Hourly planning saves TVA millions

Scott Walker

Tennessee Valley Authority

The Tennessee Valley Authority (TVA), one of the largest utilities in the United States, has implemented new operations scheduling software for hourly planning. Savings have come from optimizing the schedule to minimize operating costs. Computer simulation has, in a number of cases, encouraged the utility to idle as many as 12 units to achieve significant cost savings while ensuring generation reliability.

While hard numbers are not yet available, TVA management believes that potential savings are between 1 percent and 2 percent of the utility`s annual fuel bill, which is in the neighborhood of $1.5 billion. TVA is the nation`s largest wholesale power producer, with 28,498 MW of generating capacity. TVA`s power facilities include 11 fossil plants, 29 hydroelectric dams, three nuclear plants, four combustion-turbine plants, a pumped-storage facility and 17,000 miles of transmission lines.

TVA faces an enormous scheduling problem. It has one of the largest loads of any single utility with a large daily swing-up to 10,000 MW. In the summer, the load usually peaks between 4 p.m. and 5 p.m. while load in the winter typically peaks twice, at 7 a.m. and at 6 p.m. It`s impossible for a human scheduler to accurately weigh all of the factors-seasonal issues, weather, fuel costs, maintenance requirements, etc.-that affect the operating load. Schedulers had a natural tendency to err on the side of conservatism. In order to make absolutely sure that the utility was able to meet its load, they often operated more units than later turned out to be necessary which increased operating costs.

Over the last several years, the TVA has gradually been implementing more efficient computerized scheduling methods that can be run more frequently. TVA uses Cougerplus operations scheduling software from ABB Energy Information Systems (www.abbes.com), based in Raleigh, N.C. This program is currently used to schedule some 40 percent of all generating capacity in the United States. In the beginning planners used this tool only for limited operational costing. As they gained experience and confidence in computerized optimization scheduling, they began using Windows-based scheduling tools for variational studies and hour-to-hour decision-making.

Most recently, the TVA has implemented the software into its operational planning process, which is run 24 hours a day. The results are used for unit commitment and real-time pricing decisions. Experienced planners first review the output from the program. The reason for this approval process is that there are elements in the decision-making process that are still difficult to model, such as transmission voltage constraints. In the great majority of cases, no problems are detected and the program output becomes the operational plan.

A typical example of the savings that have been achieved by the program came when the program suggested putting several additional units on-line. One of the planners suggested that it would make more sense to leave the units off-line for several more days. Calculations were made to determine the relative cost and benefits of starting up the units. They showed that while the TVA did not need the extra capacity provided by these units, bringing them back on-line would make it possible to shift output from higher-cost to lower-cost sources of energy. Marginal costs of power production are nearly zero for hydroelectric power, quite low for nuclear power, followed by coal-fired and combustion turbine generation. The software`s recommendation to startup the unit one week early saved TVA $3/MWh for a 100 MW unit, providing total savings of $50,400 in this case.

The program allows planners to recommit and re-dispatch units to evaluate alternative operating scenarios. Over time, planners believe they are significantly reducing operating costs on a regular basis. For example, in a few cases, the scheduling software has shown they could avoid the startup and shutdown of a coal-fired generating plant by running a combustion turbine for a short period of time to overcome a load hump. By more effectively utilizing the scheduling program and a companion post-analysis tool, the utility has the potential to achieve annual savings of $15 million to $30 million each year.

The operations scheduling software is also used to calculate the TVA production cost of supplying power. This information is processed, formatted and made available to customers via a wide-area network within minutes of the time the prices were calculated. This approach makes it possible to deliver more accurate prices in a much shorter time frame than was previously possible. All in all, the move to improved optimization scheduling at TVA is having a significant impact on the utility`s bottom-line performance.

Scott Walker is an operational planning and review specialist with the Tennessee Valley Authority electric system operations department.

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The Clarion Energy Content Team is made up of editors from various publications, including POWERGRID International, Power Engineering, Renewable Energy World, Hydro Review, Smart Energy International, and Power Engineering International. Contact the content lead for this publication at Jennifer.Runyon@ClarionEvents.com.

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