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America's Clipper “uåX˜äŠÊ˜·³Ç, which is introducing a 2.5 MW turbine to the market, has developed and patented technology which it says provides advancements beyond current wind turbine variable speed methods. The new patent harnesses transistors to deliver variable speed operation which meets power quality requirements in a faster, more simplified manner. The company claims this increases wind turbine reliability and decreases maintenance costs, and that the use of proprietary permanent magnet generator technology is a key factor.

"Historically, we have learned that the use of a doubly-fed generator wired directly to dual IGBT's can induce stray currents in the generator rotor shaft, commonly damaging slip rings and brushes which, thereby, have traditionally required ongoing maintenance," says Clipper's Amir Mikhail. "Stray currents have also been responsible for pitting generator bearings, which have led to costly generator failures."

Through the Clipper variable speed system, power is delivered exclusively from the generator's stator where it is rectified to DC current. "Including the use of unique permanent magnet generator technology, the need for troublesome slip rings and brushes is negated," adds Mikhail. More than a year of testing on the one prototype yet running, at Medicine Bow, Wyoming, has validated the theory, he says.

The turbines are designed to avoid the wind industry's endemic gearbox problems, mitigating gearbox stresses through Clipper's patented Quantum Distributed Generation Drivetrain, says the company. This includes a gearbox with four output shafts, each feeding a separate generator. If one generator fails, the others can go on running. The system includes a health monitoring system and is designed for ease of maintenance.