Digital turbomachinery controls are helping Duke to maintain reliable performance and uptime through fault-tolerant operation, increased automation and greater process visualisation.
Safe operation is far and away the top concern of any nuclear plant and reliability is a cornerstone of safety. Some 30 years ago, when many nuclear plants in Europe and the USA were commissioned, analog control systems were the state of the art in ensuring plant reliability, but they did present certain constraints. When one component failed through normal wear and tear, for example, the plant’s entire system would have to be shut down, adding risk and costing hundreds of thousands of dollars a day in downtime.
The new system is based on triple modular redundant architecture
“We found that our analog turbine control system was one of the top three causes of trips and transients, primarily because its components presented a single point of failure,” said Marlon Dempsey, instruments and controls engineer for the Duke Power-owned Oconee nuclear power station in Seneca, South Carolina, USA. “We knew that introducing more redundancy at key points would enhance reliability considerably and found that digital technology could provide that redundancy while at the same time reducing the cost of downtime.”
After evaluating alternative turbomachinery control solutions, Duke Energy has begun implementing a fault-tolerant control system, which has already contributed to safe, reliable plant operation by delivering the following benefits:
- Improved technology: accurate monitoring of turbine speed and other critical variables in a nuclear facility
- Greater visibility to operations and easier configuration
- Elimination of costly cabinet rework, equipment relocation and wiring changes
- Improved data tools for operators.
The need to go digital
Duke Power has provided safe, reliable and economically-priced power to the Carolinas for a century. The company delivers approximately 7000 MW of electricity to more than 2 million customers throughout a 56 980 km2 service area while balancing the region’s growing electricity needs with care for the environment.
Duke Power operates three nuclear power stations. Oconee, the first nuclear power plant in South Carolina, has a capacity of 2538 MW. Since its inception Oconee has generated more than 495 TWh of electricity – more than any other nuclear station in the United States. In 2000, Oconee earned further distinction as the second nuclear station in the country to have its licenses renewed by the Nuclear Regulatory Commission for an additional 20 years.
Duke has a strong, proven commitment to plant safety and security and strives to keep production costs among the lowest of all US nuclear plants. Since installation, the plant has been using analog turbine controls provided by its turbine supplier, General Electric, but when GE announced that it was phasing out support for its analog controls, Duke set out to evaluate digital control alternatives. Duke compared options provided by GE to those provided by control system suppliers such as Invensys Process Systems.
Finding the right technology
Invensys’ Triconex control system appealed to Duke Energy because of its technical features and capabilities and its suitability for the nuclear industry. In addition to rating the control system on its ability to eliminate any single point of failure on a critical nuclear power production function, Duke was also seeking an easier to use alternative to the hard-panel interface, with its cumbersome switches and meters that made running tests and acquiring readings somewhat tedious. Duke found that the Triconex turbomachinery controls from Invensys met all its technology specifications along with flexibility and expertise that simplified and reduced implementation costs.
The system controls variables such as turbine speed and valve operation in communication with field sensors. It also runs quarterly turbine-valve movement tests and monthly trip tests to ensure the turbine will trip when necessary to protect the system.
The control technology is the Triconex Tricon system, the control industry’s first completely triple-redundant, ruggedized and cost-effective digital control system. Based on triple-modular redundant (TMR) architecture, Tricon uses three isolated, parallel-control systems and extensive diagnostics integrated into one system. The system uses ‘two-out-of-three’ voting to provide high integrity, error-free, uninterrupted process operation with no single point of failure.
“The controller is triple-redundant, so we no longer have a single point of failure. And the box is qualified with the Nuclear Regulatory Commission as a safety-related application that can help keep core from melting if there’s a problem. This is several safety levels above what is required for turbine control, but when you consider that the turbine can trip the entire plant, it makes sense to go for the highest quality,” Dempsey said.
Duke operates three nuclear power plants in the US Carolinas
To enable Duke’s operators to interact with the controls more effectively, Invensys also provided Wonderware InTouch human machine interface (HMI) software. The InTouch HMI enables operators to execute start-up and valve tests more efficiently than with an analog system.
In constant communication with the controller, for example, InTouch software first confirms whether it is safe to run a specified test. The system then automatically runs the test and reports on the results. If any failures occur, the software tells the operator where and why the failure occurred. The system will also abort a test if a condition arises that will impact system performance or safety.
“It would have been easy just to go with the turbine control system offered by the manufacturer,” Dempsey said. “But the Invensys hardware was not only on par with the other alternatives, they also went the extra mile to make sure the system matched our needs. With the other vendors, we would have had to make changes to our plant control system and our cable room, which simply were not viable options.”
Maintained cabinet and wiring
The Triconex engineering teams are highly experienced and qualified in delivering the rigorous testing and attention to detail necessary for safe and reliable power operations. “They actually mounted their devices inside a mock cabinet that matched the dimensions and available space of the cabinet in our cable spreading room. This extra step impressed us and proved that Invensys was serious about making sure things would fit and work properly.” Dempsey said.
Once on site, Triconex engineers conducted thorough site-acceptance testing to verify that no components were damaged during shipping. When that was done, Invensys transferred the systems from the simulated cabinet to the Duke Energy cabinet. And once installed, Triconex engineers performed a pre-start acceptance test before giving Duke Energy the OK to bring the systems online.
“Physically adapting their systems to fit within our existing cabinet and testing them three times before deployment impressed us greatly,” Dempsey said. “They used our existing wiring for their control systems with very little maneuvering room, but they solved this by performing extra design work up-front.” Triconex also provided training in new features that are not available with analog controls.
Another reason for the Triconex choice was the fact that Invensys was willing and able to retrofit the controls into the existing control room cabinets, which reduced the overall cost of the project considerably by eliminating the need for new cabling.
“Our existing cable was in good condition. Installing a whole new cabinet and pulling cable would have been both expensive and time consuming,” Dempsey said, adding that using existing cabinets is also important because other alternatives required moving to a larger control room, finding new locations for cabinets or rearranging the existing cabinets infrastructure, either approach which would have added costs and time to the overall project.
In addition to eliminating one of the top causes of trips and transients, upgrading to a digital control system gave operators improvements in routine operations, enabled them to make changes in configuration without going back to the vendor, and provided a platform which will enable them to extend automation to other processes.
Following are the improvements in routine operations that were not available with the previous analog system:
- Diagnostic software now pinpoints the location quickly and guides troubleshooting
- High-speed data collection software now scans and collects data so that Duke Energy can analyse circumstances leading up to an event
- Remote access software enables Invensys to provide off-site technical support when necessary.
The InTouch visual touch-screens help by telling system operators what’s wrong when something happens, with ample diagnostic information. “With the old system, we would know when a problem occurred but we couldn’t tell where,” Dempsey said. “This system makes it a lot easier for our technicians to locate the source of problems.”
The system also helps technicians with calibration. The analog system had individual circuit cards that took about a week to calibrate. But with the new digital system, Duke Energy doesn’t have to calibrate at all. “We’ve completely eliminated this task so personnel can now do other work,” Dempsey said.
Dempsey added. “We can make tweaks on our own because the digital technology makes it easy. Not having to rely on an outside vendor is a big security benefit. We have more control over our system. Our old analog system did not allow for changes because it was hard-wired. With the digital control system, we can easily copy the logic and enhance it.”
Duke delivers some 7000 MW of electricity to over 2 million customers
In-house engineering teams, for example, have already used the controls to automate turbine chassis shell warming, freeing operators from the task of sitting at the controls and running the process manually. They have also used the controls to improve valve movement testing, implementing a “feed-forward” control strategy rather than a “feed-back” strategy.
“With a feed-back methodology, you have to wait for a transient to occur and then react, but the system allows us to implement a feed-forward system to compensate for steam loss and minimize the effect ahead of time.”
Leading the industry
Duke Energy is among the small group of nuclear plants now effectively using digital turbine controls, although all are considering doing so as their existing analog controls continue to age.
By implementing the digital system, Duke has virtually eliminated one of the top sources of system trips on one of its three turbines. Installation on a second turbine has been completed, with a third planned for the autumn of this year. And to take full advantage of the new automation platform, Duke is also considering extending control to other systems, such as feed-water control.