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Every parameter in the industry or laboratory needs measurement. For measuring those quantities dedicated instruments are more often used. These instruments provide very accurate measurement and are reliable. But they cannot be customized. They are very much useful in industries but they cannot meet the requirements of scientists and research workers. A virtual instrument overcomes the drawbacks of traditional instruments.
Virtual instruments are fueled by the rapid advancement of the chip technology and in PC. Virtual instruments represent a fundamental shift from traditional hardware-centered instrumentation system to software-centered systems that exploit the computing power, productivity, display and connectivity capabilities of popular desktop computers and workstations. Virtual instruments are real instruments, real world data is collected, recorded and displayed, it just uses the data acquisition capabilities, processing, storage and other capabilities of a computer.
This paper explains the main concept of virtual instrumentation and its application in power engineering lab.
In industries we find many parameters to be controlled, and many electronic instruments are used to control these parameters. All these instruments are dedicated to measure or control those parameters only. They entirely differ from one another but they have one thing in common, they all are box shaped and has some controls and knobs on them. the Stand-alone electronic instruments are very powerful, expensive and designed to perform one or more specific tasks defined by the vendor. The user cannot extend or customize them. The knobs and buttons, built-in circuitry and the functions available to the user, all of these are specific to the nature of the instrument. In addition, special technology and costly components must be developed to build these instruments.
Widespread adoption of the PC over the past twenty years has given rise to a new way for scientists and engineers to measure and automate the world around them. One major development resulting from the advancement of the PC is the concept of virtual instrumentation. A virtual instrument consists of an industry-standard computer or workstation equipped with off-the-shelf application software, cost effective hardware, which together performs the function of traditional instruments. Today virtual instrumentation is used by engineers and scientists for faster application development, higher quality products at lower costs.
Virtual instruments represent a fundamental shift from traditional hardware-centered instrumentation systems towards software-centered systems that exploit the computing power, productivity, display and connectivity capabilities of popular desktop computers and workstations. Even if PC and IC technologies experienced a good growth, it is the software that makes a reality of building virtual instruments.
CONCEPT OF VIRTUAL INSTRUMENT:
Usually instrumentation manufactures provide specific functions to given architecture and fixed interfaces for measuring devices, and thus limit the application domain of these devices. In actual use much time is required for adjusting the measuring range and for saving and documenting the results. The advent of microprocessors in the measurement and instrumentation fields produced rapid modifications of measuring device technology, soon followed by the appearance of computer based measurement techniques. These techniques consists of three parts as shown in fig-1, acquisition of measurement data, conditioning and processing of analysis of measurement signals and presentation of data.
The concept of virtual instrument is frequently used in industrial measurement practice, but not always with precisely the same meaning. In one view virtual instruments are based on standard computers and represent systems for storage, processing and presentation of measurement data. In another view, a virtual instrument is computer equipped with software for a variety of uses including drivers for various peripherals, as well as A to D and D to A converters, representing an alternative to extensive conventional instruments with analog displays and electronics. Acquisition of data by a computer can be achieved in various ways and for this reason the understanding of architecture of the measuring instrument becomes important.
A virtual instrument can be defined as an integration of sensors by a PC equipped with specific DAC hardware and software to permit measurement data acquisition, processing and display. Virtual instruments are a means of integration of the display, control and centralization of complex measurement systems. Industrial instrumentation applications however require high rates, long distances and multi vendor instrument connectivity based on open industrial network protocols. In order to construct a virtual instrument it is necessary to combine the hardware and software elements which should perform data acquisition and control, data processing and data presentation in a different way to take maximum advantage of the PC, as shown in fig-2. Virtual instrumentation can use the serial communication based on RS-232 standard or the parallel communication based on GPIB standard, PC bus or VXI bus.
BASIC COMPONENTS OF VIRTUAL INSTRUMENTATION:
The basic components of all virtual instruments include a computer and a display, the virtual instrument software, bus structure and instrument hardware.
COMPUTER AND DISPLAY: these are the heart of VI systems .these systems are typically based on a personal computer with a high resolution monitor, a keyboard and a mouse. Rapid technological advancements of PC technology have greatly enhanced VI. Moving from DOS to WINDOWS gave the PC users GUI. The advances in processor performance supplied the power needed to bring the new applications with in the scope of VI.
SOFTWARE: if the computer is the heart of virtual instrument the software is their Brian. the software uniquely defines the functionality and personality of VI systems. This can be divided into several levels.
REGISTER LEVEL SOFTWARE: it requires the knowledge of inner registers structure of the device for entering the bit combination taken from instruction manual in order to program measurement functions of the device. It is the hardest way in programming and is strongly hardware dependent.
DRIVER LEVEL SOFTWARE: it is one of the most important components in measurement systems. They perform the actual communication and control of the instrument hardware in the system. They provide a medium level easy-to-use programming model that enables complete access to complex measurements capabilities of the instruments.
HIGH LEVEL TOOL SOFTWARE: currently the most popular way of programming is based on high level tool software. With easy-to-use integrated development tools, design engineers can quickly create, configure and display the measurements in a user friendly form, during product design and verification. The most popular tools are: LabVIEW, Lab Windows, HP VEE, Test Point, and Measurement Studio.
APPLICATION OF VI IN POWER ENGINEERING LABORATORY
In traditional power engineering labs the data is collected using traditional analog voltmeters, ammeters, watt meters, multimeters and oscilloscopes and we spend a lot of time in connecting the standard hardware instrumentation by which we can only measure rms voltage, current and real power. the quantities such as reactive power and phase angle are calculated on paper .since these quantities are not measured at real time we cannot observe the effect on these quantities due to changes in other parameters .if the use of VI we can see the real time effects of an experimental variable on the real power, reactive power and phasor quantities and the spectral representation of ac signals. For example we can immediately see the result of loading a motor, changing a supply voltage or changing a connection.
Each computer is equipped with a DAQ board. The DAQ card acquires data from any of the channels and passes it to LABVIEW. A prime mover is placed at each station, which enables to mechanically drive generators and to load motors. The prime mover has analog speed and torque outputs which are directly connected to DAQ card. The combination of digital acquisition, computer processing and display is required to as Virtual Instrumentation.
It is important to emphasize that virtual instruments are real instruments: no simulation is involved. Real world data is collected, processed and displayed.
SINGLE AND THREE PHASE TRANSFORMERS:
To study the characteristics of single phase transformer, voltage is applied to the primary of a single phase two winding transformer. Using a current and a voltage channel, the input current and voltage are measured. The obtained characteristics are as shown in the fig-1. a third and odd harmonics occur . Fig-2 shows the hysteresis curve, which is obtained by displaying core flux as a function of excitation current.
To study the presence of harmonics in a three phase Y-Y transformer and the effect grounding the neutral has on the harmonics, three single phase transformers are connected in a Y-Y connection as shown in fig-2. by using the current and voltage waveforms VI can compute the full spectra of each. Fourier analysis is performed on each of the acquired waveforms in order to obtain different frequency components.
The ability to study the basic waveforms does not require virtual instrumentation. This is also possible by using C R O. But by using we cannot get the phasor diagrams. Using the current and voltage waveforms, the 50Hz component is extracted and the results can be displayed on a polar plot as a phasor diagram as shown in the figure-4.
DC GENERATOR CHARACTERSTICS
By using VI we can find the characteristics of a DC generator and find the magnetization curves as shown in fig-6. In the figure different DC generators characteristics are shown, which depicts a set of three graphs of load voltage versus armature current. The top most curves represent a separately excited Dc generator. The centre curve represents a cumulative compound long shunt dc generator and third curve represents self excited dc generator. The separately excited dc generator has grouping characteristics, self excited shunt dc generator has a reduction terminal voltage as shown in the figure. The cumulative compound dc generator has both these characteristics.
Similarly we can obtain the graphs of synchronous motors and induction motors and also to any electrical machinery
Virtual instrumentation is fueled by ever advancing computer technology and it offers the power of creating and defining someoneâ€™s own system based on an open frame work. The combination of computer performance, graphical software, and modular instrumentation has led to the emergence of virtual instruments, which are substantially differ physical ancestors. Virtual instruments are manifested in different forms ranging from graphical instrument panels to complete instrument systems. Modular instrumentation building blocks are becoming more prevalent in the industry and are allowing users to develop capabilities unattainable using traditional instrument architectures. Despite these changes however, these measurement paradigm remains unaltered. This might be the proper platform for the new development.