Comparative Studies of DC/DC Converters for Solar Panel MPPT

Transcription

1 DC/DC CONVERTERS FOR SOLAR PANEL Comparative Studies of DC/DC Converters for Solar Panel MPPT Chandani Sharma 1 and Dr. Anamika Jain 2 Department of Electronics and Communication Engineering, Graphic Era University, Dehradun Uttrakhand India 1 chandani19nov@gmail.com, 2 anamikajain2829@gmail.com Abstract -- Solar DC/DC converters are the key sources to integrate PV applications in renewable energy. There appears stringent need to understand need of standalone/grid PV system requirements. As a consequence, functions like dynamic control of active and reactive power with operation exhibited over a wide range of voltage and frequency through converters becomes a necessity. This paper explains five different converter topologies and control of simulink based solar panel based on continuous and discrete GUI. In addition to power electronics, paper focuses on the specific essential factors including detection of MPPT for photovoltaic systems ideally considering STC. The synchronization of PV model is based on applied voltage at gate control. Keywords: DC/DC converters, standalone/grid system, topologies and control of converters, MPPT (Maximum Power Point Tacking), STC (Standard Test Conditions). I. INTRODUCTION A DC-to-DC converter is defined as power electronic circuit converting one form of voltage to other. These may be used whenever we want to change DC electrical power efficiently from one voltage level to another. There is emergent need of converters because unlike AC using transformer we cannot simply step up or step down DC available at the output of solar panel. In many ways, DC equivalent of a transformer may be a DC-DC converter. Typical applications of DC-DC converters are where a large solar panel driving grid systems may be used to operate a standalone application by stepping down voltage. An important consideration while studying converters is that no energy is generated inside converters. They change output voltage level depending upon feeder input voltage. Similar to transformer they may be considered about changing impedance level of input energy and vice versa. Using equations the basic power flow in a converter can be represented as, PIN = POUT + PLOSS (1) Where P IN is the power fed into the converter, P OUT is the output power and P LOSS is the power wasted inside the converter. Of course an ideal converter would have behaved in similar way as an ideal transformer. There would be no losses, and Pout would be exactly the same as Pin. We could then say that:, VIN X IIN = VOUT X IOUT (2) Or by re-arranging, we get: VOUT/VIN = IIN/IOUT (3) In other words, if we step up the voltage we step down the current, and vice-versa. There is no ideal or perfect DC-DC Converter as there is no ideal or perfect transformer. So we need the concept of efficiency given by, Efficiency (%) = P OUT /P IN (4) Generally Converters achieve an efficiency of 80-85%. However using various circuit techniques efficiency above 90% is achievable. II. TYPES OF CONVERTERS There are many different types of DC-DC converters, each tending to be suitable for some types of application when compared to others. As per convenience they can be classified into various groups. Some converters are suitable for stepping up the voltage, while others are suitable for stepping it down; a third group can be used for either stepping up or down. Another important distinction is dielectric isolation offered fully or partially by converters relative to their input and output circuits. In this study brief look at each of the main types of DC -DC converter in current use is highlighted with simulations obtained after solar panel constructed through Simulink is implemented by different converters. For solar panel mathematical details [6], [7] can be accessed. III. NON-ISOLATING CONVERTERS Since the output available from Solar panel needs to be tracked for MPP. The voltage needs to be stepped up or down by a relatively small ratio (less than 4:1) with no problem in output and input having no dielectric isolation. The non-isolating type 29

2 AKGEC INTERNATIONAL JOURNAL OF TECHNOLOGY, Vol. 5, No. 2 of converter is generally used for Solar panel applications. Some examples are 24V/12V voltage reducers, 5V/3V voltage reducers and 1.5V/5V step-up converters. Non-Isolating converters classify five main types of converter, usually called the buck, boost, buck-boost, Cuk and Sepic converters. The buck converter is used for voltage step-down or reduction, while the boost converter is used for voltage step-up or increase. The buck-boost and Cuk converters when being used for step-down or step-up, reversing polarity of voltage are known as inverters as well. (The Cuk converter is named after originator, Slobodan Cuk of Cal Tech University in California.). SEPIC (Single-ended primary-inductor converter) is a type of DC-DC converter that allows voltage at its output to be higher than, less than, or equal to that at its input. Output is not inverted for this converter. The output of Converters is controlled by the duty cycle of the control transistor. Modeling and simulation for all five converters is described in successive sections. GUI (Graphic User Interface) in continuous and discrete mode is applied to obtain learning graphs with discrete components used in MATLAB SIMULINK. Figure 1. Buck Converter. IV. SIMULINK MODELING OF CONVERTERS BUCK CONVERTER The basic circuit configuration used in the buck converter is shown in Fig.1. Only four main components are used, switching power MOSFET Q, flywheel diode D, inductor L and output filter capacitor C. A control circuit is used to monitor the output voltage, and maintain it at the desired level by switching Q1 on and off at a fixed rate known as converter s operating frequency. By varying duty cycle based on proportion of each switching period Q is turned on. Figure 1.1. Buck Converter Without Phase Delay. When Q is turned on, current begins flowing from the input source through Q and L, and then into C and the load. The Inductor L starts building magnetic field and stores energy with voltage drop across opposing or bucking part of the input voltage. Then when Q is turned off, the inductor opposes any drop in current by suddenly reversing its EMF and supplies current to the load itself via D. The DC output voltage appearing across the load is a fraction of the input voltage, and this fraction turns out to be equal to the duty cycle. So we can write: = D, (5) Or = V IN x D (6) Where D is the duty cycle defined by ratio of T ON /T, where T is the inverse of the operating frequency. Thus Buck Converter output voltage can be varied in proportion to input voltage varying the switching duty cycle. Figure 1.2. Buck Converter With Phase Delay. Figures above depict the outputs obtained from solar panel using Simulink in Matlab. 30

3 DC/DC CONVERTERS FOR SOLAR PANEL BOOST CONVERTER The basic boost converter has the components arranged differently (Fig.2) in order to step up the voltage. This Converter consists of using Q MOSFET as a high speed switch, with output voltage control by varying the switching duty cycle. Current flows from the input source through L and Q, when MOSFET is ON. The energy is stored in the inductor s Magnetic field. There is no current through D, and the load current is supplied by the charge in C. L opposes current by immediately reversing EMF when Q is turned off. Thus inductor voltage adds i.e., boosts the source voltage, and through L current is directed due to this boosted voltage now flows across D and the load, recharging C. The output voltage appears to be higher than the input voltage, and it turns out that the voltage stepup ratio is = 1/ (1-D) (7) Where 1 -D is actually the proportion of the switching cycle that Q is off, rather than on. So the step-up ratio is also /VIN = T/T OFF (8) Figure 2. Boost Converter. When simulating diagrams in Matlab scope outputs and waves appear as plotted in Fig 2.1 and 2.2. Fig 2.2. Boost Converter With Phase Delay. BUCK-BOOST CONVERTER The components of Buck and Boost converters are configured in a different way using Buck-Boost Converter (Fig.3). The voltage to be stepped either up or down, depending on the duty cycle. Here when MOSFET Q is turned on, inductor L is directly connected across the source voltage and stored energy in form of magnetic field causes current flows through it. No current can flow through D into load since its reverse biased. Capacitor C must supply the load current in this Ton phase. But when Q is turned off, L is disconnected from the source. Inductor L again opposes any tendency for the current to drop, and instantly reverses it s EMF. Hence output is avalible for phase delay making circuit functional on and off frequently. This generates a voltage that forward biases D1, and current flows into the load and to recharge C. With this configuration ratio between the output and input voltages can be expressed as = - D/ (1-D) (9) This again equates to = - T ON /T OFF (10) So the buck-boost converter steps down voltage when the duty cycle is less than 50% (i.e., Ton < Toff), and steps it up when the duty cycle is greater than 50% (Ton > Toff). Figure 2.1. Boost Converter Without Phase Delay. Figure 3. Buck-Boost Converter. 31

4 AKGEC INTERNATIONAL JOURNAL OF TECHNOLOGY, Vol. 5, No. 2 When Q is turned on, current flows from the input source through L and Q. This helps in storing energy in L creating magnetic field. Then when Q is turned off, the voltage across L is reversed. As was the case of boost converter where current flows from the input source, through L and D, making C charge up with a voltage higher than Vin and transferring stored energy in L. When Q1 is turned on again, C discharges via second L through the load, where L and C act as a smoothing filter. At the same time, energy is being stored again in first L, ready for the next cycle. Meanwhile, the ratio between the output voltage and the input voltage similar to Buck-Boost Converter can be expressed as = - D/ (1-D) = - T ON /T OFF (11) Figure 3.1. Buck-Boost Converter without Phase Delay. Here negative sign indicates voltage inversion. Similar to Buck- Boost converter, voltage can be either step up or step down through Cuk converter. The main difference being presence of the series inductors at both input and output which helps in much lower current ripple at the output of Cuk Converter. Figure 4. Cuk Converter. Figure 3.2. Buck-boost Converter with Phase Delay. CUK CONVERTER The basic circuit of a Cuk converter is shown in Fig.4. It uses an additional inductor and capacitor. The circuit configuration combines the buck and boost converters, which delivers an inverted output. The output current must pass through C, and as ripple current C must be such that it has a high ripple current rating and low ESR (equivalent series resistance), to minimize losses. Figure 4.1. Cuk Converter without Phase Delay. 32

5 DC/DC CONVERTERS FOR SOLAR PANEL Figure 5. Sepic Converter. Figure 4.2. Cuk Converter Expanded Scope without Phase Delay. Figure 5.1. Sepic Converter without Phase Delay. Figure 4.3 Cuk Converter with Phase Delay. SEPIC CONVERTER SEPIC stands for single ended primary inductor converter. This circuit is operated within a limited range because the MOSFET and diode voltages and currents are higher as compared to Buck-Boost Converter. Diode is to be kept at minimum voltage. Fig. 5 shows modeled SEPIC Converter. It can sweep up entire curve obtained from solar panel output. Figure 5.2. Cuk Converter Expanded Scope without Phase Delay. 33

6 AKGEC INTERNATIONAL JOURNAL OF TECHNOLOGY, Vol. 5, No. 2 Figure 5.3. Sepic Converter with Phase Delay. V. COMPARISONS S.No. Converter GUI Output GUI Output without phase with phase delay delay V IN = 21V 1 BUCK BOOST BUCK * *10-11 BOOST 4 CUK SEPIC *10 4 TABLE 1 COMPARISON OF DIFFERENT CONVERTERS VI. RESULTS Thus when compared, outputs obtained from scope and display unit, it is clear that using phase delay doesn t brings about much changes in Buck Converter. Moreover when compared to Input voltage difference appears to be nominal decrease in output voltage as compared to other converters. Thus with solar panel designed Buck Converter can be used to obtain a desired set point while using a controller. VII. REFERENCES [1] Tarak Salmi, Mounir Bouzguenda, Adel Gastli, Ahmed Masmoudi MATLAB/Simulink Based Modelling Of Solar Photovoltaic Cell, International Journal of Renewable Energy Research Tarak Salmi Et Al., Vol.2, No.2, [2] Jaw, Kuen Shiau, Min Yi Lee, Yu-Chen Wei, and Bo Chih Chen, Circuit Simulation for Solar Power Maximum Power Point Tracking with Different Buck-Boost Converter Topologies Ist International Conference on Energies, March [3] Robert W. Erickson, DD-DC Power Converters Wiley Encyclopedia of Electrical and Electronics Engineering June 2007 [4] Dr.P.Sangameswar Raju, Mr. G. Venkateswarlu, Simscape Model Of Photovoltaic cell, International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering, Vol. 2, Issue 5, May [5] Savita Nema, R.K.Nema, Gayatri Agnihotri, Matlab / simulink based study of photovoltaic cells / modules / array and their experimental verification, International Journal of Energy and Environment, Volume 1, Issue 3, 2010 pp [6] Islam, M.A, Mohammad, N. Khan, P.K.S, Modeling and performance analysis of a generalized photovoltaic array in Matlab Joint International Conference IN 2010 on Power Electronics, Drives and Energy Systems by IEEE. [7] Chandani Sharma, Anamika Jain; Solar Panel Mathematical Modelling using Simulink in International Journal of Engineering Research and Applications, Vol. 4, Issue 5( Version 4), May 2014, pp [8] Chandani Sharma, Anamika Jain; Simulink Based Multi Variable Solar Panel Modeling in Telkominika Indonesian Journal of Electrical Engineering, Vol. 12, Issue 8, Aug 2014 [9] Chandani Sharma, Anamika Jain; Maximum Power Point Techniques: A Review in International Journal of Recent Research in Electrical and Electronics Engineering, Vol. 1, Issue 1, April-June 2014, pp Chandani Sharma is a PhD Research Scholar at Graphic Era University, Dehradun. She received M.Tech in Communication Engg. with specialization in Image Processing from Shobhit University, Meerut. She has 7 years of academic experience. Her interest areas include Photovoltaic Systems, Soft Computing, Fuzzy Logic Control Systems and Image Processing. She published 14 International/National Journals and Conferences papers. She has been a meritorious student throughout with an active involvement in many projects and workshops/conference conduction. Dr. Anamika Jain is Professor in Electronics and Communication Engineering Department at Graphic Era University, Dehradun. She has received her PhD Degree from IIT-Roorkee with specialization in Soft Computing. Her interest areas include Artificial Intelligence, Fuzzy Control Systems and Process Control. She has a vast academic experience of 18 years. She has to her credit more than twenty publications in National and International Journals/Conferences. She is currently supervising three PhD students and more than ten M.Tech and B.Tech students. She has versatile knowledge and a contributor to Journal reviews. 34