Inside Clipper’s driveline design
Wind turbine ratings have gone from 700 MW to more than 2 MW over the past decade. As the size of the turbines increases, so do stresses on turbine gearboxes and other driveline components. The applications have a continuously varying load that is difficult to predict, so designs have to be extremely robust to meet developers’ reliability goals. High torque is normally transmitted through a three-stage planetary gearbox in the larger systems. As torque demands have risen, so have the size, and cost, of the ring gears and bearings.
Clipper took a different design approach with its Quantum Distributed Generation Drivetrain (Quantam Drive), which uses four permanent magnet generators and a multiple-path, distributed gearbox that is unique in the industry. Proprietary variable-speed technology also enables operation in a simpler, more efficient manner than standard wind turbine technologies allow (Figure 4).
4. Four of a kind. The drive train of the Liberty wind turbine splits the torque among four generators operating in parallel. The turbine can continue to operate with one generator out of service. Courtesy: Clipper Windpower Inc.
The advantages of this arrangement are clear. If one generator goes off-line, the other three continue to operate. In normal wind conditions the drop in output isn’t noticeable; only with high winds will the capacity fall—by 25%. To simplify maintenance, a single 650-kW generator can be removed and lowered to the ground by an onboard crane. Another boon to service: The high-speed gear sets can be replaced without having to remove the gearbox.
The turbine control system incorporates the high-speed microprocessors needed to execute algorithm computations, which are repeated every 50 milliseconds. Unity power factor is maintained down to a low-rated power percentage, reducing the need for volt-ampere-reactive (VAR) correction. Grid integration is achieved through power factor regulation technology with ride-through capability, which exceeds current and planned standards for electric grid operation. The control system can ride through a low-voltage condition for up to 3 seconds. It also reduces loads by anticipating resonant conditions within the drive train structure and generators.
Clipper’s Liberty I prototype began operation at a remote site in Medicine Bow, Wyo., in March 2005 (Figure 5). The site has offered up a wide range of weather conditions, including temperature extremes, high-turbulence wind squalls, lightning, ice, and snow. Together, the elements and limited maintenance infrastructure have both challenged the machine and given the testing team and maintenance crew plenty of lessons to learn, which were used in the design of Steel Winds’ turbines.
5. Continuous improvement. Clipper’s Liberty I prototype began operating at a remote site in Medicine Bow, Wyo., in March 2005. Operating the prototype under extreme conditions, including multiple lightning strikes, taught Clipper many lessons that have motivated design improvements found on the Liberty turbines installed at Steel Winds. Courtesy: Clipper Windpower Inc.
Heading for the big time
Clipper Windpower announced in early October that it has entered the competitive European offshore windpower market and unveiled its plans to develop a 7.5-MW turbine. Such a turbine would surpass Germany’s Enercon 6-MW model, which currently holds the turbine size record. The UK government’s One North East regional development agency is putting up $10 million for the two-year effort, dubbed the “Britannia Project.” The new turbine is described as a scaled-up version of the 2.5-MW Liberty wind turbine, with additional innovations suitable for such a record-size machine.
With approximately 5,500 MW of firm and contingent orders for Clipper’s Liberty 2.5-MW wind turbine through 2011, production is sold out for 2008, and 2009 sales are nearly finalized.
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