ABSTRACT
This research work presents a scheme that identifies line outage and generator outage using contigency analysis and effectively splits the network into a set of predefined islands, with a load shedding strategy to minimise the adverse effect of each outage and to ensure system security. In this research work, Newton Raphson power flow was used for the power flow analysis of the network. Also, Active Power Loading Perfomance Index was used during the contigency analysis to rank the transmission line and generator outage based on the severity of each outage. For each outage causing a line overload and voltage violation, the network splits into predefined islands and power flow analysis is performed on the new island to check the stability of the network. For each island found to be unstable, a power mismatch, and under voltage load shedding scheme is used to ensure the system stability. The developed algorithm was implemented on the IEEE 6 and 14 test bus network. From the IEEE 6 bus network, ten outages resulted in the split of the network into two islands. Also, the developed load shedding scheme was applied on each island that was found to be unstable after power flow analysis. The average voltage profile improvement of the island network over the base case was found to be 13.02% after load shed of 42.3 MW. Also, from the IEEE 14 bus network, the contigency analysis considered twenty four outage, with twenty one outage causing a split of the network into two islands. Load shedding scheme was also applied on each newly formed island found to be unstable. The average voltage profile improvement of the islands over the base case was 2.89% after load shed of 80.92 MW. The validation of this research work was performed by simulation, and comparing with the work of Soman et al,.(2015a), using load shed speed and voltage profile as performance metrics. The developed method obtained an average load shed speed improvement of 63.3% and an average voltage profile improvement of 1.01%. Also, from the results obtained, it is quite evident that the developed scheme has a better performance than Soman et al,.(2015a).
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TABLE OF CONTENTS
TITLE PAGE
DECLARATION i
CERTIFICATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT vi
TABLE OF CONTENTS vii
LIST OF FIGURES x
LIST OF TABLES xi
APPENDIX xii
LIST OF ACRONYM xiii
CHAPTER ONE: INTRODUCTION
1.1 Background of Study 1
1.2 Motivation 4
1.3 Significance of Research 4
1.4 Statement of Problem 5
1.5 Aim and Objectives 5
1.6 Dissertation Outline 5
CHAPTER TWO: LITERATURE REVIEW
2.1 Introduction 6
2.2 Overview of fundamental concept 6
2.2.1 Power System Background 6
2.2.2 Power System Blackout 6
2.2.3 Transmission System 9
2.2.4 Bus Classification 11
2.2.5 Power Flow Techniques 12
2.2.6 Contingency Analysis 16
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2.2.7 Contingency Ranking 19
2.2.8 Forced Islanding Scheme 21
2.2.9 Load Shedding Scheme 22
2.2.10 Objective Function 26
2.2.11 Standard IEEE Test Systems 27
2.3 Review of Similar Works 30
CHAPTER THREE: MATERIALS AND METHODS
3.1 Introduction 35
3.2 Materials 35
3.2.1 Simulation Environment 35
3.2.2 Transmission System Parameters 35
3.3 Methodology 36
3.3.1 Power Flow Analysis 36
3.3.2 Contingency Analysis 37
3.3.3 Predefined Island 39
3.3.4 Develop Load Shedding Scheme 40
3.4 Performance Evaluation 42
CHAPTER FOUR: RESULTS AND DISCUSSION
4.1 Introduction 43
4.2 IEEE 6 Bus Test System 43
4.2.1 Pre-contingency Analysis 43
4.2.2 Contingency Analysis 46
4.2.3 Predefined Island 50
4.3 IEEE 14 Bus System 57
4.3.1 Pre-contingency Analysis 57
4.3.2 Contingency Analysis 60
4.3.3 Predefined Island 62
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4.3.4 Load Shed Speed 66
4.4 Validation 68
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.1 Introduction 73
5.2 Conclusion 73
5.3 Significant Contribution 74
5.4 Recommendations 74
REFERENCES 75
CHAPTER ONE
INTRODUCTION
1.1 Background of Study
Power system networks mostly operate close to their stability limits as a consequence of the deregulated electricity market, growth in energy consumption and lack of expansion of transmission networks due to economic and environmental constraints. Under such operating conditions, a severe disturbance such as a loss of generating units or faults along transmission lines may lead to cascading events. Thus, the risk of collapse and blackout of the overall power system is increased (Tang et al., 2013). Security of a power system refers to the degree of risk in its ability to survive imminent disturbances (contingencies) without interruption of customer service. It relates to robustness of the system to imminent disturbances, and hence depends on the system operating condition as well as the contingent probability of disturbances. Electric power system security analysis encompasses three functions namely system monitoring, contingency analysis and corrective control in which safe island formation and load shedding are some of the corrective control (Ezhilarasi & Swarup, 2009). Power system blackout is the state when partial or complete areas of the system collapse due to cascading of failure events which causes mass scale tripping of transmission lines and generating units (Soman et al., 2015a). Islanding (also known as loss of grid or Loss-of-Mains – LoM) represents “a condition in that a portion of the power system that contains both load and generation remains energized while isolated from the remainder of the power system”. An island represents a condition where a portion of an area electric power system (EPS) is energized solely by one or more local power sources while that portion of the area EPS is electrically separated from the rest of the area EPS. Islanding appears when some part of the utility grid loses connection with the rest of the system (Banu & Istrate, 2014). Controlled islanding
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is a critical way of preventing large disturbances from propagating into the rest of the system and causing a severe breakup and blackout. In a system, more islands will lead to longer resynchronization times, and cutting more interconnections between the islands would have higher risks of introducing disturbances into the system. Thus the smallest number of islands which is ideally two should be the best islanding scheme (Lin et al., 2014). The technical issues in achieving safe and smooth operation of islanded events are speed governor response, range of operating power, voltage and frequency control, earthing or equivalent protection of the island operation, and resynchronisation to the grid. Among these technical issues, voltage and frequency control tends to occur more frequently, of which load shedding is considered the most effective technique to overcome the problem. The only means of stabilizing the voltage and frequency to their nominal values in an islanded system is by rejecting several loads through a load shedding scheme (Khamis et al., 2015). Under Voltage Load Shedding is based on the possibility of disconnecting some loads (or percentage of load) after a severe contingency, in order to relocate the operating point far from the critical value (López et al., 2016). To prevent system failure during extreme emergencies, it is recommended to execute controlled splitting of the system into stable islands with generation and load shedding schemes. Such formed islands are more stable than unintentionally formed islands and are less prone to reach conditions that lead to total blackout of the entire system (Soman et al., 2015d). One of the most important factors in the operation of any power system is the desire to maintain system availability and reliability. This ensures a secure operation of the system and improved economic operation. Power system security is the ability of the system to withstand one or more component outages with the minimal disruption of service or its quality (Nnonyelu et al., 2014). In large disturbances of power system, usually frequency decays are accompanied with voltage decays at load buses which reduce the system security, so to restore a system due to
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frequency decline actual levels of load shed by under frequency load shedding (UFLS) or under voltage load shedding (UVLS) are relative to the levels required and expected, this required levels are calculated using power system analysis (Balasubramaniam et al., 2016; Moafi et al., 2016). In addition, active power deficit is usually accompanied by a reactive power deficit. This reactive power deficit results in a sudden decrease in the voltage of all the buses which in turn requires some of the loads that decreases the frequency to be dropped (Marzband et al., 2016).
Load shedding is an emergency control action to ensure system stability, by curtailing system loads, this action only takes place when system frequency/voltage falls below a specified threshold. Typically, the load shedding attempts to balance real and reactive power supply and demand in the system (Bevrani et al., 2010). Power system experience severe voltage and frequency disturbance due to the imbalance between the generation and load after islanding occurs. The frequency/voltage rapidly decreases which may be damaging to rotating machines and other load equipment within the island, and it will be essential that certain amount of load be shed to restore system frequency/voltage to nominal value in order to prevent total system collapse (Laghari et al., 2015). When an extreme failure event occurs in a network, under voltage load shedding can be applied as an economical method for preventing voltage collapse in order to maximize the power system loadability (Majidi et al., 2014). An undervoltage load shedding scheme should address two tasks: the detection of voltage instability following a large disturbance and the determination of the amount of load to be shed. In case of short term voltage instability, the scheme should be fast (Damodhar & Krishna, 2016).
Due to the economic challenges associated with blackouts, researchers have proposed schemes to help identify outage which could lead to system collapse. Dola and Chowdhury (2006) proposed a blackout mitigation technique based on strategic tripping of overloads. Nnonyelu et al., (2014)
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presented a reliability evaluation analysis considering system line overload index (SLOI) for contingency ranking but not considering generator outage. Li et al., (2014) developed a zone division scheme based on sensitivity analysis by lumping up buses for its analysis. Soman et al. (2015a) presented an algorithm that identifies the risk during an outage and forcefully splits the system into predefined islands using DC power flow for its system analysis. Therefore, this research work proposes an improved scheme considering generation and transmission line outage for its contingency analysis, the use of Newton Raphson load flow for accuracy, and an optimum load shedding strategy to balance generation and load considering power imbalance, voltage deviation and thermal limit constraints.
1.2 Motivation
During the planning and operation of a power system, there is need for corrective control to severe contingencies in order to ensure steady power delivery to loads. Contingencies such as generator and line outage could cause voltage violation, and line limit violation which could lead to total system collapse. Thus, a need for accurate forced island formation to prevent cascading of fault, and a fast load shedding scheme as a corrective control is of importance, and a motivation for this research work.
1.3 Significance of Research
The significance of this research is the development of an improved forced island and optimal load shedding scheme to prevent total system collapse in the event of line outage and generator outage contingencies considering power imbalance, bus voltage magnitude, line maximum loading, and time to stability for the load shedding scheme. Previous researchers did not consider the use of this scheme to prevent total system collapse.
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1.4 Statement of Problem
Power system contingencies (line, transformer, generator outage) cause redistribution of power flow and violation of line and bus voltage limits, which leads to cascade failure and collapse of the entire system. The problem related to load shedding and islanding scheme is the development of a strategy to define the location, and amount of load to shed for every severe faults in order to save the system from total collapse. This research work developed a scheme for predefined island formation and optimal load shedding scheme to ensure continious supply to customers and prevent cascade failure which leads to total system collapse.
1.5 Aim and Objectives
The aim of this research is to develop improved forced island and load shedding scheme to prevent total system collapse. In order to achieve this aim, the following are objectives of the study:
1. To perform a power flow analysis using Newton Raphson power flow technique.
2. Develop a forced island and optimal load shedding scheme based on power system contingency analysis.
3. Validation by comparing the developed scheme with the work of Soman et al., (2015a),
1.6 Dissertation Outline
The introduction to this research has been presented in Chapter One. Detail review of related literatures and relevant fundamental concepts, and review of similar works published, related to this research is contained in Chapter Two. Chapter Three presents the methodology and materials used. The results obtained after applying the improved methods on the IEEE 6 bus, and also the performance of the developed scheme on IEEE 14 bus is presented in Chapter Four. Chapter Five presents conclusion, and recommendations for further work. While list of cited work, data and MATLAB codes are given in the appendix section provided at the end of this research report.
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