Function
The export cable connects the offshore and onshore substations to transmit power from the wind farm to shore. It also provides auxiliary power to the wind farm when it is not generating and provides fibre communications.
Who is involved
Export cables are manufactured by specialist suppliers contracted by the developer.
Key facts
High voltage alternating current (HVAC) export cables are now typically rated at 220 kV, allowing the export of approximately 300 MW per three-core subsea cable. Future wind farms may use higher voltages of up to 275 kV. The voltage chosen balances the cost of the cable, the number of circuits required, and the number of offshore substations required. Wind farms tend to have more than one export cable circuit for redundancy.
Medium voltage alternating current (MVAC) cables may be used for export for small wind farms close to shore. Their use for commercial-scale projects in the future is therefore unlikely, but MV export is attractive for demonstration projects.
High voltage direct current (HVDC) connections are used to connect larger projects, typically those of more than 1 GW installed capacity, and those located further from shore, typically further than 80 to 100 km.
HVDC significantly reduces losses caused by high levels of reactive power that is seen in long distance HVAC cables. which increases the net annual energy production. The full capacity of the cable system can be used for transferring active power because there is no reactive power flow in DC systems and the current flows at a constant level rather than fluctuating as a sine wave.
HVDC converter stations are expensive, and the savings from the use of HVDC cable are not realised until the cable route between the substations is 80 to 100 km. Even beyond 100 km, project-specific considerations can make the final choice complex in deciding between HVAC and HVDC. New technology is steadily reducing the cost of HVDC.
A subsea HVAC export cable is a three-core design, whereas a typical subsea HVDC system has a bipolar design with two single-core cables, a positive and a negative. For a given capacity, HVDC cables are lighter with positive implications for the ease and cost of installation. Overall export cable costs, therefore, for an HVDC offshore wind farm are usually lower than for an HVAC wind farm.
Notable differences for floating
Fixed offshore substations with static export cables are expected to be used for early floating projects., and this is the scenario described in this report.
Floating offshore wind farms using floating offshore substations will require a dynamic section of export cable. This is connected at the seabed to a static length of export cable, which will run the majority of the cable length to shore.
Soon after landfall, the subsea export cable is jointed to the onshore export cables in a transition joint bay. Onshore export cables run from the transition joint bay to the onshore substation. Onshore export cables are manufactured and laid as single-core cables, meaning that three individual onshore cables are jointed to a subsea three-core cable. Floating projects that are large and/or located far from shore are also expected to use HVDC connections.
