ABSTRACT
Flexible AC Transmission System (FACTS) devices such as Static Var Compensator (SVC) and Static Synchronous Compensator (STATCOM) when placed at the midpoint of a long transmission line play an important role in controlling the reactive power flow into the power network. This Thesis explores the effect of STATCOM and SVC on voltage stability. The Nigerian 24-bus system has been used to demonstrate the ability of STATCOM and SVC in improving the voltage stability of a power system network. The structure of STATCOM and SVC are explained and their impact on midpoint voltage regulation. Furthermore, the performance of the STATCOM is compared with that of conventional static var compensator (SVC). Newton Raphson load flow analysis was carried out on the Nigerian 24-bus 330KV network using Neplan Engineering software. It was discovered that STATCOM provided a high reactive power support than SVC and also improved the static voltage of the buses to which it was connected to, as well as other buses that were not directly connected to the STATCOM. Although SVC improved the voltages of the buses to which it was connected to as well as other buses not directly connected to it, STATCOM displayed a greater improvement of the bus voltages to which it was connected to, with STATCOM offering the highest voltage improvement of 1.0388pu while SVC offered an improvement of 1.0282pu. The real and reactive power losses in the system network were reduced when STATCOM and SVC were inserted into the network, however the real and reactive power losses were lower when STATCOM was inserted than when SVC was inserted with STATCOM having a reactive power loss of 467.2285MVar giving a total reduction of 32.01% in the reactive power loss of the network while SVC had a total reactive power loss of 481.4609MVar giving a total reduction of 29.94% in the reactive power loss in the network. Similarly, STATCOM had an active power loss of 53.8229MW giving a total reduction of 17.96% in the active power loss of the network while SVC had an active power loss of 54.2594MW giving a total reduction of 17.30% in the active power loss of the network.
TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Table of Contents vi
List of Tables ix
List of Figures x
List of Abbreviations xii
Abstract xiii
CHAPTER 1: INTRODUCTION 1
1.1 Background of the Study 1
1.2 Statement of the Problem 3
1.3 Aims 4
1.4 Objectives of the Study 4
1.5 Significance of the Study 4
1.6 Scope and limitation of the Study 5
CHAPTER 2: REVIEW OF RELATED LITERATURE 7
2.1 Flexible AC Transmission System 21
2.2 Harmonic Interactions 23
2.2.1 Multi-pulse converter configuration 24
2.2.2 Multi-level converter configuration 25
2.2.3 Pulse-width modulation 26
CHAPTER 3: MATERIALS AND METHODS 27
3.1 Materials 27
3.2 Methods 27
3.2.1 Analysis of STATCOM 28
3.3 Analysis of static Var compensator (SVC) 33
3.3.1 Components of SVC 34
3.3.2 Thyristor controlled reactor current 37
3.4 Newton Raphson’s algorithm 40
3.4.1 Nigeria 330KV 24 bus transmission network 48
3.4.2 Implementation 50
CHAPTER 4: RESULTS AND DISCUSSION 55
4.1 Load flow result 55
CHAPTER 5: CONCLUSION AND RECOMMENDATION 90
5.1 Conclusion 90
5.2 Recommendation 93
References 94
Appendices 98
LIST OF TABLES
3.0 Transmission line data of the Nigerian 24- bus network 51
3.1 Bus data of the Nigerian 24-bus system 52
4.0 Newton Raphson’s load f low result without fact devices 58
4.1: Voltage profile without fact devices 59
4.2 Line flows and losses without fact devices 62
4.3 Line losses without fact devices 64
4.4 Load flow result with STATCOM inserted into the network 67
4.5 Voltage profile with STATCOM inserted into the network 68
4.6 Line flows and losses with STATCOM 71
4.7 Line losses with STATCOM ` 73
4.8 Load flow result with SVC inserted into the network 77
4.9 Voltage profile with SVC inserted into the network 78
4.10 Line flows and losses with SVC 81
4.11 Line losses with SVC 83
4.12 Bus voltage and reactive power comparison of STATCOM and SVC 87
4.13 Total active & reactive power loss with & without STATCOM & SVC 89
5.0 Comparison of STATCOM and SVC 92
LIST OF FIGURES
2.0 Multi-Pulse staircase voltage waveform 25
3.0 One line diagram of a STATCOM 28
3.1 Equivalent circuit diagram of asynchronous condenser 29
3.2 A single phase STATCOM 30
3.3 Waveform of VPN 31
3.4 STATCOM connected to the power system via a coupling transformer 32
3.5 A typical SVC (TSC-TCR ) configuration 35
3.6 SVC structure 36
3.7 One line diagram of a 4-bus system 40
3.8 Modified equivalent circuit of the 4-bus system 41
3.9 A typical bus of the power system 46
3.10 Transmission line model for line flow 47
3.11 The 24-bus 330kv Nigerian transmission system network 49
3.12 Flow chart for the implementation 53
4.0 Line diagram of the Nigerian 330kv 24 bus networks without fact device 55
4.1 Line diagram of the Nigerian 330kv 24 bus network with STATCOM 56
4.2 Line diagram of the Nigerian 330kv 24 bus network with SVC 57
4.3 Chart of the voltage profile without any FACT device inserted 61
4.4 Chart of the line losses in MW without fact devices 65
4.5 Chart of the line losses in MVar without fact devices 66
4.6 Chart of the voltage profile with STATCOM inserted 70
4.7 Chart of the line losses in MW with STATCOM inserted 74
4.8 Chart of the line losses in MVar with STATCOM inserted 75
4.9 Chart of the voltage profile with SVC inserted 79
4.10 Chart of the line losses in MW with SVC inserted 84
4.11 Chart of the line losses in MVar with SVC inserted 85
4.12 Voltage profile comparison of STATCOM and SVC 88
4.13 Reactive power supplied by SVC and STATCOM 88
LIST OF ABBREVIATIONS
FACTS : Flexible AC transmission system
SVC : Static Var compensator
STATCOM : Static synchronous compensator
PWM : Pulse-Width modulation
VSC : Voltage source converter
TCR : Thyristor controlled reactor
TSC : Thyristor switched reactor
UPFC : Unified power flow controller
IPFC : Interline power flow controller
SSSC : Static synchronous series compensator
TCPST : Thyristor controlled PST
TCSC : Thyristor controlled series capacitor
SC : Synchronous condenser
GTO : Gate turn-off thyristor
TSR : Thyristor switched reactor
FC-TCR : Fixed capacitor-thyristor controlled reactor
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF THE STUDY
Power system stability is the ability of a power system which enables it to remain in a stable operating equilibrium under normal conditions and to return to a stable state after it has been subjected to some form of disturbance. (Anbarasan and Sanavulah, 2012). A power system becomes unstable when voltages uncontrollably decreases due to outage of Generators, sudden increment in load. (Anbarasan and Sanavulah, 2012); One of the major reasons for voltage instability is reactive power imbalance in the system. This affects the load ability of a bus in a power network.
When the load increases, there will be a corresponding decrease of the voltage at the bus. Continuous increase in the loading of the network results in shortage of reactive power. Thereafter, if the active and reactive power transfer increases, there will be a quick decrease in the voltage magnitude at the bus. As critical point is reached, heavy reactive power losses lead to a high voltage drop which consequently leads to voltage collapse. Power electronic based equipment such as FACTS (Flexible AC Transmission System) controllers with their ability to rapidly respond to system events and improve the quality of power delivered constitutes one of the technical advancements which address the operating challenges that are being presented today. Among the FACTS controllers, the one that is most advanced is the one that employs voltage sourced converter (VSC) as synchronous sources.
STATCOM is a voltage source inverter which converts a D.C input voltage into A.C output voltage so as to compensate for the reactive and active power required by the system. ( Anbarasan and Sanavulah, 2012);
FACTs devices- SVC and STATCOM can provide reactive power support.
SVCs are known to improve the properties of a power system like voltage regulation, stability limits, dynamic over voltage and under voltage control as well as var compensation.
STATCOM is purely a voltage source converter (VSC) which converts a D.C voltage to a three phase A.C voltage at a fundamental frequency of controlled magnitude and phase angle. VSCs use pulse width modulation technology which makes it capable of providing high quality ac output voltage to the grid or even to a passive load.
STATCOM is used for shunt compensation in the same manner as SVC. However, it utilizes a voltage source converter in place of shunt capacitors and reactors. The main principle in the operation of a STATCOM is that it generates a controllable A.C voltage source through a leakage transformer by a voltage source converter which is connected to a D.C capacitor. The difference in the voltage across the leakage reactance is utilized in the production of reactive and active power exchange between the STATCOM and the power system.
This thesis discuses the dynamic response of STATCOM and compares STATCOM and SVC to the A.C system conditions.
1.2 STATEMENT OF THE PROBLEM
In static voltage stability, slowly developing changes in the power system occur that eventually leads to a shortage of reactive power and declining voltage.
Voltage collapse phenomena in power system have become one of the important concerns of the power industry.
Power quality is ultimately a consumer-driven issue and the end user’s point of reference takes precedence. The ultimate importance of power quality is economic value.
A poor power quality has numerous economic impact on utilities, their customers and suppliers of load equipment. Poor power quality also has a direct economic impact on many industries and industrial consumers. This can lead to high cost of production in the industries and consequently high inflation rates.
In order to solve these problems this research was carried out on the application of STATCOM and SVC which aims at
i. Improving the system voltage
ii. Providing reactive power support with a faster response time.
iii. Reducing losses associated with the system.
iv. Preventing voltage collapse as well as voltage sags.
v. Better utilization of machines connected to the system.
vi. High performance during low voltage condition as the reactive current can be maintained constant.
1.3 AIMS
The aim of this thesis is to overcome the problem of voltage instability and poor power quality through the use of STATCOM.
1.4 OBJECTIVES OF THE STUDY
The objectives of this Thesis are listed below.
i. Review of related literatures.
ii. Apply Newton Raphson’s Algorithm in analyzing the load flow in the network.
iii. Collect line data and bus data of the 24-bus Nigerian 330KV Network from the Transmission Company of Nigeria.
iv. Use the load flow analysis to demonstrate the effect of STATCOM and SVC in reducing both the real and reactive power losses on the Transmission lines.
v. Compare the response of STATCOM with SVC using Neplan Software
1.4 SIGNIFICANCE OF THE STUDY
The need to increase the quality of power transmitted and improve the stability of the voltage cannot be overemphasized.
With electricity increasingly being considered as a commodity, transmission systems are being pushed closer to their stability and thermal limits while the focus on the quality of power delivered is greater than ever. In addition, dynamic reactive power support is becoming more important. In the deregulated utility environment, financial and market forces will demand a more optimal and profitable operation of the power system with respect to generation, transmission and distribution. Advanced technologies are paramount for reliable and secure operation of power systems.
Hence voltage stability must be improved by providing suitable reactive power compensation using SVC and STATCOM.
1.5 SCOPE AND LIMITATION OF THE STUDY
Reactive power compensation is the most effective method to improve both voltage stability and power transfer capability of the system. The control of bus voltage level is accomplished by controlling the generation, absorption and flow of reactive power. Voltage stability of a bus in the power system mainly depends on the reactive power support that the bus can receive.
When the system approaches the maximum loading point then the real and reactive power losses will be increasing rapidly. Therefore a sufficient reactive power supports have to be given to maintain the voltage stability.
In this study, typical time domain simulations will be employed to analyze the performance of power systems containing both conventional equipment and FACTS controllers (STATCOM) for voltage/var support. This approach usually requires a large number of study cases at different system operating conditions to evaluate the relationship between control parameters and voltage stability.
In addition, Matlab/ Simulink will be used to analyze the effect of STATCOM in response to voltage fluctuations at the bus. Also real time values of certain parameters which will be measured over time will be used in the simulation.
The limitation of this work is that the synchronism operation of the switches in a STATCOM results in the A.C voltage at the output. This output voltage contains many harmonics and some solution has to be found to eliminate them.
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