ABSTRACT
This work focused on Evaluating the Performance of Unified Power Flow Controller (UPFC) on
Fault Current Limitation in the Nigerian 330kV Power Network. The SIMULINK model of the
Nigerian 330kV system was developed. The model of the UPFC was developed and integrated
into the power system model. The proportional integral (PI) control parameters of the UPFC was
optimized using genetic algorithm. Results from simulation carried out showed the versatility of
the UPFC in the protective limitation of excessive fault current in the system. Evaluation carried
out indicated that the UPFC achieved an effective average of 59.23% fault current limitation.
This result was shown to have high impact for the protection of critical assets within the power
system such as circuit breakers. At a fault impedance of 0.0001Ω, the UPFC provided a 45.81%
protection margin for the type of high voltage circuit breakers used on the 330kV system. Apart
from interrupting capacity of circuit breakers, this action of the UPFC would also help to
increase the life expectancy of circuit breaker contacts between overhaul.
TABLE OF CONTENTS
Title page i
Approval Page ii
Certification iii
Title Page iv
Dedication v
Acknowledgement vi
Abstract vii
Table of Contents vii
List of Figures xi
List of Tables xiv
List of Abbreviations xv
CHAPTER ONE
1.0 INTRODUCTION 1
1.1 Background of the Study 1
1.2 Statement of the Problem 3
1.3 Aim and Objectives of the Study 3
1.4 Significance of the Study 4
1.5 Scope of the Study 4
CHAPTER TWO
2.0 LITERATURE REVIEW 5
2.1 Flexible AC Transmission System Controllers (FACTS) 5
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2.2 Basic Types of FACTS Controllers 6
2.2.1 GTO Controlled Series Capacitor 9
2.2.2 Thyristor Controlled Capacitor 10
2.2.3 Thyristor Controlled Series Reactor 12
2.2.4 Static Synchronous Compensator 14
2.2.5 Static Var Compensator 15
2.2.6 Thyristor Controlled Reactor 16
2.2.7 Thyristor Switched Reactor 16
2.2.8 Thyristor Switched Capacitor 16
2.2.9 Static Var Generator or Absorber 17
2.2.10 Static Var System 17
2.2.11 Thyristor Controlled Braking Resistor 17
2.2.12 Static Synchronous Series Compensator 20
2.2.13 Interline Power Flow Controller 20
2.2.14 Thyristor-Controlled Phase Shifting Transformer 21
2.2.15 Interphase Power Controller 22
2.2.16 Thyristor-Controlled Voltage Limiter 22
2.2.17 Thyristor-Controlled Voltage Regulator 24
2.2.18 Unified Power Flow Controller 24
2.2.19. UPFC PI Control Parameter Optimization 27
2.3.0 Genetic Algorithm 28
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2.3.1 Basic Description 29
2.3.2 Outline of the Basic Genetic Algorithm 29
2.3.3 Operators of GA 30
2.3.3.1 Encoding of a Chromosome 30
2.3.3.2 Crossover 31
2.3.3.3 Mutation 31
2.3.4 Parameter of Genetic Algorithm 32
2.3.5 Roulette Wheel Selection 32
CHAPTER THREE
3.0 METHODOLOGY 35
3.1 Model Design and Analysis 35
3.2 The UPFC Controller Model and Implementation 36
3.2.1 Control of the UPFC 41
3.2.2 The phase locked loop 43
3.2.3 Switching Logic (Pulse Generator) 44
3.3 Analysis of the Fault Current Limitation of UPFC device 50
3.4 PI Control Parameter Optimization Using Genetic Algorithm 55
3.4.1 Parameters Coding and Recoding 56
3.4.2 Initial Population Selection Operation 56
3.4.3 Crossover and Mutation 57
CHAPTER FOUR
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4.0 SIMULATION AND RESULT EVALUATION 59
4.1.0. Result Evaluation 66
4.1.1 Evaluation of System Steady State Operation with the UPFC 66
4.1.2 Evaluation of System Fault State Operation with the UPFC 69
4.2.0. Comparative Analysis 77
4.2.1. Comparison of the response time of the UPFC and the Circuit Breaker 78
4.2.2. Implication for Circuit Breaker Protection 90
CHAPTER FIVE
5.0 CONCLUSION 92
5.1 Summary 92
5.2. Conclusion 92
5.3 Recommendations 93
5.4. Suggestion for Future Work 93
Reference 94
Appendix A 99
Appendix B 100
Appendix C 111
CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
In modern power system, the increasing rate of energy demand pushes the increase in the
addition of more generation and transmission system to the grid. As an unwelcome consequences
of this, fault currents are day-to-day increasing [1]. Many utilities all over the world are
experiencing the problem of astonishing short circuit current (fault current) levels [1]. A fault is
an unintentional short circuit, or partial short-circuit, in an electric circuit. Faults on power
system are inevitable due to external or internal causes, lightning may strike the over head lines
causing insulation damage, incidences of downed or crossed power lines cause faults. During a
fault, excessive current called fault current flows high and may exceed ten times the rated current
of a piece of plant [2].
Millions of dollars are spent each year to maintain and protect the grid from potentially
destructive fault currents [3]. These large currents can damage or degrade circuit breakers and
other expensive transmission and distribution components.
It is well established that the fault current levels in a network increases proportionally with the
addition of lines and new generation [1]. This happens to be the case with the Nigerian 330kV
system, especially considering the addition of transmission and generation components to it as a
result of the National Integrated Power project (NIPP). This fact means that the short-circuit
current rating (i.e the fault current withstand) of existing transmission assets on the Nigerian
330kV system will be exceeded. Increasing rate of fault current levels on power systems cause
undesired consequences which may be summarized as follows[1]:
· Equipment is exposed to unacceptable thermal stresses;
· Equipment is exposed to unacceptable electro-dynamic forces;
· Short circuit breaking capability of high voltage circuit breakers are typically limited to
80kA[4];
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· In order to prevent equipment damage, faster circuits breakers are required. This
requirement faces both technical and economical restrictions.
· Step and touch voltages are also increased as a result of increasing short circuit levels.
This will cause safety problems to the personnel;
· Switching over voltage transients will become more severe, due to significant short
circuit currents.
These problems put more pressure on power system protection equipment and their
configuration. Furthermore the fault clearing time of conventional protection system
(Relay/Circuit breaker) is not instantaneous, for it depends on the operating time settings of over
current relay and the circuit breakers tripping time, hence a system that can swing faster into
action to limit the destructive effects of the fault current is necessary.
Due to the above-mentioned problems, the subject of fault current level reduction has gained a
considerable attention in recent years among electric utilities[5]. The idea behind this line of
protection research is to reduce the stress within the network or limit the stress over certain assets
(e.g the circuit breaker itself). A number of fault current limitation techniques have been
introduced in the iterative. Some of this protection techniques include super conducting fault
current limiter [6][7], HVDC links [8] and current limiting reactor [9] [10]. The super conducting
fault current limiters use superconducting material such as NbT and MgB2 to transfer from
superconducting state to the normal state, if exposed to high current levels. Although this limiter
seems to be an ideal fault current limiter, it is still too expensive, especially due to the cost of its
complicated cryogenic system. HVDC links are used to diminish inter-area short circuit current.
However, it is reported that this method is not economically justified. Among the excessive fault
current limiting methods indicated, the current limiting reactor is argued to be the most practical
approach. However it is reported [11] that current reactor may degrade both voltage stability and
transient stability of the power system.
Consequently a more versatile protection technique becomes necessary. Such a technique should
possess almost instantaneous response to fault and have dynamic and enhanced power control
capability. Flexible AC Transmission System (FACTS) devices fit into this requirement. The
IEEE defines FACTS as: Alternating Current Transmission System incorporating power
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electronic based and other static controllers to enhance controllability and increase power
transfer capability. Now-a-days FACTS has become a subject of interest for the power system
engineers. This technology eliminates the use of bulky, show the operation of circuit breakers,
and use highly sophisticated semiconductor devices such as Thyristors, GTOs or IGBTs
[11][12][13].
The speed, the power transfer and control capabilities of FACTS can be applied to enhance the
protection of the Nigerian 330kV transmission system from destructive over currents.
1.2 Statement of the Problem
Power systems globally are increasingly being stressed as a result of addition of transmission
lines and generators to the power system as is the case in the Nigerian 330kV system. This
creates excessive fault current far exceeding the withstand level of the existing protection system
(like relay/circuit breakers) on the 330kV system. Hence it resorts to using fault current
limitation techniques to enhance protection of existing power systems. Considering the
shortcomings of the fault current limitation techniques as briefly pointed out in the background
of this study, the problem confronting this work is the application of the Unified Power Flow
Controller (UPFC) to enhance the protection of the Nigerian 330kV power system by effectively
limiting destructive fault currents. Furthermore, this work tackles the problem of the
optimization of the control parameters of the UPFC using genetic algorithm.
1.3 Aim and Objectives of the Study
The aim of this study is to apply FACTS technology for the protection of the Nigerian 330KV
system without further upgrade or replacement of existing protection equipment. In pursuit of
this goal, this work seeks to realize the following specific objectives:
(1) To create the digital model of the Unified Power Controller (UPFC), using genetic
algorithm to optimize the control parameters for the effective limitation of excessive fault
current in the 330kV power system
(2) To model the integration of the proposed FACTS controller with the model of the
Nigerian 330kV system.
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(3) To simulate the protection dynamics of the proposed FACTS devices on the system.
(4) To carry out evaluation of the impact and performance of the proposed protection
technique on the system.
1.4 Significance of the Study
The success of this work in demonstrating the protective benefits of FACTS would go a long
way in creating justification and impetus for applying this technique on the actual 330kV
Nigerian system. Doing this would have the economic benefits of reducing the money spent by
the government annually to maintain and protect the 330kV system from destructive fault
currents.
Due to frequent voltage collapse on the 330kV system, this study should help to forge the
direction for the possible application on the use of FACTS technology not just to protect the
system but provide needed power compensation technique to the already weak system. This
possibility becomes even more appealing, as indicated in this report, since FACTS devices can
be easily integrated and requires no upgrade of the transmission assets on the 330kV system.
The work also constitutes a key contribution to the literature on the use of FACTS for protection,
especially as the information on the use and application of FACTS devices for fault current
limitation is somewhat scarce.
1.5 Scope of the Study
The work covers application of the power transfer and control capability of FACTS devices for
the protection of the 330kV system from excessive fault current. It includes the MATLAB model
of the unified power flow controller, the 330kV system and the fault current limitation analysis
of the FACTS devices within power systems. However, it does not include compensating for the
impact of FACTS dynamics on the operation of protection relays within the network.
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