Function
The cables deliver the power output from the wind turbines to the grid.
Who is involved
Cables are manufactured by specialist suppliers contracted by the developer.
Key facts
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.
Cables are laid up with insulation and armour coating around the conductors. They must have high chemical and abrasion resistance as well as tensile strength to survive the laying process and withstand wave and tidal loading for exposed sections.
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.
Currently, cables with voltages above 66 kV are only available as dry designs. Wet designs have the advantage of being lighter and more flexible but are not yet available at higher voltages. Much of the development of 66 kV subsea cables has focused on developing wet designs at high voltages than had been previously possible.
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 and failure to maintain this during transportation or installation greatly increases the risk of cable faults. The cable lengths are delivered on drums with sealed ends in order to prevent entry of moisture and other damage.
Notable differences for floating
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.
