Stable grid operation is challenging, especially when intermittent and unpredictable renewable generation is added to the generation mix. For NorthWestern Energy, the best solution was adding fast-acting gas-fired generation to its Montana electricity grid to meet required reliability standards while replacing expensive third-party contracts for ancillary services.
NorthWestern Energy’s (NWE’s) Dave Gates Generating Station (DGGS), formerly the Mill Creek Generating Station, was constructed as a “regulating reserve” plant to provide highly flexible reserve capacity for a grid with increasing nondispatchable renewable energy sources. The $185 million plant—purportedly the first built specifically for electrical transmission grid regulation duty—has increased grid reliability, provides a rapid ramp rate resource, and can operate at low loads, which minimizes fuel costs.
The 150-MW natural gas–fired plant—which was renamed in March 2011 in honor of the late Dave Gates, NWE’s vice president of wholesale operations—began commercial service on January 1, 2011 (Figure 1). It is located at Mill Creek, near Anaconda, Mont., northwest of Butte.
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| 1. Regulating reserve plant. NorthWestern Energy’s Dave Gates Generating Station consists of three Pratt & Whitney Power Systems 50-MW Swiftpacs. Also shown is the 500,000-gallon raw water storage tank on the right and the 500,000-gallon demineralized water storage tank on the left. Courtesy: NorthWestern Energy |
Defining the Need
NWE is an electric and natural gas utility that serves approximately 656,000 customers in the western two-thirds of Montana, eastern South Dakota, and central Nebraska. The company is headquartered in Sioux Falls, S.D.; its Montana general office is located in Butte.
In 1997, the Montana legislature enacted Senate Bill 390 (SB 390), the Electric Utility Industry Restructuring and Customer Choice Act, to deregulate the state’s electricity markets. To promote development of merchant generation, SB 390 required Montana Power Co. (MPC) to functionally separate its generation resources from its transmission and distribution (T&D) assets. MPC sold its generating capacity to PPL Montana in 1999, and NWE acquired MPC’s remaining T&D assets in 2002. At that time, NWE did not own any generation assets in Montana, so it relied on contracted power purchases from outside its service territory. By 2005, quickly rising power prices caught the attention of the Montana Public Service Commission and state legislators.
The Montana legislature passed House Bill 25 (HB 25) in May 2007, which allowed NWE to build and add new generation to its rate base as part of the state’s move back toward the regulated utility model. Also in 2007, NWE was faced with two problems that threatened its grid-balancing authority responsibilities: Purchased ancillary services were becoming more costly, and renewal of those contracts was just a few years away.
A self-assessment found that NWE’s most important generation need was not baseload or peaking power, but regulating reserve power. The Shaw Group was retained to perform an independent engineering assessment of technology and plant siting options for a plant designed to provide regulating services to NWE’s Montana grid.
Shaw completed site and technology recommendations in December 2007. The generation technology options were quickly narrowed to fast-acting natural gas–fired, simple cycle combustion turbines (CTs). The turbine selection process began in January 2008 and ended in July 2008 when an agreement was reached with Pratt & Whitney Power Systems (PWPS) for three Swiftpacs, based on the FT8 combustion turbine (Figure 2). In addition, Shaw was retained as the owner’s engineer for the project in the fall of 2008.
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| 2. The race goes to the swift. A close-up of the Pratt & Whitney Power Systems FT8 combustion turbine inside the enclosure shows how the output shaft on the right connects with a double-ended generator. Another FT8 is attached to the other side of the generator. The entire package is a 50-MW Swiftpac. Courtesy: Pratt & Whitney Power Systems |
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