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Offshore wind turbines are steadily increasing. This poses challenges for logistics as well as cable producers and installers, reports Anne-Katrin Wehrmann

En route to keep driving down cost for offshore wind energy, the turbines themselves are an important factor. The pace[ds_preview] of development is stunning: While the industry installed the first commercial offshore wind farms at the turn of the millennium with a turbine power of 2–3 MW, market leader Siemens Gamesa has recently reported new large scale orders for its new 8MW turbines. New systems delivering power of 10-12 MW are in development and the largest offshore wind farm operator in the world, Ørsted, already sees projects with 13–15 MW turbines on the horizon for 2024.

But the electrical power harnessed offshore has to get to land somehow and the laws of physics cannot be bent. The valid formula is power = voltage x current. This means that in case of increasing power, cables have to grow in diameter to transfer the current or, alternatively, the voltage has to be increased. The latter has been in discussion within the industry for quite a while already, but now development seems to take off.

Any bigger offshore wind farm has a transformer platform that collects and transforms the current to higher voltage levels for transfer to land. At larger distances to the coast, especially in the German part of the North Sea, converter platforms are being used. They transform direct current from the connected wind farms into alternating current and increase voltage levels further to reduce transport losses to land.

Up to now, 33 kV subsea cables were deployed for electricity transport from the turbines to the transformer platform. Currently, the first European project using a new cable system that doubles the voltage level. Energy company Vattenfall has started building a new wind farm off the Scottish coast that encompasses eleven new generation 8.4 and 8.8 MW turbines of manufacturer MHI Vestas and that uses 66 kv subsea cables.

In mid-June, also Siemens Gamesa announced to have successfully tested a 66 kV system in a small offshore wind project in Denmark. The doubled operating voltage reduces transfer losses and thus increases energy yields and efficiency, Siemens Gamesa stated. Furthermore, a reduction of investment costs during cable installation is possible by deploying the innovative »cable in pipe« wiring method, using standard onshore cables in plastic pipes. »We see 66 kV systems as the new industry standard for intra-wind-farm wiring,« a spokesperson of Siemens Gamesa said. The company already offers its turbines with different transformer types that either support 33 or 66 kV. Shortly, the 66 kV system is going to be deployed on a large scale in a British project.

Cable makers broaden portfolio

The big cable makers on the market have adapted to the new requirements. »Through growing turbine sizes we see essential changes happening in voltage levels and cable diameter of intra-wind-farm cables,« says Michael C. Hjorth, head of sales for high voltage at NKT. »Growing turbine power makes higher voltage a necessity, making 66 kV cables a natural part of the development in the offshore wind industry.« According to Hjorth, NKT constantly adapts to the changing environment and develops new technologies. 66 kV cables are already part of the portfolio, making NKT ready to serve the market. For the future, Hjorth expects hat the global demand for offshore wind energy will lead to new concepts and innovations being developed. »There will be a stronger focus on minimizing risks of expensive and unscheduled blackouts – through high-quality products, digitisation and solution for cable monitoring.«

Nexans, another pioneer in making cables for offshore wind farm, has developed 66 kv cables at his headquarters in Hannover, Germany, in recent years. In 2017, they were delivered to a test wind farm in Northern England. Based on 33 kV technology, fitting connectors for the 66 kV system have been developed. Further German cable manufacturers were not available for comment.

Reliability is trump

Developers and operators of offshore wind farms like Ørsted work closely together with cable manufacturers to find optimal solutions for every project. High on the agenda there are such topics as reliability and durability, explains Tobias Scheil, an expert for offshore cable installation at Ørsted. For operators it is most important that cables reliably transport electricity during a wind farms lifecycle. »Currently we rather see challenges in the area of quality management, documentation and the increasing expectations on engineering.«

Regarding the introduction of new voltage levels in intra-wind-farm wiring, Scheil is cautious. »It is imperative to keep looking for optimisation potential. Many developments are necessary because of the circumstances at the construction site.« An important reason for the offshore wind industry making it so far, have been standardisation concepts that helped lowering costs, Scheil says. »It is unrealistic to turn the whole thing upside down now – unless of course the saving potential is high enough.«