International Journal of Electrical Engineering & Technology (IJEET)
Volume 7, Issue 1, Jan-Feb, 2016, pp.17-29, Article ID: IJEET_07_01_002
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ENHANCEMENT OF FUNDAMENTAL RMS
OUTPUT VOLTAGE OF 5-LEVEL
CASCADED H-BRIDGE MULTILEVEL
INVERTER USING MODIFIED MULTICARRIER PWM TECHNIQUE
CHINMAYI
Asst. Prof., Department of Electrical and Electronics Engineering,
East West Institute of Technology, Bengaluru, Research Scholar,
RRC-ECE, JSSATEB, VTU, Belagavi, India
Dr. B.G. SHIVALEELAVATHI
Professor, Department of Electronics and Communication Engineering,
JSSATE, VTU, Bengaluru, India
ABSTRACT
Cascaded H-bridge Multilevel Inverter (CHBMLI) is the most suitable
topology for the PV power converters. In this paper an effort has been made to
increase the performance of CHBMLI by improving the fundamental Root
Mean Square (RMS) value of the output voltage. This work proposes a
Modified Multi Carrier PWM (MMCPWM) technique where, reference sine
wave has been replaced by ellipse wave, resulting in enhanced performances
on the fundamental rms output voltage and lower Total Harmonic Distortion
(THD). Analysis of single phase 5-level CHBMLI with and without load are
carried for the different Multi Carrier PWM (MCPWM) techniques. Results
were compared for both MCPWM and MMCPWM at different modulation
indices. The proposed MMCPWM technique emerged as a very promising
technique in enhancing the fundamental output voltage and at the same time
mitigating the problem of THD. 5-level CHBMLI with the proposed control
strategy is simulated in MATLAB/SIMULINK. The results were compared with
the existing literature for validation of the proposed control strategy.
Key words: Cascaded H-Bridge Multilevel Inverter (CHBMLI), Modified
Multi Carrier PWM (MMCPWM), Multi Carrier PWM (MCPWM), Phase
Disposition (PDPWM), Phase Shifted PWM (PSPWM), Ellipse wave.
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Chinmayi and Dr. B.G. Shivaleelavathi
Cite this Article: Chinmayi and Dr. B.G. Shivaleelavathi, Enhancement of
Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge Multilevel
Inverter Using Modified Multi-Carrier PWM Technique. International
Journal of Electrical Engineering & Technology, 7(1), 2016, pp. 17-29.
http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=7&IType=1
1. INTRODUCTION
The standard of living of a given country can be directly related to per capita energy
consumption. The per capita energy consumption is a measure of the prosperity of the
nation. Solar energy has the greatest potential of all the resources of renewable energy
and it is the most important supplies of energy especially when the other sources in
the country have depleted [1].
Therefore, the research need to be carried out to increase the efficiency of PV
power generation and to minimize the system cost. In this regard the inverters used to
convert power from DC to AC in solar power generation is having a very important
role to mitigate the problem of non sinusoidal output, high THD, high switching stress
and more number of switches. Multilevel inverters are the most promising in
overcoming the above problems. The advantages of MLI are:



Multilevel converters not only can generate the output voltages with very low
distortions, but also can reduce the dv/dt stresses.
Multilevel converters can draw input current with low distortion.
They can operate at both fundamental switching frequency and high switching
frequency PWM.
MLI topologies are classified into 3 types: Diode clamped inverters, Flying
capacitor inverters and Cascaded inverters [2].
In first two types for high levels, more number of diodes and capacitors are
required respectively so hence, the circuit will be bulky. And these two topologies
suits for single DC source input.
For a solar PV application CHBMLI are best suitable as it requires separate DC
sources for the real power conversion. From [2] [3] [4] and [5] CHBMLI is the best
suitable for solar PV application. The advantages of CHBMLI are:




CHBMLI uses less number of components compare to first two topology of MLI
No need of extra diodes and capacitors.
No voltage unbalancing problem compared to other two topology.
It has modular structure.

The transformers can be eliminated and this helps in enhancing the efficiency and
cost effectiveness.
Additional features such as its battery management capability, redundant
switching states in inverter operation and scalability make the CHBMLI the MLI of
choice [3]-[5].
The selection of appropriate PWM technique has a greater role in producing
quality output voltage and current for inverters. The main modulation techniques used
in MLI are: Multilevel Sinusoidal PWM (carrier based PWM), Space Vector PWM
and Multilevel Selective Harmonic Elimination (MSHE).For controlling the output
voltage of CHBMLI several types of modulation techniques have been proposed in
the literature, namely multistep or stair case modulation techniques, Multi Carrier
PWM (MCPWM) techniques and space vector modulation technique [4] and [6].
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Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge
Multilevel Inverter Using Modified Multi-Carrier PWM Technique
The general principle of a MCPWM technique is the comparison of a sinusoidal
waveform with a carrier waveform, this typically being a triangular waveform. The
Carrier frequency depends on the switching frequency of the converter and the
elimination of high order harmonic components of the output voltage. The multi
carrier techniques are divided into the following categories [7]:

Level shifted PWM (LSPWM): Depending on the phase relation between the
individual carriers, there are three variants in LSPWM: Alternative Phase Opposition
Disposition (APOD), Phase Opposition Disposition (POD) and Phase Disposition
(PD) [8].
The unequal device conduction periods of the LSPWM technique has resulted as
main disadvantage particularly, in photovoltaic power generation. To overcome this
problem many techniques are discussed in literature [8]-[12].

Phase Shifted PWM (PSPWM): PSPWM technique is generally used modulation
technique in CHBMLI, as it offers even power distribution among the modules and
results in uniform utilization of inverter switches within a module [12].
In this paper, a MMCPWM technique is proposed, where in, the conventional sine
wave reference is been replaced by an ellipse wave. The simulation was carried out
for MCPWM and MMCPWM techniques on single phase 5-level CHBMLI. It is
found that, for the same CHBMLI, the overall performance of MMCPWM techniques
is superior compared to conventional MCPWM techniques, in terms of fundamental
output voltage and total harmonic distortions.
2. CASCADED H-BRIDGE INVERTER TOPOLOGY
2.1. 5 level - CHBMLI
The single phase cascaded five level inverter topology is as shown in Fig.1a and the
corresponding output voltage waveform in Fig. 1b. The circuit consists of eight main
switches in two series connected H-bridge configuration S1 to S4, and S5 to S8. The
number of DC sources are two, so the output voltage of the CHBMLI is given by (1):
Vo= V1+V2
(1)
Table I shows the switching sequence of devices and the respective output
voltages.
(a)
(b)
Figure 1 a) 5-Level CHBMLI b) Output Voltage Waveform
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Chinmayi and Dr. B.G. Shivaleelavathi
Table I Switching Sequence of 5-level CHBMLI.
Switches turn on
Output Voltage level
S1, S2
+Vdc
S1,S2,S5, S6
+2Vdc
S4,S2,S8,S6
0
S3,S4
-Vdc
S3,S4,S7,S8
-2Vdc
2.2. Multi Carrier Modulation Techniques
The principle of the MCPWM technique is based on a comparison of a sinusoidal
reference waveform with triangular carrier waveforms. For n-level inverter, (n-1)
carriers are required to compare. The carriers are continuous bands around the
reference zero. They have the same amplitude, Ac and the same frequency, fc. The sine
reference waveform has a frequency fr and amplitude of Af. Comparison at each time
will generate a high if carrier signal is greater than sine else a low [7]. The amplitude
modulation index ma and modulation frequency m f can be given by:
(2)
(for PD, POD, APOD) and
(3)
(for PSPWM)
(4)
The different types of MCPWM techniques are presented in [7] - [11]. Comparing
them [7]-[12] for suitability with CHBMLI, the following are considered for the
analysis:



PSPWM: The carriers are phase shifted by 360/(n-1), where n is the output levels in
the MLI. For a 5 level-CHBMLI, the four carriers are phase shifted by 90° each and
compared with reference wave having a frequency of fundamental output voltage.
PDPWM: The (n-1) carriers of equal amplitude and frequency are in phase but
shifted vertically for n-level MLI. For 5-level- CHBMLI, four carriers are vertically
shifted and they will be in phase.
APODPWM: Similar to PDPWM, but the (n-1) carriers are phase displaced from
another by 180° alternatively.
2.3. Proposed Modulation Technique
In conventional method of MCPWM technique, the sine wave with the fundamental
frequency is applied as reference wave. The expression for the sine wave is given by,
Y= amp * sin (2*pi*f*t)
(5)
Where in the (5), amp is the amplitude and f is the frequency of the sine wave.
In the proposed MMCPWM modulation technique, the ellipse wave is compared
with carrier wave. The expressions used to generate the ellipse wave are given by:
X= x(i) + a * cos(t)
(6)
Y= y(i) + amp * sin(t)
(7)
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Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge
Multilevel Inverter Using Modified Multi-Carrier PWM Technique
Where in the (6) and (7), a is the radius of the ellipse which gives the frequency
term and amp is the amplitude of the ellipse wave.
Fig. 2 shows the comparison between the sine and ellipse wave with same
amplitude and frequency. The shape of the ellipse towards the peaks is wider than
sine, resulting in cutting more carrier waves. This increases the width of switching
pulses in the upper and lower peak of the output voltages, in turn reducing THD and
enhancing the rms of the fundamental output voltage. The improvement of output
voltage is achieved without entering the over modulation range.
Figure 2 Comparison of Sine and Ellipse wave
3. PERFORMANCE COMPARISON OF PWM TECHNIQUES
To analyze the different MCPWM techniques, single phase 5 - level CHBMLI with
R-L load is simulated in MATLAB/SIMULINK software platform. Both PSPWM
and LSPWM techniques were simulated.
In PDPWM and APODPWM techniques as the carriers are vertically shifted, the
width of the switching pulses is less at the peak of the fundamental output voltage and
also the switching frequency is almost equal in all intervals as shown in Fig. 3a. This
result in reducing the fundamental rms output voltage and increasing THD compare to
PSPWM technique. In PSPWM technique as the carriers are phase shifted, there will
be more overlapping of carrier waves leading to more switching width at peak of both
the half cycle and less in between of the fundamental output voltage as shown in Fig.
3b. The Switching pulses of PDPWM technique using sine and ellipse wave are
shown in Fig. 4a and Fig. 4b respectively, which clearly shows more switching pulse
width at peak of the reference wave in ellipse case.
(a)
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Chinmayi and Dr. B.G. Shivaleelavathi
(b)
Figure 3 Comparison of Sine and Ellipse wave for 5- level CHBMLI a) for PDPWM
technique b) for PSPWM technique
(a)
(b)
Figure 4 PDPWM Switching pulses for 5-level CHBMLI with a) Sine wave as
reference b) Ellipse wave as reference
4. PERFORMANCE COMPARISON OF OUTPUT VOLTAGE
The different modulation techniques discussed in the previous section were applied to
5- level CHBMLI and simulation was carried out using MATLAB/SIMULINK
platform. Fig. 5 shows a single phase 5 level CHBMLI simulation circuit to which an
RL-load is connected. The circuit parameters details are given in Table. II. First , all
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Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge
Multilevel Inverter Using Modified Multi-Carrier PWM Technique
the PWM techniques were simulated with Sine as reference wave having a frequency
of 50Hz. Carrier frequency was set for 1350Hz, keeping Mf = 27. PSPWM gives
better performance compared to PDPWM and APODPWM technique. The analysis
for %THD is noted by varying the modulation indices from 0.6 to 1.
Figure 5 Simulation circuit of 5-level CHBMLI
TABLE II Circuit Parameters of 5-level CHBMLI
Simulation Parameter
Values
DC Voltage, Vdc
115V
Reference frequency ( fundamental output frequency)
50Hz
Carrier frequency
1350Hz
100Ω and 5mH
RL-Load
Filter inductance
50mH
Filter capacitance
40µF
The proposed MMCPWM technique was verified by replacing the reference sine
wave to ellipse wave. In all the cases, the proposed MMCPWM result in enhanced
output voltage as it produces more switching pulse width at both the peak of output
voltage. Even the total harmonic distortion has reduced comparatively. The output
voltages were compared without load and with RL-load for PDPWM technique for
both sine and ellipse as reference on a single window as shown in Fig. 6 and Fig. 7. It
shows that the MMCPWM technique produces more peak output voltage compare to
MCPWM technique. Henceforth the RMS output voltage is comparatively more in
proposed technique.
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Chinmayi and Dr. B.G. Shivaleelavathi
(a)
(b)
Figure 6 a) No-Load output Voltage of 5-level- CHBMLI with MCPWM and
MMCPWM in a single scope b) Comparison of output voltages for a single cycle
showing the more output width of the MMCPWM
Figure 7 Output voltage of PDPWM fed 5-level CHBMLI with RL-Load.
(Comparison of Sine and Ellipse wave performance)
Fig. 8a and Fig. 8b shows the simulated output voltage for the Modified PDPWM
and Modified PSPWM techniques without load. Fig.9 shows the output voltage and
current waveform for 5-level CHBMLI with Modified PDPWM technique. The
corresponding FFT analysis of the output voltages for PDPWM and PSPWM
technique are shown in Fig. 10 and Fig. 11 respectively. The FFT analysis of the
output voltage for Modified PDPWM and Modified PSPWM are shown in Fig.12 and
Fig.13 respectively. The %THD has reduced in the proposed MMCPWM technique
for PDPWM. It is very clear from the figures, that the 3 rd harmonic component is
contributing more to THD. Fig. 14 shows the FFT list for fundamental output voltage,
peak output voltage with the corresponding %THD for PSPWM and Modified
PSPWM technique. The RMS output voltage and %THD is much better from
modulation indices of 0.8 onwards in Modified PSPWM technique.
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Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge
Multilevel Inverter Using Modified Multi-Carrier PWM Technique
(a)
(b)
Figure 8 CHBMLI No-Load Output Voltage waveform with Ellipse reference for
a) PDPWM technique b) PSPWM technique
Figure 9 Output Voltage and Current for PDPWM CHBMLI with Ellipse reference
Figure 10 FFT Spectrum of output Voltage
Voltage for PDPWM with Sine wave
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Figure 11 FFT Spectrum of Output
for PSPWM with Sine wave
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Chinmayi and Dr. B.G. Shivaleelavathi
Figure 12 FFT Spectrum of Output Voltage Figure 13 FFT Spectrum of Output
Voltage for PDPWM with Ellipse wave
for PSPWM with Ellipse wave
(a)
(b)
Figure 14 a) FFT List for Ellipse wave and b) FFT List for Sine wave for PSPWM
with ma = 0.8
The simulated fundamental RMS output voltage and % THD for different
modulation indices has been tabulated in Table III. The proposed MMCPWM
technique have improved performance in all the types of PWM techniques compare to
MCPWM technique. For PSPWM of MCPWM technique at ma = 1, %THD is low. In
MMCPWM technique the %THD is not increasing much with reduction in
modulation indices, which shows the superior performance of the proposed technique.
MMCPWM –APOD technique have much reduced harmonics compare to MCPWM –
APOD. The fundamental RMS output voltage is enhanced in all types of MMCPWM
technique. Fig.15-18 shows the graph plot for fundamental output voltage against
modulation indices in PS and PDPWM technique, where, it shows the excellent
performance of the proposed MMCPWM technique.
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Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge
Multilevel Inverter Using Modified Multi-Carrier PWM Technique
TABLE III Performance Comparison of MCPWM AND MMCPWM TECHNIQUE
for 5-level CHBMLI with RL-Load
FIVE
LEVEL
SINE WAVE
(MCPWM)
Output
%
Voltage
THD
147.4
13.08
102.8
14.68
56.81
17.5
13.18
12.01
144.8
8.55
101.4
11.36
56.85
15.04
20.74
18.87
146.6
15.43
102.2
17.02
56.52
19.53
13.15
14.6
Ma
1
0.8
0.6
0.4
1
0.8
0.6
0.4
1
0.8
0.6
0.4
PD
PWM
PS
PWM
APOD
PWM
ELLIPSE WAVE
(MMCPWM)
Output
%
Voltage
THD
163.3
10.65
114.2
11.39
64.21
11.65
16.32
5.66
163.2
10.39
113.8
9.93
64.03
9.87
23.95
11.76
163.4
11.01
114.3
11.15
63.97
11.16
15.72
5.64
%THD Vs Modulation Index
for PDPWM
200
20
150
15
100
%THD
RMS Phase Voltage
RMS Output Voltage Vs Modulation
Index for PDPWM
Sine
5
Ellipse
50
Sine 5level
10
Ellipse 5
level
0
0
0.4 0.6 0.8
0.4
0.6
0.8
1
Ma
Ma
Figure 15 Comparison of RMS output
voltage VS Modulation indices
Figure 16 Comparison of % THD Vs
Modulation indices
%THD Vs Modulation Index
PSPWM
RMS Output Voltage Vs
Modulation Index
PSPWM
20
200
15
150
100
%THD
RMS Phase Voltage
1
Sine
50
5
Ellipse
0
Ellipse 5
level
0
0.4 0.6 0.8 1
0.4 0.6 0.8
Ma
1
Ma
Figure 17 Comparison of RMS output
Voltage Vs Modulation indices
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Sine 5level
10
Figure 18 Comparison of THD Vs
Modulation indices
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Chinmayi and Dr. B.G. Shivaleelavathi
4. CONCLUSION
This paper investigates the performance analysis of three MCPWM and MMCPWM
techniques namely PSPWM, APODPWM and PDPWM for single phase 5-level
CHBMLI. The performance evaluation is carried out in terms of RMS output voltage
and %THD. Control strategy was first verified for its functioning on
MATLAB/SIMULINK software. It is found that for the same circuit parameters, the
overall performance of MMCPWM is superior to that MCPWM technique as it is
producing more fundamental RMS output voltage and hence increasing the DC bus
utilization and reducing %THD. Further, a suitable technique may be developed to
improve the performance with respect to %THD by eliminating third harmonic
component. Hence the proposed MMCPWM technique is a most promising technique
to improve the performance of a CHBMLI for a PV Power generation in renewable
energy source.
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Multilevel Inverter Using Modified Multi-Carrier PWM Technique
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