Power Electronic Transformer based Three-Phase PWM AC Drives

125 Pages·2012·3.72 MB·English

Power Electronic Transformer based
Three-Phase PWM AC Drives
A DISSERTATION
SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF THE UNIVERSITY OF MINNESOTA
BY
Kaushik Basu
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
Doctor of Philosophy
Professor Ned Mohan
December, 2012
(cid:13)c Kaushik Basu 2012
ALL RIGHTS RESERVED
Acknowledgements
I want to express my profound gratitude to Professor Mohan for being my advi-
sor. I really appreciate his encouragement and relentless support particularly in
difficult times. He always listened to my ideas and discussions with him led to key
insights. His commitment to teaching is an inspiration to any one like me who
aspires to be a teacher. Above all, he made me feel a friend, which I appreciate
from the bottom of my heart.
I express my heartfelt thanks to Professor Robbins for giving me an opportu-
nity to teach courses and being supportive atall times. Iconsider myself fortunate
to work with Professor Wollenburg during faculty workshops. I am grateful to
Professor Anderson for interesting discussions in mathematics.
I appreciate the financial support provided by Office of Naval research. Ithank
Mr Dan Dobrick for the support extended in procuring components.
I would specially like to thank Mohapatra for introducing to the idea of power
electronic transformers. I am grateful to Dr Chris Henze and professor D.T.
Shahanifortheirtimeandhelpfuldiscussionsduringthehardwareimplementation
of this project.
I thank Hari, Apurva, Shanker, Rohit, Viswesh, Ruben and Ashish for their
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help and support at various stages of my project.
I thank all my friends Arushi, Apurva, Eric, Gysler, Rashmi, Nathan, Ruben,
David, John, Ranjan, Saurav, Kartik, Srikant, Hari, Shanker, Rohit, Viswesh,
Ashish for creating a charming atmosphere in the lab.
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Dedication
To my parents who valued education above all.
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Abstract
A Transformer is used to provide galvanic isolation and to connect systems at
different voltage levels. It is one of the largest and most expensive component in
most of the high voltage and high power systems. Its size is inversely proportional
totheoperatingfrequency. Thecentralideabehindapowerelectronictransformer
(PET)alsoknownassolidstatetransformeristoreducethesizeofthetransformer
by increasing the frequency. Power electronic converters are used to change the
frequency ofoperation. Steadyreductioninthecostofthesemiconductor switches
andtheadvent ofadvancedmagneticmaterialswithvery lowlossdensity andhigh
saturation flux density implies economic viability and feasibility of a design with
high power density. Application of PET is in generation of power from renewable
energy sources, especially wind and solar. Other important application include
grid tied inverters, UPS e.t.c.
In this thesis non-resonant, single stage, bi-directional PET is considered. The
main objective of this converter is to generate adjustable speed and magnitude
pulse width modulated (PWM) ac waveforms from an ac or dc grid with a high
frequency ac link. The windings of a high frequency transformer contains leakage
inductance. Any switching transition of the power electronic converter connecting
the inductive load and the transformer requires commutation of leakage energy.
Commutation by passive means results in power loss, decrease in the frequency of
operation, distortion in the output voltage waveform, reduction in reliability and
power density.
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In this work a source based partially loss-less commutation of leakage energy
has been proposed. This technique also results in partial soft-switching. A series
of converters with novel PWM strategies have been proposed to minimize the
frequency of leakage inductance commutation. These PETs achieve most of the
important features of modern PWM ac drives including 1) Input power factor
correction, 2) Common-mode voltage suppression at the load end, 3) High quality
output voltage waveform (comparable to conventional space vector PWM modu-
latedtwolevel inverter)and4)Minimizationofoutputvoltageloss, common-mode
voltage switching and distortion of the load current waveform due to leakage in-
ductance commutation. All of the proposed topologies along with the proposed
control schemes have been analyzed and simulated in MATLAB Simulink. A
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hardware prototype has been fabricated and tested. The simulation and experi-
mental results verify the operation and advantages of the proposed topologies and
their control.
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Contents
Acknowledgements i
Dedication iii
Abstract iv
List of Tables ix
List of Figures x
1 Introduction 1
1.1 Power electronic transformers . . . . . . . . . . . . . . . . . . . . 1
1.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Current state of the art . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1 dc/ac PETs . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2 ac/ac PETs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4 The problem of leakage inductance commutation . . . . . . . . . . 9
1.4.1 Proposed solution and outline of the thesis . . . . . . . . . 13
2 Modulation 15
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2.1 Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Modulation for ac/ac topology . . . . . . . . . . . . . . . . . . . . 17
2.3 Modulation for dc/ac topology . . . . . . . . . . . . . . . . . . . . 22
3 Commutation 28
3.1 The Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4 Results 38
4.1 Experimental set up . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.1.1 Power and Driver circuits . . . . . . . . . . . . . . . . . . 39
4.1.2 Control platform and Measurement card . . . . . . . . . . 41
4.1.3 High frequency transformers . . . . . . . . . . . . . . . . . 42
4.1.4 Input filter and protection circuits . . . . . . . . . . . . . . 43
4.1.5 Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.1.6 motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.2.1 Results for the dc/ac topology . . . . . . . . . . . . . . . . 57
5 Conclusion 64
References 67
Appendix A. PETS without common-mode voltage elimination 72
A.1 Analysis and Simulation . . . . . . . . . . . . . . . . . . . . . . . 73
A.1.1 Modulation for the dc/ac topology . . . . . . . . . . . . . 73
A.1.2 Modulation of the ac/ac converter . . . . . . . . . . . . . . 77
A.1.3 Commutation for the dc/ac topology . . . . . . . . . . . . 80
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A.1.4 Simulation results of dc/ac topology . . . . . . . . . . . . 87
A.1.5 Simulation results ac/ac topology . . . . . . . . . . . . . . 88
A.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Appendix B. Inter-winding Capacitance 94
Appendix C. A PET based on indirect modulation 97
C.1 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
C.1.1 PWM Strategy for the Front End Converter . . . . . . . . 98
C.1.2 Isolation and Rectification . . . . . . . . . . . . . . . . . . 103
C.1.3 PWM strategy for the output inverter . . . . . . . . . . . 104
C.1.4 Simulation results . . . . . . . . . . . . . . . . . . . . . . . 107
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