SCAD system (overview)

SCADA SYSTEM
Overview
ENG. ALI F. AL SARRAF
ENG. ALI F. AL SARRAF E-MAIL: ALI.ALSARRAF@OUTLOOK.COM 1

What is SCADA ?
SCADA (supervisory control and data acquisition) is a system operating with coded signals over
communication channels so as to provide control of remote equipment (using typically one
communication channel per remote station).

Overview of SCADA
SCADA systems are highly distributed systems used to control geographically dispersed assets,
often scattered over thousands of square kilometers, where centralized data acquisition and
control are critical to system operation. They are used in distribution systems such as water
distribution and wastewater collection systems, oil and gas pipelines, electrical power grids, and
railway transportation systems. A SCADA control center performs centralized monitoring and
control for field sites over long-distance communications networks, including monitoring alarms
and processing status data. Based on information received from remote stations, automated or
operator-driven supervisory commands can be pushed to remote station control devices, which
are often referred to as field devices. Field devices control local operations such as opening and
closing valves and breakers, collecting data from sensor systems, and monitoring the local
environment for alarm conditions.

Overview Of DCS
DCSs are used to control industrial processes such as electric power generation, oil and gas
refineries, water and wastewater treatment, and chemical, food, and automotive production.
DCSs are integrated as a control architecture containing a supervisory level of control overseeing
multiple, integrated sub-systems that are responsible for controlling the details of a localized
process. Product and process control are usually achieved by deploying feed back or feed
forward control loops whereby key product and/or process conditions are automatically
maintained around a desired set point. To accomplish the desired product and/or process
tolerance around a specified set point, specific programmable controllers (PLC) are employed in
the field and proportional, integral, and/or differential settings on the PLC are tuned to provide
the desired tolerance as well as the rate of self-correction during process upsets. DCSs are used
extensively in process-based industries.

Overview of PLC
PLCs are computer-based solid-state devices that control industrial equipment and processes.
While PLCs are control system components used throughout SCADA and DCS systems, they are
often the primary components in smaller control system configurations used to provide
regulatory control of discrete processes such as automobile assembly lines and power plant soot
blower controls. PLCs are used extensively in almost all industrial processes.

Differences between SCADA and
DCS,PLCs
SCADA DCS and PLC
geographically dispersed field sites. sub-systems are usually located within a more
confined factory or plant-centric area
long-distance communication systems communications are usually performed using local
area network (LAN) technologies
designed to handle long-distance communication
challenges such as delays and data loss posed by
the various communication media used
usually employ greater degrees of closed loop
control than SCADA systems

ICS Operation
 Control Loop. A control loop consists of sensors for measurement, controller hardware such as
PLCs, actuators such as control valves, breakers, switches and motors, and the communication of
variables. Controlled variables are transmitted to the controller from the sensors. The controller
interprets the signals and generates corresponding manipulated variables, based on set points,
which it transmits to the actuators. Process changes from disturbances result in new sensor
signals, identifying the state of the process, to again be transmitted to the controller.
 Human-Machine Interface (HMI). Operators and engineers use HMIs to configure set points,
control algorithms, and adjust and establish parameters in the controller. The HMI also displays
process status information and historical information.

ICS Operation (cont.)
 Remote Diagnostics and Maintenance Utilities. Diagnostics and maintenance utilities are used
to prevent, identify and recover from failures.
 A typical ICS contains a proliferation of control loops, HMIs, and remote diagnostics and
maintenance tools built using an array of network protocols on layered network architectures.
Sometimes these control loops are nested and/or cascading –whereby the set point for one loop
is based on the process variable determined by another loop. Supervisory-level loops and lower-
level loops operate continuously over the duration of a process with cycle times ranging on the
order of milliseconds to minutes .

Control Components
 Control Server. The control server hosts the DCS or PLC supervisory control software that is
designed to communicate with lower-level control devices. The control server accesses
subordinate control modules over an ICS network.
 SCADA Server or Master Terminal Unit (MTU). The SCADA Server is the device that acts as the
master in a SCADA system. Remote terminal units and PLC devices (as described below) located
at remote field sites usually act as slaves.
 Remote Terminal Unit (RTU). The RTU, also called a remote telemetry unit, is special purpose
data acquisition and control unit designed to support SCADA remote stations. RTUs are field
devices often equipped with wireless radio interfaces to support remote situations where wire-
based communications are unavailable. Sometimes PLCs are implemented as field devices to
serve as RTUs; in this case, the PLC is often referred to as an RTU.

Control Components (Cont.)
 Programmable Logic Controller (PLC). The PLC is a small industrial computer originally
designed to perform the logic functions executed by electrical hardware (relays, drum switches,
and mechanical timer/counters). PLCs have evolved into controllers with the capability of
controlling complex processes, and they are used substantially in SCADA systems and DCSs.
Other controllers used at the field level are process controllers and RTUs; they provide the same
control as PLCs but are designed for specific control applications. In SCADA environments, PLCs
are often used as field devices because they are more economical, versatile, flexible, and
configurable than special-purpose RTUs.
 Intelligent Electronic Devices (IED). An IED is a “smart” sensor/actuator containing the
intelligence required to acquire data, communicate to other devices, and perform local
processing and control. An IED could combine an analog input sensor, analog output, low-level
control capabilities, a communication system, and program memory in one device. The use of
IEDs in SCADA and DCS systems allows for automatic control at the local level.

 Human-Machine Interface (HMI). The HMI is software and hardware that allows human
operators to monitor the state of a process under control, modify control settings to change the
control objective, and manually override automatic control operations in the event of an
emergency. The HMI also allows a control engineer or operator to configure set points or control
algorithms and parameters in the controller. The HMI also displays process status information,
historical information, reports, and other information to operators, administrators, managers,
business partners, and other authorized users. The location, platform, and interface may vary a
great deal. For example, an HMI could be a dedicated platform in the control center, a laptop on
a wireless LAN, or a browser on any system connected to the Internet.
 Data Historian. The data historian is a centralized database for logging all process information
within an ICS. Information stored in this database can be accessed to support various analyses,
from statistical process control to enterprise level planning.

 Input/Output (IO) Server. The IO server is a control component responsible for collecting,
buffering and providing access to process information from control sub-components such as
PLCs, RTUs and IEDs. An IO server can reside on the control server or on a separate computer
platform. IO servers are also used for interfacing third-party control components, such as an HMI
and a control server.

Network Components
 Fieldbus Network. The fieldbus network links sensors and other devices to a PLC or other
controller. Use of fieldbus technologies eliminates the need for point-to-point wiring between
the controller and each device. The sensors communicate with the fieldbus controller using a
specific protocol. The messages sent between the sensors and the controller uniquely identify
each of the sensors.
 Control Network. The control network connects the supervisory control level to lower-level
control modules.
 Communications Routers. A router is a communications device that transfers messages
between two networks. Common uses for routers include connecting a LAN to a WAN, and
connecting MTUs and RTUs to a long-distance network medium for SCADA communication.

Network Components (cont.)
 Firewall. A firewall protects devices on a network by monitoring and controlling
communication packets using predefined filtering policies. Firewalls are also useful in managing
ICS network segregation strategies.
 Modems. A modem is a device used to convert between serial digital data and a signal
suitable for transmission over a telephone line to allow devices to communicate. Modems are
often used in SCADA systems to enable long-distance serial communications between MTUs and
remote field devices. They are also used in both SCADA systems, DCSs and PLCs for gaining
remote access for operational functions such as entering command or modifying parameters,
and diagnostic purposes.
 Remote Access Points. Remote access points are distinct devices, areas and locations of a
control network for remotely configuring control systems and accessing process data. Examples
include using a personal digital assistant (PDA) to access data over a LAN through a wireless
access point, and using a laptop and modem connection to remotely access an ICS system.

SCADA SYSTEM
SCADA systems are used to control dispersed assets where centralized data acquisition is as
important as control. These systems are used in distribution systems such as water distribution
and wastewater collection systems, oil and gas pipelines, electrical utility transmission and
distribution systems, and rail and other public transportation systems. SCADA systems integrate
data acquisition systems with data transmission systems and HMI software to provide a
centralized monitoring and control system for numerous process inputs and outputs. SCADA
systems are designed to collect field information, transfer it to a central computer facility, and
display the information to the operator graphically or textually, thereby allowing the operator to
monitor or control an entire system from a central location in real time. Based on the
sophistication and setup of the individual system, control of any individual system, operation, or
task can be automatic, or it can be performed by operator commands.

SCADA Systems components
 SCADA systems consist of both hardware and software:
 Hardware
MTU placed at a control center .
 Communications equipment (e.g., radio, telephone line, cable, or satellite) .
one or more geographically distributed field sites consisting of either an RTU or a PLC .
 The software is programmed to tell the system what and when to monitor, what parameter ranges are
acceptable, and what response to initiate when parameters go outside acceptable values.

General configuration of a SCADA system
 The control center houses a control server (MTU) and the communications routers.
 Other control center components include the HMI, engineering workstations, and the data
historian, which are all connected by a LAN.
 The control center collects and logs information gathered by the field sites, displays
information to the HMI, and may generate actions based upon detected events.
 The field site performs local control of actuators and monitors sensors.
 Field sites are often equipped with a remote access capability to allow field operators to
perform remote diagnostics and repairs usually over a separate dial up or WAN connection.
 The control center is also responsible for centralized alarming, trend analyses, and reporting.

 Standard and proprietary communication protocols running over serial communications are
used to transport information between the control center and field sites using telemetry
techniques such as telephone line, cable, fiber, and radio frequency such as broadcast,
microwave and satellite.

Basic SCADA Communication Topologies
 MTU-RTU communication architectures vary among implementations. The various architectures
used, including point-to-point, series, series-star, and multi-drop.
 Point-to-point is functionally the simplest type; however, it is expensive because of the individual
channels needed for each connection.
 In a series configuration, the number of channels used is reduced; however, channel sharing has an
impact on the efficiency and complexity of SCADA operations.
 Similarly, the series-star and multi-drop configurations’ use of one channel per device results in
decreased efficiency and increased system complexity.
The four basic architectures shown in Figure 2-3 can be further augmented using dedicated
communication devices to manage communication exchange as well as message switching and
buffering.
Large SCADA systems, containing hundreds of RTUs, often employ sub-MTUs to alleviate the burden
on the primary MTU. This type of topology is shown in Figure 2-4.

BasicSCADA
CommunicationTopologies

LargeSCADA
CommunicationTopologies

SCAD system (overview)

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