International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Modified Trans-Z-Source Inverter with Continuous
Input Current and Improved Boost Factor
Swathyprakash and Rani S
Department of Electrical and Electronics Engineering
MEA Engineering College
Perinthalmanna, Kerala, India
swathyprakash92@gmail.com
Abstract—This paper presents a modified trans-Z-source
inverter which improves upon conventional Z-source and transZ-source inverters. It consists of an impedance network as in the
case of Z-source inverter. The two inductors in Z-source inverter
are replaced by two transformers. An extra inductor and two
capacitors are added to provide continuous input current. It also
suppresses high resonant current at startup. By changing turns
ratio of transformers and modulation index, different boost
factors and corresponding output voltages are obtained. The
operating principle, analysis, control method and simulation
results are presented in this paper.
Keywords—Z-source
inverter;
trans-Z-source
inverter;
transformer; voltage source inverter; current source inverter
I. INTRODUCTION
Traditionally, there exist two converters: voltage source
converters and current source converters. Due to different
limitations of these two converters, Z-source inverter was
proposed in [1]. Z-source inverters are used for single stage
power conversion with buck and boost abilities. Many Zsource topologies have been developed in different studies
which focus on its pulse width modulation strategies [2]-[4].
To overcome some disadvantages of Z-source inverters, a
class of inverters called quasi-Z-source inverters were
introduced [5] and [6], having merits such as improvement in
input current profile and reduction in ratings of passive
components.
Later, some papers have introduced topologies by adding
extra inductors, capacitors and diodes in order to increase the
boost factor and to obtain higher boost inversion capability
[7]-[9]. In order to increase the boost factor and voltage gain, a
transformer has been added to the conventional Z-source
inverter replacing the two inductors in its impedance network.
Such topology is called trans-Z-source inverter. Many papers
have been proposed analyzing these kinds of topologies [10]
and [11]. An improved trans-Z-source inverter has been
proposed in [12] which have the advantage of continuous
input current and boost inversion capability.
This paper presents a modified trans-Z-source inverter
with an increased boost factor. There are two transformers in
this topology replacing the two inductors in the conventional
Z-source inverter. Like the conventional Z- source inverter,
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
Fig. 1. Z-source inverter for fuel cell application
the modified inverter utilizes both the shoot through and non
shoot through states of the inverter bridge in order to obtain
boosted output voltage. The modified inverter has continuous
input current and high current at startup is suppressed. Both
the shoot through duty ratio and transformer turns ratio can be
adjusted over a wide range and can be boosted to a high value.
The conventional Z-source inverter and improved trans-Zsource inverter are explained in section II. Detailed operation
and analysis of modified trans-Z-source inverter is given in
section III. Section IV describes the simulation and result
analysis of the modified trans-Z-source inverter.
II. CONVENTIONAL Z-SOURCE AND IMPROVED TRANS-ZSOURCE INVERTER TOPOLOGIES
Fig. 1 shows the conventional Z-source inverter topology
for fuel cell applications. The classical Z-source inverter
consists of a unique impedance network which couples
inverter main circuit to dc source. This utilizes both shoot
through and non shoot through states of inverter bridge. As
shown in Fig.1, the impedance network consists of two
inductors L1 and L2 and two capacitors C1 and C2. The boost
factor of conventional Z-source inverter can be represented as
B=
1
1 − 2D
(1)
Fig. 2 shows the improved trans-Z-source inverter topology
with continuous input current. The two inductors in the
classical Z-source inverter are replaced by a transformer here.
Fig. 2. Improved trans-Z-source inverter with continuous input current
An inductor L3 and a capacitor C2 is provided for making the
input current continuous and to suppress resonant current at
start up. A capacitor C1 is placed at the dc link. By changing
the shoot through duty ratio and turns ratio of the transformer,
a wide range of output voltage can be obtained. The boost
factor of improved trans-Z-source inverter can be represented
as
B=
1
1 − ( 2 + n) D
Fig. 3. Modified trans-Z-source inverter with continuous input current and
improved boost factor.
(2)
The major disadvantages of the above mentioned
topologies are as follows.
1) Input current is discontinuous in the classical Z-source
inverter, thus requiring a decoupling capacitor bank or
LC input filter at the front end to eliminate
discontinuity and protect the energy source.
2) In improved trans-Z-source inverter, the basic structure
of Z-source inverter is altered. A higher transformer
turns ratio is used to get higher boosting which
increases the weight and cost of the transformer.
Fig. 5. Shoot-through state of modified trans-Z-source inverter with
continuous input current and improved boost factor
III. MODIFIED TRANS-Z-SOURCE INVERTER
Fig. 3 shows the modified trans-Z-source inverter with
continuous input current and improved boost factor. It is
created by replacing the two inductors (L1 and L2) of classical
Z-source inverter by two transformers. Apart from that, it
consists of an inductor (L3), four capacitors (C1, C2, C3 and C4)
and two diodes (D1 and D2). The main characteristics of the
proposed inverter are as follows: 1) the basic X shape is
retained; 2) only two transformers are used and a very high
boost factor can be obtained by changing the turns ratio of the
transformer; 3) the input dc current is continuous; 4) it
provides resonant current suppression.
As in the case of conventional Z-source inverter, the
modified inverter also utilizes the shoot-through zero states in
addition to the six active and two zero states. Thus the
operating principles are similar to conventional Z-source
inverters. For the purpose of analysis, the operating states are
classified to shoot-through and nonshoot-through states. Fig. 5
and Fig. 6, shows respectively the shoot through and
nonshoot-through states of modified trans-Z-source inverter.
Fig. 6. Nonshoot-through state of modified trans-Z-source inverter with
continuous input current and improved boost factor.
In shoot-through state, the inverter is shorted by both upper
and lower switching devices of any phase leg. During this
time, both diodes D1 and D2 are reverse biased.
v
v
v
v
L11
L12
(3)
= n2 vL11
(4)
=V C2
(5)
= n1 vL 21
(6)
L 21
L 22
= V C1
In nonshoot-through state, the inverter has six active states
and two zero states. The diodes are on at this time. The
corresponding equations are
TABLE I.
SIMULATION PARAMETERS
Parameters
Values
Input DC voltage
100V
Output phase voltage
135V
Transformer turns ratio
1:1
Capacitors (C1 and C2)
5mF
Capacitors (C3 and C4)
100µF
Inductor (L3)
1mH
Switching frequency
2KHz
Three phase
output filter
Lf
12mH
Cf
500µF
Three phase resistive load
v
v
L 22
L11
50Ω/phase
+ vL 21 = V C 2
(7)
+ vL12 = V C1
(8)
Fig. 7. Simulink model of modified trans-Z-source inverter with continuous
input current and improved boost factor.
The boost factor of the proposed inverter is defined by,
B=
1
1 − [ 2 + N 1 + N 2 ]D
(9)
When N1+N2=0, the inverter behaves similar to a Z-source
inverter and when N1+N2=N, it is similar to improved trans-Zsource inverter.
When using simple boost control method, the shootthrough duty cycle is,
D =1− M
(10)
Where M is the modulation index. Substituting, we obtain,
1
1 − [2 + N 1 + N 2](1 − M )
Fig. 8. Subsystem section of modified trans-Z-source inverter with continuous
input current and improved boost factor.
(11)
The switching frequency is 2KHz. Simple boost control
technique is used in the following simulation with modulation
indices of 0.7.
The peak value of phase voltage from inverter output is
represented by,
The circuit is simulated with open loop control, in which,
output voltage is controlled by varying amplitude of the
reference sine wave and the positive and negative constants,
i.e, modulation index M. The control technique used is simple
boost control. Here, M is taken as 0.7. A discrete powergui of
sample time 5*e-6 is given to increase the computation speed
of simulation. Fig. 7 shows the simulink model of open loop
operation of modified trans-Z-source inverter.
B=
v
ph
= M * B * V dc
2
(12)
The overall dc-ac inversion gain is obtained as,
G = M *B
(13)
IV. SIMULATION RESULTS
To validate the operation of the modified trans-Z-source
inverter, simulation has been carried out using MATLAB
simulation tool. A simulation model of the proposed technique
is developed in MATLAB/Simulink environment.
Fig. 8 shows the subsystem section of control circuit. The
control technique used here is simple boost control where a
positive constant and a negative constant is compared with the
triangular carrier wave to generate the shoot through pulses.
Fig. 9 shows the PWM pulses obtained from simple boost. Six
control pulses produced are given to the six switches.
Fig. 10 shows the simulation output of input current waveform
of modified trans-Z-source inverter. The input current is
continuous in nature and the starting current is reduced to a
value of 150A. Fig. 11 shows the simulation result shows the
output voltage of inverter before filtering. The output voltage
obtained is of square shaped. It has a value of about 550V. To
make this into sine wave output, a low-pass LC filter is used
on the output side of the inverter. Fig .12 shows the filtered
output of modified trans-Z-source inverter. It has a rms value
of about 135V.
V. CONCLUSION
Fig. 9. PWM control signals generated by simple boost control
A modified trans-Z-source inverter topology with
continuous input current, resonant current suppression and
improved boost factor was introduced. The simple boost
control strategy is implemented. By varying the shoot through
duty ratio and transformer turns ratio, a wide range of output
voltage is obtained. The modified inverter uses a lower
transformer turns ratio reducing the size and weight of
transformer. Simulation is done using MATLAB/simulink and
results are verified. The modified inverter can be used for
renewable energy applications where a low input voltage must
be inverted to a high ac output voltage.
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Fig. 10. Input current waveform of modified trans-Z-source inverter with
continuous input current and improved boost factor.
Fig. 11. Inverter output voltage waveform of modified trans-Z-source inverter
with continuous input current and improved boost factor.
Fig. 11. Filtered output voltage waveform of modified trans-Z-source inverter
with continuous input current and improved boost factor.
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