Minnesota’s largest electric generator is back in full service after a catastrophic accident and a 22-month repair effort costing more than $200 million, plant operator Xcel Energy said Monday.

Most of the cost to repair Unit 3 of the Sherco power plant in Becker will be covered by insurance, Xcel said. But the replacement power that Xcel purchased from other generators cost $64 million, according to an August estimate, and that expense is passed on to ratepayers.

“Restoration following such a significant event on such a large unit was extremely complex,” Dave Sparby, CEO of Xcel’s Minnesota operations, said in a statement.

The damage happened Nov. 19, 2011, as the generator was being returned to service. Turbine blades broke off, sending metal parts flying and causing a brief fire. The problem was blamed on stress corrosion linked to the blades’ original design.

Unit 3, the largest of the three coal-burning generators at Sherco, is 41 percent owned by the Southern Minnesota Municipal Power Agency, which supplies 18 municipal utilities. The agency used financial reserves to absorb the cost of buying replacement power for customers.

The other two units at Sherco, which are wholly owned by Xcel, were not damaged in the accident. Unit 3 is the largest, at 884 megawatts, and together the three units serve more than 2 million customers.

The Sherco plant is on the Mississippi River, just upriver from Xcel’s Monticello nuclear power plant, where a major upgrade was finished in July at a cost of $665 million. □