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Wind Technology: Developing the New Medium-speed 3-MW+ Class

Rostock-based German engineering consultancy W2E Wind to Energy installed a 3-MW and 3.3-MW sister prototype of its medium-speed flagship model in 2013.

With more than a decade of experience, W2E develops and operates multi-megawatt class geared turbines that feature a patented, in-house developed LARUS Compact system. According to general manager Christoph Klewitz, “[W2E] engineers perform system dynamics and turbine-grid behaviour simulations and analysis, develop specifications and layout of electrical and mechanical safety systems, and design turbine controls and plant operational management strategies.”

LARUS Compact

W2E’s portfolio comprises three mainstream multi-megawatt pitch controlled variable speed product platforms of 2 MW, 2.5 MW and 3 MW+ each.

The LARUS Compact system in the 2.05- to 3-MW+ models consists of a single rotor bearing, a cast main carrier with two separate mounting flanges, and a flexible connection between main shaft and gearbox input shaft. Its primary function is to enhance gearbox operational lifetime by decoupling rotor-bending moments and rotor torque. The rotor bearing is thereby mounted to a cast main carrier’s outer front flange, while the gearbox is connected to a separate mounting flange located behind it by a flange. "Pure" rotor torque is transmitted to the gearbox input shaft is via a coning hollow shaft coupling fitted with an annular array of flexible elastomer elements. Rotor bending moments pass directly into the tower via the hub, rotor bearing, main carrier, and yaw bearing.

The 2.5-MW (2006) and 2.05-MW platforms (2009) both feature a high-speed three-stage gearbox with non-integrated, doubly fed induction generator (DFIG), interconnected by an intermediate shaft solution incorporating a fail-safe mechanical disk brake. See Table 1 below.

Innovations

The pitch-controlled variable speed 3-MW+ W2E-120/3fc is developed for IEC class IIA low and medium wind conditions. The German Ministry for the Environment, Preservation of Nature and Reactor Safety (BMU) provided financial support due to the innovative nature of the technology project.

The W2E-120/3fc features one of the current largest 120.6-metre rotor diameters in its class (263W/m2), and is available with hub heights up to 140 meters through Fuhrländer successor company FWT. The turbine also incorporates several innovations including a load and mass optimised cast main carrier and a novel combination of LARUS Compact and medium-speed HybridDrive, a semi-integrated gearbox/generator solution. HybridDrive is a patented development by one of the world’s largest gearbox suppliers and geared drivetrain specialist Winergy of Germany for power ratings from 3 to 8 MW and was first introduced in 2011. 

HybridDrive is characterised by a two-stage planetary gearbox with flange connection to a 720-V permanent magnet generator (PMG). This flange-type solution virtually eliminates misalignment damages risks and enables easy (individual) component exchange. Compact overall system dimensions are a combined result of eliminating an intermediate connecting shaft, and skipping a gearbox high-speed stage. The HybridDrive fitted in the W2E-120/3fc’s has a total 34-tonne mass including oil supply system.

Key HybridDrive design objective was maximum operational efficiency under partial-load and peak-load conditions. Winergy reported in 2013 that gearbox and generator bench test results showed full-load efficiency figures of 98 percent for individual components, or up to 96.5 percent total system conversion efficiency. Prototype and serial tests performed during 2013 independently confirmed these values, and are claimed among the best of all existing drivetrain technology solutions.

Higher rated generator speeds translate into substantially smaller generator dimensions and reduced magnet demand, which explains the wind industry’s strong medium-speed interest following the rare earth’s price hike in early 2011.

The W2E-120/3fc operates with 450 RPM-rated generator speed, compared to around 12 RPM had the turbine been a direct drive system, and it requires only about 20 percent of rare earth elements for the magnets. 

Market requirements

The first 3.0-MW+ prototype was installed during September 2013.

“With developing this new W2E-120/fc we continue a long-term strategy in combining proven technology and a controlled level of technological innovation to meet future market requirements. LARUS Compact has already proven its capability to protect the gearbox against damages caused by rotor bending moments and the premature failures this can cause, but the combination with HybridDrive is new," explained Klewitz. "Its system integration level enables drivetrain length reductions by 35-50 percent compared to conventional non-integrated high-speed geared drive system of similar power rating. As a result W2E-120/fc nacelle dimensions could be reduced substantially resulting in lower head mass (nacelle + rotor), which in turn positively impacts dynamic behaviour of the wind turbine and allowed substantial cost reduction.”

W2E head mass (nacelle + rotor) is 185T, which includes the frequency converter and transformer both located in the nacelle. Klewitz said that elimination of the gearbox high-speed spur-gear stage boosts gearbox total efficiency and reliability due to a decreased number of moving parts. He also added that HybridDrive’s PMG plus full inverter solution will be better capable of coping with future grid code demands. At the same time, the latter issue continues to be a matter of ongoing wind industry debate, whereby the end of DFIG has been predicted already many times but so far it has not happened. 

Aerospace

New simulation tools were used for the W2E-120/fc design, including comprehensive dynamic behaviour investigation using general-purpose multibody codes methods that have proven themselves in the automotive and aerospace industries. The advanced methodology enables thorough analysis without installing prototype, and with multibody dynamics numerous single and interacting physical effects can be considered like the dynamic behaviour of a wind turbine rotor, said Klewitz. “Another main benefit of modern simulation tools is much-reduced product development time, as numerous operating procedures can be tried and tested in preparation of actual wind turbine construction. Furthermore, uncertainties in the development process are greatly reduced, which contributes to efficient wind turbine design.”

He used the cast main carrier as an example, explaining that a massive rather solid load-carrying structure as applied for earlier can be optimized to a great extent. To minimize mass, topology and shape optimisation tools are used during the design and optimisation process, which starts with a definition of available design space and a solid shape. During multiple finite element simulation iteration steps all carrier parts not experiencing loads are cut off, see illustrations top to bottom left. Next the model is evaluated with respect to manufacturability and in a final step structural analysis is performed.

"The final main carrier mass is the same compared to W2e’s 2.5 MW-class despite a 20-metre increase in rotor diameter and 500-kW higher rated output," said Klewitz. "The W2E-120/fc product outcome above all shows that new design methods and new energy conversion technologies do greatly contribute to a robust, high economic and future-proof wind turbine. Ongoing extensive field tests will prove these capabilities."

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