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  1. 1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), pp. 74-82 © IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2013): 5.5028 (Calculated by GISI) IJEET ©IAEME www.jifactor.com MPPT TECHNIQUE TO IMPROVE EFFICIENCY IN WIND-SOLAR HYBRID SYSTEM Aishwarya P. Mulmule, Rambabu A. Vatti, Pratik M. Porwal Dept. of Electronics, Vishwakarma Institute of Technology, Pune, India ABSTRACT As the environmental conditions (temperature, irradiance) vary the power output of a solar system differs which in turn reduces the efficiency of the system. This paper shows Maximum Power Point Tracking (MPPT) as one of the techniques to improve the efficiency and reliability of the Wind-Solar Hybrid System. The I-V characteristic of a solar cell is non-linear and has an exponential relationship around the region of maximum power point. By tracking the point where dP/dV=0, the power delivered can be optimized. A simple and extensively used Perturb and Observe method off the different methods available for MPPT like perturb & observe, incremental conductance, current sweep method is used and its implementation using simulation software PSIM is done. The paper further describes various simulation software like VEE Pro, LabVIEW, Visual Basic, etc. needed for graphically viewing the estimated results. KEYWORDS: Maximum Power Point Tracking Algorithm (MPPT), Observe & Perturb, VEE Pro, Wind and Solar Hybrid System. I. INTRODUCTION Solar energy is free of cost, virtually inexhaustible and doesn’t pollute the atmosphere. Technological development has led to easy conversion of solar to electrical energy using photovoltaic models (PV arrays). Maximum Power Point Technique (MPPT) is used for PV arrays. A boost is being provided in the field of applications of PV generate systems in order to mitigate environmental issues such as the green house effect and air pollution. Many MPPT algorithms and control schemes of PV generate systems have been proposed in the literature, viz., the perturb and observe method (P&O), the incremental conductance (IC) method, the constant voltage, and the current sweep method. Although various methods of MPPT control have been proposed in existing literature, the power generate efficiency is relatively low, and the total electric power generated in the grid by the solar panel is always changing with weather conditions. Different solar panels have different parameters with different thresholds and maximum values. If the weather conditions are same, the 74
  2. 2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME theoretical and simulated (estimated) results show that a piece of solar cell has maximum power at that instant. Thus it is very important to acquire the actual efficiency, output power and threshold value for a set of panel with the help of simulated results. II. MPPT A. What is MPPT? In the Maximum Power Point Tracking (MPPT) technique power grid, inverters, solar battery chargers and devices like photovoltaic cells solar panels where maximum power can be generated or stored are interconnected. Solar irradiation, temperature and total resistance hold a complex relationship which results in the I-V curve and I-V curve of solar panel gives relation between nonlinear output efficiency as shown in figure. By sampling the output of the cells and applying the suitable resistance (load), the MPPT system obtains maximum power for any given environmental condition [1]. MPPT devices are used in the electric power systems so it will give sufficient voltage and current, its regulation and filtering for driving various loads like motor, including power grids, batteries, or home appliances. The I-V curve of cells have a single operating point. The maximum power (P=I*V) output is the product of current and voltage at that operating point. From fig.1, we see that a photovoltaic cell, primarily, acts as a constant current source(CCS). However, at maximum power point region of the photovoltaic cell's , the current and voltage illustrate an inverse exponential relationship between each other. The derivative (dI/dV) slope of the I-V curve is equal to the I/V ratio (where dP/dV=0) but opposite in sign as derived under. This is known as the maximum power point (MPP) where slope of the P Vs V curve is zero [1]. ݀ܲ ൌ ݀ሺܸ ‫ ܫ כ‬ሻ ܸ݀ ܸ݀ ௗ௉ ௗ௏ ൌ For MPP, as ூௗ௏ ௗ௏ ௗ௉ ௗ௏ ൅ ௏ௗூ ௗ௏ ൌ‫ܫ‬൅ ௏ௗூ ௗ௏ ……………… (1) ൌ 0 , from equation (1) ௗூ ‫ ܫ‬ൌ െܸ ௗ௏ …………………………. (2) (a) Experimental data of KC200GT solar array at different temperatures, nearly 1000 W/m2 (b) Experimental data of KC200GT solar array at different irradiations, 25oC[3] Fig. 1 Characteristics of a solar cell [3] 75
  3. 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME B. IMPORTANCE OF MPPT In winter the amount of solar radiation received decreases because of the low angle of sun. The solar modules have to be tilted in order to minimize power loss. One example demonstrating such loss is the Recreational Vehicles (RV). Hence, use of MPPT to transfer more power is the solution.The small increase in the power (in Watt) delivered by the system adds up to provide tremendous energy (in kWh) in a year due to increased efficiency. MPPT gives the luxury of connecting PV arrays in series to generate high voltages of 600 V DC for off-grid systems [2]. C. DIFFERENT METHODS OF MPPT The time complexity of the algorithm, the implementation cost and the ease of implementation are the major factors responsible in the choice of algorithm. The different methods of MPPT as in [1] are1. Perturb and observe method(p & o method) The voltage is adjusted in steps by small amounts using a controller and power is calculated; if the power increases, voltage is increased in the same direction. If power decreases the direction of voltage increment is reversed. This is known as the perturb and observe method. The tracker oscillates about the MPP which may lead to oscillations in power output. It is also referred to as a hill climbing method. 2. Incremental conductance method The incremental conductance of the photovoltaic cells(array) is used to decide the sign of (dP/dV) ratio. The dynamic conductance (I∆ / V∆) is compared to the static conductance (I / V).This helps in computing the MPP. When these two are equal (I / V = I∆ / V∆), the output voltage is the Maximum power point voltage. The controller maintains this voltage but as the irradiation changes the process is repeated. This method calculates MMP more rapidly when compared to p & o method and requires more mathematics to be done by the controller. However, this method too gives oscillations in the output. 3. Current sweep method The I-V characteristic of the PV array is obtained and updated at fixed time intervals using a sweep waveform for the PV array current. This characteristic curve gives the MPP voltage at the same intervals. 4. Constant voltage method In the constant voltage MPPT method, the power delivered to the load is interrupted for a short time or temporarily and the open-circuit voltage is measured. Witj the help of modelling or empirial method, the maximum power point(MPP) is measured to be 0.76 percent of open-circuit voltage Voc calculated above. This voltage is maintained by the controller by constantly matching it to the fixed reference voltage Vref =x*Voc. The value of Vref may be selected with the aim of achieving optimal performance relative to other factors as well as the MPP.However, the central idea in this technique is that Vref is determined as a ratio to VOC. This method is also called as the open voltage method. III. MPPT AND P&O METHOD A. Importance of P & O method The P&O MPPT algorithm is mostly used due to its ease of implementation. It makes use of only one type of sensor which is a voltage sensor. The time complexity of the algorithm is also very low. 76
  4. 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME B. P & O method Perturb and observe method is widely used in this method. We check current and voltage at every instant in PV model and track the maximum point in the graph by equating slope to 0 (dP/dV=0). The previous power sample dictates the increments or decrements of reference current or voltage and thus next array output power. Fig. 2 shows MPP on the P-V curve of the solar cell for three different values of irradiance. Fig.2 I-V and P-V characteristic for a photovoltaic device for different irradiance.[4] Author in paper [4] developed flow chart for the algorithm as follows : 77
  5. 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME According to the algorithm, by calculating instantaneous current and voltage, the power is found out and compared with the previous value of power. If found greater, the instantaneous voltage is also compared in a similar fashion. If V(k)>V(k-1) then dP/dV >0 i.e. tracker is on the left hand side of the MPP. The tracker is perturbed towards the MPP by incrementing voltage by an amount v. If the increment value is small along with a low sampling rate, the dynamic response is low [3]. However, low increments are a must to minimize the steady state error because the operating point oscillates about the MPP. Large increments would oscillate the point to larger “amplitudes”. The response of the converter to large voltage or current variations will introduce oscillations and cause overshoot. The settling time of the converter also confuses the algorithm. Very low increments would make the algorithm too slow and take long to determine the MPP. The algorithm speed may be increased by taking the samples at a higher rate. So one must find a middle ground between the increment and the sampling rate in the P&O method[3]. IV. SIMULATION OF MPPT ALGORITHM USING PSIM A. What is PSIM ? Powersim (PSIM) is a fast simulation software used at circuit level or system level with a friendly user interface [5]. It is especially used in simulation of power converters and control circuits. The PSIM simulation package consists of three programs: circuit schematic editor SIMCAD, PSIM simulator, and waveform processing program SIMVIEW. Simview display helps in evaluating the different waveform results. Psim version 9 introduced new wind and solar modules for renewable energy simulation needs. Parameter data from datasheets can be stored in Psim for comparing with the results. Similarly wind module works along with the motor drive module[6]. B. Solar models in Psim Functional solar module works with only a few inputs from the user but doesn’t consider temperature and light intensity changes. The physical solar module, on the other hand, gives this functionality. Fig. 3 Physical model of solar cell [7] Fig. 4 Characteristics of solar cell (Physical model) 78
  6. 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME Fig. 5 Functional model of solar cell [7] Fig. 6 Characteristics of solar cell (Functional model) D. IMPLEMENTATION OF MPPT IN PSIM AND ITS OUTPUT Fig.7 Simulation Model of Perturb & Observe Algorithm for MPPT Fig. 8 (a) Pmax and Po, (b) Vcell, (c) Vload Fig. 9 Icell 79
  7. 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME V. SIMULATION SOFTWARE FOR GUI A. VEE-PRO VEE was formerly an abbreviation for Visual Engineering Environment and was developed by HP. They designated it as HP VEE. It was later renamed as Agilent VEE. It provides a graphical programming environment which offers fast measurement and analysis results. The operating system for VEE Pro is Microsoft windows. Different versions of VEE released are compatible with the latest OS. In VEE user only needs to create objects in the form of block diagrams from pull-down menus and link them using wires in the way the program flows. The development time is reduced as the software focuses on building powerful programs through graphical user interface (GUI) instead of writing hundreds of lines of complex code. In VEE Pro programming objects (devices) are created which consist of different pins, viz., data pins, sequence pins, execute pins (XEQ), control pins and error pins. The input pins are on the left side of the object and the output pins are on the right. If the flow is from the output pin of object A to input pin of object B then A and B are said to be connected. A single output pin can be multiplexed but a maximum of one connection line can be made an input to an input pin. The data input pins and execute pins connection is must. Control pins and output pins can be left unconnected. The data flow through objects is from left to right at the same time as the sequence flow is from top to bottom.Instrumentation from non-Agilent manufacturers can be interfaced in Agilent VEE giving flexibility, expandability and compatibility with the latest industry standards. Devices of National Instruments (NI) like niDAQ can be used in Agilent whereas simulation software LABView provided by NI doesn’t give this functionality for Agilent devices. Fig. 10 Example of a program flow [8] The built-in ACTIVEX automation server provides an environment for interaction with different software like EXCEL, WORD, OUTLOOK, ACCESS, etc. Agilent VEE comes with the facility of exchanging data with excel to form spreadsheets and save data as shown in fig. 11 and 12. We have used VEE Pro to generate excel sheets of data like wind speed, wind direction, temperature, etc required in the project. The in-built functions in VEE like those in MATLAB enable quick analysis of waveforms. About 1800 MATLAB functions are available through MATLAB scripts. Agilent VEE we can supervise program remotely because of built in web server. Agilent VEE Pro is incorporated with Microsoft .NET Framework (Common Language Runtime and Framework Class Libraries) that provides you with a host of functions and controls that add to your program[8]. 80
  8. 8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME Fig. 11 Pull-down menu for excel options[8] Fig. 12 An excel sheet with data [8] B. LAB-VIEW LabVIEW (Laboratory Virtual Instrument Engineering Workbench) is a system-design platform. It is a development environment for GUIs from National Instruments (NI).It is also a dataflow programming language. The programs or subroutines are known as virtual instruments (VIs). Each VI is made of following three components: 1) block diagram, 2) front panel and 3) connector panel. The front panel is made up of controls (inputs) and indicators (outputs). The back panel i.e. the block diagram contains the graphical source code, structures and functions [9]. LabVIEW also provides extensive instrumentation hardware support and drivers and abstraction layers for them. It offers an interface to .NET Framework assemblies. LabVIEW package provides large number of functions and graphical interface elements. It is also compatible with MATLAB. However, the third party standards committee such as ANSI, IEEE, ISO, etc. do not manage or specify LabVIEW. There is no LabVIEW 2011 student license available for Linux. It has a few runtime environment problems. Fig. 13 Workspace of LabVIEW [9] C. VISUAL BASIC Developed by Microsoft, it is derived from BASIC and gives rapid application development (RAD) of graphical user interface (GUI) applications. In VB (visual basic), the components are arranged visually and then given required controls, inputs, outputs and actions to be taken on them are specified. A few lines of code is written additionally which increases with the complexity of the GUI. 81
  9. 9. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME The COM components can be written in different languages and then integrated using VB[10]. Programs written in VB cannot be transferred to other operating systems easily. Execution is slow in visual basic. VI. CONCLUSION The efficiency of the solar PV panel can be increased by using the perturb & observe method of MPPT technique. In this method, the MPP is tracked by comparing power and voltage at a particular instant and its previous instant. Using appropriate increment or decrement value of voltage, the dP/dV =0 condition is met. For the simulation of MPPT algorithm PSim software can be used. Agilent VEE Pro proves helpful in generating user friendly GUIs. It can be used in the hybrid system to view the results, data and generate related excel sheets. Agilent VEE also has a built-in web server which allows you to monitor your program remotely. VII. REFERENCES 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) M. C. Cavalcanti, K. C. Oliveira, G. M. S. Azevedo, and F. A. S. Neves. Comparative study of maximum power point tracking techniques for photovoltaic systems. Brazilian Journal of Power Electronics, SOBRAEP, 12(2):163–171, 2007. N. Femia, G. Petrone, G. Spagnuolo, and M. Vitelli. Optimizing sampling rate of P&O mppt technique. In IEEE PESC’04, vol. 3, 2004. M. G. Villalva, J. R. Gazoli, and E. Ruppert F. Comprehensive approach to modeling and simulation of photovoltaic arrays. IEEE Transactions on Power Electronics, vol. 25, no. 5, pp. 1198–1208, 2009. Analysis and Simulation of the P&O MPPT Algorithm Using a Linearized PV Array Model Marcelo G. Villalva, Ernesto Ruppert F. http://www.psim-europe.com/psim_whyuse.php,accessed on 7-11-2013. http://powersimtech.com/products/psim-modules/renewable-energy/ , accessed on 7-11-2013. PSim renewable energy free demo video- http://www.youtube.com/watch?v=GSUdCVS6rcE, accessed on 7-11-2013 User manual of VEE Pro 8.5, april 18, 2008, Part Number 5989-6002EN LabVIEWTM User Manual, National Instruments, April 2003 Edition, Part Number 320999E-01 Microsoft Basic Logical Expression Evaluation. Vb.mvps.org. Retrieved 2009-06-16. Anto Joseph, Nagarajan and Antony Mary, “A Multi Converter Based Pure Solar Energy System with High Efficiency MPPT Controller”, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 4, 2013, pp. 205 - 212, ISSN Print: 0976-6545, ISSN Online: 0976-6553. M D Goudar, B. P. Patil and V. Kumar, “A Review of Improved Maximum Peak Power Tracking Algorithms for Photovoltaic Systems”, International Journal of Electrical Engineering & Technology (IJEET), Volume 1, Issue 1, 2010, pp. 85 - 107, ISSN Print: 0976-6545, ISSN Online: 0976-6553. Minakshi DebBarma, Sumita Deb and Champa Nandi, “Maximum Photovoltaic Power Tracking using Perturb & Observe Algorithm in Matlab/Simulink Environment”, International Journal of Electrical Engineering & Technology (IJEET), Volume 1, Issue 1, 2010, pp. 71 - 84, ISSN Print: 0976-6545, ISSN Online: 0976-6553. 82