Modelling Flexible AC Transmission Systems (FACTS) Devices on Weak Transmission Lines in the Nigerian Power Network

CONTENTS

CHAPTER ONE: INTRODUCTION
1.0 Background to the Study
1.1 Statement of the Problem
1.2 Aim and Objectives of the Study
1.3 Research Questions
1.4 Scope of the Study
1.5 Significance of the Study
1.6 Project Layout

CHAPTER TWO: LITERATURE REVIEW
2.0 Introduction
2.1 Brief History of Nigerian 330kV, 30Bus Interconnected Electric Power System
2.2 Concept of Transmission Line
2.3 Concept of FACTS Device
2.4 Types of FACTS Device Based on their Application
2.5 Choice of FACTS Devices/Controller
2.6 Power Flow FACTS Model

CHAPTER THREE: METHODOLOGY
3.1 Research Design
3.2 Modelled Network of the 330kV Nigeria Power Network on Matlab
3.3 Load Flow Analysis through Power Flow Analysis Tool Box
3.4 Optimization through Genetic Algorithm

CHAPTER FOUR: RESEARCH FINDINGS AND DISCUSSION
4.0 Introduction on Research Findings and Discussion
4.2 The Load Flow Analysis with Optimization Techniques
4.3 Discussion on the Case Three Optimization Approach With GA and FACTS Device

CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.0 Conclusion
5.1 Recommendation for Future Research

REFERENCES

APPENDIX A

APPENDIX B

APPENDIX C

APPENDIX D

APPENDIX E

APPENDIX F

CHAPTER ONE: INTRODUCTION

1.0 Background to the Study

The Nigeria transmission network and distribution system, supply the vast needs of electrical power to its citizenry. Due to the tremendous power requirement, there is a concern of efficient operation of the power transmission network and the associated systems [1]. Power is an essential commodity, because it drives the economy of a nation and the reliability that sustains her developmental growth. There is therefore a correlation between the standard of living and the available power supplied in a nation.

The importance of electricity cannot be compared with any other utility supply since it controls, determines and affects all other sectors of the nation's economic development. The national gross domestic product (NGDP) is seriously dependent on the quality of electric supply of the nation. With the ever growing technological world, there is a deep dependence on the continuous availability of electrical power [2]. Increase in electric usage is necessary and desirable since electrical services are essential for the nation's improved standards of living. As electricity attracts attention it brings infrastructural development to every nation and the sound national development depends on adequate provision of quality, reliable, efficient and affordable electricity. In contrast, Nigeria power system is faced with the problems of insufficient generation and transmission lines, hence resulting in the overloading and stressing the network beyond their thermal limit, due to the increasing load demand. The resultant effect is having the transmission lines voltages operating outside the allowable limit of + or - 5%, insufficient and inadequate power flow, high line losses, and damping oscillation. In an attempt to solve these challenges, the federal government through Independent Power Producers (IPP) and National Independent Power Projects (NIPP) embarked on building more generating stations and additional transmission lines. The Implication of this attempt, is that the network levels soon will become complex and will require thorough control of basic parameters such as bus voltages, power flows (active and reactive), phase angles and line currents. This is buttressed in the Fig.1.0 below;

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Fig. 1.0: Operational Limits of Transmission Lines for Different Voltage[5]

Conventionally, these controls were achieved using reactors, capacitor banks and tap changing transformers( phase shifters) to regulate the power flows.

Due to their slow response to system changes, power electronic based devices became imminent because of their speedy and accurate response to controls.

With integrated power project to solve some of these peculiar problems, the government still face some challenges such as delay in obtaining the right of ways in building transmission lines and having such generating stations located very far from load centers [4]. In an attempt to investigate both steady and transient states stability of the existing network, it was found that it is always operated very close to its thermal limit with some bus voltages below the allowable voltage limits of 313.5kV-345.5kV [4].

However, Flexible Alternating Current Transmission Systems (FACTS) devices offer a good opportunity in solving some of these issues, rather than making more investment in either building more generating stations or lines. FACTS is defined by Institute of Electrical Electronic Engineers (IEEE) as “a power electronic based system and other static equipment that provides control of one or more Alternating Current (AC) transmission system parameters to enhance controllability and increase power transfer” [4]. Improved utilization of the existing power system was provided through the application of advanced control technologies. Power electronics based system, or Flexible AC Transmission Systems (FACTS), provide proven technical solutions to address these new operating challenges being presented today.

FACTS technologies allow for improved transmission system operation with minimal infrastructure investment, environmental impact, and implementation time compared to the construction of new transmission lines [7]. In this thesis, the focus is on the effect of FACTS devices in the minimization of power losses on weak transmission line, with a case study of Nigeria power network.

1.1 Statement of the Problem

Due to the insufficient generation, inadequate transmission lines, and the slow response of the conventional means of control through the use of reactors, capacitor banks and tap changing transformers (phase shifters) to regulate power flows, there has been an overloading and stressing of the operational network of the Nigeria's power system beyond their thermal limit in an attempt to meet the increasing load demand. Hence, there is a need to improve the voltage capacity to be handled on the weak transmission lines so as to avoid stress and overloading of the operational network.

1.2 Aim and Objectives of the Study

1.2.1 Aim of the Study

To model FACTS devices on weak transmission line in the Nigeria power network and consider their effect on the bus voltages, reactive and active power using genetic algorithm(GA) approach for loss minimization. The Nigeria 330KV existing network to be considered consist of nine (9) generating stations, thirty(30)Buses and forty one (41) transmission lines which will be modelled and simulated using Matlab Version 7.10.

1.2.2 Objectives of the Study

The objectives of this research are:

1. to model 330kV Nigerian transmission line
2. to calculate load flow analysis of the entire system to determine weak transmission lines whose bus voltage is not in range of the tolerance level.
3. to investigate the impact of FACTS devices on weak transmission line
4. to recommend the best optimization technique for transmission line losses minimization.

1.3 Research Questions

For the realization of the objectives mentioned above and the aim, the following research questions were set as a guide:

1. What is the significant effect of FACTS devices on weak transmission lines?
2. Can FACTS device be used with genetic algorithm for optimization of power loss and improvement of the bus voltages?
3. What is the limitation of using just genetic algorithm without FACTS device for the optimization of power loss and the improvement of the bus voltages?

1.4 Scope of the Study

The study is limited to Nigeria 330kV existing power network with the focus on the comparison of the Bus voltages and power flow on the transmission lines when FACTS devices are incorporated and when the FACTS devices are not incorporated.

1.5 Significance of the Study

In the modern power system of Nigeria, there are several elements between the generating station and the consumers. As a result, the power network has become extensive and subjected to considerable drop in voltage on the transmission lines.

In view of the above analysis, the significance of this research work is to show that power electronic high speed control FACTS-DEVICE is a more efficient method of voltage control when compared to the conventional methods of control (through the use of reactors, capacitor banks and tap changing transformer).

1.6 Project Layout

This research work is divided into five chapters with each chapter buttressing more on minimization of power loss.

The scope of the work ,the objective and aim of the research work to be achieved is addressed in chapter one (1). Chapter two(2) focus on the literature review of other researchers on FACTS device in the improvement of the power network, the concept of FACTS device and the choice of FACTS device to be used was also addressed in chapter two (2) of this research work.

Chapter three focus on the methodology used for this study. The simulation of the 330kV Nigeria power network was done on MATLAB /SIMULINK 7.5.

Also the chapter three focused on the use of power flow analysis toolbox which is a collection of a written codes of m files that has a compatible interface with MATLAB to generate the load flow of the power network instead of using ETAP. The genetic algorithm was also discussed as an optimization tool deployed to optimize the losses on the transmission line.

Chapter four focus on the research findings with possible explanation as to some of the result obtained. Finally chapter five talks about the conclusion of this research work and highlight some areas to explore in the future.

CHAPTER TWO: LITERATURE REVIEW

2.0 Introduction

This chapter sets the scene by systematically reviewing the FACTS devices in existences and how they have been implemented by various researchers to solve the problem encountered in transmission lines. By reviewing the journals of these researchers, it paved way for the study of the effects of a particular type of FACTS- based device on weak transmission lines in the 330kV Nigerian power system network.

Based on the research conducted, in [14] FACTS devices can be broadly divided into two;

- conventional thyristor based FACTS devices
- voltage source converter based FACTS devices

Over the years, various researchers, both in Nigeria and abroad have said something about FACTS devices. Their work are still ongoing so as to find newer concept for minimizing the reason for voltage collapse by increasing the voltage stability(dynamic, transient, and steady state stability),voltage margin and voltage security in the system. Recently, it has been observed that;

- the improvement of electric power quality in Nigeria existing 330kV 28 bus electric power system can be done by using static Var compensator system[3]
- the efficiency improvement of 330kV network can be done by using the flexible alternating current transmission system devices[8]
- there is an impact of FACTS devices on transmission congestion charges in LMP – based market[11]
- the modeling , simulation and comparison of various FACTS Devices in power system can help improve the voltage stability of a power system[13].
- there is an adverse effects of FACTS on the performance of distance protection relays[9]
- the genetic algorithm can be applied on the optimal location of FACTS devices in the power system[15].

The chapter starts with the concept of a transmission line, then the concept of FACTS devices that are used for different power system applications leading to various types of FACTS implementations. A literature survey was carried out to understand the different types of FACTS devices and the concepts of series and shunt types of FACTS devices. The different types of FACTS devices; are conventional thyristor based FACTS devices and the voltage source converter based FACTS devices [9]. Finally the power flow of FACTS devices model for the study was stated.

2.1 Brief History of Nigerian 330kV, 30Bus Interconnected Electric Power System

The electrical utility is probably the largest and most complex industry in the world. The electrical engineer, who researches in this industry, will encounter challenging problems in designing future power systems to deliver increasing amounts of electrical energy in a safe, clean and economical manner [20]. The origin of the Nigerian Electric Power System can be traced to the year 1898 [21], when a small generating plant was installed in Lagos. The first power interconnection was a 132kV link constructed in 1962 between Lagos and Ibadan. By 1968, the first National grid structure emerged with the construction of the Kainji Hydro project, which supplied power via a 330kV, primarily radial type transmission network into the three numbers 132kV subsystems then existing in the Western, Northern and Eastern parts of the country.

The 330kV and 132kV systems were initially run by two separate statutory bodies: - “Nigerian Dams Authority “ (NDA) and “Electricity Corporation of Nigeria” (ECN) respectively. Central control for the 330kV Network was co-ordinated from Kainji Power Supply control room while the 132kV network was run by load dispatcher located at Ijora Power Supply Lagos. These two bodies were merged formally into single power utility- National Electric Power Authority (NEPA) on 1st April, 1972; thus ushering in centralized regulation and coordination of the entire rapidly growing 330kV and 132kV national network in Nigeria. The transmission network in Nigeria is characterised by these outages.

2.2 Concept of Transmission Line

All transmission lines in a power system are made to transport energy from power generation stations to distribution stations. Hence, they exhibit the electrical properties of resistance, inductance, capacitance, and conductance. The resistance is due to the nature of the conductor while inductance and capacitance are due to the effects of magnetic and electric field around the conductor. These parameters form the basis for the development of transmission line models used in power system studies. The shunt conductance accounts for leakage current which flows across insulators and ionized pathways in the air. The leakage currents are negligible compared to the current flowing in the transmission lines and may be neglected [6].

2.2.1. Line Load-ability

There is always an inherent trade-off between increasing utilization of the grid and security of grid’s operation. The Load-ability of a transmission line is defined as the optimum power transfer capability of a transmission line under a specified set of operating criteria[6].

2.2.2. Transmission Systems Enhancement Strategies

There are several methods which could be used to enhance the performances of transmission lines, some of them include :

1. Installation of New Transmission Line: This is usually the first option that comes to mind whenever a transmission line is limited in the amount of power it can transmit, so as to alleviate overloading by providing additional paths for power flow. It is beneficial by increasing the reliability of the transmission system. However, it has to pass through economic, political and environmental hurdles.
2. Changing the conductor of the Transmission Line/Terminal Equipment: The conductor of a particular line can be changed with a larger conductor with more power-carrying capability if the original transmission line conductor is inadequate to carry expected power flows, provided that the transmission line towers do not need to be significantly altered to support the heavier conductor. In addition, some terminal equipment may need to be upgraded to match the desired rating .
3. Conversion from Single Circuit to Double Circuit: This involves making necessary modifications to the existing transmission towers and adding a second transmission line to the structure. This option extensively reduces the line impedance and increases current-carrying capacity of the line, thereby increasing power transmission capacity of the line.
4. Voltage Upgrade: Another option is to increase the operating voltage of the transmission line, such as upgrading the voltage from 132 kV to 330 kV. In this instance, for example, the nominal rating of the line maybe drastically increased while using the same conductor. This type of improvement may require upgrading the transmission towers to meet National Electric Safety Code (NESC) clearance levels. In addition, the switching stations and substations must also be upgraded with higher voltage circuit breakers, switches, transformers, and other related equipment .
5. Reactive Power Compensation: Addition of reactive power compensation devices (especially FACTS devices) in the form of series compensation, shunt compensation or the combination of the two, (depending on the nature of the line and its identified deficiency and need) to the transmission line is also another means of enhancing the performances of a transmission line. The addition of compensation modifies the electrical impedance of the line and therefore increases the power flows across the line. This can be an effective and economical means of increasing the transmission capability as a whole, by taking advantage of transmission lines that are not loaded to their thermal limits.

2.3 Concept of FACTS Device

In a power system, the transmission of power in a transmission line is mainly dependent on the sending and receiving end voltage levels, the transmission angle and the transmission line reactance. To increase the power flow through a transmission system, one or more of the above parameters must be changed. For example, the transmission angle can be increased with the use of a phase shifting transformer but such an item of plant is costly to purchase and install, and the transformer losses must be accounted for. Increasing the transmission angle also pushes a power system closer to its stability limit, thereby increasing the likelihood of system instability. Also the transmission voltage level could be increased. However, this would only be economically feasible if permitted by existing tower construction, and it would still be very costly to upgrade system insulation and switchgear. Where such an approach is envisaged in the future, transmission lines could be constructed taking into account future operation at higher voltage levels. Power flow could also be increased by reducing the inductive reactance of the transmission system by installing fixed series capacitor. This was in the past found to be one of the most economical ways of increasing the power flow of the transmission system.

Hence, FACTS devices can be broadly applied to increase the power flow or even to change the power flow by having a higher degree of control of the three key parameters of line impedance, phase angle, and voltage magnitude. In addition, FACTS devices are used to increase the stability of the system and to regulate the system voltage.

In its most general expression, the FACTS concept is based on the substantial incorporation of power electronic devices and methods into the high-voltage side of the network, to make it electronically controllable during steady-state and transient conditions [9].Several kinds of FACTS controllers have been commissioned in various parts of the world. The most popular are the Thyristor-Controlled Phase Shifter (PS), Load Tap Changer (LTC), Thyristor-Controlled reactor (TCR), Thyristor-Controlled Series Compensator (TCSC), Interphase Power Controller (IPC), Static Synchronous Compensator (STATCOM), Solid-State Series Controllers (SSSC), Unified Power Flow Controller (UPFC), StaticVar Compensators (SVC), High-Voltage Direct Current link (HVDC) and the Hingorani's sub synchronous resonance (SSR) damper. In high-voltage transmission, the most popular FACTS equipment is the UPFC and the HVDC-VSC[9]. At the low voltage distribution level, the SVC provides the core of the following custom power equipment: the distribution STATCOM, the dynamic voltage restorer and active filters. Again in distribution system, FACTS controllers based on conventional Thyristors like TCR, SVC, and the TCSC are fast giving way for Solid-State transfer switch-controlled thyristors like STATCOM. The STATCOM provides all the functions that the SVC can provide but at a higher speed. It is more compact and requires only a fraction of the land for its installation. The STATCOM is essentially a voltage source converter (VSC) using insulated gate bipolar transistor (IGBT) interfaced to the AC system through a shunt connected transformer. The voltage source converter (VSC) is the basic building block of the new generation of power electronic converters that have emerged from the FACTS initiative. It is pertinent to state that the remit/crux of this research is the study of FACTS Controller models and procedures with which to use the most suitable FACTS controller to assess the steady-state operation of electrical power system distribution network at the fundamental frequency. The models of the aforementioned FACTS controllers are developed from first principle with strong reference to the physical structure of the FACTS controller.

2.4 Types of FACTS Device Based on their Application

FACTS controller or devices are broadly classified into four different categories based on the type of application [9]:

1. Series devices
2. Shunt devices
3. Combined series-series devices
4. Combined series-shunt devices

2.4.1 Series Devices

Series devices are power electronic based variable source of mains frequencies, sub-synchronous frequencies and harmonic frequencies. In principle all series devices inject a voltage in series with the line. For example, variable impedance multiplied by the current flowing through it represents an injected variable series voltage in the line. As long as the voltage is in phase quadrature with the line current, the series controller only supplies or consumes variable reactive power. Any other phase relationship will involve real power being supplied or consumed. Examples of series compensators are Static Synchronous Series Compensators (SSSC) and Thyristor Controlled Series Compensators (TCSC).

2.4.2 Shunt Devices

As in the case of series devices, shunt controllers may be of variable impedance, variable source, or combinations of these. In principle, shunt devices inject current into the system at the point of connection. Variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes variable reactive power. Any other phase relationship involves supply or consumption of real power.

Examples are Static Synchronous Compensator (STATCOM), Static Var Compensator (SVC) etc.

2.4.3 Combined Series-Series Devices

Combined series-series devices are normally referred to as interline power flow controllers which are a combination of separate series devices controlled in a coordinated manner. Combined series–series devices have the ability to balance both real and reactive power flows in the lines. When the DC terminals of all the controller converters are connected together for real power transfer.

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