Paper Title (use style: paper title)

Optimizing Duty-Cycle Algorithm of P & O MPPT Technique Based controller for Voltage regulation of PV array Using Buck ConverterSthitaPrajna Rath1

Arun Kumar Sahoo2

Surya Prasad Mishra3Department of Electrical EngineeringDepartment Of Electrical Engineering Department Of Electrical engineeringKrupajal Engineering College

Sudhananda Engineering & research center Gandhi Institute For Technology

Bhubaneswar, Odisha,India

Bhubaneswar,Odisha, India

Bhubaneswar, Odisha, India

sthitaprajna.rath@gmail.com

arun_bicky@yahoo.co.in surya.mishra1188@gmail.comAbstract-- The power delivered by a photovoltaic power system depends strongly on the level of sunlight, the cell temperature and the nature of the load supplied, It is therefore highly unpredictable. Therefore a maximum power tracking (MPPT) technique is needed to track the peak power to maximize the produced energy. The maximum power point in the power voltage graph is identified by an algorithm called perturbation & observation (P&O) method or Hill climbing. This algorithm will identify the suitable duty ratio in which the DCIDC converter should be operated to maximize the power output. The results confirm that the photo voltaic array with proposed MPPT controller can operate in the maximum power point for the whole range of assumed solar data (irradiance and temperature). The results of the simulation in Simulink confirm the efficiency of the proposed method. Index Terms MPPT, Solar Array, Buck Converter, P &O, Duty Cycle, 1. Introduction A photovoltaic system converts sunlight into electricity. The basic device of a photovoltaic system is the photovoltaic cell. Cells may be grouped to form panels or modules. Panels can be grouped to form large photovoltaic arrays. The term array is usually employed to describe a photovoltaic panel (with several cells connected in series and/or parallel) or a group of panels. The electricity available at the terminals of a photovoltaic array may directly feed small loads such as lighting systems and DC motors. Some applications require electronic converters to process the electricity from the photovoltaic device. Photovoltaic arrays present a nonlinear I-V characteristic with several parameters that need to be adjusted from experimental data of practical devices. The mathematical model of the photovoltaic array may be useful in the study of the dynamic analysis of converters, in the study of maximum power point tracking (MPPT) algorithms and mainly to simulate the photovoltaic system and its components.[1]

These converters may be used to regulate the voltage and current at the load, to control the power flow in grid connected systems and mainly to track the maximum powerpoint (MPP) of the device. Converter with maximum power point tracking(MPPT) feature use an algorithm to continuously detect the maximum instantaneous power of the array. As the operating conditions of the array (solar irradiation and temperature) may change randomly during the operation of the system, an MPPT algorithm is necessary so that the maximum instantaneous power can be extracted and delivered to the load.[2]

Many MPPT methods have been proposed. However, the achievement of MPPT strongly depends on the performance of the converter control and its ability to regulate the operating point of the PV array. The PV array operating point can be adjusted by regulating the voltage and current at the terminals of the array. The voltage control method is preferred because the voltage at the MPP is approximately constant. The PV current, on the other hand, changes greatly when the solar irradiation..varies .

Fig( 1) Over all block diagramIn fig. 1 the PV array feeds the DC-DC buck converter. The output of the converter is represented by a constant DC voltage source V0 that represents a battery or a DC link for another cascade converter. The buck converter serves as an interface between the PV array and the voltage V0. The MPPT block provides a voltage reference and the voltage controller regulates the PV array voltage.

The paper is organized as follows

Modeling of PV array are given in section 2 .In section 3 MPPT algorithm are discussed. Section 4 present the BUCK converter model. PWM inverter & simulation result, discussion are discussed in section 5 & 6. 7 & 8 section represent the conclusion & reference. 2. Modeling Of PV Arraya. Modeling of PV array

A typical PV cell produces less than 2W at 0.5Vapproximately; the cells are connected in series-parallel configuration on a module to produce high power. A PV array is a group of several PV modules which are electrically connected in series and parallel circuits to generate the required current and voltage. For Ns cells connected in series and Np cells connected in parallel. Then equivalent equation can be given as

-------------------------- (1)The equivalent circuit for the solar array in which the modules are arranged with Npp parallel and Nss series as is shown in figure

Fig 2.General Equivalent circuits Model of generalized PV array

The terminal equation for the current and voltage of the array becomes as follows

--(2)b. Simulink Model of PV arrayWe can simulate the PV array with an equivalent circuit model based on PV model shown in Fig.2

Fig 2.Photovoltaic array model with a current controlled source, equivalent resistorc. Simulation result of PV array

Fig 3. P-V characteristics of solar PV

Fig 4. V-I characteristics of solar PV

The output of Simulink model is shows first the P-V

characteristics of PV module and then V-I characteristics, reference to the key specifications of the NA-901 module illustrated in table 1 , the results of Simulink PV module show the excellent correspondence to the modelTABLE I. SOLAR PANNEL SPECIFICATION

T25Co

Maximum PowerPMAX131WP

Open circuit voltageV23VOC

Short circuit currentI7.2ISC

3. Mppt Algorithm

Fig 5. P & O observation techniqueFig. 5 shows the algorithm of the P&O MPPT method. As the name says, the algorithm is based on the observation of the array output power and on the perturbation of the power based on increments of the array voltage or current. The algorithm continuously increments or decrements the reference current or voltage based on the value of the previous power sample. The P&O method is claimed to have slow dynamic response and high steady state error. In fact, the dynamic response is low when a small increment value and a low sampling rate are employed. Low increments are necessary to decrease the steady state error because the P&O always makes the operating point oscillate near the MPP, but never at the MPP exactly. The lower the increment, the closer the system will be to the array MPP. The greater the increment, the faster the algorithm will work, but the steady state error will be increased. Considering that a low increment is necessary to achieve a satisfactory steady state error, the algorithm speed may be increased with a higher sampling rate. So there is always a compromise between the increment and the sampling rate in the P&O method.4. Buck Converter ModelThe simplified model of a photovoltaic (PV) array connected to a buck converter. The output voltage of the PV array is VPV.[3] This voltage must be controlled in order to keep the array operation at the maximum power point, which is accomplished by a power tracking algorithm. The maximum power point tracking (MPPT) algorithm provides the reference VPV* for the buck converter. The buck converter sets the input voltage (which is the output voltage of the PV array) to the desired value while its output voltage is fixed. In most typical applications of PV arrays the output of the array is directly connected to a load or to a battery with an approximately constant voltage. Although this kind of system is costless, the lack of the array output voltage control prohibits the system to operate at with maximum efficiency, hence energy is wasted. [4]If a power electronic converter is used to interface the PV array and the load (e.g. a battery that feeds a DC load or a DC-AC conversion stage) the best performance of the PV array may be achieved, hence the energy utilization is optimized.

Figure 6 PV array connected to a buck converter with constant output voltageThe buck circuit of this figure has a constant output voltage and its input voltage depends on the output of the PV array. The voltage Vo may be kept constant by a battery or by another electronic power converter, as mentioned before.[5]5. PWM InverterGrid-connected single-phase inverters are normally full bridge voltage source inverters as shown in Fig.7 below

Fig 7. Grid connected IGBT inverter system

The single phase grid connected inverter can be broadly can be broadly classified into two categories: inverters with isolating transformer and inverters without it, where the former ones offer better EMI capability than transformer less inverters. Here first transformer is taken into consideration. The three-phase grid connected inverter shown in fig.7 is composed of a dc voltage source (Vdc), six switches, and utility grid (Vgrid). In inverter-based DG, the produced voltage from inverter must be higher than the Vgrid. It is required to assure power flow to grid. Since Vgrid is uncontrollable, the only way of controlling the operation of the system is by controlling the current that is following into the grid [6][7].6. Simulation Result & Discussion

The overall block diagram and simulation result are discussion below

Fig 8. Over all simulation model of the systema. Simulink result of MPPT controller

Fig 9. MPPT controller output of solar PV system

Here MPPT controller shows the output of 157 watt using P & O observation technique

b. Simulink result of Duty Cycle

Fig 10. Duty Cycle In the starting, we reduced the value of duty cycle so that it can track the maximum voltage and power and on the maximum point in P-V graph the duty cycle is varying and does not go to downward. Here we see that the duty cycle is varying only between 0.32 to 0.38. so that in that value of duty cycle we get a maximum power i:e; 157 watts. c. Simulink result of PWM generator

Fig 11. Gate signal through PWM modulatorHere we get the gate signal through the PWM modulator which applies to the switch of the Buck converter. Here we take the saw tooth wave as the reference signal is compared with duty cycle. d. Simulink result of Voltage & current wave form

Fig 12. voltage wave form

Fig 13. Current wave form

Fig 14. Load current wave form7. ConclusionThis paper presents a new method for MPPT in Photovoltaic systems based on P&O algorithm. According to the proposed method cube of slope of Power vs. Voltage (P V) curve is calculated in each calculation step. Through this calculation both the direction of Photovoltaics (PV) voltage change and the step size of this change are resulted without making any slope or sign control. Moreover, a way to apply this method in PV systems which are consisted of Ns in series and Np in parallel connection PV arrays with high installed nominal power is proposed. A DC to DC buck converter is used in connection with PV arrays for achieving operation in Maximum Power Point and keeping output voltage constant. So, this method is more flexible and due to its flexibility, results power ripples elimination both in steady state and during transitions. Taking into account this, more energy is extracting from this PV system during the same time period. One more advantage of this proposed method for its simplicity of its implementation. The whole system is simulated using Mat lab/Simulink and simulation results are presented and analyzed.

8. Reference [1] honghai kuang,shengqing li,zhengqiu Wu,Discussion on advantages & disadvantages of DG connected to grid.IEEE,pp.170-173,2011[2] R. Messenger and J. Ventre, Photovoltaic Systems Engineering, CRC Press, 2000, pp.41-51[3] Satyaranjan Jena Member, IEEE, and B.Chitti Babu, Member IEEE, Amiya Kumar Naik, Gokulananda Mishra. Performance Improvement of Single-Phase GridConnected PWM Inverter Using PI with Hysteresis Current Controller 978-1 4673-0136-7/11/$26.00 2011 IEEE[4] Roland E. Best, Phase-locked loops: Theory, Design and Application, McGraw-Hill, New York, 2nd edition, 1993, pp. 93 . 104[5] Rahman, M.A.; Radwan, T.S.; Osheiba, A.M.; Lashine, A.E.; Analysis of Current Controllers for Voltage-Source Inverter IEEE Trans. On Industrial Electronics, Volume: 44 , no. 4 , Pp. 477 485, ,1997[6] Heinz Willi van der Broeck and Hans-Christoph Skudelny, Analysis and Realization of a Pulse With Modulator Based on Voltage Space Vectors, in IEEE Transactions on Industry Applications, 1988, pp. 142 . 150[7] N. Goshima, T. Kaito, M. Kawasaki, H. Koizumi, K. Kurokawa. T. Mizuno, K. Nagasaka and Y. Noda, A Novel Microcontroller for Grid-Connected Photovoltaic Systems, in IEEE Transactions on Industrial Electronics, 2006, pp.1889 . 1897_1417016428.vsd

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