BECKER, MINN. - Ten months after a catastrophic turbine failure, Xcel Energy Inc. said Tuesday it will need another six months and as much as $200 million to restore the largest electric generator in Minnesota.

Plant director Ron Brevig said it's still not clear what tore apart the turbine on Nov. 19, 2011, an accident that also triggered an oil fire at the Sherburne County Generation Station (Sherco) Unit 3.

"We haven't identified the root cause yet, but we do have the forensic investigation ongoing now," said Brevig, who expects a report from consultants by year end or shortly after.

Speaking to reporters at the plant 45 miles northwest of the Twin Cities, Brevig said that, thankfully, no one was injured in the accident, even though broken turbine blades were turned into "shrapnel" and one spinning metal part the size of a five-gallon bucket got hurled through the control room.

The 900-megawatt generator is co-owned by Southern Minnesota Municipal Power Agency, and it sits next to two undamaged units owned by Xcel. The three units can supply about 2 million homes, typically burning through about three 115-car trainloads of western coal each day.

Xcel and the municipal power agency have been largely insulated from the accident's costs. Brevig said insurance will pay for most of the repairs, and both utilities say they've been able to get replacement power at reasonable rates, thanks to weak power demand and low natural gas prices.

The damaged turbine is expected to resume service by the end of the first quarter of 2013 -- more than 16 months after the accident. About 100 in-plant workers are making repairs in shifts around the clock. Many of them are from TurbinePros, a Rogers-based unit of Toshiba that maintains and repairs turbines across the country. Turbine-maker GE also has workers on site.

How it happened

Everything seemed to be going well last November as Xcel finished an upgrade to boost Unit 3's output by about 2 percent.

Then came the day, a Saturday, to test safety mechanisms that shut down the turbine if it spins too fast. The test required operators to advance the speed beyond the usual 3,600 rounds per minute, according to Xcel's account filed with regulators.

Every indicator registered normal as a clock ticked 10 seconds past 12:39 p.m.

Then, two seconds later, vibrations spiked.

Turbine blades shaped like samurai swords ripped off their mountings, and the twisted shards sliced into other components, passing through solid metal.

Catastrophic forces bent the 80-ton rotor and hurled the bucket-sized cylinder into the control room. Thin metal sheets about the size of a napkin got mangled inside the turbine -- and all 400,000 of them would later need to be replaced.

"In about eight seconds it was over," said Brevig.

Except that oil and other flammable liquid had leaked from the turbine's seals. Within minutes, it began to burn. Brevig said there's also evidence that hydrogen inside the turbine ignited. Fire departments from Becker and neighboring communities put out the flames by mid-afternoon.

Brevig, who was 20 minutes away from the plant when the incident happened, said employees were staggered by the accident when he arrived to find fire crews at work. "It was traumatic," he said.

It was the largest turbine ever to fail at Xcel, though not the company's worst accident. Five workers were killed and three injured at an Xcel hydropower station in Georgetown, Colo., in a 2007 flash fire whose five-year anniversary, coincidentally, was on Tuesday.

At Sherco, workers initially put in 10-hour shifts dismantling the damaged turbine, even removing the giant bottom shell, which had been in place since the plant's construction in 1987 and was never intended to be moved. Truckloads of parts were sent to outside companies to inspect or repair, and Xcel hired consultants to conduct forensic metallurgical and other tests.

Some workers who had intended to retire from Sherco decided not to. "They are going to stay on and see the machine returned to service," Brevig said.

Effect on utilities

The accident had the potential to disrupt electrical service, especially for the municipal agency that owns 41 percent of Unit 3. The generator supplies more than 80 percent of the electricity used by the group's 18 municipal utilities spread from Austin to Grand Marais.

By comparison, Minneapolis-based Xcel, which operates 81 generating stations in eight states, could turn to other units for backup power, including new, large natural gas-fired generators in Minneapolis and St. Paul.

At the municipal utility group, Dan Hayes, manager of renewable energy programs and communications, said it quickly began purchasing power off the grid, and later signed contracts with power producers to assure a steady future supply.

"We haven't had to raise rates," he said in an interview. "It hasn't had much of an effect, mostly because we are fortunate enough to have it happen when market prices are pretty moderate."

David Shaffer • 612-673-7090