Flexibility from the Carolinas to South Africa

EPRI Helps Duke Energy and Eskom Fine-Tune Power Plants for Lower Loads and Other Flexible Modes

By Tom Shiel

Peter Hoeflich, Duke Energy’s director of analytical engineering, has this advice for utilities just getting started with flexible operations for their fossil power plants: “Prepare to be agile.”

Since 2015, EPRI has been helping utilities do just that. EPRI and the 15 power companies in its Mission Profile Working Group have developed a comprehensive online resource that power plant operators can draw upon to identify and address impacts of various flexible modes.

Flexible operations can adversely impact fossil plants in various ways, such as fatigue, compromised environmental controls, and component pitting and corrosion. In 2018, EPRI completed a three-volume report on impacts of flexible operations on boiler components (see Additional Resources at the end of this article). A study on avoiding turbine damage from low-load operation is slated for completion later this year. EPRI is providing utilities worldwide with these and other research results to support reliable, cost-effective service.

“Operating at lower loads or shutting down periodically can mean less revenue for fossil plants,” said EPRI Senior Program Manager Michael Caravaggio. “They’re being asked to provide the power system with more services while generating less revenue. We need to help utilities use their fossil generation more efficiently.”

Duke Energy: Optimizing Fossil and Hydropower Fleets

As an active participant in EPRI’s Mission Profile Working Group since 2015, Duke Energy applied EPRI’s systematic approach at two coal plants to reduce minimum generation levels. At the coal plants and a combined-cycle natural gas plant, Duke Energy and EPRI applied lessons from the working group to review design, operations, and maintenance data; evaluate risks to components and systems operating in flexible modes; and develop strategies to minimize those risks.

“As our renewable energy teams predicted growing solar generation in the Carolinas, we recognized that our fossil plants would have to accommodate that generation,” said Duke Energy Analytical Engineer Stephen Dean. “We wanted to optimize our fossil and hydropower fleets to benefit our customers. We’ve built our understanding of flexible operations based on insights coming out of EPRI’s working group.”

Among the biggest challenges for flexible operations is to determine how long a plant can run at a lower level without incurring damage or increasing costs. “Through collaboration with EPRI, we were able to update our models so they can help us assess the changes under consideration at our plants,” said Dean.

Through its own research and work with EPRI, Duke Energy has improved the flexibility of several plants and is planning to enhance its entire fleet of fossil and hydropower facilities. One key to the utility’s success is discussion among its plant operators and grid operators. These groups needed to determine flexible modes that are both safe for plants and supportive of grid flexibility.

Now, the utility is developing a metric for tracking flexibility improvements across the fleet. It’s also training plant operators and developing new procedures for flexible modes.

“EPRI has enabled us to apply technical knowledge to improve our risk projections, cost projections, and mitigation strategies associated with flexible operations,” said Hoeflich.

Dean pointed to additional research needs such as:

• Better predict plant impacts of flexible operations
• Assess impacts of energy storage on new power plants and future operations
• Develop techniques to expand plants’ tolerances for flexible modes while minimizing impacts on components

Eskom: Lowering Minimum Generation Levels

Eskom generates 83% of its electricity from coal-fired plants in providing approximately 90% of South Africa’s electricity. Historically, outages at the utility’s units result from planned maintenance or emergency repairs. However, as independent power producers develop more renewable generation, flexible operation of Eskom’s coal-fired units has become more important. Their current design and operating regimes are not flexible enough for load-following.

Eskom and EPRI collaborated on studies on grid flexibility and plant flexibility—in particular, the load-following capabilities of fossil generation. In 2018, Eskom conducted EPRI-designed tests on units at the 3,450-megawatt Matla Power Station and the 3,510-megawatt Tutuka Power Station to assess how much they could lower minimum generation levels. EPRI and Eskom then evaluated different configurations for equipment (such as pumps and mills) and adjusted steam temperature and flame stability parameters. The aim was to reduce minimum loads while maintaining reliable, stable operations and minimizing risk of unit trips.

“We worked with EPRI to review our plant operational parameters, define operating tolerances for flexible modes, and develop methods to enable lower load levels without leading to trips,” said Naushaad Haripersad, Eskom’s acting senior manager of plant performance and optimisation. “The results allowed us to reduce our minimum generation during testing and have the potential to lead to coal savings during periods of high renewable generation and low electricity demand.”

Eskom is reviewing its plant staff manual on when and how to run plants in various flexible modes.

“The operating staff will have to follow a process—what action to take, when to take it, and how,” Haripersad said. “This is a challenge we must address in parallel as we evaluate our plants for flexible operations.”

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