An introduction to SCADA

Introduction: distribution system automation is built on 5 pillars which are computer programming, computer aided design & drafting (CADD), system supervisory & data acquisition systems (SCADA), local networks including LAN & WAN and geographic information systems (GIS) including global positioning system (GPS) software & receivers. In this article an introduction to SCADA systems will be presented. The main purposes for the use of a SCADA system would be to collect the needed data from remote sites and even the local site, displaying them on the monitor of the master computer in the control room, storing the appropriate data to the hard drive of the master computer and allowing the control of field devices (remote or local) from the control room. Such systems are used in various fields and industries to monitor and control the operation and/or process of the plant. They are used to control lighting, elevators, HVAC (heating ventilation & air condition) and other loads found in commercial & institutional buildings. They are,also, used to automate industrial, chemical & petrochemical plants as well as the food processing factories, pulp & paper mills, pharmaceutical, plastics/rubber and primary metals plants. They are applied in the utilities field to monitor & control the water treatment & supply systems, the sewage treatment plants and the electrical power systems including the generation, transmission & distribution of electrical power. In general, the information & conditions required from the field devices can be classified into either digital (discrete) or analog (continuous) ones. Such data is collected and reported by the RTU (remote terminal unit) to the master computer in the control room through the communication link. The (remote) devices in the field that are to be controlled by the operator from the control room can be classified into continuous or on/off (discrete) control. SCADA systems vary widely from one supplier to the other. A few years ago, each system has its proprietary network & communication protocol that allowed the different devices to communicate with each other. The field devices used to communicate over the network to the master computer. When additional equipment had to be added to the network, they had to be able to communicate using the existing network & protocol, thus limiting the user choices. Nowadays, the trend is going towards open communication systems. Though the products available can handle/support more than one communication protocol, the communication protocol is becoming more standardized even the Internet protocol (TCP/IP) is used in information, data & commands passing from one device to another.Components of a SCADA system: a typical SCADA system will have 5 major components which are the master computer, the software, the RTUs, the communication link and the transducers.The master computer: it is the hardware at the control station (room). Basically, it comprises of the personal computer (PC) complete with the attached peripheral (connected) devices including the monitor, the printers, data loggers & mimic panel (having lamp & analog outputs) that allows the operator to monitor & control the field data & distributed devices/equipment over the plant or city. The PC serial port is connected to a suitable modem (system box) for communication with the field RTUs. The PC will have the appropriate operating system (the platform which the SCADA software is going to run under), the microprocessor, the hard disk, the floppy disk drive, the CD-ROM drive, the memory (RAM), the graphical adapter, support for multi-screen adapter boards and logical input/output for pen recorders, mimic panel,..etc.

Keyboard essential functions: the key board can be divided into 4 sections:1) Alphanumeric keypad that is used to enter text & numbers by the operator.2) Editing keypad which includes the the cursor control keys and certain user-defined functions.3) Auxiliary keypad which includes the user defined function keys that would reference the most important pictures in the system.4) Top row function keys which includes the system special functions and more user-defined functions.The keys of the key board will offer as a minimum the following functionality to the operator: recalling previous selected pictures (the pictures of the system or plant built using the screen builder facility provided with the SCADA to model the existing user system/plant, more about this topic in Software section later), selecting the next picture, selecting an object or screen target, preparing system for data input, verifying the data input, removing entry from alarm list, switching command, controlling remote devices, acknowledging an alarm, canceling incorrect input sequence, executing a control sequence, selecting system picture, selecting telemetry device picture, selecting hardware configuration screen (picture), selecting profile picture for an object, selecting index picture used for selecting screens & reports, selecting the chronological alarm list.Essential displayed information (on the monitor): the pictures (screens) are displayed in text and/or graphics mode, the display will have the following minimal instructions/information:1) Data & time, title of screen & page number.2) The next input steps the operator is allowed to make.3) The last alarm in the system.4) The operators input part plus the part of the screen that displays the error message if the operator enters wrong values or illegal sequences.5) The process/data of the selected zone of the modeled system.The software: it includes the following operator's tools: database organization, the screen (picture) builder, programming facility and a shell-like application.The data base control tool: the real time database contains all system data. There are 3 distinct models of data base management systems. They are the hierarchical, the network & the relational models. The first 2 are quite complex in the sense that they rely on the use of permanent internal pointers to relate records to each other. The process of inserting (entering), updating & deleting records using these types of databases requires synchronization of the pointers (a task that must be performed by the application). Relational databases rely on the actual attribute values as opposed to internal pointers to link records. Using a common attribute from each record, the linking of records is achieved. A RDBMS stores data like a collection of tables. As mentioned, common fields in different tables allow linking the tables together. Because of its simplicity & flexibility, RDBMS found a wide use in the different applications including the SCADA. Access to the data base to allow the operator to manipulate the data stored may be achieved through the ability to write to the database, to read from the database, to special write to data base and to read from data base with conversion to another data format.Screen (picture) builder: it allows the user/designer to construct the different screens (mimics) that represent the different zones of the plant/system. Each screen will have the different objects (elements) that has to be monitored (polled) and controlled. Each of the elements will display the status and/or relevant attributes that need to be verified by the operator. It is used to graphically

develop new mimics (pictures), add/delete/save new pictures, define mimic background, import other graphics files, create & save in a user defined symbol library graphical objects (elements) plus other functions as required to build screens (pictures).Programming facility: it is provided to fulfil the user's special requirements. This provision should allow the user to add to the functionality of his/her interface, manipulate files, perform complex mathematical & string operations and to construct simple control sequences using logical decisions. It may, also, be used in conjunction with event driven, cyclic or scheduled actions to further increase the capability of the application.Shell-like application: it is used to manage the display of the windows having the pictures, trends, elements (objects) properties & tables and other applications within the Workstation environment. It allows the user to control the position and location of the opened windows in a graphical environment. It may also provide the provision for the operator to build custom menus.The remote terminal units: they are the devices that accept digital, analog and counter inputs and provide digital and analog output. The inputs to the RTU are the indications/status (digital-on/off, closed/tripped,..etc), the values (analog-measurands & levels) and pulses of the field devices i.e. the information to be monitored and reported to the master computer. To effect control on the remote devices, the analog & discrete output of the RTU are connected to the pertinent devices/equipment. Discrete output is used to drive an external relay, to operate a circuit breaker (closing it or tripping it), dropping a section of the feeder by opening a pole mounted switch or a a pad-mounted switchgear switch, to disconnect a service, to switch off a motor and other similar applications. Analog output are used to remotely control devices that requires an increment adjustment or a variable set point for example opening/closing a valve, controlling a tap changer,...etc. If the RTU is intelligent, it can be programmed to make certain decisions instead of sending the information to the master computer and wait for the instruction to come back. Such devices could be considered as upgraded to the controller level. The RTU requires a power supply, has several interfaces & multiple ports with the protocol selectable on a per-port basis. The RTU may also have a microprocessor, RAM, real time clock, watchdog timer, LED indicators, internal diagnostics routines, fiberoptic interface and an internal (built-in) modem.The communication link: it can be any or a combination of any of the following: leased lines, dial up lines, cellular circuits or VHF-UHF radio. Leased lines are reliable & require less initial investment in equipment. It is used when data polled from the remote sites is required at the control station continuously in real time. The dial up lines are used when the information at the remote sites are required less often. The investment in equipment is low. The modems used are the same as those used with personal computers. The cellular circuits are similar to the dial-up except special modems & telephones are required at the remote sites (to be connected to the RTUs). VHF-UHF radio requires a transmitter-receiver & antenna for each site. Distances are limited to line-of-site from antenna to antenna. Repeaters may be required depending on the range and the attenuation level. The range is function of RF power (2 to 5 watts for narrow band & 1 watt for spread spectrum), the receiver sensitivity, the frequency selected. The 3 frequency ranges (bands) are 150 to 170 MHZ (VHF), 450 to 470 MHZ (UHF) & the 900 MHZ. The area of coverage varies due to terrain, antenna height, frequency band & vegetation. The spread spectrum uses one of three methods to spread the data signal: slow hoppers (frequency hopping), fast frequency hoppers and direct sequence.

The sensors & transducers: the sensors in the field are responsible for converting the real measured values into their corresponding electric signals that can be inputted into the transducers or the RTU. The transducers, if used in the remote monitoring & controlling system, are the intermediate stage to condition the signal from the sensors before getting it to the RTU.How a SCADA works?The data sources of the field devices at a remote site are the sensors/transducers eg. Current transformers/transducers combination, level gauges, pressure switches, valves, tap changers, breaker auxiliary contact, relay operating coil. The output from the sensor/transducer is converted to digital data through the RTU. In the case of radio telemetry, the output of the RTU is inputted into a radio modem. The modem converts the digital data into analog signal that can be transmitted over the air. If telephone lines are used as the communication medium, then the RTU output is fed into a modem (or the RTU may have a built in modem) from which the signal is put on the 2-wire (twisted pair) telephone line. At the receiving end the above process is reversed. The data recovery present at the receiving end will render the required compatible data to the master computer (or the field device). In a typical application, the master computer requests data from the remote field devices (polling), one RTU at a time. It transmits a request to the RTU to send its data. It then reverts to receive mode waiting for the data coming from the remote site. After the RTU sends the requested data it reverts to receive mode waiting for further instructions from the master station. Once the master station receives the remote site data, it goes into transmitting mode, it may send additional instructions to that site or move on to the next one to be polled. The polling process continues until all the remote sites in the system have sent their data. This process repeats indefinitely, if continuous polling is needed. The RTU can revert to sending if an alarm or a limit is exceeded (report by exception) and under control of the prevailing communication protocol sends the data to the master station.Tasks of a SCADA system: the main tasks that a SCADA system has to provide are: controlling the plant/system field devices (obviously), alarm handling, limits changing, providing more than 1 operation mode, data archiving provision, events logging and the production of report & trend charts (graphs).Controlling field devices: the system remote components are controlled from the master computer. The means of controlling the system can be any or all of the following: cursor positioning, function keys, custom designed menus and the alphanumeric keys. Questioning remote devices for specific status or values, closing or opening remote devices, adjusting field devices are just examples of what the operator is capable of doing from the master station.Alarm handling: when an unexpected event generates an alarm, various alarm indicators are provided to the operator (by the software). These indicators can be all or any of the following: audible alarm, station alarm lamp flashes on the mimic panel, the last alarm line is updated on all pictures, the alarm is inserted in the appropriate alarm list, the object symbol (element) or value flashes on the station diagram (process picture). An auto dialer/voice unit can be included which functions as follows: if an alarm signal from a remote site is not acknowledged at the master station (within a specified period of time), the computer will begin calling a customer configured phone list. Once someone answers the phone, the computer will ask for an authentication code before announcing the alarm condition. The computer records all voice activity in the system log. It can be used in conjunction with a cellular phone & a laptop.

Limits changing: the upper & lower limits for a remote measured value (that were previously set) can be changed from the master station. There is a time-out period for the operator to effect the change. If the time-out period expires, the operator must cancel the sequence he/she already entered and restart the sequence of changing from the beginning.Operation modes: the system should provide the following operation modes: operator mode (normal supervisory & data acquisition operation), programming mode (it is used by programmers/designers for changing data, system extension, program maintenance, report generation-control of network is disabled), training mode (self-explanatory) and special mode.Data Archiving: it regularly samples the values of the measurands & the states of the monitored points in the data base and storing them in history files for a predefined retention period. This task is useful for monitoring networks and in producing reports as well as providing data to the trending facility.Events logging: it is a list of all events which occur in the system & which are chronologically printed on the logger. The types of events can be: state changes of alarms & indications, time-outs, limit violation, operator input (commands, set point transmission, blocking & unblocking of remote stations, general interrogation of remote stations & simulation of measurands when a device is blocked) and system alarms (loss of communication with remote stations, hardware errors & faults in a remote stations).Production of reports & trend charts: a report consists of data which is presented to the user in a predetermined tabular format. It may be produced cyclically from the historical database. The trending facility allows the operator to generate graphic displays showing historical trend of measured data.