SCADA (Supervisory Control and Data Acquisition) systems are extensively used to control geographically distributed processes, which are often scattered over thousands of kilometers and where centralized data acquisition and control is necessary to perform system operation.
Some SCADA applications include mining industries, water distribution system and wastewater collection systems, modern process and manufacturing industries, public and private utilities, electrical power grids, oil and gas pipelines and railway transportation systems.
What is SCADA System?
As the name defines, SCADA system provides supervisory control, monitoring and management of various industrial automation systems (such as manufacturing and process control automation systems) by acquiring and analyzing the data from remote devices.
It gathers the real-time data from various remote locations or plants, presents the data on various HMIs, records and logs the data on SCADA database management.
It provides the centralized monitoring and control system by integrating data acquisition and control systems with a telemetry system (in other words with data transmission systems) and HMI software.
SCADA systems are designed to gather field information such as sensor output, status of various machines and set limits of various process variables, and to transfer it to the central computer thorough wireless communication systems.
At the monitoring side, the received information is displayed graphically or textually to the operator, thereby it allows the operator to monitor, analyze and control an entire system from a central control location.
Depending on the control system implemented, the control of any individual system can be performed manually by the operator commands or can be automatic using various closed loop control strategies.
Generic scada system
SCADA system combines both hardware and software components. SCADA hardware includes MTU (master terminal unit) which is placed in a central location, communication equipment such as telephone line, radio, cable, or satellite, and one or more RTUs (remote terminal units) or PLCs which are placed at geographically distributed field sites.
These RTUs or PLCs are connected with various sensors and actuators and are responsible for gathering the data and controlling the field parameters. The Master Terminal Unit (MTU) collects and processes the data from RTU or PLC inputs and outputs, while the PLC or RTU controls field devices or a local process.
SCADA software performs the functionalities of a SCADA such as what and when to acquire and control, storing and accessing of acquired data, calculating parameters acceptable range (set limit checking), responding to parameter violations (beyond the range), providing HMI, reporting and accounting, generating alarms, etc.
There are different SCADA vendors, some of those include Siemens, ABB, Honeywell, Rockwell, Schneider Electric, Technomatix and Tibbo Systems.
Architecture of SCADA
A typical architecture of a SCADA system is shown in below figure which describes the general configuration and major components of the SCADA. In this, control area or master station house main server which acts as MTU and communication routers.
Also, the control area includes engineering workstations, HMI stations, data servers, and data historians, which are all connected by local area network (LAN). The master station is responsible for monitoring and controlling various remote stations.
Remote stations are equipped with one or more RTUs/ PLCs which performs local monitoring through sensors (such as voltage, current, temperature and pressure) and local controlling through actuators (such as pumps, relays, and valves). The field site devices are connected through WAN network to perform remote diagnostics.
The communication or transferring of information between field sites and control center is carried through proprietary communication protocols over serial communications using telemetry techniques such as cable, telephone line and RF.
Components of SCADA or Block Diagram of SCADA:
The major components of SCADA include
Master Terminal Unit (MTU)
It is the heart of the SCADA system, which can be a dedicated computer, a Programmable Logic Controller (PLC), or a network server that communicates with remote field side RTUs. It initiates all communication, collects the data, stores the data in database, provides interfaces to operators and sends the information to other systems.
It allows the users to perform controlling functions on field devices such as breakers, switches and other actuators depending on the gathered data. It continuously communicates with other devices in master station so as to facilitate data logging, alarm processing, trending and reporting, graphical interface and security system.
Remote Terminal Units (RTUs)
RTUs gathers the information from various field sites in which they are employed. Each RTU is connected with various sensors and actuators that manage local process or field equipments.
It collects the information from various sensors and sends the information to the MTU. Also, it receives the control commands from MTU and correspondingly controls the various actuators.
Many RTUs store the data in their database and waits for a request from the MTU to send or transmit the data. In sophisticated systems, PLCs are used as RTUs which directly transfers the field data and controls the parameters without a request from the MTU. It uses a local area network to communication with various field intelligent devices.
Communication Equipment/Network
It provides the link between RTUs (in the field) to MTU (in the control center). The communication can be wired or wireless or through internet which provides bidirectional and uninterrupted communication between RTU and MTU.
SCADA systems can be connected using various communication mediums including twisted pair cables, coaxial metal cables, fiber optic cables, satellites, high frequency radio, telephone lines, and microwave radio.
The topology of the SCADA system network depends on the type of system or application it is intended for. Mostly redundant topology is recommended for critical control applications.
SCADA Software
It is an important aspect of every SCADA system which presents the information to the user and also allows the user to intervene in the process control. Many SCADA systems use commercial proprietary software upon which SCADA system is developed.
This software comprises a computer operating system which controls the central host computer hardware, communication network management, graphical generation tool for HMI, database management and report generation tools.
Other components or auxiliary equipment in central station includes HMI station, which gives graphical representation of field parameters, alarm generators to inform normal and abnormal conditions of process and recorders to produce a permanent record of analog and discrete variables on the charts.
Functions of SCADA
1. Data Acquisition
In SCADA systems, MTU performs the periodic acquisition of data from RTUs. As discussed above that the RTU can respond in either a request form the MTU or continuously transferring the data when changes of state of a parameter takes place or when limits of the parameter exceeded, even without a request from the MTU.
The data acquisition process includes internal scanning of RTU internal database, periodic RTU polling by MTU, transmission of data by RTU to MTU, scaling of data into engineering units and updating a previous value or state in the database.
2. Human Machine Interface (HMI)
SCADA products display the information on multiple screens, which combines both text and synoptic diagrams. It provides the provision for human operators to continuously monitor the operations and to intervene when necessary.
SCADA HMI software consists of library of graphical symbols to which tag names are associated for a particular device or parameter such as ON/OFF status of switch, level information on tank, etc.).
Display selection on HMI is organized mostly in a tree structure, where index pages allow human operator to select various displays using a cursor, keyboard, trackball, or touch-screen positioning techniques.
3. Supervisory Control
It is the process of controlling the equipment operations from remote locations. In SCADA systems, the MTU in the master station sends the control instructions such as set points and discrete control commands to the RTU at remote station. At the remote stations, RTU receives the commands and accordingly controls the appropriate actuator.
The supervisory control includes selection of the remote station, choosing the device to be controlled and executing the desired command such as close or trip. Most of the systems employ check-before-operate method for correct selection and operation of the equipment in the remote place.
4. Trending
All SCADA products provide trending facilities which display the gathered (real-time) or saved (historian) data on various charts. The parameters to be trended on a specific chart can be defined online or it can be predefined.
These charts are able to display one or more parameter using one or more plots. It provides the automatic scrolling of data with enhanced zoom features. Historian trending is possible with archived databases.
5. Alarm Processing
It involves in alerting the operator to unscheduled events by informing place of occurrence, time of occurrence, device ID and nature of the event.
Alarms are logically programmed on the master control station by comparing the received data with appropriate limits. It is possible to handle alarms on multiple priority level. Alarms can be suppressed either by individual or as a complete group.
6. Information Storage and Reports
SCADA stores the gathered data on either disks or permanent storage devices. The logging of data is performed on a cyclic basis, which means the time span of a rotating historical file is limited (which can be 40 days or 12 months).
Once the period is completed or the log is full, it archives the data to permanent storage device and then the information older than the file time span is discarded. This allows the user to retrieve and analyze the data whenever it is needed.
SCADA provides the report generation using SQL type queries. The historical file provides the source of information for generating various reports. SCADA also facilitates to print and archive reports.
Applications of SCADA
The characteristics such as flexibility, reliability and scalability of SCADA have made its extensive use in automating complex systems.
There are numerous real-world applications where SCADA already been successful in delivering monitoring and control solutions over a wide range of industries, ranging from energy production to agricultural systems. Some of these applications are given below.
Waste Water Treatment
The waste water treatment involves in filtering the raw water from wells or surface and to discharge the clean water into the distribution system. The processing areas in the water treatment process include low lift or raw water station, pre-treatment, filtration, high lift or treated water station and chemical injection systems.
Here, each stage is employed with PLCs as RTUs along with sensors and actuators for field level monitoring and control of operations. SCADA system enables the central monitoring thorough HMI workstations by gathering data via communication network from various stages of water treatment.
Electrical Power Distribution System
SCADA system automates the electrical distribution tasks with the use of intelligent electronic devices (IEDs) as RTUs. SCADA supervises the entire distribution system so as to reduce the duration of outages and for an optimum operation. The functions of SCADA in power distribution system include substation control, feeder control and end user load control.
RTUs/ IEDs in a substation, feeder, or end use installations gather the field data, including status of switches, transformers, circuit breakers, fault information, energy consumption and billing.
At the central monitoring side, MTU receives the data from RTUs, displays it on HMIs, provides the data trending and logging. It also maintains the desired level of currents, voltages, and power factor and correspondingly generates the alarms and sends back control signals to remote stations.
Power Generating Stations
In thermal power plants, various equipments are distributed over plant area, which all are connected with SCADA systems. The SCADA enables monitoring and control of numerous boilers, turbines and pumps.
SCADA automates and supervises the operations of the plant, including pulverizing of coal, control of steam flow, turbine start/stop status, water control into the boiler, power distribution and control.
In case of wind power plants, SCADA provides the real time visibility of the remote plant and facilitates the control of plant remotely and centrally. In this, SCADA controls the wind turbines during start-up, power production, shutdown and stopped (faulty) cases. Also, it provides online data, histories, alarms and the HMI.
Industrial Control
SCADA usage is often found in many industrial applications including manufacturing industries (steel plants), process industries, oil and gas industries, food processing, and so on.
SCADA integration into the industries processes results higher production rates, increases quality of products and provides the cost-effective operation. In these industries PLCs plays a major role in dealing with field parameters and to transfer them to the central controlling station.
