SIMULATION OF A SOLAR MPPT CHARGER USING CUK CONVERTER FOR STANDALONE APPLICATION 1 Diva Catherine, 2 Kavitha Bhaskar 1 M tech student, 2 Assisstant Professor Jyothi Engineering College, Thrissur Email : 1 catrina.diva@gmail.com, 2 kavithabhaskar@gmail.com ABSTRACT: Standalone photovoltaic system requires a proper battery charge controller. The main disadvantage of photovoltaic system is its variations in output voltage. Therefore to obtain a stable voltage from solar panels, DC- DC converters are used. This paper explains the use of cuk converter with MPPT technique in a photovoltaic system. The MPPT is implemented by incremental conductance algorithm with direct control method. This system simplifies the conventional MPPT systems by eliminating proportional integral control loop. The overall system is designed, developed and validated by using MATLAB/SIMULINK. KEYWORDS: Renewable energy, Cuk Converter, Mppt I. INTRODUCTION One of the major concerns in the power sector is the day-to-day increase in power demand but the unavailability of enough resources to meet the power demand. Demand has increased for renewable sources of energy to be utilized along with non renewable energy resources to meet the energy demand.[1-5] Renewable energy sources like wind energy and solar energy are the prime energy sources which are being utilized in this regard. Solar energy is abundantly available and that has made it possible to harvest it and utilize it properly. A photovoltaic system can be a standalone generating unit or can be a grid connected generating unit. Thus standalone system can be used to power rural areas where the availability of grids is very low.[6] Photovoltaics are semiconductor devices which convert solar irradiation in the visible spectrum to generate direct current. In order to increase the efficiency of a solar panel certain control methods used.[8-9]the use of the newest 12 power control mechanisms called the Maximum Power Point Tracking (MPPT) algorithms has led to the increase in the efficiency of operation of the solar modules and thus is effective in the field of utilization of renewable sources of energy. The main challenge of the generation side of the PV system is to generate as much power as it can. This is related to the payback and efficiency of the system. Thus in PV system efficient dc-dc converter is used to control the load power and to ensure that system operates in maximum power point.[15-16] In this paper cuk converter is used in between the PV panel and inverter. In Cuk converter the source and load side are separated via a capacitor thus energy transfer from the source side to load side occurs through capacitor which leads to less current ripples at the source and load side. The most common algorithms for Maximum Power Point (MPP) are Perturb and Observe algorithm and Incremental conductance method. In this paper cuk converter is used for maximum power point tracking. The outline of the proposed system is given in fig 1 Fig 1 Outline of the proposed system
II. PV MODULE AND MPPT The solar cell is the basic unit of a PV system. An individual solar cell produces direct current and power typically between 1 and 2 W, hardly enough to power most applications. Solar Cell or Photovoltaic (PV) cell is a device that is made up of semiconductor materials such as silicon, gallium arsenide and cadmium telluride, etc. that converts sunlight directly into electricity. The voltage of a solar cell does not depend strongly on the solar irradiance but depends primarily on the cell temperature. PV modules can be designed to operate at different voltages by connecting solar cells in series. When solar cells absorb sunlight, free electrons and holes are created at positive/negative junctions. If the positive and negative junctions of solar cell are connected to DC electrical equipment, current is delivered to operate the electrical equipment. The equivalent circuit of the PV cell is shown in figure 2. maximum power point tracking(mppt).the output of MPPT is given to dc-dc converter for extracting the output IV. SELECTION OF CONVERTER Switch Mode Power Supply topologies follow a set of rules. A very large number of converters have been proposed, which however can be seen to be minor variations of a group of basic DC- DC converters built on a set of rules.[8-9]many consider the basic group to consist of the three: BUCK, BOOST and BUCK- BOOST converters. The CUK, essentially a BOOST- BUCK converter, may not be considered as basic converter along with its variations: the SEPIC and the zeta converters.[12],[18] The buck, boost and buckboost converters all transferred energy between input and output using the inductor, analysis is based of voltage balance across the inductor.[21] The CUK converter uses capacitive energy transfer and analysis is based on current balance of the capacitor. The circuit in Fig.4 is derived from DUALITY principle on the buck - boost converter.[22-23] Fig 2 Equivalent circuit of PV The panel is modelled by a Thevenin s equivalent circuit, fig 2 which consists of a voltage source Vg connected in series with an output resistance Rg around the MPP. Both and are subject to the level of insolation and temperature. The input voltage and equivalent input resistance of the converter are and, respectively. As the input power to the tracker is equal to the output power of the panel. Module III. PARAMETERS OF PV Module used here is KC85T Pmax=87w Vmp=17.4v Imp=5.02A Voc=21.7V Isc=5.34A This is for 1000w/m^2 irradiance level In addressing the poor efficiency of PV systems, some methods are proposed, among which is a new concept called 13 Fig : 3 cuk converter If we assume that the current through the inductors is essentially ripple free we can examine the charge balance for the capacitor C1. For the transistor ON the circuit becomes Fig :4 cuk converter on state and the current in C1 is IL1. When the transistor is OFF, the diode conducts and the current in C1 becomes IL2.
Fig :4 cuk converter off state Since the steady state assumes no net capacitor voltage rise,the net current is zero. [24-25]The relations between output and input currents and voltages are given in the following: (Vo/Vin)= -(D/1-D)-----(3) (Io/ I in)= -(D/1-D)-----(4) D-duty cycle Thus the voltage ratio is the same as the buck-boost converter. The advantage of the CUK converter is that the input and output inductors create a smooth current at both sides of the converter while the buck, boost and buck-boost have at least one side with pulsed current. Some analyses of Cuk converter specifications are provided in [23], and a comparative study on different schemes of switching converters is presented in the literature [8-9] The components for the Cuk converter used in simulation and the hardware setup were selected as follows: Input inductor L1=5mH Duty cycle=50% Capacitor c1 (pv side )=47uF Filter inductor L2 = 5mH Switch : MOSFET IRF540 Freewheeling diode Capacitor c2 (filter side ) =1uF Resistive load =10 Switching frequency =10kHz Pic processor SCHEME V. MPPT CONTROL Performance of the solar-pv system not only depends on the environment conditions but also the MPPT approach holds a significant role. In case of high solar radiation situation, the PV system generates the power efficiently using an effective MPPT method.[14] A MPPT is the approach which is used in the system to 14 achieve maximum efficiency. The main objective of this controller is to regulate the operating point of the system in case of variations in environment conditions. The dcdc converter is connected with the solar-pv array to regulate the operating point through a MPPT controller. The output power of PV panel is varied due to change in solar radiation and temperature conditions and under consumer load variations. The cuk dc-dc converter is used here as shown in Fig.3. Its output voltage Vc can be regulated by controlling the duty cycle of the switch S1. Duty cycle presents the ratio of the on and off periods for a switch in one cycle. It also allows regulation of the input voltage though controlling the duty cycle. Different methods are proposed and analyzed for tracking the MPP in a solar- PV system. Here the modified incremental conductance (IC) method is used as a MPPT approach. This tracking approach uses a fix step size and it can be chosen by the tracking speed requirements. This algorithm utilized the principle of calculating the change in output power in the fixed time period and from this it determines the duty ratio as shown in Fig.5 In this modified approach, two conditions loop is used according to the change in radiation. First case is an increase in radiation and second one is a decrease in radiation. When the solar radiations increase then the voltage and current both increase and thus the V/P and I/P both are positive and when solar radiation decreases then also V/P and I/P both become positive. [14]In first case when the radiation decreases then, if one takes only first loop of V/P, the voltage and power reduce but the ratio of them becomes positive and it increases the duty ratio which is incorrect in this case. Therefore the second loop is used and in case of an increase in radiation the controller follows the first loop for determining the duty ratio. The modelling of this approach is performed in Simulink and integrated into the system as a subsystem. Fig 5 algorithm [14]
Method VI. DIRECT CONTROL Conventional MPPT systems have two independent control loops to control the MPPT. The first control loop contains the MPPT algorithm, and the second one is usually a proportional (P) or P integral (PI) controller.[15] The IncCond method makes use of instantaneous and IncCond to generate an error signal, which is zero at the MPP; however, it is not zero at most of the operating points. The main purpose of the second control loop is to make the error from MPPs near to zero. Simplicity of operation, ease of design, inexpensive maintenance, and low cost made PI controllers very popular in most linear systems. However, the MPPT system of standalone PV is a nonlinear control problem due to the nonlinearity nature of PV and unpredictable environmental conditions, and hence, PI controllers do not generally work well. In this paper, the IncCond method with direct control is selected. [16]The PI control loop is eliminated, and the duty cycle is adjusted directly in the algorithm. The control loop is simplified, and the computational time for tuning controller gains is eliminated. To compensate the lack of PI controller in the proposed system, a small marginal error of 0.002 was allowed. In this paper the second control loopis eliminated and shows that sophisticated MPPT methods do not necessarily obtain the best results, but employing them in a simple manner for complicated electronic subjects is considered necessary [3]. The feasibility of the proposed system is investigated with a dc dc converter configured as the MPPT. In, it was mentioned that the power extracted from PV modules with analog circuitry can only operate at the MPP in a predefined illumination level. VII. SIMULATION The diagram of the closed-loop system designed in MATLAB and Simulink is shown in Fig.6, which includes the PV module electrical circuit, the Cuk converter, and the MPPT algorithm. This block is simulated using the Simulink blocks available in the MATLAB library. The converter components are chosen according to the values presented previous section. The PV module is modeled using electrical characteristics to provide the output current and voltage of the PV module. The provided current and voltage are fed to the converter and the controller simultaneously. The Output of PV panel is given to MPPT controller. Triggering pulse of MPPT controller is given to IGBT switch. Then the output is fed to the load.the entire system has been modelled on MATLAB 2009a.The PI control loop is eliminated, and the duty cycle is adjusted directly in the algorithm. To compensate the lack of PI controller in the proposed system, a small marginal error of 0.002 is allowed. 15 Voltage measurement is required at the point where the PV module output is connected to the input of the Cuk converter. The voltage at this point is the operating voltage of the PV module. On the other hand, current measurement is also necessary to indicate the generated current of the PV module on each operating point. It is particularly important to determinate the atmospheric condition, which is vital in connection with the accuracy of MPP tracking. Fig 6 simulation of proposed system Fig 7 simulation of mppt algorithm VIII. SIMULATION RESULT The flowchart of the incremental conductance MPPT algorithm has been implemented in Matlab/Simulink. The figure 7 illustrated the modeling diagram for the above algorithm. The simulation results of the ouput power of the PV module and the MPPT pulse width modulated output is shown in figure 8. The diagram of figure 10 represents the whole PV system with MPPT along with the cuk converter has been implemented in the Matlab/ simulink.
Fig 8 Simulation of MPPT Fig 9 scope of generation of pulses Fig 10 simulation of output voltage The various waveforms were obtained by using the plot mechanism in MATLAB. There is a small loss of power from the solar panel side to the boost converter output side. This can attributed to the switching losses and the 16 losses in the inductor and capacitor of the boost converter. This can be seen from the plots of the respective power curves. The results also indicate that the proposed control system is capable of tracking the PV array maximum power and thus improves the efficiency of the PV system and reduces low power loss and system cost. IX. CONCLUSION A low power stand-alone solar-pv energy generation system with a cuk dc-dc converter has been designed and the performance analysis of the system has been presented using MATLAB simulation with the device currents and voltages. The MPPT method simulated in this paper is able to improve the dynamic and steady state performance of the PV system simultaneously. Through simulation it is observed that the system completes the maximum power point tracking successfully despite of fluctuations. When the external environment changes suddenly the system can track the maximum power point quickly. X. REFERENCES [1] Resource and Energy Economics - C Withagen - 1994 Elsevier [2] E. Weston, Art of utilizing solar radiant energy, September 1888. [3] A. Einstein, Concerning an heur istic point of view toward the emission and transformation of light, American Journal of Physics, vol. 33, no. 5, 1965. [4] Mission tiros - nasa science. Http:// science.nasa.gov/missions/ tiros/, Nov 2012. [5] The institute of energy conversion: The first twenty-five years: 1972-1997. Http: //www.udel.edu/iec/history.html, Nov 2012 [6] W. Xiao, W. G. Dunford, P. R. Palmer, and A. Capel, Regulation of photovoltaicvoltage, IEEE Trans. Ind. Electron., vol. 54, no. 3, pp. 1365 1374, Jun. 2007 [7] F. Liu, S. Duan, F. Liu, B. Liu, and Y. Kang, A variable step size INC MPPT method for PV systems, IEEE Trans. Ind. Electron., vol. 55, no. 7,pp. 2622 2628, Jul. 2008. [8] T. Esram and P. L. Chapman, Comparison of photovoltaic array maximum power point tracking techniques, IEEE Trans. Energy Convers.,vol. 22, no. 2, pp. 439 449, Jun. 2007. [9] K. K. Tse, B. M. T. Ho, H. S.-H. Chung, and S. Y. R. Hui, A comparative study of maximumpower-point trackers for photovoltaic panels using switchingfrequency modulation scheme, IEEE Trans. Ind. Electron., vol. 51, no. 2, pp. 410 418, Apr. 2004
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