The 860-megawatt Unit 3 at the facility, known as Sherco, was shut in November 2011. A failure during a maintenance outage led to extensive turbine, generator and associated system damage.

“An investigation by external experts found extensive cracking in one row of turbine blades due to stress corrosion,” the company said.

“The problem was a function of the original design and was not related to how the plant has been operated or maintained.”

Company president and CEO Dave Sparby said restoration and repair, which cost about $US200 ($A205) million but is expected to be covered by insurance, was extremely complex.

“Plant employees, contractors and vendors worked thousands of hours to safely and efficiently return the … unit to service.

“We implemented repairs that ensure Sherco Unit 3 returned to operation with the highest integrity and reliability,” he said.

The three-unit Sherco plant is Xcel Energy’s largest power generator in the Midwest, providing 2400 megawatts of electricity and serving more than 2 million in Minnesota.

Sherco Unit 3 is jointly owned by Xcel Energy and Southern Minnesota Municipal Power Agency.

SMMPA, made up of 18 municipally-owned electric utilities, owns 41% of the unit.