SCADA for beginners : An Introduction
What Is It
SCADA for beginners is an introductory course to students who are learning from the scratch. SCADA is acronym for Supervisory Control and Data Acquisition. It is a distributed measurement and process control system for industrial automation software application program. It is also used to gather real time data from remote locations for exercising this control on equipments and conditions. SCADA has applications in automated operations like chemical manufacturing and transport, supply systems and power generation.
The SCADA System consists of hardware and software components. The hardware collects and feeds data into a computer with SCADA software installed. The data is then processed by the computer before presenting it in a timely manner. The function of SCADA is recording and logging all events in a file that is stored in a hard disk or sending them to a printer. If conditions become hazardous, SCADA sounds warning alarm.
SCADA finds use in power plants, oil and gas refining, transportation, telecommunication, various treatment plants and in control of waste and water. Typical use of SCADA system is data collection and control at the supervisory level. Data acquisition without control is not SCADA.
The processes are in the domain of industrial, infrastructural and facilities. Industrial processes like manufacturing, fabrication, refining, etc. may be in batch or continuous, discrete or repetitive. The processes used in infrastructure like power transmission, communication, water treatment, etc. may be either public or private. Public or private facility processes involve monitoring and controlling of access and managing energy consumption in airports, buildings, space station, etc.
SCADA system is basically measurement and control system. The control after data collection is done at the supervisory level. The measurement and control is done in a large scale in any size or geographical distribution. SCADA is not a real time control but is placed in addition to real time control for controlling processes external to SCADA. A real time control like control of power consumption by a computer itself is integrated and SCADA in spite of collecting real time data from remote locations is not a real time control system in that sense.
SCADA: Systems Concept
Input/output signal hardware, network, human machine interface, controllers, database, communication and software constitute a SCADA system. SCADA comes under the scope of Instrumentation Engineering.
There are real time automated and integrated control systems like its cooling by the computer itself for quick responding to the process changes within the processes’ own time-frame. But SCADA is not critical to controlling real time process.
A system or a complete site is usually spread over a long geographical distance. SCADA is the central system for control and monitor of such a site or system. A Programmable Logic Controller (PLC) or a Remote Terminal Unit (RTU) automatically performs the main site control process. The host control functions in most cases are limited to the capability of supervisory level or site override. For example, in an industrial process, flow of cooling water through a part of the process may be controlled by PLC but the control set point for the water flow can be allowed to be changed by an operator by the SCADA system. Also any alarm condition like high temperature or flow loss may be recorded and displayed by SCADA. Through PLC the feedback control loop is closed but the total performance of the loop is monitored by SCADA. In this example of control of set point for water flow, the measured flow/level of water is read by the SCADA system before sending the set points to the two PLCs. The PLC for flow after comparing the measured flow to the set point sets the sped pump to match the flow with the set point. The other PLC for level after comparing the measured level to the set point controls the flow through a valve in a way as to match level with the set point. This is how the SCADA system controls and monitors the overall system without performing the main site control process that is done through PLC.
SCADA: Data Acquisition
PLC/RTU Level Data
The PLC/RTU performs the data acquisition like metre readings and equipment status which they communicate to the SCADA as per requirement. SCADA compiles and formats this data in a way that makes possible for a control room operator to make appropriate supervisory decision using the Human Machine Interface (HMI) to adjust or even change PLC/RTU controls. Often for studying trends or other analytical works, a commodity specific database management is developed from which data are collected.
A tag database is a distributed database typically implemented by SCADA systems. The data elements contained in such database are called tags or points. The value of a single input/output represented by a point is monitored and controlled by the SCADA system. There are two types of points, viz. hard and soft. An actual input/output connected to the system is represented by a hard point. Applications of the result of logical and mathematical operations to other hard and soft points are represented by a soft point. However in most actual implementations, this distinction between the two points are removed and each property is made conceptually a soft point that in the simplest case can be made equal to a single hard point. The storage of point values are in the form of value-timestamp combinations representing the value and the time when the value was recorded or calculated. History of a point is represented by a series of value-timestamp combination of that point. Additional tag data like PLC Register, path to field device, comments on design time, alarm information, etc. are also commonly stored.
Purchase of DCS (Distributed Control System) or SCADA can be made from a single vendor. However assembling of SCADA system from hardware/software component is more common. There are many suppliers in the market for these components. The SCADA system requires HMI packages also which have to be resourced from different suppliers.
SCADA: Human Machine Interface (HMI)
What Is HMI
Human operators control the process. It is through the HMI apparatus that the human operators receive the processed data to control the process.
A need was felt for standardizing a way for control and monitor of many remote controllers including control devices like PLC. This need in essence gave rise to HMI industry. Automatic pre-programmed control over a process is provided by a PLC. However it is hard to gather data manually from them because of their usual distribution across a plant. There was never any standardized way that PLC could present information to an operator. PLC and other controllers provide the SCADA system with the information through some form of network. In SCADA this information is combined and formatted. Linking an HMI to a database serves various purposes. Such database apart from giving diagnostic and trend-indicating data also provides management information like procedures for schedule maintenance, sensor or machine specific detailed schematics and logistic information. Troubleshooting guides and expertise are also provided to the system.
An integrated HMI/SCADA System uses protocols for communication that are open and non-proprietary. Now almost for a decade, majority of the PLC manufacturers have been offering this kind of integrated HMI/SCADA System. A large number of specialized and third party HMI packages are now available in the market. In such packages, there are in-built compatibilities with most of the major PLCs. This facilitates the technicians and electrical or mechanical engineers to configure HMI by themselves and without the necessity for a customized program developed by a software engineer. The HMI/SCADA package because of its dependability and compatibility has become very popular. It finds wide use ranging from small applications like temperature control in a room to such large applications as controlling the process in a nuclear power plant. It is one of the best ways to control and monitor remotely located data with large geographic distribution.
SCADA: Remote Terminal Units
Distributed Control System (DCS) components normally constitute a SCADA system. Remote Terminal Units or RTUs are one of the major components of a SCADA System. It can automatically execute a simple logical process without involvement of a master computer and are being increasingly used. IEC61131-3, a functional programming language is commonly used for creating programs to run on these RTUs. IEC61131-3 not being a procedural language like FORTRAN or C Programming requires minimum training. This facilitates performance of both design and implementation of a program to be executed on a RTU by the SCADA system engineers.
Functions Of RTU
The functions of RTU in a SCADA setup is multifarious. RTU is the link or connection to the physical instrument and from a switch or valve, it reads open and close status data. Apart from status data, it also reads pressure, flow, voltage and current measurements. It performs equipment control such as closing and opening of switch and valve or setting the pump speed. Analog measurement data or digital status data can also be read by RTU. After reading it can send analog set points or digital commands.
Alarm is one of the most important aspects of a SCADA System. Alarm or the digital status point indicates a value which is either normal or alarm. Alarms are so created that they can be activated only when the condition for their requirement is reached. Fuel tank empty light indicator or alarm in a car is one such example. The alarm draws the attention of the SCADA operator to that part of the system which requires immediate adjustment or even change if necessary. Often in a SCADA system, alarm activation alert managers accompany the sent emails and text messages. The real time alarm and data provide operators with the information needed to respond quickly and adjust or change processes for effective control.
SCADA: Master Station
What Is It
Master Station is one of the three components of a SCADA system. The term in a small system refers to a single computer responsible for communicating with the field equipments. In a large SCADA system, master station consists of multiple servers, distributed software applications and disaster recovery sites. Master station server and software communicate with, apart from the field equipments like RTU/PLC, HMI software running in the workstation of control room or at other place.
Presenting The Information
Information to the operating personnel is generally presented graphically by the SCADA system. This graphic representation is known as a mimic diagram. This enables the operator to have a schematic representation of the controlled plant. For example, it shows a graphic of a pump connected to a pipe from which the operator can see both running of the pipe as well as the volume of fluid being pumped through the pipe at the given moment. The operator can switch off the pump depending on the need. The flow rate of fluid in the pipe decreases over real time as shown by the HMI software. Process elements are usually represented by schematic symbols and line graphics of mimic diagrams. The process equipment is represented in animated symbols by digital photograph.
There is a drawing program in a typical SCADA system with HMI package. The operators or system maintenance people use this drawing program to make changes in the way of representation of these points in the interface. An on-screen traffic light representing actual traffic light state in the field is an example of a simple representation. A multi-projector display to represent in a skyscraper position of all the elevators is an example of a complex representation. Platforms like LINUX were not much used earlier on account of its dynamic development environment. However now all major operating systems like LINUX, UNIX and OPENVMS are used for master station servers and also HMI workstations.
SCADA: Communication Infrastructure
A SCADA System Component
Communication infrastructure is one of the components of a SCADA system. For meeting requirements of communication, combined radio and direct serial or other modern connections have been used by SCADA systems. Railways and Power Stations which are large sites very often prefer use of Ethernet or IP over Synchronous Optical Networking (SONET). The term Telemetry is used when a SCADA system performs function of remote management and monitoring.
SCADA Communication Protocols
While the earlier low bandwidth protocols are still persisting, customers now mostly want SCADA data to transmit through preset corporate network and sharing of the network with other applications. Protocol designs in SCADA are compact and are so designed as to send information to master station only in case the RTU is surveyed for information by the master station. Modbus, RP570 and Conitel are the traditional vendor specific SCADA communication protocols. Standard communication protocols include IEC61850, DNP3, Profibus and IEC60870-5-101 or 104. All major SCADA vendors recognize these protocols. Communication protocols with extensions can operate in internet protocol TCP/IP. Modbus TCP/IP has now become standard for lot of hardware manufacturers and is a widely accepted communication protocol. Although for safety and security of SCADA system, it is advisable not to connect it to internet and expose it to risk, Ethernet TCP/IP has found its way into industrial automation breaking the barriers in majority of SCADA/HMI markets.
SCADA Control Protocols
Before evolution of standards for interoperability in industry, a lot of automatic control devices like RTU were already developed. This resulted in proliferation of control protocols by various developers. Besides to keep their customers hooked, the vendors made their own protocols. Some communication between devices was originally not designed to be part of industrial network. Now there are solutions gaining wide acceptance for interconnecting diverse hardware and software permitting communication even between devices not part of original network.
SCADA: Operational Philosophy & Trends
Present day RTUs are required to perform safety related tasks on their own to control tunnel fire or other such risks. This has to be independent of operator participation or master station automation. Detailed analyses of historic data as also particular industry-related analyses are required to be performed by master station software before presenting to the operator. Because of application of stringent safety requirements in the system, the safety standards for markets have to be met by the master station software.
Risk Of Failure
If the control system fails in some installation, the resultant cost including possible loss of life would be very high. The SCADA system hardware is generally made much toughened for coping with varied temperature, voltage fluctuation and vibration. But reliability in such installations can be further enhanced if standby hardware and communication channels are provided. Such backup ensures that the backup hardware can urgently identify and replace a part that has failed and can take up automatically its function without impeding the process. Mean time between failures calculated statistically indicates reliability of the system and often this time is very high highlighting risk.
Now the preference is for matching combinations of HMI/SCADA and PLC software. Mix-match is the current trend. A manufacturer of a typical DAQ/IO in the mid nineties had their own proprietary communication protocol on offer like RS-485 that could carry suitable distance. The late nineties saw continuation of the tilt in the direction of open communication. There were on offer by I/O manufacturers open message structure support in the form of Modicon Modbus replacing RS-485. By the turn of the millennium, open interfacing like Modicon Modbus on TCP/IP was being offered by most I/O makers. With rising industrial automation, factors like protocol selection, synchronization, suitable environment, etc. came to assume important role and industrial automation saw entry of Ethernet TCP/IP.
SCADA: Security Issues
SCADA based systems may be highly vulnerable. Its security has come under scrutiny due to increasing attacks from cyber-terrorism/warfare to which it has become a prey.
The issues addressed by the researchers on security of SCADA from such cyber terrorism dwells on some major concerns enumerated below:
- Little concern is shown towards authentication/security of the SCADA networks vis-à-vis their design, operation and deployment.
- There is often a mistaken notion that use of specialized interfaces and proprietary protocol provides an obscurity to SCADA system and thus gives it benefit of security.
- It is often wrongly perceived that because of some notional physical security, SCADA networks are fully secure.
- It is also wrongly believed that due to its non-linkage with internet, SCADA is supposed to be secure.
Consequences & Prevention
SCADA systems are usually intended for handling critical processes and have a mission to achieve. Therefore in the event of any cyber-terrorism attack on this system being successful, consequence could be as worse as loss or actual physical destruction of data leading to stupendous financial loss, theft or wrong use of processes and facilities and even direct or indirect loss of life. It remains a matter of speculation if such consequences can cause enough concern for industries to avoid use of SCADA platform which is mission-intensive in its applications and they change it to opt for some more secure architecture and configuration. However such thinking at this point of time may be a little far-fetched considering the belief in many decision-making people in the world of corporate and governments that comparatively low initial cost and advantages and benefits of SCADA system still outweigh the potential risk and associated cost if inflicted with cyber terrorism. Some security vendors in response to these risks are already on the job of developing specialized industrial solutions like firewall and VPN for SCADA networks based on TCP/IP.
Simulation Of SCADA
Simple simulation of a SCADA system with Java Applet can be used for controlling a small water supply system. In the system there are water wells or bores that pump water to a storage tank. In case the storage tank is large for supplying to a town using its gravity, a transfer pump station pumps water to it. For still larger town, systems with more zones are required but the principle remains the same as for a small town. Only in case of very large cities, the water supply system is divided into many zones but still working with same guiding principle.
There is a menu bar at the bottom of the applet with options “start”, “stop”, “inhibit”, and “summer”, “autumn”, “spring” and “winter”. The use of first two options is obvious. The “inhibit” option is clicked to slow down operation of all bores and pumps when the electricity supply goes through peak tariff periods. The water supply system in this mode can be operated quite safely during winter, autumn and spring. But use of inhibit mode in summer would result in the town running out of water supply. The times when the pumps are inhibited are indicated by a display during the running of the simulation. To select between different demands situations according to seasons, the last four options as mentioned above are used. This Java simulation applet is programmed for a screen size of at least 800×600. The applet needs quite an updated version of Netscape (at least 4.07) or IE4. JDK 1.1.6 is also used in this application.
The level in the storage tank controls the two “banks” the bores are organized into. With falling tank level, bank 1 starts functioning. Further fall drives bank 2 to commence work. The storage tank is provided with a display named “Tank RTU” that indicates the time of calling bank 1 or 2 by the RTU. Finer control strategies than this simulation is adopted in real situations.
SCADA: In Electricity Supply & Distribution System
SCADA Software Package
It is a graphical package that can use Windows NT Operation System. The program provided is intended to incorporate actual AutoCAD campus diagrams and maps besides real time information at the top of this graphic background. The AutoCAD single line diagram can further be overlaid by such items as status of breakers and switches, alarms, power system parameters and event and historical data logs. The system allows detailed view of the operations by the technicians. Monthly energy reports and alarms are generated by SCADA automatically and the scope can further be enhanced with growing needs. Addition of IED meters lends more flexibility and expendability to the system.
Operators get immediate information from SCADA as and when a particular section of the distribution system goes down. Therefore the system operators can give information on the problem to the researchers, building managers and other people of the unit. The data on performance and trend are promptly, reliably and accurately provided by SCADA. The affected parts like the electrical power loop can be easily identified by the technicians for responding suitably. SCADA provides historical data that enables the engineers to improve system performance through planning and trending of the electrical load. As across the campus use of SCADA expands, more functions can be utilized by the system. For example, automatic reporting can replace manual reading of the meter. Also the operators armed with the information on real time alarms and data can respond promptly to a situation. Thus quality information can be provided to the customers proactively.
At any location of the distribution system, the failure of electrical power system needs to be identified by the power system operators immediately from remote locations. Considering the varied needs of the energy using community, there is need for more accurate historical information and real time alarm system. The demands for trending the historic data would continually increase.
SCADA: A System In Action In A Campus
Fault Location Without SCADA
Things were grim in the early nineties before introduction of SCADA technology. There was no clear-cut method to determine the magnitude of power failure in a campus. Primitive methods like waiting, guessing, luck factor, triangulation, etc. were depended on to find out the affected buildings and ways for restoring power.
Lot of people was involved before SCADA in the process of identification of power failure ranging from customer service people to building managers to the high-volt electricians. The particular branch of a circuit where the power had failed could not be known and the failed area of the distribution system could only be found by applying a method of triangulation after identifying a number of buildings without power. It took many hours and an elaborate procedure like triangulation to trace the source of this power failure. Each fault indicator on a distribution cable located underground and spread over a large area had to be examined by the attending crew by removing vaults and manhole covers. Location and restoration time of faults were thus long. SCADA system has totally eliminated this cumbersome procedure of fault location and servicing. By doing post-failure analysis, SCADA prevents recurrence of such failures.
System operators are now able to obtain immediate and remote status changes as the systems under SCADA are connected by multiple high-volt switch contacts. Other than the need to locate the fault, there was need for HV technicians to determine the existing use of specific distribution cables for the purpose of record and evaluation. Now SCADA provides detailed periodic reports enabling the switching operators to transfer load in case of necessity to other feeders in the system. This information was earlier obtained from feeder ammeters in a sub-station which took a lot of time. Before the use of remote SCADA application, there was no way the physical changes in operator switch position could be confirmed.
SCADA: System Features & Installation
SCADA has now become the most prevalent measurement and control system in any industry. The central host is the Master Terminal Unit (MTU) and the data collection and control units are the remote stations more popularly known as RTU (Remote Terminal Unit). Remote data are controlled by customs software. The control features of a modern SCADA system are mainly open-loop using long-distance communication though with occasional short-distance communication elements.
Distributed Control Systems (DCS)
These systems similar to SCADA find routine use in treatment plants and factories. While DCS functions almost like SCADA system, unlike SCADA the distribution of field data collection and control units are more confined in area for DCS. Reliable and high speed communication is normally available via LAN. Using close-loop control in significant amount is the basic feature of a DCS system. SCADA on the other hand provides wider geographic coverage and depends on different communication systems which are usually not as dependable as LAN. Open loop controls are more suited to this kind of scenario.
Compulsion of budget, logistics and various other reasons make many organizations install their SCADA system spreading over a period of time. Often integration of SCADA systems is required when different discrete SCADA systems are delivered as a result of competitive tenders from multiple vendors. Use of proprietary hardware/software/communication protocols in SCADA may pose a serious problem in such scenario. Standards for the RTU for SCADA telemetry applications are required to be specified to solve this problem. RTU hardware and software need to be provided with standard specifications. This enables the RTU for applications ranging from small systems to distributed networks of large scale like interface with SCADA networks. Use of IEC1131-3 programming language makes RTU programmable. The communication protocol normally used by RTU is DNP 3.0. For attaining equipment inter-operability, enforcement of only a few standards is required but for getting software portability, additional standards are always needed.
SCADA: New Challenges
Large changes are coming over the SCADA systems and their networking environment. The modern SCADA equipments are being fed with more and more computer power. This is because they have to meet increasing demands for information being made on them. To be more powerful, the new SCADA systems need networks like Ethernet, IP and frame relay for their operations.
Because of handling highly critical control functions, various networks like private wire/optical cable and private microwave are used by SCADA systems to ensure reliability. Only a strong network can avoid disasters like pipeline failure in oil well leaking oil from which can cause massive fires and also contaminate water with catastrophic consequences or failure of a power station causing major power breakdown.
Current SCADA networking practices are getting significant challenges from a new environment being evolved out of data communication networks. Demands for more information means higher data rates through incorporating higher computer power in the SCADA systems.
More computer power and modern networks pose higher challenges for the SCADA equipments. The points of meeting these challenges are SCADA protocols. The new SCADA environment is Ethernet/IP protocols, higher speed and frame relay. Integrating these networks with SCADA is the important challenge. The dichotomy of industrial SCADA equipments and networking technology is that whereas the average lifespan of the former is more than ten years, the latter undergoes many changes during that period as technology changes faster. It is through network protocols that the SCADA protocols are transported. However the networks and their protocols pose limitation while interacting with the SCADA protocols. Delays and short gaps in data and absence of Data Carrier Detect (DCD) transitions are caused by the characteristics of different network protocols like Ethernet, IP, etc. These lead SCADA protocols to presume links errors. As a result of this deficiency in transition of control signal, microwave radio link cannot be keyed.
SCADA: Major Protocols & Modems
In this packet protocol, the data packets do not directly correspond to the poll or response packets size of SCADA. The network thus breaks up the SCADA packet into relay packets of various frames. Delays would be caused in between frame relay data packets. When there are no actual errors, these delays or time gaps within SCADA packet would make SCADA polling system presume transmission error.
Regardless of incoming data protocols, packets of Ethernet, a packet oriented protocol is generated. Ethernet devices follow their own protocol rules depending on their needs. They do not follow the rules of the devices connected to them.
They possess same packet characteristics as Frame Relay and Ethernet. The incoming SCADA data packets bear no relations with the IP packets. Here again the SCADA system may presume error because of time gaps.
1200 BPS Modems
For more information, transferring data at a higher rate is very necessary. Most SCADA systems used this modem earlier. The system could find out through interface with remote devices whether the remote unit was operating allowing small amount of data collection and delivery. Now there is demand for more data volumes with faster processors and more memory.
Fast Poll Modems
These are 9600 bps faster modems that can meet this higher demand for data. SCADA systems mostly use this high-speed device. Voice grade frequencies have a bandwidth of around 3000 hertz. The phone lines, private wires and microwaves can operate at this frequency which is also adequate for fast poll 9600 bps modems. More data at higher data rates are delivered by fast poll modems. However additional time is required for these modems for proper acquisition of data signal. This “training time” is referred as the modem’s request to send/ clear to send (RTS/CTS) delay. The manner of modulating analog signal in case of this modem is different from 1200 bps modems. A complex technique called QAM is used by this modem for sending and decoding several bits of data tat causes some milliseconds delay.
SCADA or Supervisory Control and Data Acquisition is a distributed measurement and control system for large-scale industrial automation. SCADA has applications in automated operations like chemical manufacturing and transport, supply systems and power generation.
How SCADA Works
The SCADA technology was developed as part of Instrumentation Engineering. Monitoring systems such as SCADA have been in use for quite some time now. Such systems are collectively called DCS (Distributed Control System). DCS have conventionally been used for facilities like factories. However such systems are not effective in covering large geographical areas like those involved in gas transport systems. SCADA has been specifically developed to meet requirements covering large territories.
A SCADA system at the machine level consists of a central station for gathering data and managing the overall operation. It also has sensors (these could be Remote Terminal Units or RTUs, or Programmable Logic Controller) placed in proximity to where the action is. The RTU or the PLC collects the information locally and then passes it on to the central station which is often several miles away. RTUs and PLCs today are capable of controlling the operations within its purview through closed loop feedback systems. The central station oversees the overall performance of the one or more RTU/ PLC under its control. SCADA systems also allow operators or supervisors to change the settings as appropriate at the level of the RTU or the central station. Alarming conditions like high temperature are recorded and displayed.
Where Programming Comes In
All of this requires that physical conditions be translated into machine language and then signals that humans can read, record and analyze. Thus a full fledged SCADA system has to comprise of both hardware and software elements. Today’s sophisticated SCADA systems include input/output signal devices, control equipment, HMI (Human Machine Interface), networking, communication systems, databases and software.
Thus SCADA system development involves programming at various levels. Data collected at the RTU has to be converted into signals, and for interpreting this data an operator requires HMI. Often the data also has to be compiled and stored (history databases) for recognizing trends and analysis work. Thus customized database systems have to be developed. Networks, communication systems etc bring in more varied requirements for programming.
Add to this the fact that SCADA systems are still evolving. Industries are awakening to challenges like possibility of terrorist strikes. Thus R&D for better, more fool-proof systems is still on at both hardware and software levels. SCADA programming this way has a lot of possibilities.
As the requirements and the systems themselves are getting more complex, SCADA users today tend to source PLC, HMI and networking software from different vendors and put them all together, rather than buy them all from the vendor who manufactures the hardware. Mix-and-match is the name of the game. Thus the SCADA softwares of today have to pitch for open communications.
We have to wait and watch to see if this becomes a weak point for SCADA systems. Open systems as we know are more vulnerable to real and cyberspace attacks by terrorists. Suitable security measures like firewalls and VPN solutions could counter this threat effectively and this becomes another area with scope for a lot of work. SCADA seems to be here to stay, as its overall benefits outweigh the risk factor.