Wind Tech: Offshore wind - Siemens 6MW opts for direct drive

Vestas, Alstom, GE and Nordex have already unveiled plans for next-generation offshore wind turbines this year. Now Siemens Wind Power has added its name to the list with the SWT-6.0-120.

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In May, Siemens installed and put into operation a prototype of the new 6MW direct-drive offshore turbine with a 120-metre rotor diameter at a test site south of Hovsore in Western Jutland, Denmark.

From the outside, the cylindrical-shaped nacelle looks like a bigger version of the 3MW SWT-3.0-101, introduced in 2009. "The water-cooled permanent-magnet generator is based upon the same design principles, but is longer, has more poles and the outer diameter is 6.5 metres," explains Henrik Stiesdal, chief technology officer at Siemens. "The prototype rotor blade and rotor hub are similar to those of an SWT-3.6-120.

"Reusing major components combines the benefits of proven rotor technology and a measured power curve of up to 3.6MW output level," he adds.

Stiesdal declines to reveal specific information on the top head mass of the SWT-6.0-10, but extrapolating from the low weight of the 3MW model, the mass of the Siemens 6MW may be approximately 300 tonnes. This would compare favourably to geared 6-6.5MW class wind turbines on the market.

A cast main carrier forms the central structural element inside the nacelle, on which the generator and single rotor bearing are bolted. The multiple yaw motors are located in the horizontal plane to direct the rotor towards the prevailing wind direction. The turbine incorporates two individual power converters located above the tower and a medium-voltage transformer set in an explosion-free enclosed space inside the nacelle behind the tower.

A prominent feature of the design is a helicopter-hoisting platform integrated into the nacelle rear, with the outer-rotor generator located at the front.

The SWT-6.0-120 mass increment may be as low as 75 tonnes, despite a 67% higher power rating. Only the rectifier that converts generator AC-power into DC-power is located inside the nacelle in SWT-3.6-120 turbines. The grid-side DC-AC inverter and medium-voltage transformer are both located in the tower foot. For a 6MW power rating this would have added about 30 tonnes if these power-electronic components were put inside the nacelle.

Talking about Siemens' technology switch from geared turbines to a new direct-drive concept, Stiesdal says one of the most interesting wind industry phenomena has been variations in wind technology choice.

"In the late 1990s 600kW and 750kW stall turbine concepts were mainstream and one could believe that future wind turbines would basically all look the same," he says. "However, after 2000 several new concepts like Multibrid were introduced, and today we experience a battle between different concepts."

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