The expectation right now is that the coal-fired Sherburne County (Sherco) Unit 3 will back in service around the end of the first quarter of 2013, following an unexpected outage that began in November 2011.
Northern States Power, a unit of Xcel Energy (NYSE: XEL), on Nov. 2 filed a major rate hike request at the Minnesota Public Utilities Commission. In that voluminous filing is testimony on power plant issues from Kent Larson, Senior Vice President of Operations for Xcel Energy Services.
In November 2011 there was a significant failure at the 884-MW Sherco 3, which led to an extended outage, Larson noted. The event occurred at the end of a regularly scheduled overhaul outage, which included completion of an uprate for Unit 3. During the testing procedure, 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.
“As discussed in our May 2012 status update to the Commission, we have made significant progress in assessing the extent of the damage and determining the scope of work to repair or replace the various components affected by the event,” Larson wrote. “We have developed and implemented plans and schedules for the various restoration activities and those activities are well underway. Based on the progress to-date and the remaining restoration activities and schedule, we continue to target return of the unit to service at the end of the first quarter of 2013.”
Most of the cost of the repair work is expected to be covered by insurance, Larson said. While the unit is offline, however, the company elected to proactively take care of some additional work to avoid longer plant outages in the future. Specifically, the Sherco 3 cooling towers were scheduled to be replaced in 2014 as part of a 2014 planned outage. The cooling tower project would have extended the outage by approximately four weeks. By performing this work now while the plant is being repaired, the future outage will be limited to the standard six- to eight-week period.
Also, while assessments of the structural integrity of the cooling towers concluded that the existing towers would last until the scheduled replacement in 2014, that conclusion was based on the towers maintaining normal operation. Due to the Sherco 3 turbine failure, the towers would have remained dry (non-operational) until the Unit 3 restoration was complete, which put it at risk for dry rot.
Utility in pretty good shape already in terms of major projects
Larson noted that overall, Northern States Power has been upgrading and refurbishing its electric generation fleet for several years, such as the Metro Emissions Reduction Project (MERP) efforts at the coal-fired High Bridge, A.S. King, and Riverside plant. “As a result, we believe we have a portfolio of generation units that are well positioned to reliably meet most generation needs for years to come, while also meeting currently expected environmental regulations,” he wrote. “Given these past investments and slow projected future sales growth, our near-term focus for Energy Supply is on maintaining the health and reliability of our existing assets, as opposed to building new generation capacity.”
Xcel’s $1bn voluntary MERP initiative involved significantly reduced air emissions from three Twin Cities-area coal plants – High Bridge, Allen S. King and Riverside – while increasing electricity output by around 300 MW. The MERP proposal was authorized in 2002 by the Minnesota commission. The 588-MW King plant was rehabbed under MERP, while High Bridge and Riverside were converted to natural gas.
Examples of projects planned for 2013 include the replacement of one selective catalytic reduction (SCR) catalyst layer at the A.S. King plant, the modification of scrubber modules to reduce SO2 emissions on Sherco Units 1 and 2, and the installation of mercury reduction systems on Sherco Units 1 and 2.
Examples of reliability-focused projects planned for 2013 include replacement of the boiler reheat section and boiler water wall sections at the A.S. King plant to reduce unplanned, forced outages resulting from tube leaks in those aging sections of the boiler. Plans also include replacement of Number 27 High Pressure Feed Water Heater (HP FWH) on Sherco Unit 2.
The King boiler reheater section replacement was considered during the scoping process for MERP. Results of the 2004 technical assessments showed that the reheater had enough remaining life to warrant delaying replacement for eight to 10 years. Some repairs were performed during the rehabilitation in 2006 and 2007 to correct deficiencies and to allow the reheater to operate until replacement parts can be installed in 2013.
In 2012, the company replaced the Sherco Unit 1 boiler nose arch during the planned Unit 1 overhaul to reduce boiler tube failures in that section of the boiler and improve plant reliability. The boiler nose arch is a section of the boiler that directs hot flue gases into the pendant sections located at the top of the boiler. Due to its location, the nose arch is exposed to hot flue gas in addition to coal ash and fly ash. Thus, the nose arch is in a highly corrosive and abrasive environment and subject to boiler tube failures due to coal ash and fly ash abrasion. The boiler nose arch on Sherco Unit 2 was replaced during the planned outage in 2010 and that unit, along with Unit 1, has not experienced a forced outage due to an arch boiler tube failure since their replacement, Larson said.
Examples of other projects planned for 2013 include:
- replacement of the coal conveying control system at the A.S. King plant;
- the replacement of #22 Main Station Auxiliary Transformer on Sherco Unit 2, which is at the end of life; and
- beginning the replacement of the boiler Couton Bottom on Sherco Unit 1, which is the original boiler section and is approaching the end of life.
An example of a project that addresses equipment obsolescence is the replacement of electro-hydraulic control (EHC) systems on all three turbines at the Sherco plant. The EHC system controls the amount of steam entering the turbine, thereby regulating the output of the unit.
“The Company recognizes the Commission may require us to examine the feasibility and cost-effectiveness of continuing to operate Sherco 1 and 2 units as part of our next Resource Plan,” Larson noted. “The planned capital projects will allow the Company to continue to operate the plants reliably in the near term while any evaluation process occurs.”
The company recently updated plans for the gas-fired, 60-MW Granite City peaking plant and concluded that additional work will be needed to continue operations through 2018. “Specifically, a new operating control system for Units 1 and 2 is required at an estimated total cost of $294,000 through 2013, as the existing control system is obsolete and no longer supported by the manufacturer,” Larson wrote. “If the system is not replaced, control cards will continue to fail and extended outages will occur. Due to the timing of the decision to continue operations at Granite City, this project was not included in the 2013 capital budget.”