Simulation and study of multilevel inverter (report)

SIMULATION AND STUDY OF MULTILEVEL
INVERTER
A PROJECT REPORT
Submitted in partial fulfillment of the
requirements for the award of the degree
of
BACHELOR OF TECHNOLOGY
DEPARTMENT OF ELECTRICAL ENGINEERING
ii

ABSTRACT
Nowadays multilevel inverters are a very attractive solution for medium-voltage high-
power conversion applications; they convert DC power to AC power at required output
voltage and frequency level. Three-phase Multi Level Inverter (MLI) are used in many
medium and high-power applications such as motor drives and grid connected systems.
There are numerous Pulse Width Modulation (PWM) techniques for MLIs. In this project,
a three phase five level inverter has been simulated using Sinusoidal PWM technique using
MATLAB/Simulink. This technique is recommended to improve the performance of
inverter and to eliminate the filtering requirements.
The topology used in this case is cascaded H Bridge. Each H bridge is supplied by
a separate DC source. And the DC voltages used here are identical, so it is called as a
cascaded multilevel inverter. If the voltages used are different then the circuit can be called
as a hybrid multilevel inverter.
The multilevel inverter has been used to reduce the harmonic contents. The inverter
with a large number of steps can generate a high-quality output waveform, which has been
compared with the earlier simulated model of three level inverter.
Iii

TABLE OF CONTENTS
Declaration i
Acknowledgement ii
Abstract iii
Chapter 1 Page No.
INTRODUCTION
1.1 Literature Review 2-4
1.2 Objectives 4
1.3 Problem Formulation 4
1.4 Solution Methodologies 5
1.5 Pulse Width Modulation 5-7
Chapter 2
INVERTER
2.1 Introduction to Inverter 8
2.2 Concept of Multi Level Inverter 8-9
2.3 Multilevel Inverter Topologies 9-13
2.4 Three level Inverter 13
2.4.1 Simulation Model 14
2.4.2 Simulation Results 15
2.4.3 FFT Analysis 16
Chapter 3
ProjectDescription
3.1 Five level Inverter 17
3.2 Simulation Model 17
3.3 Simulation Results 19
3.4 Advantages 20-21
3.5 Disadvantages 21
Chapter 4
CONCLUSION AND FUTURE SCOPE
4.1 Comparison 22
4.2 Conclusion & Discussion 22
4.3 Future scope 22-23
REFERENCES 24

LIST OF FIGURES
Figure No. Title Page No.
1 PWM Technique 6
2 Diode Clamped MLI 11
3 Flying Capacitor MLI 12
4 Cascaded H-Bridge MLI 13
5 Circuit Design for 3 Phase 3 Level MLI 14
6 Switching Scheme for 3 Phase 3 Level MLI 14
7 Output Voltage Waveform for 3 Level MLI 15
8 FFT Analysis of Voltage Signal 16
9 Simulation Model of 3 Phase 5 Level MLI 17
10 Switching Scheme for 3 Phase 5 Level MLI 18
11 Output Voltage Waveform for 5 Level MLI 19
12 FFT Analysis of Voltage Signal 20

1
CHAPTER 1
INTRODUCTION
An inverter is an electrical device which converts the DC voltage into AC voltage so that it
can be used by common appliances. The inverter output can be single phase or polyphase and
can have sine wave, square wave, PWM wave, stepped wave or quasi square wave at the
output. Voltage-fed converters are extensively used and found application in AC motor drives,
AC Uninterruptable Power Supplies (UPS) etc.
When AC loads are fed through inverters it is required that the output voltage of desired
magnitude and frequency be achieved. A variable output voltage can be obtained by varying
the input dc voltage. On the other hand, if the dc input voltage is fixed and it is not controllable,
a variable output voltage can be obtained by varying the gain of the inverter, which is normally
accomplished by pulse-width-modulation (PWM) control within the inverter.
A two-level Inverter creates three different voltages for the load i.e. suppose we are
providing Vdc as an input to a two-level inverter then it will provide + Vdc/2 and – Vdc/2 on
output. In order to build an AC voltage, these three newly generated voltages are usually
switched. Although this method of creating AC is effective but it has few drawbacks as it
creates harmonic distortions in the output voltage and also has a high dv/dt as compared to
that of a multilevel inverter.
The concept of multilevel Inverter (MLI) is kind of modification of two-level inverter.
In multilevel inverters we don’t deal with the two-level voltages instead in order to create a
smoother stepped output waveform, more than two voltage levels are combined together and
the output waveform obtained in this case has lower dv/dt and also lower harmonic distortions.
The inverters which produce an output voltage or a current with levels either 0, +V,-V, +V/2,-
V/2 are known as five level inverters. In high-power and high-voltage applications two-level
inverters however have some limitations in operating at high frequency mainly due to
switching losses and constraints of device rating. This is where multilevel inverters are
advantageous.

2
1.1 LITERATURE REVIEW
Sr. No Paper Author Name Year Contribution Gap/Scope
1 Generalized
structure of a
Multilevel
PWM inverter,
Pradeep M.
Bhagwat
1983 A generalized
structure of a
multilevel
voltage source
thyristor
inverter is
proposed.
Multilevel
Inverter
Topologies
2 High
performance
current control
techniques for
applications to
multilevel
high-power
voltage source
inverters,
university of
Genova (Italy)
M.
Marchesoni
1989 High-
performance
current control
techniques
have been
developed
Extension to
three phase
system is
possible.
3 Cascade
Multilevel
Inverters for
Utility
Applications,
Oak Ridge
National
Laboratory,
Tennessee
F. Z. Peng, J.
W. McKeever
& D. J. Adams
1997 Cascade
multilevel
inverters have
been
developed
Lower output
voltage rating
than
conventional
multilevel
Inverter, that
can be raised
further.
.4 Novel
Multilevel
Inverter
Carrier-Based
PWM
Methods,
School of
Electrical and
Leon M.
Tolbert
& Thomas G.
Habetler
1998 Two novel
carrier-based
multilevel
PWM schemes
are presented
This topology
can be used to
enable better
switch
utilization.

3
Computer
Engineering
Atlanta
5 Extension of
PWM Space
Vector
Technique for
Voltage Source
Inverters
Multilevel
Current-
Controlled
J. Mahdavi',
A. Agah, A.
M. Ranjbar &
H. A. Toliyat'
1999 A current
control
method using
SV for voltage
source
inverters was
discussed
The proposed
method can be
implemented by
application of
microprocessor-
based
controllers.
6 Multilevel
Inverter with
Series
Connection of
H-Bridge
Cells, Power
Electronics
Research
laboratory,
National
Yunlin
University of
Science and
Technology
Bor-Ren Lin,
Yuan-Po
Chien & Hsin-
Hung Lu
1999 A novel
converter
topology, a
three-phase
switching
mode rectifier
(SMR) and a
three-phase
multilevel
inverter with
separately dc
power
supplies, is
proposed
Different circuit
topologies can
used for
switching mode
dc/dc
converters.
7 A Generalized
Multilevel
Inverter
Topology with
Self Voltage
Balancing, Oak
Ridge,
Tennessee
Fang Z. Peng 2000 Generalized
inverter
topology
The existing
multilevel
inverter
topologies can
be derived from
generalized
inverter
topology.

4
8 New Multilevel
Inverter
Topology with
reduced
number of
Switches using
Advanced
Modulation
Strategies
S. Nagaraja
Rao, D.V.
Ashok Kumar
& Ch. Sai
Babu
2013 New class of
three phase
seven level
inverter based
on a multilevel
DC link
(MLDCL) and
a bridge
inverter to
reduce the
number of
switches.
MLDCL can be
used for both
Flying
capacitor and
Diode clamped
inverter for
their
performance
enhancement.
9 A Brief review
on multilevel
inverter
topologies
Amol K.
Koshti & M.
N.Rao
2017 The brief
review of
multilevel
inverter
topologies and
introduction to
control
strategies used
for MLI’S
Reduction of
switches in case
of various types
of Multilevel
Inverter
1.2 OBJECTIVES
In order to overcome the problem of distortion in the output voltage waveform caused due
to the presence of harmonics, this project proposes the modelling of a Five-Level Inverter
using MATLAB/SIMULINK. The objectives of the work are:
1) To study and simulate a three phase five level Voltage Source Inverter.
2) Comparison in performance of three-level and five-level inverters.
3) To understand and implement Pulse Width Modulation for minimization of filtering
requirements.

5
1.3 PROBLEM FORMULATION
1) In high-power and high-voltage applications three-level inverters have some limitations in
operating at high frequency.
2) Three-level inverter incorporates considerable values of Total Harmonic Distortion
(THD).
1.4 SOLUTION METHODOLOGIES
Implementation of Five-Level Inverter so as to overcome the limitations associated with the
use of low-level inverters:
1)Increasing the inverter levels will result in an output waveform which is closer to the
sinusoidal waveform.
2) The unique structure of Multi-Level inverter allows them to reach high voltages with low
harmonic distortion without the use of transformers.
1.5 PULSE WIDTH MODULATION
As an inverter contains electronic switches, it is possible to control the output voltage as well
as optimize the harmonics by performing multiple switching within the inverter. By
changing the pulse width, we can change the average output voltage seen by a circuit. This
technique is called Pulse Width Modulation.
1) In PWM, the comparison of a control (modulating) signal is carried out with a high
frequency carrier signal.
2) Whenever the amplitude of the control signal is greater than the carrier signal, the output
is positive and when the carrier is greater, then the output is negative.

6
(Fig.1- Pulse Width Modulation Technique)
1.5.1 Terminologies:
1) Modulation index (m):
m=Vcontrol/Vtri
Where,
Vcontrol is the peak value of the modulating wave and
Vtri is the peak value of the carrier wave.
Ideally, m can be varied between 0 and 1 to give a linear relation between the modulating
and the output wave.

7
2) Pulse Width:
Pulse width represents the width of the pulse per half cycle.
3) Duty Cycle:
The term duty cycle describes the proportion of 'ON' time to the regular interval or 'period'
of time; a low duty cycle corresponds to low power, because the power is off for most of the
time. Duty cycle is expressed in percentage. When a digital signal is on half of the time and
off the other half of the time, the digital signal has a duty cycle of 50% and resembles a
"square" wave.

8
CHAPTER 2
2.1 INTRODUCTION TO INVERTER
A dc-to-ac converter whose output is of desired output voltage and frequency is called an
inverter.
Based on their operation the inverters can be broadly classified into two categories:
1) Voltage Source Inverter (VSI)
2) Current Source Inverter (CSI)
A voltage source inverter is one where the independently controlled ac output is a voltage
waveform. And a current source inverter is one where the independently controlled ac output
is a current waveform.
In this project, we have implemented a five level Voltage Source Inverter using cascaded
H Bridge topology. The switching scheme uses Sinusoidal Pulse Width Modulation.
Some industrial applications of inverters are: adjustable- speed ac drives, induction heating,
stand by air-craft power supplies, UPS (uninterruptible power supplies) for computers,
HVDC transmission lines etc.
In voltage fed converters, the semiconductor devices always remain forward biased
due to the dc supply voltage and therefore self-controlled forward or asymmetric blocking
devices such as IGBTs, GTOs, BJTs, power MOSFETs etc. Force commutated thyristor
circuits were used earlier, but now they have become obsolete. A feedback diode is always
connected across the device to have free reverse current flow. One important characteristic
of a voltage fed converter is that the AC fabricated voltage wave is not affected by the load
parameters.

9
2.2 MULTILEVEL INVERTERS
A Multi-Level Inverter consists of more than two levels in its output voltage waveform.
Numerous industrial applications have begun to require higher power apparatus in recent
years. Some medium voltage motor drives and utility applications require medium voltage and
megawatt power level. For a medium voltage grid, it is troublesome to connect only one power
semiconductor switch directly. As a result, a multilevel power converter structure has been
introduced as an alternative in high power and medium voltage situations. A multilevel
converter not only achieves high power ratings, but also enables the use of renewable energy
sources. Renewable energy sources such as photovoltaic, wind, and fuel cells can be easily
interfaced to a multilevel converter system for a high-power application.
The concept of multilevel converters has been introduced since 1975.The term multilevel
began with the three-level converter. Subsequently, several multilevel converter topologies
have been developed. However, the elementary concept of a multilevel converter to achieve
higher power is to use a series of power semiconductor switches with several lower voltage
dc sources to perform the power conversion by synthesizing a staircase voltage waveform.
Capacitors, batteries, and renewable energy voltage sources can be used as the multiple dc
voltage sources. The commutation of the power switches aggregates these multiple dc sources
in order to achieve high voltage at the output; however, the rated voltage of the power
semiconductor switches depends only upon the rating of the dc voltage sources to which they
are connected.
2.3 MULTILEVEL INVERTER TOPOLOGIES
Plentiful multilevel converter topologies have been proposed during the last two decades.
Contemporary research has engaged novel converter topologies and unique modulation
schemes. Moreover, three different major multilevel converter structures have been reported
in the literature:
1) Diode clamped (neutral-clamped)
2) Flying capacitors (capacitor clamped) and
3) Cascaded H-bridges converter with separate dc sources.

10
All share the same property, which is that the output filter can be dramatically reduced,
and the usual bandwidth limit induced by the switching frequency can be reconsidered.
Moreover, abundant modulation techniques and control paradigms have been developed
for multilevel converters such as sinusoidal pulse width modulation (SPWM), selective
harmonic elimination (SHE-PWM), space vector modulation (SVM), and others. In
addition, many multilevel converter applications focus on industrial medium-voltage
motor drives, utility interface for renewable energy systems, Flexible AC Transmission
System (FACTS), and traction drive systems etc.
2.3.1 NEUTRAL POINT CLAMPED INVERTER
The Neutral Point Clamped or Diode-Clamped Multilevel Inverter uses series
connected capacitors to divide up the DC bus voltage into a set of various voltage
levels. To produce (m) levels of the phase voltage, an m level diode clamp inverter
needs (m-1) capacitors on the dc bus. In this project, Diode-Clamped multilevel inverter
topology is used. The power circuit configuration of the three-phase multilevel inverter
is as shown in fig.
It is a modification of basic 2-Level Inverter configuration; where an auxiliary
circuit constitutes one switching element and full- bridge diodes. The dc source is split
into two equal parts by capacitor banks; hence forming the neutral point. For each
phase, one terminal of the auxiliary circuit is connected to the neutral point, while the
other terminal of the auxiliary circuit is hooked to the center of the respective phase-leg.
In all, only three active switches are utilized per phase leg.
This second type of converter presents the following Advantages:
1)When M is very high, the distortion level is so low that the use of filters is unnecessary.
2)Constraints on the switches are low because the switching frequency may be lower than
500 Hz (there is a possibility of switching at the line frequency).
3)Reactive power flow can be controlled.

11
The main Disadvantages are:
1)The number of diodes becomes excessively high with the increase in level.
2)It is more difficult to control the power flow of each converter.
(Fig.2 -Diode Clamped Multi-Level Inverter)
2.3.2 FLYING CAPACITOR INVERTER
Figure shows the structure of a flying-capacitor type converter. We notice that
compared to NPC-type converters a high number of auxiliary capacitors are needed, for
M level (M-1) main capacitors and (M-1)*(M-2)/2 auxiliary capacitors.
The main advantages of this type of converter are:
1)For a high M level, the use of a filter is unnecessary.
2)Control of active and reactive power flow is possible.
The drawbacks are:
1) The number of capacitors is very high.

12
2) Control of the system becomes difficult with the increase of M.
(Fig.3 -Flying capacitor type Multi Level Inverter)
2.3.3 CASCADED TYPE MULTILEVEL INVERTER
This type of converter does not need any transformer clamping diodes, or flying
capacitors; each bridge converter generates three levels of voltages (E; 0, and ÿE). For a
three-phase configuration, the cascaded converters can be connected in star or delta. It
has the following advantages:
1)It uses fewer components than the other types.
2)It has a simple control, since the converters present the same structure.
3)However, the main drawback is that it needs separate dc sources for the conversion of
the active power, which limits its use. Its configuration can be represented as:

13
(Fig.4 – Cascaded H- Bridge Multi Level Inverter)
2.4 THREE LEVEL INVERTER
A three-level inverter is a modification of basic two-level inverter. The topology used here is
that of cascaded H bridge inverter. In this, separate DC sources are used for each H bridge. If
the value of DC voltage is same in all the bridges, then it can simply be called as a cascaded
multilevel inverter. In case of different DC voltage being used in different H bridges, then it
is called a Hybrid multilevel inverter.

14
2.4.1 SIMULATION MODEL
(Fig 5: Circuit diagram of three phase three level inverter)
(Fig.6- Switching scheme for the three-level inverter)

15
2.4.2 SIMULATION RESULTS
(Fig.7- Output voltage waveform of three-level inverter)

16
2.4.3 FFT ANALYSIS
(Fig.8- FFT analysis of voltage signal)

17
CHAPTER 3
PROJECTDESCRIPTION
3.1 FIVE LEVEL INVERTERE
In the Five level Inverter model, the following scheme has been implemented:
1) Switching Scheme– Sinusoidal PWM
2) Power Supply – 100V DC each
3) Switches – IGBT
4) Load – R-L load
5) Measurements
3.2 SIMULATION MODEL
(Fig.9-Simulation model of three phase five-level inverter)

18
SWITCHING SCHEME
In the switching scheme, Sinusoidal PWM has been implemented. In this, we have used one
triangular wave and two sine waves for producing switching pulses for a single phase. The
triangular wave is compared to the one sine wave and another sine wave is used with phase
reversal and is also compared to the same triangular wave to obtain the required switching
signal for the switches. The output after comparison has been used in switching of the
switches.
The switching scheme is shown below:
(Fig.10- Switching scheme for the five-level inverter)

19
3.3 SIMULATION RESULTS
(Fig.11- Output voltage waveform of five-level inverter)

20
3.4 FFT ANALYSIS
(Fig.12- FFT analysis of voltage signal)
3.4 ADVANTAGES
A multilevel converter has several advantages over a conventional two-level converter that
uses high switching frequency pulse width modulation (PWM). The attractive features of a
multilevel converter can be briefly summarized as follows.
1) Staircase waveform quality: Multilevel converters not only can generate the output voltages
with very low distortion, but also can reduce the dv/dt stresses; therefore, electromagnetic
compatibility (EMC) problems can be reduced.

21
2) Common-mode (CM) voltage: Multilevel converters produce smaller CM voltage;
therefore, the stress in the bearings of a motor connected to a multilevel motor drive can be
reduced. Furthermore, CM voltage can be eliminated by using advanced modulation strategies
such as that proposed in.
3) Input current: Multilevel converters can draw input current with low distortion. Switching
frequency: Multilevel converters can operate at both fundamental
4) switching frequency and high switching frequency PWM. It should be noted that lower
switching frequency usually means lower switching loss and higher efficiency.
3.5 DISADVANTAGES
Unfortunately, multilevel converters do have some disadvantages. One particular
disadvantage is the greater number of power semiconductor switches needed. Although lower
voltage rated switches can be utilized in a multilevel converter, each switch requires a related
gate drive circuit. This may cause the overall system to be more expensive and complex.

22
CHAPTER 4
CONCLUSION AND FUTURE SCOPE
4.1 COMPARISON
After Simulation, The Total Harmonic Distortion of Three-Level Inverter was around 35.41%
and that of Five level Inverter is 27.91% by assuming the same constraints in both the models.
Three level Inverter is used for low power ratings whereas Five level Inverter is used for high
power ratings. The number of switches used in an Inverter will be increased by increasing the
number of levels of the inverter i.e. Three-level inverter has a smaller number of switches in
comparison to that of Five-level Inverter.
4.2 CONCLUSION & DISCUSSION
In this project the simulation of Three-phase Five-level cascaded H Bridge inverter is carried
out using Sinusoidal Pulse Width Modulation Technique through MATLAB / Simulink. In
this Project, Five-levels of AC output Voltage are generated using a Five-Level Inverter.
Efficiency and losses of five level inverter has been carried out in this project. Earlier
simulated Three-phase Three level Inverter results has been compared to three phase five level
inverter and it is found that the total harmonic distortion has been reduced by 21.1% and
efficiency has been increased by 2.44%. As the number of level increases, THD decreases and
active power increases. The output waveforms of the voltage and load current are also
approximated sine wave.
Increasing the number of voltage levels in the inverter without requiring higher rating
on individual devices can increase power rating. The unique structure of multilevel voltage
source inverters allows them to reach high voltages with low harmonics without the use of
transformers or series-connected synchronized-switching devices. The harmonic content of
the output voltage decreases significantly.

23
4.3 FUTURE SCOPE
The total harmonic distortion can be decreased by increasing the level of the Inverter so we
can go for more levels to get the best output with zero or minimum distortion. The control
technique for multilevel power converters can be further implemented using more efficient
means such as Space Vector Pulse Width Modulation and it can be further generalized to
higher levels and other class of power converters and inverters. The levels of multilevel
configuration can be increased and further improvements in terms of performance and power
quality issues can be broadly studied and could be implemented with hardware circuits.

24
REFERENCES
[1] K. Panda, Y. Suresh, “Research on Cascade Multilevel Inverter with Single DC Source
by using Three-phase Transformers”, Electrical Power and Energy System, Vol 40, March
2012, pp. 9-20.
[2] Rodriguez J, Lai S, and Peng FZ. Multilevel inverters: A survey of topologies, controls,
and applications. IEEE Trans. Ind. Electron; vol. 49, no. 4, pp. 724–738: Aug. 2002.
[3] Carrara et al G. A new multilevel PWM method: A theoretical Analysis. IEEE Trans Power
Electronics; Vol. 7, No. 3, pp. 497- 505: July 1992.
[4] Fang Lin Luo Hong Ye, Advanced DC/AC Inverters (Applications in renewable
energy),2013:140-149.
[5] Tolbert L, Peng FZ, Habetler T. Multilevel converters for large electric drives. IEEE Trans.
Ind. Applicat; vol. 35, pp. 36–44: Jan.-Feb. 1999.
[6] Kouro S,Rebolledo J,Rodriguez J. Reduced Switching Frequency Modulation Algorithm
for High Power Multilevel Inverter. IEEE Trans on industrial Electronics; vol 54, no.5: Oct
2007
[7] Maynard TA, Foch H. Multilevel conversion: high voltage choppers and voltage source
inverters. In Proc. of IEEE Power Electron. Spec. Conf., 1992; Vol. 1, pp. 397-403.
[8]Fukuda S, Suzuki K. Harmonic evaluations of carrier based pwm methods using harmonics
distortion determining factor. IEEE Trans. Power Electr.; Vol. 24, No.2, pp. 1: Nov. 2009.
[9]Carrara et al G. A new multilevel PWM method: A theoretical Analysis. IEEE Trans Power
Electronics; Vol. 7, No. 3, pp. 497- 505: July 1992.
[10] Maynard TA, Foch H. Multilevel conversion: high voltage choppers and voltage source
inverters. In Proc. of IEEE Power Electron. Spec. Conf., 1992; Vol. 1, pp. 397-403.

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