The accident that wrecked Xcel Energy’s largest electric generator in 2011 has been traced to cracks on turbine blades that broke loose from a spinning rotor and caused catastrophic damage that is still being repaired, says a new report.

The Minneapolis-based utility, in an update submitted to state regulators, also said it expects the coal-fired Sherco unit 3 in Becker, Minn., to be back online by Sept. 30 — seven weeks before the two-year anniversary of the Nov. 19, 2011, accident.

Xcel said an investigation found “extensive cracking” in one row of turbine blades at their attachment points. The utility’s report attributed the cracks to “stress corrosion,” and said the problem is “a function of the original design,” not how the plant has been operated. The blades were replaced in 1999, about 12 years after the plant went on line.

Plant director Ron Brevig said “there is no way to know” how many of the blades triggered the catastrophe, but it would only take one. The result was a massive imbalance on the rotor, hurling metal parts from the machine, the report said. No one was hurt, and a fire that erupted was quickly put out, but the generator has been down ever since.

About 150 workers are repairing the 900-megawatt generator, which is co-owned by Southern Minnesota Municipal Power Agency (SMMPA), he said. Two adjacent generators owned by Xcel were not damaged.

“It has been huge,” said John Loubier, vice president of TurbinePros, a Rogers-based turbine-repair company that is one of the local contractors helping to reassemble the machine. “I don’t think anyone expected the scope and duration of this project. It was a catastrophic event. There is no template for it.”

He said TurbinePros, founded in 2009, has had 50 to 75 workers on the project at any one time.

In March, Xcel said it had spent $146 million on the repairs, much of which will be covered by insurance. Xcel and SMMPA have purchased power from the electrical grid during the rebuilding project.

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