Maximum power point tracking of pv arrays under partial shading condition using sepic converter

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Special Issue: 07 | May-2014, Available @ http://www.ijret.org 398
MAXIMUM POWER POINT TRACKING OF PV ARRAYS UNDER
PARTIAL SHADING CONDITION USING SEPIC CONVERTER
Sreekumar1
A V, Arun Rajendren2
1
M.Tech Student, Department of EEE, Amrita School of Engineering, Kerala, India
2
Assistant Professor, Department of EEE, Amrita School of Engineering, Kerala, India
Abstract
There exists a variety of maximum power point tracking (MPPT) techniques, each having its own merits and demerits. Under partial-
shading conditions, the conventional tracking techniques fail to guarantee successful tracking of the global maximum power, ie the
conventional MPPT methods such as perturb and observe and incremental conductance may converge on local maximum power point
resulting in significant reduction of power generated. This paper discusses about an improved technique for tracking global maximum
power point of photovoltaic arrays that has better performance under partial shading conditions. The first stage in this method is to
find out global maximum power point among the local maxima. Once the global maximum power point is found then by adjusting the
duty ratio, the voltage corresponding to maximum power can be found out. The control is then transferred to perturb and observe
algorithm stage. This technique could be applied for both stand alone and grid connected PV system. A comparison study between a
SEPIC converter and a buck boost converter with the above mentioned algorithm has also been carried out in order to verify the
performance of both the converters. The above mentioned converters have been designed for 150W at a switching frequency of 10
KHz. Modified algorithm has been simulated using MATLAB/Simulink and results are obtained. Partial shading condition was
modelled in MATLAB/Simscape and analysed the solar array characteristics under various shading conditions. From the simulation
results it was found that the SEPIC converter is much more efficient and is highly suitable for photo voltaic applications.
Key Words: Partial shading, DC-DC converters, Global maximum power point, SEPIC converter.
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1. INTRODUCTION
The demand for energy is increasing day by day with the
growth of world population. However, the natural energy
resources don’t grow with time instead gets depleted due to
over usage. This has lead to a hike in energy cost and an
increase in the emission of greenhouse gases. Solar energy has
been identified as the most abundant resource of future energy
and is becoming a strong competent to fossil fuel. Recent
advancements in photovoltaic (PV) technology made access to
solar energy more economical [1] than in the past. The future
of Indian energy sector is expected to be dominated by solar
energy.
Photovoltaic cells when connected together form a panel and a
number of panels contribute to form a solar array. The PV
array consists of a number of panels connected in series and
parallel topologies. With varying levels of irradiation during
the day, the array output can vary in a wide range. This effect
is expected. But unexpected shading effects due to dusts,
clouds, leafs, branches of trees and buildings causing shading
on cells or part of modules or panels. Under these partial
shading conditions, the Power versus voltage characteristics of
the solar array will contain one global maximum along with
many local maxima. The global maxima correspond to
maximum power while the others correspond to much lower
powers [2]. Around 30% power loss will take place even
though only one cell in the PV module is shaded. As the
number of shaded cells increases, the amount of power loss
also will be increases (nearly 80%). Under partial shading
conditions, conventional MPPT methods may not be able to
track maximum power irrespective of the change in irradiance
conditions. At the local maximum power point it may
converge resulting in reduction of PV panel output. Due to this
reason the overall PV system efficiency gets degraded [3]-[4].
An efficient MPPT system which can be used even under
partial shading conditions efficiently is discussed in this paper.
Organization of this paper is as follows: a description about
the MPPT system is given in Section II, SEPIC converter
characteristics is dealt in section III, section IV presents the
implementation of partial shading effect, Global MPPT is
described in section V and simulation results are presented in
section VI.
2. SYSTEM DESCRIPTION
Basic block diagram of photovoltaic maximum power point
tracking system implemented using SEPIC converter is shown
in Fig.1. System load is supplied from solar panel with the
help of SEPIC converter. The array voltage VPV and current
IPV is sensed by suitable sensors and is given as input to the
MPPT controller. The basic idea of MPPT technique is to
adjust the load impedance of the panel there by forcing the

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panel to operate at the maximum power point of the P-V
curve. Load impedance is adjusted by changing the duty ratio
of the converter which is connected as an interface between
the panel and the load.
In this work, SEPIC serves as the dc-dc converter and its duty
ratio is adjusted to track maximum power transfer from PV
array to the load.
Fig.-1: General block diagram of an MPPT system
Under partial shading conditions multiple local maxima will
appear on the power-voltage characteristics of solar PV
system in that only one will be global maximum power point.
The above mentioned situation is shown in the power-voltage
curve of partially shaded array in Fig.2. The conventional
MPPT methods converge at local maximum power points and
the efficiency of the solar PV system reduces considerably.
Fig-2: Power vs Voltage for a partially shaded PV array
The above mentioned MPPT system can be applied to either
grid connected solar PV applications or standalone
applications. Here Single Ended Primary Inductor (SEPIC)
converter is used as DC-DC power converter so that the output
voltage can be made constant irrespective of the input voltage
variations.
The global MPPT technique used here is performed in three
consecutive steps,
a) Constant input power mode
b) PV array voltage regulation
c) Perturb and observe (P&O) stage
The switch is controlled by either PWM1 or PWM2 control
signals provided from the MPPT controller. This aspect is
explained in the following section.
3. SEPIC CONVERTER
Since the SEPIC converter is a modified form of an ordinary
buck-boost converter, it has some advantages [5]-[6] over
conventional buck boost converters. The circuit diagram of
single-ended primary inductance converter as shown in Fig.3
C2 LOAD
L1
L2
C1
Vs
Fig-3: Circuit diagram of SEPIC-converter
The output of a SEPIC converter is non-inverting but the
output of a buck boost converter is inverting. In this SEPIC
converter, MPP tracking is much easier because the amount of
input current ripple present is non-pulsating. Since the switch
present in SEPIC converter is directly connected to ground
only low side driving is required which is easier than high side
driving used in buck-boost converter. Additionally, the SEPIC
converter is highly efficient compared to buck-boost
converter.
The voltage gain of a SEPIC converter is given by,
1
o pv
D
V V
D
 

(1)
Where
on
s
t
D
T
 is the duty ratio of the converter switch, Vo is
the output voltage of the converter and Vpv is the input voltage
which is fed from the solar array. The control signal of PWM1
is produced by comparing the panel current Ipv with a control

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signal, which is a sawtooth wave derived from the basic
equations of SEPIC converter.
For stand-alone applications, the output voltage Vo has to be
constant even if it is connected to the grid or to the battery
bank. For stand-alone battery charging application [7]-[8], the
SEPIC converter acts in buck mode for reducing the panel
voltage to constant value. But for grid interface, the SEPIC
converter has to work in the boost mode. The input power of
the converter can be controlled by varying the amplitude of
the control signal and it is called constant input power mode
operation.
4. IMPLEMENTATION OF PARTIAL SHADING
EFFECT
A Solar panel has been simulated in MATLAB/Simscape and
is discussed in this section. Twenty solar cells are connected in
series to form a string which acts as a module. Each cell
having an open circuit voltage Voc= 0.9 V and short circuit
current Isc = 0.63 A. The combination of such modules forms a
solar panel. A number of panels interconnected forms solar
array, of maximum power 39.2 W at Vm = 65.6 V and Im=0.59
A. In order to create partial shading effect using Simulink, a
diode is connected in antiparallel to each cell. This is done to
avoid avalanche breakdown which would create hot spots on
the solar cell which may lead to the damage of entire solar
cell.
At a standard irradiance condition of 1000W/m2
the solar cell
produces rated power which can be observed from volt-current
and volt-power characteristics. Fig.4 shows the solar cell
modeling for partial shading conditions.
Fig-4: Modeling of solar cell for partial shading condition
The irradiance value can be changed manually and the
corresponding variation in irradiance can be seen. As the
irradiance value decreases from 1000W/m2
the panel voltage
and current also reduces considerably which further results in
the reduction of panel power output. In order to overcome this
condition, modified maximum power point tracking method as
explained in this paper can be used to track available
maximum power from the panel. Fig.5 shows the Simulink
model of the solar panel under partial shading conditions.
Fig-5: Simscape model of solar panel
Fig.6 shows the Power - Voltage characteristics under normal
irradiation conditions (1000W/ m2
) without partial shading.
The magnitude of voltage (VM) corresponding to maximum
power (PM) is also shown.
Fig-6: Power-voltage curve under 1000 W/m2
The Current-Voltage characteristics of the designed panel are
shown in Fig.7 at 1000W/m2
. Current and voltage magnitudes
corresponding to maximum power is represented as IM and
VM respectively.

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Fig- 7: Current-voltage curve under 1000 W/m2
The Power-Voltage characteristics of the designed solar panel
under Partial shading condition created using Simulink is
shown in Fig.8.
Fig-8: Power-voltage curve under partial shading condition
Positions of global and local maximum power points are
marked in the figure. Similarly current voltage characteristics
of panel under partially shaded condition are shown in Fig.9
with global and local maximum power points marked in it.
Fig-9: Current-voltage curve under partial shading condition
It can be observed that under partial shading condition,
multiple maximum power points has been created. Out of
these maximum power points only one will be global
maximum power point and the remaining power points will be
local maximum power points.
5. GLOBAL MAXIMUM POWER POINT
TRACKING
The maximum power point tracking method under partial
shading condition [9] is explained in this section.Fig.10 shows
the flowchart of the global maximum power point tracking
algorithm. The entire logic can be divided in to two parts
a) Global search algorithm
b) Local search algorithm
Initially the global tracking process is performed and the
voltage magnitude corresponding to each maximum power
point is identified. There will be one global maximum power
point among the local maximum power points. The value of
voltage magnitude corresponding to maximum power is stored
in microcontroller memory. The control algorithm adjusts the
duty cycle to adjust the converter input impedance to make it
operate at global maximum power point. Once the global
maximum power point has been reached then the control is
moved from global loop and enters to local loop.
The local loop contains some ordinary MPP tracking method
such as Perturb & Observe, Incremental Conductance etc. The
SEPIC converter is switched by using either PWM1 or PWM2.
PWM1 signal is generated with the help of global tracking
loop and PWM2 signal is generated using local tracking loop.

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Initialization of
program variables
Pold = 0
Dnew = 0.1
Measure Vnew , Inew
Vmax = Vold
Calculate Power
Pnew = Vnew * Inew
Pnew < Pold
Adjust the ampletude
of control signal till
GMPP reached
Dnew = D
old -dD
Measure Vnew
Vnew< Vmax
Timer
interrupt
NO
YES
YES
Global MPPT
Voltage regulation
NO
NO
YES
Local MPP
{P & O / IC}
Fig-10: Flowchart of the GMPPT algorithm
Perturb and observe is the most common MPPT method. It is
simple and easy to develop; only a few parameters are
required for calculation. Once the global maximum power
point has been found out then the control is locate to P & O
loop and the tracking is done around global maximum power
point.
According to Perturb and Observe algorithm [10] due to a
perturbation in output voltage by a small increment, if the
resulting changes in power ΔP is positive, then we can move
in the direction of MPP and we keep on perturbing in the same
direction. If the value ΔP is negative, we are going away from
the direction of MPP and the sign of perturbation supplied has
to be changed. The P&O algorithm operates by periodically
perturbing the operating voltage and comparing it with the
previous instant. If the power difference ΔP and the voltage
difference ΔV, both in the positive direction then there is an
increase in the array voltage. If either the voltage difference or
the power difference is in the negative direction then there is a
decrease in the array voltage. If both the voltage and power
difference are in the negative direction then there is a increase
in the array voltage.
START
Measure V(K), I(K)
P(K)=V(K)*I(K)
dP>0
Decrease array
voltage
Increase array
voltage
NO YES
NO
Update history
Decrease array
voltage
Increase array
voltage
V(K)-V(K-1)
>0
YES
NO
V(K)-V(K-1)
<0
Fig-11: Flowchart of the P & O algorithm
6. SIMULATION RESULTS
The SEPIC converter has been selected for this MPPT
application, after comparing its performance with buck boost
converter. Simulation was performed using
MATLAB/SIMULINK. To understand the difference, MPPT
was performed with the help of SEPIC and Buck-Boost
converter. Both the converters has been designed for 150 W
and 20 % inductor current ripple. Global MPPT technique
was employed here. Simulation diagrams of SEPIC and Buck-
Boost converter supplied from solar panel is shown in Fig.12
and Fig.13 respectively
Fig-12: Simulink model of the MPPT controller using SEPIC

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Fig-13: Simulink model of the MPPT controller using Buck-
Boost converter
Design parameters and ratings of the SEPIC and Buck-Boost
converter is as per table 1.
Table I: Converter Parameters
Parameter Value
SEPIC Buck-boost
L1 1107μH 864μH
C1 518μF 69μF
L2 1107μH
C2 518μF
Power 150 W 150 W
Vs 30 to 40 V 30 to 40 V
Vo 36 V 36 V
fs 10 KHZ 10 KHZ
From the simulation results first difference which was
observed is in the output voltage of the converters. Buck-
Boost Converter produces an inverted output voltage while the
SEPIC has non inverted output. The output voltage waveforms
of both converters are shown in Fig.14.
Fig-14:Output voltages of SEPIC and Buck-Boost converter
Similarly the converters were also compared for efficiency by
measuring their output power . For 1000 W/m2
irradiance and
25 o
C ambient temperature, the output power obtained is
138.5W in the case of a SEPIC converter as shown in Fig.15.
Similarly for a buck boost converter, the output power is only
133W. Thus from Fig.15 it is clear that maximum power
tracked using SEPIC converter is greater than buck-boost
converter. Hence the efficiency of SEPIC converter is higher
than buck-boost converter.
Fig-15:Output power of SEPIC and Buck-Boost converter
In Fig.16, different irradiance condition and the corresponding
power using MPPT technique is shown. Different irradiance
values have also been shown in Fig.16.
Fig-16: Maximum panel power and tracked maximum power
under different irradiance conditions at 298 K
Initially, the temperature value is given as 298 K. The energy
production efficiency of solar panel is very much affected due
to temperature rise. As the temperature of the panel increases
the open circuit voltage will be reduced but the short circuit
current will increase slightly, so that the net power produced
will reduce which in turn reduces the overall efficiency.

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The maximum power from the panel and the output power of
the SEPIC converter under different irradiance conditions and
at a temperature of 350K is shown in Fig.17. From these
graphs it can be seen that under varying irradiance condition
the maximum power obtained from the panel is also varying.
The voltage which gives maximum power point also varies in
accordance with change in irradiance. The performance of
SEPIC converter is better compared to buck-boost converter
and is most suited for photo voltaic applications.
Fig-17: Maximum panel power and tracked maximum power
under different irradiance conditions at 338 K
7. CONCLUSIONS
Under partial shading conditions and rapidly varying
irradiance conditions conventional MPPT methods fail to track
real maximum power point. To overcome this situation, an
improvement in conventional MPP tracking algorithm is done
and integrated with SEPIC converter. The above mentioned
MPPT system is able to track the real maximum power point
under constant and varying weather conditions. The
comparative study with buck-boost converter shows the added
advantage of SEPIC converter for photo voltaic applications.
REFERENCES
[1] “Trends in photovoltaic applications. Survey report of
selected IEA countries between 1992 and 2006,” Int.
Energy Agency Photovoltaic Power Syst.,Paris, France,
Tech. Rep. IEA-PVPS T1-16:2007, 2007
[2] H. Patel and V. Agarwal, “Maximum power point
tracking scheme for PV systems operating under
partially shaded conditions,” IEEE Trans.
Ind.Electron., vol. 55, no. 4, pp. 1689–1698, Apr. 2008.
[3] E.V. Paraskevadaki and S. A. Papathanassiou,
“Evaluation of MPP voltage and power of mc-Si PV
modules in partial shading conditions,” IEEETrans.
Energy Convers. vol. 26, no. 3, pp. 923–932, Sep.
2011.
[4] Y. H. Ji ,Suwon, D.Y. Jung , C. Yue Won , B. K Lee
“Maximum Power Point Tracking Method for PV
Array under Partially Shaded Condition”, IEEE
International Conferenceon, Sept. 2009 , Page(s):307 –
312
[5] D. W. Hart “Power Electronics” 1ST ed., New York:
2011.
[6] J. Betten, “Benefits of a coupled-inductor SEPIC
converter”, Power Management.
[7] S. J. Chiang, H.J. Shieh, and M.C. Chen, “Modelling
and Control of PV Charger System with SEPIC
Converter”, IEEE Trans. Ind. Electron, vol.56, no. 11,
pp.4344-4353, Nov. 2009.
[8] N. Mohan, T. Undeland, and W. Robbins, Power
Electronics: Converters, Applications and Design,
2nd ed., New York:Wiley, 11995, pp. 164–172.
[9] E. Koutroulis, ”A New Technique for Tracking the
Global Maximum Power Point of PV Arrays
Operating Under Partial-Shading Conditions”, IEEE
journal of Photovolt, vol. 2, no. 2, april 2012.
[10] S. Alsadi, B. Alsayid, “Maximum Power Point
Tracking Simulation for Photovoltaic Systems Using
Perturb and Observe Algorithm” International Journal
of Engineering and Innovative Technology (IJEIT)
vol. 2, Issue 6, Dec. 2012.
BIOGRAPHIES
Sreekumar A.V. received his B.Tech degree in
Electrical and Electronics Engineering from
University of Calicut, Kerala. He is currently
pursuing his M.Tech at Amrita School of
Engineering, Amritapuri, Kollam, Kerala. His
areas of interest are Power Electronics, Power System and
renewable energy.
Arun Rajendran received his B.Tech degree in
Electrical and Electronics Engineering from
College of Engineering Cusat, Kerala and
M.Tech degree in Power-System from College
of Engineering, Trivandrum. Currently he is
working as Assistant Professor in Department of Electrical
and Electronics Engineering Amrita School of Engineering,
Amritapuri, Kollam, Kerala. His areas of interest are Power
Electronics and Power System stability.

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Maximum power point tracking of pv arrays under partial shading condition using sepic converter