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SCADA (Supervisory Control and Data Acquisition): SCADA is a collection of equipment that provides monitoring and control of process and parameters of a central site. As a consequence of huge industries and plants an operator has to monitor and control thousands of sensors. As the definition itself says this system supervises and controls real time data gathered remote site. SCADA system is used to monitor and control a plant or equipment in industries. Supervisory control and data acquisition (SCADA) technology collects real-time data from virtually any environment where there is a need to monitor machinery or processes. As such, it is purely software package that is positioned on top of hardware to which it is interfaced, in general via Programmable Logic Controllers (PLC'S), or other commercial hardware modules. SCADA system can be built by different kinds of technologies and protocols.
Why SCADA is used?
If a fault occurs in a huge network operator has to find the fault and he has to rush to the fault location without delay to avoid interruption of work. By using a SCADA system an operator can monitor and control the whole process from his seat by giving proper instructions.SCADA systems are used where human control is impractical. And they can rectified or controlled faster than a human being. This systems are used in the huge networks with multiple control factors, and the control factors vary in fraction of seconds where an operator control is not possible, than human beings can comfortably manage. Real time monitoring and control of a system optimizes efficiency and profitability. Automation eliminates human errors achieving better productivity, reliability and optimization with lesser utilization of resources and time. By using this kind of automation the system can be monitored uninteruptly without breaks.. The entire system can be monitored over a network, from a central location through the Internet or a dial-up line or telephone line.Cables and wired communication was used in older days but this is limited to small area.In power plants to monitor transmission SCADA is used in this scenario using cables LAN for communication is not possible so wireless communication is used.
Where is SCADA used?
Areas of SCADA systems control:
Power Systems: SCADA is used in Electric power generation, transmission and distribution to analyze load flow analysis, to predict the control operation within the statutory limits as prescribed.
Water and sewage: SCADA is used to monitor and regulate water flow, Water Leak detection and location, Treatment plants integrated local / remote control , Pumping station automatic management
Functionality - alarm detecting and event monitoring, data acquisition, network operator interface, non real time control, Data bases and data logging, use of MMI, logging/archiving, report generation, automation.
SCADA systems consist of:
There are three main classification in SCADA system:
RTU's (Remote Telemetry Units),
HMI (Human Machine Interface).
These operations are performed by Master station, MMI/HMI and RTU or PLCs,Communication network.
RTUs or PLCs: Theses are Data interface devices which interface to field sensing devices. These are small computerized units will be located at remote areas where data/control should be taken these devices are interfaced to sensors and switches and gathers reports from them and delivers data . Remote telemetry unit converts electric signals at remote site to digital signals as the sensors and control relays are simple electric devices they can't communicate with protocols on their own .Remote telemetry unit (RTU) acts as an interface between the sensors and the SCADA network. Most of the controlling in SCADA system is done by RTU's and PLC's.
Communication Network : A communication network used to transfer monitored or controlled data between monitoring devices ,controlling units and servers or central computers in the SCADA or master station. The media for Communication system may be radio, telephone lines, cable, satellite, etc combination of any of these.
Master Station : A central host computer server or servers (sometimes called a SCADA Center, master station, or Master Terminal Unit (MTU). It is the main body of the Master station serves as the central processing unit for the SCADA system. Master station consists of various modules like Historical Management System, Trend Management System, Log Management System, Alarm Management System etc. MTU predicts human interface to the system and automates regulation of the managed system in response to sensor inputs.
HMI/MMI :It is an interface where data is processed and presented to the operator to monitor and control. This acts as an interface between SCADA system and operator so it is called Human Machine Interface or Man Machine Interface. A Human-Machine Interface is the system which ensures process data from master system to a network operator, and by which the operator controls the process.
How SCADA Systems Work
SCADA system functionality is divided into four functions:
1. Data acquisition
2. Data Communication
3. Data presentation
Data Acquisition:Data acquisition is collection of real time data.With the adverse impact of huge equipments and plants SCADA system has to monitor hundreds of sensors.The entire system can be monitored with data acquisition. Data acquisition is the information technology It is designed to collection of data , recording the collected data, storing and analyzing the data gathered from electronic devices such as sensors or gauges. Data acquisition is meant for digitizing of electric signals and processing of multiple sensor or signal inputs. Supervisory and data acquisition is an application that collects data from remote site for processing from a centralized computer. common goal of acquiring, analyzing, and presenting information. Data Acquisition systems can also form partÂ of a process control system by use of appropriateÂ software provides direct digital control ofÂ various process. Data acquisition systems incorporate signals, sensors, actuators, signal conditioning, data acquisition devices, and application software.
Data acquisition software variedly used from simple data-collection to high level programs for measuring and controlling process. DAQ software capture device data from scales, gauges and barcode-scanners in real time. Data acquisition software reads device data from TCP/IP protocol. This software is used to control processes such as starting or stopping a counter, sending control command to an analog-to-digital converter, gathering data points at regular intervals from a sensor or a switch. Some of the complex data acquisition software is used for presenting the data gathered in the form of various representations, likely graphical and pictorial pictorial representations such as graphs or three-dimensional graphics.Data monitored is saved in the database. The HMI of a SCADA system is where processed data is presented to be monitored by a human operator.
Data Communication: In real world, for monitoring multiple systems from a central location, we need a communications network to transfer all the data collected from remote area to central station and processed data from central station to HMI/MMI Vice-Versa. Earlier SCADA networks are communicated over radio telephone lines, modem or dedicated serial lines. Now we are using Ethernet, IP and in huge plants wireless communication is used.To transfer data gathered securely this is SCADA data is encoded and transferred over the secure protocols. These are designed to be very compact.. RTU converts the electrical signals from the equipments like sensors and relays to digital values.RTU emphasis like a interface between the sensors in the remote location and the SCADA system. Remote telemetry unit RTU are used to transmit data from remote area to central station and the RTU receives control commands in protocol format from the central and converts electrical signals and transfers to the appropriate control relays.
Data Presentation: The data received from RTU's or PLC's will be displayed to the network operator over computer system in central station , human machine interface(HMI).Central station monitors the whole system like sensors. It predicts the operator the operation of the system as graphical representation or pictorial representation. It alerts the operator when there is a malfunctioning in the system with alarms.
Control : The data gathered from remote site is controlled automatically mostly by Remote Terminal Units ("RTUs") or by Programmable Logic Controllers ("PLCs") with predefined set of values. An operator with Human Machine Interface (HMI) monitors the situation and he will pass the respective control commands. SCADA system may allow operators to change the predefined parameters values for the flow, circuit breaker operation, starting and stopping of a counter ,and enable alarm conditions, high temperature, to be displayed and recorded. The whole control can be achieved from a central location without any risk. The performance of the system after passing the control commands is monitored.
SCADA APPLICTION IN POWER SYSTEM:
By using SCADA system, large network having several generating stations and substations and large load centers is controlled from centralized load dispatch. Scada system emphasis on electricity utility remotely-monitor co-ordinate, control transmission and distribution components and devices in real time from a remote area with acquisition of data for analysis and planning from one control area. Most SCADA systems run either on a UNIX variant or on VMS.
In this system the real time data from the power system is gathered from transducers which converts the a.c. signals from the Current transducers and Power transducers to the dc signal proportional to the measured value for the respective parameters is converted to the digital signals. These transducers are interfaced with RTU (Remote Terminal Unit), which is located at the generating stations or substations the data is transmitted to the load control centre through power line carrier system (PLCC), fiber optics communication and microwave channels. Thus the data acquisition is done. The data, which is processed by the computer systems employing energy management software, provides automatic and remote control of the network at the load control centre. Then the instructions from the load control area are transmitted to the control rooms of the substation and generating station for executing appropriate action.
Digital computers and microprocessor installed in the control rooms of large substation generating station and load control area are used for data collection, data monitoring and a automatic control.
Thus, by employing SCADA system to power distribution network provides integrated approach to the power system protection, operation control and monitoring automatically with least intervention of control room operator.
SCADA FUNCTIONALITY IN POWER SYSTEMS
Data acquisition: Provides measurements and status to operators.
Representing plots and measurements on selected time intervals.
Supervisory control: Enables operators remotely control devices circuit breakers and relays.
SCADA employs network operators to control circuit breakers disconnect switches change transformer taps and phase shifter position remotely. It enables operators to monitor the generation and high voltage transmission system to predict over loads or out of limit voltages. It
Monitors all switch gear position, substation loads and voltages, capacitor banks, tie line flows and interchange schedules. It detects through telemetry the failures and errors in bilateral communication links between computer and remote equipment. These functions are scanned every fraction of second precisely.
SCADA provides status and measurements for distribution feeders at substation. It renders customer meter, implement time of day pricing and switch customer equipment to manage load and improves functionality of distribution control centre.
SCADA is extensively used for compilation of extensive data and management of distribution systems. Tripping due to human errors can be avoided eminently.
By using SCADA system manual errors and oversights are made almost negligible, the system provides periodic reports that help in analysis of performance of power system. Distribution network monitoring functions with geographical mapping, fault location. At generation 'distributed control' has reduced the cabling cost within the plant and has potential of replacing control rooms with distributed CRT/keyboard stations. Data Acquisition collects data from particular points in the power system converts these into engineering units. All these inputs are brought through cables to terminals. This information is supplied to perform following functions.
1. a) ANALOG: Continuous Electrical Signals Ex. Active Power (MW), Reactive Power (MVAR), Voltage (KV), Frequency (Hz).etc.
b) DIGITAL: Switching Signals High (1) or Low (0) Signal Ex. Breaker Close (high) or Open (low), Isolator Closed (high) or Open (low).
2. Process- a) The signals are converted into digital format. b) Implement protocol between Master and Slave. c) It operates with Real Time Operating System (RTO).
3. Output- a) The results are exposed with user friendly environment.
b) Through displays can be possible to control the substation and generating station.
Display on CRT screen
Graphical display of plant sub-systems
Trending of analog variables
Generating control signals.
The CRT's in control room enables operator to display of alarms, plant variables. To view the information of plant along with real time variables and status information. The limits of all variables are checked if any variable is out of range an alarm is raised. The status changes and time of occurrences are maintained accurately in order to maintain quality analysis efficiently all the equipments are periodically calculated.
AUTOMATIC SUB-STATION CONTROL
The electrical energy is transferred from large generating stations to distant load centers via various sub-stations. In every sub-station certain supervision, control and protection functions are necessary. Every substation has a control room. The relay and protection panels and control panels are installed in the control room. The various circuit breakers, tap changers and other devices are controlled by corresponding control-relay panels. In a small independent sub-station, the supervision and operation for normal service can be carried out by the operator with the aid of analogue and digital control systems in the plant. The breakers can be operated by remote control from the control room. During faults and abnormal conditions, the breakers are operated by protective relays automatically. Thus, the primary control in sub-station is of two categories.
Normal routine operation by operators command.
Automatic operation by action of protective relays and control systems.
SUB STATION CONTROL FUNCTION ARRANGED THROUGH SCADA SYSTEM
1. Alarm Functions
To sound alarm/annunciation regarding dangerous, uncommon events such as abnormal values of process parameters, fire, illegal entry in premises, over temperatures, low voltage of auxiliary supply, unusual happening etc. Alarms are obtained from data logger and are for alerting this operator in the control room.
2. Control and Indication
Control of two position devices such as circuit-breakers, isolators, earthing-switches, starters. Indication of ON/OFF state of the devices on control board/mimic diagrams.
Control of position of devices having positions (closed, middle open) e.g. values, input settings, indication of position on control panels.
Control positions of multi-position device e.g. tap changer, indication of position on control panels.
Indication without control.
Control without indication: e.g. raise or lower control of generator load by automatic load frequency control.
Set-point control to provide set point to a controller located at remote sub-station.
3. Data collection, recording, display.
4. Sequential operation of devices with predetermined time and conditions for operation of various devices
Auto-reclosing of circuit-breakers operation O-CO-Time-CO
Operation of circuit-breaker, isolator and earthing switch in a particular sequence during opening of circuit and another sequence during closing of circuit.
5. By means of SCADA system, the operator in control centre can cause operations in a remote sub-station. The possible remote operations include:
Opening and closing of switching devices I
Tap-changing of transformers (voltage control)
Switching of capacitor banks (voltage control)
Load shedding (load frequency control)
6. Some of the remote operations are made automatic by one-line computer based system without human intervention e.g. Net work islanding, Backup protection. The automatic control function are segregated into :
Transmission line automatic function
Distribution system automatic functions
Thus by using SCADA system increase in capacity utilization in generation . ii.Saving energy and raw materials due to increased operational efficiency.SCADA system monitors and controls the grid and improves effeciancies and plays key role in maintaining the power system security.By using SCADA system monitoring and controlling can be done from anywhere in world by using wireless networks.
Viewing and monitoring of real time data.
Time synchronization with GPS
Powersystem co-ordination and Control through communication.
Alarms for any abnormal condition in the system.
Studies can be done.
Optimum Power Utilization through computers.
Planning of electrical power position in the state.
Minimization of Man Power.
Monitoring the theft of power in any position of transmission.
Cost of Saving Revenues.
Faster restoration of normal revenue.
Minimizing outages of equipment.
User friendly report generation.