Function
The network of array cables transfers power from the wind turbines to the offshore substation. It also provides auxiliary power to the turbines when they are not generating and provides fibre communications.
Who is involved
Array cables are manufactured by specialist suppliers contracted by the developer.
Key facts
Array networks are most often designed as “strings” which connect several turbines to the substation. They can also be designed in loops to increase redundancy.
Each turbine is linked to the next with at least 1.5 km of array cable, assuming a 15 MW turbine. Array cables are typically rated at 66 kV. In the next few years, array cable voltages are expected to increase to 132 kV. This is to accommodate turbines rated at and above 16 MW more efficiently and to reduce the number of array cable strings required.
Array cables are typically supplied by the manufacturer with cable accessories, although the production of accessories may be outsourced. Cable protection may be included in the array cable supplier’s scope, but it is more often part of the installer’s scope. Cables may be supplied as pre-cut lengths or as a continuous length, depending on the project’s requirements.
Notable differences for floating
Array cables have a dynamic cable length between the seabed and the floating substructures. The dynamic cable length typically follows a lazy wave configuration to accommodate dynamic movement of the floating substructure, including lateral excursion (the horizontal movement of a floating offshore wind turbine). It must also accommodate the loads resulting from the cable being exposed to the whole water column, as well as withstanding abrasion from the seabed. At the seabed, the cable is either buried or sits on the seabed anchored using rocks or protective matting.
The dynamic section of array cable for floating offshore wind farms is incorporated in one of three ways:
- A single length of dynamic cable between turbines
- Dynamic lengths at each turbine connected to a static length in between using either field joints or connectors, or
- A single cable assembly using dynamic cable at each end with a length of static cable in between, assembled using factory joints (so manufactured and installed as a single length of cable).
The final choice depends on the trade-off between the relative costs of static and dynamic cables, the additional costs of using field joints or connectors, and the introduction of additional potential points of failure at field joints or connectors.
In deep water array cables could be suspended across their whole length. This would put greater loading on the cable due to water current-induced movement of the cable but would reduce the length of cable required. Floating projects to date have not used this approach so the water depth at which this becomes attractive is not well understood, but it is likely to be in water depths of around 500 m.
