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Offshore AC Substation

Offshore Alternating Current (AC) substations are offshore platforms containing electrical components to connect an offshore energy generation project (e.g. a windfarm) to the onshore electricity network or to an AC/DC converter station.

Subsea AC Transmission Cable

Subsea cables transfer electricity from the offshore Alternating Current (AC) substation or the offshore AC/DC converter station to shore. Commonly called ‘export cables’, to distinguish from the smaller lower voltage ‘inter-array cables’ which connect the wind turbines to each other and to the offshore substation(s).

Landfall and Transition Pit

Transition pits are required to allow connection of subsea cables to onshore cables. These are usually located close to landfall.

Buried Cable

Onshore cabling is required to transfer the power from the offshore wind farm to the onshore electricity network. Cabling will typically run from landfall or the transition pit to the onshore substation.

Onshore AC Substation

Onshore Alternating Current (AC) substations house the electrical equipment for switching and protection of the electrical system. In most cases these substations contain transformers to step up electrical voltages to connect to the onshore electricity transmission system.

Overhead Lines

Overhead lines transmit electricity at high voltages on land, from sources of generation to areas of demand (and ultimately to end customers via local distribution companies). The onshore transmission system operates at Alternating Current (AC).

Offshore Wind Turbine Generators

Offshore wind turbines harness wind power to generate electricity.

Transmission Infrastructure – Alternating Current

The onshore transmission system in the UK operates using Alternating Current (AC) technology.  This is also the predominant technology used at present to connect offshore generation projects.  This is a proven technology and there is a relatively well established supply chain in place, for cabling and related electrical infrastructure.

Offshore AC Substation

Purpose

To provide the same functions as onshore electrical substations: switching devices to connect or disconnect equipment, protection equipment to respond to faults, and transformation to higher voltages for either transmission to shore or feeding an AC/DC converter station.  

Subsea AC Transmission Cables

Purpose

To transfer the power from the offshore AC substation or the offshore AC/DC converter station to shore. Commonly called ‘export cables’, to distinguish from the smaller ‘inter-array cables’ which connect the wind turbines to each other and to the offshore substation(s). Depending on project design, there may also be a requirement for cables between substation platforms (if there are more than one). For the purposes of this document, any cables between substations are considered similar to export cables.

Landfall and Transition pit

Purpose

To bring the subsea cables to shore, and connect to buried onshore cables or overhead lines.

Buried Cable

Purpose

To transfer the power from the offshore wind farm and connect from landfall or transition pit to the onshore electricity network.  

Onshore AC Substation

Purpose

To house electrical equipment for switching and protection of the electrical system. In most cases it also steps up electrical voltages to connect to the onshore electricity transmission system.

Overhead Lines

Purpose

To transmit electrical power on land. 

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Transmission Infrastructure – Direct Current

Illustrative connection scenario using Direct Current (DC) transmission infrastructure

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FAQs

For information on frequently asked questions about this document, please read on.

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