during transportation, installation, and operation greatly increases the risk of damaging the cable, potentially leading to cable faults.
Floating offshore wind farms make extensive use of dynamic cables. These are designed to be exposed in the water column and to withstand the movement of floating substructures, subjecting them to greater fatigue loading than static cables. Compared to static cables, dynamic cables have:
- Sheathing over insulation using materials other than lead
- An additional layer of armouring, and
- Polyethylene outer sheath instead of polypropylene yarn.
Cable suppliers have invested significantly in dynamic designs to support the development of the floating offshore wind sector.