Functional view on I&C

To keep a plant parameter within design limits, accurate and reliable information about the parameter is needed. This information is provided by measurements using sensors. Depending on the type of parameter, e.g., temperature, pressure, flow rate, level, and on the requirements and constraints, e.g., accuracy, response time, and environmental conditions, a broad variety of sensors may be used. The measurement of the plant parameter is compared with the design set point and, based on the deviation from this set point, corrective actions are taken by controlling suitable actuators.

HUMAN-SYSTEM INTERFACE

ACTUATORS MEASUREMENTS

SETPOINTS LIMITS CONTROL

PROCESS VALUES

ENVIRONMENT NUCLEAR POWER PLANT

I&C

POWER GENERATION (HEAT TRANSPORT)

MANUAL CONTROLS

AUTOMATIC CONTROLS

PLANT INFORMATION

FIG. 1. High level overview of I&C main functions.

Figure 2 shows the block diagram of an I&C function. The value of a plant parameter is measured by means of a sensor. Signals from sensors may vary significantly depending on the kind of process parameters and the type of selected sensors. To process sensor signals in a uniform way it is necessary to normalize sensor signals. This is done in the signal conditioning and data acquisition block. After conditioning, signals can be processed in a uniform way. Signal processing depends strongly on the plant parameter being controlled and on the underlying plant process. Signal processing may involve, for example, scaling, linearization, or filtering of the measurement and the calculation of the deviation between the pant parameter and the designed set point. The result of the signal processing is used to control an actuator.

Figure 3 shows an I&C function from the physical point of view. Analog and digital I&C systems are distinguished by the way in which signal processing and actuator control is performed. Analog I&C systems use analog voltages or currents and analog electronics to process the signals and to control the actuator. Digital I&C systems do the signal processing and the actuator control by means of computer processors, using a binary representation of the measured and controlled parameters. From the functional point of view both solutions are similar but from the physical point of view, the differences are significant.

Sensor

Signal conditioning Data acquisition

Actuator Signal

processing

Component control

Human System Interface

FIG. 2. Block diagram of a typical I&C function.

Sensor Controller Actuator

Digital controller ^A

D Processor

D A

+ -

Setpoint Analog controller

Setpoint

Block diagram

Legend:

A/D: Analog to Digital Converter D/A: Digital to

Analog Converter

FIG. 3. Analog versus digital I&C systems.

Figure 4 shows an elementary I&C function in the context of the related process elements. The water level in the tank is controlled by changing inlet flow into the tank. In this simple example the water level is measured by a level sensor and the deviation between the measured level and the designed set point is calculated by the closed loop controller. The actuator of this simple I&C function is a valve in the inlet pipe. If the deviation is negative (level is high) the valve will be closed by the controller so that the inlet flow will decrease. In the opposite direction, if the deviation is positive the valve will be opened so that the inlet flow will increase. If the controller is well designed the water level in the tank will always be kept very close to the designed set point.

Figure 5 shows a simplified functional overview of the I&C in a NPP.

To ensure a safe and reliable plant operation under all plant conditions, I&C systems have to monitor and control hundreds or thousands of plant parameters. Thus, nuclear power plant I&C systems are complex.

Subdividing the plant I&C according to its functions facilitates understanding of the entire system.

The most common way to subdivide I&C by functional groups is the following:

— Sensors: to interface with the physical processes within a plant and to continuously take measurements of plant variables such as neutron flux, temperature, pressure, flow, etc.;

— Operational control, regulation and monitoring systems: to process measurement data, to manage plant operation, and to optimize plant performance. Surveillance and diagnostic systems that monitor sensor signals for abnormalities are important parts of operational monitoring systems;

M

Closed loop controller

FIG. 4. Level control: An elementary I&C function.

NPP processes to be monitored and controlled M

Safety functions

• Reactivity control

• Heat removal

• Prevention of radiation release

Operational control

• Reactor power control

• Turbine generator control

• Water steam cycle control

• Surveillance and diagnostics Human System Interface

• Information

• Alarm

• Manual control Operators

Redundant Trains

A=actuation M=measurement

A M A

FIG. 5. Functional overview of NPP I&C.

— Safety systems: to keep the plant in a safe operating envelope in case of any postulated initiating event (design basis accident);

— Communication systems: to provide data and information transfer through wires, fibre optics, wireless networks or digital data protocols;

— Human-system interfaces (HSIs): to provide information to and interaction with plant operating personnel;

— Actuators (e.g., valves and motors): to adjust the plant’s physical processes.

In control rooms and at control panels in the field the I&C systems and the plant operators meet at the human-system interface.

At a fundamentally lower level, the functionality that is embodied in the I&C system architecture can be decomposed into several elements such as:

The data acquisition functionality addresses all signals from the control, safety and monitoring systems, while the actuator activation functionality is limited to the control and safety systems. The validation functionality addresses signals, commands, and system performance. The arbitration functionality addresses redundant inputs or outputs, commands from redundant or diverse controllers, and status indicators from various monitoring and diagnostic modules. The control functionality includes direct plant or system control and supervisory control of the NPP control system itself. The limitation functionality involves maintaining plant conditions within an acceptable boundary and inhibiting control system actions when necessary. The checking functionality can address computational results, input and output consistency, and plant/system response. The monitoring functionality includes status, response, and condition or health of the control system, components, and the plant, and it provides diagnostic and prognostic information. The command functionality is directed toward configuration and action of control loops and diagnostic modules. The prediction functionality addresses identification of plant/system state, expected response to prospective actions, remaining useful life of components, and incipient operational events or failures. The communication functionality involves control and measurement signals to and from the field devices, information and commands within the control system, and status and interactions between the NPP automatic control system and operators. The fault management and configuration management functionalities are interrelated and depend on two principal design characteristics. These are the ability of the designer to anticipate a full range of faults and the degree of autonomy enabled by the control system design. Not all of these functionalities are present in the I&C system architecture at every NPP.