Function
The cables deliver the power output from the wind turbines to the grid.
What it costs*
About £345 million for a 1 GW wind farm.
Who supplies them
Cable manufacturers include Apollo, Hydro Group, Hellenic Cables, ICS2, JDR Cable Systems, LS Cable & System, Nexans, NKT, Prysmian, Sumitomo Electric and TKF.
Key facts
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.
In terms of production, a single extrusion line can produce around 200 km of core per year (this equates to around 40 cm per minute).
The cable lengths are delivered on drums with sealed ends in order to prevent entry of moisture and other damage.
To avoid unnecessary handling, cables that will be installed subsea are ideally loaded directly onto an installation vessel from the factory.