The Battle of Obsolescence

Issue 9 and Volume 108.

Cooperation between control system vendors and TXU Power’s engineering staff enabled Comanche Peakto effectively manage the replacement of obsolete analog controls with digital controls on Units 1 and 2.

TXU Power’s Comanche Peak nuclear power plant is embarking on a comprehensive program to replace its analog instrumentation and controls (I&C) systems with state—of—the—art digital technology. The effort is driven by the need to solve obsolescence issues, while at the same time achieve improvements in component reliability and overall plant performance.

The digital upgrade program began with several interrelated studies to determine the best approach for making the necessary upgrades. Both short—term and long—term needs were assessed. A study carried out by the utility addressed the following:

  • Expected reliability of the technology
  • Product of choice
  • Long-term supplier support
  • Parts availability
  • The capability of event triggered fault recording
  • Operator interface
  • Total cost of ownership

The study concluded that switching to digital equipment was the right direction for Comanche Peak and the sequence of upgrades was determined by the risk significance of the system and its overall impact on station reliability. TXU decided to replace many of the I&C systems at the plant, and the main generator and emergency diesel generator (EDG) excitation systems with digital controls.

For the upgrades, TXU Power entered into an alliance partnership with the integrated operating team of Framatome ANP and Siemens. This alliance offered TXU Power the advantage of having direct access to all supplier cost and technical data for more efficient project execution. Under the agreement, supplier profits are tied to achieving target values for key performance indicators established for each project.

Comanche Peak’s Emergency Generator Control Panels. Photo Courtesy: Framatome ANP, Inc.
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Typically, these indicators include total project cost, schedule adherence, and the overall quality and reliability of the installed product. If performance indicator target values are reached, or exceeded, the alliance partners share in the financial gain. Should targets be missed, penalties can be applied up to the point where the supplier receives no profit. This approach offers TXU Power a maximum of cost predictability and control.

Turbine Controls

The turbine generator analog control system, installed during the plant’s construction in the 1980s, was obsolete and replacement parts were difficult to obtain or unavailable. As a result, TXU Power decided to replace existing instrumentation and controls systems with Siemens’ TELEPERM digital platform. The study also defined the migration strategy with timelines and estimated costs. Planning began in 2001. The Siemen’s system was selected for its flexibility, automation capabilities and the integrated operating team’s commitment to provide 25—year support for spare parts, technical services and access to upgrades.

The scope of the upgrade included the electro—hydraulic controller, the speed target unit, the turbine—stress evaluator, seal steam control, generator temperature control, leakage water control and moisture separator re—heater. In addition, monitoring and controls for the various functions were provided through the OM 690 System, which serves as the human machine interface. Modifications to the main control room included installation of several new operator terminals.

To support the planned upgrades, changes were first made to the plant simulator for turbine controls and turbine voltage regulation. In order to facilitate operator training, these changes were completed six months in advance of the plant modifications.

A significant amount of planning and design engineering was required to support physical modifications to the plant. Engineers from Comanche Peak and the alliance partner were all involved in the engineering design of the system. The integrated team was responsible for design inputs and details, specifications for plant layout including control room modifications, cabinet positions, cable and raceway modifications, and associated seismic and anchorage calculations.

Turbine control upgrades were installed during fall 2003 and spring 2004 on Unit 2 and Unit 1 respectively. The team performed factory and site acceptance tests on the equipment and conducted necessary plant personnel training to prepare for the outage. Throughout the entire modification process, lessons learned were captured and incorporated into project plans on a go—forward basis.

As a result of the upgrades, the plant turbine reliability has significantly improved. The operations staff has experienced improved ability to control the turbine at low power compared to the manipulations required with the analog system. Upgrades to the turbine protection system, including extended turbine protection, electronic generator protection and the turbine trip system, will be made during plant outages in 2005.

Main GeneratorVoltage Regulator

The voltage regulator system for Comanche Peak’s 1,350 MVA main generators was replaced during the same outages as were the turbine controls. Comanche Peak performed extensive system integration testing of the new voltage regulator system and the new turbine control system. With the hardware available for reference on site, responsible maintenance engineers had optimum conditions for creating the test procedures. Subsequently, every system was tested individually, with every piece of hardware and software function covered by the tests. The individual tests were followed by system integration testing with the turbine control system.

At Comanche Peak the voltage regulation system consists of two independent voltage regulator channels and two independent power channels (thyristor bridges). The rated regulator output current is 144 amps with the possibility of a maximum short—term ceiling current of 225 amps.

Mobile Training Center for Hands—On Training. Photo Courtesy: Framatome ANP, Inc.
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Each of the regulator channels has a dedicated power controller, and the power controllers feed the field winding of the main exciter. Only one channel is active at a time. The second channel runs in standby mode. If one channel fails, the control is switched over automatically to the non—defective channel without interruption.

The regulator is implemented in an entirely redundant two—channel configuration each consisting of a controller and power converter. Both channels are operated in parallel, one active and the other in hot standby. The controllers are built using the configurable digital control system SIMADYN D. This system is especially suitable for fast control and calculation with sampling rates of up to once per millisecond. Each of the two SIMADYN D systems controls a dedicated power section SITOR decoder acting as line side converters.

Emergency DieselGenerator (EDG)Excitation Unit

The EDG excitation unit’s OEM PORTEC discontinued its 10 CFR 50, Appendix B, quality assurance programs in 1994 and ceased all product manufacturing, repairs and engineering support in 1998. At that time, TXU Power purchased some spare components and a complete set of design documents from the OEM.

The design documents had sufficient information to manufacture the majority of the components. Unfortunately, this approach was determined to be costly and time consuming. Additional modifications were also needed in order to add fault recording and diagnostic capabilities to the analog system. Due to the limited capabilities of the current system, trouble—shooting efforts were time consuming due to the lack of data after a fault or disturbance had been identified.

Because of continuing operating issues with the 7Wwel, 6,900V EDG, TXU Power decided to replace the static exciter voltage regulator, power potential transformers, current transformers and linear reactors, generator control devices and protective relays.

In order to solicit input from other utilities and to have a basis of support from within the industry, TXU Power invited other utilities and industry experts to a joint conference in 2002. The goal was to determine the advantages and disadvantages of various solutions and design concepts associated with each solution. After this meeting, TXU Power decided proceed with a digital excitation system.

TXU was seeking higher reliability, compatibility with other plant control systems that would eventually be upgraded to digital systems, and commitment by the alliance partner to support the delivered digital solution for the lifetime of plant. After evaluating a number of available options, Comanche Peak selected Siemens’ THYRIPART excitation system, a safety related system qualified by Framatome ANP in the U.S.A.

The excitation system is a load dependent static excitation system powered by the generator voltage and current. A major portion of the field current is provided through a passive, analog power excitation circuit, which maintains the generator output voltage to within +/—2%. A digital voltage regulator, shunted to the field winding, fine—tunes the generator output to +/— 0.5%.

A key feature of the excitation system is its ability to continue operating even if the AVR has been shutdown and automatically isolated after a failure. Additional key features of the system include:

  • Visual fault analysis and alarm records to aid operator response
  • Full trace recording and fault recording coupled with on board diagnostics
  • Simple structure of the passive power excitation circuit which limits the possibility of system failure
  • Expected reduction of maintenance and calibration efforts
  • Clear text display on cabinet front door for system status and alarms
  • Communication to remote locations possible

When work started in October 2002, significant challenges existed, including design of the system, manufacture and testing of a prototype, testing of the production units, and writing of the modification package and procedures. All of this work, including Institute of Nuclear Power Operations (INPO) grade training and installation, was completed in 18 months. One of the expected challenges involved plant engineers and technicians working with digital systems for the first time. However, the challenge was met with extra training to familiarize plant engineers with the new digital technology.

Partial view of the THYRIPART electronics and generator controls rack and rear side of instrument door. Photo Courtesy: Framatome ANP, Inc.
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Lessons Learned

A significant amount of work was needed in commercial grade dedication. This process would assure that system components to be installed at the plant would perform as well as the prototype. Although consideration was given to allowing credit for system testing, a more conservative route was chosen. Only after individual component tests were completed was the entire system tested.

Extra testing meant that a larger—than—expected amount of test plans, procedures and reports had to be written. Changes to nuclear power plant equipment require a great deal of documentation, so resource planning and scheduling took that into consideration.

Training for plant engineers and technicians is a significant part of a digital upgrade project, both in preparing plant staff for project planning and review and operation of the installed systems. INPO—accredited training requires following strict guidelines and regulations in the development and delivery of training programs and is more stringent and costly than standard vendor training.

Training equipment, INPO certified professional trainers and administrative processes were needed to provide this type of training. Framatome ANP provided its mobile training center, complete with classroom space and digital equipment to facilitate specialized hands—on instruction. The mobile training center allowed plant staff to stay on site and avoid travel expenses during the digital upgrade projects.

Proper planning and preparation insured that the installation of the Unit 2 turbine controls and main generator voltage regulator upgrades were accomplished during a record 25—day outage. During the installation and commissioning of Unit 2, the team initiated a condition report within the Comanche Peak system to capture lessons learned as the effort progressed. In preparation for Unit 1 installation and commissioning, the team used the lessons learned to ensure an even smoother effort. The result was dramatic. The Unit 1 upgrades were completed on a similar schedule, with less manpower and with no impact to the critical path.