International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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SINGLE-STAGE NON-ISOLATED SEMI Z-SOURCE INVERTER
FOR RENEWABLE SYSTEMS
1
S. Swapnil, 2A.V.V. Sudhakar, 3Lokesh. N
1
1,2,3
P.G Scholar, 2,3Associate Professor
Dept. of EEE, SR Engineering College, Ananthasagar,Warangal
Email : Swapnil06229@gmail.com
Abstract: This paper presents an economic and doubly
grounded non-isolated semi Z-source inverter for a single
phase photovoltaic (PV) systems. This semi Z-source
inverter employs only two switches to get the same output
voltage of the traditional single phase Z-source/quasi
Z-source inverter. The shoot-through zero state is not
applicable to the proposed semi Z-source inverter unlike the
traditional Z-source inverter. Because of doubly grounded
feature of semi Z-source inverter, the leakage current in
input dc source is reduced and so rectifies the safety and
electromagnetic interference problems, which are major
issues in traditional Z-source inverter. Thus it is a preferred
feature for non-isolated grid-connected inverters, especially
in PV applications. A revised nonlinear sinusoidal pulse
width modulation method is also proposed to generate the
desired duty cycle to achieve a sinusoidal voltage reference.
A single phase semi Z-source inverter is simulated using
MATLAB and its results are provided to verify the features
of the proposed circuit.
These inverters can be categorized into two: isolated
inverters and non-isolated inverters. For low voltage grid
or power levels below 20 kW, non-isolated inverters are
being used. These non-isolated inverters can again be
classified into two types: Single stage inverter and two
stage inverter. Single stage inverter topologies are
preferred because of its reduced complexity and reduced
cost. If the input dc source of non-isolated inverter and
grid do not have same ground, then the input dc source
may have large leakage current, which will cause safety
and electromagnetic interference problems [3], [4], [5].
In order to solve the aforementioned problem, either extra
switches have to be used which will increase the cost and
the complexity of the system or doubly grounded
converters have to be used [3], [4], [5]-[7].
For the considerations of safety, cost and system
simplicity, Z-source and quasi Z-source inverters are
proposed [8-9]. The Z-source converter employs a unique
impedance network (or circuit) to couple the converter
main circuit to the power source, thus providing unique
features that cannot be obtained in the traditional
voltage-source (or voltage-fed) and current-source (or
current-fed) converters where a capacitor and inductor are
used respectively. The Z-source converter overcomes the
conceptual and theoretical barriers and limitations of the
traditional voltage-source converter (abbreviated as
V-source converter) and current-source converter
(abbreviated as I-source converter) and provides a novel
power conversion concept.
Keywords: Non-isolated inverter, Photovoltaic, quasi
Z-source, semi Z-source, Grid-connected, sinusoidal pulse
width modulation.
I. INTRODUCTION
Electricity is a major commodity for the socio-economic
development of any country. The major part of electricity
is developed mainly from the fossil fuel. These fossil fuels
have severe impact over the atmosphere and even these
are limited. Due to these limitations renewable energy
resources are becoming more and more popular,
nowadays. Many renewable energy distributed power
generators give dc output voltage, so inverter has to be
interfaced with the ac grid. Many inverter topologies have
been proposed and reviewed recently [1]-[2].
The proposed semi Z-source inverter can achieve the
same output voltage of the traditional Z-source inverter,
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with only two active switches. Both proposed and
traditional circuits share the same form of impedance
network. But it is placed at ac side in the proposed circuit,
which is smaller in size than the impedance network
placed at dc side in the traditional Z-source inverter. The
modulation strategy of the proposed circuit is also
different, which does not use sinusoidal voltage reference
with extra shoot through reference to achieve the
sinusoidal output voltage. But the traditional circuit uses
the above said modulation strategy to get the sinusoidal
output voltage. The proposed circuit uses a modified
voltage reference (which is explained and derived in the
further sections) instead of the sinusoidal reference to
achieve the desired form of output voltage.
III. PROPOSED SEMI Z-SOURCE
INVERTERS
A. Principle of Operation
Fig. 2 shows the two states‘ equivalent circuit in one
switching period using semi-quasi-Z-source inverter as an
example for analysis. Fig. 3(a) shows state I when switch
S1 is conducted. During this period, capacitor C1 and the
input voltage source charge the two inductors, and the
inductor current is increased. Fig. 3(b) shows state II
when switch S2 is conducted. During this period, the two
inductors become the source and the inductor current is
decreased. The inductor current reference and the
capacitor voltage reference direction are marked in the
figure for the following steady-state equation derivation.
According to the inductor voltage second balance and the
capacitor charge balance equations, we can have the
following steady-state equations:
II. TRADITIONAL Z-SOURCE INVERTER
AND ITS MODULATION STRATEGY
A. Principle of Operation
Fig. 1 shows the traditional single-phase Z-source
H-bridge inverter and its modulation method. Simple
boost control is used as an example. The correct
conduction time of each switch of two-phase legs is
generalized by two sinusoid voltage references compared
with a triangle carrier voltage. The two sinusoid voltage
references vA* and vB∗ are 180 degree phase shift from
each other. Two straight lines vP∗ and vN∗ are used to
generalize the shoot-through zero state. When the carrier
is higher than the upper straight line, phase leg A goes to
shoot-through state, whereas phase leg B goes to
shoot-through state when the lower straight line is greater
than the carrier [10]. By controlling the shoot-through
duty cycle, the traditional Z-source inverter can achieve
the different voltage gain.
voltage _ gain 
VC1 
V0 1  2 D

......................(1)
Vin 1  D
D
Vin .............................................(2)
1 D
I L2   I 0 .....................................................(3)
I L1  
D
I 0 ...........................................(4)
1 D
Fig.2 proposed single-phase semi-z-source inverters.
(a) semi z-source inverter. (b) semi-quasi-z-source
inverter
Fig.1. Traditional single-phase Z-source H-bridge
inverter and its modulation method.
Fig.3.semi-quasi-z-source operation modes in one
switching period (a) state I S1 is ON.(b) state II S2 is ON.
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the duty cycle cannot be changed in a sinusoid manner. So
a new duty cycle reference has to be used to generate the
correct sinusoid output voltage.
B. Modulation Strategy
Modulation methods for the proposed semi-Z-source
inverters and the traditional single-phase Z-source
inverter are compared in this section. Fig. 8 shows the
proposed modified SPWM method of semi-Z-source
inverters. Instead of using the sinusoid voltage reference,
a modified voltage reference as derived in (8) is used as
the reference signal for the conduction of switch S2 in
order to output the sinusoid voltage. When the reference
is greater than the carrier, switch S2 is turned ON;
otherwise, S2 is turned OFF. And the gate signal of S1 is
complementary with switch S2. The modified voltage
reference as derived in (7) can be also used directly to
generate the gate signal of S1. But in real implantation,
the gate signal generation of S2 needs less calculation of
DSP, which is usually preferred. So Fig. 8 uses the
generation of gate signal S2 as an example. The
modulation index of the modified SPWM method is also
in the range of 0–1. Fig. 8 shows the situation when the
modulation index M = 2/3 as an example. Fig. 9 shows the
duty cycle operation region with different output voltages
when the modulation index is equal to 1. It can be
concluded from Fig. 9 that in order to get the sinusoidal
output voltage, the duty cycle D is limited in the region
(0–2/3).
Fig.4. Duty cycle operation region of the proposed
semi-Z-source inverters.
The notation ‗D‘ stands for duty cycle of switch S1 and
the switches S1, S2 are conducted in complementary
manner. Fig. 4 shows the voltage gain curve of the
proposed semi-Z-source inverters. By operating switch S1
with duty cycle changing from 0 to 0.667, the proposed
inverters are able to output the same voltage range (+Vin
to -Vin ) as the full-bridge inverter, as shown in Fig. 5
with red solid line. When the duty cycle of S1 changes
from (0–0.5), the inverter can output the positive output
voltage; when the duty cycle of S1 changes from
(0.5–0.667), the inverter can output the negative output
voltage. When the duty cycle is equal to 0.5, the
semi-Z-source inverters are able to output zero voltage.
The output voltage of the inverter can be represented by
V0  V sin wt........................................(5)
And the modulation index can be defined as
M
V
...............................................(6)
Vin
Combining (5) and (6) into (1), we can get
D
Fig 5. Proposed Modified SPWM Method for Semi
Z-Source Inverters
1  M sin wt
...................................(7)
2  M sin wt
IV. SIMULATION RESULTS
The word ―data‖ is plural, not singular. The subscript for
the permeability of vacuum µ0 is zero, not a lowercase
letter ―o.‖ The term for residual magnetization is
―remanence‖; the adjective is ―remanent‖; do not write
―remnance‖ or ―remnant.‖ Use the word ―micrometer‖
instead of ―micron.‖ A graph within a graph is an ―inset,‖
not an ―insert.‖ The word ―alternatively‖ is preferred to
the word ―alternately‖ (unless you really mean something
that alternates). Use the word ―whereas‖ instead of
―while‖ (unless you are referring to simultaneous events).
Do not use the word ―essentially‖ to mean
―approximately‖ or ―effectively.‖ Do not use the word
D1 = 1− D is the duty cycle of S2 and is derived as
D1 
1
.......................................(8)
2  M sin wt
The sinusoid output voltage can be achieved by using a
sinusoidal changed duty cycle, because of linear relation
between input and output voltage of the full-bridge
inverter. But the output voltage and the input voltage of
the semi-Z-source inverter are no longer in a linear
relation. In order to achieve the sinusoid output voltage,
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―issue‖ as a euphemism for ―problem.‖ When
compositions are not specified, separate chemical
symbols by en-dashes; for example, ―NiMn‖ indicates the
intermetallic compound Ni0.5Mn0.5 whereas ―Ni–Mn‖
indicates an alloy of some composition NixMn1-x.
Be aware of the different meanings of the homophones
―affect‖ (usually a verb) and ―effect‖ (usually a noun),
―complement‖ and ―compliment,‖ ―discreet‖ and
―discrete,‖ ―principal‖ (e.g., ―principal investigator‖) and
―principle‖ (e.g., ―principle of measurement‖). Do not
confuse ―imply‖ and ―infer.‖
Input voltage
Prefixes such as ―non,‖ ―sub,‖ ―micro,‖ ―multi,‖ and
―"ultra‖ are not independent words; they should be joined
to the words they modify, usually without a hyphen. There
is no period after the ―et‖ in the Latin abbreviation ―et al.‖
(it is also italicized). The abbreviation ―i.e.,‖ means ―that
is,‖ and the abbreviation ―e.g.,‖ means ―for example‖
(these abbreviations are not italicized).
An excellent style manual and source of information for
science writers is [9].
Output voltage & current
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sincere thanks to Prof. P. VENKATESHWARLU,
Principal for his support throughout my paper. I would
like to thank Prof. C. VENKATESH, Head of the
Department of Electrical and Electronics Engineering for
his constant motivation and support during the course of
my paper.
GATE PULSES
I express my gratitude and sincere thanks to my paper
guide Assoc. Prof. A. V. V. SUDHAKAR, and Asst .Prof
N.LOKESH I truly appreciate and value their esteemed
guidance and encouragement from the beginning to end of
this work. I am indebted to them for having helped me
shape the problem and providing insights towards the
solution.
I am gratefully acknowledging the help of incharge for the
department of library of Electrical Engineering for his
support during the paper.
VDS1, VDS2
Modulation
index(m=vo/vin)
0.95
0.94
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Output
voltage vo
38v
37.6v
36v
32v
28v
24v
20v
16v
12v
8v
4v
0v
I also thank all the teaching and non-teaching staff for
their co-operation to me and I also thank my friends for
their support.
I wish to express my gratitude to my Parents, whose love
and encouragement have supported me throughout my
education
REFERENCES
Table for modulation index and output voltage
[1]
M. Calais and V. G. Agelidis, ―Multilevel
converters for single-phase grid connected
photovoltaic systems—An overview,‖ in Proc.
IEEE Int. Symp.
[2]
Y. Xue, L. Chang, S. B. Kjaer, J. Bordonau, and T.
Shimizu, ―Topologies of single-phase inverters for
small distributed power generators: An overview,‖
IEEE Trans. Power Electron., vol. 19, no. 5, pp.
1305–1314,Sep. 2004
[3]
J. M. A. Myrzik and M. Calais, ―String and module
integrated inverters for single-phase grid
connected photovoltaic systems—A review,‖ in
2003 IEEE Bologna PowerTech Conf. Proc., Jun.,
vol. 2, p. 8.
[4]
S. Araujo, P. Zacharias, and R. Mallwitz, ―Highly
efficient single-phase transformerless inverters for
grid-connected photovoltaic systems,‖ IEEE
Trans. Ind. Electron., vol. 57, no. 9, pp.
3118–3128, Sep. 2010.
[5]
O. Lopez, F. D. Freijedo, A. G. Yepes, P.
Fernandez-Comesaa, J.
Malvar,R. Teodorescu,
and J. Doval-Gandoy, ―Eliminating ground current
in a transformerless photovoltaic application,‖
In the above table we can observe modulation index and
output voltage are linear
V. CONCLUSION
In this paper, an economic single stage single phase
non-isolated semi Z-source inverter is proposed, which is
suitable for a PV panel in low voltage grid connected
applications. The proposed semi Z-source inverter is able
to achieve the same output voltage as the traditional
converter, with only two active switches. The input dc
source and the output ac voltage share the same ground,
which effectively eliminates the leakage current caused by
the PV panel. A modified SPWM method is also proposed
to solve the nonlinear voltage gain problem of the
semi-Z-source inverter. A single phase semi Z-source
inverter has been designed and simulated using
MATLAB.
ACKNOWLEDGMENT
This is our pleasure to express our immense gratitude and
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International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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IEEE Trans. Energy Convers.,vol. 25, no. 1, pp.
140–147, Mar. 2010.
[6]
[7]
H. Patel and V. Agarwal, ―A single-stage
single-phase transformer-less doubly grounded
grid-connected PV interface,‖ IEEE Trans. Energy
Convers.,vol. 24, no. 1, pp. 93–101, Mar. 2009.
R. Gonzalez, E. Gubia, J. Lopez, and L.Marroyo,
―Transformerless singlephase multilevel-based
photovoltaic inverter,‖ IEEE Trans. Ind.
Electron.,vol. 55, no. 7, pp. 2694–2702, Jul. 2008.
[8]
J. Anderson and F. Z. Peng, ―A class of
quasi-Z-source inverters,‖ in Proc.IEEE Ind. Appl.
Soc. Annu. Meeting, 2008, pp. 1–7.
[9]
J. Anderson and F. Z. Peng, ―Four quasi-Z-Source
inverters,‖ in Proc.IEEE Power Electron. Spec.
Conf., 2008, pp. 2743–2749.
[10]
Y. Huang, M. Shen, F. Z. Peng, and J. Wang,
―Z-source inverter
for residential photovoltaic
systems,‖ IEEE Trans. Power Electron., vol.
21,no. 6, pp. 1776–1782, Nov. 2006.
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