Incident Report

Subject:                   Power Plant Fire following Steam Turbine Failure

Date of Email report:   Wed 04/01/2012

Report Detail:

The effect of last month’s major fire at the Sherco 3 coal-fired electric generating plant in Becker on Princeton electrical rates may not be known for about six months, Princeton Public Utilities Commission (PUC) manager Dave Thompson said this week. The coal-fired electric generating plant in Becker has three units which are known as Sherco 1, 2 and 3, and the Southern Minnesota Municipal Power Association (SMMPA) owns the number 3 Sherco unit with Xcel Energy. That fact is important because the Princeton PUC is a member of SMMPA. On Nov. 19 a failure occurred within the steam turbine and generator in Sherco 3 and a fire broke out there. Fire departments put out the fire and there were no serious injuries. All the safety and fire protection systems operated as designed. The Sherco 3 unit had already been shut down since sometime in September for a major overhaul and so the Princeton PUC was already buying power on the open market. But now it is uncertain how long the Sherco 3 unit will be down to tear apart the equipment and replace and repair, Thompson said this week. The SMMPA board met in the city of Wells  on Dec. 14 and Thompson said the Sherco 3 fire was a topic. It was determined, Thompson said, that the Sherco 3 unit will remain shut down for another six months under the best scenario. However, it would be two to three years under the worst scenario, he added. Thompson left no doubt that having to buy power elsewhere during the shutdown of the Sherco 3 unit could lead to higher rates for electrical customers of the Princeton PUC. In the worst case, the rates would go up as much as 23 percent, but in the best case wouldn’t rise at all, said Thompson. Probably what will happen is there would be a single-digit increase, he added. The Princeton PUC still has its diesel-powered electric generating plant if ever there is a problem getting electricity for the city, but the cost of that electricity is higher, Thompson said. But in the meanwhile, the electricity Princeton uses could be coming from either the nuclear-powered electric generation plant in Monticello, or from Sherco units 1 and 2.

Attached are pictures and documentation of an incident at the Xcel Sherco Unit 3 Power Plant in Becker, MN (largest MW unit producer in the State). The unit was in a test phase after an extensive and expensive outage. The Steam driven Turbine exploded. Two electricians on the turbine floor heard noise and started running toward Unit 2 when they felt the heat blast. No major injuries occurred.

First update (edited exerpts):

• At approximately 12:39 PM this afternoon we experienced a significant failure of the Sherco Unit 3 turbine, generator and exciter.
• As part of the start-up plan the unit had operated normally at 240 MWs all last night.
• This morning we took the unit off-line for turbine overspeed testing and turbine valve tightness testing.
• We anticipated that testing evolution to take a few hours and then bring the unit back on-line for the next sequence of testing.
• At 12:38 the unit was off-line and operating at about 3960 RPM when a significant failure occurred.
• While it's hard to tell at this time where the failure originated is clear that the HP/IP sections of the turbine, the generator and exciter were affected and the exciter catastrophically failed, along with turbine-to-generator and generator-to-exciter couplings.
  
• Both ends of the unit were involved in fire. Turbine oil and insulation (on the HP/IP end) along with the turbine front standard were involved in fire.
  
• The Becker and Clear Lake fire departments responded and by 3:30 all fires were out.
  
• All personnel on-site were evacuated and accounted for.
  
• No significant injuries occurred although one contract employee was taken to the hospital with minor smoke inhalation.
  
• The turbine lube oil sump (10,000 gallons) basically emptied and mixed with water from the turbine fire suppression system and fire department activities.

Excerpts from an area newspaper article:

The largest unit at Xcel Energy’s coal-fired power generating plant in Becker, MN will be offline at least until spring as the company tries to figure out what caused a fire in one of its turbines last month. The Sherburne County Generating Station, or Sherco, will continue to operate its other two units, Tom Hoen, Xcel spokesman, said Friday. But the 900-megawatt Unit 3 will remain offline as internal investigators disassemble it to figure out what is broken and what needs to be repaired or replaced, Hoen said. “As of right now, we don’t even know what the damage level is,” he said. Hoen said it will likely be “months, not weeks” before the unit is back up and running. Unit 3 had been shut down since mid-September for routine maintenance. Crews were running tests Nov. 19 when there was an equipment failure and a fire started outside of a turbine, Hoen said. There were no serious injuries.

Additional Documentation: