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International energy agency

Introduction

According to the survey of International Energy Agency (IEA) energy need in the world would be over 50% higher in 2030 than today with an increase in average annual rate of 1.8% per year(Valsalam, SR, Sathyan & SS;2008).With the increase in demand for power generation,distribution and transmission of power has to be optimized(M. Ordean). With the advances of electronic and software technologies, the supervisory control and data acquisition (SCADA) system is widely used in plant automation(Yao, Albert W.L). Modern Information Technology based Supervisory Control and Data Acquisition Systems (SCADA) assume greater significance in this context to derive maximum efficiency in power plant operations by ensuring optimal use of available resources. (Valsalam, S. Rominus;2008). By using SCADA system a large network having several generating stations and substations and large load centers is controlled from central load dispatch centre. This literature review provides an overview of SCADA , automation of power plant using scada ,optimizing power generation and automation of power distribution using scada.

SCADA (Supervisory Control and Data Acquisition): SCADA system gather and analyze data for real-time control(Farris, Jeffrey J;2004). As the definition itself says this system supervises and controls real time data gathered remote site. SCADA system is used to monitor, co-ordinate and control operation of plant or equipment in industries(KOTHARI, D. P. & NAGRATH, I. J.; 2004). 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.

Why SCADA is used?

As a consequence of huge industries and plants an operator has to monitor and control thousands of sensors. 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 huge in networks where human control is impractical and they can be rectified or controlled faster than an operator. 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.

Where SCADA is 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

Why SCADA used in power system?

As a result of increase in demand of the power, these systems has reached statutory limits of their maximum capacity and were proving inadequate to meet the sufficient operational requirements and respond to a growing uncertainty in utility power reliability and increase in losses in the event of power failure. This scenario leads to the replacement and up gradation of the complete SCADA system in power plant for automation (Chan, E.-K., Ebenhoh, H. & AG, S; 1992). Due to increase in power demand results in continuous network expansion and effort to provide a more reliable supply to its customers has failed to fill this gap SCADA is used in distribution system(Ghoshal & Koustuv;1997).

Overview of SCADA components:

SCADA system is implemented to facilitate data acquisition, monitoring, control and remote control of a process(Valsalam, SR, Sathyan & SS; 2008).

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.

SCADA CONTROL IN POWER GENERATION:

BENEFITS

High profit
High final product quality
Minimization of wastage in production
Minimization of energy usage
Maximization of overall production rate

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.

OPERATION:

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.

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).
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).
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
Data logging
Alarm generation
Event logging
Trending of analog variables
Performance calculation
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

Electrical power is transferred to load centres from huge generating station through various substations.Electric energy is transferred to large distances so an human monitoring will be impractical. So in every sub-station certain supervision, control and protection functions are inevitable this can be done by SCADA.The majority of the data for SCADA system in power system is gathered from substation. Every substation has a control room in which 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.

SCADA system detects disturbance cuts off the faulty portion of the network.The protection relay and control unit have communication bus and acts as a data transfer to local system.All the data acquisition devices are connected through common communication bus.The protection relays, control units provide the system with following data.

Measured Electrical quantities
Position data for switching device
Alarm data
Digital input values
Operation counters
Disturbance data
Device setting and parameter data

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

e.g.

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 :

Interconnection functions
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.

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