Modeling and analysis performance simulation performance of multilevel inverter using bipolar and unipolar switching schemes.
FAKULTI KEJURUTERAAN ELEKTRIK
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
LAPORAN PROJEK
SARJANA MUDA
MODELING AND ANALYSIS PERFORMANCE SIMULATION
PERFORMANCE OF MULTILEVEL INVERTER USING BIPOLAR
AND UNIPOLAR SWITCHING SCHEMES
NG SEE MAN
Bachelor of Electrical Engineering (Power Electronic and Drive)
June 2014
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MODELING AND ANALYSIS PERFORMANCE SIMULATION OF MULTILEVEL
INVERTER USING BIPOLAR AND UNIPOLAR SWITCHING SCHEMES
NG SEE MAN
A report submitted in partial fulfilment of the requirements for degree of Bachelor of
Electrical Engineering (Power Electronic and Drive)
Faculty of Electrical Engineering
UNIVERSITI TEKNIKAL MALAYSIA MELAKA
(3)
I declare that this report entitle “Modeling and Analysis Performance Simulation of Multilevel
Inverter Using Bipolar and Unipolar Switching Schemes” is the result of my own research
except as cited in the references. The report has not been accepted for any degree and is not
concurrently submitted in candidature of any other degree.
Signature
: ………..
Name
: NG SEE MAN
(4)
“I hereby declare that I have read through this report entitle “Modeling and Analysis
Performance Simulation of Multilevel Inverter Using Bipolar and Unipolar Switching
Schemes” and found that it has comply the partial fulfilment for awarding the degree of
Bachelor of Electrical Engineering (Power Electronic and D
rive)”
Signature
: ………
Supervisor‟s Name
: En. Musa Yusup Lada
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i
ACKNOWLEDGEMENT
I would like to express my great appreciation to my supervisor, Mr. Musa Bin Yusup
Lada for his patience guidance and enthusiastic encouragement throughout the duration of this
project.
The supervision and patient guidance that he gave truly help the progression and
smoothness of my final year project. The support and hard work are much appreciated indeed.
Next, I would like to extend my thanks to all my course mates for their supports and time
during my final year project namely Joycelyn Goh, Sharul Nidzam, Hazwal, Shafiq and Izwan as
well. We help each other and discuss together when we encounter some confusing problem on our
projects.
Last but not least, I would like to show my gratitude to my family for their encouraging
and supporting in completion my final year project. Without their support, I am not able to
complete my project successfully.
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ii
ABSTRACT
Multilevel inverter is one of the popular converter topologies used in high power
medium- voltage (MV) drives due to able operate at high direct current (DC) voltage which is
achieved using series connection of power semiconductor switches. Multilevel inverter can
generate output voltage with very low harmonic distortion and synthesis a staircase voltage
output waveform by having multiple voltage level. Various topologies of multilevel inverters
are introduced; however, in this project, the cascaded seven-level multilevel inverter will be
discussed due to easy control method implementation. Unipolar and bipolar switching
schemes are used to analyze the performance of seven-level cascaded H-bridge inverter. Both
of these SPWM voltage modulation type are selected because these method able to double the
switching frequency of the inverter voltage effectively. The major problem in designing
multilevel inverter is to design filtering in order to filter the low harmonic which is very hard
to eliminate. Bipolar switching PWM inverter is able to filter the low frequency harmonics
compared to the unipolar switching schemes inverter. From the simulation results, it can be
clearly seen that by using bipolar switching scheme, the total current harmonic distortion
(THD
i) and voltage harmonic distortion (THD
V) for R, RL and RC loads are low compared to
unipolar switching scheme. Therefore, the bipolar switching scheme is more suitable because
the common domestic loads used are RL load. MATLAB Simulink is used to model the seven
levels cascaded multilevel PWM inverter
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ABSTRAK
Penyongsang berperingkat adalah salah satu topologi penukar kuasa arus terus yang
popular dalam voltan sederhana kuasa tinggi dengan menggunakan penyambungan siri suis
–
suis semikonductor kuasa. Penyongsang berperingkat dapat menjanakan voltan yang
berherotan harmonic yang sangat rendah dan bentuk gelombang voltan tangga yang berbilang.
Pelbagai topologi penyongsang berperingkat telah diperkenalkan, walaubagaimanapun, dalam
projek ini, tahap tujuh penyongsang melata berperingkat akan dibincangkan kerana
pelaksanaan kaedah pengawalan lebih mudah. Pensuisan dwikutub dab ekakutub digunakan
untuk menganalisi prestasi tujuh peringkat penyongsan berperingkat. Dua persuisan modulasi
ini dipilih kerana kaedah ini mampu menggandakan frekuensi utama voltan penyongsang
dengan berkesan. Masalah utama dalam mereka penyongsang berperingkat adalah menapis
herotan harmonik yang rendah kerana herotan harmonik yang rendah sangat sukar untuk
ditapis. Dwikutub persuisan lebih mudah menapis herotan harmonik ini berbanding dengan
persuisan ekakutub. Keseluruhan keputusan simulasi, dengan jelasnya herotan harmonik arus
dan voltan penggunaan R, RL dan RC muatan adalah rendah berbanding dengan persuisan
ekakutub.Lantarannya, persuisan dwikutub adalah lebih sesuai digunakan kerana
muatan-muatan domestik adalah RL muatan-muatan. Simulink Matlab digunakan untuk memperagakan tujuh
tahap penyongsang berperingkat
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iv
TABLE OF CONTENT
ABSTRACT ... ii
ABSTRAK ...iii
TABLE OF CONTENT ... iv
LIST OF FIGURES ... vi
LIST OF TABLES ... vii
LIST OF ABREVATIONS ... viii
CHAPTER 1 ... 1
INTRODUCTION... 1
1.1 Background ... 1
1.2 Problem Statement ... 2
1.3 Objective ... 2
1.4 Scope ... 3
1.5 Report Outline ... 3
CHAPTER 2 ... 4
LITERATURE REVIEW ... 4
2.1 Introduction ... 4
2.2 Classification of Inverter... 4
2.2.1 Current-source Inverter (CSI) ... 5
2.2.2 Voltage
–
Source Inverter (VSI) ... 5
2.2.2.1 Single-phase Half-bridge Inverter ... 6
2.2.2.2 Single-phase Full- bridge Inverter ... 7
2.2.2.3 Square Wave Inverter ... 9
2.2.2.4 Pulse- width Modulated Inverter ... 10
2.3 General Topologies of Multilevel Inverter ... 10
2.3.1 Neutral Point Clamped Multilevel Inverter (NPC-MLI) ... 11
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2.3.3 Cascaded H-bridge Multilevel Inverter (CHB-MLI) ... 13
2.4 Multilevel Inverter Modulation Control Schemes ... 13
2.4.1 SPWM with Bipolar Switching ... 14
2.4.2 SPWM with Unipolar Switching ... 15
2.4.3 Comparison between Unipolar and Bipolar Switched Multilevel Inverter ... 17
2.4.4 PWM Consideration ... 17
2.4.4.1 Frequency Modulation Ratio, m
f... 17
2.4.4.2 Amplitude Modulation Ratio, m
a... 18
2.4.5 Sinusoidal PWM (SPWM)... 18
2.4.5.1 Phase- Shifted PWM (PSPWM) ... 19
2.4.5.2 Level- Shifted PWM (LSPWM) ... 20
2.6 Power Quality ... 21
2.6.1 Harmonic Distortion Definition ... 22
CHAPTER 3 ... 25
DESIGN METHODOLOGY ... 25
3.1 Introduction ... 25
3.2 Research Methodology ... 25
3.2.1 Flowchart ... 25
3.3 Control Switching Technique Used ... 27
3.4 Single Phase and Three-Phase Inverter ... 28
3.4.1 Simulink Block... 28
3.4.1.1 Unipolar Single Phase Inverter... 30
3.4.1.2 Unipolar three-phase inverter ... 31
3.4.1.3 Bipolar single-phase inverter ... 32
3.4.1.4 Bipolar three-phase inverter ... 33
3.5 Cascaded Seven-level Multilevel Inverter ... 34
3.5.1 Simulink Block... 34
3.5.1.1 Unipolar Three Phase Seven-Level Multilevel Inverter ... 36
3.5.1.2 Bipolar Three-Phase Seven-Level Multilevel Inverter ... 37
CHAPTER 4 ... 40
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vi
4.1 Introduction ... 40
4.2 Simulation Result for Unipolar and Bipolar Single Phase Inverter ... 40
4.2.1 Unipolar Single-phase Inverter ... 40
4.2.1.1 Simulation Result using R-load for Unipolar Single Phase Inverter ... 42
4.2.1.2 Simulation Result using RL-load for Unipolar Single Phase Inverter ... 44
4.2.1.3 Simulation Result using RC-load for Unipolar Single Phase Inverter ... 46
4.2.2 Bipolar Single-phase Inverter ... 48
4.2.2.1 Simulation Result using R-load for Bipolar Single Phase Inverter ... 50
4.2.2.2 Simulation Result using RL-load for Bipolar Single Phase Inverter... 52
4.2.2.2 Simulation Result using RC-load for Bipolar Single Phase Inverter ... 55
4.3 Simulation Result for Unipolar and Bipolar Three Phase Inverter ... 57
4.3.1 Unipolar Three-phase Inverter ... 57
4.3.3.1 Simulation Result using R-load for Unipolar Three Phase Inverter ... 58
4.3.3.2 Simulation Result using RL-load for Unipolar Three Phase Inverter ... 61
4.3.3.3 Simulation Result using RC-load for Unipolar Three Phase Inverter ... 65
4.3.4 Bipolar Three-phase Inverter ... 68
4.3.4.1 Simulation Result using R-load for Bipolar Three Phase Inverter ... 69
4.3.4.2 Simulation Result using RL-load for Bipolar Three Phase Inverter ... 73
4.3.4.3 Simulation Result using RC-load For Bipolar Three Phase Inverter ... 76
4.4 Simulation Result for Cascaded Seven-Level Multilevel Inverter ... 81
4.4.1 Unipolar Three Phase Seven-Level Multilevel Inverter ... 81
4.4.1.1 Simulation Result using R-load for Cascaded Seven-Level Multilevel Inverter .. 82
4.4.1.2 Simulation Result using RL-load for Cascaded Seven- Level Multilevel Inverter
... 84
4.4.1.3 Simulation Result using RC-load for Cascaded Seven-Level Multilevel Inverter 86
4.4.2 Bipolar Three Phase Seven-Level Multilevel Inverter ... 89
4.4.2.1 Simulation Result using R-load for Cascaded Seven-Level Multilevel Inverter .. 90
4.4.2.2 Simulation Result using RL-load for Cascaded Seven-Level Multilevel Inverter 93
4.4.2.3 Simulation Result using RC-load for Cascaded Seven-Level Multilevel Inverter 95
CHAPTER 5 ... 100
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vii
REFERENCE ... 101
PUBLICATION ... 103
APPENDIX ... 104
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viii
LIST OF FIGURES
Figure 2.1: Block Diagram of Inverter... 4
Figure 2.2: Circuit Configuration of CSI inverter ... 5
Figure 2.3: Circuit Configuration of VSI Inverter ... 5
Figure 2.4: Circuit Configuration of a Single-phase Half-bridge Inverter ... 6
Figure 2.5: (a) Gating Signal for Switch S1 (b) Gating Signal for Switch S2... 7
Figure 2.6: Output Voltage of Single phase half-bridge inverter... 7
Figure 2.7: Circuit Configuration of a single-phase, full-bridge inverter ... 8
Figure 2.8: (a) Gating signal for switch S1,S2 (b) Gating signal for switch S3,S4 ... 8
Figure 2.9: Output voltage for single phase, full-bridge inverter ... 9
Figure 2.10: Square Wave Inverter Output for Harmonic Control ... 9
Figure 2.11: Three –level Neutral Point Clamped Topology ... 11
Figure 2.12: Three-level Flying Capacitor Topology ... 12
Figure 2.13: Five-level Cascaded H-bridge Topology ... 13
Figure 2.14: Multilevel converter modulation methods ... 14
Figure 2.15: (a) Switching pattern (b) Output waveform ... 15
Figure 2.16: (a) Switching pattern (b) Output Waveform ... 16
Figure 2.17: Multi-carrier SPWM control techniques ... 19
Figure 2.18: Level-shifted multicarrier modulation: (a) PD, (b) POD, and (c) APOD. ... 21
Figure 3.1: Flowchart of research methodology ... 26
Figure 3.2: Block Diagram of Unipolar and Bipolar Single Phase Inverter ... 28
Figure 3.3: Block Diagram of Unipolar and Bipolar Three-Phase Inverter... 29
Figure 3.4: Block diagram of unipolar single phase inverter ... 31
Figure 3.5: Block diagram of unipolar single-phase PWM generation ... 31
Figure 3.6: Block diagram of unipolar three-phase inverter ... 31
Figure 3.7: Block diagram of unipolar three-phase PWM generation ... 32
Figure 3.8: Block diagram of bipolar single-phase inverter ... 32
Figure 3.9: Block diagram of bipolar single-phase PWM generation ... 33
Figure 3.10: Block diagram of bipolar three phase inverter ... 33
Figure 3.11: Block diagram of bipolar three phase PWM generation ... 34
Figure 3.12: Block Diagram of Seven-Level Cascaded Multilevel Inverter ... 35
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Figure 3.14: PWM Generation of H-Bridge 1 ... 36
Figure 3.15: PWM Generation of H-Bridge 2 ... 37
Figure 3.16: MATLAB Simulink model of bipolar seven-level multilevel inverter ... 38
Figure 3.17: PWM Generation of H-Bridge 1 ... 38
Figure 3.18: PWM Generation of H-Bridge 2 ... 39
Figure 4.1: Reference and carrier signals under condition mf = 50 , ma = 0.75 ... 41
Figure 4.2: PWM compensator signal ... 41
Figure 4.3: Current waveform (R-load) ... 42
Figure 4.4:Voltage waveform (R-load) ... 42
Figure 4.5: THD for load current... 43
Figure 4.6: THD for load voltage ... 43
Figure 4.7: Current waveform (RL-load) ... 44
Figure 4.8: Voltage waveform (RL-Load) ... 45
Figure 4.9: THD for load current... 45
Figure 4.10: THD for load voltage ... 46
Figure 4.11: Current Waveform (RC-load) ... 47
Figure 4.12: Voltage Waveform (RC-load) ... 47
Figure 4.13: THD for load current ... 48
Figure 4.14: THD for load voltage ... 48
Figure 4.15: Sinusoidal reference and triangular carrier mf = 50 , ma = 0.75 ... 49
Figure 4.16: PWM Compensator Signal ... 49
Figure 4.17: Current Waveform (R load) ... 50
Figure 4.18: Voltage Waveform (R load) ... 51
Figure 4.19: THD for load current ... 51
Figure 4.20: THD for load voltage ... 52
Figure 4.21: Current Waveform (RL-load) ... 53
Figure 4.22: Voltage Waveform (RL-load) ... 53
Figure 4.23: THD for load current ... 54
Figure 4.24: THD for load voltage ... 54
Figure 4.25: Current Waveform (RC-load) ... 55
Figure 4.26: Voltage Waveform (RC-load) ... 56
Figure 4.27: THD for load current ... 56
Figure 4.28: THD for load voltage ... 56
Figure 4.29: Carrier and reference waves for PWM operation for the three phase inverter under condition mf = 100, ma = 0.9 ... 57
Figure 4.30: PWM Compensator Signal ... 58
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Figure 4.32: Line-to-line voltage (R-load ... 59
Figure 4.33: Line-to-neutral voltage (R-load) ... 60
Figure 4.34: THD for phase current... 60
Figure 4.35: THD for phase voltage ... 61
Figure 4.36: THD for Line-to-line voltage ... 61
Figure 4.37: Three phase current (RL-load) ... 62
Figure 4.38: Line-to-neutral voltage (RL-load) ... 63
Figure 4.39: Line-to-line voltage (RL-load) ... 63
Figure 4.40: THD for phase current... 64
Figure 4.41: THD for phase voltage ... 64
Figure 4.42: THD for Line-to-line voltage ... 65
Figure 4.43: Three phase current (RC-load)... 66
Figure 4.44: Line-to-line voltage (RC-load) ... 66
Figure 4.45: Line-to-neutral voltage (RC-load)... 67
Figure 4.46: THD for Phase Current ... 67
Figure 4.47: THD for Line-to-line Voltage ... 68
Figure 4.48: THD for Line-to-neutral voltage ... 68
Figure 4.49: PWM compensator signal... 69
Figure 4.50: Three phase current (R load) ... 70
Figure 4.51: Line-to-line voltage (R load) ... 70
Figure 4.52: Line-to-neutral voltage (R-load) ... 71
Figure 4.53: THD for phase current... 71
Figure 4.54: THD for Line-to-line voltage ... 72
Figure 4.55: THD for Line-to-neutral Voltage ... 72
Figure 4.56: Three phase current (RL-Load) ... 73
Figure 4.57: Line-to-line voltage (RL-load) ... 74
Figure 4.58: Line-to-neutral voltage (RL-load) ... 74
Figure 4.59: THD for phase current... 75
Figure 4.60: THD for Line-to-neutral voltage ... 75
Figure 4.61: THD for Line-to-line Voltage ... 76
Figure 4.62: Three phase current (RC-load)... 77
Figure 4.63: Line-to-line voltage (RC- load) ... 77
Figure 4.64: Line-to-neutral voltage (RC- load) ... 78
Figure 4.65: THD for Phase Current ... 78
Figure 4.66: THD for Line-to-neutral voltage ... 79
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Figure 4.68: Phase Dissipation (PD multicarrier modulation for Seven-level inverter under condition of mf = 100
and ma = 0.6 ... 81
Figure 4.69: PWM Compensator Signal (Upper) ... 82
Figure 4.70: PWM Compensator Signal (Lower) ... 82
Figure 4.71: Three phase current (R-load) ... 83
Figure 4.72: Line-to-neutral voltage (R-load) ... 83
Figure 4.73: THD for phase current (R-load) ... 84
Figure 4.74: THD for phase voltage (R load) ... 84
Figure 4.75: Three phase current (RL load) ... 85
Figure 4.76: Line-to-neutral Voltage (RL load) ... 85
Figure 4.77: THD for phase current (RL load) ... 86
Figure 4.78: THD for phase voltage ( RL load) ... 86
Figure 4.79: Three Phase Current (RC load) ... 87
Figure 4.80: Line-to-neutral Voltage (RC load) ... 87
Figure 4.81: THD for phase current (RC load)... 88
Figure 4.82: THD for phase voltage (RC load) ... 88
Figure 4.83: PWM Compensator Signal (Upper) ... 90
Figure 4.84: PWM compensator signal (Lower) ... 90
Figure 4.85: Three phase current (R load) ... 91
Figure 4.86: Line-to-neutral voltage (R-load) ... 91
Figure 4.87: THD for phase current (R-load) ... 92
Figure 4.88: THD for Phase Voltage (R-load) ... 92
Figure 4.89: Three phase current (RL load) ... 93
Figure 4.90: Line-to-neutral voltage (RL load) ... 94
Figure 4.91: THD for Phase Current (RL load) ... 94
Figure 4.92: THD for Phase Voltage (RL load) ... 95
Figure 4.93: Three phase current (RC load) ... 96
Figure 4.94: Line-to-neutral Voltage (RC load) ... 96
Figure 4.95: THD for Phase Current (RC load) ... 97
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LIST OF TABLES
Table 3.1: Simulation Parameters of Unipolar and Bipolar Single Phase Inverter
29
Table 3.2: Block Diagram of Unipolar and Bipolar Three Phase Inverter
30
Table 3.3: Simulation Parameters of Unipolar and Bipolar Cascaded Seven-level Multilevel
Inveter
35
Table 4.1: Total Harmonic Distortion in Single-phase Inverter
80
Table 4.2: Total Harmonic Distortion in Three-phase Inverter
80
Table 4.3: Voltage and Current Harmonic of Unipolar Sinusoidal PWM Inverter
89
Table 4.4: Voltage and Current Harmonic of Bipolar Sinusoidal PWM Inverter
98
Table 4.5: Voltage and Current Harmonic of Sinusoidal PWM Inverter
98
Table 4.6: Voltage and Current Harmonics of Sinusoidal PWM Inverter In 3-Level and
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LIST OF ABBREVATIONS
AC
–
Alternating Current
APOD
–
Alternative Phase Opposition Dissipation
CSI
–
Current Source Inverter
DC
–
Direct Current
GTO
–
Gate Turn-Off Thyristor
MLI
–
Multilevel Inverter
PD
–
Phase Dissipation
POD
–
Phase Opposition Disposition
PWM
–
Pulse Width Modulation
THD
–
Total Harmonic Distortion
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1
CHAPTER 1
INTRODUCTION
1.1 Background
Inverters are circuits that convert direct current (DC) to alternating current (AC). More
precisely, inverters transfer power from a DC source to an AC load. Since the main objective
of the inverter is to use a DC voltage source to supply a load requiring AC, it is useful to
describe the quality of AC output.
In the electrical power distribution system, there are many non-linear loads drawing
non-sinusoidal current. The quality of a non-sinusoidal wave can be expressed in terms of total
harmonic distortion (THD). If non-sinusoidal current passes through a different kind of
impedance, it will produce voltage and current harmonics. The voltage and current harmonics
can cause additional losses, overheating and overloading of the load.
Multilevel inverter has the advantages of generating better output quality by using
pulse width modulation (PWM) technique. This is because PWM inverters are able to
eliminate unwanted harmonic by using suitable design of PWM scheme such as bipolar and
unipolar schemes.
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2
1.2 Problem Statement
In the past, square wave inverters were widely used in independent wind or solar
power systems and some industrial applications with lower requirement on power quality. In
square wave inverter, the harmonics and output voltage amplitude could not be controlled by
the user. In addition, in AC power distribution system, harmonics occur when the normal
electric current waveform is distorted by non-linear loads such as computer equipment with
switched-mode power supplies, variable speed motors and drive, fluorescent lamp ballasts and
others.
Harmonic decreases significantly in power quality and live cycle of electrical
equipment. Basically, harmonic distortion will increase resistive losses, voltage stresses and
excessive voltage distortion on power distribution system.
Therefore, the multilevel inverter is used to produce multilevel-output voltages which
are purely sinusoidal or synthesis a staircase voltage waveform and thus reduce harmonic
content. Higher frequency harmonics are easier to filter than harmonics near the fundamental
frequency. Bipolar and unipolar switching schemes are chosen because these methods able to
double the switching frequency of the inverter. However, some of low frequency harmonics
are formed due to non-linear loads, therefore, bipolar switched multilevel inverter is proposed
due to able to filter the low frequency harmonics More number of levels of multilevel inverter
will give better performance in term of total harmonic distortion.
1.3 Objective
The objectives of this project are:
1.
To model there phase seven-level multilevel inverter using bipolar and unipolar
switching technique by using Matlab Simulink Software.
2.
To analyze and evaluate the simulation of seven-level of multilevel inverter by using
bipolar and unipolar switching schemes.
3.
To validate the THD for bipolar and unipolar switching schemes for multilevel inverter
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3
1.4 Scope
This project focuses on the performance of seven-level multilevel inverter on
minimizing the harmonics distortion. The Matlab Simulink is used to model the seven-level
cascaded multilevel inverter. The multilevel inverter is used for various loads testing such as R,
RL and RC load. The performance of seven levels cascaded inverter will be analyzed by using
bipolar and unipolar switching schemes. Meanwhile, the simulation results are compared and
validated that the bipolar seven-level cascaded inverter has the minimum harmonic distortion
.
1.5 Report Outline
This report contains five chapters and start with the introduction of project which is
multilevel inverter. The following five chapters of this project are arranged as follows:
Chapter 1 covers the short explanation about this research project, problem statement,
objective and scope.
Chapter 2 covers the theoretical background of this project including the detail about the
types of inverter, the general topologies of multilevel inverter, multilevel inverter modulation
control schemes, PWM consideration, power quality and total harmonic distortion definition.
Chapter 3 states about the research methodology. This chapter consists of the flowchart of
project and the switching methods used in this project and the simulation results which are the
designing Simulink block by using bipolar and unipolar switching schemes.
Chapter 4 discusses the simulation result by using different non-linear loads. Harmonic
analysis for the non-linear loads will be discussed to evaluate the performance of the inverter.
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4
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
Inverters are circuits that convert a DC power into an AC power at a desired output
voltage and frequency. Figure 2.1 shows the block diagram of inverter. Inverters can be in
single phase or multiphase and they deliver bipolar current waveform and allow for
bi-directional power flow [1]. Inverters are widely used in various applications requiring variable
voltage and variable frequency AC supply. Some important applications are adjustable-speed
AC motor drives, DC motor drives; uninterruptible power supplies (UPS), induction heating,
standby power supply and high voltage DC transmission systems [2].
Figure 2.1: Block Diagram of Inverter
2.2 Classification of Inverter
There are different basis types of inverters. The two major types of inverters are
current-source inverter (CSI) and voltage-source inverter (VSI). Their input sources are either
a constant current or constant voltage.
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5
2.2.1 Current-source Inverter (CSI)
Current-
source inverter is fed by a „current source‟ with an inductor in series with a dc
source. Therefore, the supply current does not change quickly. Figure 2.2 shows the block
diagram of CSI inverter. CSI inverters are in general constructed with gate-turnoff thyristor
(GTO) and refer to high power levels. The load current is varied by controlling the DC input
voltage to the current source inverter. CSI are used in a very high-power AC drives [3].
Figure 2.2: Circuit Configuration of CSI inverter
2.2.2 Voltage
–
Source Inverter (VSI)
Voltage-source inverter is fed by a DC source of small internal impedance. VSI
inverters are constructed with insulated-gate bipolar transistor (IGBTs) or GTO. The AC
terminal output voltage remains almost constant irrespective of the load current drawn. There
are two types of VSI which are square wave inverter and PWM inverter [3].
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6
2.2.2.1 Single-phase Half-bridge Inverter
The basic topology of a single-phase is half-bridge inverter fed by a DC voltage source
as shown in Figure 2.4. S1 and S2 are gate-commutated power semiconductor switches. When
closed, these switches conduct current flows. The gating signal of switches, S
1and S
2are
shown in Figure 2.5. When S
1is closed, the load voltage is Vdc/2. When S
2is closed, the load
voltage is
–
Vdc/2.Thus, a square wave or bipolar pulse width modulated output voltage can be
produced. The output voltage of half-bridge is shown as Figure 2.6 too [3].
V/2
V/2
Vload Io
S1
S2 load
Figure 2.4: Circuit Configuration of a Single-phase Half-bridge Inverter
S1
T/2
T
t
0
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7
S2
T/2
T
t
0
(b)
Figure 2.5: (a) Gating Signal for Switch S
1(b) Gating Signal for Switch S
2T/2
T
t
0
Vdc/2
-Vdc/2
V
Figure 2.6: Output Voltage of Single phase half-bridge inverter
2.2.2.2 Single-phase Full- bridge Inverter
The basic topology of a single-phase is full-bridge inverter fed by a DC voltage source
as shown in Figure 2.7. The inverter uses two pairs of controlled switches (S
1S
2and S
3S
4) and
two pairs of diodes (D
1D
2and D
3D
4). The devices of one pair operate simultaneously.
However, in reality, there must have switching transition time or blanking time to control the
closing and opening of switches Overlapping opening of switches will result in short circuit or
shoot-through fault. The gating signals of the switch-pairs S
1S
2and S
3S
4are shown in Figure
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2 1.2 Problem Statement
In the past, square wave inverters were widely used in independent wind or solar power systems and some industrial applications with lower requirement on power quality. In square wave inverter, the harmonics and output voltage amplitude could not be controlled by the user. In addition, in AC power distribution system, harmonics occur when the normal electric current waveform is distorted by non-linear loads such as computer equipment with switched-mode power supplies, variable speed motors and drive, fluorescent lamp ballasts and others.
Harmonic decreases significantly in power quality and live cycle of electrical equipment. Basically, harmonic distortion will increase resistive losses, voltage stresses and excessive voltage distortion on power distribution system.
Therefore, the multilevel inverter is used to produce multilevel-output voltages which are purely sinusoidal or synthesis a staircase voltage waveform and thus reduce harmonic content. Higher frequency harmonics are easier to filter than harmonics near the fundamental frequency. Bipolar and unipolar switching schemes are chosen because these methods able to double the switching frequency of the inverter. However, some of low frequency harmonics are formed due to non-linear loads, therefore, bipolar switched multilevel inverter is proposed due to able to filter the low frequency harmonics More number of levels of multilevel inverter will give better performance in term of total harmonic distortion.
1.3 Objective
The objectives of this project are:
1. To model there phase seven-level multilevel inverter using bipolar and unipolar switching technique by using Matlab Simulink Software.
2. To analyze and evaluate the simulation of seven-level of multilevel inverter by using bipolar and unipolar switching schemes.
3. To validate the THD for bipolar and unipolar switching schemes for multilevel inverter by using varieties of load testing.
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3 1.4 Scope
This project focuses on the performance of seven-level multilevel inverter on minimizing the harmonics distortion. The Matlab Simulink is used to model the seven-level cascaded multilevel inverter. The multilevel inverter is used for various loads testing such as R, RL and RC load. The performance of seven levels cascaded inverter will be analyzed by using bipolar and unipolar switching schemes. Meanwhile, the simulation results are compared and validated that the bipolar seven-level cascaded inverter has the minimum harmonic distortion
.
1.5 Report Outline
This report contains five chapters and start with the introduction of project which is multilevel inverter. The following five chapters of this project are arranged as follows:
Chapter 1 covers the short explanation about this research project, problem statement, objective and scope.
Chapter 2 covers the theoretical background of this project including the detail about the types of inverter, the general topologies of multilevel inverter, multilevel inverter modulation control schemes, PWM consideration, power quality and total harmonic distortion definition. Chapter 3 states about the research methodology. This chapter consists of the flowchart of project and the switching methods used in this project and the simulation results which are the designing Simulink block by using bipolar and unipolar switching schemes.
Chapter 4 discusses the simulation result by using different non-linear loads. Harmonic analysis for the non-linear loads will be discussed to evaluate the performance of the inverter. Chapter 5 is the summary of this project.
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4 CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
Inverters are circuits that convert a DC power into an AC power at a desired output voltage and frequency. Figure 2.1 shows the block diagram of inverter. Inverters can be in single phase or multiphase and they deliver bipolar current waveform and allow for bi-directional power flow [1]. Inverters are widely used in various applications requiring variable voltage and variable frequency AC supply. Some important applications are adjustable-speed AC motor drives, DC motor drives; uninterruptible power supplies (UPS), induction heating, standby power supply and high voltage DC transmission systems [2].
Figure 2.1: Block Diagram of Inverter
2.2 Classification of Inverter
There are different basis types of inverters. The two major types of inverters are current-source inverter (CSI) and voltage-source inverter (VSI). Their input sources are either a constant current or constant voltage.
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5 2.2.1 Current-source Inverter (CSI)
Current-source inverter is fed by a „current source‟ with an inductor in series with a dc source. Therefore, the supply current does not change quickly. Figure 2.2 shows the block diagram of CSI inverter. CSI inverters are in general constructed with gate-turnoff thyristor (GTO) and refer to high power levels. The load current is varied by controlling the DC input voltage to the current source inverter. CSI are used in a very high-power AC drives [3].
Figure 2.2: Circuit Configuration of CSI inverter
2.2.2 Voltage – Source Inverter (VSI)
Voltage-source inverter is fed by a DC source of small internal impedance. VSI inverters are constructed with insulated-gate bipolar transistor (IGBTs) or GTO. The AC terminal output voltage remains almost constant irrespective of the load current drawn. There are two types of VSI which are square wave inverter and PWM inverter [3].
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6 2.2.2.1 Single-phase Half-bridge Inverter
The basic topology of a single-phase is half-bridge inverter fed by a DC voltage source as shown in Figure 2.4. S1 and S2 are gate-commutated power semiconductor switches. When closed, these switches conduct current flows. The gating signal of switches, S1 and S2 are
shown in Figure 2.5. When S1 is closed, the load voltage is Vdc/2. When S2 is closed, the load
voltage is –Vdc/2.Thus, a square wave or bipolar pulse width modulated output voltage can be produced. The output voltage of half-bridge is shown as Figure 2.6 too [3].
V/2
V/2
Vload Io
S1
S2 load
Figure 2.4: Circuit Configuration of a Single-phase Half-bridge Inverter
S1
T/2
T
t
0
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7
S2
T/2
T
t
0
(b)
Figure 2.5: (a) Gating Signal for Switch S1 (b) Gating Signal for Switch S2
T/2
T
t
0
Vdc/2
-Vdc/2
V
Figure 2.6: Output Voltage of Single phase half-bridge inverter
2.2.2.2 Single-phase Full- bridge Inverter
The basic topology of a single-phase is full-bridge inverter fed by a DC voltage source as shown in Figure 2.7. The inverter uses two pairs of controlled switches (S1S2 and S3S4) and
two pairs of diodes (D1D2 and D3D4). The devices of one pair operate simultaneously.
However, in reality, there must have switching transition time or blanking time to control the closing and opening of switches Overlapping opening of switches will result in short circuit or shoot-through fault. The gating signals of the switch-pairs S1S2 and S3S4 are shown in Figure