The onshore substation transforms power to grid voltage, for example up to 400 kV. Where a HVDC export cable is used, the substation converts the power to three-phase AC. It also provides switchgear to protect the grid from the wind farm, and vice versa, for fault conditions.

What it costs

About £37 million for a 450 MW floating offshore wind farm.

Who supplies them

They are generally contracted to the same main contractor as the B.4 Offshore substation.

Onshore substation.
Onshore substation. Image courtesy of ScottishPower Renewables. All rights reserved.

Key facts

There are no fundamental differences between onshore substations for fixed or floating offshore wind farms.

The onshore substation is often the first part of the wind farm to be built, about a year before offshore construction. In some cases, work may start ahead of FID for the wind farm to mitigate the risk of stranded generation assets.

Typically, they are two parts to the substation: the wind farm side owned by the offshore transmission owner (OFTO in the UK) and the grid side owned by the relevant grid operator (National Grid Electricity Transmission in England and Wales, SSE Networks or SP Energy Networks in Scotland, or Northern Ireland Electricity Networks).

The wind farm side of the substation is larger, consisting of the majority of the electrical system and a building with a control room, office and storage. The grid side of the substation may be an extension to an existing facility or a new one if this is not practical.

Many of the electrical components are similar in specification to the offshore substation, but constraints on weight and space are not as critical. The substation will contain metering equipment to measure electricity exported to the grid.

The onshore substation is ideally located close to the offshore export cable landfall to limit the length of the onshore cable route, but it may be up to 60 km from landfall.

The area of the onshore substation is likely to be about 5 ha for an HVAC system and 7.5 ha for an HVDC system.

The onshore substation is likely to be contracted to a supplier of transmission systems with a substantial amount of the work contracted to a civil engineering contractor.

Guide to a Floating Offshore Wind Farm