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Close up -- Northern Power plans 8MW offshore turbine

UNITED STATES: After introducing an upgraded direct-driven 2.3MW turbine model in 2010, Northern Power Systems (NPS) is taking a leap forward and is developing an 8MW direct-drive offshore turbine with a 175-metre rotor diameter. Eize de Vries spoke with NPS president of utility wind Troy Patton about the development process and potential pitfalls in developing the turbine.

Founded in 1974 Vermont-based NPS is one of the world’s oldest wind-turbine manufacturers. For a number of years, its sole offering was a 100kW direct-drive turbine. Then in 2008, this 100kW model was refitted with an in-house designed permanent-magnet direct-drive generator. A second version, with a wind-diesel hybrid system for remote cold-climate applications like Alaska, was also produced.

The 8MW NPS 8.0-175 is based on the recent 2.3MW turbine model, which has a 93-metre rotor diameter. Speaking about prototype and product development strategy, Patton said: "One of our main findings was that the turbine technology concept scales very well. We are now working on a 2.2MW sister version with 100-metre rotor diameter and another with 110-112m rotor size and 2.0/2.1MW power rating. Equally encouraging was that in the first 3-4 months of operation we achieved already a satisfying 97-98% availability."

The three NPS 2MW+ turbine models feature a standardised generator comprising a liquid-cooled stator built in segments and an air-cooled inner rotor with permanent magnets. Electrically, the generator is split into two halves, with each section connected to its own power electronic converter. Patton added: "A larger 3.XMW onshore turbine model is in development. It is based upon the same generator layout with unchanged diameter but increased length."

A much bigger but again segmented liquid-cooled direct-drive generator is being developed for the NPS 8.0-175. In common with smaller models, individual stator segments as well as converter modules can be exchanged with the aid of an inboard crane

The 33-34.5kV medium-voltage transformer will be located in a nacelle compartment behind the tower. Patton thereby considers the overall benefits linked to full power conversion equipment placement up tower superior to what he describes as minor loads increase penalty due to extra mass.

Patton further regards optimised generator torque-to-weight ratio of key importance both at component and turbine level: "A lightweight generator positively impacts nacelle, tower and foundation mass and cost. Our liquid-cooled generator design simultaneously enables the application of relatively lower grade permanent magnets, which utilise a small fraction of the rare earth dysprosium used in competing generator designs.  The near elimination of dysprosium results in magnets that are less expensive and easier to procure from multiple sources."

A generator that uses lower-grade magnets requires careful thermal management of magnet temperature, which is integral to the NPS generator design. "Liquid-cooling is by far superior to air-cooling in terms of cooling performance and NPS aims at combining this inherent benefit with the use of lower-grade magnets and retaining a favourable generator mass," says Patton. "In our advanced power lab different stator wire slot shapes, pole ratios and pole placing alternatives can be tested and optimised in a rapid-loop sequence. This is made possible by the segmented stator design, which, for instance, avoids having to build a complete new stator for each new experiment."

Finally, the NPS 8.0-175 in development will be an IEC WC I turbine for wind speeds up to 10m/s, the 175-metre rotor diameter resulting in a specific power ratio of 0.33 kW/m2. This value is comparable to the specifications of three other recently announced offshore concepts — the Vestas V164-7.0MW and the 6MW Nordex and Alstom turbines each with 150-metre rotor diameter.

Patton concludes: "For the simulation process, design and construction of such very long blades for a direct-drive application is less complex compared to geared-turbine equivalents due to marked differences in dynamic modes and mode shapes. But it is possible that we initially decide for a smaller rotor diameter in the 150-metre range as prototype blades of around 73 metres long are now becoming available."

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