High-Power Converters and AC Drives E-Book

Bin Wu

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: IEEE Press Series on Power Engineering
• Sprache: Englisch

Seitenzahl: 617

Veröffentlichungsjahr: 2016

Beschreibung

A comprehensive reference of the latest developments in MV drive technology in the area of power converter topologies This new edition reflects the recent technological advancements in the MV drive industry, such as advanced multilevel converters and drive configurations. It includes three new chapters, Control of Synchronous Motor Drives, Transformerless MV Drives, and Matrix Converter Fed Drives. In addition, there are extensively revised chapters on Multilevel Voltage Source Inverters and Voltage Source Inverter-Fed Drives. This book includes a systematic analysis on a variety of high-power multilevel converters, illustrates important concepts with simulations and experiments, introduces various megawatt drives produced by world leading drive manufacturers, and addresses practical problems and their mitigations methods. This new edition: * Provides an in-depth discussion and analysis of various control schemes for the MV synchronous motor drives * Examines new technologies developed to eliminate the isolation transformer in the MV drives * Discusses the operating principle and modulation schemes of matrix converter (MC) topology and multi-module cascaded matrix converters (CMCs) for MV drives, and their application in commercial MV drives Bin Wu is a Professor and Senior NSERC/Rockwell Automation Industrial Research Chair in Power Electronics and Electric Drives at Ryerson University, Canada. He is a fellow of Institute of Electrical and Electronics Engineers (IEEE), Engineering Institute of Canada (EIC), and Canadian Academy of Engineering (CAE). Dr. Wu has published more than 400 papers and holds more than 30 granted/pending US/European patents. He co-authored several books including Power Conversion and Control of Wind Energy Systems and Model Predictive Control of Wind Energy Conversion Systems (both by Wiley-IEEE Press). Mehdi Narimani is a Postdoctoral Research Associate with the Department of Electrical and computer Engineering at Ryerson University, Canada, and Rockwell Automation Canada. He is a senior member of IEEE. Dr. Narimani is author/co-author of more than 50 technical papers and four US/European patents (issued/pending review). His current research interests include power conversion, high power converters, control of power electronics, and renewable energy systems.

Leseprobe

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Tariq Samad, Editor in Chief

George W. Arnold

Xiaoou Li

Ray Perez

Giancarlo Fortino

Vladimir Lumelsky

Linda Shafer

Dmitry Goldgof

Pui-In Mak

Zidong Wang

Ekram Hossain

Jeffrey Nanzer

MengChu Zhou

HIGH-POWER CONVERTERS AND AC DRIVES

Second Edition

Bin Wu Mehdi Narimani

Copyright © 2017 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data is available.

ISBN: 978-1-119-15603-1

CONTENTS

About the Authors

Preface and Acknowledgments

List of Abbreviations

Part One Introduction

Chapter 1 Introduction

1.1 Overview of High-Power Drives

1.2 Technical Requirements and Challenges

1.3 Converter Configurations

1.4 Industrial MV Drives

1.5 Summary

References

Appendix

Chapter 2 High-Power Semiconductor Devices

2.1 Introduction

2.2 High-Power Switching Devices

2.3 Operation of Series Connected Devices

2.4 Summary

References

Part Two Multipulse Diode and SCR Rectifiers

Chapter 3 Multipulse Diode Rectifiers

3.1 Introduction

3.2 Six-Pulse Diode Rectifier

3.3 Series-Type Multipulse Diode Rectifiers

3.4 Separate-Type Multipulse Diode Rectifiers

3.5 Summary

References

Chapter 4 Multipulse SCR Rectifiers

4.1 Introduction

4.2 Six-Pulse SCR Rectifier

4.3 12-Pulse SCR Rectifier

4.4 18- and 24-Pulse SCR Rectifiers

4.5 Summary

References

Chapter 5 Phase–Shifting Transformers

5.1 Introduction

5.2 Y/Z Phase–Shifting Transformers

5.3 Δ/Z Transformers

5.4 Harmonic Current Cancellation

5.5 Summary

Part Three Multilevel Voltage Source Converters

Chapter 6 Two–Level Voltage Source Inverter

6.1 Introduction

6.2 Sinusoidal PWM

6.3 Space Vector Modulation

6.4 Summary

References

Chapter 7 Cascaded H–Bridge Multilevel Inverters

7.1 Introduction

7.2 H–Bridge Inverter

7.3 Multilevel Inverter Topologies

7.4 Carrier–Based PWM Schemes

7.5 Staircase Modulation

7.6 Summary

References

Chapter 8 Diode–Clamped Multilevel Inverters

8.1 Introduction

8.2 Three–Level Inverter

8.3 Space Vector Modulation

8.4 Neutral–Point Voltage Control

8.5 Carrier–Based PWM Scheme and Neutral–Point Voltage Control

8.6 Other Space Vector Modulation Algorithms

8.7 High–Level Diode–Clamped Inverters

8.8 NPC/H–Bridge Inverter

8.9 Summary

References

Appendix

Chapter 9 Other Multilevel Voltage Source Inverters

9.1 Introduction

9.2 Multilevel Flying-Capacitor Inverter

9.3 Active Neutral-Point Clamped Inverter

9.4 Neutral-Point Piloted Inverter

9.5 Nested Neutral-Point Clamped Inverter

9.6 Modular Multilevel Converter

9.7 Summary

References

Part Four PWM Current Source Converters

Chapter 10 PWM Current Source Inverters

10.1 Introduction

10.2 PWM Current Source Inverter

10.3 Space Vector Modulation

10.4 Parallel Current Source Inverters

10.5 Load-Commutated Inverter (LCI)

10.6 Summary

References

Appendix

Chapter 11 PWM Current Source Rectifiers

11.1 Introduction

11.2 Single-Bridge Current Source Rectifier

11.3 Dual-Bridge Current Source Rectifier

11.4 Power Factor Control

11.5 Active Damping Control

11.6 Summary

References

Appendix

Part Five High-Power AC Drives

Chapter 12 Voltage Source Inverter Fed Drives

12.1 Introduction

12.2 Two-Level VSI-Based MV Drives

12.3 Neutral Point Clamped (NPC) Inverter Fed Drives

12.4 Multilevel Cascaded H-Bridge (CHB) Inverter Fed Drives

12.5 NPC/H-Bridge Inverter Fed Drives

12.6 ANPC Inverter Fed Drive

12.7 MMC Inverter Fed Drive

12.8 10 KV-Class Drives with Multilevel Converters

12.9 Summary

References

Chapter 13 Current Source Inverter Fed Drives

13.1 Introduction

13.2 CSI Drives With PWM Rectifiers

13.3 Transformerless CSI Drive for Standard AC Motors

13.4 CSI Drive with Multipulse SCR Rectifier

13.5 LCI Drives for Synchronous Motors

13.6 Summary

References

Chapter 14 Control of Induction Motor Drives

14.1 Introduction

14.2 Reference Frame Transformation

14.3 Induction Motor Dynamic Models

14.4 Principle of Field Oriented Control (FOC)

14.5 Direct Field Oriented Control

14.6 Indirect Field Oriented Control

14.7 FOC for CSI Fed Drives

14.8 Direct Torque Control (DTC)

14.9 Summary

References

Chapter 15 Control of Synchronous Motor Drives

15.1 Introduction

15.2 Modeling of Synchronous Motor

15.3 VSC FED SM Drive with zero

d

-axis current (ZDC) Control

15.4 VSC FED SM Drive with MTPA Control

15.5 VSC FED SM Drive with DTC Scheme

15.6 Control of CSC FED SM Drives

15.7 Summary

References

Appendix

Part Six Special Topics on MV Drives

Chapter 16 Matrix Converter Fed MV Drives

16.1 Introduction

16.2 Classic Matrix Converter (MC)

16.3 Three-Module Matrix Converter

16.4 Multi-Module Cascaded Matrix Converter (CMC)

16.5 Multi-Module CMC Fed MV Drive

16.6 Summary

References

Chapter 17 Transformerless MV Drives

17.1 Introduction

17.2 Common-Mode Voltage Issues and Conventional Solution

17.3 CM Voltage Reduction in Multilevel Vsc

17.4 Transformerless Drives with Multilevel vsc

17.5 Transformerless CSI Fed Drives

17.6 Summary

References

Index

IEEE Press Series on Power Engineering

EULA

List of Tables

Chapter 1

Table 1.4-1

Chapter 2

Table 2.2-1

Table 2.2-2

Table 2.2-3

Table 2.2-4

Table 2.2-5

Table 2.3-1

Chapter 5

Table 5.2-1

Table 5.3-1

Chapter 6

Table 6.3-1

Table 6.3-2

Table 6.3-3

Table 6.3-4

Table 6.3-5

Table 6.3-6

Chapter 7

Table 7.3-1

Table 7.3-2

Table 7.4-1

Chapter 8

Table 8.2-1

Table 8.3-1

Table 8.3-2

Table 8.3-3

Table 8.3-4

Table 8.4-1

Table 8.7-1

Table 8.7-2

Chapter 9

Table 9.2-1

Table 9.3-1

Table 9.3-2

Table 9.4

Table 9.5-1

Table 9.5-2

Table 9.5-3

Table 9.5-4

Table 9.6-1

Table 9.6-2

Chapter 10

Table 10.3-1

Table 10.3-2

Table 10.4-1

Table 10.4-2

Chapter 11

Table 11.2-1

Table 11.5-1

Table 11.5-2

Table A-1

Table A-2

Chapter 12

Table 12.3-1

Table 12.3-2

Table 12.4-1

Table 12.6-1

Chapter 13

Table 13.2-1

Chapter 14

Table 14.5-1

Table 14.8-1

Table 14.8-2

Table 14.8-3

Chapter 15

Table 15.3-1

Table 15.5-1

Table 15.5-2

Table 15.6-1

Table A-1

A-2

Chapter 16

Table 16.3-1

Table 16.3-2

Table 16.4-1

Table 16.5-1

Chapter 17

Table 17.3-1

Table 17.3-2

Table 17.3-3

Table 17.3-4

Table 17.3-5

Table 17.3-6

Table 17.3-7

Guide

Cover

Table of Contents

Preface

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About the Authors

Bin Wu graduated from Donghua University, Shanghai, China in 1978, and received his M.A.Sc. and Ph.D. degrees in electrical and computer engineering from the University of Toronto, Canada in 1989 and 1993, respectively. He joined Ryerson University in 1993, where he is currently a Professor and Senior NSERC/Rockwell Automation Industrial Research Chair in Power Electronics and Electric Drives. Dr. Wu has published more than 400 technical papers, authored/coauthored several Wiley-IEEE Press books, and holds more than 30 granted/pending US/European patents in the area of power electronics, medium-voltage drives, and renewable energy systems.

Dr. Wu received the Gold Medal of the Governor General of Canada in 1993, Premier's Research Excellence Award in 2001, NSERC Synergy Award for Innovation in 2002, Ryerson Distinguished Scholar Award in 2003, Ryerson YSGS Outstanding Contribution to Graduate Education Award, and Professional Engineers Ontario (PEO) Engineering Excellence Medal in 2014. He is a fellow of Institute of Electrical and Electronics Engineers (IEEE), Engineering Institute of Canada (EIC), and Canadian Academy of Engineering (CAE).

Mehdi Narimani received his Ph.D. degree from University of Western Ontario, Ontario, Canada in 2012 and received his B.S. and M.S. degrees from Isfahan University of Technology (IUT), Isfahan, Iran in 1999 and 2002, respectively. He is currently assistant professor at the Department of Electrical and Computer Engineering at McMaster University, Hamilton, Ontario, Canada. Prior joining McMaster University, Dr. Narimani was a Power Electronics Engineer at Rockwell Automation Canada, in Cambridge, Ontario. He also worked as a faculty member of Isfahan University of Technology from 2002 to 2009 where he was involved in design and implementation of several industrial projects.

Dr. Narimani has published more than 55 journal and conference proceeding papers, and holds more than four issued/pending US/European patents. His current research interests include power conversion, high power converters, control of power electronics converters, and renewable energy Systems. Dr. Narimani is a senior member of Institute of Electrical and Electronics Engineers (IEEE).

Preface and Acknowledgments

There have been a number of new developments in high-power medium-voltage (MV) drive technology since 2006 when the first edition of this book was published. The second edition of the book incorporates these developments by adding three new chapters and revising two existing chapters.

The new chapters include Chapter 15 Control of Synchronous Motor Drives, where various control schemes for the synchronous motor drives are presented; Chapter 16 Matrix Converter Fed Drives, where multi-modular cascaded matrix converters developed for the MV drive are analyzed, and Chapter 17 Transformerless MV Drives, in which the technologies for the elimination of the isolation transformer in the MV drive are elaborated. Two chapters are extensively revised, including Chapter 9 Other Multilevel Voltage Source Inverters and Chapter 12 Voltage Source Inverter Fed Drives, where a number of newly developed converter topologies and drive configurations have been added.

The second edition of the book contains 6 parts with 17 chapters. Part 1 (Introduction) is composed of two chapters. Chapter 1 provides an overview of high-power converters, drive configurations, and typical applications. Chapter 2 introduces high-power semiconductor devices.

Part 2 (Multipulse Diode and SCR Rectifiers) contains three chapters on multipulse rectifiers, which are widely used in high-power drives as front-end converters. Chapter 3 deals with multipulse diode rectifiers, Chapter 4 addresses multipulse SCR rectifiers, and Chapter 5 introduces phase-shifting transformers used in the multipulse rectifiers.

Part 3 (Multilevel Voltage Source Converters) is composed of four chapters dealing with a variety of high-power voltage source converters. Chapter 6 introduces modulation techniques for a two-level inverter, which provides a basis for developing modulation schemes for multilevel inverters. Chapter 7 focuses on cascaded H-bridge inverters. Chapter 8 presents a detailed analysis on neutral point clamped multilevel inverters. Chapter 9 introduces other multilevel converter topologies that are recently developed for the MV drive.

Part 4 (PWM Current Source Converters) has two chapters for high-power PWM current source converters. Chapter 10 focuses on the switching schemes for the current source inverters whereas Chapter 11 puts more emphasis on power factor and active damping controls for the current source rectifiers.

Part 5 (High-Power AC Drives) consists of four chapters on high-power drive systems. Chapter 12 presents configurations of voltage source inverter fed MV drives while Chapter 13 is on current source inverter based drives. Chapter 14 presents advance control schemes for induction motor MV drives, including field-oriented control and direct torque control. Chapter 15 deals with advanced control schemes for synchronous motor MV drives such as maximum torque per amp control and direct torque control.

Part 6 (Special Topics on MV Drives) has two chapters on the state-of-the-art MV drives. Chapter 16 focuses on multi-modular cascaded matrix converter topologies and matrix converter fed drives. Chapter 17 presents transformerless MV drive configurations for both current and voltage source inverter fed drives.

The second edition of the book presents the latest technology in the field, provides design guidance with tables, charts, and graphs, addresses practical problems and their mitigation methods, and illustrates important concepts with computer simulations and experiments. It can serve as a reference for academic researchers, practicing engineers, and other professionals. This book also provides adequate technical background and can be adopted as a textbook for a graduate-level course in power electronics and ac drives.

Finally, we would like to express our deep gratitude to our colleagues at Rockwell Automation Canada, particularly, Dr. Navid Zargari, for more than 20 years of research collaboration in developing advanced MV drive technologies. We are grateful to our postdoctoral fellows and graduate students in the Laboratory for Electric Drive Applications and Research (LEDAR) at Ryerson University for their assistance in preparing the manuscript of this book. In particular, we would like to thank Drs. Jiacheng Wang and Kai Tian for their great help in preparing Chapters 16 and 17. Our special thanks go to Wiley/IEEE press editor, Ms. Mary Hatcher, for her precious help and support. We also express our sincere appreciation to the Wiley Editorial Program Assistants, Mr. Brady Chin and Ms. Divya Narayanan for their kind help and assistance.

BIN WU

MEHDI NARIMANI

Toronto

List of Abbreviations

ABB

Asea–Brown–Boveri

AFE

Active front end

ANPC

Active neutral point clamped

APOD

Alternative phase opposite disposition

CHB

Cascade H-bridge

CM

Common mode

CMC

Cascaded matrix converter

CMV

Common mode voltage

CSC

Current source converter

CSI

Current source inverter

CSR

Current source rectifier

C-SVM

Conventional space vector modulation

DCC

Diode clamped converter

DF

Distortion factor

DFE

Diode front end

DM

Differential mode

DPF

Displacement power factor

DSP

Digital signal processor

DTC

Direct torque control

emf

Electromotive force

EMI

Electromagnetic interference

ETO

Emitter turn-off thyristor

FC

Flying capacitor

FOC

Field-oriented control

FPGA

Field programmable gate array

GAN

Gallium nitride

GCT

Gate commutated thyristor (also known as integrated gate commutated thyristor)

GTO

Gate turn-off thyristor

HPF

High pass filter

HVDC

High-voltage dc current

IEEE

Institute of Electrical and Electronics Engineers

IEGT

Injection enhanced gate transistor

IGBT

Insulated gate bipolar transistor

IM

Induction motor

IPD

In-phase disposition

LCI

Load commutated inverter

LPF

Low pass filter

MCT

MOS-controlled thyristor

MC

Matrix converter

MMC

Modular multilevel converter

MOSFET

Metal-oxide semiconductor field-effect transistor

MTPA

Maximum torque per ampere

MV

Medium voltage (2.3KV to 13.8 KV)

NPC

Neutral point clamped

NPP

Neutral point piloted

NNPC

Nested neutral point clamped

PCBB

Power converter building block

PF

Power factor (DF × DPF)

PFC

Power factor compensator

PI

Proportional and integral

PLL

Phase-locked loop

PM

Permanent magnet

PMSM

Permanent magnet synchronous motor

POD

Phase opposite disposition

PS-SPWM

Phase-shifted sinusoidal pulse width modulation

PWM

Pulse width modulation

pu

Per unit

RCM

Reduction common mode

rms

Root mean square

rpm

Revolutions per minute

SCR

Silicon-controlled rectifier (thyristor)

SHE

Selective harmonic elimination

Si

Silicon

SiC

Silicon carbide

SIT

Static induction thyristor

SM

Synchronous motor

SPWM

Sinusoidal pulse width modulation

STATCOM

Static synchronous compensator

SVM

Space vector modulation

THD

Total harmonic distortion

TPWM

Trapezoidal pulse width modulation

VBC

Voltage balancing control

VOC

Voltage-oriented control

VSC

Voltage source converter

VSI

Voltage source inverter

VSR

Voltage source rectifier

VZD

Voltage zero crossing detector

WRSM

Wound-rotor synchronous motor

ZDC

Zero d-axis current

Part OneIntroduction

Chapter 1Introduction

1.1 Overview of High-Power Drives

The development of high-power converters and medium voltage (MV) drives started in the mid 1980s when 4500 V gate turn off (GTO) thyristors became commercially available [1]. The GTO was the standard for the MV drive until the advent of high-power insulated gate bipolar transistors (IGBTs) and gate commutated thyristors (GCTs) in the 1990s [2, 3]. These switching devices have rapidly progressed into the main areas of high-power electronics due to their superior switching characteristics, reduced power losses, and ease of gate control.

The MV drives cover power ratings from 0.4 to 40 MW at the medium voltage level of 2.3–13.8 kV. The power rating can be extended to 100 MW, where synchronous motor drives with load commutated inverters (LCIs) are often used [4]. However, the majority of the installed MV drives are in the 1–4 MW range with voltage ratings from 3.3 to 6.6 kV as illustrated in Fig. 1.1-1.

Figure 1.1-1 Voltage and power ranges of the MV drive.

The high-power MV drives have found widespread applications in industry. They can be used for pipeline pumps in the petrochemical industry [5], fans in the cement industry [6], pumps in water pumping stations [7], traction applications in the transportation industry [8], steel rolling mills in the metals industry [9], and other applications [10, 11]. A summary of the MV drive applications is given in the appendix of this chapter [12].

Market research has shown that around 85% of the MV drive applications are for pumps, fans, compressors, and conveyors [13]. The technical requirements for these drives are relatively simple and can be accomplished by a standard MV drive. As shown in Fig. 1.1-2, only 15% of the total installed drives are non-standard drives.

Figure 1.1-2 MV drive market survey.

One of the major markets for the MV drive is for retrofit applications. Although with the advancements of high-power converter technology, the variable-speed MV drives have been widely accepted in industry over the last three decades, many of the MV motors still operate in the field at a fixed speed. When large fans, pumps, or compressors are driven by a fixed-speed motor, the control of air or liquid flow is normally achieved by mechanical methods, such as throttling control, inlet dampers, and flow control valves, resulting in a substantial amount of energy loss. The installation of the MV drive can lead to significant savings on energy cost. It was reported that the use of the variable-speed MV drive resulted in a payback time of the investment from 1 to 2 ½ years [7].

The use of the MV drive can also increase productivity in some applications. A case was reported from a cement plant where the speed of a large fan was made adjustable by an MV drive [11]. The collected dust on the fan blades operated at a fixed speed had to be cleaned regularly, leading to a significant downtime per year for maintenance. With variable-speed operation, the blades only had to be cleaned at the standstill of the production once a year. The increase in productivity together with the energy savings resulted in a payback time of the investment within 6 months.

Figure 1.1-3 shows a general block diagram of the MV drive. Depending on the system requirements and the type of the converters employed, the line- and motor-side filters are optional. A phase-shifting transformer with multiple secondary windings is often used mainly for the reduction of line current distortion.

Figure 1.1-3 General block diagram of the MV drive.

The rectifier converts the utility supply voltage to a dc voltage with a fixed or adjustable magnitude. The commonly used rectifier topologies include multipulse diode rectifiers, multipulse SCR rectifiers, or pulse-width-modulated (PWM) rectifiers. The dc filter can simply be a capacitor that provides a stiff dc voltage in voltage source drives or an inductor that smoothes the dc current in current source drives.

The inverter can be generally classified into voltage source inverter (VSI) and current source inverter (CSI). The VSI converts the dc voltage to a three-phase ac voltage with adjustable magnitude and frequency whereas the CSI converts the dc current to an adjustable three-phase ac current. A variety of inverter topologies have been developed for the MV drive, most of which will be analyzed in this book.

1.2 Technical Requirements and Challenges

The technical requirements and challenges for the MV drive differ in many aspects from those for the low voltage (≤ 600 V) ac drives. Some of them that must be addressed in the MV drive may not even be an issue for the low voltage drives. These requirements and challenges can be generally divided into four groups: the requirements related to the power quality of line-side converters, the challenges associated with the design of motor-side converters, the constraints of the switching devices, and the drive system requirements.

1.2.1 Line-Side Requirements

(a) Line Current Distortion

The rectifier normally produces distorted line currents and also causes notches in voltage waveforms. The distorted current and voltage waveforms can cause numerous problems such as nuisance tripping of computer controlled industrial processes, overheating of transformers, equipment failure, computer data loss, and malfunction of communications equipment. Nuisance tripping of industrial assembly lines often leads to expensive downtime and ruined product. There exist certain guidelines for harmonic regulation, such as European Standard IEC1000 and IEEE Standard 519-2014 [14]. The rectifier used in the MV drive should comply with these guidelines.

(b) Input Power Factor

High input power factor is a general requirement for all electric equipment. This requirement is especially important for the MV drive due to its high power rating.

(c) LC Resonance Suppression

For the MV drives using line-side capacitors for current THD reduction or power factor compensation, the capacitors form LC resonant circuits with the line inductance of the system. The LC resonant modes may be excited by the harmonic voltages in the utility supply or harmonic currents produced by the rectifier. Since the utility supply at the medium voltage level normally has very little line resistance, the lightly damped LC resonances may cause severe oscillations or over-voltages that may destroy the switching devices and other components in the rectifier circuits. The LC resonance issue should be addressed when the drive system is designed.

1.2.2 Motor-Side Challenges

(a) dv/dt and Wave Reflections

Fast switching speed of the semiconductor devices results in high dv/dt at the rising and falling edges of the inverter output voltage waveform. Depending on the magnitude of the inverter dc bus voltage and speed of the switching device, the dv/dt can well exceed 10, 000 V/μs. The high dv/dt in the inverter output voltage can cause premature failure of the motor winding insulation due to partial discharges. It induces rotor shaft voltages through stray capacitances between the stator and rotor. The shaft voltage produces a current flowing into the shaft bearing, leading to early bearing failure. The high dv/dt also causes electromagnetic emission in the cables connecting the motor to the inverter, affecting the operation of nearby sensitive electronic equipment.

To make the matter worse, the high dv/dt may cause voltage doubling effect at the rising and falling edges of the motor voltage waveform due to wave reflections in long cables. The reflections are caused by the mismatch between the wave impedance of the cable and the impedances at its inverter and motor ends, and can double the voltage on the motor terminals at each switching transient if the cable length exceeds a certain limit. The critical cable length for 500 V/μs is in the 100 m range, for 1000 V/μs in the 50 m range and for 10, 000 V/μs in the 5 m range [15].

(b) Common-Mode Voltage Stress

The switching action of the rectifier and inverter normally generates common-mode voltages [16]. The common-mode voltages are essentially zero-sequence voltages superimposed with switching noise. If not mitigated, they will appear on the neutral of the motor with respect to ground, which should be zero when the motor is powered by a three-phase balanced utility supply. Further, the motor line-to-ground voltage, which should be equal to the motor line-to-neutral (phase) voltage, can be substantially increased due to the common-mode voltages, leading to the premature failure of the motor winding insulation system. As a consequence, the motor life expectancy is shortened.