Balance of Plant

The main components within the Balance of Plant are:

1. Array Cables
2. Export Cables
3. Cable Protection

Cables:

Deliver the power output from the wind turbines to shore. The standard design of subsea cable used in offshore wind is made up of a stranded, profiled conductor with a combination of sealing layers, XLPE (cross linked polyethylene) insulation and armoring, giving a single 3-core cable for AC use, for example.

Following are the types of cables needed for wind turbines:

1. Inter array cables
2. Export cables
   1. HVAC Cables
   2. HVDC (High Voltage Direct Current) cables

Inter array cable:

The Inter array cable connects the turbines to the offshore substation. Each turbine has of the order of 1 km of array cable associated with it, depending on turbine size and spacing.

Main manufacturers: ABB, JDR Cable Systems, Draka, Nexans, NKT, NSW, Parker Scanrope and Prysmian.

Typical specifications for a large turbine:

Rating: 33-36kV

Mass: 20kg/m

Export Cable:

The export cable connects the offshore and onshore substation. Most offshore wind farms are relatively close to shore and have one offshore substation with an AC export cable run to shore.

An AC export is 3-core whereas a typical HCDC is of bipolar design with two single-core higher-voltage cables, meaning that for a given capacity HVDC cables are lighter with implications for the ease and cost of installation with subsequent savings in cable costs.

HVAC cables suffer significant losses over longer distances due to reactive power flow. HVDC is used for long distance transmission because the full capacity of the cable system can be used for transferring active power (i.e. no reactive power flow).

Cables are laid up with insulation and armor coating around the conductors; they must have high chemical and abrasion resistance as well as tensile strength to survive the laying process.

Main manufacturers: ABB, Nexans, NKT and Prysmian

Typical specifications for a large turbine:

Rating (AC export cable): 132 kV

Mass: 60 kg/m (for 132 kV cable)

Components: Conductors, insulators and mechanical & chemical protection

Cable protection:

Cable protection is used at vulnerable locations, to protect the cables from the wave and tidal action. J-tube seals provide a seal at the ends of the J-tube to prevent seawater entering the J-tube. Passive seals consist of a series of disks that are pulled up into the J-tube. Active seals require inflation after they have been pulled through into the J-tube, requiring an ROV. Seals are not used in all cases, but a sealed J-tube may be filled with a corrosion inhibitor.

A bend restrictor prevents damage caused by excessive bending. Cable stiffeners are also used for protection. If made from steel they effectively weigh down the exposed cable. Cable mats are also used to protect exposed areas of cable, such as when cables cross and they cannot be buried. Mats are typically made of concrete or polyurethane.

Components: J-tube seals. Bend restrictors. Stiffeners. Cable mats