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Transcript
i LIGHTNING SIMULATION STUDY ON LINE SURGE ARRESTERS AND PROTECTION DESIGN OF SIMPLE STRUCTURES NOOR SHAHIDA BT JAMOSHID A project report submitted in fulfillment in partial fulfillment of the requirements for the award of the degree of Master of Engineering (Electrical - Power) Faculty of Electrical Engineering Universiti Teknologi Malaysia MAY 2008 iii To my beloved mother and father iv ACKNOWLEDGEMENT Praise be to Allah s.w.t. to Whom we seek help and guidance and under His benevolence we exist and without His help this project could not have been accomplished. I would like to acknowledge the contributions of my respectful supervisor, Associate Professor Dr. Zulkarnain Bin Abdul Malek for his time, support and advice throughout this project. Without his support this proposal may not have come to fruition. I also would like to thank all my friends for the numerous ideas and helpful hands throughout this project. I wish to thank the grateful individuals from TNB Transmission Line group. Lastly, I am deeply grateful to my parents Jamoshid Bin Paramuthullah and Leha Binti Bahadur Khan, as well as to my sister and brothers for a support and care throughout my journey of education. v ABSTRACT There was a recent incidence where a direct lightning strike on the earth shielding conductor of a 275/132kV quadruple circuit transmission line had caused the breakage of the conductor at four points. Three short conductors connecting the line arrester installed on the 132kV line were not affected. The location of the affected arrester was not at the nearest tower to the point of strike but at the adjacent tower. The arresters at the nearest tower were not affected. This phenomenon was studied using ATP-EMTP simulation. Transmission tower is modeled according to the multi storey tower proposed by Masaru Ishii which was validated through theory and calculation. Simulation results show that the phenomenon cannot be conclusively reproduced within the ATP-EMTP simulation. Study indicating the fact that the phenomenon may be a one-off special case event. Overhead line is modeled by applying the PI subroutine file. This project also study the protection of simple structures from lightning strikes. The most common and simplest form of lightning protection is by using a vertical rod which has the function of intercepting a lightning stroke before it can strike a nearby object it is protecting, and then discharging the current to ground. In this simulation study, 1500 strokes were applied in a square plot ground area of 1km² and the number of flashes to ground per square kilometer per year (Ng) is 15 strokes/ km²/year. A Monte-Carlo technique is used to manipulate the statistical distribution of lightning strokes. The program is written in C-language using MATLAB simulation. vi ABSTRAK Baru-baru ini, satu kejadian telah berlaku di mana panahan petir pada talian bumi, talian penghantaran atas 275/132kV litar berkembar empat (quadruple circuit) telah menyebabkan talian bumi terputus kepada empat bahagian. Penangkap kilat pada bahagian bawah talian 132kV pada menara talian penghantaran yang berdekatan tidak berfungsi, sebaliknya penangkap kilat pada menara bersebelah yang berfungsi. Menara penghantaran dimodel berdasarkan kepada model bertingkat yang dicadangkan oleh Masaru Ishii. Model disahkan melalui kiraan dan teori. Keputusan daripada simulasi kajian yang dijalankan tidak dapat membuktikan kejadian ini berlaku melalui ATP-EMTP. Aturcara Simulasi ATP-EMTP telah digunakan dalam mengkaji panahan petir terhadap litar berkembar empat. Talian atas dimodelkan dengan menggunakan model PI yang sedia ada dalam EMTP. Simulasi menunjukkan fenomena di atas tidal dapat ditunjukkan melalui simulasi dan ia mungkin merupakan kes terpencil. Projek ini juga mengkaji perlindungan daripada struktur yang mudah terhadap panahan kilat. Struktur yang asas dan mudah untuk perlindungan petir ialah dengan menggunakan rod tegak dimana ia berfungsi memintas penahan petir sebelum ia memanah kawasan sekitar yang dilindungi dan kemudian menyahcas arus ke bumi. Untuk kajian simulasi ini, 1500 panahan telah dikenakan pada segiempat sama yang berukuran 1 panjang dan lebar kawasan bumi. Bilangan panahan ke bumi per per tahun (Ng) adalah sebanyak 15 panahan. Teknik Monte-Carlo telah digunakan untuk manipulasi statistik taburan panahan petir. Program ini menggunakan bahasa C dalam Simulasi MATLAB. vii TABLE OF CONTENTS CHAPTER 1 2 TITLE PAGE DECLARATION ii DEDICATION iii ACKNOWLEDGEMENTS iv ABSTRACT v ABSTRAK vi TABLE OF CONTENTS vii LIST OF TABLES x LIST OF FIGURES xi LIST OF ABBREVIATIONS xiv LIST OF SYMBOLS xv LIST OF APPENDICES xvii INTRODUCTION 1 1.1 Introduction 1 1.2 Problem Statement 2 1.3 Objective 4 1.4 Scope of Project 5 1.5 Organization of Thesis 5 LITERATURE REVIEW 6 2.1 Lightning Problem for Transmission Line 6 2.2 Effects on Transmission Line Protection 7 2.2.1 7 2.3 Backflashover Travelling Wave viii 2.4 Lightning Current 8 2.4.1 9 Characterization of the lightning discharge 2.5 Line Insulation Flashovers Model 11 2.6 Ground Flash Density 16 2.7 Tower Footing Resistance 16 2.8 Transmission Line Tower 17 2.8.1 Development of Tower Model 17 2.8.2 Tower model 18 2.9 Surge arrester 21 2.10 Transmission Line Model 24 2.11 Monte Carlo Simulation 25 2.11.1 The 3-Dimensional Electrogeometric Model 26 2.11.2 3-Dimensional Simulation of Fields of Influence 26 2.11.3 3-Dimensional Modeling of The Lightning Stroke 27 3 2.11.4 Ground Flash Density 30 2.11.5 Shielding Effect of a Vertical Rod 30 METHODOLOGY 31 3.1 ATP-EMTP Simulation 31 3.2 Typical EMTP Applications 32 3.3 Creating Simulation File 33 3.4 Creating Punch File 35 3.5 Simulation 36 3.6 Plot File 37 3.7 Transmission line 37 3.8 Transmission tower 38 3.9 Insulator String 44 3.10 Lightning source selection 44 3.11 Monte Carlo Simulation 47 3.12 Project Flow 50 ix 4 SIMULATION RESULT AND DISCUSSION 53 4.1 Introduction 53 4.2 Line Surge Arrester Study 54 4.2.1 Transmission tower 54 4.2.2 Transmission Line and Tower Circuit Model on 55 EMTP Simulation 4.3 5 Lightning Protection of Structures 64 4.3.1 64 Simple Structure Protection Result CONCLUSIONS AND RECOMMENDATIONS 74 5.1 Conclusions 73 5.2 Recommendations 75 REFERENCES 76 Appendix A 80 x LIST OF TABLES TABLE NO. TITLE PAGE 2.1 Flashover rate for different circuit without line surge arrester 23 2.2 Flashover rate for different circuit with line surge arrester 23 3.1 Parameter of the 275/132kV quadruple tower model 42 4.1 Voltage between each phase and insulator string at tower 3 63 4.2 Voltage between each phase and insulator string at tower 4 63 4.3 Lightning stroke with effective striking distance 71 xi LIST OF FIGURES FIGURE NO. 1.1 TITLE Transmission line had caused the breakage of the conductor PAGE 3 at four portions 1.2 The direct stroke on shield wire between T70-T71 affected 3 Three TLAs installed at T69 and T68 2.1 Reflection and refraction at tower after lightning strike 8 2.2 Lightning current shape, according to IEEE guidelines 10 2.3 Peak current magnitude (kA) versus flashover rate 10 2.4 Rise time lightning current versus flashover rate 11 2.5 Critical flashover voltage for 275/132kV transmission line 12 2.6 The back flashover mechanism. 14 2.7 Model used for string of insulator up 275/132kV. 14 2.8 Kawai tower model 18 2.9 Mathematical calculation for multistory tower model 20 2.10 Multiconductor vertical line model 20 2.11 Line arrester installed on 275/132kV 22 2.12 Transmission line model 23 2.13 Fields of influence of a vertical rod and ground. Rs and rsg 28 are the effective striking distances of the vertical rod and ground 2.14 Fields of influence of horizontal wire and ground 28 2.15 Fields of influence of rectangular block and ground 29 2.16 Display of lightning strokes (represented by dots) terminating 30 on structure (vertical rod) and surrounding ground - plan view xii 3.1 Overview of ATPDraw commands and function 32 3.2 Data window for simulation setting 34 3.3 Data window for inserting the parameter 35 3.4 Data window for transmission line 36 3.5 Transmission line model 39 3.6 Multistorey transmission tower 39 3.7 M.Ishii’s tower model for a double line tower 40 3.8 Tower equivalent radius 41 3.9 Modified M.Ishii’s tower model for a quadruple circuit line 43 tower modeling 3.10 Insulation string model 44 3.11 Waveform of fast front voltage surge using Heidler model, 45 20kV with 0.5µs fast front time 3.12 Waveform of voltage using DC model, 20kV with 0.5µs fast 46 front time 3.13 Voltage at tower top by using a DC source as input 47 3.14 Flow chart of Monte Carlo simulation on transmission line 49 3.15 Project flow chart 51 3.16 Protection of simple structures due to lightning strikes 52 4.1 Complete multistorey model 54 4.2 Voltage at tower top, tower base and each crossarm of the tower 55 4.3 The simulation circuit of 275/132kV multistory quadruple 56 transmission line, transmission tower with EMTP 4.4 Voltage at red phase and insulator string tower 3 (275kV) 57 4.5 Voltage at blue phase and insulator string tower 3 (275kV) 57 4.6 Voltage at yellow phase and insulator string tower 3 (275kV) 58 4.7 Voltage at red phase and insulator string tower 3 (132kV) 58 4.8 Voltage at blue phase and insulator string tower 3(132kV) 59 4.9 Voltage at yellow phase and insulator string tower 3(132kV) 59 4.10 Voltage at red phase and insulator string tower 4 (275kV) 60 4.11 Voltage at blue phase and insulator string tower 4 (275kV) 60 xiii 4.12 Voltage at yellow phase and insulator string tower 4 (275kV) 61 4.13 Voltage at red phase and insulator string tower 4 (132kV) 61 4.14 Voltage at blue phase and insulator string tower 4 (132kV) 62 4.15 Voltage at yellow phase and insulator string tower 4 (132kV) 62 4.16 Lightning Surge Arrester Configuration L-Arrangement 64 4.17 Display of lighting strokes at surrounding ground-plan view 65 4.18 Display of lightning strokes (represented by dots) 66 terminating on structure (vertical rod), and surrounding ground-plan view with current 2.5kA and 5kA. 4.19 Vertical rod and its effective striking with current 2.5kA 69 4.20 Vertical rod and its effective striking with current 5kA 69 4.21 Vertical rod and its effective striking with current 10kA 70 4.22 Vertical rod and its effective striking with current 15kA 70 4.23 Field of influence of a rectangular block above ground which 72 can be used to represent a building structure or a patch of trees with current 2.5kA with 2 dimensional electrogeomatric model. 4.24 Field of influence of vertical cylinder can be used to represent a building structure or a patch of trees with current 2.5kA (3 dimensional electrogeomatric model). 72 xiv LIST OF ABBREVIATIONS ATP - Alternative Transient Program EMTP - Electromagnetic Transient Program TLA - Transmission Line Arrester TD - Thunder Days CIGRE - International Conference on Large High-Voltage Electric System IEEE - Institute Electrical and Electronics Engineers LCC - Line Cable Constant R-L - Resistance and Inductance SiC - Silicon Carbide DC - Direct Current xv LIST OF SYMBOLS V - Voltage θ - Angle Ω - Ohm I - Current kV - Kilo-Volt m/µs - Meter per Micro-second R - Resistance L - Inductance C - Capacitance µs - Micro-second kA - Kilo-Ampere mH - Millie-Henry µF - Micro-Farad t - Time % - Percent - Tower surge impedance - Attenuation coefficient - Damping coefficient - Height - Probability current H xvi Ng - Field of influenced of object - Number of flashes to ground per square kilometer per year xvii LIST OF APPENDICES APPENDIX A TITLE 1) 275/132kV Transmission line and PAGE 81 Transmission Tower Model - EMTP 2) Matlab Simulation of lightning strokes 81 (represented by dots) terminating on Structure (vertical rod), and surrounding ground-plan view with current 3) Matlab Simulation of lightning strokes 84 (represented by dots) terminating on structure (vertical rod 4) Matlab Simulation of field of influence of vertical cylinder can be used to represent a building structure 88 1 CHAPTER 1 INTRODUCTION 1.1 Introduction High overvoltage transients caused by lightning is considered a major source of disturbances in high voltage transmission line systems. There is a consensus that lightning starts from the charge separation process (positive and negative), which is due to transportation of lightweight particles to higher regions by the rapid updrafts of moist air, usually in hot humid areas. This charge separation is known as the vertical thunderstorm dipole. It can be performed within the cloud or between the cloud and the earth which creates electric fields that eventually bring out the breakdown known as lightning. The overvoltage introduced by lightning have traditionally been estimated using conventional and simplified methods. More involved calculations become possible with digital computer programs such as Electromagnetic Transients Program (EMTP). In such a program, each power system component can be modelled in great detail. 2 The characteristics of lightning surges on overhead transmission lines, which result from lightning strokes, depend on how there are caused. They can be broadly divided into four types: a) Tower/ground wire surge - The stroke terminates on the tower structure/ground wires without any flashover to the phase conductors. b) Shielding failure - The stroke passes through the protective zone of the ground wires and terminates on the phase conductors. c) Back flashover - The same as a), but followed by a flashover to the phase conductors. This type of flashover is called back flashover. d) Shielding failure flashover – The same as b), but followed by a forward flashover to the ground/ground wires or tower. 1.2 Problem Statement Part 1: Lightning Simulation Study on Line Surge Arresters. A recent incidence from direct lightning strike on the shielding conductor of a 275/132kV quadruple circuit transmission line had caused the breakage of the conductor at four portions. This incident happened between transmission line Pulu to Serdang(275kV) and Balakong to Serdang(132kV). Figure 1.1 shows a direct stroke on the earth wire between two towers has caused the wire to snap into 4 portions. Line arresters are installed on the 132kV lines. The location of the affected arrester was not that closest to the point of strike but rather further down at the next tower. The arrester at the nearest tower was not effected. Figure 1.2 shows the tower locations. 3 Figure 1.1 Transmission line had caused the breakage of the conductor at four portions[1] Figure 1.2 The direct stroke on shield wire between T70-T71 affected three TLAs installed at T69 and T68 [1] 4 Part 2: Protection Design of Simple Structure There are standard methods to design and install the lightning protection devices for structures. Among the concepts used is the rolling sphere method which determines the exposed areas to lightning strikes. Lightning rods, usually the conventional Franklin rods, are installed on top of buildings and structures is protect the exposed areas from lightning threats. The rolling sphere method described above is based on a number of assumptions such as the average lightning peak current, which may limit the protection reliability to a certain condition only. This simulation work aims to consider all possible lightning current magnitudes and the corresponding ground flash density. The simulation is run for long time (teens or hundreds of years) and this is possible using a computer simulation. The performance of the designed lightning protection can then be studied. 1.3 Objective The objectives of this project are: 1) To study and investigate a recent incident where a direct lightning strike on the earth shielding conductor of a 275/132kV quadruple circuit transmission line as below: a) Arrester at the nearest to the point of strike is not effected rather further down at the next tower. b) Lightning strike at shielding wire caused the breakage of conductors at four points. 5 2) To develop a program to simulate the probability nature of lightning strike using Monte Carlo Simulation and to simulate the lightning protection of simple structures. 1.4 Scope of Project Design and analysis: ¾ Modeling 275/132kV Quadruple Circuit Transmission Line use ATP-EMTP Simulation ¾ Monte Carlo Simulation using MATLAB 1.5 Organization of Thesis The thesis is organized in the following manner. Chapter 2 describes the literature review of the project which includes the lightning strikes phenomenon on transmission line and transmission tower, and the protection design of simple structures. Chapter 3 describes on the methodologies used. Results and discussion are described in Chapter 4 followed by conclusions in Chapter 5. 6 CHAPTER 2 LITERATURE REVIEW 2.1 Lightning Problem for Transmission Line Lightning strokes to transmission line and tower of 275/132kV quadruple circuit are classified into two groups which are direct stroke and induced voltage. Direct stroke is the phenomenon of thunder cloud directly discharge into transmission line and it is considered the major source of disturbance in transmission line system [3]. Induced voltage is introduced when the thunderstorm generates negative charges and the earth objects develop induced positive charges. When cloud discharges to some earthed objects other than the transmission line, the line is left with a huge concentration of charge (positive) which cannot leak instantaneously. The transmission line and the ground will act as a huge capacitor charged with a positive charge and hence overvoltage occurs due to these induced charges [3,6]. This phenomenon is not so critical for system voltages more than 66kV. 7 2.2 Effects on Transmission Line Protection When a direct lightning stroke occurs, lightning current of large amplitude will be injected into the transmission line. Lightning can strike on transmission lines in many ways. However, only the lightning strokes, which can cause transients on phase conductors of the transmission line, may influence the surge arrester. They are: direct stroke to a phase conductor and strike to the overhead shield wire or tower, which then flashes over to the phase conductor [10]. 2.2.1 Backflashover When lightning strikes a tower, a traveling voltage is generated which travels back and forth along the tower, being reflected at the tower footing and at the tower top, thus raising the voltage at the cross-arms and stressing the insulators. The insulator will flashover if this transient voltage exceeds its withstand level (backflash). Backflashover voltages are generated by multiple reflections along the struck tower and also along the shield wire for shield lines at the adjacent towers. The backflashover voltage across insulator for the struck tower is not straight forward. The peak voltage will be directly proportional to the peak current [7]. 8 2.3 Travelling Wave Traveling wave occurs when lightning strikes a transmission line shielding conductor, phase conductor or tower. A high current surge is injected as the lightning strikes. The impulse voltage and current waves divide and propagate in both directions from the stroke terminal at a velocity of approximately 300 meters per microsecond with magnitudes determined by the stroke current and line surge impedance [6]. Figure 2.1 2.4 Reflection and refraction at tower after lightning strike Lightning Current Wave shape and amplitude of lightning current are influenced by some stochastic factors, including geographic location, geologic conditions, climate and weather, etc. Thus, they change every time. But investigations show that although the lightning currents differ every time in waveform and magnitude, all exhibit the basic characteristics of a double-exponent wave. It can be given by: 9 (2-1) where: I, is the amplitude of the lightning current; α, ß are attenuation coefficients. [8] 2.4.1 Characterization of The Lightning Discharge The lightning discharge current is defined by its shape and characteristic parameters. Given the random nature of lightning, the parameters identifying each stroke follow probabilistic laws which have to be considered. IEEE guidelines consider a triangular shape, it can be shown in Figure 2.2. The current amplitude follows a probabilistic law given by the cumulative probability of exceeding the amplitude I, : [12] (2-2) where I is given in kA. 10 Figure 2.2 Lightning current shape, according to IEEE guidelines (negative polarity) . Peak current amplitude (lightning) and rise time of lightning stroke can effect to the overvoltage that occur in transmission line because the higher peak current magnitude and shorter front time will increase the overvoltage. It can be shown in Figure 2.3 and Figure 2.4. This will lead to backflashover [11]. Figure 2.3 Peak current magnitude (kA) versus flashover rate 11 Figure 2.4 2.5 Rise time lightning current versus flashover rate Line Insulation Flashover Model The leader propagation model is used to represent line insulation flashovers[14]: (2-3) where: - Leader velocity (m/s) d - Gap distance (m) - Leader length (m) u(t) - Applied voltage (kV) Eo= 520 (kV/m) 12 The critical flashover voltages U50% of 275 kV and 132kV circuits are 1120 kV and 880kV respectively. Flashover voltage of all line insulators in the simulated section is randomly varied, according to the normal distribution. Standard deviation for the line insulation flashover voltage was 3% [2]. Figure 2.5 Critical flashover voltage for 275/132kV transmission line Line insulators from tower to conductor can be represented as a capacitor. The tower to conductor has equivalent capacitance of about 80 pF for 132kV lines [12]. The transient-voltage withstands level of a power apparatus is not a unique number. An apparatus may withstand a high transient voltage which has a short duration even it has failed to withstand a lower transient voltage with longer duration. This characteristic of the insulator is known as the volt-time characteristic of the insulation. However, a simplified expression for the insulator voltage withstand capability can be calculated as below [12]: 13 (2-4) where: - a flashover voltage (kV), - 400*L, - 710*L, - elapsed time after lightning stroke, µs. The back flashover mechanism of the insulators can be represented by volt-time curves. When a back flashover might occur, a parallel switch is applied. If the voltage across the insulator exceeds the insulator voltage withstand capability, the back flashover occurs. The back flashover is simulated by closing the parallel switch. Once the back flashover occurs, the voltage across insulator goes down to zero. Figure 2.6 and Figure 2.7 show the insulator model and the waveform of voltage across insulator, when back flashover occurs at 4 μsec [4]. 14 Figure 2.6 Model used for string of insulator up 275/132kV. Figure 2.7 The back flashover mechanism 15 2.6 Ground Flash Density The Ground Flash Density, Ng, has a linear effect on lightning outage rates. There have been important developments in measurements of Ng, in the 1980s. Based on a power-law regression between CIGRE Lightning Flash Counter readings and local thunder days (TD) values for the same period [8]. Ng is given as: Ng = 0.04 T The flash/100km/year, (2-5) , is used to calculate total hit on the transmission line which is given by: (2-6) where: h = average conductor height, m b = overhead ground wire separation distance, m Ng = ground flash density, flashes/ Na = flashes/100km/year /year 16 2.7 Tower Footing Resistance The tower footing behavior is characterized by a lumped resistance. This resistance is constant according to IEEE guidelines, while in CIGRÉ the effect of soil ionization is taken into account. The decrease of the tower footing resistance when the lightning current amplitude exceeds a critical value Ig is given by [9]: (2-7) where R0 is the low current footing resistance (non-ionized soil) and the critical value of the lightning current is given by the soil ionization threshold field, Eg, using the equation: (2-8) where: Ro = low current footing resistance (Ω) Ri = tower footing resistance (Ω) ρ = soil resistivity (Ωm) I = impulse current (kA) Ig = soil ionization limit current (kA) Eg = soil ionization critical electric field (kV/m) [ Eg = 400 (kV/m] 17 2.8 Transmission Line Tower A direct stroke to a transmission line is very rare and most of the lightning strikes to the top of a transmission tower. As a result, in calculation of lightning, tower models have been developed using a theoretical approach or an experimental work. The accurate representation of the transmission tower has been the subject of much discussion. In lightning surge simulations, the tower model used can range from simple lumped inductances or resistance to complicated nonuniform transmission line circuits. Representation of the tower as a lumped element is only valid if surge current rise time is long compared to surge travel time in the tower. So for a steep-front wave the tower must be modeled as a distributed parameter element [4]. 2.8.1 Development of Tower Model Several formulas for the tower surge impedance have been used in the past. Wagner’s and Hileman’s model indicates that the tower impedance varies as the wave travels from top to bottom, being lowest at the tower top and increasing as the wave travel down the tower [9]. Kawai later performed measurements on isolated tower (without ground wires connected) and obtained similar result, although the magnitudes were appreciably lower [9]. Later on Chisholm et al. performed some experiments and found that the tower response to a horizontal current, resulting from a midspan stroke, is different from the response to a vertical surge, where the tower impedance decrease from top to bottom [9]. All these result are obtained considering the tower alone, without ground wires connected [9]. 18 Next, Ishii et al, measured the surge response of the typical double circuit 500kV transmission tower, with ground wires, for vertical stroke current. Based on this measurement, they developed a multistorey transmission tower model to be used in the multiconductor analysis with ElectroMagnetic Transients Program (EMTP). The multistorey transmission tower model consists of distributed parameter lines representing tower surge impedance and parallel R-L circuits representing an attenuation of a travelling wave along the tower [5]. 2.8.2 Tower Model The surge impedance expression proposed by Sargent [5] has been widely used as a tower model for traveling wave calculation. According to this expression, the tower under measurement is approximated by a cone, and a surge impedance of 170Ω is obtained for this shape. In this case, it is treated that the velocity of surge propagation in the tower is equal to the velocity of light (300 m/µs) and there is no surge attenuation. On the other hand, a surge impedance of 100Ω to 115Ω, a surge propagation velocity of 210 to 240 m/µs and a surge attenuation coefficient of 0.8 to 0.9 obtained by Kawai et al. through experiments on an actual tower used as second model [5]. Figure 2.8 Kawai tower model [5] 19 In the new model an inductance is connected parallel with the resistance determining the attenuation coefficient, enabling a more accurate approximation of the characteristic of the wave tail. This inductance is a parameter to determine the shape of the wave tail, and has nothing to do with the lumped inductance often used to represent the tower itself. The damping resistance is determined from the resistance per unit length of a transmission line calculated from the postulated surge attenuation coefficient of a tower [13]. The transmission line tower model, used in simulation is presented in Figure 2.9. The value of R can be obtained by calculating and dividing the tower into upper and lower truncated cones as shown in Figure 2.10. Section of the tower from the bottom crossarm to the ground is represented as propagation element, which is defined by the surge impedance ZT and wave propagation speed on the tower was taken to be equal to the velocity of light. Sections on the tower top [between tower top and top crossarm and between crossarms] modeled as inductance branches. Branch inductance is determined according to the section length, tower surge impedance and the propagation velocity. In the parallel to the inductance branches a damping resistors are introduced [19]. 20 Figure 2.9 Mathematical calculation for multistore tower model Figure 2.10 Tower equivalent model 21 2.9 Surge Arrester Four general classes of devices that have been used to limit over voltage and permit low (more economical) insulation levels of equipment [7]: ¾ Spark gaps ¾ Expulsion-type arresters ¾ Gapped valve-type arrester ¾ Gapless-Metal oxide arrester Overvoltage protective devices use spark gaps connected in series made with a nonlinear silicon carbide (SiC) material. The spark gaps provided high impedance during normal conditions. Nowadays, the physical construction of modern high voltage surge arrester consists of metal oxide discs inside a porcelain or polymer insulator. The use of line surge arresters to improve transmission line lightning performance or to avoid double circuit outages has increased over the last decade. Many line surge arresters are in service today and substantial service experience has been accumulated. The majority of line surge arresters are installed on lines having nominal voltages between 44kV and 138kV, but the application of this type of technology has been extended to the distribution lines and also to the transmission lines up to 500kV. Line surge arresters are installed on 132kV lines, mainly to reduce double circuit outage rate. Line surge arresters are normally installed on all phase conductors of one circuit of the double circuit line. Arresters are installed on all towers of the considered 132kV line as shown in Figure 2.11. With this arrester installation configuration, double 22 circuit outages are eliminated, but there exists possibility to have flashovers on the circuit without arresters [2]. Figure 2.11 Line arrester installed on 275/132kV Lightning stroke performance of the line without line surge arresters is presented in Table 1 (per circuit flashovers). As expected, the majority of the flashovers happen on 132kV circuits. Line lightning performance strongly depends on the tower footing resistance. For the tower footing resistance less than 10Ω, zero flashover rate is obtained (line is equipped with two shield wires with a negative shielding angle) [2]. 23 Table 2.1 Flashover rate for different circuit without line surge arrester (flashover/100km/year). Refer to Figure 2.6 for location of C1, C2, C3 and C4. Table 2 Line double circuit flashover rate different arrester installation configuration (Flashover/100km/year) The number of double circuit flashovers depends on the tower footing resistance, and may reach value of 35 % of the line total flashover rate, for the tower footing resistance of 40Ω. The number of the triple circuit flashovers (simultaneous flashovers 24 on two 132kV circuit and on one 275kV) is very low. The best improvement in the line total flashover rate is obtained by the installation of the arrester on the bottom conductors of both 132kV circuit and on the one top conductor of one 132kV circuit (the best three arrester installation configuration) [2]. When line surge arresters are installed on all phase conductors of one 132kV circuit, double circuit flashover are completely eliminated (actual installation on the considered transmission line). But, it is to note that with this arrester installation configuration line total flashover rate remains high. Arrester installation configuration with the arresters on the bottom conductors of both 132 kV circuits and on the one top conductor of one 132 kV circuit is very attractive, because this configuration substantially reduce line total flashover rate, reducing in the same time line double circuit flashover rate [2]. 2.10 Transmission Line Model Figure 2.12 Transmission line model 25 There are five types of the line/cable in ATP (EMTP) which are[16]: 1. Bergeron: Constant parameter KCLee or Clark models 2. PI: Nominal PI-equivalent (short lines) 3. JMarti: Frequency dependent model with constant transformation matrix 4. Noda: Frequency dependent model 5. Semlyen: Frequency dependent simple fitted model. J.Marti is a suitable model to represent the multiphase transmission line. This model considers frequency attenuation, the geometrical and material of the conductor including skin effect and conductor bundling and the corresponding electrical data are calculated automatically by ATP-EMTP program. It also generates high order frequency dependent model for overhead line and cables. 2.11 Monte Carlo Simulation A Monte Carlo method is a technique that involves using random numbers and probability to solve problems. The term Monte Carlo Method was coined by S. Ulam and Nicholas Metropolis in reference to games of chance, a popular attraction in Monte Carlo, Monaco. It is a method for iteratively evaluating a deterministic model using sets of random numbers as inputs. This method is often used when the model is complex, nonlinear, or involves more than just a couple uncertain parameters. Monte Carlo technique can be used in order to build the computer program for the evaluation of the performance of overhead lightning shielding system. Analysis of atmospheric overvoltage in power plants or transmission line there was always a problem how to 26 determine amplitude of the lightning current which is striking the protected object and cause overvoltage. Development a computer program to represent an algorithm which will determine the mentioned amplitude in same range for entered protected object is necessary. The program is based on a statistical Monte Carlo analysis on the 3dimensionally simulated system. 2.11.1 The 3-Dimensional Electrogeometric Model The basic feature of the 2-dimensional electrogeometric model of Whitehead is the simple criterion of shortest path (from the leader tip) determines the target point in protection on structure. This target point of the lightning stroke is determined when the tip of the descending leader reaches a point when the distance from the leader tip to the protective target point equals the striking distance. The field of influence of any structure to a descending lightning leader is hence described by arcs with centers at the various parts of the structures having a radius equal to its striking distance [17]. 2.11.2 3-Dimensional Simulation of Fields of Influence To extend the 2-dimensional EG model to a 3-dimensional system, fields of influence of a structure described by its space of influence whose extreme radius is defined by its striking distance are now considered. For example, the field of influence of a vertical rod can be described by a vertical cylinder with a hemispherical top, both having a radius equal to its effective striking distance r as illustrated in Figure 2.13. 27 Similarly, the fields of influence of a horizontal wire above ground can be represented by a horizontal cylinder (Figure 2.14). Figure 2.15 also illustrates the fields of influence of a rectangular block above ground which can be used to represent a building structure or a patch of trees, etc. In all cases, the field of influence of the ground plane is represented by a horizontal plane at its effective striking distance rs above the ground. The termination point of the lightning stroke is determined on the basis that an object will be struck if its field of influence is meet first by the leader tip on its way to ground. As in the case of the example given in Figure 2.13, stroke A will terminate on the rod and stroke B will terminate on the ground [17]. 2.11.3 3-Dimensional modeling of the Lightning Stroke The lightning stroke is characterized principally by the lightning leader approach angle and stroke current magnitude. The probability density function of the vertical angle of approach of the lightning stroke is given by [17] (2-9) 28 Figure 2.13 Fields of influence of a vertical rod and ground. Rs and rsg are the effective striking distances of the vertical rod and ground respectively [17] Figure 2.14 Fields of influence of horizontal wire and ground [17] 29 Figure 2.15 Fields of influence of rectangular block and ground [17] To fully describe the stroke in 3 dimensions, a horizontal angle having a uniform probability distribution of between 0 and 360 degrees is incorporated. The AIEE current distribution used is represented by an array with 250 current values stored in a data file. The IEEE WG distribution is given by [17] (2-9) where I is the stroke current in kA and P(1) is the probability of current exceeding I. Striking distance is related to stroke current magnitude. (2-10) where I is in kA and is in meters. 30 2.11.4 Ground Flash Density The frequency of strokes to an area under study is determined by the ground flash density which is the number of ground discharges per square kilometer per year. The shielding failure rate of a shielding system is a function of the ground flash density. The distribution of all prospective ground discharges within the area of study is taken to be uniform as there is no reason to consider otherwise [17]. 2.11.5 Shielding Effect of a Vertical Rod The most common and simplest form of lightning protection is using a vertical rod which has the function of intercepting a lightning stroke before it can strike a nearby object it is protecting, and then discharging the current to ground [17]. Figure 2.16 Display of lightning strokes (represented by dots) terminating on structure (vertical rod) and surrounding ground - plan view [17] 31 CHAPTER 3 METHODOLOGY 3.1 ATP-EMTP Simulation The Alternative Transients Program (ATP) is considered to be one of the most widely used universal program system for digital simulation of transient phenomena of electromagnetic as well as electromechanical nature in electric power systems. With this digital program, complex networks and control systems of arbitrary structure can be simulated. ATP has extensive modeling capabilities and additional important features besides the computation of transients. ATPDraw for Windows is a graphical, mouse-driven preprocessor to the ATP version of the Electromagnetic Transients Program (EMTP). In ATP Draw the user can construct the digital model of the circuit to be simulated using the mouse and selecting predefined components from anextensive palette, interactively. Then ATP Draw generates the input file for the ATP simulation in the appropriate format based on built circuit. Figure 3.1 shows an overview of ATPDraw commands and functions. 32 Main menu Tool bar icons Component tool bar Circuit window Component selection menu Figure 3.1 3.2 Overview of ATPDraw commands and function Typical EMTP Applications ATP-EMTP is used world-wide for switching and lightning surge analysis, insulation coordination and shaft torsional oscillation studies, protective relay modeling, harmonic and power quality studies, HVDC and FACTS modeling. Typical EMTP studies are: ¾ Lightning overvoltage studies ¾ Switching transients and faults 33 ¾ Statistical and systematic overvoltage studies ¾ Very fast transients in GIS and groundings ¾ Machine modeling ¾ Transient stability, motor startup ¾ Shaft torsional oscillations ¾ Transformer and shunt reactor/capacitor switching ¾ Ferroresonance ¾ Power electronic applications ¾ Circuit breaker duty (electric arc), current chopping ¾ FACTS devices: STATCOM, SVC, UPFC, TCSC modeling ¾ Harmonic analysis, network resonances ¾ Protection device testing 3.3 Creating Simulation File Simulation file is created by keying the parameter of the circuit into the components which are called out to the circuit window. A data window will pop out after clicking on that component and the required parameters for the component will show up. The input data can directly inserted to the special column provided in the data window. The EMTP input data structure consists of several important parts that consist of the simulation setting or called miscellaneous data cards as shown in Figure 3.2. It control the simulation setting as time interval between processing loop, the maximum simulation and several frequency parameter that effected the inductance and capacitance value in the branch section. The second part of the input data is called branch segment. In this segment, the parameter of the transformer, transmission line, and basic element 34 are placed in the special columns provided in the data window by clicking on that element. Figure 3.2 Data window for simulation setting. The third part is the source segment where all the source parameter are placed. The procedure to insert the data is same with branch segment as shown in Figure 3.3. This included the impulse and ramp type source that important in transient study. The final part is the plot segment and this is where the voltage at different nodes are requested for plotting purpose. This step is carried out with the probe components located at the measured nodes. 35 Figure 3.3 3.4 Data window for inserting the parameter Creating Punch File When involving with frequency dependent overhead line or underground cable, the characteristic matrics would have to punch by EMTP. In creating the punch file, two simple steps have to be, the first process is to locate the parameter for the requested apparatus in appropriate location in the data window. In order to create the punch file, the second steps involve the punching process using the EMTP software. This process is quiet similar to the simulation process but the result from the computation are the punch file usually with extension of *.pch instead of the*.pl4 file obtained from normal simulation. This file could further be pasted inside the main input data file by connecting directly the component to the system circuit in circuit window 36 and thus automatically called out when needed by “INCLUDE” command inside the input data file of the EMTP simulation. Figure 3.4 shows the data window for transmission line. Figure 3.4 3.5 Data window for transmission line Simulation Simulation involve the simplest procedure involving the used of the EMTP command line. By clicking the “Run ATP” command or simply press F2, the simulation process can now begin. The time needed to finish the simulation depends on the complexity of the simulation file, number of branch that are requested to be plotted, time interval between computation loop and the maximum time of the simulation. Some complex simulation will take about three hours to finish and consume large amount of computer main memory. 37 3.6 Plot File As the result of the request node in the simulation, a *.pl4 file will be created after the simulation has ended. This file can be plotted using the external software specially design for viewing the result such as PCPlot and TPPlot that usually support three plot data for each graph. This chart viewing software especially for ATP versions of EMTP can only be used in MSDOS environment and with DBOS simulation software running. There is another new plotting program called plotXY to generate scientific line plots using data collected from *.pl4 file. 3.7 Transmission Line Simulation on overhead transmission line is conducted through PI subroutine file in EMTP. This model considers the geometrical and material of the conductor including skin effect and conductor bundling and the corresponding electrical data are calculated automatically by the LINE CONSTANTS, CABLE CONSTANTS or CABLE PARAMETER (LCC) subroutine file. PI setup is a supporting routine to generate frequency dependent model data for overhead line and cables. It is also generates high order frequency dependent model for overhead line and cables. Figure 3.5 Transmission line model 38 In this simulation the PI model was used. The geometrical and material data for the overhead line conductors are specified as below [16]: Phase no: Phase number 0 = ground wire (eliminated) RESIS : Conductor resistance at DC (with skin effect) or at Freq. Init. (no skin effect) REACT : The frequency independent reactance for one unit spacing (meter/foot). Only available with no skin effect. Rout : Outer radius (cm or inch) of one conductor Rin : Inner radius of one conductor. Only available with skin effect. Horiz: Horizontal distance (m or foot) from the center of bundle to a user selectable reference line. VTower: Vertical bundle height at tower (m or foot). VMid : Vertical bundle height at mid-span (m or foot). The height h=2/3* VMid +1/VTower is used in the calculations. Separ : Distance between conductors in a bundle (cm or inch) Alpha : Angular position of one of the conductors in a bundle, measured counterclockwise from the horizontal NB: 3.8 Number of conductor in a bundle Transmission Tower The transmission model consists of seven sections divided at the upper, middle and lower phase cross arm positions (not including insulator strings) is shown in Figure 3.6. Each section consists of a loss free transmission line and a lumped constant consisting of a damping resistance shunted by an inductance 39 Figure 3.6 Multistorey transmission tower The surge impedance takes into account of the tower configuration, the height and the radius of the tower. There are shown in Figure 3.7 and Figure 3.8. The parameters of the 275/132kV quadruple tower model is shown in Table 3.1. These data are determined by using the following equations. (3-1) (3-2) (3-3) (3-4) where: = Tower surge impedance = Attenuation coefficient 40 = Damping coefficient V = Surge propagation velocity R = Resistance r = Radius of tower H = Height L = Inductance Figure 3.7 M.Ishii’s tower model for a double line tower 41 Figure 3.8 Tower equivalent radius 42 Table 3.1: Parameters of the 275/132kV quadruple tower model Name Symbol Value Tower surge impedance 85Ω Propagation velocity 300m/µs Attenuation coefficient 0.7 Damping coefficient 1 Damping resistance (Ω) Damping Inductance (µH) Height R1 2.85 R2 5.65 R3 5.65 R4 8.25 R5 3.95 R6 3.95 R7 30.31 L1 0.9 L2 1.8 L3 1.8 L4 2.62 L5 1.26 L6 1.26 L7 9.64 H1 2.8 H2 5.55 H3 5.55 H4 8.1 H5 3.88 H6 3.88 H7 17.95 43 One of the important aspects which must be considered in tower modeling is to simulate the transmitted wave from the tower top and the reflected wave from the tower base. The surge will propagate from the tower top and will reflect from the tower base. The surge impedance of the tower is represented by a distributed parameter, Z, which takes into account of surge velocity, tower height, and the surge impedance on the transmitted and reflected wave. The modification of M.Ishii’s tower model for a quadruple circuit line tower modeling is shown in Figure 3.9. Figure 3.9 modeling Modified M.Ishii’s tower model for a quadruple circuit line tower 44 3.9 Insulator String The insulators are represented by capacitors in parallel with voltage dependent flashover switches connected between the respective phases and the tower. This is shown in Figure 3.10. In this study, a capacitance value of 80 pF was used. Figure 3.10 3.10 Insulation string model Lightning Source Selection The lightning source was simulated by using Heidler model with 20kA magnitude and 0.5µs front time. The current surge is a single stroke with positive polarity. The current source can be represented by the following equation and the wave shape of the fast front current surge by using Heidler model is shown in Figure 3.11. (3-5) 45 where: Amp = Multiplicative number in (A) or (V) of the function, does not represent peak value of surge. Tf = the front duration in (sec), which is interval between t=0 to time of the function peak. Ta = the stroke duration in (sec), which is interval between t=0 and the point on the tail where the function amplitude has fallen to 37% of its peak value. N = factor influencing the rate rise of the function. A 20MV DC type source was used as the lightning input step voltage. It is injected at the middle point on the earth wire between tower 2 and tower 3. Figure 3.12 shows the input voltage waveform (MV) (µs) Figure 3.11 Waveform of fast front voltage surge using Heidler model, 20MV with 0.5µs fast front time 46 Figure 3.12 Waveform of voltage using DC model, 20MV with 0.5µs fast front time Figure 3.13 shows voltage at the tower top when using a DC model source as an input. Waveform of multistorey tower is influenced by the surge attenuation. The surge will propagate from the tower top to the tower base. From Figure 3.13, voltage at the tower top rose approximately to 1.7MV. The traveling wave will travel to the tower base in 0.3µs and the tower base voltage at that point is -1.0MV. After that, the wave will reflect to the tower top at 0.6µs time scale and voltage rose up to 1.0MV. This phenomenon will be repeated and can be explained by using the lattice diagram. 47 Figure 3.13 3.11 Voltage at tower top by using a DC model source as input Monte Carlo Simulation The lightning performance of an overhead line can be measured by the flashover rate, usually expressed as the number of flashovers by 100 km and year. Due to the random nature of lightning, an accurate evaluation of the lightning performance must be based on a statistical approach. A Monte Carlo simulation is the most usual method for this purpose. The computation of flashover rate and shielding failure rate at transmission line will be performed by using a Monte Carlo Simulation. The main aspect of the Monte Carlo procedure embedded into the ATP can be summarized as follows: a) The calculation of random values includes the parameters of the lightning stroke phase conductor voltages, the footing resistance and the insulator strength. b) Overvoltage calculations are performed once the point of impact has been determined. 48 c) If a flashover occurs in an insulator string, the run is stopped and the flashover rate is updated. d) The convergence of the Monte Carlo method is checked by comparing the probability density function of all random variables to their theoretical functions; the procedure is stopped when they match within the specified error. The overall procedure is illustrated in Figure 3.14. Note that for a specific design, the lightning parameters as well as the soil resistivity are allowed to vary in accordance to known distribution functions. For each sample, a two part analysis is performed. The first part determines the lightning termination point (and thus the probability of shielding failure). For this purpose, the electrogeometric model for lightning termination is used. This method determines the probability of shielding failure for any power line in a general terrain. Figure 3.14 illustrates the basis of the method. The lightning streamer is assumed to propagate from the top with equal distribution per unit area. When it approaches the power line, it will terminate at the nearest point within the striking distance of the lightning. From this construction, the probability of shielding failure is computed. 49 N=1 Generate sample of lightning parameter based on ground flash density Generate a sample of soil resistivity Store result the maximum overvoltage flashover (Nmax) N=N+1 No Is N > Nmax Yes Generate reports Figure 3.14 Flow chart of Monte Carlo simulation on transmission line 50 3.12 Project Flow The project focuses on the model of 275/132kV quadruple transmission line and transmission tower to investigate the performance of transmission line due to lightning strike. Protection of simple structure is done by using MATLAB Simulation. The overall project flow is shown in Figure 3.15 and Figure 3.16 shows the protection of simple structure (vertical rod) due to lightning strikes flow chart. 51 Start Literature Review Literature work and review on the surge analysis of the transmission line and tower and protection of simple structure from lightning strike Design and Analysis - - Modeling and Simulation ATP-EMTP Simulation model of transmission line Monte-Carlo Simulation Result Analysis and Evaluation Analysis of transmission line performance due lightning strike Protection of simple structure due to lightning strike System Optimization Report Writing Done Figure 3.15 Project flow chart 52 Generate sample of lightning parameter based on ground flash density Calculate striking distance (rs) based on lightning current amplitude and calculate high of the vertical rod Lightning current amplitude= Lightning current use in calculate striking distance (rs) No Lightning terminated on ground Yes Lightning terminated on vertical rod Figure 3.16 Protection of simple structure (vertical rod) due to lightning strikes. 53 CHAPTER 4 SIMULATIONS RESULTS AND DISCUSSIONS 4.1 Introduction This chapter presents the results of the simulations carried out, namely the ATPEMTP simulation for surge arrester study and the MATLAB simulation for the lightning protection study. For the surge arrester study, a 275/132kV quadruple transmission line system consisting the transmission line and 5 towers was simulated. Two source model were used, namely the Heidler model and DC model. The transmission tower was modeled according to modified M.Ishii’s model. The lightning protection study incorporating the Monte Carlo probability concept was simulated using MATLAB simulation. The equation described in section 2.11.3 were utilized. 54 4.2 Line Surge Arrester Study 4.2.1 Transmission Tower X01 L1 X02 L2 X03 L3 X04 L4 X05 L5 X06 L6 X07 L7 X08 RT R1 R2 Figure 4.1 R3 R4 R5 R6 R7 Complete multistorey model Figure 4.1 shows a complete multistorey tower model simulated in the ATPEMTP program. A lightning strike with 20kA peak and 0.5µs fast front time was chosen as the input. The lightning current surge was injected in the top of a standalone tower. The parameters of the tower model are as shown in Table 3.1. Figure 4.2 shows the resultant output voltages at the tower top, tower base and at each crossarm of the tower. The purpose of this simulation is to show the traveling waves propagate from the tower top to the tower base. As can be seen in Figure 4.2, the voltages along the tower is reducing starting at the tower top towards the tower base. There is also a slight time delay due to propagation delay. 55 V:X01 Figure 4.2 4.2.2 V:X02 V:X03 V:X04 V:X05 V:X06 V:X07 V:X08 Voltage at tower top, tower base and each crossarm of the tower Transmission Line and Tower Circuit Model on EMTP Simulation Figure 4.3 shows the simulation circuit of 275/132kV quadruple circuit transmission lines connected to the transmission towers. In the simulation, 5 towers was used. A lightning surge of DC type with a peak voltage of 20 MV was injected on the earth wire at midspan. 56 To tower 1 To tower 4,5 R 275kV B Y R 132kV B Y Tower 2 Figure 4.3 Tower 3 The simulation circuit of 275/132kV quadruple circuit transmission line, and transmission towers. Figures 4.4 to 4.9 show the voltage oscillograms at each of the crossarm position corresponding to each conductor as well as corresponding voltage across the insulator strings of 275kV and 132kV circuits at tower 3. Voltages at red phase and blue phase at circuits 275kV in Figure 4.4 and Figure 4.5 show the same voltage swing pattern. The voltage of red phase rose about 4MV which is maximum voltage approximately 1.8µs and maximum insulator string voltage at each phase is 2MV. Then, Figure 4.6 and Figure 4.7 show the voltage at yellow phase circuit 275kV and red phase circuit 132kV. As can be seen, the voltage rose sharply to 2.25MV which is maximum voltage approximately 2.5µs and insulator string at each phase is swing between 0.1MV to 0.1MV. Figure 4.8 and figure 4.9 show the same phase voltage pattern and insulator voltage swing. As can be seen in Figure 4.8, the maximum blue phase voltage and insulator string at circuit 132kV are 1.25MV at 4.4µs and 0.5MV at 9 µs. Figure 4.9 57 shows the maximum yellow phase voltage and insulator string is 1.5MV at 4.4µs and 0.4MV at 9 µs. Figure 4.4 Voltage at red phase and insulator string tower 3 (275kV) Figure 4.5 Voltage at blue phase and insulator string tower 3 (275kV) 58 Figure 4.6 Figure 4.7 Voltage at yellow phase and insulator string tower 3 (275kV) Voltage at red phase and insulator string tower 3 (132kV) 59 Figure 4.8 Figure 4.9 Voltage at blue phase and insulator string tower 3(132kV) Voltage at yellow phase and insulator string tower 3(132kV) Figure 4.10 to Figure 4.15 show the voltage oscillograms at each crossarm for tower 4. Figure 4.10 shows the maximum red phase voltage and insulator string at circuit 275kV are 1.1MV at 4.4µs and 0.5MV at 9 µs. Then, Figure 4.11 and Figure 4.12 show the maximum voltage at blue and yellow phase, and insulator string at circuits 275kV have same voltage which is 1.2MV at 6µs for phase voltage and 0.1MV for insulator string. Voltage at red phase at circuit 132kV is decreased by 0.3MV but still show the same pattern voltage swing in Figure 4.13. Figure 4.14 shows the voltage at blue phase and insulator string at circuit 132kV. The maximum phase voltage is 1.1MV 60 at 4.5µs and for insulator string is 0.4MV. This follows by Figure 4.15 which is the maximum yellow phase voltage is 1.2MV at 6µs and insulator string is 0.1MV Figure 4.10 Voltage at red phase and insulator string tower 4 (275kV) Figure 4.11 Voltage at blue phase and insulator string tower 4 (275kV) 61 Figure 4.12 Figure 4.13 Voltage at yellow phase and insulator string tower 4 (275kV) Voltage at red phase and insulator string tower 4 (132kV) 62 Figure 4.14 Figure 4.15 Voltage at blue phase and insulator string tower 4 (132kV) Voltage at yellow phase and insulator string tower 4 (132kV) Table 4.1 shows the tabulated data for the maximum voltage at each phase voltage and insulator string at tower 3. Table 4.2 shows the tabulated data for maximum voltage at each insulator string at tower 4. From Table 4.1 and Table 4.2, it can be seen that the differential voltage between phase voltage and string insulator at tower 3 is higher than tower 4. The configuration of surge arrester is shown in Figure 4.16 using L arrangement of arrester. Based on critical flashover voltage, it shows that all surge arresters at both towers which are tower 3 and tower 4 at circuit 132kV were affected by the lightning strike. 63 Table 4.1 Tower 3 Voltage between each phase and insulator string at Tower 3 Phase Insulator Voltage(MV) Voltage (MV) Phase Voltage – String Insulation Voltage (MV) R phase (275kV) 4 2 2 B phase (275kV) 4 2 2 Y phase (275kV) 2.25 0.1 2.15 R phase (132kV) 2.25 0.1 2.15 B phase (132kV) 1.25 0.5 0.75 Y phase (132kV) 1.5 0.4 1.1 Table 4.2 Tower 4 Voltage between each phase and insulator string at Tower 4 Phase Insulator Voltage(MV) Voltage (MV) Phase Voltage – String Insulation Voltage (MV) R phase (275kV) 1.1 0.5 0.6 B phase (275kV) 1.2 0.1 1.1 Y phase (275kV) 1.2 0.1 1.1 R phase (132kV) 0.9 0.1 0.8 B phase (132kV) 1.1 0.4 0.7 Y phase (132kV) 1.2 0.05 1.15 64 Figure 4.16 - With surge arrester installation - Without surge arrester installation Lightning Surge Arrester Configuration L-Arrangement 4.3 Lightning Protection of Structures Study 4.3.1 Simple Structure Protection Result The most common and simplest form of lightning protection is by using a vertical rod which has the function of intercepting a lightning stroke before it can strike a nearby object it is protecting, and then discharging the current to ground. In this simulation study, 1500 strokes were a applied in square ground with an area of 1 km² and number of flashes to ground per square kilometer per year, Ng = 15 strokes/ km²/year. Figure 4.17 shows the distribution of lightning flashes on a 1 km² ground area with an Ng of 15 strikes/ km²/year over 100 year period. 65 Figure 4.17 Distribution of lightning flashes on a 1 km² ground area with an Ng of 15 strikes/ km²/year over 100 year period. Figure 4.18 (a) and (b) show the distribution of lightning flashes on a 1 km² ground area with an Ng of 15 strikes/ km²/year over 100 year period with lightning strokes terminating on structure which is vertical rod with current 2.5kA, 10kA and 20kA. The striking distance is influenced by the lightning current. For this study, the range of current is between 2.5-20kA. According to Figure 4.18 (c), the higher lightning current will bring wider striking distance, it can be seen corresponding to 20kA current. 66 (a) (b) Figure 4.18 Display of lightning strokes terminating on vertical rod 67 (c) Figure 4.18 Display of lightning strokes terminating on vertical rod (cont.) Figure 4.19 to Figure 4.22 show the lightning strokes (represented by dots) terminating on structure (vertical rod), and the surrounding ground-plan view with of current 2.5, 5 and 10kA. It shows that the protection area is influenced by the current magnitude which is lightning strike. A vertical rod is placed in the middle of a square plot of ground of area of 1 and 1500 strokes were applied to the area under study. In this study, the concepts of electromagnetic model which is “striking distance”( are applied. Rolling sphere method was used for the determination of protection radius (or target point) of lightning stokes. Here any point or surface on a structure touch by rolling sphere whose radius equals striking distance is protected from lightning strike. The analysis of the performance of any lightning shielding system is complicated by the fact that the occurrence and nature of lightning is statistical and that structures their surroundings are asymmetrical. Analysis on 3 dimensional system are required. The radius of effective striking is calculated to determine the height of the rod 68 that can withstand from the lightning strike. is field of influenced of object where is given by: (4-1) The protection area is influenced by the lightning stroke current and height of the rod. Height (h) of rod above a flat roof or horizontal plane are considered to protect points on that plane up to a horizontal distance r from a horizontal conductor or to horizontal radius r from a vertical rod, where r is given by: (4-2) where r and h in meters The lightning discharge current is defined by its shape and characteristic parameters. Given the random nature of lightning, the parameters identifying each stroke follow probabilistic laws which have to be considered in IEEE guidelines. The current amplitude follows a probabilistic law given by the cumulative probability of exceeding the amplitude I, . The probability current exceeding I(P(I)), where I is the stroke current in kA is given by: (4-3) 69 Figure 4.19 Figure 4.20 Vertical rod and its effective striking with current 2.5kA Vertical rod and its effective striking with current 5kA 70 Figure 4.21 Vertical rod and its effective striking with current 10kA Figure 4.22 Vertical rod and its effective striking with current 15kA Table 4.3 shows lightning stroke (kA) with effective striking ( , height of rod (h) and probability of lightning strikes (P). The protection area is influenced by the 71 lightning stroke current and height of the rod. The radius of effective striking is able to determine the height of the rod that can with stand from the lightning strike. Lightning stroke (kA) with effective striking ( Table 4.3 , height of rod (h) and probability of lightning strike (P). Lightning stroke (kA) Height of rod (h) Probability of (meter) (meter) lightning strike 2.5 15 2 0.9986 5 22 5 0.9914 10 36 18 0.9499 15 78 45 0.8685 Effective striking Figure 4.23 and Figure 4.24 show the basic implementation of 3-dimensional of the electrogeometric model of the lightning stroke on structures such as building that required to be protected. In both cases, the field of influence of the ground plane is represented by a horizontal plane at its effective striking distance (equation 2-9) above the ground. The termination point of the lightning stroke is determined on the basis that an object will be struck if its field of influence is met first by the leader tip on its way to ground. Other strokes will terminate on the ground if they do not meet the field of influence. 72 Figure 4.23 Field of influence of a rectangular block above ground which can be used to represent a building structure or a patch of trees with current 2.5kA with 2 dimensional electrogeomatric model. Figure 4.24 Field of influence of vertical cylinder can be used to represent a building structure or a patch of trees with current 2.5kA (3 dimensional electrogeomatric model). 73 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 5.1 Conclusion The ATP-EMTP simulation study is supposed to show that the arrester at the nearest point of strike is not effected rather the ones further down at the next tower. This can also be explained from the travelling wave theory where at the exact location of the strike, the current splits into 2 (I/2). As it travels to the next tower, the traveling surge induced coupled voltage, which is a fraction of the traveling voltage. As a result, the total stress is higher at the adjacent towers compared to the exact location. In the incident sited, when lightning stroke the earth wire, the wire snapped and fell. Both portions then broke again due to high current and caused the breakage of the conductor at four portions. Based on the result obtained, all surge arrester at both tower (tower 3 and tower 4) were affected by the lightning strike. From the simulation results, differential voltage between the phase conductor and the crossarm at the insulator string for each phase shows that the adjacent tower which is tower 4, has less differential voltage than nearest tower (tower 3). Therefore, the simulation results show that the 74 phenomenon cannot be conclusively reproduced within the ATP-EMTP simulation. This may indicate that the phenomenon may be a one-off special case event. A study which focus on many factors such as circuit outage, flashover and backflashover, insulation failure and shielding failure has been done. The ATP-EMTP simulation program has been used to carry out the study and the results explain the phenomenon from theoretical and practical points of view. For transmission line modeling, the configuration of the overhead line must be known such as number, location and spacing between conductors. Besides that, skin effect and other properties may also be considered in the model. The configuration of the tower structure such as height and radius must be known. The phenomenon which may include the travelling wave effect (reflection etc) has been studied to prove that when a lightning strikes, the arrester at the nearest to the point of strike is not effected rather the one further down at the next tower. Monte Carlo concept has been used to estimate the probability of lightning strikes and lightning protection of simple structures. The most common and simplest form of lightning protection is by the use of a vertical rod which has the function of intercepting a lightning stroke before it can strike a nearby object it is protecting, and then discharging the current to ground. The analytic method used is based on a 3dimensional implementation of the electrogeometric model. The protection area is influenced by the lightning stroke current and height of the rod. The radius of effective striking is able determine the height of the rod that can withstand the lightning strike. 75 5.2 Recommendation Based on the simulation study on 275/132kV quadruple transmission line, below are two computer softwares are suitable to be used in future study : ¾ Sigma slp is PC Windows based software, which has been specially developed to enable quick and easy determination of transmission line lightning performance. This software provide an alternative way to bring the precise result on 275/132kV quadruple circuit transmission line in term of to prove that the surge arrester location of the affected arrester was not at the nearest tower to the point of strike but affected at the adjacent tower. The arresters at the nearest tower were not affected. ¾ A computer program MFASP (Multiple Flashover Across Same Phase in Different Towers) is developed by [18] which is able to simulate the multiple flashovers across all phases including on the same phase in different towers. In protection of simple structure a computer program for the evaluation of the lightning performance to protect structure such as shielding lines, transmission lines, buildings and tree patches from lightning stroke is recommended to be built. The analytic method used is based on a 3-dimensional implementation of the electrogeometric model. Monte-Carlo technique also can be done using C language to manipulate the statistical distribution of the lightning stroke. 76 REFERENCES [1] Iryani Mohamed Rawi, “Tripping report- Post mortem study on the root cause of earth wire failure between T70-T71 and TLA(gapless type) at T68 &T69 for 132kV BLKG-SRDG line”, Engineering Department (Lines and cable)TNB Transmission Division, 2007 [2] Y.A Wahab, Z.Z Abidin and S.Sadovic, “Line Surge Arrester Application on the Quadruple Circuit Transmission Line”, IEEE Bologna Power Tect Conference, June 23, 2003. [3] C.A.Nucci and F.Rachidi, “ Lightning Induced Voltage”, IEEE Transmission and Distribution Conference, April 14 , 1999. [4] M. T. Correia de Barros, J. Festas, H. Milheiras, N. Felizardo (IST - Universidade Técnica deLisboa / Instituto da Energia - INTERG), M. 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Neumann, “Backflashover Analysis for 110-kV Lines at MultiCircuit Overhead Line Towers, “Presented at the International Conference on Power Systems Transients (IPST’07) in Lyon, France on June 4-7, 2007 [15] P. Yadee and S. Premrudeepreechacharn, “Analysis of Tower Footing Resistance Effected Back Flashover Across Insulator in a Transmission System”, Presented at the International Conference on Power Systems Transients (IPST’07) in Lyon, France on June 4-7, 2007 [16] ATPDRAW version 3.5for Windows 9x/NT/2000/XP. Users' Manual [17] A.C.Liew, C.M.Gui and Sr. M. I, “Performance Assessment of Lightning Shielding Systems”,1990 [18] A.C.Liew and J.P.Wang, “Multiple Flashovers Across Same Phase In Different Towers”,1996 [19] T. Yamada, A. Mochizuki, J. Sawada, E. Zaima T. Kawamura A. Ametani M. Ishii S. Kato Markus Junker, Thomas Schneider, Max J. Ammanno, Andreas T. Schwarzbacher, and Kai-Uwe Lauterbach, “Experiment Evaluation Of A UHV Tower Model For Surge Analysis” IEEE Transactions on Power Delivery, Vol. 10, No. 1, January 1995 79 80 APPENDIX A 1) 275/132kV Transmission line and Transmission Tower Model - EMTP 2) Matlab Simulation of lightning strokes represented by dots) terminating on structure (vertical rod), and surrounding ground-plan view with current 3) Matlab Simulation of lightning strokes represented by dots) terminating on tructure (vertical rod 4) Matlab Simulation of field of influence of vertical cylinder can be used to represent a building structure 1) 275/132kV Transmission line and Transmission Tower Model – EMTP 81 Tower 1 2) Tower 2 Tower 3 Tower 4 Tower 5 Matlab Simulation of lightning strokes (represented by dots) terminating on structure (vertical rod), and surrounding ground-plan view with current data=1500; %random lightning stike r4=0.2; r5=0.2; a=0.4; % coordinate x b=0.5;% coordinate y c=0.6; % coordinate x d=0.5;% coordinate y %start simulation x=rand(data,1); y=rand(data,1); [lat,lon] = SCIRCLE1(a,b,r4); [p,t] = SCIRCLE1(c,d,r5); s=size(lat) s=size(p) %data for circle 82 for h=1:s for k=1:data; if x(k)>=c&y(k)>=d if x(k)<=p(h)& y(k)<=t(h) x(k)=c; y(k)=d; end end if x(k)<=c&y(k)>=d if x(k)>=p(h)& y(k)<=t(h) x(k)=c; y(k)=d; end end if x(k)<=c&y(k)<=d if x(k)>=p(h)& y(k)>=t(h) x(k)=c; y(k)=d; end end if x(k)>=c&y(k)<=d if x(k)<=p(h)& y(k)>=t(h) x(k)=c; y(k)=d; end end end end for i=1:s for j=1:data; if x(j)>=a&y(j)>=b if x(j)<lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; end end 83 if x(j)<=a&y(j)>=b if x(j)>=lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; end end if x(j)<=a&y(j)<=b if x(j)>=lat(i)& y(j)>=lon(i) x(j)=a; y(j)=b; end end if x(j)>=a&y(j)<=b if x(j)<=lat(i)& y(j)>=lon(i) x(j)=a; y(j)=b; end end end end scatter(x,y,2) plot(p,t,'r') hold scatter(x,y,2) plot(lat,lon,'r') 3) Matlab Simulation of lightning strokes (represented by dots) terminating on structure (vertical rod) and surrounding ground-plan view with current 84 r=0.1 [X,Y,Z] = cylinder(r); X=X+0.5; Y=Y+0.5; h=1.0;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h; surf(X,Y,z) hold r2=0.005 [X,Y,Z] = cylinder(r2); X=X+0.6; Y=Y+0.5; Z=Z+1.0; h=0.5;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h surf(X,Y,z) r3=0.005 [X,Y,Z] = cylinder(r3); X=X+0.4; Y=Y+0.5; Z=Z+1.0; h=0.5;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h surf(X,Y,z) r4=0.1000005 [X,Y,Z] = cylinder(r4); X=X+0.5; Z=Z+0.5; Y=Y+0.5; 85 h=0.1;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h surf(X,Y,z) r=0.2; %radius data=1500; %random lightning strike a=0.4; % coordinate x b=0.5;% coordinate y c=0.6; % coordinate x d=0.5;% coordinate y %start simulation x=rand(data,1); y=rand(data,1); [lat,lon] = SCIRCLE1(a,b,r); s=size(lat) %data for circle for i=1:s; for j=1:data; if x(j)>=a&y(j)>=b if x(j)<=lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; end end if x(j)<=a&y(j)>=b if x(j)>=lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; end end if x(j)<=a&y(j)<=b if x(j)>=lat(i)& y(j)>=lon(i) x(j)=a; y(j)=b; end end if x(j)>=a&y(j)<=b if x(j)<=lat(i)& y(j)>=lon(i) 86 x(j)=a; y(j)=b; end end end end scatter(x,y,2) plot(lat,lon,'r') Rod r=0.005 [X,Y,Z] = cylinder(r); X=X+0.5; Y=Y+0.5; h=50;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h surf(X,Y,z) hold r=47/100; %radius data=1500; %random lightning stike a=0.5; % coordinate x b=0.5;% coordinate y %start simulation x=rand(data,1); y=rand(data,1); [lat,lon] = SCIRCLE1(a,b,r); s=size(lat) %data for circle for i=1:s; for j=1:data; if x(j)>=a&y(j)>=b if x(j)<=lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; 87 end end if x(j)<=a&y(j)>=b if x(j)>=lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; end end if x(j)<=a&y(j)<=b if x(j)>=lat(i)& y(j)>=lon(i) x(j)=a; y(j)=b; end end if x(j)>=a&y(j)<=b if x(j)<=lat(i)& y(j)>=lon(i) x(j)=a; y(j)=b; end end end end scatter(x,y,2) plot(lat,lon,'r') 5) Matlab Simulation of lightning strokes (represented by dots) terminating on structure (vertical rod) r=0.1 [X,Y,Z] = cylinder(r); X=X+0.5; Y=Y+0.5; 88 h=1.0;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h; surf(X,Y,z) hold r2=0.005 [X,Y,Z] = cylinder(r2); X=X+0.6; Y=Y+0.5; Z=Z+1.0; h=0.5;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h surf(X,Y,z) r3=0.005 [X,Y,Z] = cylinder(r3); X=X+0.4; Y=Y+0.5; Z=Z+1.0; h=0.5;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h surf(X,Y,z) r4=0.1000005 [X,Y,Z] = cylinder(r4); X=X+0.5; Z=Z+0.5; Y=Y+0.5; h=0.1;%high of rod h=h-1; z=Z+h; z(1,:)=z(1,:)-h surf(X,Y,z) 89 hold data=1500; %random lightning stike r=0.2, r=0.2; a=0.4; % coordinate x b=0.2;% coordinate y c=0.6; % coordinate x d=0.7;% coordinate y %start simulation x=rand(data,1); y=rand(data,1); [lat,lon] = SCIRCLE1(a,b,r1); [p,t] = SCIRCLE1(c,d,r2); s=size(lat) %data for circle s=size(p) for h=1:s for k=1:data; if x(k)>=c&y(k)>=d if x(k)<=p(h)& y(k)<=t(h) x(k)=c; y(k)=d; end end if x(k)<=c&y(k)>=d if x(k)>=p(h)& y(k)<=t(h) x(k)=c; y(k)=d; end end if x(k)<=c&y(k)<=d if x(k)>=p(h)& y(k)>=t(h) x(k)=c; y(k)=d; end end 90 if x(k)>=c&y(k)<=d if x(k)<=p(h)& y(k)>=t(h) x(k)=c; y(k)=d; end end end end for i=1:s for j=1:data; if x(j)>=a&y(j)>=b if x(j)<=lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; end end if x(j)<=a&y(j)>=b if x(j)>=lat(i)& y(j)<=lon(i) x(j)=a; y(j)=b; end end if x(j)<=a&y(j)<=b if x(j)>=lat(i)& y(j)>=lon(i) x(j)=a; y(j)=b; end end if x(j)>=a&y(j)<=b if x(j)<=lat(i)& y(j)>=lon(i) x(j)=a; y(j)=b; end end end end scatter(x,y,2) plot(p,t,'r') 91 hold scatter(x,y,2) plot(lat,lon,'r')