A Virtual Measurement Instrument for Three Phase Electrical Networks Analysis

Transcription

1 A Virtual Measurement nstrument for Three Phase Electrical Networks Analysis ALEXANDRU BALO ADRAN PANA Department of Power Engineering POLTEHNCA University of Timisoara Piata Victoriei, No., Timisoara ROMANA Abstract: - With the development of data acquisition and transmission systems, virtual laboratories can be an important element for the professional improvement of students. The paper presents a virtual three-phase measurement instrument developed using LabView graphic language and a National nstruments acquisition board. The virtual instrument can be used for various laboratory works regarding three phase electrical network analysis like: the study of the parameters for the electrical lines and transformers, particular operating conditions of transmission lines. The diagram blocks used and the corresponding front panels are described in the paper. Key-Words: - virtual instrument, block diagram, front panel, LabView, electrical networks ntroduction The term Virtual nstrumentation means the use of a computer equipped with specialized input and output devices to simulate the characteristics and functioning of an instrument or measuring system, test or data recorders. Virtual instruments make use of transducers and sensors to get in touch with the physical quantity measured by any system of signal conditioning and analog-digital conversion circuits []. The difference compared with "classical" measurement systems is that this time all of the processing and analysis functions of measured values, the storage of this information and their transmission to the human user are made by computer and not by dedicated equipments. The software has performed these functions in most cases, a graphical user interface having the same look as the front panel of a meter. This is why these applications are called virtual instruments. LabVEW programs are called virtual instruments because by their form and their mode of operation mimics the actual measurement and control instrumentation. A virtual instrument has an interactive user interface, a source code equivalent, and supports a hierarchy along with other virtual instruments. However they are identical to functions in conventional programming languages, and sometimes are integrated into these []. Virtual instrument can be easily adapted for the use for a wide range of applications in power engineering [], [4]. The paper is organized as follows: Section proposes the architecture of data acquisition and processing system that is used in the laboratory, Section presents the three phase virtual measurement instrument realized by the authors and which can be used for different studies on the three phase electrical networks operating conditions, while Section V reports the conclusions. The architecture of data acquisition and processing system By way of structuring, a data acquisition system contains elements that must meet three basic functions: Convert physical quantities characteristic of a signal process that can be measured; Measuring the output signals of sensors and transducers to enable information extraction; Data processing and presentation in a form that allows analysis of the process. Hardware and software products for data acquisition were designed to be compatible with existing computer systems were then incorporated. Therefore most of the modern data acquisition systems use a PC as a controller thus leading to a structure almost typed, which contains mainly the following components: SBN:

2 sensors and transducers that convert physical size appropriate to study physical phenomena in an electrical signal that is sent either to an adjustment circuit, either directly to data acquisition card; adjusting the signal circuits (such as to be compatible with the input circuits analog-digital converter) which create isolation, conversion, filtering and / or amplifying the signal from the transducer; a data acquisition system that includes multiplexers, analog-digital converters; a computer system; software for data acquisition and processing. Fig.. The structure of the data acquisition and processing system. Virtual nstrument for data acquisition and processing All programs created using LabVEW are called virtual instruments because the appearance and operations performed are similar with classical instruments. n any case, virtual instruments are related to the functions of conventional programming languages. A virtual instrument consists of the following components: nteractive user interface type; Data chart; con connections that allow the virtual instrument to be called by another virtual instrument from a higher level. Specifically, virtual instruments are structured as shown below: User interactive interface of the virtual instrument is the front panel as it simulates the front panel of a physical gauge. The front panel may include: snaps, buttons, graphics and other controls and indicators; The virtual instruments receive instructions from a block diagram, which is built in graphics mode G. The block diagram is a visual solution to the problem of programming. The block diagram is also the source for virtual instrument. Below are the block diagrams and front panels of the main components of the corresponding virtual instrument created. To start the measurement operation, you must first select the desired channels depending on their behalf as they were configured. Acquired signals on all channels and stored as a matrix unit will be separated according to channel number in order to use them as input in other virtual instruments. Since the adjusting circuits provide an output signal voltage of 0 V maximum, a ratio must be established so that the virtual instrument show exactly the same value as the primary circuit. For the presented case, the ratio for channel voltage transformer is 7.4. Corresponding to voltage wave, virtual instrument used in LabView library, provides output amplitude and signal phase respectively. To show the rms value, the amplitude is divided by, Fig.a). To obtain the voltage phase, depending on the quadrant in which it is, we used the received signal phase in degrees and we used the procedures presented in Fig.b), the rule being that the phases should be between 0 and +80 degrees counterclockwise ( quadrants and ), and between 0 and -80 clockwise (quadrants and V). The current channels used the same procedure, but in this case the transformer ratio is.85. a) b) Fig.. Voltage measurement: a) rms value, b) phase. For the representation of voltage and current phasor on the same chart, we used the rms values and their phases. n order to draw the two phasors, we fix it first in the origin of the coordinate axes, Fig.. SBN:

3 Fig. 4. Determination of the complex voltage. Fig.. Corresponding block diagram for the voltage and current phasor representation. To obtain the complex current, respectively the real part and the imaginary part of current, a virtual instruments defined in LabView library was used. These tools use the amplitude and the phase in radians, so the transformation presented in Fig.4 was done. The same procedure was applied to the voltage channel. Using the rms values of voltage and current, the apparent power was calculated, and to calculate the active and reactive power, it must be determined first the phase shift between voltage and current. Thus, given the fact that the voltage and current can be in any of the four quadrants, and their phases are determined as described above and shown in Fig.b), in order to determine the phase shift between voltage and current, the procedure shown in Fig.5 was used. Fig. 5. Determination of electric powers. The rule is that an inductive phase shift is negative, the imaginary part of an inductive complex current is negative and the inductive reactive power is positive. For the same capacitive amounts, the sign must be contrary. Having established the phase shift between voltage and current, two predefined virtual instruments for the trigonometric functions sinus and cosinus were used. To express the current in sequence amounts, the values for the complex currents were first determinate using a predefined LabView tool. After that, the values for the zero-sequence, positive-sequence and negative-sequence were determined using the expressions below: h = ( + + ) d = ( + a + a ) () i = ( + a + a ) where, a = + j, respectively = a j. Accordingly, the virtual instruments have been created using the three-phase input SBN:

4 complex currents, and the complex value corresponding to the operator a. Fig.6 presents the procedure for the positive-sequence current. Using the expressions (), the procedures for the zero-sequence and negative-sequence are similar. To test this virtual three-phase wattmeter kit, it was installed in a Y connection three-phase RLC circuit corresponding to phases, and. The results are presented via the corresponding front panel in Fig.7. Fig. 6. Determination of positive-sequence current. Fig. 7. The front panel corresponding to the virtual three-phase wattmeter kit. 4 Conclusion Data acquisition boards with multiple input channels represent a useful tool for measuring and monitoring electrical energy parameters. Using virtual instrumentation desired user builds his own instrument, implementing both the front panel and the functionality to fully meet their needs. LabView is a programming environment used primarily for making measurements and monitoring of automated processes. Some of the main advantages of using this program are outlined below: reducing the number of connections, this simplifies the circuit required to the study of operating regimes of power lines; on a single interface of the program are available the values: voltage, current, active power and reactive power, in addition to the waveforms, which highlights the nature of current: inductive or capacitive; as a virtual instrument can be used as sub_virtual instrument in other sites is provided great flexibility in generating other virtual SBN:

5 instruments using the already constructed parts of programs. The virtual instrument can be used for various laboratory works regarding three phase electrical network analysis, like: the study of the parameters for the electrical lines and transformers, particular operating conditions of transmission lines. Acknowledgment This paper was supported by the project "Develop and support multidisciplinary postdoctoral programs in primordial technical areas of national strategy of the research - development - innovation" 4D-POSTDOC, contract nr. POSDRU/89/.5/S/560, project co-funded from European Social Fund through Sectorial Operational Program Human Resources References: [] J.A.B. Grimoni, Using LabVEW in a Mini Power System Model Allowing Remote Access and New mplementations, nternational Conference on Engineering Education CEE 007, -7 Sept [] S. Vergura, E. Natangelo, Labview-Matlab integration for analyzing energy data of PV plants, nternational Conference on Renewable Energies and Power Quality (CREPQ 0), -5 March 00. [] J. Barros, M. de Apraiz, R.. Diego, A virtual measurement instrument for electrical power quality analysis using wavelets, Measurement, No.4, pp.98-07, 009. [4] M.N.N. Santos, M.E.L. Tostes, R.D.S. Silva, R.S. Fadul, Software Based on LabView for Monitoring and Analysis Some Power Quality Parameters, The 5th nternational Conference on ntelligent System Applications to Power Systems, Curitiba, Brazil, 8- November 009. [5] P. Bilik, L. Koval, J. Hajduk, CompactRO Embedded System in Power Quality Analysis, Proceedings of the nternational Multiconference on Computer Science and nformation Technology, Wisla, Poland, pp , 0- October 008. SBN: