Four Onboard Generators Mean Almost Never Having to Shut Down

The Liberty 2.5 MW wind turbine from Clipper Windpower, Carpenteria, Calif., is Windpower Engineering’s first Turbine of the Month because its unusual engineering features address common industry headaches, most of them in unscheduled maintenance. Downtime often comes from gearbox stresses, swapping out large components, and unscheduled crane call-outs. The company’s Quantum Drive distributed drive train is a two-stage helical, load-splitting gearbox, with four (yes, four) separate permanent-magnet synchronous generators, and controls for variable-speed operation with power conversion.

In the Liberty, Clipper engineers uses a compact, two-stage helical, distributed load powertrain for higher efficiency than that provided by three-stage designs. Also, the company uses four smaller high-speed shafts to distribute torque to the four generators, thereby reducing potential for premature bearing failure and decreasing the time and cost associated with generator service and repair.

The permanent magnet generators are said to have advantages over induction generators in terms of increased power density, and increased efficiencies at lower wind speeds. The four-generator configuration and control lets the turbine continue operating even with one or more generators are removed for repair. The small, compact generators can be replaced using an onboard hoist instead of an expensive ground-based crane. Two IP54 cooling configurations, one water-to-air and the other air-to-air cooling, offer arrangements for mild or harsh operating environments. Effective generator cooling maintains temperature at less than Class F rise under all conditions. Lower operating temperature provides for increased reliability, and longer life due to a greater thermal margin.

The company adds that two pre-loaded, low-speed taper-roller main bearings absorb thrust loads and mitigate problematic axial motion and main-shaft misalignment associated with low-speed bearing failures. In addition, high-speed gear sets are in “cartridge” form so maintenance personnel can replace gear sets using the onboard hoist without removing the gearbox. This removes more need for ground-based cranes.

In addition, the design uses no slip rings or brushes. Historically, these have been a maintenance issue requiring replacement more frequently than anticipated.

There are no moving parts on the rotor in the rotating frame, thus eliminating the need for complex replacements of brushes, rotating rectifiers, and exciters typical found in doubly-fed generators.

Lastly, the design’s variable speed generator and inverter completely decouples the generator from the grid, eliminating grid induced drivetrain torque excursions. The company says it designed the Liberty turbine in partnership with the U.S. Department of Energy and its National Renewable Energy Lab, which provided development funding and support for drivetrain and blade testing. The DOE awarded the company an ‘Outstanding Research and Development Partnership Award’ in 2007.