The cable core contains the conductor through which power is transferred. The rest of the core consists of screens, insulation, and sheathing to protect the conductor and prevent short circuits.

Who supplies them

Cable cores are typically manufactured by the cable manufacturer. Usually, complete cable cores are manufactured and assembled at the same site to reduce transportation costs of the different components.

Key facts

The conductor may be stranded copper or aluminium. Both have low resistance, excellent conductivity, are ductile, and are relatively resistant to corrosion. Copper has a higher conductivity, 60% greater than aluminium for the same cross section, but is more expensive and the price is more volatile. Aluminium is lighter, and therefore easier to handle.

Copper has better fatigue performance than aluminium, and small diameter strands have greater flexibility (hence resistance to fatigue) than larger strands. These are important considerations for the dynamic array sections.

The conductor screen is a semiconducting tape that surrounds the conductor, maintains a uniform electric field, and minimises electrostatic stresses on the cables.

Most subsea cables used in offshore wind are insulated with cross-linked polyethylene (XLPE). This is due to its excellent strength and rigidity. Ethylene propylene rubber (EPR) has also been used for array cable insulation. It is more flexible than XLPE but has higher dielectric losses.

Surrounding the insulation is a further screen, similar to the conductor screen.

Lead has historically been used for sheathing static subsea cable, but it does not have the fatigue resistance to cope with the additional mechanical stresses placed on cables in a dynamic environment. Alternatives are being researched to develop HV dynamic cables. Environmental concerns around the use of lead are also driving research into alternative sheathing materials. MV 66 kV cables can be wet-designed and so do not require water-blocking barriers.

The cable should at least have a conductor cross-section adequate to meet the system requirements for power transmission capacity. Energy losses can be reduced by using a larger conductor with a greater current carrying capacity but at a greater capital cost.

A 66 kV AC subsea cable conductor typically has a cross-sectional area of between 150 mm2 and 800 mm2 with 13 mm of insulation.

What’s in it

Guide to a Floating Offshore Wind Farm