Dogger Bank Wind Farms, a 50:50 joint venture of Equinor (formerly Statoil) and SSE Renewables, has named Aibel and ABB as suppliers of the HVDC transmission system for its Creyke Beck A and Creyke Beck B offshore wind farms – the UK’s first offshore wind application of HVDC.

The wind farms, each 1200 MW, are located in the Crown Estate ‘Round 3’ Dogger Bank offshore wind park zone in the UK North Sea.

The choice of HVDC was dictated by the distance of Dogger Bank from the coast, about 130 km.

At distances of around 100 km and above, HVDC is generally considered preferable to AC. But Niklas Persson, MD, Grid Integration business line, ABB Power Grids*, points out that voltage source converter (VSC) based HVDC, as planned for the Dogger Bank farms, brings a number of ancillary benefits to the grid, in addition to simply power transmission from the offshore turbines. These include capabilities for very fast regulation of frequency, voltage and power flow, as well as provision of blackstart. With a VSC based HVDC system and its land based converter “you get kind of a Statcom or large SVC onshore that can be used to regulate and stabilise the local grid, and increase loading on lines by improving control of the system.”

Under separate contracts placed by the Equinor/SSE joint venture, ABB will provide its VSC based HVDC Light technology, while Aibel will build, transport and install the offshore platforms (one per farm), in water depths of 20-35m.

Both companies also have options (to be confirmed in due course) to provide platform and transmission systems for the Equinor/SSE Teesside A wind farm, also 1200 MW and located in the Dogger Bank zone.

The preferred cable supplier for all three projects is NKT (which acquired ABB’s HV cable business three years ago).

The three Dogger Bank projects were successful in securing 15-year inflation indexed contracts in the latest UK CfD (contracts for difference) allocation round, the UK government auction mechanism designed to support “less established” low carbon generation technology. The “strike price” for Creyke Beck A was 39.650 £/MWh and that for Creyke Beck B and Teesside A 41.611 £/MWh. A final investment decision has however yet to be made but is expected next year, with first power generation in 2023.

Aibel and ABB entered into a strategic partnership in 2016 to work together on offshore wind connections, with ABB focusing on the power electronics and Aibel on the offshore platforms, and collaborating in-depth on optimal integration of the two. The idea is for each company to build on its strengths and focus on its core business, in part a response to ABB’s experience of taking on a wider, EPC type, role in some of the earlier German offshore HVDC projects, which employed gargantuan converter platforms and, not surprisingly, suffered delays and cost overruns.

The strategic partnership “ensures each participant does what it does best”, said Niklas Persson and “doesn’t take on scope that it is not used to.”

The Dogger Bank HVDC technology will also benefit from the evolution of power electronics that has occurred over the last
15 years or so and experience gained with German North Sea offshore HVDC projects, including DolWin 5, awarded in May 2019 to a consortium of Aibel and Keppel FELS, as main contractor, with ABB as subcontractor.

Four out of the ten German offshore HVDC projects awarded to date use ABB’s HVDC Light, and the three in operation are now recording high availabilities.

ABB can also draw on experience with HVDC Light power-from-shore schemes for Norwegian offshore oil and gas platforms.

There has been steady progress in reducing the tonnage, footprint and complexity of the converter platforms, with more compact power electronics, while at the same time improving electrical performance, continuously increasing voltage and power ratings, and minimising losses.

The converter platforms to be deployed at Dogger Bank will have a “lean design”, with a steel jacket structure and no living quarters or helideck. The platforms will be normally unmanned, operated from shore and accessed only by a service operations vessel.

The turbines (monopole-mounted 12 MW GE Haliade X machines) will supply AC power to the converter stations at 66 kV (the same array voltage to be used at DolWin 5, and the new de facto industry standard), eliminating the need for offshore AC substations, a further simplification designed to reduce the costs of offshore wind.

* ABB’s Power Grids business will be divested to Hitachi in 2020.