optimal pulse width modulation of multilevel inverters for medium voltage drives amarendra ...

209 Pages·2016·28.45 MB·English

OPTIMAL PULSE WIDTH MODULATION OF
MULTILEVEL INVERTERS FOR MEDIUM
VOLTAGE DRIVES
AMARENDRA EDPUGANTI
NATIONAL UNIVERSITY OF SINGAPORE
2015
OPTIMAL PULSE WIDTH MODULATION OF
MULTILEVEL INVERTERS FOR MEDIUM
VOLTAGE DRIVES
AMARENDRA EDPUGANTI
(M.Tech, IIT Kanpur, India)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF
PHILOSOPHY
DEPARTMENT OF ELECTRICAL AND
COMPUTER ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2015
DECLARATION
I hereby declare that this thesis is my original work and it has been
written by me in its entirety. I have duly acknowledged all the sources of
information which have been used in the thesis.
This thesis has also not been submitted for any degree in any university
previously.
Amarendra Edpuganti
September 7, 20L5
i
Acknowledgements
It is my great pleasure to remember and appreciate all those directly or in-
directly involved in successful completion of the thesis. First of all, I would
like to express my sincere gratitude to my thesis supervisor Dr. Akshay
Kumar Rathore for his esteemed guidance, encouragement and trust in my
abilitythroughoutstudy. Hissubjectexpertise, perfectplanningandvision
has been greatly instrumental in completing the thesis within three years.
I am also grateful for his kind support in obtaining internship position at
Rolls-Royce, Singapore and part-time research job in university.
I would like to thank National University of Singapore for giving me the
opportunity to pursue doctoral studies and awarding research scholarship.
I would also like to thank lab officers Mr. Y.C. Woo, Mr. M. Chandra, Mr.
Seow, Mr. AbdulJalil, andMr. Teofortheirsupportindoingexperimental
work. I would like to thank Dr. Amit Gupta for giving me the internship
opportunity at Rolls-Royce, Singapore and Michael Zagrodnik for deep
technical discussions during internship.
I am also grateful to my colleagues Pan Xuewei, Radha, Ravi, Gnana,
Debjani, Satarupa, Devendra, Anirban, Suvendu, Dorai babu, Kanakesh,
Divya, Anurag, Siva, Prasanna, Jeevan, and Kalpani for their support and
interactions during studies. I am also indebted to my friends Rajasekhar,
Ashok, Suvendu, Himanshu Singh, Arjun, Vishal, Gopal, Manish Kanjikar,
Shiva,LalanKumar,Santosh,Kamesh,Virendra,andBalaKumarfortheir
support in having peaceful stay in Singapore.
I am obliged to my parents for showing me the path to pursue doctoral
studies. I am specially grateful to my wife Maheswari for her unconditional
love and support throughout my thesis.
ii
I owe a huge gratitude to my spiritual master H.H. Radhanath Swami
for giving me the vedic wisdom to pursue a meaningful life and also for
inspiring his disciples to become an ideal example in the society. I am
specially grateful to H.G Devaki Nandan prabhu for his valuable guidance
through out my stay in Singapore. I am also grateful to H.G Shankar
Pandit Pr for his maturity and wisdom. Most of all, I thank their lordships
Sri Sri Jagannath Baladeva Subhadra for giving me the opportunity and
intelligence to carry out the research work.
Contents
Acknowledgements i
Table of Contents iii
Summary vi
List of Tables viii
List of Figures x
List of Acronyms xix
List of Symbols xxi
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Literature Survey . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1 LDSF modulation techniques . . . . . . . . . . . . . 5
1.2.2 Emerging MLC topologies . . . . . . . . . . . . . . . 20
1.3 Performance comparison of LDSF modulation techniques . . 25
1.4 Research Problem and Objectives . . . . . . . . . . . . . . . 27
1.5 Thesis Contributions . . . . . . . . . . . . . . . . . . . . . . 28
1.6 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.7 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . 30
2 Generalized SOP Technique 31
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2 Basics of SOP . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3 Estimation of pulse number, N . . . . . . . . . . . . . . . . 33
2.4 Optimization of switching angles . . . . . . . . . . . . . . . 35
2.4.1 Multilevel Structures . . . . . . . . . . . . . . . . . . 37
2.4.2 Optimization Algorithm . . . . . . . . . . . . . . . . 38
CONTENTS iv
2.4.3 Optimization Results . . . . . . . . . . . . . . . . . . 41
2.5 Allocation of Optimal Switching Angles . . . . . . . . . . . . 43
2.6 SOP of Cascaded 7L MLC . . . . . . . . . . . . . . . . . . . 44
2.6.1 Circuit Topology and Operation . . . . . . . . . . . . 44
2.6.2 Implementation of SOP technique . . . . . . . . . . . 46
2.6.3 Experimental results . . . . . . . . . . . . . . . . . . 52
2.7 SOP of Cascaded 9L MLC . . . . . . . . . . . . . . . . . . . 62
2.7.1 Circuit topology and Operation . . . . . . . . . . . . 63
2.7.2 Implementation of SOP Technique . . . . . . . . . . 63
2.7.3 Experimental Results . . . . . . . . . . . . . . . . . . 71
2.7.4 Performance Comparison . . . . . . . . . . . . . . . . 83
2.8 Summary and Conclusions . . . . . . . . . . . . . . . . . . . 84
3 Modified Generalized SOP Technique 85
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.2 Modified Generalized SOP . . . . . . . . . . . . . . . . . . . 87
3.2.1 New method for estimation of N . . . . . . . . . . . 87
3.2.2 Optimization Results . . . . . . . . . . . . . . . . . . 90
3.3 Experimental Results for 7L MLC . . . . . . . . . . . . . . . 92
3.4 Experimental Results for 9L MLC . . . . . . . . . . . . . . . 102
3.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . 105
4 Modified SOP Technique for MMC Topology 114
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.2 Circuit Topology and Operation . . . . . . . . . . . . . . . . 115
4.3 Modified SOP Technique . . . . . . . . . . . . . . . . . . . . 116
4.3.1 Calculation of N . . . . . . . . . . . . . . . . . . . . 117
4.3.2 Optimization of switching angles . . . . . . . . . . . 119
4.3.3 Allocation of Optimal Switching Angles . . . . . . . . 120
4.4 Experimental results . . . . . . . . . . . . . . . . . . . . . . 122
4.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . 127
5 Enhanced SOP Technique for Dual Inverter Based MLC
Topologies 128
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
CONTENTS v
5.2 Circuit topologies and Operation . . . . . . . . . . . . . . . 129
5.3 Enhanced SOP technique . . . . . . . . . . . . . . . . . . . . 132
5.3.1 Computation of pulse number N . . . . . . . . . . . 134
5.3.2 Optimization of Switching Angles . . . . . . . . . . . 139
5.3.3 Allocation of Optimal Switching Angles . . . . . . . . 140
5.4 Experimental results . . . . . . . . . . . . . . . . . . . . . . 142
5.4.1 D2L Inverter . . . . . . . . . . . . . . . . . . . . . . 143
5.4.2 D3L Inverter . . . . . . . . . . . . . . . . . . . . . . 146
5.4.3 Dual 3L-MMC . . . . . . . . . . . . . . . . . . . . . 149
5.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . 153
6 Conclusions and Future Work 155
6.1 Summary and Conclusions . . . . . . . . . . . . . . . . . . . 155
6.2 Summary of thesis contributions . . . . . . . . . . . . . . . . 158
6.3 Scope of Future Work . . . . . . . . . . . . . . . . . . . . . . 158
Bibliography 161
List of Publications 184
vi
Summary
Lowdeviceswitchingfrequencymodulationofmultilevelinverters(MLIs)
for medium voltage drives is essential to increase semiconductor device uti-
lization, minimize cooling requirements and achieve higher overall system
efficiency. However, there exists a trade-off between device switching fre-
quency and harmonic distortion of machine stator currents. Therefore,
classical modulation techniques such as sinusoidal pulsewidth modulation
and space vector modulation techniques utilize higher device switching fre-
quency (around 1 kHz) to generate good quality of machine stator cur-
rents. Based on the literature survey of several low device switching fre-
quencymodulationtechniques,ithasbeenidentifiedthatsynchronousopti-
mal pulsewidth modulation (SOP) is an effective and emerging modulation
technique to achieve better quality of machine stator currents.
Till now, SOP has been demonstrated for classical three-level and five-
level MLIs. On the other hand, the present trend in MV high power drives
is to use MLIs with higher number of voltage levels to achieve better qual-
ity of output voltage waveforms. Also, several new topologies have been
proposed as an alternative to classical MLI topologies and some of them
found industrial relevance as well. Therefore, there is a need to analyze
and propose modifications to the state-of-the-art SOP technique in order
to modulate MLIs with any number of voltage levels and also emerging
MLI topologies.
One of the main goal of the thesis is to develop systematic analysis
to implement state-of-the-art SOP technique for MLIs with any number
of voltage levels. The developed analysis has been utilized to modulate
cascaded seven-level and nine-level MLIs with peak device switching fre-
quency limited to rated fundamental frequency (50 Hz). However, it has
been noticed that power semiconductor devices operate at unequal device
switching frequency and peak device switching frequency is not limited to
desired value. Subsequently, modified SOP technique has been proposed to
overcome these issues. The proposed technique has been utilized to mod-
vii
ulate seven or higher-level MLIs with peak device switching frequency set
at rated fundamental frequency (50 Hz).
Also, the thesis has explored the feasibility of implementing SOP tech-
nique for one of the emerging topology known as modular multilevel con-
verter (MMC). One of the main challenges for modulation of MMC is to
achieve balanced floating capacitor voltages. Based on the study, a system-
atic algorithm has been developed to implement SOP for MMC topology.
The proposed technique has been demonstrated for five-level MMC with
peak device switching frequency set at 2000 Hz.
In addition, the thesis has investigated the issue of common-mode cur-
rents in machine stator windings for the dual inverter based MLI fed open-
end stator winding induction motor drives. An enhanced SOP technique
has been proposed to achieve high quality stator currents with elimination
of common-mode currents, while operating power semiconductor devices
at few hundred Hertz. Finally, dual nL-MMC topology has been proposed
for open-end stator winding induction motor drives, which can be operated
from a single dc source and generates any number of voltage levels. The
proposed technique has been demonstrated experimentally with low power
prototypes of dual two-level, dual three-level and dual three-level MMC
topologies.