Substations

1. Offshore substations:

Offshore substations are used to reduce electrical losses by increasing the voltage before export of power to shore. Some wind farms will have more than one substation to increase security of export.

Although many substations are not being used primarily as service platforms, they will still have a modestly equipped workshop. In time, it is anticipated that substations placed far from shore will have additional functions, such as providing refuge, temporary or permanent accommodation.

Main manufacturers:

• Electrical components: ABB, Siemens T&D, AREVA T&D.
• Structure: Bladt, Harland and Wolff, McNulty Offshore and Heerema.
• Structural designers: Atkins and Ramboll.

Typical specifications for a large wind turbine:

Weight: up to 2,000 tons

Platform level: 25m above the sea

Area: 800 m²

Input capacity: 500MW

Components: Electrical system. Facilities.  Structure.

1.1 Electrical system:

Integrates AC power output from individual turbines and transforms voltage from (say) 33kV to (say) 132kV for export to onshore substation, else converts to DC for onward transmission.

It is anticipated that offshore substations 50 miles or more from the onshore substation will use HVDC to reduce transmission losses. HVDC systems currently only operate point-to-point and require the use of converters at each substation (onshore and offshore).

ABB, Siemens and Areva have proprietary HVDC systems.

Key components include:

Backup diesel generator rated at around 300kW to provides power to the substation in the event of loss of high voltage feed via export cable. It also plays a role in the commissioning of the wind farm.

Switchgear sets to isolate separately the array and export connections to the substation.

Transformers (if AC) in order to transform to higher voltage for onward transmission. A typical offshore substation has multiple transformers to improve the availability. Transformers are oilcooled, requiring the use of fire and blast protection.

Converters (if HVDC) in order to convert to DC for onward transmission.

Reactors to improve stability of the local grid system.

Earthing to provide an excellent electrical connection between the electrical components and the substation structure.

Panels, cable trays, tracks, clamps and supports to protect electrical items.

Main manufacturers:

• ABB, Siemens T&D, EDF, Areva T&D.
• Transformers: Above plus Tironi.

Components:  Backup generator. Switchgear. Transformers (if AC). Converters (if HVDC). Reactors. Earthing materials. Panels, cable trays, tracks, clamps and supports.

1.1 Facilities:

Support the operation and maintenance of the substation and wind farm more generally.

Fire and blast protection is required because the transformers contain oil and coolants and present a fire risk. They need to be protected from fires elsewhere on the platform. Since most substations have two transformers they also need to be protected from each other.

Typically transformers must be protected to H60 fire protection and 0.3 Barg blast protection. Any accommodation areas need to be protected, along with escape routes. All supply is bespoke.

Steel protection typically is double-skinned at up to 20cm thick with insulation, corrugations and fire-resistant coatings. Composite structures are generally thinner (4-15cm). Double thickness protection will be used between the transformers.

An on-board crane to lift from a service vessel typically has a load capacity of around 2.5 tons.

Also required is a control room, accommodation, water and fuel tanks, low-voltage power supplies, navigational aids and safety system.

Main manufacturers:

• Crane: Kenz Figee.
• Fire and blast protection: Mech-Tools (steel) and SCS (composites).
• Supply of general facilities is often local to assembly of the substation.

Components: Fire and blast protection systems. Crane. Control room. Accommodation (refuge, temporary or permanent). Water tank. Low voltage supplies. Navigation aids. Fuel tanks. Safety system.

1.1 Structure:

Provides support and protection for the electrical and other systems. The steel structure is complex, with many safety considerations and services incorporated.

For a small substation, the foundation may be similar to a turbine foundation, but with a different loading pattern. For a large substation, distributed piles or a jacket is preferable.

A helipad is generally specified to enable helicopter landing. Offshore helipads are generally aluminum to minimize corrosion and weight. An accident during take-off or landing can result in hundreds of liters of jet-fuel spilling from ruptured fuel tanks so stringent safety regulations are in place with the requirement for an integrated firefighting system. The use of helicopters for single-crew transfer is not expected to be routine.

Access by vessel is similar to that for a turbine.

Main manufacturers:

• Structure: BiFab, Bladt, Harland and Wolff, Heerema and McNulty Offshore.
• Helipad: Bayards, Aluminium Offshore and other suppliers to the oil and gas industry.

Components: Foundation. Helipad. Platform access.

1.1 Onshore substation:

Transforms power to grid voltage, say 400kV. Where a high voltage DC export cable, the substation will convert the power three phase AC.

Many of the electrical components will be similar in specification to the offshore substation.

There are the same constraints with weight and space as the offshore substation.

The substation will contain metering equipment to monitor the input to the grid

Components: Similar electrical systems to offshore substation. Buildings and other facilities are simplified compared to offshore. Metering equipment.

The Crown Estate. “A Guide to an Offshore Wind Farm”. Provides technical and policy-related information on offshore wind turbines. This is the source used for all the information in this document on wind turbine parts.