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In an investigation in the aftermath of a wind farm accident last month, experts said a North Dakota wind turbine’s rotor and blades crashed to the ground because they were not in proper alignment with a power shaft atop the turbine’s steel tower. That is what caused the rotor’s connecting bolts to fail.

The March 14 accident north of Rugby will prompt more frequent inspections of other turbines, said Scott Winneguth, director of wind plant engineering for Iberdrola Renewables Inc. of Portland, Ore.

Winneguth told North Dakota’s Public Service Commission investigators were unsure whether the problem resulted from the turbine’s operation or reflected an assembly flaw.

He said the accident was “very out of the ordinary” and “a singular event” that did not indicate a broader problem.

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Normal maintenance procedures, Winneguth said, “are not designed to detect this sort of misalignment.”

Commissioner Kevin Cramer said the information would be useful in evaluating future requests for locating North Dakota wind farms.

“They seem to have figured out what created the failure on the one turbine,” Cramer said. “I’m certainly encouraged they didn’t have a bunch of other ones to report to us.”

The turbine was one of 71 that make up an Iberdrola wind energy project in Pierce County, in north-central North Dakota, that is capable of generating 149 megawatts of power.

The turbine first went into commercial service in December 2009, said Mark Perryman, an Iberdrola managing director for field services.

The turbine’s rotor, which has three long blades, attaches to its main power shaft with 48 bolts. The connecting surfaces of the rotor hub and main shaft were not in proper alignment, which eventually caused the bolts to fail, said Winneguth and Duncan Koerbel, an executive for the turbine’s manufacturer, Suzlon Wind Energy Corp.

Winneguth said the 70 turbines in the Rugby project underwent inspection and each of their 3,360 bolts checked. They ended up replacing seven bolts on four of the turbines.

Koerbel said the 70 turbines resumed operation within a week. The affected tower suffered a dent from a falling blade, but they will not need to replace it, he said.

Suzlon has about 7,600 wind turbines in operation worldwide, including about 1,800 of the S88 model involved in the Rugby accident. There are about 1,100 S88 models operating in the United States alone, Koerbel said.

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