 Open Access
 Total Downloads : 309
 Authors : A. Vinothkumar, S. Selvakumar, M. Vigneshkumar
 Paper ID : IJERTV3IS20857
 Volume & Issue : Volume 03, Issue 02 (February 2014)
 Published (First Online): 27022014
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Modelling and Analysis of Photo Voltaic Cell Fed Seven Level MultiString Inverter
A. Vinothkumar1, S. Selvakumar 2, M. Vigneshkumar3
1. Assistant Professor, Department of Electrical and Electronics Engineering, P.A college of Engineering and Technology, Pollachi.
2. M.E (Power Electronics and Drives) student, P.A college of Engineering and Technology, Pollachi.
3. Assistant Professor, Department of Electrical and Electronics Engineering, P.A college of Engineering and Technology, Pollachi.
Abstract –There is strong trend in the photovoltaic (PV) inverter technology to use transformer less topologies in order to acquire higher efficiencies. This paper presents a singlephase seven level grid connected PV inverter with two reference signals that were identical to each other with an offset that was equivalent to the amplitude of the triangular carrier signal were used to generate PWM signals for the switches. Multistring inverter based system gives better voltage regulation and efficiency compare to the multilevel inverters. Seven level multistring inverter consists of two auxiliary switches and diodes. The inverter produces output voltage in seven levels Vdc, Vdc/3, 2Vdc/3, 0,Vdc/3,2Vdc/3,Vdc. The validity of the propose inverter is verified through simulation.
Keyword – Pulse Width modulation (PWM), Photo Voltaic (PV) Source, Maximum Power Point (MPP).

INTRODUCTION
PV inverter, which is the heart of a PV system, is used to convert dc power obtained from PV modules into ac power to be fed into the grid. Improving the output waveform of the Inverter reduces its respective harmonic content and, hence, size of the filter used and the level of the Electromagnetic Interference (EMI) generated by switching operation of the inverter. In recent years, multilevel inverters have become more attractive for researchers and manufacturers due to their advantages over conventional three level PWM Inverters. They offer improved output waveforms, smaller filter size and lower EMI, lower Total Harmonic Distortion (THD).

CIRCUIT DESCRIPTION OF THE PROPOSED SYSTEM
The proposed singlephase sevenlevel multistring inverter circuit is shown in Fig 1. It consists of three strings of DC – DC step up converter connected to common dc bus, an auxiliary circuit, and conventional fullbridge inverter configuration. Input sources, PV strings are connected to the inverter via dcdc boost converters. The dcdc boost converters are used to track the Maximum power point
tracking (MPPT) independently and to step up inverter output voltage. The multistring approach is adopted because each dcdc converter can independently perform MPP tracking (MPPT) for its PV strings. This will compensate for mismatches in panels of like manufacture, which can be up to 2.5%. It further offers the advantage of allowing panels to be given different orientations and so open up new possibilities in architectural applications. Another advantage of multistring conguration is that the mixing of different sources becomes possible, i.e., existing PV panel strings could be extended by adding new higher output panels without compromising the overall string reliability or performance [1]. Depending on the design, each converter module may be able to isolate its connected power source so that the wiring of series or parallel connection of these strings can be performed safely. The powersource converter connection is a safe lowvoltage connection. The dcdc boost converters are connected in parallel to avoid high dcbus voltage, which will eventually increase the size of the capacitors and the inverters cost. Therefore, only three capacitors with equal capacitance rating are used as the dc bus, and the other dcdc boost converters are connected to this dc bus, as shown in Fig.1. A ltering inductance Lf is used to lter the current injected into the grid. The injected current must be sinusoidal with low harmonic distortion. In order to generate sinusoidal current, a sinusoidal PWM is used because it is one of the most effective methods.
A sinusoidal PWM is obtained by comparing a high frequency carrier signal with a lowfrequency sinusoidal signal, which is the modulating or reference signal.
The carrier has a constant period; therefore, the switches have constant switching frequency. The switching instant is determined from the crossing of the carrier and the modulating signal.
The table 2.1 gives the switching operation of the Seven Level inverter. The voltage levels in the truth table are Vdc, Vdc/3, 2Vdc/3, 0, Vdc/3, 2Vdc/3, Vdc. The switching operation of the above voltage levels for the switches will be on (1) and off (0) according to the input need to be given. In Pulse width modulation technique the input can be given as in
the table 1. The on and off can be done according the output required.
Fig.2. Switching pattern for the single phase seven level inverter

PWM MODULATION
Modulation index given as,
M a for a sevenlevel PWM inverter is
Fig.1. Circuit diagram of Multistring inverter
Where,
M a
Am
2 Ac
(1)
Ac is the peaktopeak value of carrier
Voltage Levels
S1
S2
S3
S4
S5
S6
Vdc
0
0
1
0
0
1
Vdc/3
0
1
0
0
0
1
2Vdc/3
1
0
0
0
0
1
0
0
0
1
1
0
0
Vdc/3
0
0
0
1
1
0
2Vdc/3
0
1
0
0
1
0
Vdc
1
0
0
0
1
0
Am is the peak value of voltage reference Vref .
As two reference signals are identical to each other, (1)
can be expressed in terms of the amplitude of carrier signal
Vc by replacing
Ac withVc , and
Am Vref 1 Vref 2 Vref
then,
M Vref
2Vc
(2)
If M 1, higher harmonics in the phase waveform is obtained. Therefore, M is maintained between zero and one. Here two reference signals Vref 1 and Vref 2 are compared with
the carrier signal at a time. If Vref
exceeds the peak amplitude
Table 2.1 Truth table for the 7level multistring inverter
of carrier signal Vcarrier , then Vref 2 will be compared with the carrier signal until it reaches zero. At this point onward, Vref 1
takes over the comparison process until it exceeds Vcarrier . This will lead to a switching pattern, as shown in Fig 2. Switches S4S9 will be switching at the rate of the carrier signal frequency, while S7 and S8 will operate at a frequency that is equivalent to the fundamental frequency. Table 1 illustrates the level of Vinv during S4S9 switch on and off.
Section 4 illustrating the Simulink model of the PV cell is given in the appendix.

CHARACTERISTICS OF PV CELLS

Characteristics of PV cell at constant temperature
degr cent
ee igrade
10
00 W/m2
80
60
40
0 W/m2
0 W/m2
0 W/m2
20
0 W/
m2
160
From the above characteristics (Fig.4, Fig.5, Fig.6, Fig.7) curves the power generation continuously varies along with two main factors, which are known as cell temperature and irradiance. In this work MPPT technique is used for finding
140
120
OUTPUT POWER
100
80
60
40
20
0
Tc=25
0 10 20 30 40 50 60 70 80
OUTPUT VOLTAGE
the maximum output at various instant of time.


MAXIMUM POWER POINT TRACKING CONTROL (MPPT)
In order to operate the PV cells at the maximum power point, several techniques has been suggested in the literature
.Some of them are,

Look up table Method
Fig.4. Power and voltage waveform at constant temperature for PV cells
100
0 W/m2
T
C=25 DEG
CENTIGRA
80
0 W/m2
60
0 W/m2
400
W/m2
200
W/m2
3
DE
2.5
OUTPUT CURRENT
2
1.5
1
0.5
0
0 10 20 30 40 50 60 70 80
OUTPUT VOLTAGE
Fig.5. Current and voltage waveform at constant temperature for PV cells

Characteristics of PV cell at constant irradiance
I
RRADIAN
CE = 1000
W/m2
25 DE
G. CENTI
0
DEG CEN
TIGRADE
50 DEG
CENTIGR
75 DEG
CENTIGR
180
160

Perturbation and Observation methods

Model based computation methods

Artificial intelligence techniques
Model based Computation methods are of two types they are,

Voltage based MPPT (VMPPT)

Current Based MPPT (CMPPT)

In this work focus is made on Current Based MPPT (CMPPT).The CMPPT method simplifies the entire control structure of the power conditioning system and uses an inherent current source characteristic of solar cell arrays. Therefore it exhibits robust and fast response under rapidly changing environmental conditions.
A. Current Based Maximum Power Point Tracking Control
140
GRADE
The main idea behind current based MPPT is that the
POWER OUTPUT
120
100
ADE
current at the maximum point
Imp has a strong relationship
80 with the short circuit current Isc . Isc can either be measured
ADE
60
40
on line under different operating conditions or can be computed from a validated model.
20
0
0 10 20 30 40 50 60 70 80 90
OUTPUT VOLTAGE
3
2.5
Model Response Curve Fitting
Fig.6. Power and voltage waveform at constant irradiance for PV cell
y = 0.948*x
Current at Maximum Power (Imp)
2
0 c
entigrade
25
centigra
de
10
00 W/m2
50 ce
ntigrade
75 centi
grade
3
2.5
OUTPUT CURRENT
2
1.5
1
0.5
1.5
1
0.5
0
0.5 1 1.5 2 2.5 3
SHORT CIRCUIT CURRENT(Isc)
Fig.8. Current based control
Fig.8 represents the current based control scheme which
0
0 10 20 30 40 50 60 70 80 90
OUTPUT VOLTAGE
Fig.7.Current and voltage waveform at constant irradiance for PV cells
gives the linear relationship between short circuit current and current at maximum power Imp using curve fitting.
Isc


SIMULINK MODEL OF SEVEN LEVEL INVERTER
The feasibility of the proposed approach is verified using computer simulations. A model of the sevenlevel inverter is constructed in MATLABSimulink software. A new strategy with reduced number of switches is employed. Cascaded H bridge 7 level inverter requires 12 switches to get

RESULT AND DISCUSSION
A. Switching Pattern for the Single Phase sevenlevel inverter
seven level output voltage and with the proposed topology requirement is reduced to 6 switches. The new topology has the advantage of its reduced number of switching devices (switches) compared to conventional cascaded Hbridge multilevel inverter, and can be extended to any number of levels. The simulink model of seven level multistring inverter is represented in the Fig.9
A. Simulation diagram of multistring inverter
Carrier With Two References
Voltage in Volts
2
0
2
0 0.005 0.01 0.015 0.02
Control Signal to Switch 2
Voltage in Volts
2
0
2
0 0.005 0.01 0.015 0.02
Control Signal to Switch 4
Voltage in Volts
2
0
2
0 0.005 0.01 0.015 0.02
Control Signal to Switch 1
Voltage in Volts
2
0
2
0 0.005 0.01 0.015 0.02
Control Signal to Switch 3
Voltage in Volts
2
0
2
0 0.005 0.01 0.015 0.02
Control Signal to Switch 5
Voltage in Volts
2
0
2
0 0.005 0.01 0.015 0.02
Fig.10. Switching pattern for seven level inverter
Fig.10 represents the switching pattern for seven level inverter. Here two reference signals Vref 1 and Vref 2 will take turns to be compared with the carrier signal at a time. If Vref exceeds the peak amplitude of carrier signalVcarrier , then Vref 2 will be compared with the carrier signal until it reaches
zero. At this point onward, Vref 1
takes over the comparison
Fig.9. Simulink model of Seven level multi string inverter
The above simulink model in Fig.9 could recreate the characteristic curves shown in Fig.6 and Fig.7 of section V when correctly inserted into the progam.
The life time of the PV panel depends on the environmental conditions at which the panel is installed. Aging effect is unavoidable but it can be minimized using antiaging agents like ethylene vinyl acetate. This serve to improve the life time of PV panel to certain extent. The exact life time of the PV panel is unpredictable as it depends upon the field conditions and quality of manufacturing.
process until it exceedsVcarrier . This will lead to a switching pattern, as shown in Fig.10. Switches S4S9 will be switching at the rate of the carrier signal frequency, while S7 and S8 will operate at a frequency that is equivalent to the fundamental frequency.
Fig.11 Simulation results for seven level inverter output voltages
(M 0.5)

CONCLUSION
This paper has presented a singlephase multistring sevenlevel inverter for PV application. A novel PWM control scheme with two reference signals and a carrier signal has been used to generate the PWM switching signals. The circuit topology, control algorithm, and operating principle of the proposed inverter have been analyzed in detail. The configuration is suitable for PV application as the PV strings operate independently and later expansion is possible.

SCOPE FOR THE FUTURE WORK
The proposed model is simulated using MATLAB.The simulation results indicate the functioning of the proposed model. The future work is to implement the proposed model in hardware. The proposed model can be improved by increasing the level of the inverter output.
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APPENDIX SIMULINK MODEL OF PV CELL
Fig.3 Line diagram of PV cell

Multistring fivelevel inverter with novel PWM control scheme for PV
Application, Nasrudin A.Rahim, IEEE, and JeyrajSelvaraj, IEEE transactions on industrial electronics,vol 57,n0.6,June 2010.
Where,
I IL

ID
(3)

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S. Vazquez, J. I. Leon, L. G.Franquelo, J. J. Padilla, and J. M. Carrasco,
DC voltageratio control strategy for multilevel cascaded converters
I = Output Current in Amps
I L = Photo Generated Current in Amps
I D = Diode Current in amps
By Shockley equation, current diverted through diode is,
U IR
(4)
C pv = overall heat capacity of PV cell/Module
I D Io exp s 1
nkT / q kin, pv = transmittance absorption product of PV cells
Where,
Io = Reverse Saturation Current
Kloss
Ta
= overall heat loss coefficient
= ambient temperature
n = Diode Ideality Factor k = Boltzmanns Constant T = Absolute Temperature
q = Elementary Charge
For silicon of 250C nkT / q =0.0259 volts=,
A = effective area of PV cell/Module
I I
U IRs
(5)
D o exp 1
Substituting above equation in equation (3)we get,
I I

I exp U IRs
L o
1 (6)
where nkT / q
completion factor.
is known as Thermal voltage timing
I
Photo generated current IL is calculated by
I
L
L,ref

I ,SC T
Tc,ref
(7)
c
Where, ref
= Irradiance (W / m2 )
ref
= reference irradiance (1000W / m2 )
IL,ref = Light current at reference condition
Tc = PV cell temperature
Tc,ref = Reference temperature
I ,SC = Temperature coefficient of the short circuit current (A/ C)
Saturation Current Io is given by
T 273 3 e N T 273 (8)
o o,ref
I I c,ref exp gap s 1 c,ref
Where,
Tc 273
qref
Tc 273
Io,ref = saturation current at the reference condition (A)
egap = band gap of the material (1.17eV for Si)
Ns = number of cells in series of the PV module
q = charge of the electron
ref = value of at the reference Condition
Thermal Model of Photovoltaic Cell is
C dTc k
U x I K
T T
(9)
pv dt
Where,
in, pv A
loss c a