February 1, 2010

The Advanced Digital Fieldbus Option for Nuclear Plants

Jack Y. Zhao, U.S. Nuclear Regulatory Commission

Digital Fieldbus Technologies and Their Benefits

Digital fieldbus technology can be defined as a communication protocol that allows two-way digital communications among various control devices such as those used in control room equipment. Many types of fieldbus architectures are available for use, including FF, Profibus, ControlNet, DeviceNet, AS-i, World-FIP, and Interbus. Among the available fieldbus technologies, FF and Profibus have been widely acknowledged, accepted, and used because of their simplicity, flexibility, and international and interoperable standards.

Compared with conventional control systems (CCSs) that use point-to-point hardwired technology, a fully digital control system (DCS) using FF or Profibus offers the following lifecycle benefits.

Reduced Costs. The following five factors illustrate why digital fieldbus technology reduces costs significantly — although the required terminators, connectors, and power conditioners may offset the savings to some extent:

• Because digital fieldbus technology allows multiple devices to use a single wire pair, less wire and conduit, fewer cable trays, and smaller control cabinets are needed.

• Standard function blocks, which are used in digital fieldbus technology to implement control strategies, reduce the amount of system hardware needed, including input/output (I/O) converters, power supplies, and cabinets.

• Multivariable digital fieldbus transmitters with close-coupled mounting eliminate the need for temperature transmitters.

• Power loads on control room equipment and the size of control cabinets are reduced because of the reduction in the number of I/O cards that are required.

• Reduced hardware simplifies start-up and parameterization operations and reduces installation time, commissioning time, and construction costs.

Improved System Operation. Digital fieldbus technology allows multiple variables from each device to be brought into the control system for remote diagnostics, archival activities, trend analysis, process optimization studies, report generation, predictive maintenance, remote configuration, and asset management.

Decreased Risks of System Failure and Increased Operating Reliability. The self-test and communication capabilities of microprocessor-based digital fieldbus devices help reduce downtime and improve plant safety. Plant operations and maintenance personnel can be notified upon detection of abnormal conditions or the need for preventive maintenance, allowing them to initiate corrective action quickly and safely.

Enhanced Plant Performance and Asset Management. Digital fieldbus technology enables asset management functions such as diagnostics, calibration, identification, and other maintenance management operations to "mine" massive amounts of information from field devices in real time.

Increased Flexibility and Interoperability. Because digital fieldbus technology has an open protocol, different subsystems from different suppliers in a nuclear power plant can be easily integrated, and information can be accessed without the need for custom programming.