Close up - Turbine upgrade fuels offshore ambition

GERMANY: Repower's 6M offshore turbine aims to build on the success of its 5MW predecessor, one of the largest at its launch in 2004. Eize de Vries climbs into a prototype to see what makes these big turbines different from their competitors and weighs up the company's plans.

A Repower 5M turbine being installed at Thornton Bank
A Repower 5M turbine being installed at Thornton Bank

On the morning of November 5, 2010, high winds swept across northern Germany. Heavy rain over several days had soaked the area's flat agricultural land surrounding the turbine that directly faces the strong North Sea winds. Interesting weather to take a peek at the inside of a nacelle 120 metres above the ground.

German wind turbine maker Repower installed the 5M prototype at Brunsbuttel, north-west of Hamburg, in 2004. A combination of the machine's 5MW power rating and record 126-metre rotor diameter gave it an edge over its competitors. A few years later, the company is launching its scaled-up 6.15MW successor, the 6M.

Repower Systems was the result, in 2001, of the merger of three relatively small German wind companies: engineering consultancy Pro + Pro and wind turbine manufacturers BWU and Jacobs Energie. The new company's research and development team had fewer than 20 engineers, compared with the 500 the company employs today, but plenty of ambition. Work on developing the 5M turbine started that same year.

The wind industry at the time did not have much experience of developing turbines of that size and even less insight into the actual loads they would encounter. The combination of issues that had to be tackled represented a formidable design challenge.

Full-power operation

At the Brunsbuttel site, Repower's 5M/6M product manager Benjamin Johannsen says the 5M turbine has been generating power mostly at rated output level for several days. Thanks to consistently strong wind conditions at this onshore location, the prototype has generated a total 68GWh over six years, despite being repeatedly stopped for client visits and research tasks. No main component has had to be replaced on this ground-breaking machine.

Company safety rules require the turbine to be shut down while we visit the nacelle. Wind speed at 120-metre hub height was 16 metres per second (m/s)that morning, just below the maximum 18 m/s that would have prevented us from using the elevators for safety reasons.

Johannsen urges us to take off our coats and sweaters before going up: the temperature inside the nacelle will be about 40 degrees Celsius, he warns. The strong winds have kept the machine operating at full power for long periods of time and will prevent us from opening any roof hatches. Noise regulations require the cooling fans to be switched off during our visit.

A 5M nacelle visit starts with an elevator ride that covers most of the distance; the final part is a climb up a ladder. At the nacelle bottom there is a massive yaw bearing with multiple brakes and motors, which ensures the rotor is directed towards the prevailing wind direction.

Once we reach the nacelle, its dimensions are truly impressive. Eighteen metres long, six metres wide and six metres high, the huge nacelle is full of components. Because of its non-integrated, fast-speed geared drive system, the 5M's main parts are arranged along a line. Viewed from the rotor, the main shaft supported by two large bearings is located in front of the nacelle. Behind it is the gearbox connected with a flange to the main shaft. If needed, the gearbox can be replaced in one day without having to remove the rotor or use a shaft clamping device. This was, in fact, one of the key design demands.

The gearbox output shaft is located off-centre and connects to the generator via an intermediate shaft. The space near the side-mounted generator is filled with electrical and electronic components including a frequency converter and switchgear. A medium-voltage transformer is at the back of the nacelle. A ladder leads to a helicopter-hoisting platform on top of the nacelle, which is to be used for personnel access.

Offshore spec

Because the 5M prototype is an offshore turbine, the nacelle is completely sealed to protect mechanical, electrical and electronic components from harmful exposure to the harsh marine environment. The design of the cooling systems for gearbox, generator and modular converters protects the sensitive internal instruments through the use of heat exchangers. The turbine is fitted with a duplicate temperatureand speed-sensing system designed to take over immediately if the principal system fails.

A hatch in the nacelle front provides access to the internal hub. Before entering the hub, the service brake on the intermediate shaft and a rotor pin lock the rotor. For internal inspection, the 61.5-metre-long blade is positioned horizontally, allowing service technicians to walk fully upright for about 30 meters and then crawl further. When the turbine needs to run during servicing, all rotating components are covered for health and safety reasons.

The next step

With nacelle and rotor - the top head mass - weighing approximately 430 tonnes, the 5M is not a lightweight machine. The 6M turbine is conceptually identical to its predecessor. The top head mass has increased by 4% to just below 450 tonnes. The unchanged chassis is a welded steel structure split into a heavy-duty main front section containing the main shaft and gearbox and a rear section often called a generator chassis.

Most turbine manufacturers apply a cast main chassis. Repower, by contrast, argues that foundry capacity for extremely large and complex castings is still limited and can easily turn into supply chain bottlenecks when the components market is tight.

New features

A key design change in the 6M is a newly developed gearbox which, despite a 20% torque increase, weighs nearly the same as its 5M equivalent. Even major repairs such as replacing bearings can be conducted without removing the gearbox - a unique design feature that will reduce downtime and operating costs. The 6M has a 6.15MW double-fed induction generator with a 6.6kV voltage level, and a matching power converter.

Another new main component is an RE 61.5 rotor blade that was developed in-house. The innovative feature of these blades is a patented fully integrated spoiler near the cylindrical blade root.

Compared with the 5M, the 6M is quoted as being capable of achieving a yield increase of 12-15% at 10m/s mean wind speed, 10-13% at 9m/s and 8-12% at 8m/s.

RE 61.5 blades are being manufactured in the seaport town of Bremerhaven at a facility named PowerBlades, a joint venture of Repower Systems (51%) and German rotor-blade manufacturer SGL Rotec. Series manufacture started in 2010 and will grow to 300 units by 2012, matching Repower's plans to produce 100 of its 5M/6M turbines in 2012.

Offshore push

Repower expects to reap the benefits of a significant increase in both the number and capacity of offshore projects. The company won two major orders, each for 48 of its 6M turbines at the Nordsee Ost project in German waters and Phases II and III of Belgium's Thornton Bank, both planned for 2012-13.

In 2009, Repower signed a framework agreement with RWE Innogy for more than 1.2GW of capacity in 5M and 6M machines to be installed at the German Nordsee 1 wind farm over 2011-15. The deal is worth EUR2 billion.

Repower is not relying only on its turbines and innovative customised steel jacket foundation solution to make its mark on the burgeoning offshore market. From its base in Bremerhaven, the Repower Offshore Service Solution will soon offer service and maintenance for up to 240 turbines a year in the main European wind markets.


Repower's chief technology officer, Matthias Schubert (pictured), has for many years been involved in a range of wind turbine concepts and drive system choices. The development focus within Repower, from the early 1.5MW MD 70/77 series to the 5M/6M models, has always been on pitch-controlled fast-speed geared turbine models.

Schubert is a firm believer in the geared drive system philosophy, which relies on a double-fed induction generator. "In combination with a partial converter (30-35% of the turbine's power rating), this generator type offers the most cost-effective electrical power-conversion solution," he says. "The solution can also meet any future grid requirement as, in our view, everything needed is already included."

Regional variations

He argues that the only reason for changing to a permanent-magnet generator could be that it is a global system, allowing the same generator type to be employed in 50Hz regions such as Europe and 60Hz countries like the US. But Schubert shares another observation: "Three-phase electric machines in the US grid operate at 575V, whereas a higher 690V level is used in Europe. In the US, therefore, thicker copper cables are required due to the lower voltage. If similar generators are used elsewhere, our customers in Europe pay more than needed." Schubert adds that Repower sells separate turbine models for 50Hz and 60Hz markets.

Schubert's views on the new industry trend away from high-speed geared systems to medium-speed ones are clear: "With medium-speed drive systems, the cheapest third gearbox stage is eliminated and traded in for an expensive lower-speed generator. To me, that does not make sense." If there is a problem within the fast-speed third stage of a conventional gearbox, he adds, it can usually be detected easily: "Issues in the low-speed and medium-speed planetary gear stages are by comparison much harder to detect. At the same time, gearbox research and development has progressed greatly. We at Repower have not faced major gearbox failures in the past six or seven years."

Schubert believes the wind market should have the greatest influence on discussions about technology options. "Our customers do not want to pay for innovation but for a reliable proven concept," he concludes.

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