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Vol.06,Issue.06,
September-2014,
Pages:465-472
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Improved Trans Z-Source Inverter with Continuous Input Current and
Boost Inversion Capability for Renewable Energy Resources E. TEJASWINI
1, T. PARMESHWAR
2, P. NAGESWARARAO
3
1PG Scholar, Dept of EEE, Vidya Jyothi Institute of Technology, Hyderabad, India, Email: [email protected]. 2Asst Prof, Dept of EEE, Vidya Jyothi Institute of Technology, Hyderabad, India, Email: [email protected].
3Assoc Prof, Dept of EEE, Vidya Jyothi Institute of Technology, Hyderabad, India.
Abstract: Energy resources and their utilization are a prominent issue of this century. The problems of natural source
depletion, environmental effects and the rising demand for new energy have been fervently discussed in recent years. Some
modified impedance source networks are proposed from time to time for increasing the output voltage gain. One such
impedance network is Improved Trans Z Source Inverter which has high boost voltage that improves upon the conventional
inverters. The proposed Improved Trans Z Source Inverter maintains all the main features of Z Source Inverter with the added
advantages like increased voltage gain, reduced voltage stress, continuous input current, boost inversion capability, higher
modulation index, lower current ripple etc. This is a novel concept of high step up inverter based on the transformer to improve
the input current profile. The Improved Trans Z Source Inverter can suppress resonant current at startup, which might destroy
the devices. Keeping in view the above advantages, the proposed concept has topologies that suit solar cell and fuel cell
applications and can be extended towards the implementation and analysis with PV cells and fuel cells which account for
renewable energy resources and a brief comparison between PV and Fuel cells is also implemented in this paper.
Keywords: Improved Trans Z Source Inverter.
I. INTRODUCTION
The rapidly increasing environmental degradation
across the globe is posing a major challenge to develop
commercially feasible alternative sources of electrical
energy generation. Energy resources and their utilization
are a prominent issue of this century. The problems of
natural source depletion, environmental effects and the
rising demand for new energy have been fervently
discussed in recent years. It is believed that the distributed
generation market will be more than $30 billion by the year
2015. Due to environmental concerns, more effort is now
being put into clean and distributed power like geothermal,
wind power, fuel cells, and photovoltaic (PV) that directly
uses the energy from the sun to generate electricity. Thus, a
huge research effort is being conducted worldwide to come
up with a solution in developing an environmentally
benign and long-term sustainable solution in electric power
generation. The major players in renewable energy
generation are photovoltaic (PV), wind farms, fuel cell,
and biomass. These distributed power generation sources
are widely accepted for micro grid applications.
The worldwide grid-connected PV system grows at a
rate of 25% every year. As the energy from the sun is free,
the major cost of photovoltaic generation is the installation
cost, which is mainly composed of the costs of solar
modules and the interface converter system, also called the
power conditioning system (PCS). A fuel cell combines
hydrogen and oxygen to produce electricity, heat, and
water. Fuel cells are often compared to batteries. Both
convert the energy produced by a chemical reaction into
usable electric power.
Fig1. PV Array with Dc to Dc Converter.
Fig2. Fuel cell with Dc to Dc Converter.
With the development of solar cell and fuel cell
technologies, the price of their modules has dropped
dramatically. To lower the cost of the PCSs has become a
E. TEJASWINI, T. PARMESHWAR, P. NAGESWARARAO
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.06, September-2014, Pages: 465-472
very urgent issue of grid connected PV systems. However,
the reliability of the micro grid relies upon the interfacing
power converter. Thus the proper power regulation from
the interfacing power converter will ensure a stable and
reliable micro grid system. Fig 1 and fig 2 show pv array
and fuel cell with pcs integration to the grid.
II.TRENDS IN Z-SOURCE INVERTER
The use of renewable-energy generating systems has
recently increased dramatically due to the exhaustion of
fossil fuels and their impact on the environment. The
unregulated output power of renewable energy sources
should be regulated through inverters to satisfy the
conditions for connection to the grid. A Z-source inverter
(ZSI) has been proposed to overcome the disadvantages of
the conventional scheme with a unique impedance
network. A ZSI can buck or boost the input voltage using
the shoot-through state and the modulation index in a
single stage. Since no dead time is needed, the output
voltage is free from voltage distortion. Due to these
advantages, the ZSI has been applied to single stage
conversion applications, such as PV systems, fuel cell
systems and ac motor drive systems.
Fig3. QZSI with continuous input current.
The quasi-Z-source inverter (QZSI) is similar to the ZSI
presented above, but has several advantages including, in
various combinations (see fig 3); lower component ratings,
reduced source stress, reduced component count and
simplified control strategies. The extended boost Z-source
inverters add inductors, capacitors, and diodes to the Z
impendence network in order to produce a high dc-link
voltage for the main power circuit from a very low input dc
voltage. A combination of the Z-source inverter and
switched-inductor structure, called the switched-inductor
Z-source inverter, provides strong step-up inversion to
overcome the boost limitation of the classical Z-source
inverter. In order to overcome the inconvenience of inrush
current suppression at startup of the switched-inductor Z-
source inverter, a switched-inductor quasi- Z-source
inverter is proposed in which provides continuous input
current, reduced passive component count, reduced voltage
stress on the capacitors, lower shoot-through current, and
lower current stress on inductors and diodes, in comparison
to the switched-inductor Z-source inverter for the same
input and output voltages.
The trans-Z source/-quasi-Z-source inverter is extended
to various structures in cascade topologies and parallel
operations for high power conversion system. In order to
improve the input current profile, an inductor–capacitor–
capacitor–transformer Z-source inverter (LCCT-ZSI) in
used one more inductor and one more capacitor in
comparison with the trans-Z-source/-quasi- Z-source
inverters. All topologies suit solar cell and fuel cell
applications, since they require high voltage gain in order
to match the source voltage to the line voltage. The input
dc current is discontinuous in the trans-Z-source inverter
and has a high ripple in the trans-quasi-Z-source inverter,
thus requiring a decoupling capacitor bank or an LC input
filter at the front end to eliminate current discontinuity and
protect the energy source. As shown in fig 4, the trans-
quasi-Z-source inverter can suppress the resonant current
at startup while the trans-Z-source inverter cannot, and the
resulting voltage and current spike can destroy the devices.
The startup resonant problem of the trans-Z-source inverter
occurs because a huge resonant current flows to the diode,
transformer windings, capacitor and body diode of the
insulated gate bipolar transistors (IGBTs).
Fig4. Trans z source inverter with input filter.
III. IMPROVED TRANS Z SOURCE INVERTER
The converter shown in section I need to provide high
voltage gain for higher load demand. Therefore some
modified impedance source networks are proposed from
time to time for increasing the output voltage gain. One
such impedance network is Improved Trans Z Source
Inverter which has high boost voltage that improves upon
the conventional inverters. The proposed Improved Trans
Z Source Inverter maintains all the main features of Z
Source Inverter with the added advantages like increased
voltage gain, reduced voltage stress, continuous input
current, boost inversion capability, higher modulation
index, lower current ripple etc. This is a novel concept of
high step up inverter based on the transformer to improve
the input current profile. The Improved Trans Z Source
Inverter can suppress resonant current at startup, which
might destroy the devices (see fig 5).
Improved Trans Z-Source Inverter with Continuous Input Current and Boost Inversion Capability for Renewable
Energy Resources
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.06, September-2014, Pages: 465-472
Fig 5 Improved trans z-source inverter.
Fig6. Basic block diagram for integration of impedance
network with renewable energy resource.
Keeping in view the above advantages, the proposed
concept has topologies that suit solar cell and fuel cell
applications and can be extended towards the
implementation and analysis with PV cells and fuel cells
which account for renewable energy resources and a brief
comparison between pv and fuel cells is also implemented.
Fig 6 Block diagram shows the Implementation of
Improved Trans Z Source Inverter for Renewable Energy
Resources
IV. PV CIRCUIT IMPLEMENTATION
There are several power converter topologies employed
in PV systems; however, they differ by several
characteristics: two stage or single-stage, with transformer
or transformer less, and with a two-level or multilevel
inverter. Single-stage inverters are becoming more
attractive in comparison to two-stage models due to their
compactness, low cost, and reliability. However, the
conventional inverter has to be oversized to cope with the
wide PV array voltage changes because a PV panel
presents low output voltage with a wide range of variation
based on irradiation and temperature, usually at a range of
1 : 2. To interface the low voltage output of an inverter to
the grid, a bulky low-frequency transformer is necessary at
the cost of a large size, decrease in efficiency, loud
acoustic noise, and high cost. The two-stage inverter
applies a boost dc/dc converter instead of a transformer in
order to minimize the required KVA rating of the inverter
and boost the wide range of voltage to a constant desired
value. Unfortunately, the switch in the dc/dc converter
becomes the cost and efficiency killer of the system.
Transformer less topologies especially deserves attention
because of their higher efficiency, smaller size and weight,
and a lower price for the PV system. The Z-source inverter
(ZSI), as a single-stage power converter with a step-
up/down function, allows a wide range of PV voltages, and
has been reported in applications in PV systems (see fig 7).
Fig7. Improved Trans z source inverter with source as
PV Module.
A. Operating Modes
Fig8. (a) equivalent circuit (b) shoot through state (c)
non shoot through state.
It can handle the PV dc voltage variation in a wide range
without overrating the inverter, as well as implement
voltage boost and inversion simultaneously in a single
power conversion stage, thus minimizing system cost and
reducing component count and cost, and improving the
reliability. Recently proposed qZSIs have some new
attractive advantages that are more suitable for application
E. TEJASWINI, T. PARMESHWAR, P. NAGESWARARAO
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.06, September-2014, Pages: 465-472
in PV systems. This will make the PV system much
simpler and lower its cost because the qZSI: 1) draws a
constant current from the PV panel, thus no need for extra
filtering capacitors. 2) Features lower component
(capacitor) rating; and 3) reduces switching ripples to the
PV panels.
The improved inverter has extra shoot-through zero
states in addition to the traditional six active and two zero
states in a classical Z-source inverter. For the purpose of
analysis, the operating states are simplified into shoot-
through and nonshoot-through states. Fig.8 (a) shows the
equivalent circuits of the improved trans-Zsource inverter
with pv module implementation. In the shoot-through state,
as shown in Fig. 8(b), the inverter side is shorted by both
the upper and lower switching devices of any phase leg.
During the shoot-through state, the diode D is OFF. We
thus obtain the following:
vL1 = VC1 (1)
vL2 = nvL1 = nVC1 (2)
vL3 = Vpv + VC2. (3)
In the nonshoot-through state, as shown in Fig.8(c), the
improved inverter has six active states and two zero states
of the inverter main circuit. During the nonshoot-through
state, D is on. The corresponding voltages across the
primary and secondary windings of the transformers in this
state are vL1 non and vL2 non. We obtain
vL1 non + vL2 non = −VC2 (4)
vL3 = Vpv − VC1 − vL2 non (5)
vPN = VC1 − vL1 non. (6)
Applying the volt-second balance principle toL1and L2, (1)
and (2) yield
vL1 non =−D VC1/(1 – D) (7)
vL2 non =−nD VC1/(1 – D) (8)
Substituting (7) and (8) into (4), we have
VC2 =(1 + n)D VC1/(1 – D) (9)
Applying the volt-second balance principle to L3, (3), (5),
and (9) yield
VC1 =(1 – D) Vpv/(1 − (2 + n)D) (10)
VC2 =(1 + n)D Vpv /1 − (2 + n)D (11)
The peak dc-link voltage across the inverter’s main
circuit is expressed in (6) and can be rewritten as
VPN =1/(1 − (2 + n)D) (12)
Vpv = BVdc. (13)
The boost factor of the proposed inverter B is defined by
B =1/(1 − (2 + n)D)=1/(1 − (2 + n)T0/T). (14)
From (14), when n = 0, B = 1/(1 − 2D), the improved
inverter becomes the classical Z-source inverter. When n ≥
1, the boost ability of the improved trans-Z-source inverter
is higher than that of the trans-Z-source, trans-quasi-Z-
source, and classical Z-source inverters. On the other hand,
the improved inverter uses a smaller shoot-through duty
cycle at the same boost factor in comparison with the
conventional trans-Z-source/-quasi-Z source inverters.
V. FUEL CELL IMPLEMENTATION
Recently, energy management based on classical PI
controllers has been proposed. This strategy is based on the
control of the main performance parameters such as the
battery state of charge (SOC), the super capacitor voltage
or DC bus voltage using PI controllers. The knowledge of
an expert is not necessary and the PI controllers can be
easily tuned online for better tracking. The load power is
distributed in such a way to allow the fuel cell system to
provide the steady state load demand. The frequency
decoupling strategy ensures the fuel cell provide low
frequency demand while the other energy sources deal with
high frequency demand. This is achieved through the use
of low pass filters; wavelet or fast Fourier transforms
(FFT) techniques. This strategy improves the life time of
the fuel cell system as the dynamic stress on the fuel
supply system is prevented. Here, the fuel cell system
supplies a nearly constant mean load power while the other
energy sources discharge or recharge when the load power
is above or below its mean value respectively (see fig 9).
Fig9. Improved Trans z source inverter with source as
fuel cell.
A. Operating Modes
The improved inverter has extra shoot-through zero
states in addition to the traditional six active and two zero
states in a classical Z-source inverter. For the purpose of
analysis, the operating states are simplified into shoot-
through and nonshoot-through states. Fig 10 (a) shows the
equivalent circuits of the improved trans-Zsource inverter
with pv module implementation. In the shoot-through state,
as shown in Fig. 10(b), the inverter side is shorted by both
the upper and lower switching devices of any phase leg.
During the shoot-through state, the diode D is OFF. We
thus obtain the equations similar to those derived in section
IV for a voltage of Vfc for fuel cell.
Improved Trans Z-Source Inverter with Continuous Input Current and Boost Inversion Capability for Renewable
Energy Resources
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.06, September-2014, Pages: 465-472
Fig 10 (a) equivalent circuit (b) shoot through state (c)
non shoot through state.
VI. SIMULATION RESULTS.
A. PV Module Simulation Results
E. TEJASWINI, T. PARMESHWAR, P. NAGESWARARAO
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.06, September-2014, Pages: 465-472
Fig11. Simulation results using PV module.
B. Fuel Cell Simulation Results
Improved Trans Z-Source Inverter with Continuous Input Current and Boost Inversion Capability for Renewable
Energy Resources
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.06, September-2014, Pages: 465-472
Fig12. Simulation results using fuel cell: output phase
voltage Va; three phase voltage and current; dc link
voltage Vpn; input current Iin; shoot through current Ish;
capacitor voltages steady state- dc link voltage; diode
voltage; input current; shoot through current.
Figure11 and 12 shows the simulation results using PV
module and fuel cell. We selected the simulation
Parameters L3 =1 mh, C1=C2 =1000 μf, Lf =1.5 mh,Cf =
10 μf, and R = 50 Ω/phase. The turn ratio of the
transformer is 2. The magnetic inductance measured from
the primary side was set to 0.737 mh. The leakage
inductance was set to 0.5 μh. The switching frequency was
10 khz, the input was 100 Vdc, And the output phase
voltage was 115 Vrms to meet the grid-tied requirement.
Constant boost control was used. When using the constant
boost control method for the improved inverter, According
to M = 0.95 to produce the output phase Voltage of 115
Vrms from the 100 V input dc voltage. Thus, we obtain D
= 0.1772, B = 3.42, G = 3.25, VPN = 342 V, VC1 = 281 V,
and VC2 = 181 V for the improved Trans-Z-source
inverter. Adding of one more inductor and one more
Capacitor makes more resonances in the improved inverter.
The conventional trans-quasi-Z-source inverter has the
same simulation results as the trans-Z-source inverter,
except for eliminating the huge resonant current at startup,
lower voltage stress on the capacitor, and a different input
current drawn from the dc source. These results can also
be enhanced by using different semiconductors in case of
pv cells and their trend over coming years is shown in fig
13 and a comparison graph between pv and fuel cell is
shown in the graph below in fig 14.
Fig13. Trend in pv cells for different semiconductors.
Fig14. A comparison between pv and fuel cell for a
given load demand in case of improved trans z source
inverter.
VII. CONCLUSION
The proposed concept aims to improve the trans-Z-source
inverter with the following main characteristics: high boost
voltage inversion ability, continuous input current, and
resonance suppression at startup. Compared with the
conventional trans- Z-source and trans-quasi-Z-source
inverters, for the same transformer turn ratio and input and
output voltage, the improved inverter has a higher
modulation index with reduced voltage stress on the dc
link, lower current stress flow to the transformer windings
and diode, and lower input current ripple. If the modulation
index is kept fixed, the improved inverter uses a lower
transformer turn ratio to produce the same input and output
voltage compared to the conventional trans-Z-source/-
E. TEJASWINI, T. PARMESHWAR, P. NAGESWARARAO
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.06, September-2014, Pages: 465-472
quasi-Z-source inverters. As a result, the size and weight of
the transformer in the improved inverter can be reduced.
The improved inverter application to fuel cells and
photovoltaic cells is shown in this paper, where a low input
voltage must be inverted to a high ac output voltage and a
comparison between pv and fuel cells simulation results
can be done for the case of improved trans z source
inverter.
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Author’s Profile:
E. Tejaswini received B-Tech
Degree in Electrical and Electronics
Engineering from Sridevi Womens
Engineering College Hyderabad,
India. She is pursuing her Post
Graduation from Vidya Jyothi
Institute of Technology, Hyderabad,
India.
T.Parameshwar received B-Tech
Degree in Electrical and Electronics
Engineering from VNR Viganan
Jyothi Institute of Technology
Hyderabad, India. He did his Post
Graduate in Electrical Power System
from Jawaharlal Institute of
Technology University Anatapuram,
India. He has 6 years of experience in teaching. Currently
he is with Vidya Jyothi Institute of Technology Hyderabad,
India, as an Assistant Professor in Electrical and
Electronics Engineering.
P. Nageswararao is an associate
professor working with Vidya Jyothi
Institute of Technology, India in the
Electrical and Electronics
Engineering. His areas of interests are
Fuzzy Logic Applications in Power
Systems, FACTS Controllers in
Power Systems.