A badly damaged electric generating turbine in Becker, Minn., is turning out to be harder than expected to repair.
Xcel Energy Inc., the Minneapolis-based electric utility that co-owns and operates the massive machine, said Wednesday that it’s uncertain when repairs will be finished, though it expects it will be this year.
Part of the problem? One major repair had to be done twice.
The accident on Nov. 19, 2011, at the coal-fired Sherburne County (Sherco) Unit 3 ripped turbine blades off their mountings, hurled twisted shards and other metal out of the machine, bent an 80-ton rotor and triggered a fire. No one was hurt, but the generator — the largest in the state — has been shut down since.
For months, the machine has been spread open like a whale-sized fillet, with engineers crawling around the innards to rebuild it. Last fall, Xcel said it believed the work would be finished in the first quarter of 2013, but that timetable has been scrapped.
“We are dealing with an event that we don’t have any precedent for,” said Laura McCarten, a regional vice president for Xcel. “This is new territory for us. We don’t have real good benchmarks for recovering from an event like this. We are very far along. We learned a lot, but we still have significant work to complete.”
Xcel officials submitted a written update on the repairs Tuesday to Minnesota regulators, and elaborated in interviews Wednesday with the Star Tribune.
Plant director Ron Brevig said an analysis of the cause of the accident is still being reviewed by forensic experts, including insurance industry consultants. The findings should be released in the “not-too-distant future,” he said.
The most recent cost estimate for the repair job is $130 million, but that may change, though Brevig declined to speculate on whether it will go up or down. Xcel has said most of the repairs should be covered by insurance.
Xcel said it will have spent $33 million on replacement power from the electric grid in the year ending this month. Xcel officials said they are analyzing how that would have compared to what it would have cost to purchase coal and burn it to generate power.
‘Better do it again’
In its regulatory filing, Xcel disclosed that one of the most painstaking jobs — replacing or “restacking” about 400,000 small metal plates in the generator — had to be almost completely redone. The initial results were outside the margin of error of 1/32nd of an inch.
The repair work first required raising the giant generator from its horizontal position, a job that took weeks of planning, two weeks to set up the lifting equipment and two days for the actual lift, the filing said. After placing the 400,000 plates, the unit was set down again. Then the process was repeated after the problems were discovered.
“This is the largest generator of this type that has been repaired like that and I think we had some misalignment when we set it horizontal,” Brevig said. “We didn’t have it within the tolerance we wanted, and we decided we better do it again.”
Xcel also disclosed that it has ordered new rotors — giant spinning parts that were repaired, but that still contain minor damage. The company said the various damaged parts of Sherco 3 were from 55 percent to 95 percent restored.
Two undamaged generating units at the plant 45 miles northwest of the Twin Cities are still operating. The damaged 900-megawatt generator is co-owned with Southern Minnesota Municipal Power Agency. The three units can supply about 2 million homes with electricity.