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Gearbox problems in Vermont

The gearboxes in the 11 Zond Z-40 wind turbines installed in Vermont late last year are being replaced. The $11.5 million project, erected for Green Mountain Power Corp (GMP), was to have been on line by the end of 1996. Instead new gearboxes were already being manufactured in late March, says GMP project consultant John Zimmerman. Since that will probably take until late spring, the project is not expected to be on line until summer, he adds.

The area's wet climate is thought to be a contributor to the gearbox problems, says Zimmerman, although the exact cause of damage is not yet known. Corrosion had started inside the gearboxes, possibly because the turbines had been "locked off" and the rotor's slight rocking meant that lubricant was worn off the gear teeth where they meshed, says GMP in a statement. Transporting the turbines on land may also have been a contributing factor.

The first problem was noticed in late November. Technicians had started "ready to operate" tests, using a portable generator as utility power was not yet at the site. The high speed shaft in the transmission (gearbox) on the sixth turbine became damaged during the "hard brake" test. One of the gears "slipped" on its shaft, and one of the internal gear assemblies had to be replaced. The turbine was reinstalled in December, having been shipped back to Wisconsin where the gearbox was replaced. The cause is not yet understood, says GMP. Zond declines to respond to questions.

When acceptance tests resumed, several broken blade bolts were found in two turbines. GMP stresses that it was not a high percentage that were broken, as there are more than 2600 bolts. Zond, nonetheless, decided to replace all the bolts. Then, when the turbines were brought on line in late February, an "unusual thumping sound" came from the gearboxes. Corrosion was found where the gear teeth meshed, potentially serious as the damage could spread to other parts of the drive-train when the turbines are operated under load.

Eventually the turbines will have to be taken down and shipped out of state so that the gearboxes can be replaced. In the interim, ten of the 11 are in "virtual" operation at up to 20 rpm. (One has a more serious problem). But they are not producing power. "This is a relatively small delay considering the complexity and climatic challenges of the project," GMP concludes. "It is only a small blip in the 30 year life of the plant." GMP also stresses that Zond has responded well and quickly to all the problems.

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