DC-DC Buck-Converter for MPPT of PV System

Transcription

1 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) DC-DC Buck-Converter for MPPT of P System Dhananjay Choudhary, Anmol atna Saxena 2,,2 Department of Electrical Enineerin, Madhav Institute of Technoloy & Science, Gwalior, India Abstract Althouh solar enery is available throuhout the day its insolation varies from mornin to evenin and with chanin climatic conditions. As the efficiency of solar P panel is low it becomes mandatory to extract maximum power from the P panel at any iven period of time. Several maximum power point trackin (MPPT) techniques are proposed for the purpose. Incremental conductance MPPT technique has hiher steady-state accuracy and environmental adaptability. This paper investiates implementation issues of Incremental conductance MPPT alorithm. Hih frequency DC-DC Buck converter is used to interface P panel with load. The Matlab Simulink model of the system is developed and results are validated with experimental results obtained usin laboratory prototype of the system. Keywords Continuous Conduction Mode (CCM), DC-DC Buck Converter, Incremental Conductance (IC), Maximum Power Point Trackin (MPPT), Photovoltaic (P) I. INTODUCTION The depletion of fossil fuel resources on a worldwide basis has necessitated an urent search for alternative enery sources to meet up the present day demands. Amon all the renewable enery option solar enery is clean inexhaustible and environment friendly potential source. The harnessin of solar enery usin P modules comes with its own problems that arise from the chane in insolation conditions. These chanes in insolation condition severely affect the efficiency and output power of the P modules []. A lot of research has been done to improve the efficiency of the P modules. A number of methods to track the maximum power point of a P module have been proposed to overcome the limitation of efficiency [2]. A MPPT is used for extractin the maximum power from the solar P module and transferrin that power to the load. And a dc-dc converter (step up / step down) act as an interface between the load and the P module as it serves the purpose of transferrin maximum power from the solar P module to the load. By chanin the duty cycle the load impedance is matched with source impedance to attain the maximum power from the P panel [3]. Fi. shows the basic block diaram of DC-DC Converter operatin at MPP. Different maximum power point trackin alorithms have been proposed includin fractional open circuit voltae, fractional short circuit current, perturb and observe (P&O), incremental conductance, and artificial-intellience-based alorithms [4]-[5]. These alorithms may vary in there complexity, efficiency, cost, and there potential application. In hih power applications, the cost of MPPT control is low as aainst the cost of the photovoltaic (P) array and power converters. However, for low power applications, the implementation cost of an MPPT alorithm is a concern [6]- [7]. The most commonly used hill-climbin MPPT technique is the P&O alorithm. The P&O alorithm perturbs the operatin point of the P enerator by increasin or decreasin a control parameter by a small amount and measurin the P array output before and after the perturbation. If the power increases, the alorithm continues to perturb the system in the same direction; otherwise the system is perturbed in the opposite direction [8]. Another very popular hill-climbin MPPT alorithm is the incremental conductance. This MPPT alorithm is based on the fact that the power-voltae curve of a P enerator at constant solar insolation and cell temperature levels has normally only one MPP [9]-[]. At this point the derivative of the power with respect to the voltae equals zero which means that the sum of instantaneous conductance (I pv / pv ) and the incremental conductance (di pv /d pv ) equals zero. On the riht-hand side of the MPP, the sum of the instantaneous conductance is neative while on the left-hand side of the MPP, the sum is positive. The incremental conductance alorithm compares the instantaneous conductance of a P enerator with its incremental conductance and decides whether to increase or decrease a control parameter accordinly []. An improved MPPT alorithm for P sources was proposed to reduce the trackin time where a dc-dc boost converter was used to track the MPP and was brouht out that trackin performance depends upon the trackin alorithm used [2]. In this paper, the trackin capability of incremental with buck converter conductance at different insolation level is presented and the simulation results are verified with the experimental results. 83

2 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) SOA PANE Ipv pv DC-DC Converter MPPT out Iout Fi. DC-DC converter for operation at the MPP OAD II. MODEING AND CHAACTEISTICS OF P MODUE The phenomenenon named photovoltaic effect consists mainly transformin the solar liht in electric enery by means of the semi conductor devices named photovoltaic cells. Fi. 2 shows the equivalent circuit diaram of a sinle solar cell. Here, I is the photo current source with a ph reverse connected diode. S and resistance respectively. As the value of is nelected in the analysis [3]. Sh are series and shunt s Sh is very lare it I Where I is the reverse saturation current of the diode, q is the electron chare, is the voltae across the diode, K is d 9 Boltzmann constant.38 JK / and T is the junction temperature in Kelvin (k). Substitutin eq. (2) in () ives I I I e (3) ph ( qd / KT ) Substitutin voltae across diode (3) ives d Is N in eq. I I I e (4) ph ( q( Is )/ NKT ) ) Where, is the P cell voltae, T is the temperature (in Kelvin) and N is the diode ideality factor []. The typical I- and P- characteristics of the P module are shown in Fi. 3 & 4. Fi. 3 I- characteristics of a P cell I ph D sh Fi.2 Sinle diode model of a P cell The output current from the photovoltaic array is I I I () d ph d I I e (2) ( qvd / KT ) 84 Fi. 4 P- characteristics of a P cell

3 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) Here, Short-circuit current (I sc ) is the maximum output current, open-circuit voltae (oc) is the maximum output voltae, MPP current (I m ) is the maximum power point current, MPP voltae ( m ) is the maximum power point voltae and MPP Power (P m ) is the power at maximum power point these all values are at certain sunliht and temperature of the P module. i C ic o III. DC-DC BUCK CONETE The dc-dc buck converter converts a hiher dc input voltae to lower dc output voltae. The basic dc-dc buck converter topoloy is shown in Fi. 5. It consists of a controlled switch S W, an uncontrolled switch diode (D), an inductor, an capacitance C and a load resistance [4]. S w D C (b) Fi.6.Buck Converter Cicruit when switch:- (a) turns ON (b) turns OFF In the description of converter operation it is assumed that all the components are ideal and also the converter operates in Continuous conduction mode (CCM). In CCM operation the inductor current flows continuously over one switchin period. The switch is either ON or OFF accordin to the switchin position this results in two circuit states. The first sub-circuit state is when the switch is turned ON, the diode is reverse biased and inductor current flows throuh the switch, which can be shown in Fi. 6(a). The second subcircuit state is when the switch is turned OFF and current freewheels throuh the diode [5-6] which is shown in Fi. 6(b). Fi. 5 DC-DC buck converter topoloy _ I + C (a) o When switch S W is ON and D is reverse biased, then inductor current i and capacitor voltae C are iven by eq. (5) and (6). di dt (5) o dv dvc ic (6) dt dt C When the switch is OFF and D is forward biased, i and capacitor voltae C are iven by eq. (7) and (8). di dt (7) dv dvc ic (8) dt dt C The state space representation for converter circuit confiuration can be expressed as iven in eq. (9) 85

4 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) dx Ax BU when Swis closed dt A2x B2Uwhen Swis opened T T X [ x x ] [ i ] (9) Where 2 c Equation (3) is the condition to achieve the maximum power point, when the variance of the output conductance is equal to the neative of the output conductance, the module will work at the maximum power point. The flow chart of the incremental conductance is shown in Fi. 7 [2 ]. The state transition matrix A, and B input matrix are system matrices as shown in eq. () arc a r A A2, B, B2 ar C a C C Where a r C I. INCEMENTA CONDUCTANCE MPPT AGOITHM () A typical solar panel converts about 3-4 % of the incident solar insolation in to electrical enery [7]. Maximum power point trackin technique is used to improve the efficiency of the solar panel. Accordin to maximum power transfer theorem, the power output of a circuit is maximum when the Thevenin impedance of the circuit (source impedance) matches with the load impedance. Hence the problem of trackin the maximum power point reduces to an impedance matchin problem. There are several techniques to track the MPPT but this paper deal with Incremental conductance. A. Incremental Conductance Alorithm Incremental Conductance method uses the information of source voltae and current to find the desired operatin point. From the P- curve of a P module shown in Fi. 4 it is clear that slope is zero at maximum point [7], so the formulas are as follows dp d I mpp d d di I MPP d di I MPP d () (2) (3) 86 No chane Inc. duty ratio STAT P array voltae and current samplin (k)=(k)-(k-) I(k)=I(k)-I(k-) v= NO di/dv=-i/ NO di/dv>-i/ NO Dec. Duty ratio Update History (k-)=(k) I(k-)=I(k) END I= I> Dec. Duty ratio Inc. Duty ratio No chane Fi. 7 Flow chart of the Incremental Conductance method In this flow chart, (k) is the new detection voltae and I(k) is the new detection current, (k-) and I(k-) is previous detection values. When the new value is read in to the proram, it calculates the previous value compare with the new one, and then determine the voltae differentials is zero or not, accordin the voltae differentials is zero, the current difference can be determined zero or not. If both of them are zero, it shows that they have the same value of impedance and the value of duty ratio will remain the same as before. If the voltae differential is zero, but the current differential is not zero, it shows that the insolation has chaned. When the difference of the current values is reater than zero, duty ratio will increase, when the difference of the current value is less than zero the duty ratio will decrease. If the voltae differential is not zero

5 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) determine it whether satisfy the eq. 3 or not, when eq. 3 is satisfied the slope of the power curve will be zero that means the system is operatin at MPP, if the variance of conductance is reater than the neative conductance values, it means the slope of the power curve is positive and the duty ratio is to be increased, otherwise it should be decreased [8]. The motive is to automatically find the voltae MPP or current I MPP at which a P module should operate to obtain the maximum power output P MPP under a iven temperature and irradiance. As switch-mode dc-dc converter is used as intermediate power processor, it chanes the values of current and voltae levels such that maximum power can be extracted from a P module. A buck converter can be used to maximize the power transfer throuh the use of impedance matchin. An application of this is in a maximum power point trackin commonly used in photovoltaic system. For electric power eq. (4) is as follow I I (4) Where:- is the output voltae, I is the output current, is the power efficiency (ranin from to ), is the input voltae, I is the input current By ohm s law I I i (5) D (7) Where, D is the duty cycle Substitutin the value of in eq. (6) we et: 2 2 D / / i This reduces eq. (8) to D 2 i And finally (8) (9) D (2) / i The eq. (2) shows that it is possible to adjust the impedance ratio by adjustin the duty cycle. This is very useful in calculatin the MPPT because insolation chanes very dynamically. Table- P Panel Specification P panel parameters alues oc 22.4 Isc.3A Pmax. 2watt mpp 7.77 Impp.26A Tolerence ±5% Where is the output impedance, impedance. Substitutin these expressions for I and power equation (4) yields: 2 2 i i is the input I in to the (6) As we know that for the continuous mode, (where I >). ESUTS & DISCUSSION The Matlab Simulink model of the complete system is shown in Fi. 8. The complete model is divided in to three parts for easy understandin i.e. P model, IC MPPT method & DC-DC Buck converter, in P panel total of 36 individual cells are connected in series to yield a voltaep 8. The I- and P- characteristics of the model under varyin solar insolation condition with fixed ambient temperature are shown in Fi. (9) & () respectively. It is observed that both open circuit voltae 87

6 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) and short-circuit current ets reduced at lower solar insolation levels. alorithm based on the solar insolation and prevailin ambient condition. The dc-dc buck converter is desined to operate at 25 khz. Fi. 8 Simulink block diaram of complete P system Fi. INC MPPT simulation setup Fi. 2 shows the chane in solar insolation from 5 w/m 2 to w/m 2 at 25 c. The variation in input power and voltae is shown in Fi. 3&4 respectively. The variation in output power, voltae and current is shown in Fi. 5, 6 & 7 respectively. Fi. 9 Characteristic curve of (Ipv-pv) at different solar insolation Fi. 2 Chane is solar insolation Fi. Characteristics curve of (P pv- pv) at different solar insolation The power system block set model of DC-DC buck converter supplied with P panel is shown in Fi.. The duty ratio for the converter is decided by the MPPT 88

7 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) Fi.3. Input power Fi. 6 Output power Fi. 4 Input voltae Fi. 5 Output current 89 Fi. 7 Duty ratio variation A fixed resistance of =4.5ohm is connected across the load side of the buck converter to match the input resistance in because for the buck converter input resistance is always reater than the load resistance and the relation of input resistance with load resistance and duty ratio is iven by eq. (2). D in 2 (2) At simulation time.6 there is an increase in solar insolation from 5 w/m 2 to w/m 2 and it is clear from the above Fi. (3-6) that output voltae is decreasin and output current is increasin and from Fi. 7 it is also clear that at simulation time.6 when insolation are chanin the duty ratio is adjustin itself to produce maximum power at different insolation.

8 Website: (ISSN , ISO 9:28 Certified Journal, olume 4, Issue 7, July 24) To verify proposed theory a laboratory prototype P system is constructed as shown in fi. 8. The artificial sun is realized by the incandescent lamp set and the solar insolation on the P panel is chaned by chanin the manitude of AC supply voltae.the prototype converter circuit parameter values are: =5µH, C=5µF. The semiconductor devices used for makin DC-DC buck converter are an International rectifier IF 54 Power mosfet, a Motorola MU 84 fast recovery diode. An international rectifier I2 ate driver drives the switchin device Mosfet. IC 6N36 is used for isolation purpose. DC-SUPPY COMPUTE DSP- BOAD OSCIOSCOPE DIE CICUIT BUCK- CONETE BATTEY AUTO- TANSFOME PHOTO-OTAIC MODUE In Fi. 5 and fi. 7 shows the output voltae and duty ratio values for MPPT. Experimental results shown in Fi. 8 also verify the IC alorithm, as it is clear from the Fi. 9 for the duty ratio 58% and input voltae of 8 volt the achieved output voltae is 8. olt. This verifies the laboratory prototype model results with the Simulation results. I. CONCUSION A standalone P system connected with buck converter and IC alorithm for extractin maximum power at different environmental condition is proposed. We realize that the basic function of DC-DC converter in P system is like intermediate power processor which chanes the current and voltae levels such that maximum power can be extracted from the P array. Chanin voltae and current level is nothin but convertin a iven fixed load to a variable load. Hence the results obtained at different environmental condition are found to be satisfactory as they validate the experimental results too. MPPT alorithm responds in very fast manner and its accuracy is hiher that it reaches to steady state very quickly. ACKNOWEDGMENT The authors sincerely thank Director MITS, Gwalior, India for his support durin this research work. Fi.8. Experimental setup of P system Fi. 9. Output voltae of buck converter 82 EFEENCES [] Gules,., De Pellerin Pacheco, J., Hey, H.., Imhoff, J., A maximum power point trackin system with parallel connection for P Stand-Alone Application, IEEE Trans. on Industrial Electronics, vol.55,no. 7, pp. 2674, 2683, July 28. [2] Masoum, M. A S, Dehbonei, H., Fuchs, E. F., Theoretical and Experimental Analyses of Photovoltaic systems with voltae and current-based maximum power-point trackin, IEEE Trans. On Enery Conservation, vol. 7, no. 4, pp. 54, 522, Dec. 22. [3] Subudhi, Bidyadhar and Pradhanhen, aseswari A comparative study on maximum power point trackin technique for photovoltaic power system, IEEE Trans. on Sustainable enery., vol. 4, pp , Jan.23. [4] Mohammed A. Elendy, Bashar ahawi, and David J. Atkinson, Assessment of the incremental conductance maximum power point trackin alorithm, IEEE Trans. on sustainable enery., vol. 4, pp. 8-7, Jan. 23. [5] Shihon Qin, Min Wan, Ten Chen, and Xianlin Yao, Comparative analysis of incremental conductance and perturb-andobservation methods to implement MPPT in photovoltaic system, IEEE International Conference on Electrical and control enineerin (ICECE)., pp , Sept.2. [6] Snehamoy Dhar, Sridhar and Geraldine Mathew, Implementation of pv cell based standalone solar power system employin incremental conductance MPPT alorithm, International conferece on circuits,power and computin technoloies (ICCPCT).,pp ,23.

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