Explaining the exact cause of the gearbox damage, Gates says it occurred before the wind turbines were connected to the substation. Because the rotor is so large -- 40 metres -- slight vibration occurred while the rotors were held stationary by a mechanical brake. This vibration wore a miniscule indentation in the gear tooth, perhaps as little as 1/1000 of an inch. Over time, the thin film of oil on the gear separated from it, allowing oxidisation. The following fretting corrosion resulted in rust and flat spots on the gears. Once the turbines were energised, the gear contact was not smooth and a knocking sound could be heard.
"Green Mountain Power wants these turbines to last a while," which is why they were shut down, says Gates. The actual gearbox repair is just a matter of grinding the surface of the gear, he adds. But Zond decided to replace the entire drive trains.
"We never experienced anything like this before," says Gates, noting that Zond had used the same installation techniques since 1981. "This is an example of learn as you go." Gates adds that in future care will be taken to ensure the grid connection is ready before hoisting the rotors onto the turbines, thus avoiding such periods of stress on the gear teeth.