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CHAPTER 7
CONCLUSIONS
7.1
GENERAL
The thesis is an attempt to arrive at an optimum solar photovoltaic
system configuration and the most appropriate inverter topologies for the
same. Towards that, end simulation and experimental investigations listed
below have been undertaken:
Modelling, simulation and experimental verification of solar
photovoltaic systems.
Comparative evaluations are carried out for existing
conventional voltage source inverter (VSI), diode assisted Cuk
based VSI and embedded Z source inverter topologies by
simulation results, supported with hardware realization.
Simulations are carried out for existing Z source inverter
topologies under different switching schemes suited for
photovoltaic systems.
The proposed switched inductor/capacitor quasi Z source
inverter topologies and its variants are analyzed for its better
voltage inversion and boosting capabilities through simulation
and experimental investigation is carried out for extended
switched inductor quasi Z source inverter through scaled down
laboratory prototype.
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Three modified H bridge multilevel Z source inverter
topologies are proposed to offer better voltage boosting
capability. Experimental results are provided for the proposed
modified H bridge Z source inverter with maximum constant
boost switching scheme employing third harmonic injection
for better voltage boost inversion.
A three phase quasi Z source network based DC/DC converter
with zigzag transformer is proposed with better conversion
ability and compared with other existing traditional Z source
inverter based DC/DC converter topologies.
Experimental investigation are carried out for proposed three
level improved Z source inverter and its performance is
compared with existing two level improved Z source inverter
to ensure reduction in Z capacitor voltage stress at the time of
start with inherent inrush current limitation ability.
7.2
CONTRIBUTION OF THE THESIS
In Chapter 2, solar photovoltaic system characteristics and its
performance are analyzed through simulation and experimental verification,
which are listed below.
Simulation is carried out for solar photovoltaic (SPV)
module to obtain its module parameters and I – V and P – V
characteristic curves under standard test conditions.
Simulation studies are carried out for SPV modules
connected in different array configurations under standard
test / partial shaded conditions to obtain its I – V and P – V
characteristic curves.
For the given PV system, best possible PV array configuration
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can be identified based on its maximum output power and
voltage.
Experimental verification of the SPV system under standard
test / partial shaded conditions are done to find the operating
condition of the PV inverter systems.
In chapter 3, inverter topologies like diode assisted Cuk based
buck-boost VSI and embedded Z source inverter topologies has been analyzed
on the basis of its output voltage waveform quality and voltage boosting
capability through simulation and experimental hardware.
Further diode assisted SEPIC based buck-boost VSI and various Z
source inverter topologies are analyzed for their voltage boosting ability under
different switching schemes through simulation to find voltage and current
stress across L & C.
Experimental validation is also carried out for conventional voltage
source inverter (VSI) in FPGA platform.
Specifically, investigations carried out reveal the following:
Output voltage of embedded Z source inverter have better
voltage boosting ability with smoother waveform when
compared with diode assisted Cuk based VSI’s which is
confirmed through simulation and experimental validation.
Selection of L and C decides the boosting capability of the Z
source inverter.
Z source inverter can accommodate the inputs of different
values of the given PV system and produce the same output
with lesser total harmonic distortion.
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The recent topologies of Z source inverters handle two or
more sources independently and feed power to the load, even
in the failure of any one of the sources.
The discussed inverter topologies operated in both the
microcontroller and FPGA platform ensure fast and flexible
operation.
Extensive investigations have been carried out for various
topologies of quasi Z source inverters (qZSI) and many new qZSI topologies
are proposed. Based on the simulation results, experimental validation is
carried for proposed extended switched inductor quasi Z source inverter and
the findings are reported in chapter 4 as listed below.
Extended boost qZSI topologies for both continuous and
discontinuous current offers better voltage inversion and
boosting ability and also these topologies are expanded to
have increased voltage gain suited for photovoltaic systems.
A new family of switched inductor/capacitor qZSI topologies
is proposed and open loop simulation is carried out under
various switching schemes.
Experimental result are obtained for the proposed extended
switched inductor quasi Z source inverter which has better
inversion and voltage boosting ability with reduced stress on
the passive components at start conditions.
In chapter 5, a detailed simulation is carried out for the topological
analysis of multilevel Z source inverters and quasi Z source network based
DC/DC converters and new topologies are proposed found to have better
operation and their findings are listed below as:
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Multilevel Z source inverter topologies offers several
advantages
like
improved
boosting
capability,
better
waveform quality when compared with other ZSI topologies.
Also the proposed H bridge Z source inverter topologies offer
better voltage boosting capability which is confirmed with
experimental results provided for the proposed modified H
bridge Z source inverter with maximum constant boost
switching scheme employing third harmonic injection.
Additionally, the different topologies of Z source inverter
based DC/DC converters are analyzed. It is observed through
simulation studies that the boost conversion ability of the
proposed three phase quasi Z source network based DC/DC
converter with zigzag transformer has better conversion
compared to the other traditional Z source inverter based
DC/DC converter topologies.
Through the extensive simulation studies carried out so far, Z
source concept can be applied to any power conditioning unit
and it is found that many Z source conversion circuits can be
derived.
In chapter 6, simulation studies reveal that improved two level and
the proposed three level topologies of Z source inverter have better voltage
boosting ability across the inverter bridge, and also ensures reduction in Z
capacitor voltage stress and have inherent inrush current limitation ability.
Experimental results validate that the proposed three level
improved Z source inverter exhibits better inversion ability and also
encourages the utilizing of low voltage Z capacitor, thereby reducing system
volume and cost.
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7.3
SCOPE FOR FUTURE WORK
Development of grid connected model of the proposed Z
source inverters can be demonstrated with new control
strategies employing MPPT techniques for solar photovoltaic
systems.
The proposed Z source multilevel inverters can be connected
to Z source inverter based DC/DC converters and its
integration with suitable rectifier circuits and necessary
modulation strategies is left for future investigations.