The North Dakota Public Service Commission plans a Nov. 26 hearing on a pair of April applications from Northern States Power d/b/a Xcel Energy (NYSE: XEL) for approval of a new gas-fired combustion turbine (CT) at the existing Black Dog plant in Minnesota, and two CTs at a greenfield site in North Dakota.
The utility applied with the North Dakota commission for an Advance Determination of Prudence (ADP) for its proposal to add three 215-MW natural-gas-fired, simple cycle, CTs to its system. The first CT would be constructed at Xcel’s Black Dog plant in Burnsville, Minn., called Black Dog Unit 6, with service beginning in 2017. The second and third CTs would be constructed at an undetermined new plant site to be located in the Red River Valley near Hankinson, N.D. They would be called Red River Valley Units 1 and 2 and would enter service beginning in 2018 and 2019.
Xcel is also separately requesting that the North Dakota commission grant a Certificate of Public Convenience and Necessity for Red River Valley Units 1 and 2.
Separately, Xcel is pursuing approval of these three CTs within a pending resource plan case at the Minnesota Public Utilities Commission.
Unit 6 at Black Dog would substantially replace the coal-fired capacity at this site, which is scheduled to retire in 2015. The Black Dog plant site allows the company to maximize the use of existing infrastructure and maintains generation within its largest load center, which enhances operating reliability.
The model of F class combustion turbines now commercially available has fast start capability, which allows it to reach 150 MW in 10 minutes from a cold start, operate in a range of at least 50% to 100% load while meeting emission limits, and achieve faster ramp rates over the load range, Xcel noted. In addition, the maintenance and overhaul cycles have been significantly improved. The base performance, with respect to full load capacity and heat rate, has also been improved.
The combustion turbine units will be integrated into Xcel’s remote dispatch control center. “We expect to use the units for peaking load service, dispatching them after all lower cost and ‘must run’ units,” the company said. “They are expected to be dispatched primarily during higher system load periods in the summer and winter months, with an annual capacity factor of between four and ten percent.”
More project details by plant are:
Black Dog Unit 6
The Black Dog plant is currently a coal- and gas-fired generating station. The original Unit 1 boiler/turbine and the Unit 2 boiler, installed at the site in the 1950s and fired on coal, were repowered with a natural gas combined-cycle unit (Unit 5), which includes a natural gas combustion turbine-generator combined with a heat recovery steam generator that delivers steam to the Unit 2 steam turbine and generator. The repowering project, completed in summer 2002, increased output from the two original units by more than 100 MW.
Black Dog Units 3 and 4, which utilize coal as the primary fuel, were put into service in 1955 and 1960. The boilers, turbines and generators are essentially original equipment which have been well maintained and operated. However, operating data shows a declining availability as the units continue to age. After examining the costs necessary to continue to operate these units reliably, and the cost of the pollution controls that will be needed for continued operation, the current plan is to retire these last coal units in 2015.
Black Dog Unit 6 will be in the existing powerhouse, in the area where Unit 4 currently is located. The exhaust stack will be about 200 feet tall and located adjacent to Unit 6, in the area of the existing Unit 4 boiler. The new unit will be connected to the existing 115 kV substation. Minor modifications to the existing 115 kV switchyard will be required to connect it to the transmission system. No upgrades of the 115 kV transmission system are required since Unit 6 will utilize some of the outlet capacity from retired Units 3 and 4, and a new interconnection request with the Midcontinent ISO is not required.
The output of Black Dog Unit 6 depends on ambient weather conditions (primarily temperature and humidity) and altitude. For purposes of this application, nominal generating capacity is considered to be about 215 MW at summer ambient conditions.
Unit 6 will be fueled entirely by natural gas. CenterPoint Energy currently serves the plant site. “We will be securing additional natural gas supply through a competitive process beginning in early 2014,” Xcel wrote. “We anticipate that the successful bidder may need to file for a route permit and other necessary permits to replace the existing pipeline serving the plant with a new higher pressure natural gas line running from the Cedar Town Border station to the plant.”
Decommissioning, demolition, and removal of the Unit 4 turbine, generator, boiler and other components will be completed prior to constructing Unit 6. In order to allow the construction of Unit 6 to begin when needed, it will be necessary to take Unit 4 out of service in September 2014. Unit 6 will be constructed in 2015 and 2016. Start-up of the unit would occur in early 2017. Unit 6 is expected to be in commercial operation late in the first quarter of 2017.
Unit 6 will be operated and maintained by the staff that will be retained for Units 2 and 5 (the existing 1×1 combined cycle facility) after the retirement of Units 3 and 4. No additional staff are planned to accommodate the new unit. The service life of Unit 6 is anticipated to be in excess of 35 years. Annual availability will be greater than 95%.
Red River Valley
A specific site for the two Red River Valley units in southeast North Dakota has not been selected at this time. Xcel reviewed the option of locating the plant in the vicinity of Fargo, but determined this would involve increased costs to connect the plant site to a natural gas pipeline, as well as to connect it to outlet transmission. The Hankinson area provides access to the 230 kV transmission system and is near a major natural gas pipeline.
The proposed facility would consist of two 215-MW combustion turbines with the necessary infrastructure to accommodate a full time operating and maintenance staff. The layout would allow for two combustion turbines to be installed, and for conversion of the two units to a combined-cycle configuration in the future. It is anticipated that the tallest structure within the plant will be the stacks, at approximately 65 feet. The combustion turbines and building are all expected to be less than 40 feet in height.
The output of the units depends on ambient weather conditions (primarily temperature and humidity). For purposes of this application, nominal capacity is considered to be about 214 MW at summer ambient conditions.
The layout of the facility allows for addition of distillate oil storage and handling if a future need develops to have oil as the backup fuel. The Hankinson siting area is near the Alliance interstate gas pipeline. Multiple parties utilize this line to transport gas, and indicated a willingness and ability to provide gas service. “We anticipate securing the necessary natural gas supply through a competitive process beginning in 2014,” Xcel said. “Water supply will either be from an on-site well or provided by truck.”
The Red River Valley plant would connect to the transmission network by either expanding the existing Otter Tail Power Hankinson 230 kV substation or building a new 230 kV substation at another location. Xcel anticipates a new double circuit 230 kV line will connect the plant to the interconnection substation and transmission system.