This document is the final year project report submitted by four students for their Bachelor of Science in Electrical Engineering degree. It presents the design and simulation of a cascaded multilevel inverter structure using two DC sources to produce a five-level sinusoidal output waveform through pulse width modulation. The objectives are to minimize switching losses, reduce voltage stress on switches, lower harmonic distortion to improve efficiency for grid integration without requiring expensive active filters.

1. 1
Cascaded Multilevel Inverter
FINAL YEAR PROJECT REPORT
Submitted by
Muhammad Shoaib 16-E-795
Khuram Nazir 16-E-797
Ali Muzaffar 16-E-816
M. Mohsin Abdullah 16-E-794
Supervised by
Engr. Ahsan Zafar
DEPARTMENT OF ELECTRICAL ENGINEERING
Session 2016-2020
FACULTY OF ENGINEERING, Lahore Leads
University, Lahore

2. 2
DECLARATION
We are students of Bachelor of Science in Electrical Engineering, University College of
Engineering Science and Technology, Lahore Leads University.
We hereby declare that no portion of the work referred to in this Project Report has been
submitted in support of an application for another degree or qualification to any other university
or other institute of learning. If any act of plagiarism found, we are fully responsible for every
disciplinary action against us depending upon the seriousness of the proven offence, even the
cancellation of our degree by the Disciplinary Committee.
Muhammad Shoaib _________________________
Khuram Nazir _________________________
Ali Muzaffar _________________________
Mohsin Abdullah _________________________

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CERTIFICATE
The project under titled “Cascaded Multilevel Inverter” has been completed under the supervision
of “Engr Ahsan Zafar” and successfully presented. It has been approved by all the members of
FYP assessment committee Electrical Engineering Department, University College of Engineering
Sciences and Technology, Lahore Leads University.
This project is partial requirement for the completion of BSC Electrical Engineering degree.
__________________________ __________________________
Engr. Ahsan Zafar Dr. Muhammad Saeed Khan
(Member Assessment Committee) (Supervisor)
___________________________ __________________________
Engr. Mahmood Ahmad Engr. Ms. Saba Zia
(Member Assessment Committee) (Member Assessment Committee)
___________________________
Dr. Monir Ahmad
(CHAIR)
(Electrical Engineering Department)

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ACKNOWLEDGEMENTS
In the name of Allah, the most Kind and most Merciful.
First and foremost, we have to thank our parents for their love and support throughout our lives.
Thank you both for giving us the strength to reach for the stars and chase our dreams.
We would like to sincerely thank our supervisor, Engr Ahsan Zafar for his guidance and support
throughout this study, and especially for his confidence in us. His comments and questions were
very beneficial in my completion of the manuscript and especially at interview time. We learned
from his insight a lot. We express our heartfelt gratefulness for his guide and support that we
believed we learned from the best.
To all our friends, thank you all for understanding and encouragement in many moments of crises.
Your friendship makes our life a wonderful experience, we cannot list all the names here, but all
of you are always in our mind.

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DEDICATION
This project is dedicated to our parents for their endless love, support and encouragement
throughout over life in every hardship and in good times. We can’t even imagine being here
without their motivation and courage. Finally, this thesis is dedicated to all those who believe in
the richness of learning.

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ABSTRACT
This project presents a new cascaded Switch Mode Multilevel Inverter Structure for making the
efficient use of renewable energy sources. Two different DC sources are used to feed the inverter
to get the sinusoidal output consisting of 5 levels resulting into a waveform use pulse width
modulation(PWM).The objective is to minimize the switching losses and voltage stress on
electronic switches and to make the inverter cost-effective with the removal of expensive active
filters for producing low harmonic distortion in the output and to make it highly efficient for grid
connectivity . A 5-level inverter creates 0, ±vdc, ±vdc/2 different voltages for load.

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Contents
Cascaded Multilevel Inverter.........................................................................................................................1
DECLARATION..........................................................................................................................................2
CERTIFICATE............................................................................................................................................3
ACKNOWLEDGEMENTS...........................................................................................................................4
DEDICATION.............................................................................................................................................5
ABSTRACT................................................................................................................................................6
List of Figures.......................................................................................................................................9
List of Tables ......................................................................................................................................10
CHAPTER 1..............................................................................................................................................11
INTRODUCTION...............................................................................................................................11
1.1 Literature Summary.....................................................................................................................12
1.2 Objectives of the Project.................................................................................................................13
1.3 PROJECT OVERVIEW .....................................................................................................................14
1.4 Industrial Applications ...................................................................................................................14
1.5 The need for Inverter Circuit...........................................................................................................15
1.6 Inverter.........................................................................................................................................15
1.6.1 Single Phase Bridge Inverter........................................................................................................15
1.7.2 Three Phase Inverter....................................................................................................................17
1.7 Function of IGBT ..........................................................................................................................22
CHAPTER 2..............................................................................................................................................23
LITERATURE SUMMERY ................................................................................................................23
2.1 Types of Multilevel Inverters..........................................................................................................23
2.1.1 Cascaded H-Bridge Multilevel Inverter.........................................................................................23
2.1.2 Diode Clamped Multilevel Inverter ..............................................................................................24
2.1.3 Flying Capacitor Multilevel Inverter.............................................................................................25
2.2 Reducing of harmonics of the inverter output ..........................................................................................26
2.2.1 PERFORMANCE PARAMETERS..............................................................................................27
2.2.2 Total harmonic distortion,THD....................................................................................................27
2.2.3 Distortion factor, DF .....................................................................................................................27
2.2.4 Lower-order harmonic, LOH..........................................................................................................27
CHAPTER 3..............................................................................................................................................28
METHODOLOGY..............................................................................................................................28
3.1 Techniques of Pulse Width Modulation ...........................................................................................28
3.1.1 Single Pulse Width Modulation....................................................................................................28
3.1.2 Multiple Pulse Width Modulation ................................................................................................29
3.1.3 Sinusoidal Pulse Width Modulation..............................................................................................30
3.1.4 Sinusoidal PWM for Three Phase Inverter ....................................................................................32
3.1.5 Modified Sinusoidal Pulse Width Modulation ...............................................................................33
3.1.6 Third Harmonic Pulse Width Modulation .....................................................................................33

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3.1.7 600
Pulse Width Modulations .......................................................................................................34
3.1.8 Space Vector Pulse Width Modulation .........................................................................................34
3.1.9 1800
Conduction .........................................................................................................................35
CHAPTER 4..............................................................................................................................................38
SIMULATION....................................................................................................................................38
4.1.1 Introduction................................................................................................................................38
4.1.2 Role of Simulation in Design .......................................................................................................39
4.1.3 SIMPOWERSYSTEMS..............................................................................................................39
4.1.4 Simulation Circuit.......................................................................................................................40
4.1.5 Simulation of Conventional Five Level Cascaded Multilevel Inverter.............................................40
CHAPTER 5..............................................................................................................................................42
Conclusion and Future Work.......................................................................................................................42
5.1 Conclusion ....................................................................................................................................42
5.2 Future Work..................................................................................................................................42
References .................................................................................................................................................43

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List of Figures
Fig(1.0) Project Overview……………………………………………………………………15
Fig (1.1) Single phase bridge inverter……………………………………………………..…16
Fig (1.2) Waveforms single phase bridge inverter…………………………………………..17
Fig (1.3) Three phase inverter………………………………………………………………..18
Fig (1.4) Three phase inverter ……………………………………………………….………19
Fig (1.5) line-to-line voltage waveform……………………………………………………...19
Fig (1.6) Three phase inverter ……………………………………………………………….20
Fig (1.7) Line-to-line voltage waveform…………………………………………………….20
Fig (1.8) waveforms for 180 degree conduction……………………………………………...23
Fig (3.1) Waveform single PWM……………………………………………………………..30
Fig (3.2) Waveform MPWM…………………………………………………………..……...31
Fig (3.3) Waveform SPWM………………………………..………………………………....32
Fig (3.4) SVM line to neutral voltage…………………………..………………………………35
Fig (3.5) Waveforms gating signal 180 PWM………………………………………………….37
Fig (4.1) simulation of cascaded H-Bridge…………………………………..……………......41
Fig (4.2) output waveform of simulation…….………………………………………………..42
Fig (4.3) FFT analysis of output waveform……………………….…………….……………42

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List of Tables
Table (1.1) Switch state for single phase bridge inverter…………………………………..17
Table (2.1) Sequence of MOSFET switching ………………………..…………………….24
Table (3.1) SVM switching states…………………………………………………………..36
Table (3.2) Switch states……………………………………………………………………37

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CHAPTER 1
INTRODUCTION
A multilevel inverter is a power electronic device which is capable of providing the desired AC
output by using the different low-level DC links as input. Multilevel inverters are the modifications
in simple inverters. They are the efficient alternatives in the medium and high-power applications.
In industries, there are different kinds of loads and their multilevel inverters serve a key purpose for
driving these loads. High power can be obtained through these inverters using different low DC
inputs which can be obtained from battery banks, renewable resources such as solar, wind,
geothermal, super capacitors etc. Multilevel inverters use several switches to generate high power
output with low dv/dt stress on each electronic switch.[1]
Multilevel inverters have got many applications in renewable energy systems along with electric
vehicles, machine drives and FACTS devices. Keeping this regard, researchers are trying to improve
the structure of multilevel inverters in order to reduce the components and voltage rating of switches.
So, to generate the pure pure sinusoidal wave with the simplest algorithm, hour of need is to improve
the hardware configuration of a inverter. The conventional H-Bridge inverter is already being used
in industries for many years but they have many problems including high harmonic content and very
low compatibility for voltage. As compared to a simple two-level inverter, a multilevel inverter
serves many advantages such as low voltage stress on electronic switches, low Total Harmonic
Distortion (THD) and very less electromagnetic interference etc.
Multilevel Inverters are now being preferred in high power and medium voltage applications due
to less voltage stress on switches. There are many topologies to implement multilevel inverters such
as Cascaded H-Bridge Multilevel Inverter, Diode Clamped Multilevel Inverter and Flying Capacitor
Multilevel Inverter. Multilevel Inverters need to have either isolated DC power sources or complex
balancing circuitry along with control in order to balance the voltage levels of multilevel inverter. [2]

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1.1 Literature Summary
Nowadays, Green energy systems such as solar cell, wind turbine, tidal energy converter, etc. are
connected with the micro-grid system. Micro-grid source then again gradually substitutes itself and is
normally available throughout an unremitting source. A renewable energy source, sunlight can be
directly converted into DC electrical power by using a solar panel or somehow collect the solar energy
to convert it into heat and generate steam to operate an electrical generator. In the case of renewable
wind energy, the wind passes through a tunnel or directly rotted a big propeller bleeds to move the
generator to produce the AC/DC electrical power [3].
Similarly, the other renewable energy sources are converted into electrical power to supply the micro-
grid systems with the respective suitable techniques. The micro grid system is an essential for power
supply system because it is free, available and a small region power sharing system between normal
power sources. The micro grid is a localized grouping power system consists two types of radial
feeders such as sensitive-load feeders and non-sensitive-load feeders [4].
The domestic loads are connected to the sensitive load feeder’s side and the non-sensitive-load feeders
are used any error fault occurred due to the main grid. However, a micro-grid is confined to an open
system which interconnects between transmission and distribution electrical energy systems, such as
wind power, fuel generator and solar energy with storage strategies like flywheels, batteries and
electric capacitors rely on both high and low voltages [5].
An electrical energy distribution system consists of energy storage systems, local loads and distributed
generators that can work on building or main grid-connected modes. Inversion is a process that
changes the input DC to output AC using the desired output current, voltage and frequency. An
electrical power inverter circuit can perform this type of alteration. The terms voltage-bolstered and
current-sustained are used as a part of a reference to power electrical inverter circuits [6].
A voltage sustained electrical power inverter is one within that the DC input voltage or current is
fundamentally consistent and free of the load current strained. However, the inverter brings up the
load voltage through the current strained structure is fixed by the load. An electrical power inverter is
a voltage feed inverter also called as voltage source inverter, where the output AC waveform is a sine
wave or others types of the voltage waveform. The output voltage waveform is staying unaffected
with the inverter load [7].

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The multi-level inverter is turning out to be exceptionally appealing for commercial enterprise
application systems because of their high current rating, high voltage rating, and high efficiency. In
this inverter, the overall performance is very efficient because of the system produces less harmonic,
switching loss and low cost. As the quantitative measure of level increases, the output voltage
waveform is additionally increasing. The SPWM control method is used to the control the
semiconductor switches and synchronizes phase between inverter with the utility grid [8].
The consistently developing power utilization, overload the supply micro-grid by making issues likes
micro-grid insecurity, power, security blackouts, the breakdown of power quality, and so on. The
single-phase voltage source inverter is applied to the neural network controller, LC filter and voltage
sensor. A multiple feedback controller for PWM inverter is developed by root-locus method. As a
result, in this method can reduce switching loss with linear loading condition, dynamic, efficient
response to any disturbance, change in load and very few steady state errors [9].
1.2 Objectives of the Project
The objectives of our project are as follows
• Simulate cascaded H-bridge multilevel inverter on MATLAB
• To develop a hardware of cascaded H-bridge multilevel inverter that can produce Five-levels of
output.

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1.3 PROJECT OVERVIEW
Fig. 1.0: Project Overview
1.4 Industrial Applications
Today the rising demand of electricity in every field has made the inverters the more important device
in power electronics. The energy crisis has also triggered the excessive use of inverter modules that
convert DC into AC supply using the energy stored in batteries. Many fields use inverter as an
inseparable core such as UPS, DC power source utilization, Induction heating, HVDC power
transmission, Variable frequency drives, and modern electric vehicles. The designed inverter can also
be used for any kind of linear loads. Also, modified form of the design can also be implemented in
UPS or can be used for driving nonlinear loads.

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1.5 The need for Inverter Circuit
When it is required to provide AC power for a load from a DC supply as the only source of power
for example in the case of solar power, there is need for conversion of the available DC energy to
AC. Most industrial and domestic application utilize AC energy hence the need for the conversion.
1.6 Inverter
Conversion of DC to AC is called inversion. The function of an inverter is to change a DC input
voltage to an AC output voltage of desired magnitude and frequency. The output voltage can be
variable or fixed. Variable output voltage is obtained by two ways:
 Varying input dc voltage and keeping the gain of an inverter constant.
 Keeping input dc voltage constant but varying the gain of the inverter which uses the
advantage of pulse width modulation (PWM). An inverter is called voltage fed invert
(VFI) if input voltage is kept constant and current fed inverter (CFI) if input current is
kept constant. Inverters can be single phase or three phase.
1.6.1 Single Phase Bridge Inverter
A single-phase bridge inverter is shown in figure (1.1). It consists of four choppers. When transistor
T1 and T4 are turned on simultaneously the input voltage Vdc appears across load. And when
transistors T3 and T2 are turned on simultaneously input voltage appears across load but now the
voltage is reversed i.e. -Vdc. There are four diodes used in parallel to transistor in head to tail fashion
and they acts to feed energy back to the source and they are known as feed-back diodes.
Figure. 1.1: Single phase bridge inverter
The output voltage waveform is shown in figure (1.2). When transistors T1 and T4 are conducting,

16. 16
input voltage Vdc appears across the load and inversely when transistors T3 and T2 are conducting
the input voltage appears across the load but now voltage changes its polarity i.e. Vdc is obtained in
the output.
Figure1.2: Waveforms single phase bridge inverter
The following table 1.1 shows how the switching of the transistors is being obtained and how
individual switch state output voltage is being delivered. When S1, S4 are conducting and S2, S3
are off we name it switch state 10 and output voltage is Vdc.
Table (1.1) Switch state for single phase bridge inverter
State State
no.
Switch
state
Va0 Vb0 V0 Components
Conducting
S1,s4
on
S3,s4
off
1 10 Vdc -Vdc Vs S1 ,s2 if
I0>0
D1,D2 if
I0<0
S3,s2
on
S1,S2
off
2 01 -
Vdc
Vdc -
Vs
D3,D4 if
I0>0
S3,S4 if
i0<0
S1,S3
on
S4,S2
off
3 11 Vdc Vdc 0 S1,D3 if
I0>0
D1,S3 if
i0<0
S2,s4
on
4 00 -
Vdc
-Vdc 0 D4,S2 if
I0>0

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S3,s1
off
D2,S4 if
I0<0
All are
off
5 Off -
Vdc
Vdc
Vdc
-Vdc
-
Vs
Vs
D3,D4 if
i0>0
D2,D1 if
i0<0
1.7.2 Three Phase Inverter
Single phase inverter covers low range power applications. Meanwhile, 3-phase inverters are usually
used for a high-power application. The 3-phase inverters generally are used for supplying 3-phase
load especially in AC motor drives and uninterruptible AC power supplies. A 3-phase output can be
obtained from a configuration of six transistors as shown in Figure (1.3.a). Three phase inverter
consists of six transistors and six diodes and its operation is such that three transistors are ON
simultaneously. The diodes are put parallel to each transistor to feed-back energy to the source. No
two-transistor connected to same leg will be switched ON at same time since this causes short circuit
across the DC line between them. In order to get balanced three phase output voltage, we either use
180-degree conduction or 120-degree conduction.
For ∆ connected load the phase currents can be obtained directly from line-to-line voltages. Once
the phase currents are known, the line currents can be determined. For Y-connected load, the line to
neutral voltages must be determined to find line currents. There are three modes of operation in a
half cycle.
Figure (1.1) Three phase inverter
The three modes of operation are as follows and associated waveforms of the phase-to-phase
voltages are also shown

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Figure 1.2: Three phase inverter
Figure (1.3): line-to-line voltage waveform

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Figure (1.4) Three phase inverter
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Figure (1.7) line-to-line voltage waveform

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Figure Three phase inverter
Figure Line-to-line voltage waveform

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Figure (1.10) Line-to-line voltage waveform

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Figure (1.8) waveforms for 180 degree conduction
1.7 Function of IGBT
The IGBT combines an isolated-gate FET for the control input and a bipolar power transistor as a switch
in a single device. The IGBT is used in medium- to high-power applications like switched
mode power supplies, traction motor control and induction heating.

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CHAPTER 2
LITERATURE SUMMERY
2.1 Types of Multilevel Inverters
There are three different conventional topologies for multilevel inverters:
• Cascaded H-Bridge Multilevel Inverter
• Diode Clamped Multilevel Inverter
• Flying Capacitor Multilevel Inverter
2.1.1 CascadedH-Bridge Multilevel Inverter
In this topology many H-bridge inverters are connected in series so we can get a sinusoidal voltage at the
output. This structure helps us to achieve output target with a smaller number of devices as compared to
the other topologies. The 0H-bridge name is given due the combination formed by capacitor and switches
and a separate input DC voltage is given to each H-bridge. Each H-bridge can give different voltages like
negative, positive and zero voltage. Cascaded multilevel inverters eliminates the requirement of
transformers which are needed in the other methods of inverters but they require huge number of isolated
voltages to feed each bridge. A cascaded H-bridge multilevel inverter is shown in Figure. Each bridge
contains four switching devices that can result in negative, positive or zero voltage. Its manufacturing is
fast and cheap comparatively and the need of separate voltage sources is its major drawback. The weight
and price of the inverter are less than the other two types of inverters. For clamping purpose, we do not
need any diodes or capacitors and can obtain a quite sinusoidal wave without any use of filter.[10]
Figure (2.1) Cascaded H-Bridge

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Table (2.1) sequence of IGBT switching
Applications
Cascaded H-bridge multilevel inverters are used in active filters, DC power source utilization, power
factor compensators and in motor drivers. One of the main applications of this kind of inverter is their
interfacing with number of different renewable energy resources.
2.1.2 Diode Clamped Multilevel Inverter
The diode clamped multilevel inverter topology is proposed in 1981 and they are also famous with the name neutral
point inverters. The main idea of this topology is to reduce the number of voltage stresses needed by the use of
diodes. The voltage stress across each switch and each capacitor is Vdc. There will be need of 2(n - 1) switching
devices, (n - 1) voltage sources and (n - 1) (n - 2) diodes for n level diode clamped inverter. For example: For a 5-
level diode clamped inverter: n = 5. Therefore, the required number of switches: 2(n - 1) = 8, number of diodes
required: (n - 1) (n - 2) = 12 and number of capacitors: (n - 1) = 4. Three legs with a common DC bus is present in
three phase diode clamped inverter and it minimizes the capacitance requirements. This voltage is further
subdivided via capacitors into switches. The switches are in the form of pairs so it requires that turning on one of
the switches from the pair the other one should be turned off. One of the other advantages is pre-charging the
capacitors as a group. For a single inverter real power flow is difficult because the intermediate dc levels will try
to discharge or overcharge without taking into account the precise control and monitoring [11]. A 5-level diode
clamped inverter is shown in Figure.

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Figure (2.2) Diode camped Multilevel inverter
Applications
This topology has been proposed a number of times by many different researchers and
is very popular. It can be used with motors of high power where speed is relatively
medium. It can be used in volt ampere reactive compensation. It can also be used for
efficient high-power Interfacing of AC and DC power line.
2.1.3 Flying CapacitorMultilevel Inverter
The main idea behind this topology it to limit the power devices voltage by the use of
capacitors. Its configuration is much similar to diode clamped multilevel inverter except
the division of input DC voltage by the capacitors. It has been designed theoretically for
infinite voltage levels but due to some limitations it gives lesser number of levels going
upto just 6 levels. There are switching devices which are mostly transistors in each leg
and every limb contains cells which consists of two power switches and a capacitor. For
clamping, capacitors are used in this topology.[12]
In an inverter of n cells there will 2n switches and 2n + 1 levels of different voltages including negative
voltage levels with a zero level as well. The capacitor float with respect to potential of earth that is why
they are called flying capacitor based multilevel inverters. The placement of the capacitors in the flying
capacitor based multilevel inverter assures that the stress of the voltage across each device is same that
is Vdc= (n - 1). The availability of phase dismissal for voltage balancing and the reactive and real power
control is one of the advantages of this topology. The increasing number of capacitors results in the
voltage sags and short time outages. The other disadvantages include the complex startup, the pre-

26. 26
charging of capacitors to a same level and packaging becomes difficult for increasing number of levels.
As compared to diodes these capacitors are bulky and expensive. A flying capacitor multilevel inverter
is shown on Figure. This type of inverter has poor quality performance.[13]
Figure (2.3) Flying capacitor Multilevel Inverter
Applications
These inverters can be used to control induction motor by using the technique of direct
torque control circuit and static volt ampere reactive compensation. Flying capacitor
based multilevel inverters can also be used as sinusoidal current rectifiers.
2.2 Reducing of harmonics of the inverter output
The inverter output waveform may vary depending on the application and the circuit used. In most
cases an AC load requires sinusoidal output but the majority of the inverter produces square wave
voltages. Therefore, appropriate means are used to alter the waveforms of the inverter output to a
more or less sinusoidal wave shape.[14]
Harmonic attenuation can be achieved by the following methods:
 Resonating the load
 Using LC filter
 Using pulse width modulation
 Using Poly-phase inverters.

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2.2.1 PERFORMANCEPARAMETERS
The output of a practical inverter usually contains harmonics therefore; the quality of an inverter
is usually evaluated in terms of the following performance parameters.
2.2.2 Totalharmonic distortion, THD
This is the measure of closeness in shape between a waveform and its fundamental components.
It is defined as:
TDH =
2.2.3 Distortion factor, DF
It is a measure of effectiveness in reducing unwanted harmonics without having to specify the
values of a second order filter. DF indicates the amount of harmonic distortion that remains in a
particular waveform after the harmonics of that waveform have been subjected to a second order
attenuation.
DF =
The distortion of an individual (or nth) harmonic component is defined as: DFn =
2.2.4 Lower-order harmonic, LOH
This is that harmonic component whose frequency is closest to the fundamental frequency, and
its amplitude is greater than or equal to 3% of the fundamental component. [15]

28. 28
CHAPTER 3
METHODOLOGY
3.1 Techniques of Pulse Width Modulation
To obtain a near sinusoidal output waveform and to eliminate certain harmonics from the output
different types of modulation techniques are used. In many industrial applications to control the
output voltage of inverters variations with input dc voltages are made, to regulate voltage of
inverters and satisfy the constant volts and frequency control requirement. There are various
techniques to vary the inverter gain. The most efficient method of controlling the output voltage
is to incorporate PWM control within the inverters.[16]
The commonly used techniques are:
 Single pulse width modulation
 Multiple-pulse width modulation
 Sinusoidal pulse width modulation
 Modified sinusoidal pulse width modulation
 Third harmonic pulse width modulation
 600 pulse width modulation 0
 Space vector pulse width modulation
3.1.1 Single Pulse Width Modulation
In single pulse width modulation control, there is only one pulse per half-cycle and the width of
the pulse is verified to control the inverter output voltage. The gating signals are generated by
comparing a rectangular reference signal of amplitude Ar with rectangular carrier signal Ac. The
frequency of the reference signal determines the fundamental frequency of output voltage. The
instantaneous output voltage is The ratio of Ar to ac is the control variable
and defined as the amplitude modulation index. The amplitude modulation index M is given as:
The rms output voltage can be found from
By varying Ar from 0 to Ac, the pulse width α can be modified from 00 to 1800 and the rms output
voltage Vo from 0 to Vs. The Fourier series of output voltage is obtained by:

29. 29
Due to the symmetry of output voltage along the x axis, the even harmonics are absent.
Figure (3.1) waveform single PWM
3.1.2 Multiple Pulse Width Modulation
In multiple pulse width modulation technology, waveforms that contain a number of narrow pulses
are used. The frequency of these narrow pulses is called switching or carrier frequency. The
MPWM technology is used in inverters driving variable frequency motor control systems. This
allows wide range of output voltages and frequency adjustments. Moreover, the MPWM
technology overall improves the quality of the waveforms. The harmonic content can be reduced
by using several pulses in each half cycle of output voltage. These types of modulation are also
known as multiple pulse width modulation or uniform pulse width modulation. The control signal
to turn on and off switches is obtained by comparing a reference signal with a triangular carrier
signal. The number of pulses per half cycle is given by
Where is defined as the frequency modulation ratio. When the modulation index
M is varied from 0 to 1, the pulse width varies from 0 to and the output voltage magnitude
varies from 0 to V. For example, in the half bridge inverter Q1 is turned on and off P number of
times in each positive cycle to cause P number of equal width output voltage pulses. Similarly, Q2
is controlled to cause P number of pulses of same width in each negative cycle. If the pulse width
is δ, then the rms voltage is given by

30. 30
Figure (3.2) Waveform MPWM
The general form of a Fourier series for instantaneous output voltage is:
The coefficient Bn in equation can determined by considering a pair of pulses such that the positive
pulse of duration δ starts at and the negative one of the same widths starts at
. The effect of all pulses can be combined together to obtain the effective output voltage.
i.e.
We can write computer program to calculate the harmonics in the output voltage using the above
equation. Lower order harmonics are reduced by multiple pulse width modulation. But, the
increased switching in each half cycle increase the magnitude of higher order harmonics and the
switching power losses in the switches. However, higher order harmonics can be filtered
easily.[17-18]
3.1.3 SinusoidalPulse Width Modulation
The SPWM technique is the most common traditional one and it is used in practical inverters due
to its low harmonic profile in the inverter output voltage. In sinusoidal pulse-width-modulation
technique (SPWM), the width of each pulse is varied by generating a sinusoidal reference signal
instead of a rectangular reference signal.

31. 31
This sinusoidal pulse width- modulation technique gives a harmonic profile of lower distortion
factor compared to that of multiple pulse-width modulation and single pulse width modulation
techniques.
Figure (3.3) Waveform SPWM
The sinusoidal reference signal (Vr) at required frequency is compared with a triangular carrier
signal (Vc) to produce the switching control signals. These signals control the ON-state and OFF
state of the switching device the triangular carrier signal is utilized to generate multiple pulses per
output cycle, and varied pulse widths are obtained due to applying the sinusoidal reference signal.
The amplitude ratio of the reference signal (Ar) to the carrier signal (Ac) controls the modulation
index (M) and then the inverter output voltage. In the SPWM, there are variable pulse widths and
the width of each pulse (σk) depends on its order (k). Therefore, the effective value of the inverter
output voltage is given in terms of (σk).
The rms output voltage can be varied by varying the modulation index ( ). It can be
observed that the area of each pulse corresponds approximately to the area under the sine wave
between the adjacent midpoints of OFF periods on the gating signals. If δm is the width of mth
pulse, the rms output voltage is given by

32. 32
For n = 1,3,5……
The output voltage of an inverter contains harmonics. The PWM pushes the harmonics into a high-
frequency range i.e. the switching frequency fc and its multiples. That is around harmonics mf,
2mf, 3mf, and so on. The frequencies at which the voltage harmonics occur can be related by: fn
= (jmf±k) fc Where the nth harmonic equals the kth sideband of jth sideband of jth times the
frequency to modulation ratio mf.
n = jmf±k
= 2jp±k for j = 1,2,3….and k = 1,3,5….
The peak fundamental output voltage for PWM and SPWM control can be found approximately
from
Vm1 = dVs …. for 0≤d≤1.0
If d = 1 gives maximum peak amplitude of the fundamental output voltage. The operation beyond
(d = 1) is called over modulation [19].
3.1.4 SinusoidalPWM for Three Phase Inverter
For three phase inverters there are three sinusoidal reference waves (Vra,Vrb,Vrc) each shifted by
1200. A carrier wave is compared with reference signal corresponding to a phase to generate the
gating signals for that phase. Comparing the carrier signal Vcr with reference phases Vra, Vrb and
Vrc produces g1,g3 and g5 respectively. The instantaneous line to line output voltage is
. The output voltage is generated by eliminating the condition that two switching
devices in the same arm cannot conduct at the same time.
The normalized carrier frequency should be odd multiples of three. Thus, all phase-voltage are
identical, but 1200 out of phase without even harmonics. Moreover, harmonics at frequencies
multiple of three are identical in amplitude and phase in all phases. For instance, if the ninth
harmonic voltage is in phase a
The corresponding ninth harmonic in phase b will be,
Thus, the ac output line voltage does not contain the ninth harmonic.
Therefore, for odd multiples of three times the normalized carrier frequency , the harmonics in

33. 33
the AC output voltage appear at normalized frequencies centered around and it multiplies,
specially at n = jmf±k where j = 1,3,5……. for k = 2,4,6….; and j = 2, 4,.... for k = 1,5,7…. such
that n is not a multiple of three. For nearly sinusoidal AC load current, the harmonics in the DC
link current are at frequencies given by:
where j = 0,2,4…. for k = 1,5,7…and j = 1,3,5…for k = 2,4,6…,such that 𝑛 = 𝑗𝑚𝑓 ± 𝑘 is positive
and not a multiple of three. Because the maximum amplitude of the fundamental phase voltage in
the linear region (M ≤ 1) is 𝑉𝑠/2, the maximum amplitude of the fundamental ac output line voltage
is 𝑉𝑎𝑏1 = √3 𝑉𝑠/2. Therefore, one can write the peak amplitude as:
𝑉𝑎𝑏1 = 𝑀√3 𝑉𝑠/2 For 0 <M< 1
To further increase the amplitude of the load voltage, the amplitude of the modulating signal can
be made higher than the amplitude of the carrier signal, which leads to over modulation. [20]
3.1.5 ModifiedSinusoidal Pulse Width Modulation
In sinusoidal pulse width modulation, the width of the pulses nearer the peak of the sine wave does
not change significantly when modulation index is varied. This is because of characteristic of the
sine wave. In modified sinusoidal pulse width modulation, the carrier wave is applied during the
first and last 600 intervals in each half cycle. This modification increases the magnitude of the
fundamental component of output voltage and also further reduces the harmonics. It also reduces
the number of switching of power semiconductors in each half cycle and thereby reduces the
switching power loss.
3.1.6 Third Harmonic Pulse Width Modulation
The third harmonic PWM is similar to the selected injection method, and it is implemented in the
same manner as sinusoidal PWM. The difference is that the reference ac waveform is not
sinusoidal but consists of both a fundamental component and a third harmonic component. As a
result, the peak to peak amplitude of the resulting reference function does not exceed the DC
supply voltage𝑉𝑠, but the fundamental component is higher than the available supply𝑉𝑠. The
presence of exactly the same third harmonic component in each phase results in an effective
cancellation of the third harmonic component in the neutral terminal, and the line to neutral phase
voltages (Van, Vbn and Vcn) are all sinusoidal with peak amplitude of
𝑉𝑝 = 𝑉𝑠/√3 = 0.57735𝑉𝑠.

34. 34
The fundamental component is the same peak amplitude 𝑉𝑝1 = 0.57735𝑉𝑠 and the peak line
voltage is 𝑉𝐿 = √3𝑉𝑝 = 3 ∗ 0.57735 = 𝑉𝑠. This is approximately 15.5% higher in amplitude
than that achieved by the sinusoidal PWM. Therefore, the third harmonic PWM provides better
utilization of the dc supply voltage than the sinusoidal PWM does.[21]
3.1.7 600
Pulse Width Modulations
The 600 PWM is similar to the modified PWM. The idea behind 600 PWM is to “flat top” the
waveform from 600 to 1200 and 2400 to 3000. The power devices are held ON for one third of the
cycle (when at full voltage) and have reduced switching losses. All triple harmonics (3rd, 9th, 15th,
21st, 27th, etc.) are absent in the three phase voltages. The 600PWM creates a larger fundamental
(2/√3) and utilizes more of the available DC voltage (phased voltage 𝑉𝑝 = 0.57735𝑉𝑠 and line
voltage 𝑉𝐿 = 𝑉𝑠) than does sinusoidal PWM. The output waveform can be approximated by the
fundamental and the first few terms.
3.1.8 Space VectorPulse Width Modulation
Space vector modulation (SVM) is quite different from the PWM methods. With PWMs, the
inverter can be thought of as three separate push – pull drives stages, which create each phase
waveform independently. SVM, however, treats the inverter as a single unit, especially; the
inverter can be driven to eight unique states. Modulation is accomplished by switching the state of
the inverter. This is done in each sampling period by properly selecting the switch states of the
inverter and the calculation of the appropriate time period for each state.
Figure (3.4) SVM line to neutral voltage

35. 35
Table (3.1) SVM switching states
3.1.9 1800
Conduction
In 180° conduction each transistor conducts 180. Generally, there are six modes of operation in a
cycle and the duration of each mode is 60. The gating signals shown in fig are shifted from each
other by 60 to obtain three phase balanced voltages. The transistors are numbered in a sequence of
gating (e.g. 231, 234, 345, 561, and 612).
In order to avoid undefined states in the VSI, and undefined AC output line voltages, switches
between upper leg and lower leg of the inverter cannot be switched off simultaneously as this will
result in voltages that will depend upon respective line current polarity. In addition, it is also would
result in a short circuit across the DC link voltage supply which will damage the inverter system
if the switches is switching on simultaneously. There are six modes of operating the switches,
where in a cycle the phase shift of each mode is 60º. In order to generate a desired voltage
waveform, the transistor conduction moves from one state to another. The gating signals shown in
Figure (3.5) are shifted from each other by 60º to obtain 3-phase balanced (fundamental) voltages.
The load can be connected in wye or delta connection. The line current is determined when the
phase current is known. For a wye connected load, the line to neutral voltages must be determined
to find the phase current. [22]

36. 36
Figure (3.5) Waveforms gating signal 180 PWM
Table (3.2) Switch states
State State
no
Switch
state
Vab Vbc Vca
S1,s2,s6
on
S4,s5,s3
off
1 100 Vdc 0 -Vs
S2,s3,s1
on
S5,s6,s4
off
2 110 0 Vdc -Vdc
S3,s4,s2
on
S6,s1,s5
off
3 010 -Vdc Vdc 0
S4,s5,s3
on
4 011 -Vdc 0 Vdc

37. 37
S1,s2,s6
off
S5,s6,s4
on
S2,s3
,s1 off
5 001 0 -Vdc Vdc
S6,s1,s5
on
S3,s4,s2
off
6 101 Vdc -Vdc 0
S1,s3,s5
on
S4,s6,s2
7 111 0 0 0
S4,s6,s2
on
S1,s3,s5
off
8 000 0 0 0

38. 38
CHAPTER 4
SIMULATION
4.1.1 Introduction
MATLAB and other mathematical computation tools are computer programs that combine computation
and visualization power that make them particularly useful tools for engineers. MATLAB is both a
computer programming language and a software environment for using that language effectively. The
name MATLAB stands for Matrix laboratory, because the system was designed to make matrix
computations particularly easy. The MATLAB environment allows the user to manage variables, import
and export data, perform calculations, generate plots, and develop and manage files for use with
MATLAB. The MATLAB environment is an interactive environment:
• Single-line commands can be entered and executed, the results displayed and observed, and then a
second command can be executed that interacts with results from the first command that remain in
memory. This means that you can type commands at the MATLAB prompt and get answers immediately,
which is very useful for simple problems.
• MATLAB is an executable program, developed in a high-level language, which interprets user
commands.
• Portions of the MATLAB program execute in response to the user input, results are displayed, and the
program waits for additional user input.
• When a command is entered that doesn’t meet the command rules, an error message is displayed. The
corrected command can then be entered. • Use of this environment doesn’t require the compile-link/load-
execution process described above for high-level languages. While this interactive, line-by-line execution
of MATLAB commands is convenient for simple computational tasks, a process of preparation and
execution of programs called scripts is employed for more complicated computational tasks:
• A script is list of MATLAB commands, prepared with a text editor.
• MATLAB executes a script by reading a command from the script file, executing it, and then repeating
the process on the next command in the script file.
• Errors in the syntax of a command are detected when MATLAB attempts to execute the command. A
syntax error message is displayed and execution of the script is halted.
• When syntax errors are encountered, the user must edit the script file to correct the error
and then direct MATLAB to execute the script again.
• The script may execute without syntax errors, but produce incorrect results when a logic
error has been made in writing the script, which also requires that the script be edited and

39. 39
execution re-initiated.
• Script preparation and debugging is thus similar to the compile-link/load-execution
process required for in the development of programs in a high-level language[23-29].
4.1.2 Role ofSimulation in Design
Electrical power systems are combinations of electrical and electromechanical devices like
motors and generators. Engineers working in this discipline are constantly improving the
performance of the systems. Requirements for drastically improved efficiency have forced
power system designers to use power electronic devices and sophisticate control system
concepts that tax traditional analysis tools and techniques. Further complicating the
analyst role is the fact that the system is often so nonlinear that the only way to
understand it is through simulation. Land based power generation from hydroelectric, steam or other
devices are not the only use of power systems. A common attribute of these systems is their use of power
electronics and control systems to achieve their performance objectives.
SIMULINK is a software package for modelling, simulating, and analyzing dynamical systems. It
supports linear and nonlinear systems, modelled in continuous time, sampled time, or a hybrid of the two.
Systems can also be multi rate, i.e., have different parts that are sampled or updated at different rates. For
modelling, SIMULINK provides a graphical user interface (GUI) for building models as block diagrams,
using click-and-drag mouse operations. With this interface, you can draw the models just as you would
with pencil and paper (or as most textbook depict them). This is a far cry from previous simulation
packages that require you to formulate differential equations and difference equations in a language or
program. SIMULINK includes a comprehensive block library of sinks, sources, linear and nonlinear
components, and connectors. You can also customize and create your own blocks.
Models are hierarchical, the models are built using both top-down and bottom-up approaches the system
can viewed at a high level, then double-click on blocks to go 5 down through the levels to see increasing
levels of model detail. This approach provides insight into how a model is organized and how its parts
interact. After a model is defined, it can simulate, using a choice of integration methods, either from the
SIMULINK menus or by entering commands in MATLAB's command window. The menus are
particularly convenient for interactive work, while the command line approach is very useful for running
a batch of simulations. Using scopes and other display blocks, the simulation results can see while the
simulation is running [30]
4.1.3 SIMPOWERSYSTEMS
SimPowerSystems and SimMechanics of the Physical Modeling product family work together with
Simulink to model electrical, mechanical and control systems. SimPowerSystems is a modern tool that
allows scientists and engineers to rapidly and easily build models that simulate power systems.

40. 40
SimPowerSystems uses the Simulink environment, allowing building a model using simple click and
drag procedures. Not only can draw the circuit topology rapidly, but analysis of the circuit can include
its interactions with mechanical, thermal, control, and other disciplines. This is possible because all the
electrical parts of the simulation interact with the extensive Simulink modeling library. Since Simulink
uses MATLAB as its computational engine, designers can also use MATLAB toolboxes and Simulink
block sets. SimPowerSystems and SimMechanics share a special Physical Modeling block and
connection line interface.
4.1.4 Simulation Circuit
The simulation is done using IGBT switches and control signals like Phase Disposition (PD) Pulse Width
Modulation and Digital Pulse Width Modulation is implemented
4.1.5 Simulation of Conventional Five Level CascadedMultilevel Inverter
Here in this circuit each h bridge will produce different output voltage namely +Vdc,-Vdc,+Vdc/2,-Vdc/2
and 0V. A voltage of 12V dc is applied to each H-bridge configuration to get an output voltage of 230V.
The output is viewed using the scope block which is used give the output of the simulated circuit in
MATLAB. A sine wave of amplitude 1 and frequency 2*pi*50 act as the reference signal is used to
compare with triangular carrier signal which is produced by repeating sequence block parameter. The
modulation index of the reference signal and comparator signal is 0.8. When the reference signal is
greater than the comparator signal, we get the output signal which is given to the gate section of the IGBT
used as switches.
Figure (4.1) Simulation of five level Cascaded Multilevel Inverter

41. 41
Figure (4.2) Simulation Output Waveform of The Five Level Cascaded Multilevel Inverter
Figure (4.3) FFT Analysis of Five Level Cascaded Multilevel Inverter
On FFT analyzing we get to know that its THD is 29.15%

42. 42
CHAPTER 5
Conclusion and Future Work
5.1 Conclusion
Microcontroller based multilevel inverter is simulated using MATLAB/Simulink and the
hardware is implemented. We carried out simulation of 5-level inverters in cascaded H-bridge
topology. By using still more control strategy we can use this multi-level inverter topology for more
levels and hence it will make to almost much nearer to sine wave, which intern reduces
harmonics and which can be used for motor applications, variable speed control, in wind mills etc.
5.2 Future Work
In this research work, new PWM switching schemes and new circuit topologies are developed
and analyzed for cascaded multilevel inverter. In future work, optimal inverter circuit
configuration and switch combination can be developed for reduced switch multilevel
inverter topologies. High voltage circuit configuration for reduced switch multilevel
inverters can be developed. Intelligent methods such as particle swarm optimization, genetic
algorithm and differential evolutionary algorithm, etc. can be used to calculate the optimal
switching angle of the inverter switches to eliminate specific harmonics in the output voltage.
pulse disposition(pd) pulse width modulation technique can be developed for reduced switch
multilevel inverter circuit topologies.

43. 43
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