ISSN 2249-6343
International Journal of Computer Technology and Electronics Engineering (IJCTEE)
Volume 2, Issue 1
Performance Analysis of Cascaded Z-Source Multilevel
Inverter using Third Harmonic Injection PWM
A. Shanmuga priyaa1, Dr. R. Seyezhai2, Dr. B. L. Mathur3
Abstract— Multilevel Inverter (MLI) has been recognized
as an attractive topology for high voltage DC-AC
conversion. Function of a multilevel inverter is to synthesize
a desired voltage wave shape from several levels of DC
voltages. But the main drawback of MLI is its output voltage
amplitude is limited to DC sources voltage summation. To
overcome this limitation, a five-level cascaded multilevel
inverter based Z-source inverter has been proposed in this
paper. In the proposed topology output voltage amplitude
can be boosted with Z network shoot-through (ST) state
control. This paper focuses on third harmonic injection
pulse width modulation (PWM) strategy for the proposed
topology. Performance parameters of Z -Source MLI have
been analyzed for third harmonic injection modulation
strategy. A simulation model of conventional cascaded
multilevel inverter and Z-source MLI has been built in
MATLAB/SIMULINK and its performance has been
analyzed.
The Z-source inverter utilizes Z impedance network
between the DC source and inverter circuitry to achieve
buck-boost operation. The Z-Source inverter utilizes
shoot-through state to boost the input dc voltage of
inverter switches when both switches in the same phase
leg are on. The Z-Source inverters with respect to
traditional inverters are lower costs, reliable, lower
complexity and higher efficiency.
In this paper, a cascaded five level Z-Source inverter is
proposed for renewable energy systems and it employs Z
network between the DC source and inverter circuitry to
achieve boost operation. The output voltage of proposed
inverter can be controlled using modulation index and
shoot through state.
Cascaded Z–Source Multilevel inverter is analysed
with third harmonic injection PWM technique.
Performance parameters have been analysed for cascaded
Z-Source MLI. The performance of Z-Source Multilevel
inverter is compared with the conventional cascaded
multilevel inverter. Simulation of the circuit
configurations
have
been
performed
in
MATLAB/SIMULINK.
Keyword —Multi Level Inverter, Total Harmonic
Distortion, Pulse Width Modulation, Shoot-Through, BuckBoost.
I. INTRODUCTION
In recent years, the multilevel voltage inverter is
widely used in high power applications such as large
induction motor drives, UPS systems and Flexible AC
Transmission Systems (FACTS). Multilevel inverter
obtains a desired output voltage from several levels of
input DC voltage sources. With an increasing number of
DC voltage sources, the inverter voltage output waveform
level increases. The multilevel inverters have more
advantages which include lower semiconductor voltage
stress, better harmonic performance, low Electro
Magnetic Interference (EMI) and lower switching losses.
Despite these advantages, multilevel inverters output
voltage amplitude is limited to DC sources voltage
summation. An intermediate DC to DC converter is
required for the boost or buck operation of MLI output
voltage. Occurring of short circuit can destroy multilevel
inverters. To solve these problems, multilevel inverter
based Z-source inverter is proposed in this paper.
II. CASCADED MULTILEVEL Z-SOURCE INVERTER
The circuit diagram of cascaded Z-Source five-level
inverter is shown in Fig. 1.It consists of series singlephase H-bridge inverter units, Z impedances and DC
voltage sources. DC sources can be obtained from
batteries, fuel cells, solar cells [1]. Each H-bridge ZSource inverter can generate three different output
voltage +Vin, 0, -Vin. Output voltage can be higher than
the input voltage when boost factor, B>1. This topology
has an extra switching state: shoot through state as
compared to cascaded H-bridge inverters. During the
shoot-through state, the output voltages of Z networks are
zero.
______________________________________________
__
1
PG student (Power Electronics and Drives), EEE Dept.
Associate Professor, EEE Dept.
3
Professor, EEE Dept.
SSN College of Engineering, Kalavakkam, Chennai
Affiliated to Anna University,Chennai ,India – 603 110
1
shan30121988@gmail.com, 2seyezhair@ssn.edu.in
2
Fig.1 Circuit diagram of 5-Level cascaded Z-Source MLI
143
ISSN 2249-6343
International Journal of Computer Technology and Electronics Engineering (IJCTEE)
Volume 2, Issue 1
The sinusoidal reference signal can be injected by a
third harmonic with a magnitude equal to 25% of the
fundamental. This method employs two straight lines that
are greater than or less than the peak value of the
reference signal to control the shoot-through duty ratio.
Inverter operates in shoot-through whenever the
triangular carrier signal is higher than the positive straight
line or lower than the negative straight line [7]. The
frequency of the modulating signal is taken as 50Hz.The
frequency of the triangular signal can be calculated by
Frequency modulation index, mf which is given by,
TABLE I:CONDUCTION TABLE
VOLTAGE LEVEL
VOLTAGE
ON SWITCHES
OUTPUT
Level 2 (non shoot-
2Vin
S3,S4,S5 ,S6
Vin
S1,S3,S5 ,S6
Level 1 (shoot-through)
Vin
S1,S2, S3, S4, S5, S6
Level 1 (non shoot-
Vin
S3,S4,S5 ,S7
through)
Level 1 (non shootthrough)
through)
Level 1 (shoot-through)
Vin
mf 
S3,S4,S5 ,S6, S7, S8
Level 0 (zero state)
0 (V)
S1,S3,S5 ,S7
Level 0 (shoot-through)
0 (V)
S1,S2,S3 ,S4,S5,S7
Level 0 (shoot-through)
0 (V)
S1,S3,S5 ,S6,S7,S8
-Vin
S1,S3,S7 ,S8
Level -1 (shoot-through)
-Vin
S1, S2, S3, S4, S7,S8
Level -1 (non shoot-
-Vin
S1,S2,S5 ,S7
Level -1 (shoot-through)
-Vin
S1, S2, S5, S6, S7,S8
Level -2 (non shoot-
-2Vin
S1,S2,S7,S8
(1)
Where fc is the frequency of the carrier signal and fo is the
frequency of sinusoidal and modulating signals. Output
voltage depends on the boost factor [8],
B
Level -1 (non shoot-
fc
fo
1
through)
1

2(Vca  V p )
Vca
1
2T
1  sh
T
(2)
where ,
Vca - Peak value of the triangular waveform
Vp - Amplitude of the constant
Tsh - Total shoot-through state period
T - Period of switching
through)
through)
The reference and carrier waveform for the
proposed third harmonic injection PWM is shown in
Fig. 2 followed by the gating pattern shown in Fig. 3.
Gating pattern has been generated using
MATLAB/SIMULINK.
A. Circuit Operation
Circuit operation consists of two modes namely shoot
-through and non shoot-through states [2]. In shootthrough (ST) switching state of Z-Source MLI, upper and
lower bridges of the same leg is turned on having the
output voltage of zero .During non shoot-through state
opposite pairs of legs of both the bridge conducts. In ST
state the two inductors are
being charged by the capacitors and in non shoot-through
state the inductors and input DC source transfer energy to
the capacitors and load [3]. Conduction table is shown in
Table I.
III. THIRD HARMONIC INJECTION PWM
Among the several PWM techniques reported in the
literatures [4]-[6], the method proposed in this paper is
third harmonic injection PWM technique. In this method
four phase shifted carrier triangular signals are compared
with one modulating signal to produce switching PWM
pulses. The reference modulating signal is not sinusoidal
but consists of both fundamental component and third
harmonic component.
Fig. 2 Third harmonic injection PWM technique
144
ISSN 2249-6343
International Journal of Computer Technology and Electronics Engineering (IJCTEE)
Volume 2, Issue 1
Fig.5 Load voltage waveform without filter
Fig.3 PWM pulse generation for Third harmonic injection PWM
technique
IV. SIMULATION RESULTS
A. Matlab Simulation Circuit
Fig.4 shows Matlab/ Simulink of Z-Source cascaded
MLI using Third harmonic Injection PWM with Boost
factor = 1.25, ma=0.8, RL Load where R=50Ω and
L=24mH, Input voltage Vdc=75V, Z impedances,
L1=L2=L3=L4=L=40mH and C1= C2= C3= C4=
6600µF .Simulink circuit is shown with LC filter having
L=30mH and C=150µF.This LC filter can act as an
electrical resonator, an electrical analogue of a tuning
fork, storing electrical energy oscillating at the
circuit's resonant frequency.
Fig. 6 FFT spectrum of load voltage waveform without filter
Fig.7 Load voltage waveform with filter
Fig. 4 MATLAB circuit of Z-Source cascaded MLI using
Third harmonic injection PWM
Fig.5 and Fig.6 shows the load voltage waveform
without filter and its FFT spectrum . Fig.7 and Fig.8
shows the load voltage waveform with filter and its
FFT spectrum respectively Figs.9 &10 show the load
current waveform with filter and its FFT spectrum.
Fig. 8 FFT spectrum of load voltage waveform with filter
145
ISSN 2249-6343
International Journal of Computer Technology and Electronics Engineering (IJCTEE)
Volume 2, Issue 1
Fig.9 Load current waveform with filter
Fig. 11 Effect of modulation index on Total Harmonic Distortion
for Third harmonic injection technique
B.
Inductor Current Ripple
Inductor current ripple is calculated across the
inductor of Z network of the inverter and it is
shown in Table II.
TABLE II
EFFECT OF MODULATION INDEX ON INDUCTOR CURRENT RIPPLE
Fig. 10 FFT spectrum of load current waveform with filter
S.No
ma
1
0.8
Inductor current ripple for
Third harmonic injection
technique
0.5000
2
0.73
0.5631
3
0.67
0.7000
4
0.56
1.2000
C.
Capacitor voltage ripple
Capacitor voltage ripple is calculated across the
capacitor of Z network of the inverter and it is
shown in Table III.
V. PERFORMANCE PARAMETERS OF CASCADED
Z-SOURCE MLI
Performance parameters of Z -Source multilevel
Inverter are analysed for various modulation index for
given boost factor. They are Total Harmonic Distortion,
inductor current ripple of Z-Source inverter, capacitor
voltage ripple of Z-Source inverter, voltage gain and
voltage stress [9],[10].
TABLE III
EFFECT OF MODULATION INDEX ON CAPACITOR VOLTAGE RIPPLE
A.
Total Harmonic Distortion(THD)
THD of a signal is a measurement of the harmonic
distortion present and is defined as the ratio of the sum
of the powers of all harmonic components to the power
of the fundamental frequency. THD is calculated for
various modulation index values and the comparison is
shown in Fig 11.From the figure it is shown that THD
is low for the chosen ma of 0.8.
146
S.No
ma
Capacitor voltage ripple for
Third harmonic injection
technique
1
0.8
0.0188
2
0.73
0.0120
3
0.67
0.0091
4
0.56
0.0026
ISSN 2249-6343
International Journal of Computer Technology and Electronics Engineering (IJCTEE)
Volume 2, Issue 1
D. Voltage Gain(G)
Voltage gain, G is calculated for third harmonic
injection PWM technique. In Fig.12 Voltage gain is
compared with different modulation indices and it can be
seen that voltage gain is higher for chosen ma=0.8.
Voltage gain, G is given by,
G
2Vac
Vin
VI. COMPARISON OF FIVE LEVEL Z-SOURCE MULTILEVEL
INVERTER WITH CONVENTIONAL MULTILEVEL
INVERTER
Total Harmonic Distortion and output voltage of
five level Z-Source MLI is compared with five level
conventional cascaded MLI. It is found that THD value
of Z-Source MLI is more than cascaded MLI [12], [13].
(3)
A.
where, Vac=RMS value of output voltage
Vin=Input voltage
Matlab Simulation Circuit
Fig.14 shows Matlab/ Simulink circuit of cascaded
MLI. Simulations are implemented with ma=0.8, RL
Load where R=50Ω and L=24mH and Input voltage
Vdc=75V. Simulink circuit is shown with LC filter
having L=30mH and C=150µF
Fig. 12 Effect of modulation index on voltage gain for
Third harmonic injection technique
E. Voltage Stress
Voltage Stress is calculated from voltage gain which
is shown in Fig 13. Voltage stress increases with voltage
gain. Voltage stress is given by[11],
Voltage Stress = 2 
1
G
Fig. 14 Simulink circuit of cascaded MLI
(4)
B.
where, G=Voltage gain
Simulation Results of Cascaded MLI
Fig.15 and Fig.16 shows the load voltage
waveform without and with filter respectively. Fig.17
shows the load current waveform with filter.
Fig.13 Effect of voltage gain on voltage stress for Third harmonic
injection PWM technique
Fig.15 Load voltage waveform without filter
147
ISSN 2249-6343
International Journal of Computer Technology and Electronics Engineering (IJCTEE)
Volume 2, Issue 1
VII.
CONCLUSION
This paper has investigated a Z-source cascaded
multilevel inverter. Z-Source cascaded multilevel inverter
gives higher output voltage through its Z network. Boost
operation is achieved by the shoot through state of ZSource MLI. Unipolar PWM technique has been
employed for Z-MLI which gives a reduced voltage
stress. The performance of the proposed Z-MLI has been
compared with the conventional MLI. From the results, it
is found that Z-MLI with third harmonic injection PWM
provides a higher RMS value of the output voltage, higher
voltage gain, reduced voltage stress and avoids the
intermediate boost DC-DC converter.
ACKNOWLEDGMENT
Fig.16 Load voltage waveform with filter
The authors wish to thank the management of SSN
institutions for providing the computational facilities to
carry out this work.
REFERENCES
[1]
[2]
Fig.17 Load current waveform with filter
[3]
C. Output Voltage Comparison
Output voltage and other parameters are compared
between Z-Source MLI and conventional cascaded MLI
for third harmonic injection PWM technique. This
comparison is shown in Table IV.
[4]
[5]
TABLE IV
COMPARISON BETWEEN Z-SOURCE CASCADED MLI AND
CONVENTINAL CASCADED MLI
PARAMETERS
Z-SOURCE
CONVENTIONAL
CASCADED MLI
CASCADED MLI
Modulation index
0.8
0.8
RMS value of
219.7
[6]
[7]
[8]
142.36
fundamental output
[9]
voltage(V)
RMS value of
217.8
141.42
Boost factor
1.25
1
THD(% with filter)
1.76
1.32
output voltage(V)
[10]
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needs two converters for boosting the output voltage [14],
[15].
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Volume 2, Issue 1
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AUTHOR’S BIOGRAPHY
Dr.R.Seyezhai obtained her B.E. (Electronics &
Communication Engineering) from Noorul Islam
College of Engineering, Nagercoil in 1996 and
her M.E in Power Electronics & Drives from
Shanmugha College of Engineering, Thanjavur
in 1998 and Ph.D from Anna University,
Chennai, in 2010. She has been working in the
teaching field for about 13 Years. She has
published 75 papers in the area of Power
Electronics & Drives. Her areas of interest
include SiC Power Devices & Multilevel
Inverters.
Dr.B.L.Mathur obtained his B.E. (Electrical
Engineering) from University of Rajasthan, in
1962 and his M.Tech in Power Systems from
IIT, Bombay in 1964.He completed his Ph.D. in
1979 from IISc, Bangalore. His Ph.D. thesis was
adjudged as the best for application to industries
in the year 1979 and won gold medal. He has
been working in the teaching field for about 44
Years. He takes immense interest in designing
Electronic circuits. He has published 50 papers
in National and International journals and 85 in
National and International conferences. His
areas of interest include Power Devices, Power
Converters, Computer Architecture and FACTS.
A.Shanmuga priyaa obtained her B.E.
(Electrical & Electronics Engineering) from
St.Josephs College of Engineering, chennai in
2010 and presently doing her IInd year M.E in
Power Electronics & Drives in SSN College of
Engineering, Chennai.Her areas of interest
include Power electronics & Multilevel
Inverters.
.
149