Documenting Control System Functionality for Digital Control Implementations

Abstract

In past CANDU designs, plant control was accomplished by a combination of digital control computers, analogue controllers, and hardwired relay logic. Functionality for these various control systems, each using different hardware, was documented in varied formats such as text based program specifications, relay logic diagrams, and other various specification documents. The choice of formats was influenced by the hardware used and often required different specialized skills for different applications. The programmable electronic systems in new CANDU designs are realized in a manner consistent with latest international standards (e.g., the IEC 61513 standard [1]). New CANDU designs make extensive use of modern digital control technology, with the benefit that functionality can be implemented on a limited number of control platforms, reducing development and maintenance cost. This approach can take advantage of tools that allow the plant control system functional and performance requirements to be documented using graphical representations. Modern graphical methods supplemented by information databases can be used to provide a clear and comprehensive set of requirements for software and system development. Overview diagrams of system functionality provide a common understanding of the system boundaries and interfaces. Important requirements are readily traced through the development process. This improved reviewability helps to ensure consistency with the safety and and production design requirements of the system. Encapsulation of commonly used functions into custom-defined function blocks, such as typical motor control centre interfaces, process interlocks, median selects etc, eases the burden on designers to understand and analyze the detailed functionality of each instance of use of this logic. A library of encapsulated functions will be established for complex functions that are reused in the control logic development. By encapsulation and standardisation of such complex functions, the time required for development and verification activities can be reduced significantly. The library functions can be pre-validated and re-used as trusted components. This reduces development time and minimizes errors. Maintainability is also improved.