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Transforming offshore wind energy

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ABB partners with wind energy operator to generate power at world-beating efficiency.

The ABB Power Grids business has won an order from turbine manufacturer MHI Vestas Offshore Wind to supply one hundred of its reliable, energy efficient and compact WindSTAR transformers for installation in wind turbines in the North Sea. The 295 km² Moray East offshore wind farm will be capable of providing enough clean energy to power up to one million households.

ABB will supply its WindSTAR transformers for Moray Offshore Renewable Power’s highly competitive Moray East offshore wind project. The enormous wind turbines – up to 204 m tall to the turbine’s blade-tip – will each generate 9.5 MW of power at 690 volts and the transformers will convert it to 66 kV, a world record for wind farms.

The increased voltage of the turbine-generated electricity enables efficient transmission with a significant reduction in losses and increased efficiency. This step change in renewable power generation also brings lifecycle benefits and cost optimisation. Compared to coal generation, the wind farm could save up to 3.3 million tons of carbon dioxide per annum.

The Moray East wind farm will have a capacity to generate 950 megawatts (MW) of renewable wind power in the Outer Moray Firth, 22 km off the coast of Scotland.

In terms of cost efficiency, it will supply low-carbon electricity at £57.50 per MW-hour, less than half the cost of power generated by comparable offshore wind farms currently under construction.

The site generates power from the UK’s natural wind resources and therefore helps the UK to reduce its use of fossil fuels, thus improving security of supply as well as driving down costs for consumers.

Moray East is the first of two wind farms in development in the Moray Firth Zone and is approved for expansion of up to 1,116 MW. Connection to the national grid will be via a new substation to be constructed at New Deer adjacent to the existing transmission line.

The project was granted consent by the Scottish government in 2014, as part of an ambitious plan to connect a significant amount of renewable energy to the GB transmission network over the next 10 years.

Transformers for offshore wind

Offshore wind has the advantage of stronger and more consistent wind speeds than onshore, making offshore conditions more conducive to high-quality power output than is possible in onshore.

As wind speed increases, small changes in wind speed also yield large steps in energy production: a turbine in a 25km/h wind can generate nearly twice as much energy as a turbine in a 20km/h wind. In addition, there are fewer neighbours to contest development of new wind parks.

These advantages have resulted in offshore wind farms growing in scale and wind turbines are becoming larger and more powerful to take advantage of the strong winds. At the same time, wind energy operators are keen to step up voltage to minimise transmission losses and ensure efficient production. Consequently, an increasing number of operators like MHI Vestas are stepping up their offshore collection networks to 66 kV, compared with the previous best-in-class of 33 kV.

Custom design

Harvesting power from the wind at sea requires state-of-the-art technology, as the corrosive nature of sea water and salty, damp sea air can have a severe impact on equipment. The technology also has to be extremely reliable, as poor weather often hampers access to the turbine platforms to perform maintenance. This means that offshore wind turbines and associated equipment have to be more rugged than their onshore counterparts to ensure asset availability and minimise maintenance costs in the long term.

Offshore wind transformers therefore need to be designed to reduce losses and operate in environments with high vibrations, as well as in harsh ambient conditions where salt, sand and 100% relative humidity are common. They must be compact and lightweight to minimise the structural engineering required to support the asset.

The nature of wind power means that it does not blow according to any regular schedule or wind speed, so turbine output energy varies continually. The transformers must therefore be able to withstand sudden movements and variable electrical loading.

In addition, offshore wind farms are often sited in environmentally sensitive areas, so operators must build installations to minimise the potential for environmental risk. As a

result, transformers like ABB’s WindSTAR are manufactured with biodegradable and totally environmentally sound ester insulation fluid. Their fabrication is based on proven marine technologies from the offshore oil and gas industry, where safety, environmental protection and high availability are critical.

“The WindSTAR transformers are custom designed to meet specific application needs under the mechanical and structural constraints of offshore wind farms, making them a critical factor in offshore wind electricity generation. As such, they contribute to ensure an economically feasible and sustainable future for the industry,” says Laurent Favre, managing director of ABB’s Transformers business line.


Renewable energy technologies provide the main pathway to universal energy access. With wind turbines having already reached ratings of 8-10 MW, offshore wind turbine output is forecast to attain record-level rated capacities of 12 MW by 2020. The transition from 33 kV to 66 kV transmission voltages will further reduce Levelised Cost of Energy (LCoE), encourage ongoing technology improvements, and create more efficient wind generation systems.

Energy efficiency, high transformer availability and reliability are important to help reduce operational and maintenance costs in order to minimise LCoE – a key determinant in the investment case of renewable energy installations.

Any repair or maintenance activity is subject to weather and sea conditions, and in winter it may take weeks to access an offshore wind turbine. If a crane is required for a major replacement, it is also subject to the availability of appropriate vessels. This means weeks or months of lost energy income on top of the repair cost.

High transformer availability thus ensures that operators will be able to make the most of their wind production. And by minimising energy losses using an energy efficient transformer, operators will also make the most of their investment.

Extending the MHI Vestas affiliation

The new contract for the Moray East project extends ABB’s well established relationship with MHI Vestas. In the last year, ABB has won two more orders from MHI Vestas Offshore Wind for the supply of similar WindSTAR transformers to two other large offshore wind projects.

Borssele III and IV: Netherlands

Borssele III and IV, 22 km off the coast of Zeeland in the Netherlands, comprise the second phase of the Borssele wind farm, which is located in shallow water of 38 m deep across an area of 138 km2.

Due for completion in 2020, the new phase will provide 731 MW generating capacity from 9.5 MW turbines, which is enough clean energy to power more than 800,000 households.

Once completed, Borsselle will be the largest offshore wind farm in the world with a total generating capacity of 1,484 MW.

This is more than double the size of the world’s current largest wind farm, the Walney Extension wind farm in the Irish Sea with a capacity of 659 MW.

Windfloat Atlantic: Portugal

Windfloat Atlantic, a floating, offshore wind project 20 km off the coast of Viana de Castelo where the sea is 100 m deep, features the world’s largest and most powerful wind turbines ever installed on a floating platform. The 8.4 MW wind turbines boast an imposing tip height of 190 me, nearly the length of two football fields.

A major benefit of the floating system is that it avoids the complex offshore operations associated with installing traditional fixed structures, thus reducing the potential environmental impact.

The platform and wind turbine will be entirely built and assembled on land and later towed out to their final location.

The WindSTAR power transformers have the benefit of being specifically engineered to be extra resilient against strong vibrations and extreme and sudden movements encountered on floating wind farms.

In addition, WindSTAR transformers are already in operation in UK waters in the five-turbine 41.5 MW Blyth Offshore Demonstrator, as well as in the 11-turbine 92.4 MW European Offshore Wind Deployment Centre (EOWDS) off the coast near Aberdeen. Blyth was commissioned in 2017 and EOWDS in late 2018.

In 2017, ABB also delivered its specialty transformers for the 258 MW Burbo Bank Extension offshore wind farm in the Irish Sea, where they are in operation inside MHI Vestas’ 8 MW turbines.

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