Power Quality Analyzer for Three Phase Systems

Transcription

1 International Conference on Renewable Energy Research and Applications Madrid, Spain, -3 October 3 Power Quality Analyzer for Three Phase Systems Ibrahim Sefa, Necmi Altın Electrical and Electronics Engineering, Faculty of Technology, Gazi University Ankara, Turkey isefa@gazi.edu.tr, naltin@gazi.edu.tr Erdem Asa, Kerim Colak Electrical and Computer Engineering, Polytechnic Institute of New York University New York, USA ea45@nyu.edu, kc353@nyu.edu Abstract In this study, a MATLAB based simulator is designed which can be used for education in power quality and harmonics. The designed simulator is user-interactive and also can be used in undergraduate and graduate laboratory studies. The harmonic components that are entered by the user are visualized graphically. Furthermore, power quality indices such as total harmonic distortion, distortion power factor, and active, reactive, and distortion power are calculated and current waveforms are plotted. In addition, instantaneous and average powers are graphically revealed. Keywords-power quality; harmonics; MATLAB I. INTRODUCTION Today, the number of non-linear loads such as power electronics converters and their power levels is increasing rapidly. These converter systems draw currents containing harmonic components []. The harmonics produced by power systems, rectifiers, transformers, and motors have been identified as early as the 9s, even though they have not been considered important. While harmonics did not affect the operation of the power system, it was not a necessity to meet the requirements of power systems in the past. Today, the increase in the non-linear loads causes energy losses in motors and transformers, communication disturbances, and false tripping of circuit breakers. Moreover, sensitive equipment may not work properly because of the disturbed signal and electricity interferes in the system. The efficiency of the electricity distribution system is also affected adversely []. In these cases, the importance of harmonics in the power distribution system increases every day []. In order to limit the negative effects of harmonic currents in the electric power system, international standards are published such as IEEE , EN 6-3- [3,4]. It is necessary for students to obtain basic information about harmonics and their impacts on power system to reduce these negative effects in the future. The revision of course contents has become a necessity due to the rapid development of technology. Previously, little attention was given to power quality and harmonics in the power electronics and circuit analysis courses. However, nowadays, the non-linear loads and power electronics converters that are used with increasing amounts of power each day negatively affect power system harmonics level [5]. With the new additions to the contents of the course, spending time on these issues brings reduction of time to be allocated to each topic. This raises the need for more effective use of learning methods. In order to ensure more effective learning, information technology is frequently used as a method [6]. For this purpose, various educational tools and simulators have been developed using different programs. These developed systems make education independent from the location and time. So, these educational tools have an important role in improving the effectiveness of the training [7]. One of the programs used in developing computer-based training tool is MATLAB. MATLAB is a software development tool for technical calculations and mathematical analysis of problems. Also, MATLAB GUIDE (Graphical User Interface Development Environment) toolbox allows visualization of information and provides ease to use and develop graphical user interfaces [8]. In this paper, a computer based simulator for learning harmonics, including power quality and power system, has been developed by using MATLAB-GUIDE. The user can define load current (harmonic current components which occur and their proportions) fed from a sinusoidal voltage source. The waveforms of the load current, supply voltage and active, reactive, and apparent power can be sketched by the user. In addition, active, reactive, apparent power, power factor, displacement power factor, total harmonic distortion, and power quality terms can be calculated in the program interfaces. Current waveforms and harmonic components are visualized with this simulator. The effects of different power system parameters like power factor can be compared through monitoring. The simulator is independent of location since it is a computer based system. II. POWER SYSTEM HARMONICS In recent years, increasing power ratio of nonlinear loads, such as the rectifier shown in Fig. has increased interest in power quality and harmonics in power system. Periodic and non-sinusoidal waveform with Fourier expansions of current is () t is expressed as a sum of sinusoidal harmonic components in () [,9,]: Here, i dc Eqs.-4. ( ω ω ) () i () t = I + a cosn t + b sinn t s dc n n n =,,3..., a n, and b n terms are computed with the help of ICRERA 3

2 International Conference on Renewable Energy Research and Applications Madrid, Spain, -3 October 3 where I s represents the fundamental component, while others are harmonic components. The effective values of the harmonic components can be found with Eq. 9: Figure. Nonlinear load sample T idc = is () t dt T () an = is()cos t ( nωt) dωt π (3) bn = is()sin t ( nωt) dωt π (4) Generally, the current signal is symmetric, and therefore the DC component is not included ( i dc =). In this case, sine and cosine functions that are the same frequency can be rewritten as a single sine function as given in Eq. 5: s() = sn sin + n n =,,3... ( ω ϕ ) (5) i t I n t In this equation, ϕn and I sn are given in Eq. 6 and Eq. 7: a ϕ tan n n = b (6) n I sn an + bn = (7) The effective value of the current can be calculated as follows: I = s is() t dωt = Isn sin( nωt ϕn ) dωt + n =,,3... = Isn sin( nωt+ ϕn ) Ism sin( mωt+ ϕm ) dωt n= m= Isn Is Is Is3... (8) n= = = I = I + I + I + (9) h s s3 s4... The relative amount of harmonic components is expressed with a factor called Total Harmonic Distortion (THD). THD is the proportion of the sum of all the harmonic components effective values with effective value of the fundamental component as shown in Eq. : I sn s s n= h I I I THD = = = I I I s s s () The power factor of the load fed from AC power system is explained with Eq. : Pavg PF = () VI s s where P avg is given with Eq. : P cos avg = V I ϕ s s () It is seen that, unlike linear loads, the power factor different from displacement power factor. In nonlinear load condition, additional factor called distortion factor (DF) which is given in Eq. 3 affects the power factor: DF = (3) + THD The load current and AC supply voltage are sinusoidal; the power factor is defined as the cosine of angle between the current and voltage. But that is not valid if the form of the current is not sine wave. In other words, the current contains harmonic components. Fed with sinusoidal voltage and the current with containing harmonic components of a load, the power factor can be computed using Eq. and Eq. 3: VsIs cosϕ Is Iscosϕ PF = = cosϕ = (4) VI I I + I + I +.. s s s s s s3 The case of V s = V s is only valid while the voltage is sinusoidal. As seen in Eq. 4, the harmonic components of current increase the current s effective value; thus this causes the power factor to decrease. III. DEVELOPED POWER QUALITY SIMULATOR Using the MATLAB-GUIDE a simulator is prepared for power quality training with three-phase balanced/unbalanced non-linear loads. The results of the proposed simulator are calculated from the numerical data to be entered into the prepared user interface. Obtained results are given both numerically and graphically. The proposed simulator is useful for graduate and undergraduate educational purposes. In addition, operations which conventional measuring devices ICRERA 3 3

3 International Conference on Renewable Energy Research and Applications Madrid, Spain, -3 October 3 could not realize can be achieved with taking numerical data from measuring instruments, and it can be displayed on the screen. Login window of the proposed simulator which can be designed for 3phase systems, is shown in Fig.. After the user login "Main Page" is shown where the data entries are appears on the screen as given in Fig. 3. In the main page, data entry fields can be either phase-to-neutral or phase-to-phase for three-phase voltage values, and values of the harmonic current component with their phase angle are available to be entered. The fundamental component with 3 rd, 5 th, 7 th, 9 th, th, and 3 th harmonic component's values can be entered in this area. The less effective high grade harmonics in the total harmonic distortion are neglected. Each harmonic component can be left out of the transaction by pressing Eject next to the entry field boxes. Thus, the impact of each harmonic component on to the total harmonic distortion and the current waveform can be detected. By pressing the "Calculate" button, the three phase load current effective value, active, reactive and apparent power values, THD %, displacement power factor ( cosϕ ), and power factor (PF) values are calculated for the voltage are calculated and shown on "Results" region of the main page. By the help of these calculations, THD values and the effective values of the load currents, active power, reactive power, power factor and harmonic components, and the effect on the change of variables can be observed easily. Figure. Power quality simulator login window In addition, some of the harmonic components can be left out of the calculation by selecting the Eject box in the main page. Therefore, in case of a filtered harmonic component in a real application, variation of the current, power, power factor, and THD can be observed. The conventional measuring devices and analyzers cannot provide this feature. Figure 3. Power quality simulator data entry window ICRERA 3 4

4 International Conference on Renewable Energy Research and Applications Madrid, Spain, -3 October 3 Buttons in the "Graphics" section, which is divided into five sequences, provides sketch waveforms of the selected parameters. There is a button in the "Current and Voltage" section for each of the three-phase current and voltage wave shapes in order to graphically plot in a new window. The user can plot phase current and voltage using these buttons. This process demonstrates the impact of the harmonic component on to the current waveform and also ensures to plot graphs that data measured by the user in a real application. The real application values using three-phase power quality analyzer Fluke 434 are received and the current-voltage waveforms are drawn by the simulator. The Fluke 434 three-phase power analyzer screenshots are presented with simulator results in Fig. 4. The "Current and the Component" sections are defined for each of the three phase currents. When these buttons are clicked, the load current is calculated, and the current harmonic components waveforms are depicted according to their angles. Waveforms of the load current and its harmonic components of one of the three phases which is plotted based on the values are given in Fig. 5. This operation can also be performed separately for the other two phases. Thus, how the harmonic components can affect the current wave form is demonstrated. The selected harmonic components that have the Eject box clicked are not shown. So that, if any of the harmonic component is filtered, effects the current waveform effect can be monitored. The "Power" button on the main page in "Graphics" section plots the power values which are calculated with entered values as graphical expressions. This process can be realized for each one of the active, reactive, and apparent powers. In this way, the effect of the harmonic components to the power values of the system. Thus, a better understanding about the effects of harmonics on power values is provided. In Fig. 6, instantaneous value and mean value of active power; active, reactive, distortion and apparent powers in vector form and Fluke 434 screen which shows the measured power values are given. These graphs show the values for one phase. Current and Components [A] The Current and Component Waveforms Figure 4. Waveforms of the load current and its harmonic components. The total power can be found with the sum of the three phase power values. As can be seen, the developed simulator program is able to provide active, reactive, and apparent power during a period of time that cannot be seen with measuring instruments. When the "I a I b I c I n " button in the "Current" section is clicked, the three phase and the neutral current waveforms are depicted in a new page. In Fig. 7, three-phase load current plotted by the proposed simulator and the three-phase currents waveforms measured by Fluke 434, with the neutral current waveforms are indicated. The Harmonic button in the Harmonic Analysis section plots bar graphs of the harmonic components. At this stage, the user is required to choose one of the options inside the panel, which are the THD (f %) (relative to fundamental component) and THD (r %) (relative to the effective value of the total load current). In Fig. 8, the harmonic components results are compared with the Fluke 434 power analyzer and simulator according to the effective value of the total current and main component. Phase A: Voltage and Current Waveforms Voltage [V] Current [A] Figure 5. The current-voltage waveforms a) phase A current and voltage waveforms by simulator b) phase A current and voltage waveforms by Fluke 434 ICRERA 3 5

5 International Conference on Renewable Energy Research and Applications Madrid, Spain, -3 October 3 4 The Power Waveforms 8 Power Apparent Power [S] Active Power [P] Reactive Power [Q] Distortion Power [D] Apparent Power [S] Distortion Power [D] Active Power [P] Reactive Power [Q] (c) Figure 6. Power component graphs a) the power waveforms b) Fluke 434 power values c) triangle power vector forms 5 Three Phase and Neutral Current Waveforms Current [A] Figure 7. Three phase and neutral currents a) Simulator b)fluke 434 ICRERA 3 6

6 International Conference on Renewable Energy Research and Applications Madrid, Spain, -3 October 3 Amplitude [Ih/Ia] Amplitude [Ih/Ib] Amplitude [Ih/Ic] Amlitude [Ih/Ia] Amlitude [Ih/Ib] Amplitude [Ih/Ic] Bar Graph of the Harmonic Components According to Total Current Bar Graph of the Harmonic Components According to Main Current IV. RESULTS In this study, a MATLAB based simulator for education purposes is designed in response to growing interest in the power quality and harmonics due to the negative impact on the power system. The effects of the harmonics on system power components, losses and efficiency are comprehended, and power calculation is visualized for nonlinear loads through developed training tool. Also, students can access this software on their personal computers to continue educational studies outside of the laboratory. The developed simulator calculates and visualizes the power and power quality parameters in the case of filtering some selected harmonics components of the system. This allows the preliminary studies and validation of power filtering studies. REFERENCES [] Ellis, R. G., "Harmonic Analysis of Industrial Power Systems, IEEE Transaction on Industry Applications, 3.,., 47-4, 999. [] Lin, H. C., An Internet-Based Graphical Programming Toll for Teaching Power System Harmonic Measurement IEEE Transaction on Education, 49., 3., 44-44, 6. [3] IEEE Std , IEEE Recommended practice and requirements for harmonic control in electrical power systems, Newyork IEEE, 99 [4] IEC 6-3-4, Limitation of harmonic current in low-voltage power supply systems for equipment with rated current greater than 6 A, 998. [5] Benetazzo L., Bertocco M., Ferraris F., Ferrero, A, Offelli C., Parvis M. ve Piuri V. A Web Based Distributed Virtual Educational Laboratory" IEEE Transactions on Instrumentation and Measurement, 49.,., ,. [6] Hart, D. W., Circuit Simulation as an Aid in Teaching the Principles of Power Electronics, IEEE Transaction on Education, 36.,., -6, 993. [7] Demirba,., Irmak, E. ve Çolak., A Web Based Educational Tool for Simulation of Induction Motor" Journal of Polytechnic, Vol. 9., No. 4., 33-3, 6 (In Turkish). [8] MATLAB Getting Started Guide, 8. [9] Mohan, N., Undeland, T.M., Robbins, W. P., Power Electronics, Converters, Applications and Design, Joe Wiley and Sons Inc., London, 989. [] Hart, D. W., Introduction to Power Electronics, Prentice Hall, 997, (c) Figure 8. a) Bar graph of the harmonic components according to total current values obtained from proposed simulator b) Bar graph of the harmonic components according to total current values measured with Fluke 434 c) Bar graph of the harmonic components according to main component current values obtained from proposed simulator. ICRERA 3 7