International Journal of Scientific & Engineering Research, Volume 6, Issue 4, April-2015 1692

ISSN 2229-5518

SIMULINK Model of PV array with MPPT using

Cuk converter

Ramyaka Sammangi, Raghunadha Sastry R, Sambasiva Rao N

Abstract— solar energy is most useful for sustainable development. The efficiency of Photovoltaic array will be maximum if it is operated at maximum power point (MPP). The output of Photovoltaic array is depends on temperature and irradiation levels so it is difficult to track MPP.Maximum Power Point Tracker (MPPT) is an algorithmic technique that tracks MPP and force the PV array to operate at its maximum power point. In this paper we focuses on improving Perturb and Observe (P&O) and Incremental Conductance method of MPPT techniques by eliminating its drawbacks like oscillations at maxi- mum power point, complexity . A general model of Photovoltaic system with proposed MPPT controller and Cuk converter is implemented on Matlab and a comparision between proposed P&O and Incremental Conductance MPPT techniques under constant and variable irradiation levels is done, which proves that Incremental Conductance MPPT algorithm is robust and efficient for changing irradiation conditions and P&O is simple and efficient tech- nique for constant irradiation condition.

Index Terms— Cuk converter, Incremental Conductance (IC), Irradiation, Maximum Power Point Tracker (MPPT), Photovoltaic array (PV

array), Perturb & Observe (P&O), Photovoltaic array, Temperature.

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ne of the major problems in the power sector is unbal- ance in supply and demand due to the depletion of con- ventional energy resources.Usage of fossil fuels causes serious environmental pollution. So focus is increased towards the utilisation of renewable energy. Thus the growing demand on

electricity, the limited stock and rising prices of conventional sources such as coal, petroleum etc has led to the use of re- newable energy sources.

The solar has advantages like less operational and

maintenance cost, bulk availability and ecofriendliness.With

all its advantages solar is gaining more attention.

The solar efficiency is increased by tracking its MPP

and using MPPT algorithm this can be achieved. A number of algorithms are developed to track the maximum power point efficiently. Among all MPPT methods, Perturb and Observe (P&O) method and Incremental Conductance method are the most used MPPT algorithms.

The proposed topology consists of control strategy of interconnection between photovoltaic array with MMPT and Cuk converter to load as shown in below figure 1. The power

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• *Ramyaka Sammangi is currently pursuing masters degree program in power electronics and Drives engineering in NRI Institute of Technology, Agiripalli, Affiliated to Jawaharlal Nehru Technological University Kakinada, India.*

PH- 07893363866.

E-mail: ramyaka.sammangi@mail.com

• *Raghunadha Sastry R received Master’s Degree in Power Electronics & Power*

Systems Engineering from Acharya Nagarjuna University, Guntur, India.

Presently he is working as Assistant Professor in Electrical Department, NRI Institute of Technology, Agiripalli, India.

PH- 09490337572

E-mail: raghunath.rrs@gmail.com

• *Sambasiva Rao N received PhD from Jawaharla Nehru Technological Universi-*

ty Hyderabad, received Master’s degree in Power Systems from Jawaharlala

Nehru Technological University Hyderabad. Presently he is working as Head

Of the Department in Electrical and Electronics Engineering, NRI Institute of

Technology, Agiripalli, India. PH- 09494055169

E-mail: nsraohodeee@gmail.com

Injected from PV panel to load through two stages. In the first stage PV array is connected to Cuk converter in order to improve the dc voltage level of PV panel and it is used to set the PV operat- ing point at maximum through MPPT Controller.

Fig1: Block diagram of PV array connected to load

Finally, in the second stage the dc power from the Cuk con- verter is connected to load. This proposed topology is simulated in MATLAB/SIMULINK.

PV cell works under the principle of Photovoltaic Effect. Pho-

tovoltaic Arrays are formed by combinations of series and

parallel connection of PV solar cells. A simple solar cell equiv-

alent circuit model is shown in below figure 2a.

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Fig 2a: Equivalent circuit of PV Cell

I=n p I ph - n p I rs [exp ( ko( v/ns ))-1]

Where I is the PV array output current, V is the PV output voltage, Iph is the cell photocurrent that is proportional to solar irradiation, ns represents the number of PV cells Connected in series and np represents the number of such strings connected in parallel.

I ph = [ I scr + ki (T-T r )]

I scr is the cell short-circuit current at reference temperature and

radiation,

ki is short circuit current temperature coefficient,

T is the Nominal Temperature i.e 295.15K (25C) Tr is the cell reference temperature,

S is solar irradiation in

I rs is the cell reverse saturation current that mainly depends on

the temperature, K is a constant.

I rs = I rr exp

I rr is the reverse saturation at Tr

EG is the band-gap energy of the semiconductor used in the cell.

A is the area of photovoltaic cell.

k is Bolttzmann constant i.e.

1.3806503*10-23jouls/kelvin

q is electron charge i.e. 1.607*10-19c

The performance characteristics of a PV Module depend on its basic materials, manufacturing technology and operating con- ditions. Short circuit point (Isc ), Maximum power point (Impp and Vmpp ) and Open circuit point (Voc ) are the three important points of these curves. The I-V curve is obtained by controlling the load current.. By increasing the load current from zero to its maximum value, the operating point moves from open cir- cuit voltage at zero current to the shot circuit current at zero voltage. The series of all the measured pairs yields the I-V characteristic curve of module.

Fig 2b: PV Array Characteristics V (X axis) Vs I (Y axis)

An increase in solar radiation causes the output current to in- crease and the horizontal part of the curve moves upward.

Fig 2c: Characteristics of PV Array with Variation of Insolation

Fig 2d: Characteristics of PV Array incorporating effect of

Temperature

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An increase in cell temperature causes the voltage to move leftward, while decreasing temperature produces the opposite effect. Thus, the I-V curves display how a photovolta- ic module responds to all possible loads under different solar radiation and cell temperature conditions. The maximum power points the line which is positioned at the knees of the I- V curves, has a nearly constant output voltage at varying solar radiation conditions.

Practically the efficiency of solar cell is very less nearly 15%. Maximum power point tracking technique is used to improve the efficiency of the solar panel. According to Maximum Pow- er Transfer theorem the power output of a circuit is maxi- mumwhen the Thevenin impedance of the circuit (source im- pedance) matches with the load impedance. Hence the prob- lem of tracking the maximum power point reduces to an im- pedance matching problem. So, for tracking that MPP, MPPT algorithm is using.

start

Read V(k) and I(k) from panel

P(k)=V(k)*I(k)

Delay P(k)

&V(k) by k-1 instant

P(k-1),V(k-1)

△P=P(k)-P(k-1)

△V=V(k)-V(k-1)

Yes No

△P>0

Yes No Yes No

△V<0 △V>0

D=D+△D D=D-△D D=D-△D D=D+△D

Fig3a: Operating Point of PVA

In P&O method, the operating voltage is sampled and the al- gorithm changes the operating voltage in the required direc- tion. The iteration is continued until the algorithm finally reaches the MPP. This technique is simple to implement but

the major drawback is the occasional deviation from th maxi- mum operating point in case of rapidly changing atmospheric conditions.

To switch

Fig3b: flow chart of P&O Method

The Incremental Conductance (Inc-Cond) algorithm is based on the fact that the slope of the curve power vs. Voltage (cur- rent) of the PV module is zero at the MPP, positive (negative)* . *on the left of it and negative (positive) on the right. This meth- od tracks rapidly under varying irradiation conditions. It is more accurate than P&O method. It requires complex and costly control circuits.

Fig 3c: IC method on a P-V curve of a solar module

From the P –V curve by comparing the conductance at each sampling time, the MPPT will track the maximum power

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of the PV module. The flow chart of IC method shown in below figure. It makes use of instantaneous and incremental

I S I L

conductance to generate an error signal which is zero at MPP;

however it is not zero at most of the operating points, but it

make the error from the MPPs near to zero.

+

POWER SORCE

DC/DC

+

RESISTIVE

L LOAD

Measure V(k+1),I(k+1) - -

Yes

△I=I[k+1]-I[k]

△V=V[k+1]-V[k]

dV>0 ?

No

dI/dV=

-I[k+1]/V[k+1]

Yes

dI=0 ?

Yes

Fig 4: Converter acting as maximum power point tracker

When proposing an MPPT choosing a converter with high efficiency is the main thing as its output voltage is also one of the factor that helps to note the efficiency of PV system.

Converter in the MPPT system is to maximize the

input of DC voltage. In term of maximized the output voltage by step up or step down the input voltage the cuk converter is

No No

Yes | dI/dV> | No | Yes | |||||

-I[k+1]/V[k+1] No | dV>0 ? | |||||||

D=D+dX | D=D-dX | D=D-dX | D=D+dX |

V[k]=V[k+1] I[k]=I[k+1]

Fig 3d: Flow chart of IC method

The IC method is more efficient and accurate than PO method. Also, it is good and adjusts the module voltage auto- matically without any oscillations to rapidly changing atmos- pheric conditions and esy lo locate the maximum power point. But, the IC method is more complex and costly when com- pared to P&O method. These are the main differences between IC and P&O methods.

The fundamental power conversion unit of Photovoltaic sys- tem is solar cell. The basic operation of a solar cell is character- ized by its I-V curve.Through this experimental measurement precision of electrical parameters of a PV cell, module or array can be made. This measure gives information for the design, installation and maintenance of photovoltaic systems. The experimental measure of the I-V characteristic curve can be performed using DC-DC converters in Continuous Conduc- tion Mode (CCM). The switching power of DC-DC converters are widely used in photovoltaic systems to transform DC power from a voltage level to another, and for tracking the maximum power point (MPPT).

ideally to be choose in the MPPT design compared to the Buck

converter, Boost converter because cuk converter has low switching losses and highest efficiency among non-isolated DC-DC converters. It also can provide a better input and out- put current characteristic due to the inductor on the both input and output stage.

Fig gives the detailed simulink model of PV system and ac- cepts irradiance and temperature as variable parameters and outputs the I-V characteristic and P-V characteristic

Fig 5a: Simulink model of PV module for change in irradiation

The I-V characteristisc of photo voltaic sysytem strongly depends on irradiation nd temperature. The beloe figure shows the simulation results of PV array for different values of irradiation and temperature.

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Fig 5b: PV Array characteristics

Fig 5c: Simulink model of MPPT

First irradiation level is 1000w/m²; at t=0.1s second irradiation level suddenly changes to 600w/m² and then back to 1000w/m² at t=0.3s. The step size of duty cycle is chosen to be 0.02 so the converter can smoothly track the MPP.

Fig 5d: Plot showing the varying duty ratio with time for Per- turb and Observe MPPT algorithm

Fi 5e:. Plot showing the varying duty ratio with time for In- cremental Conductance MPPT algorithm

The above figure shows the change in duty cycle ad- justed by MPPT Controller to extract maximum power from the module for P&O and Incremental Conductance MPPT techniques.

The duty ratio is adjusted by MPPT controller so that PV sys- tem will always work near MPP (Maximum Power Point) and the adjusted ratio is given to the Cuk converter. Initially duty ratio D = 0.6654. Simulation of the modified P&O and Incre- mental Conductance method in combination with Cuk con- verter under stable environmental conditions was done. The MPPT manages to adjust the duty ratio D very quickly such that a stable power output is reached. After this point the out- put power does not change any further since the solar panel operates under constant conditions.

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Fig 5h: Current and Voltage waveforms of Cuk converter for

Incremental Conductance MPPT Controller

Fig 5f: SIMULINK diagram of Cuk converter

The MPPT, however, continues to readjust the duty ratio D and herefore prevents a deviation from the optimum operat- ing point.

Fig 5g: Current and Voltage waveforms of Cuk converter for

Perturb &Observe MPPT Controller

The above two figures represents the output voltage and current waveforms of Cuk converter with modified P&O

and Incremental Conductance MPPT controllers.

Finally we conclude that the system developed is capable of extracting maximum power from the photovoltaic module at the same time providing a regulated DC supply. The proposed PV system and MPPT was simulated. The Incremental Con- ductance method is more efficient compare to all MPPT meth- ods because panel terminal voltage is changed according to its value relative to the MPP voltage and it offers good performance under rapidly changing atmospheric conditions. On the other hand, cuk converter has low switching losses and highest efficiency among all non-isolated DC-DC converters. It also provides a better input and output current characteristic due to the inductor on both input and output stage. Thus cuk converter configuration is a proper choice to be employed in designing the MPPT. MPPT techniques are applied on various PV applications such as space satellite, solar vehicles and solar water pumping etc.

7 **A**CKNOWLEDGMENT

Iam thankful to management of NRI Institute of Technology, Agiripalli and Head of the Department of Electrical and Elec- tronics Engineering Prof .N. Sambasiva Rao, and my guide Mr. R. Raghunadha Sastry with whom I had useful discus- sions. Any suggestions for further improvement of this topic are most welcome.

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