By Calvin K. Tang, Technical Manager; Thomas S. Pietryka, Program Manager; and Bill Pantis, Product Manager; Westinghouse Electric Co.
Exelon partnered with Westinghouse to design and commission a digital electro-hydraulic control (DEHC) system for the Limerick Generating Station Unit 1 boiling water reactor (BWR), a first-time application of the Westinghouse Ovation™ system for control, protection and monitoring of the main turbine and pressure control in a BWR. The work was also completed on Limerick Unit 2.
Unit 1 and Unit 2 began commercial operation in 1986 and 1990, respectively, with mechanical overspeed trip devices and an electrical backup overspeed trip system. Each BWR unit had been uprated to a reactor thermal power of 3,515 MWt, with a rated electrical power output of 1,245 MWe. The original high-pressure turbine and three low-pressure turbines had been replaced with turbines during the 1998-1999 time period, but had been operating with the original analog EHC system.
Limerick Unit 2 front standard before installation.
All photos courtesy of Westinghouse Electric Co.
To address the issues of analog control system obsolescence, reliability, maintainability and monitorability, power plants started to upgrade their turbine control systems in the late 1990s using various digital platforms. Typically, the turbine original equipment manufacturers performed the digital system upgrades, but in recent years, utilities have looked to third parties, who have gained the necessary expertise to compete and have successfully implemented DEHC upgrades.
The analog EHC system in Limerick Unit 1 was replaced with a Westinghouse DEHC system during the spring 2014 outage. The Unit 2 EHC system was then replaced and began operation in early May 2015.
|Limerick Unit 2 front standard after installation – back fit into the existing turbine configuration.|
A dynamic model of the Limerick BWR was used to develop control system logic, alarms, and operator interface graphics; to test the application software; and for factory acceptance testing. The integration of the dynamic model and the control software running in the distributed control system development system allowed verification of system design functions, operator reviews and validation of system operability, operating procedure development and Westinghouse testing of the system with full customer engagement prior to system installation.
Additionally, the existing turbine mechanical overspeed trip devices in the turbine front standard were replaced with a separate and diverse on-line testable two-out-of-three overspeed trip system and speed sensors. A pair of redundant hydraulic trip blocks was used for tripping the turbine to provide on-line testability of the trip solenoids and trip block operability.
|Digital electro-hydraulic control monitor in the main control room.|
The installation of certain equipment and cable runs were completed with the plant on-line, with no impact on the outage schedule. The demolition of existing analog equipment, control panel modifications in the main control room (MCR), front standard modifications and modification acceptance testing were completed during the actual 2014 outage. Finally, power ascension testing with the new system installed was performed.
A team approach between utility, vendor and sub-suppliers was used. The team included experienced personnel from both Exelon and Westinghouse who know the plant design, problems, interfaces and processes. Key stakeholders were included from day one.
|The team built a mock-up to the same size to ensure that they could complete the work within 10 days.|
Lessons learned from Limerick past projects and Westinghouse past and ongoing turbine control projects were applied to the Unit 1 upgrade, which went very well. Several hundred additional lessons learned from the Unit 1 upgrade were recorded in great detail for application to the Unit 2 upgrade, and tracked to completion through a weekly action items process. The result was the normal contract requirement that allows a 10-day window for installation of all equipment was reduced to eight and a half days for Unit 2.
Exelon and Westinghouse partnered again to upgrade Exelon’s Peach Bottom Atomic Power Station during the November 2015 outage. Westinghouse also will use this same control system in its AP1000® nuclear power plants currently being built in the United States and China.
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