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 Authors : Mr. G. Venkateswarlu, Dr.Psangameswar Raju, P.Giriprasad Singh
 Paper ID : IJERTV1IS7217
 Volume & Issue : Volume 01, Issue 07 (September 2012)
 Published (First Online): 25092012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
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Analasis Of Photovoltaic Cell An Application Of A Level Shifted Cascaded Multilevel Inverter
Analasis Of Photovoltaic Cell An Application Of A Level Shifted Cascaded Multilevel Inverter
*Prof in dept of EEE, Narayana Engg College, Nellore gaddam.
**Professor, S.V.Univesity, Tirupati.
***M.Tech Student Scholar Narayana and Engg. , Nellore A.P, India
Abstract The past few years have seen many milestones in the development of streamlined, standardized requirements for utility interconnection of smallscale renewable generating facilities, particularly solar photovoltaic (PV) systems. The PV array generates DC power with a variable voltage and current. It is possible to supply DC loads directly from the PV array. Direct coupling of PV array and load is typically applied for applications exhibit intrinsic storage capabilities like water pumping or cooling systems. If AC loads are to be supplied, an inverter is required, transforming the DC power from the PV array to AC power at prescribed voltage and frequency. Battery is needed as a storage device that supplies to the load incase of indirect supply of power from the PV array. This paper focuses on the grid connected PV system. These systems are usually connected to the low voltage distribution grid. Today, grid connected PV systems, they always supply the maximum available power while voltage and frequency are determined by the mains. They are cheaper than the standalone systems and are easy to operate, as they require no storage equipment.
KeywordsAmplitude modulation (AM), dcac power conversion, insulatedgate bipolar transistor (IGBT), power electronics, pulsewidth modulation, voltagesource converter (VSC)

Introduction
The energy sources we have become accustomed tothe fossil fuels and nuclear fission have ceased to be the easy answers to our evergrowing need for
electric power. Burning oil, coal and natural gas pumps nitrogen oxide, sulfur dioxide, and mercury and
other toxic metals into our atmosphere, directly cause environmental pollution. Nuclear fission produces radioactive waste, material that will remain deadly for thousands of years, for which we have yet to discover a safe method of storage. As we learn more about the interconnectedness of all the ecosystems that permit and sustain life on our planet, the poisonous
results of the various pollutants created by the use of these fuels are becoming increasingly harder to justify.
In a gridconnected PV system, PV modules, wired together to form a PV array, pass DC electricity through an inverter to convert it into AC power. If the PV system AC power is greater than the owner's needs, the inverter sends the surplus to the utility grid for use by others. The utility provides AC power to the owner at night and during times when the owner's requirements exceed the capability of the PV system.Using gridconnected PV power can have economic as well as environmental advantages. Where utility power is available, consumers can use a grid connected PV system to supply some of the power they need and use utilitygenerated power at night and on very cloudy days. When the PV system supplies power to the grid as well as to a specific building or piece of equipment, the utility becomes a kind of storage device or battery for PVgenerated power.
Distributed power generation systems (DPGS) are widely exploited according to the development of renewable energy systems [1,2]. DPGSs cover wide power ranges from 1kW class residential applications to several hundred MW class generation parks. Medium and small scale DPGSs are normally connected to grid systems through utility interactive inverters that inject grid current by current control mode operation
II MODEL FOR PV CELL
The building block of the PV array is the solar cell, which is
basically a pn semiconductor junction that directly converts light energy into electricity. The equivalent circuit is shown in Fig. 1
Fig. 1 Equivalent circuit for a PV cell.
To simulate a PV array, a PV simulation model which was used based on the following equation:
Where IPV is the PV array output current (A); VPV is the PV array output voltage (V); ns is the number of cells connected in series; np is the number of strings connected in parallel; q is the charge of an electron; k is Boltzmanns constant; A is the pn junction ideality factor; T is the cell temperature (K); and Irs is the cell reverse saturation current. The factor A in Eq. (1) determines the cell deviation from the ideal pn junction characteristics. The ideal value ranges between 1 and 5 and in our case, A equals 2.15. The cell reverse saturation current Irs varies with temperature and the photocurrent Iph depends on the solar radiation and the cell temperature as shown in the following equation:
III . Cascaded HBridge Multilevel Inverter Full HBridge
Fig.2 shows the Full HBridge Configuration.
By using single HBridge we can get 3 voltage levels. The number output voltage levels of cascaded Full H Bridge are given by 2n+1 and voltage step of each level is given by Vdc/n. Where n is number of H bridges connected in cascaded. The switching table is given in Table 1 and 2.
Switches Turn ON
Voltage Level
S1,S2
Vdc
S3,S4
Vdc
S4,D2
0
Table 1. Switching table for HBridge
Switches Turn On
Voltage Level
S1, S2
Vdc
S1,S2,S5,S6
2Vdc
S4,D2,S8,D6
0
S3,S4
Vdc
S3,S4,S7,S8
2Vdc
where Iscr is the cell shortcircuit current at reference
temperature and radiation, ki is the shortcircuit current temperature coefficient, and s is the solar radiation in mW/cm2.
Table 2. Switching table for Cascaded HBridge
which remains the same as that for the phaseshifted modulation scheme. For PID modulation, the multilevel converter with multilevel requires (m1) triangular carriers with same amplitude and frequency. The frequency modulation index mf which can be expressed as:
mf = fcr/ fm
where fm is modulating frequency and fcr are carrier waves frequency. The amplitude modulation
index ma is defined by
ma = Vm / Vcr (m1) for 0 ma 1
Figure. 2 Full HBridge

LEVEL SHIFTED PWM METHOD
Fig.3 Level shifted carrier and reference
The Level shifted carrier pulse width modulation. An mlevel Cascaded Hbridge inverter using level shifted modulation requires (m1) triangular carriers, all having the same frequency and amplitude. The frequency modulation index is given by
mf = fcr/ fm,
Where Vm is the peak value of the modulating wave and Vcr is the peak value of the each carrier wave [1].
The amplitude modulation index, ma is 1 and the frequency modulation index, mf is 6. The triggering circuit is designed based on the three phase sinusoidal modulation waves, Va, Vb, and Vc Three of the sine wave sources have been obtained with same amplitude and frequency but displaced 120Â° out of the phase with each others. For carriers wave sources block parameters, the time values of each carrier waves are set to [0 1/600 1/300] while the outputs values are set according to the disposition of carrier waves. After comparing, the output sinals of comparator are transmitted to the IGBTs.

SIMULATION RESULTS

5.1 Modeling of Cascaded HBridge Multilevel Converter
Fig.7 shows the Matlab/Simulink Model of five level Cascaded HBridge multilevel converter. Each Hbridge DC voltage is 50 V. In order to generate three phase output such legs are connected in star/delta. Each leg gating pulses are displaced by 120 degrees.
Figure. 4 Matlab/SImulink Model of CHB with grid connected
Fig.3 Shows the simulink model of the cascaded multilevel inverter which is connected to the grid. This is a closed system with PI controller.
Figure. 5Carrier Signals of Phase Shifted Carrier PWM
Fig.5 shows the Phase shifted Carrier PWM wave form. Here four carriers each are phase shifted by 90 degrees are compared with sine wave.
Fig.6 Source voltage
Fig.6shows the output voltage of the PV module
Fig.7DCDC output voltage
Fig7shows the DCDc output voltage waveform which is maintained constant as the input to the CMI is a constant DC
Fig.8Five level phase voltage
Fig.8shows the phase voltage of level shifted carrier PWM CHB inverter. Fig.12 shows the line voltage of level shifted carrier PWM CHB inverter. Here phase voltage has five voltage levels where as line voltage has nine voltage levels.
Figure. 9 Nine Level Line Voltage
This paper presents a cascaded multilevel inverter with photovoltaic module connected to the system. The multilevel inverter is triggered by using the level shifted PWM method, with closed loop system to maintain the synchronization with the grid. Finally a five level single Hbridge is proposed. A SIMULINK based model is developed and Simulation results are presented.

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