An Investigation into Pulse-Width Modulated AC Electric Drives with Open-End Winding Machines

72 Pages·2013·1.82 MB·English

An Investigation into Pulse-Width Modulated AC Electric
Drives with Open-End Winding Machines
A THESIS
SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF THE UNIVERSITY OF MINNESOTA
BY
Apurva Somani
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Advisor: Prof. Ned Mohan
January, 2013
(cid:13)c Apurva Somani 2013
ALL RIGHTS RESERVED
Acknowledgements
I would like to thank my advisor Prof. Ned Mohan for giving me the opportunity to
work on my research project and for his continued support and guidance. I consider
myself very lucky to have worked in his research group and his passion for research and
education has been very inspirational.
I would also like to thank Prof. William Robbins, Prof. Bruce Wollenberg and Prof.
Zongxuan Sun for being in my oral examination committee and for their valuable input
toward my research.
Financial support for the project from the Office of Naval Research (ONR) is also
gratefully acknowledged.
Thanks are also due to my colleagues in the Power Electronics Research Lab for
their support, time and camaraderie.
On a personal front, I would like to thank my grandma, parents, sister and wife for
their unconditional love and sacrifice.
i
Dedication
To my grandma.
ii
Abstract
Pulse-width modulated (PWM) ac drives have foundnumerous applications in industry
and energy generation. Such drives offer advantages of higher efficiency and a wider
rangeof operation as compared to line-connected machines. Thereare, however, certain
disadvantages associated with PWM ac drives. These drives utilize bulky electrolytic
capacitors in their power electronic sections which are costly. Also, the PWM inverter
generates a switching common-mode voltage at the machine terminals. This causes
spurious ground currents and harmful bearing currents through capacitively coupled
paths to ground. Conventional ac machines used in electric drives are either star-
or delta-connected and the machine has three terminals which are fed using a power
electronic converter. In open-end winding machines, this star or delta connection is
opened and the machine now has six terminals. These six terminals are then fed using
twothree-phasepowerconverters. Therearecertainadvantagestotheopen-endwinding
method, such as common-mode voltage reduction and increase in the voltage transfer
ratio. Open-end winding ac drives have been investigated in this thesis. Different
modulationstrategies havebeencomparedforthebestperformanceintermofcommon-
mode characteristics and output waveform quality. The inherent issue of circulating
currents has been investigated and solutions have been proposed. Drive structures
without dc-link capacitors have been proposed, analysed and their performance has
been validated and evaluated.
iii
Contents
Acknowledgements i
Dedication ii
Abstract iii
List of Tables vi
List of Figures vii
1 Introduction 1
1.1 PWM Converters and Electric Machines . . . . . . . . . . . . . . . . . . 2
2 Modulation Strategies for Direct-Link Drive for Open-End Winding
AC Machines 4
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Direct-Link Drive and Its Operation . . . . . . . . . . . . . . . . . . . . 5
2.3 Modulation of Direct-Link Drive . . . . . . . . . . . . . . . . . . . . . . 6
2.3.1 Space vector based modulation strategy . . . . . . . . . . . . . . 7
2.3.2 Carrier-based modulation . . . . . . . . . . . . . . . . . . . . . . 8
2.3.3 Space vector modulation using difference vector . . . . . . . . . . 10
2.4 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.5 Hardware Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
iv
3 Circulating Currents in PWM Open-End Winding AC Machines 18
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2 The Dual Two-Level Inverter . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2.1 Effect of device voltage drops . . . . . . . . . . . . . . . . . . . . 22
3.2.2 Effect of dead-time . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.3 Direct Matrix Converter based Open-End Winding Drive . . . . . . . . 26
3.3.1 Effect of device voltage drops . . . . . . . . . . . . . . . . . . . . 27
3.3.2 Effect of dead-time . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.4 Application to Open-End Winding Induction Machine Drives and Results 31
3.4.1 Dual two-level inverter based open-end winding drive . . . . . . 32
3.4.2 Direct matrix converter based open-end winding drive . . . . . . 34
3.5 Suppression of Circulating Currents . . . . . . . . . . . . . . . . . . . . 36
3.5.1 Common-mode chokes . . . . . . . . . . . . . . . . . . . . . . . . 36
3.5.2 Dead-time compensation . . . . . . . . . . . . . . . . . . . . . . . 36
3.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4 Comparison of Modulation Strategies 40
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.2 Modulation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.2.1 Space Vector PWM . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.2.2 Space Vector PWM with Dead-time Compensation . . . . . . . . 41
4.2.3 Carrier PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.3 Shifted Carrier Modulation . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5 Indirect Matrix Converter Based Open-End Winding Drive 47
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.2 Application to Open-End Winding Drives . . . . . . . . . . . . . . . . . 47
6 Conclusion 53
6.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
References 56
v
List of Tables
2.1 Mapping of resultant vectors to individual inverters . . . . . . . . . . . . 8
2.2 Mapping of resultant zero vectors to individual inverters . . . . . . . . . 8
2.3 System Parameters used for simulation . . . . . . . . . . . . . . . . . . . 11
2.4 System Parameters for experimental setup . . . . . . . . . . . . . . . . . 15
3.1 Common-mode voltage across the windings in dual two-level inverter . . 24
3.2 Common-mode voltage across the windings for Direct Matrix Converter
based Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
vi
List of Figures
1.1 Parasitic coupling in an electric drive system . . . . . . . . . . . . . . . 2
2.1 Direct-link drive for open-end winding ac machines . . . . . . . . . . . . 5
2.2 (a) Direct-link voltage for direct-link drive (b) Input current for direct-
link drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Resultant space vectors across open-end windings . . . . . . . . . . . . . 7
2.4 Space vector modulation using difference vector . . . . . . . . . . . . . . 10
2.5 Output phase voltage (v ) and output phase current (i ) for space
out out
vector-based PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.6 Input phase voltage (v ) and input phase current (i ) for space vector-
in in
based PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.7 Common-mode voltage at machine terminal (v ) and across phase
CM1
windings (v ) for space vector-based PWM . . . . . . . . . . . . . . . 13
CM
2.8 Output phase voltage (v ) and output phase current (i ) for carrier-
out out
based PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.9 Input phase voltage (v ) and input phase current (i ) for carrier-based
in in
PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.10 Common-mode voltage at machine terminal (v ) and across phase
CM1
windings (v ) for carrier-based PWM . . . . . . . . . . . . . . . . . . 14
CM
2.11 Photograph of experimental setup . . . . . . . . . . . . . . . . . . . . . 15
2.12 Output phase voltage (v ) and output phase current (i ) for space
out out
vector-based PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.13 Input phase voltage (v ) and input phase current (i ) for space vector-
in in
based PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.14 Common-mode voltage at machine terminal for space vector-based PWM 17
vii
2.15 Deadtime effect on common-mode voltage for space vector-based PWM 17
3.1 Dual two-level inverter and its equivalent circuit . . . . . . . . . . . . . 21
3.2 Individual and resultant vectors of dual two-level inverter . . . . . . . . 22
3.3 Equivalent circuit of dual-inverter topology for (i > 0, i > 0 and i < 0) 23
A B C
3.4 Phase currents and v due to device voltage-drops . . . . . . . . . . . 24
cm12
3.5 Switching diagram for INV2 in sector 1 with (i > 0, i > 0 and i < 0) 26
A B C
3.6 Direct matrix converter based open-end winding drive . . . . . . . . . . 26
3.7 Equivalent circuit for (i > 0, i > 0 and i < 0) . . . . . . . . . . . . 27
A B C
3.8 Individual and resultant vectors for direct matrix converter based open-
end winding drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.9 Switching sequence of MC1 with four-step commutation . . . . . . . . . 30
3.10 Clamp-circuit commutation of matrix converter drive . . . . . . . . . . . 31
3.11 Zero sequence equivalent circuit for an induction motor . . . . . . . . . 31
3.12 ‘Glitches’incommon-modevoltageduetodead-timeanddeviceswitching
characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.13 Phase current, common-mode voltage and zero-sequence current for dual
two-level inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.14 Phase currents and zero-sequence current in no-load induction motor . . 34
3.15 Effect of T /T on circulating currents . . . . . . . . . . . . . . . . . . . 35
d s
3.16 Circulating current in direct matrix converter based open-end winding
drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.17 Circulating currents in direct-link drive for open-end winding machines . 36
3.18 ‘Glitches’ in common-mode voltage removed by dead-time compensation 38
3.19 Comparison of current spectra (zero-sequence currents are at 3rd har-
monic which is 60 Hz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.1 Phase current for space vector modulation . . . . . . . . . . . . . . . . . 41
4.2 Shaft voltage and ground current for space vector modulation . . . . . . 42
4.3 Phase current for space vector modulation with dead-time compensation 42
4.4 Shaft voltage and ground current for space vector modulation with dead-
time compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.5 Phase current for carrier modulation . . . . . . . . . . . . . . . . . . . . 43
4.6 Shaft voltage and ground current for carrier modulation . . . . . . . . . 44
viii