Friday, November 26, 2021

How Low Can You Go?

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As low-load operations and flexibility take on increased importance, improved plant controls and automation can help

Before Mike Seela started his current job leading Tri-State Generation and Transmission Association’s efforts to improve the performance and reliability of its generation fleet, he worked as a power plant control room operator for 25 years. Seela knows from experience the tension that exists between plant operators and the utility personnel who sell electricity to energy markets.

“That tension has been around forever,” said Seela, senior monitoring and diagnostics analyst for Tri-State, which provides electric generation and transmission to 45 cooperatives in Colorado, New Mexico, Nebraska, and Wyoming. “The market guys have their job providing power, and the operators have to protect their generation units and operate them safely and reliably. Those interests don’t converge very often.”

In recent years, it has become even more difficult to reconcile market demands with safe, reliable power plant operations—and not just for Tri-State. To help accommodate more grid-connected, intermittent renewable energy, traditional fossil fuel generation facilities designed to provide baseload power are increasingly operating at varying loads. In many instances, large coal plants have to shut down and start back up to take advantage of economic opportunities presented by the inevitable ebbs and flows of renewable generation.

“We are asking Mack trucks to operate like sports cars now,” said Steve Seachman, an EPRI expert on improving plant automation and equipment health. “You’re not going to push a single button and start a coal plant in minutes.”

The Stresses of Starting and Stopping Baseload Plants

Quickly shutting down a large power plant in response to market conditions is technically challenging and presents significant potential risks and costs. Most operating coal plants in America were built between 1950 and 1990 and have decades-old components.

The large temperature fluctuations accompanying startups and shutdowns put extra stress on boilers, pumps, pipes, and other metal equipment. “Temperature swings cause equipment to expand and contract,” said Seachman. “The more you can minimize those temperature swings on any piece of equipment, the less stress you’re putting on it and the more life you’re keeping in it.”

As plant operators start up or shut down a plant, they need to watch as many as ten screens in the control room to track parameters such as steam temperatures, airflow, and water levels. At the same time, they must assess whether the changes could damage equipment or lead to an outage. This can be stressful.

“These different variables are all moving together at once, and trying to remember which ones you need to keep an eye on and when can get really tricky,” said Seachman.

Less Stress Using Controls and Automation to Run at Low Load

When generating and selling electricity isn’t economically advantageous, a less stressful alternative to shutting down and starting up power plants is to keep them running at low load, which does not expose equipment to temperature fluctuations.

Operating at low load enables the faster ramp-ups necessary to take advantage of market opportunities: a plant at low load can reach full power in a few hours instead of a day or two when it has been completely shut down. For plant operators, this approach also avoids the stress of starting and shutting down a unit.

In 2018, EPRI started research on how to use a plant’s existing distributed control and automation systems to achieve stable low loads while improving the predictability and ease of ramping generation up and down to meet demand.

Researchers identified enhancements such as adjusting controls to implement and optimize sliding pressure operation. This mode reduces the energy used to produce steam in a boiler to align with the lower energy requirements of a turbine operating at a low load. Sliding pressure operation replaces the more traditional practice of running the boiler at a constant high pressure.

Another technique identified by EPRI involves improving fuel flow control, either augmenting flow to increase a plant’s load (an approach known as overfiring) or reducing flow to lower load (underfiring). Control system modifications to more precisely control underfiring can avoid potential control valve instability when reducing plant load.

EPRI also identified ways that automation can be used to decrease load to minimum levels and increase it again when demand is high. Such automation, known as sequential control, might include starting the mill where coal is pulverized and dried before it is blown into the plant’s furnace. Starting a car is based on a similar type of sequence automation. “You turn the key in the ignition, and a lot of things happen automatically and in sequence—fuel enters the engine chamber, the fuel pump starts operating, oil begins to circulate, the power steering and water pump get going, and more,” said Seachman.

Power generator steam turbine
EPRI Collaboration with Tri-State

While existing distributed control systems made by companies like Siemens and Emerson can be adjusted to enhance low-load operations, many plant operators have been hesitant to use some of these features because they automate decisions traditionally made by people. “Until recently, plant operators in the U.S. haven’t had an appetite for these modifications,” said Seachman. “Now there is more interest because they’re looking for ways to make operations more consistent and reduce operator burden.”

Seachman and other EPRI staff work directly with plants to improve low-load operation and lower minimum loads using existing controls. They visit the site, interview plant personnel, and recommend control modifications.

EPRI worked with Tri-State to assess the controls at a 450-megawatt coal plant in its generation fleet. Tri-State approached EPRI for assistance for two main reasons: it had recently installed a new control system, and the plant lacked best practices for low-load operations instead of relying on operator experience.

“We didn’t have much confidence that the control system was configured optimally, and we don’t have a control engineer—the plant operator with the experience to consistently make control changes,” said Ron Bisbee, generation performance manager at Tri-State. “We have control technicians, but they’re hesitant to dive in and make control changes because they lack the experience and formal training that control engineers have.”

Tri-State implemented all six of EPRI’s recommended control modifications. One was sliding pressure operation, which enabled more efficient low-load operations. Another was the automation of boiler feed pump balancing, which makes ramping more predictable and reduces operator workload.

The control modifications resulted in several improvements:

  • A 25% reduction in the unit’s stable minimum load (from 200 megawatts to 150 megawatts)
  • An increase of more than 1% in the plant’s heat rate during low-load operations
  • An increase in the rate that the plant ramps up from minimum load operations

The positive results prompted Tri-State to incorporate the modifications into other units at the same power plant.

There also was an intangible benefit to the collaboration between EPRI and Tri-State. When the project started, plant operators were skeptical about whether it would deliver any value. But after they worked with EPRI and saw substantial improvements, that skepticism evaporated.

Key EPRI Technical Experts:

Steve Seachman
For more information, contact

Artwork by Josh McKible