What is the meaning of array cable ?

Any of a set of submarine power cables connecting wind turbines in an offshore wind farm.



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.


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 fiber communications.

What it costs*

About £32 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.




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 with 224 m rotor diameter and seven times diameter spacing between turbines.

Array cables have a dynamic cable length between the sea bed 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 sea bed. At the sea bed, the cable is either buried or sits on the sea bed anchored using rocks or protective matting (see I.2 for further information).

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.

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 more efficiently turbines rated at and above 16 MW 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.

Some larger cable manufacturers have cable installation equipment and vessels (see I.2 for further information), but EPCI array cable packages have typically been led by marine contractors.



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