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.

What it costs

About £88 million for a 450 MW floating offshore wind farm with cable lengths described in Table 1.

Who supplies them

Hellenic Cables, LS Cable & System, Nexans, NKT, Prysmian and Sumitomo Electric.

There are other cable manufacturers based in China and Japan, but they have yet to be used widely for UK projects.

Static export cable.
Static export cable. Image courtesy of JDR. All rights reserved.

Key facts

As dynamic export cables are not yet proven, 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 sea bed 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 (see I.3 for further information). 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.

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. For example, there are already 10 HVDC substations operating in German waters. Floating projects that are large and/or located far from shore are also expected to use HVDC connections.

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. This is because the voltage is at a steady maximum – it is not at a lower average value because it is alternating – and none of the cable’s capacity is taken up by carrying reactive power. Overall export cable costs, therefore, for an HVDC offshore wind farm are usually lower than for an HVAC wind farm.

The first commercial scale HVDC projects in UK, for example the fixed Dogger Bank projects, are using 320 kV export cables. A pair of single-core 320 kV cables can export up to 1,200 MW per pair. In time, the voltage may increase to 525 kV for even larger projects.

A static 220 kV three-core copper AC export cable has a mass of approximately 110 kg/m.

Two static 320 kV single-core copper DC export cables have a mass of approximately 80 kg/m.

HV dynamic export cables are not yet available but are the subject of industry research.

Several cable manufacturers have cable installation equipment and vessels (see I.2 for further information) and typically lead export cable EPCI packages.

Guide to a Floating Offshore Wind Farm