Northern States Power d/b/a Xcel Energy (NYSE: XEL) told the Minnesota Public Utilities Commission in a June 6 update that rebuilds of parts of the coal-fired Sherburne County (Sherco) Unit 3 are nearing completion.
Unit 3 went down due to mechanical problems in November 2011 and has been offline since. “We continue to make good progress on our restoration efforts and now expect Unit 3 to return to service in early September, slightly earlier than we noted in our last update in this docket on May 17, 2013,” said the company. That prior restart target date had been Sept. 30.
The 884-MW Unit 3 went off-line in November 2011 while being restarted after a repair shutdown. During the testing procedure after unit restart, specifically the overspeed test, the turbine and generator instrumentation showed vibration levels significantly above normal, and the unit was shut down. The vibration damaged many of the steam, oil, and hydrogen seals in the turbines and generator, and caused a fire.
Sherco is a three-unit, 2,400-MW plant. It burns low-sulfur Western coal from mines in Montana and Wyoming. The plant normally burns more than 9 million tons of coal a year. Westmoreland Coal (NasdaqGM: WLB) is a major supplier out of its Absaloka strip mine in Montana.
The status of various repair projects includes:
- Generator Rotor and Stator (85% complete) – Assembly of the generator continues. The generator end shields (end bells) were delivered in late May and trial installation has started. After completion of the end shields trial fitting and testing, the rotor will be inserted. Xcel forecasts the generator to achieve mechanical completion in mid-July, earlier than expected.
- High Pressure and Reheat Turbines (82% complete) – Pipe welding to the high pressure and reheat turbines continues. The extraction steam lines have been welded to the reheat turbine and the cold reheat steam leads have been welded to the high pressure turbine. The final load arm checks and “Tops-On” and “Tops-Off” alignments have been completed. The loading and alignment of the inner casings and lower HP/IP diaphragms has started. The high pressure and reheat turbines should be substantially assembled by late July, which is ahead of the prior schedule.
- Low Pressure Turbines (LP-A and LP-B) (85% complete) – Final fit up, alignment, and installation of internal components continues. Final diaphragm adjustments and alignments have started. Assembly of the two low pressure turbines is forecasted to be substantially complete by late July.
- Generator Exciter (75% complete) – The trial assembly of the rotor, bearings, and end shields revealed the need for minor machining changes, which have been completed. The final assembly is in progress.
- Balance of Plant Systems and Components – The balance of plant systems and components, like piping and cable, continue to be installed and restored.