Main Turbine Control System
The main turbine control system provides control and protection of the main turbine by balancing steam power input with generator output load. The digital upgrade of the main turbine control system involved installation of a Triconex triple-module-redundant system (Figures 2 and 3). The triple redundancy built into the modular design dramatically increased the fault tolerance of the system and eliminated plant trips and transients due to main turbine control system equipment failures. For example, the turbine valve position indication is now performed by redundant position demand indication such that the failure of an indication does not result in a plant transient. This new feature has prevented several reactor trips.
2. Triple hitter. The main turbine control system cabinet following the upgrade with a triple-modular-redundant system. Courtesy: Duke Energy Corp.
3. Touch-smart equipment. The new main turbine control system has a touch screen human machine interface. Courtesy: Duke Energy Corp.
The improved performance of the digital system has also been evident during required equipment testing during power operation. With the original main turbine control system, the facility experienced reactor power swings of 3% to 4% during main steam stop valve tests, but with the upgraded digital system the reactor power typically varies 0.5% or less during a test. This more stable reactor response provides both operational and nuclear safety benefits. In addition, the main turbine control system design automated numerous manual operator activities to minimize the need for operator actions during chest warming, shell warming, and turbine start-up.
The main turbine control system digital upgrade was completed in 2004.
Control Rod Drive System
The control rod drive (CRD) system receives inputs from the ICS on desired control rod position and manipulates control rods to the demanded position. The digital upgrade of the CRD system utilized an AREVA-designed system on a triple-modular-redundant platform from Triplex (Figures 4 and 5). The triple redundancy built into the modular design dramatically increased the system’s fault tolerance.
4. Keeping things under control. Here’s what the control rod drive system cabinets look like after the upgrade with a triple-modular-redundant system. This picture shows processor modules and input /output modules. Courtesy: Duke Energy Corp.
5. Strength in numbers. The facility has redundant single-rod power supplies for each control rod. Courtesy: Duke Energy Corp.
For example, the upgraded CRD system experienced the failure of a fuse disconnect in the AC power supply system, but this failure did not result in a reactor trip. As another example, failures of control rod power supplies have occurred with the digital CRD system. However, the redundant power supplies kept the CRD energized and thereby prevented dropped control rod events that could have led to reactor trips. In addition to better fault tolerance, design functions were added to the CRD system to improve system performance.
The CRD system digital upgrade is a nuclear industry first and was completed in 2008.
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