02-03-2011, 04:14 PM
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INTRODUCTION
1.1 Supervisory Control and Data Acquisition System:
Supervisory control is nothing but the ability to exercise control over a specific device, and to confirm its performance in accordance with the directed action. Let us define a supervisory control system as a collection of equipment that will provide an operator at a remote location with enough information to determine the status of a unit and to cause actions or operations to take place at that facility without being physically present.
Contemporary SCADA systems exhibit predominantly open-loop control characteristics and utilize predominantly long distance communications, although some elements of closed-loop control may also be present. SCADA systems generally cover larger geographic areas, and rely on a variety of communications systems that are normally less reliable than a LAN. Closed loop control in this situation is less desirable. It is used to monitor and control plant or equipment. Scada is a combination of hardware and software. It needs a man machine inter phase like a personnel computer
1.2 Main Functions of SCADA
• DATA ACQUISITION
• SUPERVISORY CONTROL AND MONITORING
• ALARMS
• INFORMATION STORAGE AND REPORTS
• DATA PROCESSING
• SECURITY ANALYSIS
1.2.1 Data Acquisition
The primary function of SCADA system is to automatically collect data from the field using various types of sensors. The data is acquired by means of Current Transformers, Potential Transformers, Transducers and various other methods.
There are two basic modes of capture of input data. These are:
• Scheduled Capture, whereby the local units are polled on a regular basis and all input data are transferred
• Change of state capture, whereby only input data which have changed are transferred.
1.2.2 Supervisory Control and Monitoring
One of the main functions of SCADA system is to allow the entire process to be monitored and controlled with graphical user interface. The operator can interact and supervise a process from the operator console. The continuous monitoring can also ensure that the system retains its smooth operation by taking protective action. It is up to the individual specifications set, that decides how much the SCADA controls and monitors.
1.2.3 Alarms
All the data scanned by the Central monitoring station is processed so that the system detects the abnormal conditions and if present alerts the operator in the form of audio-visual indication thereby calling for the intervention.
1.2.4 Information Storage and Reports
Record-keeping has always been an important task in the operation of electric systems. Accurate records are necessary to satisfy legal and governmental requirements, for accounting purpose, for support and forecasting of future system operations, and for engineering planning purposes.
1.2.5 Data Processing
Data Processing means a conversion of data from raw form into the form that is useful for calculation and presentation. Data Processing is responsible for converting Analog values from raw data to engineering units. It is also responsible for converting digital status points to a system convention of device states (0-closed, 1-open).
1.2.6 Security Analysis
The system security of any process may be defined as the ability of the system to operate in normal state even with the occurrence of specified contingencies. System security analysis is generally broken down into following three functions:
• System monitoring: SCADA provides up to date information regarding the condition of the process.
• Contingency analysis: Sometimes abnormalities give the operator very less time to react. SCADA system provides contingency analysis, which consists of actions to be taken by the operator in advance. Thus it allows the system to operate defensively.
• Corrective action analysis: It allows the operator to take appropriate operating action in the event of contingency in order to ensure the smooth functioning of the process.
1.3 Features of SCADA
• SIMULATION OPTION
• DATA IMPORT/EXPORT OPTION
• FLEXIBILITY
• FORECASTING
• JOB MANAGEMENT
1.3.1 Simulation Option
SCADA system contains the simulator option which allows the operator to have a hand-on experience in dealing with the day to day problems occurring in the plant by creating the environs similar to that of the main process The operators are trained in this environment which depicts the same behavior of the plant and helps them, in understanding the plant operation in better version. The simulation method included in SCADA allows the testing of new control sequences before they are taken into operation.
1.3.2 Data Import/Export Option
In case of future expansion of the existing SCADA system or for acquiring data from another SCADA database the import feature allows the transfer of all the point configuration data via a ‘comma separated variable’ files.
Management of such data is possible using applications such as Microsoft excel Microsoft access, a text editor, or a database report writer.
1.3.3 Flexibility
For optimization of any pre-existing system it is very important to possess tools by which an existing system could be tailored according to the changes taking place. SCADA has a unique feature which allows the user to mould the system according to the demands thus making it more flexible. This ensures the plant optimization. Also while installing a new SCADA system these tools can help in properly meeting the requirements of the process.
1.3.4 Forecasting
Forecasting is the ability to predict the future state of a system by studying the previously collected data. Forecasting feature of SCADA systems allows the operator to visualize the state of the system well in advance, hence the operator has enough time to manage the system properly.
1.3.5 Job Management
The SCADA system can be programmed to do all the functions in the plant in a proper order. Also the tasks can be properly sequenced and executed to allow the most efficient task scheduling for proper utilization of man and machinery of the plant. Thus ensuring the most optimum utilization of the resources. Through proper job management and thus the conformance to international standards the safety and security of the plant and the personnel is also ensured.
1.4 Need Of SCADA In Our Project:
In a any power plant the turbine rotates and it gives mechanical power. That mechanical power is converted into electrical power through generator by faradays law of electromagnetic induction at Indian standard frequency. That is 50HZ.
The voltage of electrical output is maintained constant in power system for any load demand. But load is continuously varies with rising (or) falling trends. Consequently the voltage also varies with the load and similar to frequency also.
1.5 Problems Caused By Change in Voltage and Frequency & Its Remedies:
1. The change in voltage levels may cause severe damage to the load equipments at that instant the load equipment cannot be able to work efficiently.
2. Load output also varies with variation in input voltage levels.
3. The change in the frequency causes change in speed of consumers and also affects the plant production process.
4. The change in frequency damages the sensitive parts in the plant.
To minimize these problems must maintain the constant voltage & frequency in the power plant. In this project we implemented an control strategy of voltage stabilization for excitation voltage in the alternator, why because? ,whenever the excitation voltage varies the output voltage of the alternator also varies. So to meet the load fluctuations we need to control the excitation voltage of the alternator.
In this project, we also monitor the fluctuations in the frequency, due to non-linear loads the source frequency varies. We achieve these control and monitoring techniques of voltage and frequency by using SCADA technology.