Function
Offshore substations are used to reduce electrical losses before export of power to shore. 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.
Who is involved
Offshore substations and its components are manufactured by specialist suppliers contracted by the developer.
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 tonnes. 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 tonnes. 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.
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.
Typically, a single substation can support the input of about 500 MW. In some circumstances, the greater cost of higher capacity cables can be offset by savings on substation hardware. Although many offshore substations are not being used primarily as service platforms, they will still have a modestly equipped workshop and frequently a helideck.
The offshore substation is either owned by the developer or Ireland’s transmission operator, EirGrid, depending on the auction round of the project. The wind farm owner has access and responsibility for the array cable entry and wind farm switchgear.
