Power Electronics for Mitigation of Voltage Sags and Improved Control of AC Power Systems

293 Pages·2000·11.38 MB·English

TRITA-EES-0003
ISSN 1100-1607
Power Electronics for Mitigation of
Voltage Sags and Improved Control of
AC Power Systems
Marcio Magalhães de Oliveira
Stockholm 2000
Doctoral Dissertation
Royal Institute of Technology
Department of Electric Power Engineering
Electric Power Systems
Abstract
The thesis deals with the application of compensators and switches based on
power electronics in AC transmission and distribution systems. The objective of
the studied devices/equipment is the power flow and voltage control in
transmission systems and the mitigation of voltage sags and momentary
interruptions to critical loads in distribution systems.
For validating the power electronics based devices/equipment described in the
thesis, scaled models at a real-time simulator have been built. Simulation results
of these models are presented and discussed in the thesis.
The equipment studied in the thesis exploit the fast control actions that can be
taken by power electronics devices, which are much faster than the speed of
conventional equipment and protection systems, based on electromechanical
devices. In this way, the power quality of distribution systems is improved,
regarding duration and magnitude of voltage sags (dips) and momentary
interruptions, which are the most relevant types of disturbances in distribution
systems.
The thesis presents some compensators based on forced-commutation voltage-
source converters for correcting voltage sags and swells to critical loads. The
series converter, usually denoted Dynamic Voltage Restorer (DVR), has been
proved suitable for the task of compensating voltage sags in the supply network.
The use of solid-state devices as circuit breakers in distribution systems has also
been studied with the objective of achieving fast interruption or limitation of
fault currents. The location and practical aspects for the installation of these
solid-state breakers are presented. It has been shown that a configuration based
on shunt and series connected solid-state devices with controllable turn-off
capability can also provide voltage sag mitigation, without the need of
transformers and large energy storage elements.
The operation and control of two Flexible AC Transmission System (FACTS)
devices for voltage and power flow control in transmission systems, namely the
Static Synchronous Compensator (STATCOM) and the Unified Power Flow
Controller (UPFC), respectively, are also studied. A faster response compared to
traditional equipment consisting of mechanically based/switched elements is then
achieved. This allows a more flexible control of power flow and a secure loading
of transmission lines to levels nearer to their thermal limits. The behaviour of
these devices during faults in the transmission system is also presented.
Keywords: power electronics, power quality, voltage sags, voltage-source
converters, Custom Power, FACTS, real-time simulations, solid-state devices.
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Preface
The utilization of power electronics in high power AC transmission and
distribution systems for improving their performance is the main topic of the
thesis. In this context, forced-commutated voltage-source converters and current-
limiting devices based on power electronics have been emphasized.
From a chronological perspective, worldwide research in the area of high power
voltage-source converters has been initially proposed to transmission systems,
among the compensators included in the category of “Flexible AC System
Transmissions Systems (FACTS)” devices. Research in this area resulted in a
Technical Licentiate thesis, in which a shunt voltage-source converter was
combined with Thyristor Switched Capacitors for reactive power compensation.
The second stage of the research, reported in the present work, consisted of
studying the Unified Power Flow Controller (UPFC) configuration, still
considered the most complete FACTS device.
The progressive deregulation of the energy market worldwide has led to intensive
research in the power quality of distribution systems, in cooperation with the
utilities and some of their main customers. In principle, the same equipment
employed as FACTS devices has also originated the concept of “Custom Power”,
although with different objectives. Research in this field has mostly been
documented in this thesis, where the topics of fault currents and voltage sags
(dips) - considered the primary issues of customers dissatisfaction - have been
emphasized. Although being relevant topics to a better utilization of the power
systems, the subjects of harmonics and flicker are out of the scope of the thesis.
In the chapters treating both transmission and distribution systems, the
presentation sequence has followed the chronological order of research, i.e.
transmission systems are firstly treated.
The basic questions answered in the thesis are:
(cid:1) How can high power voltage-source converters improve voltage and power
flow control in transmission systems?
(cid:1) How can these converters be employed in distribution systems for reducing
the impact of voltage sags (dips) to critical plants/loads?
(cid:1) What are the benefits and consequences of introducing solid-state switches
into distribution systems?
In order to evaluate the techniques described in the thesis, real-time models
scaled to low power have been built-up. Along the whole research period,
considerable time has been spent in the development of these models. However,
a lot could be learned from this practical experience and the models have been
very useful to a better understanding of the behaviour of the compensators and
the power systems as a whole.
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Acknowledgements
It has been an honor and a pleasure to have Professor Åke Ekström as supervisor. In
addition to his huge knowledge and experience, I could also enjoy his support and
patience during the hardest moments of the research work and writing of the thesis. His
broad technical skills, sense of humor, and positive way of criticizing have been very
instructive and will remain with me as a model for the future. In the end, I feel proud to
have had a very positive and instructive cooperation not only with the professor figure
but also with the person Åke Ekström.
Financial supports from SSF (“Stiftelsen för Strategisk Forskning”) for the project
“Future Distribution Systems with SiC Semiconductor Based Apparatus and IT-
Technologies” within the “Silicon Carbide Electronics Programme (SiCEP)” and from
ABB Power Systems AB, Svenska Kraftnät, Elforsk, and Vattenfall Utveckling for the
project “Evaluation of Performance of a Unified Power Flow Controller (UPFC)” are
gratefully acknowledged.
I would like to thank Mr. Anders Petersson of Birka Energi Elnät AB for the very
positive cooperation during the studied cases, as also for clarifying many doubts and
answering many faxes and e-mails. Acknowledgements are also due to the participant
members of the different projects I have been involved in for very interesting and
helpful discussions.
In addition to the hard work, very pleasant and unforgettable moments have been
experienced in the former division of High Power Electronics. I could also enjoy the
friendly atmosphere in the division of Electric Power Systems during the last year.
Thanks to all former and current colleagues! Special thanks go to Tech. Dr. Peter
Bennich and Mr. Anders Wikström for the discussions on current-limiting devices and
other techniques, as also for companionship on the “body-pump” sections.
Acknowledgements are also due to Tech. Lic. Rémy Kolessar for the discussions about
semiconductor devices, help and advices with computers, and mainly for the football
talks. I have also truly appreciated the help from Mr. Valery Knyazkin in proofreading
part of the thesis, supplying me with very useful comments.
Many interesting and fruitful discussions with Tech. Dr. Ying-Jiang Häfner and Tech.
Lic. Anders Lindberg, former Ph.D. students, resulted in some papers and many ideas to
the thesis. However, the most important is to have kept their friendship after they started
their professional careers in the “real world”, namely at ABB Power Systems AB and
InMotion Technologies AB, respectively.
Precious help at the real-time simulator models should be acknowledged to Mr. Anders
Björklund, for building up the extra cubicles and solving the “ghostly” problems that
sometimes emerged from nowhere. Assistance from Mrs. Margaretha Surjadi on non-
technical practical matters is also to be acknowledged.
I cannot forget the help and the discussions in the beginning of my research in Sweden
with Tech. Lic. Lars Lindberg, nowadays at InMotion Technologies AB (“Sync-Pulses”
and “transfer-functions” will never be forgotten).
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Along my stay at KTH, Prof. Willis F. Long of University of Wisconsin-Madison has
given me wise advices and a helping hand when they were mostly required, which I am
very grateful to.
I would like to thank Mr. Marcio Szechtman - formerly R&D Director at CEPEL -, who
was the main responsible person in Brazil for giving me the opportunity of coming to
Sweden for my Ph.D. studies. I want also to express my deepest gratitude to his
successor at CEPEL, Dr. Luis A.S. Pilotto, who has always shown confidence and
interest on my research work. His encouraging words, especially in the end of the thesis
writing, will never be forgotten. From CEPEL, many thanks also go to my great friend
André Bianco for keeping our e-mail discussions, especially those about our lovely
country and our football clubs.
Looking backwards to my early days as researcher, I realize that my interest on power
electronics arose thanks to Prof. Edson H. Watanabe at COPPE/UFRJ in Brazil. I am
grateful to many discussions we had and his dedication as a teacher and former
supervisor.
Although I have had the chance of meeting extremely nice and friendly people in
Sweden, our thoughts go always to our families and friends in Brazil. Millions of thanks
to our dear friends Ms. Sueli Monte and Dr. Luis George de Oliveira Bello. Your songs,
poems and messages have been guidance along our lives (“Tudo é a vontade de
Deus!”). Thanks for all support to our souls!
My dear parents Arthur (in memorian) and Marlene gave their best for bringing me up
with the lessons that are not included in any type of book. Their endless encouragement
and support remain in my heart. For them, my deep love and affection forever. The best
way to reciprocate their efforts is trying to educate their grandchild in the same way. My
thoughts go also to my dear sister Ana Carolina who has been an outstanding daughter,
sister, mother, godmother to my son, and economist.
The final and major acknowledgements are due to the persons that have proved how
wonderful life can be on its most simple moments: my “super-wife” Claudia and my
adorable son Bruno. My lovely wife has encouraged me at the many downs with a
single phrase: “Don’t give up, the reward will come!” She has been wrong since the
most valuable treasure has been with me: to have always found a warm hug and a big
smile from both of them when I really needed them. Nothing can be more precious than
your love and understanding. To my son Bruno: you do not know yet how important our
one-to-one matches and plays have been for relaxing and giving me the chance of
learning a lot with you. These moments will be always on my mind!
Marcio Magalhães de Oliveira
Stockholm, September 2000
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List of Acronyms
Acronyms Description
AC Alternative Current
ASD Adjustable Speed Drive
ASVC Advanced Static Var Compensator
CBEMA Computer Business Equipment Manufacturer Association
CLD Current-Limiting Device
CVT Constant Voltage Transformer
DC Direct Current
DPM Double Port Memory
DSP Digital Signal Processor
DVR Dynamic Voltage Restorer
EMTDC Electromagnetic Transients DC Program
EMTP Electromagnetic Transients Program
FACTS Flexible AC Systems
GTO Gate Turn-Off Thyristor
HTS High Temperature Superconductor
HV High Voltage
HVDC High Voltage Direct Current
IGBT Insulated Gate Bipolar Transistor
IGCT Integrated Gate Commutated Thyristor
ITIC Information Technology Industry Council
LCA Logic Cell Array
LTS Low Temperature Superconductor
LV Low Voltage
MOS Metal-Oxide-Semiconductor
MOSFET Metal-Oxide-Semiconductor Field Effect Transistor
MV Medium Voltage
NPC Neutral Point Clamped
PLC Programmable Logic Controller
PLL Phase-Locked-Loop
PWM Pulse-Width Modulation
rms Root Mean Square
Si Silicon
SiC Silicon Carbide
SiCEP Silicon Carbide Electronic Programme
SCFCL Superconducting Fault Current Limiter
SMES Superconducting Magnetic Energy Storage
SPFC Series Power Flow Controller
SPWM Sinusoidal Pulse-Width Modulation
SSB Solid-State Breaker
SSR Subsynchronous Resonance
SSSC Static Synchronous Series Compensator
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SSTS Solid-State Transfer Switch
STATCOM Static Synchronous Compensator
STATCON Static Condenser
SVC Static Var Compensator
TCR Thyristor Controlled Reactor
TCSC Thyristor Controller Series Capacitor
TNA Transient Network Analyzer
TSC Thyristor Switched Capacitor
UPFC Unified Power Flow Controller
UPS Uninterruptible Power Supply
VSC Voltage-Source Converter
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List of Contents
Abstract ...............................................................................................................iii
Preface .................................................................................................................. v
Acknowledgements ........................................................................................... vii
List of Acronyms ................................................................................................ ix
List of Contents .................................................................................................. xi
1 Introduction ................................................................................................ 1
1.1 Background .......................................................................................... 1
1.2 Motivation of the Work ....................................................................... 2
1.3 Contributions of the Work ................................................................... 4
1.4 Thesis Organization ............................................................................. 5
2 Terms and Events Related to Power Quality .......................................... 9
2.1 Introduction ......................................................................................... 9
2.2 What is Power Quality? ..................................................................... 10
2.3 Power Quality Problems .................................................................... 12
2.3.1 Long Duration Voltage Variations ..........................................12
2.3.2 Short Duration Voltage Variations ..........................................13
2.3.3 Other Disturbances ..................................................................16
2.4 Causes of Power Quality Problems ................................................... 17
2.5 Importance of Protection Systems ..................................................... 18
2.6 Summary ............................................................................................ 22
3 Overview of Voltage Sags and Interruptions ........................................ 25
3.1 Introduction ....................................................................................... 25
3.2 CBEMA Curve .................................................................................. 27
3.3 Classification of Voltage Sags ........................................................... 29
3.3.1 Field Measurements ................................................................ 31
3.4 Critical Equipment ............................................................................. 34
3.4.1 Electromechanical Relays and Motor Contactors ...................34
3.4.2 Programmable Logic Controllers (PLC) .................................35
3.4.3 Adjustable Speed Drives (ASD) ............................................. 35
3.5 Normally Used Measures against Voltage Sags and Interruptions ....37
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3.6 Summary ............................................................................................ 39
4 High Power Compensators Based on Power Electronics ..................... 41
4.1 Introduction ....................................................................................... 41
4.2 Thyristor-Based Compensators ......................................................... 43
4.3 Forced-Commutated Voltage-Source Converter (VSC) ....................44
4.4 Shunt Connected VSC ....................................................................... 47
4.4.1 Voltage/Reactive Power Control ............................................ 48
4.4.2 Backup Power Supply ............................................................ 50
4.4.3 Active Filtering ....................................................................... 51
4.4.4 Flicker Mitigation ................................................................... 52
4.5 Summary of Shunt Compensators ..................................................... 53
4.6 Series Connected VSC .......................................................................53
4.6.1 Series Connected VSC in Transmission Systems ...................54
4.6.2 Series Connected VSC in Distribution Systems ..................... 56
4.7 Summary of Series Compensators ..................................................... 60
4.8 Hybrid Converters ..............................................................................61
4.9 Solid-State Tap-Changers .................................................................. 64
4.10 Energy Storage Devices .....................................................................65
4.10.1 DC Capacitors .........................................................................66
4.10.2 Batteries .................................................................................. 67
4.10.3 Supercapacitors ....................................................................... 67
4.10.4 Superconducting Magnetic Energy Storage (SMES) ............. 68
4.10.5 Flywheel ................................................................................. 68
4.11 Summary ............................................................................................ 69
5 Solid-State Breakers and Current Limiting Devices ............................ 73
5.1 Introduction ....................................................................................... 73
5.2 Non-Solid-State Devices for Current Limitation/Interruption ...........74
5.3 Location of Solid-State Breakers in Distribution Systems .................76
5.4 Possible Configurations of a Solid-State Breaker ..............................80
5.4.1 Comments about the Turn-Off Device ................................... 81
5.4.2 Interruption and/or Limitation of the Fault Current ................82
5.4.3 Possible Configurations .......................................................... 85
5.5 Solid-State Transfer-Switch ...............................................................87
5.6 Summary ............................................................................................ 92
6 The Analog Real-Time Simulator ........................................................... 95
6.1 Introduction ....................................................................................... 95
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6.2 Model of the Shunt and Series Voltage-Source Converters .............. 97
6.2.1 Implementation of the Control System for the Converters .....99
6.2.2 Synchronization DSP – Phase-Locked-Loop (PLL) .............104
6.2.3 DSP Boards of the Series Converter .....................................107
6.2.3 DSP Boards of the Shunt Converter ..................................... 109
6.3 Model of Mechanical and Solid-State Breakers .............................. 110
6.4 Summary .......................................................................................... 111
7 Shunt Connected Voltage-Source Converter ...................................... 113
7.1 Introduction ..................................................................................... 113
7.2 Fundamental Frequency Switching ................................................. 113
7.2.1 Twelve-Pulse Configuration ................................................. 117
7.2.2 Size of the DC Capacitor ...................................................... 119
7.3 Control System for the Twelve-Pulse VSC ..................................... 121
7.4 Studied Configuration of a Transmission System ........................... 122
7.4.1 Simulation Cases ...................................................................123
7.5 Studied Configuration of a Shunt VSC Applied in a Distribution .. 130
System
7.5.1 Simulation Case .................................................................... 131
7.6 Summary .......................................................................................... 133
8 Series Connected Voltage-Source Converter ....................................... 135
8.1 Introduction ..................................................................................... 135
8.2 Series VSC in Transmission Systems .............................................. 136
8.2.1 Development of a Mathematical Model ............................... 136
8.2.2 Closed-Loop Theoretical Model ........................................... 141
8.2.3 Simplified Control System ................................................... 141
8.2.4 Control System Including Decoupling ................................. 146
8.2.5 Power Flow Control ..............................................................151
8.3 Application in Distribution Systems – Dynamic Voltage ............... 156
Restorer (DVR)
8.3.1 Practical Aspects ...................................................................159
8.3.2 DVR Control System ............................................................ 161
8.3.3 Simulated Cases .................................................................... 164
8.4 Summary .......................................................................................... 168
9 Hybrid Compensators and Topologies ................................................. 171
9.1 Introduction ..................................................................................... 171
9.2 Unified Power Flow Controller (UPFC) ......................................... 172
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