B.3 Offshore substation

Function

The offshore substation connects the array cable system to the export cables. It contains a step-up transformer and power factor compensation equipment to reduce losses. For longer export cables the substation may also convert the power from alternating current (AC) to direct current (DC) to minimise losses further. It also provides switchgear to protect the grid from the wind farm, and vice versa, for fault conditions.

What it costs*

About £238 million for a 1 GW wind farm, considering an HVAC system.

Who supplies them

Electrical system: Hitachi Energy, GE Grid Solutions and Siemens Energy.

Topside structure: Aibel, CS Wind, Smulders and Seatrium.

Systems integrators: Engie Fabricom, Iberdrola, Ørsted, Petrofac and Semco.

Structural designers: Arup, Atkins, ISC and Ramboll.

Key facts

Offshore substations consist of a main electrical power system, auxiliary systems, a topside structure to house the systems, and a foundation. Offshore substations are often delivered as one element of a contract to connect the wind farm generating assets to the onshore transmission grid.

An HVAC substation topside (everything above the substructure) weighs between 1,200 and 3,000 t. A 450 MW wind farm is likely to have one offshore substation. Single HVAC substations of up to approximately 700 MW have been used.

An HVDC substation topside weighs between 12,000 and 18,000 t. A 1 GW wind farm would only have one HVDC offshore substation but could be connected to the turbines by several AC convertor stations which would transform the 66 kV output from the turbines up to 132 kV or higher to feed the HVDC substation. It is unlikely to be commercially attractive to use an HVDC connection for a single 450 MW wind farm that is 60 km from shore.

A developer typically works closely with its chosen HV engineer after the turbine has been chosen to optimise the export system as a key opportunity to reduce the cost of energy. By reducing the number of circuits, the substations need less switchgear and fewer transformers. This provides an opportunity to dispense with a substation or to reduce topside and foundation costs.

Standardisation of offshore substation design offers the potential to lower costs, although few developers have the project pipelines to justify the upfront costs.

With 66 kV subsea cables, near-shore wind farms up to 300 MW can be built without an offshore substation.

A typical HVAC platform is about 25 m above the sea and has an area of 800 m². 

Although many offshore substations are not being used primarily as service platforms, they will still have a modestly equipped workshop and frequently a helideck.

In the UK, the offshore substation is ultimately owned and operated by a transmission operator (OFTO), although the wind farm owner has access and responsibility for the array cable entry and wind farm switchgear.