TABLE OF CONTENTS
Title page i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Table of Contents vi
List of Tables ix
List of Figures xi
Abstract xiii
CHAPTER 1: INTRODUCTION
1.1 Background of the Study 1
1.2 Statement of Problem 3
1.3 Aim and Objective of the Study 4
1.4 Significance of Study 4
1.5 Scope of Study 5
1.6 Thesis Outline 5
CHAPTER 2: LITERATURE REVIEW
2.1 Power System Structure 6
2.2 Load Flow Analysis 7
2.2.1 Gauss-Seidel iterative method for load flow analysis 7
2.2.2 Newton Raphson iterative method for load flow analysis 9
2.3 Power System Concept and Protection Philosophy 13
2.4 Power System Protection Structure 14
2.4.1 Functional requirements of power system protection system 15
2.4.2 Basic protection scheme components 16
2.4.2.1 Current transformer (CT) 17
2.4.2.2 Voltage transformer (VT) 17
2.4.2.3 Protective relays 18
2.4.2.4 Circuit breakers 19
2.4.2.5 Auxiliary DC supply 20
2.5 Fault Analysis Study 20
2.5.1 Fault types and causes 21
2.5.2 Transmission line transient fault 22
2.6 The Protection Coordination Study 24
2.6.1 Overcurrent protection study 25
2.7 Relay Coordination 29
2.7.1 Methods of coordination 29
2.7.1.1 Time graded systems 30
2.7.1.2 Current graded system 31
2.7.1.3 Time and current graded system 31
2.8 Time Current Characteristics Curves 33
2.9 Relay Coordination Procedure 33
2.9.1 Guidelines for correct relay coordination 34
2.10 Review of Related Work 34
CHAPTER 3: MATERIALS AND METHODS
3.1 Materials 42
3.2 Methods 42
3.2.1 Modelling of Umuahia 132/33 kV transmission substation 44
3.2.2 Bus loading equation model 46
3.3 The Umuahia 132/33 kV Transmission Sub-Station 48
3.4 Inverse Definite Minimum Time (IDMT) Overcurrent Relay
Equations 49
3.5 Data for the Analysis 51
3.6 Analysis of the Network 55
3.6.1 Fault level (Fault MVA) calculation of the station transformer 56
3.6.2 Three - phase short circuit calculation 57
3.6.3 Algorithm for relay coordination setting 58
3.6.4 Relay coordination 58
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Results of Three - Phase Short Circuit Analysis 60
4.1.1 When fault is located at Afara 33 kV feeder end 61
4.1.2 When fault is located at the 33 kV bus 63
4.1.3 When Fault is located at 132kV Side 63
4.2 Results of Relay Coordination Analysis 66
4.2.1 Fault located at Afara 33 kV feeder end 67
4.2.2 Fault is located at 33 kV bus 74
4.2.3 Fault at 132Kv side (just before T1 transformer) 81
4.2.3.1 The existing 132kV side network 81
4.2.3.2 The modified 132kV side network 86
4.3 Relay Setting Comparison Results 91
4.3.1 Relay characteristic curve comparison for 33 kV feeder and 33 kV bus bar relays 92
4.3.2 Relay characteristic curve comparison relays at the 132kVside 93
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion 95
5.2 Recommendation 97
5.3 Contribution to Knowledge 97
REFERENCES
LIST OF TABLES
2.1: Fault type and causes 21
3.1: Outgoing feeder characteristics 48
3.2: Line capacities 51
3.3: Feeder capacities 53
3.4: Feeder breakers properties 53
3.5: Line Circuit breaker properties 54
3.6: 132/33 kV transformer properties 54
3.7: Current transformer properties 54
3.8: Overcurrent relay properties 55
4.1: Three-phase short circuit fault results at different locations 60
4.2: Sequence-of-operation event summary report for standard inverse Setting 68
4.3: Sequence-of-operation event summary report 70
4.4: Sequence-of-operation event summary report 72
4.5: Sequence-of-operation event summary report for standard inverse setting 75
4.6: Sequence-of-operation event summary report for very inverse Setting 77
4.7: Sequence-of-operation event summary report for extremely inverse setting 79
4.8: Sequence-of-operation event summary report for standard inverse setting 82
4.9: Sequence-of-operation event summary report for very inverse setting 83
4.10: Sequence-of-operation event summary report for extremely inverse setting 85
4.11: Sequence-of-operation event summary report for standard inverse setting 87
4.12: Sequence-of-operation event summary report for very inverse setting 89
4.13: Sequence-of-operation event summary report for extremely inverse setting 90
4.14: Relay Setting Comparison for Relays R16, R 17, R18, R 19 and R 21 for the three considered characteristic curves 92
4.15: Relay setting comparison for 132kV side relays R1, R2 and R 26 93
LIST OF FIGURES
2.1: A single line diagram of a typical power system 6
2.2: A typical power system protection zone 14
2.3: Basic arrangement of a protection system 16
2.4: Capacitive voltage transformers 18
2.5: Symmetrical three-phase short circuit 22
2.6: Transmission line model 22
2.7: Short circuit current waveform on a transmission line 24
2.8: Characteristics of various over-current relay (a) definite time, (b) IDMT, (c) very inverse and (d) extremely inverse 29
2.9: Principal of time graded system of protection for a radial feeder 30
3.1: A two bus system 43
3.2: The i-th bus of a power system 47
3.3: Single line diagram of Umuahia 132/33 kV sub-station 52
4.1: Network diagram for when Fault is located at Afara 33 kV
Feeder End. 61
4.2: Three phase short circuit fault on Afara 33 kV line 62
4.3: Three phase short circuit fault on 33 kV bus section 63
4.4: Network diagram for when fault is located at 33 kV bus section 64
4.5: Network diagram for when fault is located at the 132kV side 65
4.6: Three phase short circuit fault at the 132kV side 66
4.7: Relay coordination simulation for fault on Afara feeder 68
4.8: TCC curve of relay 16 and 21 for standard inverse setting 69
4.9: TCC curve of relay 16 and 21 for very inverse setting 71
4.10: TCC curve of relay 16 and 21 extremely inverse setting 73
4.11: Relay coordination for fault on 33 kV bus 74
4.12: TCC curve of Relay 16, 17, 18 and 19 for standard inverse setting 76
4.13: TCC curve of relay 16, 17, 18 and 19 for very inverse setting 78
4.14: TCC curve of relay 16, 17, 18 and 19 for extremely inverse setting 80
4.15: Relay coordination operation simulation for the existing network 81
4.16: TCC curve of relay 1 and 2 for standard inverse setting 82
4.17: TCC curve of relay 1 and 2 for very inverse setting 84
4.18: TCC curve of relay 1 and 2 for extremely inverse setting 85
4.19: Relay coordination operation simulation for the modified network 86
4.20: TCC curve of relay 1, 2 and 26 for standard inverse setting 88
4.21: TCC curve of relay 1, 2 and 26 for very inverse setting 89
4.22: TCC curve of relay 1, 2 and 26 for extremely inverse setting 91
4.23: Relay setting comparison for relays 16, 17, 18, 19 and 21 93
4.24: Relay setting comparison for relays 1, 2 and 26 94
ABSTRACT
Quality electricity supply in Nigeria is yet to be enjoyed by all due to certain technical and non-technical factors. It is important to note that every power system consists of various expensive equipment which need to be protected from dangerous fault currents. A properly coordinated power system protection is that which isolates only the faulted part of the network. For proper coordination, there should be a time delay between the primary and secondary protection system. This time gap between primary and secondary protection is known as Coordination Time Interval (CTI). The data for the modeling and simulation of overcurrent relay coordination for a typical 132/33 kV transmission station is gotten from the Umuahia 132/33 kV substation of the Transmission Company of Nigeria (TCN). The Electric Transient Analyzer Program (ETAP 16.0) software was used for the modeling and analysis. A three-phase short circuit test was conducted on the network at three different locations namely: Afara 33 kV feeder, the 33 kV bus section and at the 132 kV bus. The overall maximum peak short circuit current of 20.15 kA was recorded. Afterwards, a detailed sequence of operation of the station’s overcurrent relays was carried out at these locations for standard inverse relay characteristics, very inverse relay characteristics and extremely inverse relay characteristics. Results showed that for a fault injected at Afara 33 kV feeder, relays R16 and R21’s, time current curve (TCC) for the standard inverse relay setting, very inverse relay setting and extremely inverse relay setting curves, indicated a fault current of 3.141 kA, 3.153 kA and 3.153 kA respectively which lasted for 3.51 s, 3.48 s and 3.48 s on the network respectively. Similarly, the results showed that for a fault injected on the 33 kV busbar, R16, R17, R18 and R19’s, time current curve (TCC) for the standard inverse relay setting, very inverse relay setting and extremely inverse relay setting curves, indicated a fault current of 7.746 kA which lasted for 3.57 s, 3.57 s and 3.28 s on the network respectively. Moreover, it was shown that when a fault was injected at 132kV bus, R1 and R2’s, time current curves (TCC) for the standard inverse relay setting, very inverse relay setting and extremely inverse relay setting curves, indicated a fault current of 2.843 kA which lasted for 3.25 s, 3.47 s and 3.48 s on the network respectively. Finally, it was seen that it is best to use relays with the extreme inverse characteristics at points closer to the source - as a faster trip time of 0.0952 s was recorded at the 132kV side, whereas the very inverse relay setting and standard inverse relay setting characteristics should be used downstream, as trip times of 3.51 s and 1.9 s respectively were recorded at locations farther from the source.
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF THE STUDY
The importance of electricity to mankind can never be over emphasized. Energy in electrical form has proved to be the most convenient, useful, cleaner reliable, safe, and controllable and desired form of secondary energy. Electrical energy and power is used universally since the beginning of 20th century. The electrical energy has become essential for modern civilization (Uppal and Rao, 2009). Nowadays electricity is used in almost every home in the world. Despite its vital role in civilization, electricity when not properly handled can be lethal.
The electrical power system consists of three main parts namely generation, transmission and distribution, with voltage levels ranging from 240 V to 400 kV or greater. Electrical faults such as overcurrent are an inherent factor of the power system infrastructure especially on the transmission lines where in most cases are bare conductors. The imbalance due to these faults has an impact on power behavior and as a result, other accessories connected to the power system are harmed (Abdul-Wadood et al., 2018). In order to eliminate these problems, protective measures should be taken into account. These protective measures are handled by a branch of power system called Power system protection. The art or science of continuously monitoring the power system, detecting the presence of a fault, and initiating the proper tripping of a circuit breaker to isolate the faulty section of the electrical network is known as power system protection (Mohammed and Lazim, 2008).
Some main objectives of power system protection include limiting the extent and duration of service interruptions when equipment failure, human error, or adverse natural events occurs on any portion of the system, minimizing damage to the system components involved in the failure and prevention of human injury.
Protection engineering is also concerned with the design and operation of protection schemes. The protection scheme chosen for any portion of the system depends on the kind of fault anticipated in that part. Different kinds of faults exist such as short circuit fault, earth fault, open circuit fault, etc. The faults can either be single phase, double phase or three phase faults. Causes of these faults can be as result of Insulation breakdown, Lightning, animals bridging the phases, Dig-ups for underground cables, Poles collapsing, Conductors breaking, Vehicle impact, Wind borne debris, mal-operation by personnel etc. (Madhav University, 2021).
Power system protection involves selecting suitable actuation settings for different protective devices such as relays. A typical power system such as 132/33 kV sub-transmission station comprises numerous protective devices. These numerous protective devices ought to be well coordinated in order to ensure that the vital and most essential function of each protective device is fulfilled along with the requirement of sensitivity, selectivity, reliability and speed. When setting or selecting protective devices, the operating times of all the devices in response to several intensities of trip signals (for instance overcurrent) are compared with each other. The objective, of course is to design a selectively coordinated electrical power system such that the resulting forced power outage is only limited to the faulty portion of the network in the cause of fault clearing. Considering the above, the job of a protection engineer becomes tedious and rigorous as a new or revised coordination study is required whenever there is a change in the maximum short circuit current of a power station due to the introduction of large loads, replacement of existing equipment with larger equipment, or when fault shuts down a large portion of the power system, or when protective devices are upgraded (Mohammed and Lazim, 2008).
With the above challenges, it becomes imperative for computer models to be developed in other to reduce the time and energy spent in manually configuring these protective devices. This thesis is primarily concerned with the coordination of overcurrent relays for a typical 132/33 kV station. Relay coordination entails that a relay nearest the fault point should be activated first and if it fails, then backup relay must be operated in sequence to provide back-up protection.
Electrical Transient Analysis Program (ETAP) is a full spectrum analytical engineering software that will be used to critically analyze the coordination of protective overcurrent relays in a typical sub-transmission station. This will be achieved by first modeling the switchyard of such station and then followed by series of simulations to ascertain the best relay settings/configurations that will present a well -coordinated 132/33 kV Transmission switchyard with little or no false tripping or outages.
1.3 STATEMENT OF PROBLEM
Electrical power supply in Nigeria has been known to be epileptic due to series of outages thus making the system unpredictable. It is important to note that some of these outages are due to protective relay operations. Protective relays when not well coordinated definitely result in false tripping of electrical circuits thereby isolating healthy sections of the power system from supply.
There is need to model and simulate the overcurrent protection coordination of 132/33 kV Power stations and explore how it can be improved upon for better power supply to Umuahia and its environs.
1.3 AIM AND OBJECTIVES OF THE STUDY
The aim of the thesis is to model and simulate a coordinated power system protection using Overcurrent Relay (OCR).
The objectives of this study are:
i. To model and simulate the Umuahia 132/33 kV power system network for protective relay coordination.
ii. To evaluate the Umuahia 132/33 kVnetwork behavior with respect to its overcurrent protection during fault conditions at different locations.
iii. To model a modified protection scheme in Umuahia 132/33 kV power station for improved power system security.
1.7 SIGNIFICANCE OF STUDY
Power system protection is achieved using protective relays. Protective relays have to be well configured so as to achieve the desired goals. Configuration of these relays entails rigorous and tedious processes. Thus the results obtained from this research will go a long way to assisting protection engineers in the Nigeria’s electricity company in discharging their duties with ease. This on the long run will eliminate or reduce the occurrence of false tripping of healthy part of the electrical network due to protective relay action. The findings from this study can also serve as a base for individuals who intend taking protective relays as a research work. For an industrialist, the short falls exposed in this work can be worked upon to develop better and more efficient protective relays.
1.8 SCOPE OF STUDY
The study is limited to Umuahia 132/33 kV Transmission Substation. This is because of the physical layout of the substation switchyard which presents a radial arrangement for its outgoing 33 kV feeders. The Umuahia 132/33 kV transmission substation is chosen because of its relevance in power supply to Michael Okpara University of Agriculture, Umudike (MOUA). An improvement in the station’s protection will definitely have positive impact in her wheeling capacity to the MOUAU and Abia state at large.
1.9 THESIS OUTLINE
Chapter one, contains the introduction to this thesis, its aim and objectives as well as the scope and motivations. In Chapter two, the review of the related literature is presented. Different aspects of power system protection are discussed in details herein. Chapter three discusses the methodology. A description of the materials used for the research and the implementation methods applied in realizing the results are described here in details. Chapter four deals with the research results as obtained from the simulation of the developed model and deductions made from the results. Chapter five, deals with conclusion, recommendations, suggestion and contribution to knowledge. List of references and relevant appendices are provided at the end of this thesis.
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