Abstract:
The application of multilevel converters for traction drive systems is being investigated. The main advantage of this kind of topology is that it can generate almost perfect current or voltage waveforms, because it is modulated by amplitude instead of pulse-width. That means that the pulsating torque generated by harmonics can be eliminated, and power losses into the machine due to harmonic currents can also be eliminated. Another advantage of this kind of drive is that the switching frequency and power rating of the semiconductors is reduced considerably. The amplitude modulation is based on a cascade of N converters scaled in a “trinary” form (three-state “H” converters). In the chain of N converters of each phase (N-Stage Converter), there is a “Master converter” that manages more than 80% of the total power, and N-1 “Slave converters” that take the rest of the power (less than 20%). One important drawback of this kind of arrangement is that it needs isolated power sources for each one of the N converters, and also for each phase. This paper shows that this problem can be overcome by using isolated motor windings for each phase of the traction motor (which is easy to get in normal machines), and by using low-power high-frequency, bidirectional switching power supplies for the “slave converters”. Simulations using PSIM (Power Electronics Simulator) have demonstrated the feasibility to build drive converters for electric vehicles using multilevel inverters. They are being
compared with inverters using the conventional PWM technique. The multilevel converter used in the simulations, works with only four inverters (N=4): one Master and three Slaves. In both the cases (PWM and multilevel), the traction motors have a rating of 80 kW, and the battery pack supply is 240Vdc. The battery pack is connected to the master converters of each phase in parallel, and to the slaves through isolated bi-directional switching power supplies.
Introduction
Power Electronics technologies contribute with important part in the development of electric vehicles. On the other hand, the PWM techniques used today to control modern static converters for electric traction, do not give perfect waveforms, which strongly depend on switching frequency of the power semiconductors. Normally, voltage (or current in dual devices) moves to discrete values, forcing the design of machines with good isolation, and sometimes loads with inductances in excess of the required value. In other words, neither voltage nor current are as expected. This also means harmonic contamination, additional power losses, torque ripple, and high frequency noise that can affect the controllers. All these reasons have generated many research works on the topic of PWM modulation .
Multi-stage converters work more like amplitude modulation rather than pulse modulation, and this fact makes the outputs of the converter very much cleaner. This way of operation allows having almost perfect currents, and very good voltage waveforms, eliminating most of the undesirable harmonics. And even better, the bridges of each converter work at a very low switching frequency, which gives the possibility to work with low speed semiconductors, and to generate low switching frequency losses. The objective of this paper is to show the advantages of multi-stage converters for all kind of applications. The drawbacks of requiring isolated power supplies is solved using different techniques, which depend on the type of application, and based on the fact that the first converter, called Master, takes more than 80% of the total power delivered to the load. A four-stage converter using three-state power modules, which gives 81 different levels of voltage amplitude, is studied. The results are compared with conventional PWM modulators working at a switching frequency of 10kHz. All the load parameters of both types of converters are set at the same values.
BASICS OF MULTISTAGE CONVERTERS:
Basic Principle:
The circuit of fig.1 shows the basic topology of one converter used for the implementation of multistage converters. It is based on the simple, four switches converter, used for single-phase inverters or dual converters. These converters are able to produce three levels of voltage in the load: +Vdc, -Vdc, and Zero.
Multi-Stage Connection:
The multi-stage connection can be implemented with two, three, or any number of three-level modules. The figure 2 displays the main components of a four-stage converter, which is being analysed in this work. The figure only shows one of the three phases of the complete system. As can be seen, the dc power supplies of the four modules are isolated, and the dc supplies are scaled with levels of voltage in power of three. The scaling of voltages in power of three allows having, with only four converters, 81 (3^4) different levels of voltage: 40 levels of positive values, 40 levels of negative values, and zero. The converter located at the bottom of the figure has the bigger voltage, and will be called Master. The rest of the modules will be the Slaves. The Master works at the lower switching frequency, which is an additional advantage of this topology.
With 81 levels of voltage, a four-stage converter can follow a sinusoidal waveform in a very precise way, as shown in figure 3. It can control the load voltage as an AM device (Amplitude Modulation). The figure 3 shows different levels of amplitude, which are obtained through the control of the gates of the power transistors in each one of the four converters.
Power Distribution:
One of the good advantages of the strategy described here for multiconverters is that most of the power delivered comes from the Master. The example of figure 5 shows the power distribution in one phase of the four-stage converter, feeding a pure resistive load with sinusoidal voltage. A little more than 80% of the real power is delivered by the Master converter, and only 20% for the Slaves. Even more, the second and third slave only deliver 5% of the total power. That means, the dc power sources needed by the Slaves are small.
This characteristic makes possible to feed the Slaves with low power, isolated power sources, fed by a common power supply from the Master. These power sources need to be bi-directional, because the power factor of the load can produce negative active power in some of the Slaves. The figure 6 shows a bi-directional dc-dc power supply, which can be used for this purpose .
Another attribute of the multi-stage configuration, which is possible to see in the oscillograms of figures 4 and 5, is the very low switching frequency of each converter. But even better, the Master, which carries most of the power, operates at the lower switching frequency. Then, the larger the power of the unit, the lower the switching frequency. In large traction applications, like buses, electric locomotives or ships, the Master can be implemented with GTOs, and the Slaves with IGBTs.To avoid the three Masters having to be isolated one from each other, the three windings of the machine have to be fed independently (no electrical connection between them). The complete configuration, using Bidirectional DC-DC Converters, and isolated windings for the traction motor is shown in figure 7.
Traction system configuration using a four-stage multilevel converter
Simulation Results:
The following results show a comparison between PWM strategy and a four-stage multilevel converter. These results have been obtained using the software called PSIM , which has demonstrated its reliability for almost 10 years of simulations, which have been corroborated with real experimental results. Shunt active power filters, static var compensators, sinusoidal voltage power supplies, high power rectifiers, and machine drives have previously been simulated with PSIM.
Machine drives with sinusoidal supply:
With PWM techniques, it is not possible to implement a sinusoidal voltage power supply. The multiconverter topology, scaled in power of three, with a few quantity of inverters, can generate a very good sinusoidal voltage waveform. A four-stage converter can generate 81 steps of voltage levels, as was shown in figure 3. Forty positive levels, forty negative levels, and zero. The figure 8 shows the current of the load when is fed with a PWM power supply, and with a four-stage power supply. The switching frequency of the PWM inverter is 15 kHz, and the supply frequency is 50 Hz. It is clear the difference: the current in the four-stage converter is almost harmonic-free. This system can operate at all output frequencies.
Armature currents from a pwm voltage source and a four-stage voltage source
Despite the system looks complicated, it can be adequately integrated. It is important to remember that the DC-DC converters are small power devices. For example, for 60 kW traction system for an electric vehicle, the DC-DC converters for the first Slave are only 3 kW each. For the second Slave are 0.8kW, and for the third Slave only 200 W each. These converters can be small today with a switching frequency link of hundreds of kHz.
As the quality of sinusoidal current waveforms was already showed in figure 12, the figure 13 shows a comparison of current for a brushless dc motor, using PWM converter, and four level converter. Again, the quality of the current obtained with the last technology is superior.
Machine drives with non-sinusoidal supply.
Multilevel converters can also be used as choppers for controlling dc motors. In this case, only one phase is needed, and is used for armature control. With the four-stage converter, the dc voltage of the armature can also be controlled with 81 levels: 40 for motoring, 40 for regenerative braking, and zero.Another important application is with brushless dc motors, because they need a special voltage modulation to get the typical trapezoidal waveform of the armature current. In figure 9, a comparison between PWM and a four-stage multilevel converter is displayed.
Conclusions:
A four-stage multilevel inverter, using three-state converters for electric traction applications, has been analyzed. The advantages and drawbacks of this kind of converter have been displayed. The problem related with galvanic isolation, have been overcome by using isolated, bidirectional dc power supplies,which can be fed from a common power source from the Master. This solution becomes practical because the Master takes more than 80% of the total active power required by the system. The rest of the converters, called “Slaves”, need to convert very low power, and then those dc supplies are small. Different simulations were shown and compared with similar results obtained with conventional PWM converters. The topology looks applicable not only for electric vehicles, but also for large traction equipment such as electric buses, electric locomotives and ships.
Sunday, February 17, 2008
SCADA
ABSTRACT
==========
SCADA is the acronym for Supervisory Control And Data Acquisition. The term refers to a large-scale, distributed measurement (and control) system. SCADA systems are used to monitor or to control chemical or transport processes, in municipal water supply systems, to control electric power generation, transmission and distribution, gas and oil pipelines, and other distributed processes.
An industrial SCADA system will be used for the development of the controls of the four LHC experiments.
So what is SCADA?
It is used to monitor and control plant or equipment. The control may be automatic, or initiated by operator commands. The data acquisition is accomplished firstly by the RTU's scanning the field inputs connected to the RTU (it may be also called a PLC - programmable logic controller). This is usually at a fast rate. The central host will scan the RTU's (usually at a slower rate.) The data is processed to detect alarm conditions, and if an alarm is present, it will be displayed on special alarm lists.
Data can be of three main types.Analogue data (ie real numbers) will be trended (ie placed in graphs). Digital data (on/off) may have alarms attached to one state or the other. Pulse data (eg counting revolutions of a meter) is normally accumulated or counted.The primary interface to the operator is a graphical display (mimic) which shows a representation of the plant or equipment in graphical form. Live data is shown as graphical shapes (foreground) over a static background. As the data changes in the field, the foreground is updated.
Example: a valve may be shown as open or closed. Analog data can be shown either as a number, or graphically. The system may have many such displays, and the operator can select from the relevant ones at any time.
Contents
========
1. Systems concepts
2. Human Machine Interface
3. Hardware solutions
4. System components
4.1 Remote Terminal Unit (RTU)
4.2 Master Station
5. Operational philosophy
6. Communication infrastructure and methods
7. Future trends in SCADA
Systems concepts
A SCADA system includes input/output signal hardware, controllers, HMI, networks, communication, database and software.
The term SCADA usually refers to a central system that monitors and controls a complete site or a system spread out over a long distance (kilometres/miles). The bulk of the site control is actually performed automatically by a Remote Terminal Unit (RTU) or by a Programmable Logic Controller (PLC). Host control functions are almost always restricted to basic site over-ride or supervisory level capability. For example, a PLC may control the flow of cooling water through part of an industrial process, but the SCADA system may allow an operator to change the control set point for the flow, and will allow any alarm conditions such as loss of flow or high temperature to be recorded and displayed. The feedback control loop is closed through the RTU or PLC; the SCADA system monitors the overall performance of that loop.
Data acquisition begins at the RTU or PLC level and includes meter readings and equipment statuses that are communicated to SCADA as required. Data is then compiled and formatted in such a way that a control room operator using the HMI can make appropriate supervisory decisions that may be required to adjust or over-ride normal RTU (PLC) controls. Data may also be collected in to a Historian, often built on a commodity Database Management System, to allow trending and other analytical work.
SCADA systems typically implement a distributed database, commonly referred to as a tag database, which contains data elements called tags or points. A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of an actual input or output connected to the system, while a soft point represents the result of logic and math operations applied to other hard and soft points. Most implementations conceptually remove this distinction by making every property a "soft" point (expression) that can equal a single "hard" point in the simplest case. Point values are normally stored as value-timestamp combinations; the value and the timestamp when the value was recorded or calculated. A series of value-timestamp combinations is the history of that point. It's also common to store additional metadata with tags such as: path to field device and PLC register, design time comments, and even alarming information.
It is possible to purchase a SCADA system, or Distributed Control System (DCS) from a single supplier. It is more common to assemble a SCADA system from hardware and software components like Allen-Bradley or GE PLCs, HMI packages from Wonderware, Rockwell Automation, Inductive Automation, Citect, or GE. Communication typically happens over ethernet.
Human Machine Interface
A Human-Machine Interface or HMI is the apparatus which presents process data to a human operator, and through which the human operator controls the process.
The HMI industry was essentially born out of a need for a standardized way to monitor and to control multiple remote controllers, PLCs and other control devices. While a PLC does provide automated, pre-programmed control over a process, they are usually distributed across a plant, making it difficult to gather data from them manually. Historically PLCs had no standardized way to present information to an operator. The SCADA system gathers information from the PLCs and other controllers via some form of network, and combines and formats the information. An HMI may also be linked to a database, to provide trending, diagnostic data, and management information such as scheduled maintenance procedures, logistic information, detailed schematics for a particular sensor or machine, and expert-system troubleshooting guides. Since about 1998, virtually all major PLC manufacturers have offered integrated HMI/SCADA systems, many of them using open and non-proprietary communications protocols. Numerous specialized third-party HMI/SCADA packages, offering built-in compatibility with most major PLCs, have also entered the market, allowing mechanical engineers, electrical engineers and technicians to configure HMIs themselves, without the need for a custom-made program written by a software developer.
SCADA is popular, due to its compatibility and reliability. It is used in small applications, like controlling the temperature of a room, to large applications, such as the control of nuclear power plants.
Hardware solutions
SCADA solutions often have Distributed Control System (DCS) components. Use of "smart" RTUs or PLCs, which are capable of autonomously executing simple logic processes without involving the master computer, is increasing. A functional block programming language, IEC 61131-3, is frequently used to create programs which run on these RTUs and PLCs. Unlike a procedural language such as the C programming language or FORTRAN, IEC 61131-3 has minimal training requirements by virtue of resembling historic physical control arrays. This allows SCADA system engineers to perform both the design and implementation of a program to be executed on a RTU or PLC.
System components
The three components of a SCADA system are:
Multiple Remote Terminal Units (also known as RTUs or Outstations).
Master Station and HMI Computer(s).
Communication infrastructure
Remote Terminal Unit (RTU)
The RTU connects to physical equipment, and reads status data such as the open/closed status from a switch or a valve, reads measurements such as pressure, flow, voltage or current. By sending signals to equipment the RTU can control equipment, such as opening or closing a switch or a valve, or setting the speed of a pump.
The RTU can read digital status data or analogue measurement data, and send out digital commands or analogue setpoints.
An important part of most SCADA implementations are alarms. An alarm is a digital status point that has either the value NORMAL or ALARM. Alarms can be created in such a way that when their requirements are met, they are activated. An example of an alarm is the "fuel tank empty" light in a car. The SCADA operator's attention is drawn to the part of the system requiring attention by the alarm. Emails and text messages are often sent along with an alarm activation alerting managers along with the SCADA operator.
Master Station
The term "Master Station" refers to the servers and software responsible for communicating with the field equipment (RTUs, PLCs, etc), and then to the HMI software running on workstations in the control room, or elsewhere. In smaller SCADA systems, the master station may be composed of a single PC. In larger SCADA systems, the master station may include multiple servers, distributed software applications, and disaster recovery sites.
The SCADA system usually presents the information to the operating personnel graphically, in the form of a mimic diagram. This means that the operator can see a schematic representation of the plant being controlled. For example, a picture of a pump connected to a pipe can show the operator that the pump is running and how much fluid it is pumping through the pipe at the moment. The operator can then switch the pump off. The HMI software will show the flow rate of the fluid in the pipe decrease in real time. Mimic diagrams may consist of line graphics and schematic symbols to represent process elements, or may consist of digital photographs of the process equipment overlain with animated symbols.
The HMI package for the SCADA system typically includes a drawing program that the operators or system maintenance personnel use to change the way these points are represented in the interface. These representations can be as simple as an on-screen traffic light, which represents the state of an actual traffic light in the field, or as complex as a multi-projector display representing the position of all of the elevators in a skyscraper or all of the trains on a railway. Initially, more "open" platforms such as Linux were not as widely used due to the highly dynamic development environment and because a SCADA customer that was able to afford the field hardware and devices to be controlled could usually also purchase UNIX or OpenVMS licenses. Today, all major operating systems are used for both master station servers and HMI workstations.
Operational philosophy
Instead of relying on operator intervention, or master station automation, RTUs may now be required to operate on their own to control tunnel fires or perform other safety-related tasks. The master station software is required to do more analysis of data before presenting it to operators including historical analysis and analysis associated with particular industry requirements. Safety requirements are now being applied to the system as a whole and even master station software must meet stringent safety standards for some markets.
For some installations, the costs that would result from the control system failing is extremely high. Possibly even lives could be lost. Hardware for SCADA systems is generally ruggedized to withstand temperature, vibration, and voltage extremes, but in these installations reliability is enhanced by having redundant hardware and communications channels. A failing part can be quickly identified and its functionality automatically taken over by backup hardware. A failed part can often be replaced without interrupting the process. The reliability of such systems can be calculated statistically and is stated as the mean time to failure, which is a variant of mean time between failures. The calculated mean time to failure of such high reliability systems can be in the centuries.
Communication infrastructure and methods
SCADA systems have traditionally used combinations of radio and direct serial or modem connections to meet communication requirements, although Ethernet and IP over SONET is also frequently used at large sites such as railways and power stations.
This has also come under threat with some customers wanting SCADA data to travel over their pre-established corporate networks or to share the network with other applications. The legacy of the early low-bandwidth protocols remains, though. SCADA protocols are designed to be very compact and many are designed to send information to the master station only when the master station polls the RTU. Typical legacy SCADA protocols include Modbus, RP-570 and Conitel. These communication protocols are all SCADA-vendor specific. Standard protocols are IEC 60870-5-101 or 104, Profibus and DNP3.
These communication protocols are standardised and recognised by all major SCADA vendors. Many of these protocols now contain extensions to operate over TCP/IP, although it is good security engineering practice to avoid connecting SCADA systems to the Internet so the attack surface is reduced.
RTUs and other automatic controller devices were being developed before the advent of industry wide standards for interoperability. The result is that developers and their management created a multitude of control protocols. Among the larger vendors, there was also the incentive to create their own protocol to "lock in" their customer base. A list of automation protocols is being compiled here. industrial firewall and VPN solutions for TCP/IP based SCADA networks.
Future trends in SCADA
The trend is for PLC and HMI/SCADA software to be more "mix-and-match". In the mid 1990s, the typical DAQ I/O manufacturer offered their own proprietary communications protocols over a suitable-distance carrier like RS-485. Towards the late 1990s, the shift towards open communications continued with I/O manufacturers offering support of open message structures like Modicon MODBUS over RS-485, and by 2000 most I/O makers offered completely open interfacing such as Modicon MODBUS over TCP/IP. The primary barriers of Ethernet TCP/IP's entrance into industrial automation (determinism, synchronization, protocol,selection,environment suitability) are still a concern to a few extremely specialized applications, but for the vast majority of HMI/SCADA markets these barriers have been broken.
Recently, however, the very existence of SCADA based systems has come into question as they are increasingly seen as extremely vulnerable to cyberwarfare/cyberterrorism attacks. Given the mission critical nature of a large number of SCADA systems, such attacks could, in a worse case scenario, cause massive financial losses through loss of data or actual physical destruction, misuse or theft, even loss of life, either directly or indirectly. Whether such concerns will cause a move away from the use of SCADA systems for mission critical applications towards more secure architectures and configurations remains to be seen, given that at least some influential people in corporate and governmental circles believe that the benefits and lower initial costs of SCADA based systems still outweigh potential costs and risks.
CONCLUSION
Potential benefits of SCADA
The benefits one can expect from adopting a SCADA system for the control of experimental physics facilities can be summarised as follows:
a rich functionality and extensive development facilities. The amount of effort invested in SCADA product amounts to 50 to 100 p-years!
the amount of specific development that needs to be performed by the end-user is limited, especially with suitable engineering.
Enhance reliability and robustness.
technical support and maintenance by the vendor.
For large collaborations, as for the CERN LHC experiments, using a SCADA system for their controls ensures a common framework not only for the development of the specific applications but also for operating the detectors. Operators experience the same "look and feel" whatever part of the experiment they control.
REFERENCES
· Note: this article is based on a very similar one that has been published in the Proceedings of the 7th International Conference on Accelerator and Large Experimental Physics Control Systems, held in Trieste, Italy, 4 - 8 Oct. 1999.
[1] A.Daneels, W.Salter, "Technology Survey Summary of Study Report", IT-CO/98-08-09, CERN, Geneva 26th Aug 1998.
[2] A.Daneels, W.Salter, "Selection and Evaluation of Commercial SCADA Systems for the Controls of the CERN LHC Experiments", Proceedings of the 1999 International Conference on Accelerator and Large Experimental Physics Control Systems, Trieste, 1999, p.353.
[3] G.Baribaud et al., "Recommendations for the Use of Fieldbuses at CERN in the LHC Era", Proceedings of the 1997 International Conference on Accelerator and Large Experimental Physics Control Systems, Beijing, 1997, p.285.
[4] R.Barillere et al., "Results of the OPC Evaluation done within the JCOP for the Control of the LHC Experiments", Proceedings of the 1999 International Conference on Accelerator and Large Experimental Physics Control Systems, Trieste, 1999, p.511.
==========
SCADA is the acronym for Supervisory Control And Data Acquisition. The term refers to a large-scale, distributed measurement (and control) system. SCADA systems are used to monitor or to control chemical or transport processes, in municipal water supply systems, to control electric power generation, transmission and distribution, gas and oil pipelines, and other distributed processes.
An industrial SCADA system will be used for the development of the controls of the four LHC experiments.
So what is SCADA?
It is used to monitor and control plant or equipment. The control may be automatic, or initiated by operator commands. The data acquisition is accomplished firstly by the RTU's scanning the field inputs connected to the RTU (it may be also called a PLC - programmable logic controller). This is usually at a fast rate. The central host will scan the RTU's (usually at a slower rate.) The data is processed to detect alarm conditions, and if an alarm is present, it will be displayed on special alarm lists.
Data can be of three main types.Analogue data (ie real numbers) will be trended (ie placed in graphs). Digital data (on/off) may have alarms attached to one state or the other. Pulse data (eg counting revolutions of a meter) is normally accumulated or counted.The primary interface to the operator is a graphical display (mimic) which shows a representation of the plant or equipment in graphical form. Live data is shown as graphical shapes (foreground) over a static background. As the data changes in the field, the foreground is updated.
Example: a valve may be shown as open or closed. Analog data can be shown either as a number, or graphically. The system may have many such displays, and the operator can select from the relevant ones at any time.
Contents
========
1. Systems concepts
2. Human Machine Interface
3. Hardware solutions
4. System components
4.1 Remote Terminal Unit (RTU)
4.2 Master Station
5. Operational philosophy
6. Communication infrastructure and methods
7. Future trends in SCADA
Systems concepts
A SCADA system includes input/output signal hardware, controllers, HMI, networks, communication, database and software.
The term SCADA usually refers to a central system that monitors and controls a complete site or a system spread out over a long distance (kilometres/miles). The bulk of the site control is actually performed automatically by a Remote Terminal Unit (RTU) or by a Programmable Logic Controller (PLC). Host control functions are almost always restricted to basic site over-ride or supervisory level capability. For example, a PLC may control the flow of cooling water through part of an industrial process, but the SCADA system may allow an operator to change the control set point for the flow, and will allow any alarm conditions such as loss of flow or high temperature to be recorded and displayed. The feedback control loop is closed through the RTU or PLC; the SCADA system monitors the overall performance of that loop.
Data acquisition begins at the RTU or PLC level and includes meter readings and equipment statuses that are communicated to SCADA as required. Data is then compiled and formatted in such a way that a control room operator using the HMI can make appropriate supervisory decisions that may be required to adjust or over-ride normal RTU (PLC) controls. Data may also be collected in to a Historian, often built on a commodity Database Management System, to allow trending and other analytical work.
SCADA systems typically implement a distributed database, commonly referred to as a tag database, which contains data elements called tags or points. A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of an actual input or output connected to the system, while a soft point represents the result of logic and math operations applied to other hard and soft points. Most implementations conceptually remove this distinction by making every property a "soft" point (expression) that can equal a single "hard" point in the simplest case. Point values are normally stored as value-timestamp combinations; the value and the timestamp when the value was recorded or calculated. A series of value-timestamp combinations is the history of that point. It's also common to store additional metadata with tags such as: path to field device and PLC register, design time comments, and even alarming information.
It is possible to purchase a SCADA system, or Distributed Control System (DCS) from a single supplier. It is more common to assemble a SCADA system from hardware and software components like Allen-Bradley or GE PLCs, HMI packages from Wonderware, Rockwell Automation, Inductive Automation, Citect, or GE. Communication typically happens over ethernet.
Human Machine Interface
A Human-Machine Interface or HMI is the apparatus which presents process data to a human operator, and through which the human operator controls the process.
The HMI industry was essentially born out of a need for a standardized way to monitor and to control multiple remote controllers, PLCs and other control devices. While a PLC does provide automated, pre-programmed control over a process, they are usually distributed across a plant, making it difficult to gather data from them manually. Historically PLCs had no standardized way to present information to an operator. The SCADA system gathers information from the PLCs and other controllers via some form of network, and combines and formats the information. An HMI may also be linked to a database, to provide trending, diagnostic data, and management information such as scheduled maintenance procedures, logistic information, detailed schematics for a particular sensor or machine, and expert-system troubleshooting guides. Since about 1998, virtually all major PLC manufacturers have offered integrated HMI/SCADA systems, many of them using open and non-proprietary communications protocols. Numerous specialized third-party HMI/SCADA packages, offering built-in compatibility with most major PLCs, have also entered the market, allowing mechanical engineers, electrical engineers and technicians to configure HMIs themselves, without the need for a custom-made program written by a software developer.
SCADA is popular, due to its compatibility and reliability. It is used in small applications, like controlling the temperature of a room, to large applications, such as the control of nuclear power plants.
Hardware solutions
SCADA solutions often have Distributed Control System (DCS) components. Use of "smart" RTUs or PLCs, which are capable of autonomously executing simple logic processes without involving the master computer, is increasing. A functional block programming language, IEC 61131-3, is frequently used to create programs which run on these RTUs and PLCs. Unlike a procedural language such as the C programming language or FORTRAN, IEC 61131-3 has minimal training requirements by virtue of resembling historic physical control arrays. This allows SCADA system engineers to perform both the design and implementation of a program to be executed on a RTU or PLC.
System components
The three components of a SCADA system are:
Multiple Remote Terminal Units (also known as RTUs or Outstations).
Master Station and HMI Computer(s).
Communication infrastructure
Remote Terminal Unit (RTU)
The RTU connects to physical equipment, and reads status data such as the open/closed status from a switch or a valve, reads measurements such as pressure, flow, voltage or current. By sending signals to equipment the RTU can control equipment, such as opening or closing a switch or a valve, or setting the speed of a pump.
The RTU can read digital status data or analogue measurement data, and send out digital commands or analogue setpoints.
An important part of most SCADA implementations are alarms. An alarm is a digital status point that has either the value NORMAL or ALARM. Alarms can be created in such a way that when their requirements are met, they are activated. An example of an alarm is the "fuel tank empty" light in a car. The SCADA operator's attention is drawn to the part of the system requiring attention by the alarm. Emails and text messages are often sent along with an alarm activation alerting managers along with the SCADA operator.
Master Station
The term "Master Station" refers to the servers and software responsible for communicating with the field equipment (RTUs, PLCs, etc), and then to the HMI software running on workstations in the control room, or elsewhere. In smaller SCADA systems, the master station may be composed of a single PC. In larger SCADA systems, the master station may include multiple servers, distributed software applications, and disaster recovery sites.
The SCADA system usually presents the information to the operating personnel graphically, in the form of a mimic diagram. This means that the operator can see a schematic representation of the plant being controlled. For example, a picture of a pump connected to a pipe can show the operator that the pump is running and how much fluid it is pumping through the pipe at the moment. The operator can then switch the pump off. The HMI software will show the flow rate of the fluid in the pipe decrease in real time. Mimic diagrams may consist of line graphics and schematic symbols to represent process elements, or may consist of digital photographs of the process equipment overlain with animated symbols.
The HMI package for the SCADA system typically includes a drawing program that the operators or system maintenance personnel use to change the way these points are represented in the interface. These representations can be as simple as an on-screen traffic light, which represents the state of an actual traffic light in the field, or as complex as a multi-projector display representing the position of all of the elevators in a skyscraper or all of the trains on a railway. Initially, more "open" platforms such as Linux were not as widely used due to the highly dynamic development environment and because a SCADA customer that was able to afford the field hardware and devices to be controlled could usually also purchase UNIX or OpenVMS licenses. Today, all major operating systems are used for both master station servers and HMI workstations.
Operational philosophy
Instead of relying on operator intervention, or master station automation, RTUs may now be required to operate on their own to control tunnel fires or perform other safety-related tasks. The master station software is required to do more analysis of data before presenting it to operators including historical analysis and analysis associated with particular industry requirements. Safety requirements are now being applied to the system as a whole and even master station software must meet stringent safety standards for some markets.
For some installations, the costs that would result from the control system failing is extremely high. Possibly even lives could be lost. Hardware for SCADA systems is generally ruggedized to withstand temperature, vibration, and voltage extremes, but in these installations reliability is enhanced by having redundant hardware and communications channels. A failing part can be quickly identified and its functionality automatically taken over by backup hardware. A failed part can often be replaced without interrupting the process. The reliability of such systems can be calculated statistically and is stated as the mean time to failure, which is a variant of mean time between failures. The calculated mean time to failure of such high reliability systems can be in the centuries.
Communication infrastructure and methods
SCADA systems have traditionally used combinations of radio and direct serial or modem connections to meet communication requirements, although Ethernet and IP over SONET is also frequently used at large sites such as railways and power stations.
This has also come under threat with some customers wanting SCADA data to travel over their pre-established corporate networks or to share the network with other applications. The legacy of the early low-bandwidth protocols remains, though. SCADA protocols are designed to be very compact and many are designed to send information to the master station only when the master station polls the RTU. Typical legacy SCADA protocols include Modbus, RP-570 and Conitel. These communication protocols are all SCADA-vendor specific. Standard protocols are IEC 60870-5-101 or 104, Profibus and DNP3.
These communication protocols are standardised and recognised by all major SCADA vendors. Many of these protocols now contain extensions to operate over TCP/IP, although it is good security engineering practice to avoid connecting SCADA systems to the Internet so the attack surface is reduced.
RTUs and other automatic controller devices were being developed before the advent of industry wide standards for interoperability. The result is that developers and their management created a multitude of control protocols. Among the larger vendors, there was also the incentive to create their own protocol to "lock in" their customer base. A list of automation protocols is being compiled here. industrial firewall and VPN solutions for TCP/IP based SCADA networks.
Future trends in SCADA
The trend is for PLC and HMI/SCADA software to be more "mix-and-match". In the mid 1990s, the typical DAQ I/O manufacturer offered their own proprietary communications protocols over a suitable-distance carrier like RS-485. Towards the late 1990s, the shift towards open communications continued with I/O manufacturers offering support of open message structures like Modicon MODBUS over RS-485, and by 2000 most I/O makers offered completely open interfacing such as Modicon MODBUS over TCP/IP. The primary barriers of Ethernet TCP/IP's entrance into industrial automation (determinism, synchronization, protocol,selection,environment suitability) are still a concern to a few extremely specialized applications, but for the vast majority of HMI/SCADA markets these barriers have been broken.
Recently, however, the very existence of SCADA based systems has come into question as they are increasingly seen as extremely vulnerable to cyberwarfare/cyberterrorism attacks. Given the mission critical nature of a large number of SCADA systems, such attacks could, in a worse case scenario, cause massive financial losses through loss of data or actual physical destruction, misuse or theft, even loss of life, either directly or indirectly. Whether such concerns will cause a move away from the use of SCADA systems for mission critical applications towards more secure architectures and configurations remains to be seen, given that at least some influential people in corporate and governmental circles believe that the benefits and lower initial costs of SCADA based systems still outweigh potential costs and risks.
CONCLUSION
Potential benefits of SCADA
The benefits one can expect from adopting a SCADA system for the control of experimental physics facilities can be summarised as follows:
a rich functionality and extensive development facilities. The amount of effort invested in SCADA product amounts to 50 to 100 p-years!
the amount of specific development that needs to be performed by the end-user is limited, especially with suitable engineering.
Enhance reliability and robustness.
technical support and maintenance by the vendor.
For large collaborations, as for the CERN LHC experiments, using a SCADA system for their controls ensures a common framework not only for the development of the specific applications but also for operating the detectors. Operators experience the same "look and feel" whatever part of the experiment they control.
REFERENCES
· Note: this article is based on a very similar one that has been published in the Proceedings of the 7th International Conference on Accelerator and Large Experimental Physics Control Systems, held in Trieste, Italy, 4 - 8 Oct. 1999.
[1] A.Daneels, W.Salter, "Technology Survey Summary of Study Report", IT-CO/98-08-09, CERN, Geneva 26th Aug 1998.
[2] A.Daneels, W.Salter, "Selection and Evaluation of Commercial SCADA Systems for the Controls of the CERN LHC Experiments", Proceedings of the 1999 International Conference on Accelerator and Large Experimental Physics Control Systems, Trieste, 1999, p.353.
[3] G.Baribaud et al., "Recommendations for the Use of Fieldbuses at CERN in the LHC Era", Proceedings of the 1997 International Conference on Accelerator and Large Experimental Physics Control Systems, Beijing, 1997, p.285.
[4] R.Barillere et al., "Results of the OPC Evaluation done within the JCOP for the Control of the LHC Experiments", Proceedings of the 1999 International Conference on Accelerator and Large Experimental Physics Control Systems, Trieste, 1999, p.511.
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