A methodology for determining minimum operating temperatures
2021’s Winter Storm Uri illustrated the critical importance of maintaining thermal power plant reliability when temperatures plummet. The storm brought record cold and snowfall to areas of the U.S. where frigid temperatures and frozen precipitation are exceedingly rare. In Texas, the storm caused outages that resulted in millions of people losing electricity and dozens of deaths due to hypothermia, accidents, carbon monoxide poisoning, and other medical emergencies.
Understanding minimum operating temperatures during cold weather is a key ingredient in thermal power plant reliability. At the most basic level, minimum operating temperatures are the lowest temperatures at which thermal plants can safely start up and reliably function. Turbines, boilers, pipes, tubing, and other equipment can freeze and become less efficient or even stop working when temperatures fall below that minimum temperature.
This is why it’s so important to be able to accurately determine a thermal plant’s minimum operating temperature, both for the sake of the plant itself and for the customers it serves. “At the fundamental level, it’s about safety for society,” said Mike Caravaggio, EPRI’s vice president of fleet reliability, who leads research on the long-term reliability of existing non-nuclear power plants. “It’s about the ability of the bulk energy supply to reliably serve load on the coldest day when you need more electricity than ever, and everything is more stressed in the extreme cold.”
It’s not just about maintaining reliability in a single power plant. In worst-case scenarios, such as Winter Storm Uri, an individual generating unit that freezes up and ceases operations can trigger cascading failures due to frequency variations. “When it gets really bad, everybody survives together, or the system can collapse,” Caravaggio said.
In 2019, before Winter Storm Uri, the North American Electric Reliability Corporation (NERC) began to increase cold weather emergency preparedness requirements under Standard EOP-011. NERC’s Standard EOP-012 was developed in the aftermath of Uri to address reliability in extreme cold more explicitly. This included defining a generator’s minimum operating temperatures, which can vary significantly depending on the equipment used and any winterization steps taken. For example, a plant with a powerhouse providing temperature control and protection to turbines, boilers, and other critical equipment can operate reliably in cold weather in a way that facilities lacking a powerhouse can’t.
This means that complying with NERC’s standards and consistently ensuring plant reliability in extreme cold will vary from facility to facility, depending on its geographic location, fuel and asset type, and winterization steps. Nuances include whether coal and natural gas plants are exposed to ambient temperatures. Many natural gas plants are exposed to the elements as are coal plants located in southern states, which can also experience frigid temperatures. Effective winterization can be helped significantly with the aid of a methodology to guide how to determine the minimum operating temperature at any plant. In the absence of a methodology, plant operators have relied on tribal knowledge—knowledge that, while often very effective, is disappearing as large numbers of utility employees reach retirement age.
“This knowledge is in people’s heads,” Caravaggio said. “Jim knows the minimum operating temperature and knows what to do when it starts to get cold to keep the plant running. But Jim is retiring, and that knowledge is going with him, so the question is how to institutionalize that expertise and keep it at the plant.”
A Framework to Benefit the Industry
Sunflower Electric Power Corporation in Kansas recognized the need for a structured approach to determining thermal plant minimum operating temperatures and approached EPRI to develop one. From the beginning of the engagement, Sunflower wanted the resulting framework to be shared publicly so the entire industry could benefit.
The result is a framework that utilities can follow to develop minimum operating temperatures, which is presented in the report, Cold Weather Operation: Evaluating Minimum Temperatures for Thermal Generation. Sunflower was awarded an EPRI Technology Transfer Award for the project.
To develop the methodology, EPRI conducted an industry survey and leveraged both existing EPRI research and NERC resources focused on cold weather operations. The creation of a high-level framework to guide the development of site-specific minimum operating temperatures had to consider numerous challenges. For example, most thermal plants in the U.S. have been in operation for decades. If minimum startup or operating temperatures were documented when the plants were initially designed, they may no longer be available or relevant due to changes, such as the addition of environmental control equipment.
Other wrinkles in developing a framework for determining the startup and operating temperatures of thermal plants are related to how individual plants are used. For instance, many power plants across the country are used briefly and infrequently—usually in the summer—to meet peak demand or to provide ancillary services to the bulk energy system. In 2021, for example, about 20,000 megawatts of steam turbine, combined cycle and combustion turbine-based capacity in the Electric Reliability Council of Texas (ERCOT) had a capacity factor under 10 percent.
Further complicating the determination of minimum operating temperatures is that conditions that often accompany cold weather, such as snowfall, high winds, and ice accumulation, can alter the temperatures at which units reliably startup and run. Also important to keep in mind is that thermal plants are made up of myriad interconnected components. “Every detail matters,” Caravaggio said. “Water freezes. And what is a thermal plant? It’s a plant that takes water and makes it into steam. So, it’s miles of piping and tubing that can’t freeze.”
Sunflower Electric Power Corporation has acted since the development of the framework. “They created procedures and hired an EPC (engineering, procurement, and construction) company to help them winterize their systems,” Caravaggio said.
Not only is the framework public and available for other utilities to use, but the research that went into its development also informed EPRI’s Climate READi initiative by providing guidance for reliable operations in extreme cold and how to comply with NERC’s extreme cold weather standard. “The cold weather aspect of the project benefitted significantly from all the work we did with Sunflower,” Caravaggio said.
EPRI Technical Expert:
Michael Caravaggio
For more information, contact techexpert@eprijournal.com.
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