ABSTRACT:
“Good people all, of every sort, give ear unto my song”, this was the cry made by thousands of operator running to and fro to detect, analyze and report the fault to there senior officials and wait for their instructions to be followed. Finally someone heard
their cry and a master system was born. The energy is generated, transmitted, distributed and finally utilized. At every stage certain supervision, control and protection is necessary. This paper deals with basic SCADA system, its components, basic block diagram, functions, application and advantages of SCADA.
SCADA helps in monitoring the system and have alarm generation facility incase of faults, which helps in real time analysis. This paper deals with the data logging ability of SCADA enabling off line analysis (M.I.S. report). Thus remote control over power system is achieved by SCADA, which is effectively depicted in this paper.
INTRODUCTION:
These days sophisticated working environment has become a need of the hour. Analysis of the following problem would make the need more specific. Central Railways cover a large and extensive area and efficient operation of such large network is must. Consider that circuit breaker near remote place has tripped and the supply has been cutoff. The power supply has to be restored as early as possible so that the route is cleared for the other trains and the smooth flow of trains is possible.
Hence the question of centralized monitoring and control arises. If the person back in control room knows the position of circuit breakers and could control it from there itself, the power is restored quickly and hence disruption of train route is avoided. To achieve this control data is to be acquired precisely and accurately. Thus the above problem shows us that there is need of centralized control and monitoring system, which in this age is achieved by using the SCADA system.
WHAT IS SCADA?
SCADA is abbreviation of supervisory control and data acquisition. As the name implies SCADA system supervises, acquires and control data received from a distant data source from the control center. SCADA system is located in the control center and is operated in the scanning mode, communicating between the CONTROL CENTER and the REMOTE STATION by means of two-way communication channels. Such a supervisory control and data acquisition system is intended to facilitate the work of operator by acquiring and compiling information as well as locating, identifying and reporting faults. On the basis of information received, the operator makes necessary decisions via the control system he can then perform different control operations in power stations or influence the processing of the information acquired.
FUNCTIONS OF SCADA
Thus from above we can infer that the functions performed by SCADA are;
- Data acquisition.
- Data transmission.
- Data logging.
- Monitoring.
- Alarm.
- Control and indication.
- Display.
FUNCTIONAL UNITS OF SCADA:
Following equipments perform the above functions;
- Data collection equipment.
- Data transmission / telemetric equipment.
- Remote terminal unit.
- Data loggers.
- Data presentation equipments.
- The figure shown below represents the simplest SCADA configuration employing single computer
Data collection equipment:
Data consists of electrical and mechanical variables, on/off states, analogue/digital quantities, and change of state, sequence of events, time of occurrence and several other data, which the control room operators would like to know.
Data collection equipment acquires it by means of CTs, VTs, Transducers and other forms of collecting information and converts the collected data into electrical form so as to enable easy measurement and transmission of data. Data is collected at low level (10mV) or high level (5V).
Data transmission equipment (Telemetry):
Tele means remote and telemetry refers to the science of measurement from a remote location. It comprises of electronic equipment, which converts data received from transducers into analogue or digital signals, and transmits it to the control room for the use of computers. Data can be transmitted by means of;
- Telephone lines.
- Separate cables.
- Power line carrier.
- Radio waves.
Telemetry for supervision and control
The method of transmission of data depends on the kind of information transmitted, quantity of information to be transmitted, available transmission channels and the degree of security demanded against error or loss of information.
Remote terminal unit:
The unit which records and checks the signals, measured values and meter reading before transmitting them to control station and on opposite side to transmit command, set point values and other signals to the switchgear and actuator is called remote terminal unit.
Data processor and logger:
The large number of electrical/ mechanical/ other data are scanned at required interval, recorded and displayed as per the requirement. Data loggers perform the following functions:
- Input scanning.
- Signal amplification
- A/D conversion
- Recording
- Programming
- Display
- Data logger with interface
Input scanner is generally a multi-way device that selects input signals at regular periodic intervals in a sequence. Slow varying quantities are scanned with longer period of intervals; fast varying quantities are scanned at shorter intervals of time. Output of scanner is given to A/D converter. Digital signals are given to C.P.U. , Analogue or digital signals are received in control room which are processed for measurement, recording, display and control by the instrumentation system in the control room. Display provides the operator with these informations
The above system is known as MMI (man machine interface) as it helps in interaction between man and machine.
Data presentation equipment:
Data to the operator in two forms:
Tabulated values of parameters, measured values and computed characteristics.
Graphical display representing status of equipments in the form of mimic diagram.
Various types of display available to the operator are alpha-numeric displays, single parameter displays, mimic diagram display and alarm display.
Advantages of SCADA system:
- Flexible, simple, reliable.
- Efficient with less manpower.
- Security.
- Self-checking and readability.
- Portable and cost efficient.
Applications of SCADA system:
- Inside power plant.
- On power plant.
- Industrial establishment.
- Load dispatch center.
- Railways.
SCADA IN RAILWAYS:
Railways employ Redundant SCADA system. In this scheme computer keeps on scanning all the RTUs in cyclic order with unique station address for change in state or analogue parameter. All RTUs receive this changed signal; RTU with matching station address only acknowledges it and replies back. All other RTUs skip the signal.
Any change in the status of the device transferred back to master station and displayed on VDU alarm. Commands given from the remote control center for any operation has been given priority when a command is given for a particular post. Computer suspends the cyclic scanning and comes to the post where command is to executed. Master computer in this case forms a command packet in digital form containing station address and operations to be performed and send it to front end processor. FEP then modulates this signal into frequency and transmits the same on communication lines to the RTU. Modem card on the RTU receives this command, again converts it into digital form and gives it to the CPU at RTU.
CPU analyses the operation to be performed and accordingly the concerned relays are operated for opening or closing of the devices. The change in the status of the device is sent back to the master station in similar fashion. After completion of this process the computer resumes its scanning process in cyclic order.
CONCLUSION:
From the above paper we can summarize that the SCADA system supplements the control system and protection system to form an integrated system, which is compact, economical and versatile. From the knowledge of SCADA one can know the energy distribution pattern, contribution of each load to the overall consumption and identifying the major energy wasting areas. SCADA provides futuristic approach there-by enabling further expansion of the system.
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