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Confidence in superior performance of medium-speed drivetrain solution

At the company's Nottingham headquarters, Windpower Offshore was able to learn more about the latest advances the company has made in drivetrain modelling, simulation and design.

Viewed from the outside, a Butterfly drivetrain resembles an irregular tube subdivided into two or three flanged main component sections.

One version comprises a main shaft housing at the front, flanged to the gearbox and the generator. This assembly is mounted on to a lightweight cast support structure called cradle mainframe. A second version comprises a cast main carrier with single rotor bearing and flanged gearbox-generator assembly mounted inside.

A crucial advantage of flanged components is that they nearly eliminate the risk of static and dynamic misalignment.

The Butterfly 'tube' configuration consists of easy to assemble/replace self-contained modules. It is structurally strong for lifting in position as a single assembly.

From theory to practice

The Butterfly design's theoretical basis is the result of a comprehensive research and development project incorporating a 6MW offshore drivetrain cost model.

The study compared the lifecycle-based Cost of Energy (CoE) performance of four different drivetrain alternatives:

  1. direct drive
  2. low-speed (one gear stage)
  3. medium-speed (two gear stages)
  4. high speed (three gear stages)

All four feature a permanent magnet generator (PMG).

Conventional high-speed drivetrains with asynchronous or doubly fed induction generators dominate the offshore market at present. The Siemens 3.6MW series and 5MW/6MW REpower are two key representatives of each of these categories.

The Romax study found that direct drive produced the highest CoE, followed by high speed and low speed, while medium speed achieved the best CoE performance.

A more detailed direct comparison between high-speed and medium-speed drivetrains reveals that the cost of a high-speed gearbox is a smaller proportion of overall cost (11% versus 14%). However, a substantially higher proportion of the cost is attributed to lifecycle operations and maintenance (O&M), 59% versus 33%.

Generator costs were compared too. For the high-speed systems, they account for 6% of overall cost (versus 16% in medium-speed, where the generators are larger and heavier). The generator O&M cost comparison resulted in 24% versus 37%.

According to Romax's head of sales Europe & strategic consulting, Yann Rageul, the substantially higher O&M costs for medium-speed generators can be explained by the fact that most failures are linked to electrical components.

On the other hand, the high-speed stage is eliminated in medium-speed gearboxes. This can account for 67% of all gearbox failures. Overall, the lifecycle cost of a medium-speed drivetrain is predicted to be 27% lower than that of a high-speed drivetrain - assuming they are both used at 6MW power rating.

History & development

Butterfly is undoubtedly a clever innovative product development, but Spanish turbine manufacturer Gamesa should be given credit for being the first to introduce a tube-shaped medium-speed drivetrain with PMG in a 4.5MW turbine. Danish manufacturer Vestas also uses a functionally comparable 'tube' configuration for its new 8MW offshore turbine.

But the Butterfly is a different and better proposition, according to Rageul. "These and other companies offer semi- or fully integrated medium-speed drivetrains as dedicated solutions, with all main components originating from fixed suppliers."

Using RomaxWind software tools gives third-party clients full supply chain independence by offering a wide choice in generator, gearbox, brake, and coupling suppliers. The use of Romax's CoE calculator further enhances optimal drivetrain selection.

Romax is a leading name in the automotive sector: 18 out of the top 20 manufacturers in that industry use the company's drivetrain development software tools, Rageul, told Windpower Offshore.

Since 2005, these tools have also been employed for wind turbine drivetrain analysis and design, gear and bearing assessment, gear geometry optimisation, and gearbox and drive system failure investigations.

The first Romax 1.5MW gearbox design was delivered to a third-party client in 2005, and a first semi-integrated high-speed 2MW drivetrain in 2007. The latter features cast housing with integrated rotor bearing and main shaft as an assembly connected to the gearbox by a flange. A separate output shaft connects gearbox and generator.

"Our overall aim is to remove known failure modes and address potential risks of any new layout through advanced simulation before the first hardware is built," adds Rageul. "Drivetrain structural integrity is thereby guaranteed to exceed certification requirements", he concludes confidently.