HIGH-FREQUENCY IMPEDANCE CHARACTERISTICS AND HEALTH CONDITION MONITORING OF OVERHEAD POWER LINES. A Thesis. Presented to

Transcription

1 HIGH-FREQUENCY IMPEDANCE CHARACTERISTICS AND HEALTH CONDITION MONITORING OF OVERHEAD POWER LINES A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment Of the Requirements for the Degree Master of Science Filmon A Habtemariam August, 2016

2 HIGH-FREQUENCY IMPEDANCE CHARACTERISTICS AND HEALTH CONDITION MONITORING OF OVERHEAD POWER LINES Filmon A Habtemariam Thesis Approved: Accepted: Advisor Dr. J. Alexis De Abreu Garcia Interim Department Chair Dr. Joan Carletta Committee Member Dr. Yilmaz Sozer Interim Dean of the College Dr. Donald Visco Committee Member Dr. Nathan Ida Dean of the Graduate School Dr. Chand Midha Date ii

3 ABSTRACT In an age where functioning businesses are almost inconceivable in the absence of electric power, power interruption that can even last for a short period of time can cause tremendous losses of productivity, material, and revenue. Hence, electric power systems should be highly reliable. For reliable operation of power systems, continuous power line health monitoring should be conducted. By continuously monitoring the power lines, critical damages can be detected in their early stage of development. This allows appropriate measures to be applied on time; thereby improving the reliability of the power system while significantly reducing its maintenance costs. Overhead transmission lines are an intricate part of the power system, and their health condition is very important for reliable operation of the overall system. In this thesis, an approach to health monitoring of electric power lines using high frequency impedance characteristics is presented. This technique can detect faults in the power lines, and monitor their health condition. In order to identify damaged and faulty power lines, highfrequency impedance analysis is performed by observing the effects of damages and faults on the line impedance. A threshold value is set to determine if the power line needs replacement due to excessive damage. The proposed technique is verified through Matlab TM simulations and experimental tests. Here, three 28-foot long conductors are considered. iii

4 DEDICATION I dedicate my dissertation work to my family, friends, and advisors who supported me throughout the course of my degree. iv

5 ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my advisor, Dr. J. Alexis De Abreu Garcia, for the continuous support of my graduate studies and related research, for his patience, motivation, and immense knowledge. His guidance helped me in all the time of research and writing of this thesis. His advice and support of my career have been priceless. I would like to thank Dr. Yilmaz Sozer for his help, guidance, and providing me with an excellent atmosphere for doing research at the Alternative Energy Lab. I would also like to thank my committee member, Dr. Nathan Ida. I would also like to acknowledge all my friends for their continuous help and providing the best educational atmosphere; they made my stay here a beautiful and rewarding experience. I offer my regards and blessings to all of those who supported me in any respect during the completion of the thesis. At the end, I would like to express my heart-felt gratitude to my family for their support and encouragement at all the times. v

6 TABLE OF CONTENTS Page LIST OF FIGURES... xi CHAPTER I. INTRODUCTION Overhead Power Lines Overhead Line Conductors Electric Power Line Monitoring Proposed Method for Power Line Impedance Characteristics and Health Condition Thesis Outline...11 II. LITERATURE REVIEW Introduction Causes and Types of Overhead Power Line Damages Causes of Conductor Damage Aeolian Vibration Conductor Galloping Sway Oscillation Unbalanced Loading...17 vi

7 Types of Conductor Damages Abrasion Fretting Wear Fatigue Break Tensile Break Overhead Power Line Monitoring 24 III. OVERHEAD POWER LINE IMPEDANCE MODELING AND PARAMETER ESTIMATION Introduction Power Line Analysis High Frequency Modeling of Overhead Power Line Conductors Power Line Impedance and Admittance Ground Return Path Impedance Estimation of Transmission Line Parameters from Measurements Comparison between Simulated and Measured Power Line Conductor Impedance High-Frequency Simulation of Power Line Conductor damages...44 IV. HIGH FREQUENCY IMPEDANCE CHARACTERISTICS AND HEALTH CONDITION OF OVERHEAD POWER LINE CONDUCTORS Introduction Experimental Setup and Results of Overhead Power Line Conductor Impedance Measurements Impedance Measurement Test Plan Experimental Results for the Impedance Measurements of Overhead Power Line Conductors...52 vii

8 Test-1&2: Impedance of the good, bad-1, and bad-2 conductors measured at points A and C...53 Test-3 & 4: Impedance of the good, bad-1 and bad-2 conductors measured at points B and D...57 Test-5 & 6: Impedance of the good conductor and bad conductor-2 measured at points A and C while shorted at point EE...60 Test-7 & 8: Impedance of the good conductor and bad conductor-2 measured at points A and C while shorted at point FF...62 Test-9: Impedance measurement using signal generator...63 V. CONCLUSION AND FUTURE WORK Conclusions Future Work...67 REFERENCES...68 viii

9 LIST OF FIGURES Figure Page 1.1. Electric power system Overhead power lines Aluminum conductor reinforced with steel (ACRS) Conductor bundling Single phase line with reference conductor Line parameters in a single-phase line Aeolian vibration Amplitude vs. time Galloping of overhead power line Abrasion damage Abrasion damage at spacer Abrasion damage at loose hand tie Severe strands due to abrasion Restoration of abraded conductor wear from the spreader rod clip Damaged conductor: Presence of blackish debris due to the presence of aluminum oxides (Al2O3) Fretting wear Fatigue breaks...23 x

10 2.11. Tensile break Principle of wireless monitoring system for overhead power lines Monitoring point configuration Overhead power line defect detection using ultrasonic pulses Setup for defect detection of transmission lines Piezoelectric ring transducer attached to a transmission line Sag monitoring system with transmit and receive PLC-SAG units Sample calibration curve for the principal PLC-SAG signal Single phase line with reference conductor Schematic representation of an elemental length (dz) of a transmission line Equivalent electrical model at high frequency Matlab/Simulink model to estimate parameters of transmission line Distributed line model for the transmission line parameter estimation Line to earth impedance magnitude (physical measurements result) Line to earth impedance magnitude (Simulated result) Line to Earth Plane Impedance Model Line to earth impedance magnitude for conductor sagging Impedance percentage difference for conductor sagging Line to earth impedance magnitude for conductor abrasion Impedance percentage difference for conductor abrasion Experimental set-up for the overhead conductors Measurement plan for Good and Bad conductors Impedance of the Good, bad conductor-1 and bad conductor-2 at points A and C...54 vii

11 4.4. Impedance percentage difference between Good and Bad 1 conductors at first end Impedance percentage difference between Good and Bad 2 conductors at first end Phase angle of the impedances of the good, bad-1 and bad-2 conductors Impedance of the Good, Bad-1 and Bad-2 conductors at points B and D Impedance percentage difference between Good and Bad-1 conductors at point B and D Impedance percentage difference between Good and Bad-2 conductors at point B and D Impedance of the good and bad-2 conductors when shorted at point EE Impedance percentage difference between the good conductor and bad conductor-2 when shorted at point EE Impedance of good conductor and bad conductor-2 at the first end: shorted FF Impedance percentage difference between the good conductor and bad conductor-2 at the first end: shorted FF Impedance of the Good conductor, Bad conductor-1, and Bad conductor-2 measured using the signal generator...64 viii

12 CHAPTER I INTRODUCTION The main aim of electric-power transmission systems is to carry electrical energy, from generating power plants to the distribution substations located near demand centers which ultimately deliver electrical energy to customers. When the transmission lines are interconnected with each other, they create transmission networks which are important for the economical operation of the overall system. During normal operations, the transmission networks allow economic dispatch of electrical energy between regions of the power system networks. During emergencies, electric power is being transferred between the regions of the network so the possibilities of power interruptions are minimized. Both the transmission and distribution networks are termed as the power-grid in the United States [1]. The power grid continuously delivers electrical power to customers even if a transmission line is disconnected in one portion of the grid. The grid has the ability to redirect itself through other parts of the transmission lines in the network. Figure 1.1 shows a general diagram of an electric power system. Figure 1.1: Electric power system [1]. 1

13 Usually, electrical energy is transmitted by using high-voltage three-phase overhead power transmission lines. In rare occasions, such as electrification of railway systems, single phase overhead transmission lines are used. For long transmission lines which have a length of hundreds of miles, or submarine cables which have a length 30 miles or more, a high-voltage direct current system is used due to its higher efficiency [1]. To decrease the amount of energy losses due to the flow of current, power transmission lines use very high voltages (usually 132 KV to 755 KV) to transfer electrical energy. At substation centers, the high-voltages are stepped down to a lower value. Low-voltage transmission lines transfer the electrical energy from the high-voltage transmission lines to secondary substations. The secondary substations further reduce the voltage levels to between 1000 volts and 69 KV. Finally, the low voltage levels are used by individual customers both residential and commercial. The low voltage electrical energy is normally transmitted either by overhead power lines or underground lines. The parallel interconnection of several power generating stations to deliver electrical energy to customers is called the electric grid. The electric grid increases reliability of power supplies, and also ensures economical operation of power systems. Being a very important part of a nation s infrastructure, the grid plays a crucial role in improving the way of life of citizens. The electric grid in the United State provides electrical energy to millions of people located in different places. As technology advances, we have become increasingly dependent on secure electricity supplies to carry out our daily tasks. A modern electric grid having a continuous monitoring of its condition is expected to be more secure, reliable, and able to recover from external and internal disruptions easily. This will improve the quality of service to millions of people who depend on 2

14 reliable power supplies. Power interruptions and fluctuations cost huge amounts of money in a year in terms of lost productivity, lost revenue, and damage to the grid architecture. Hence, studying power outages and finding solutions are very important steps towards avoiding these ever increasing costs. Despite the fact that electric power system investment has increased, power line damages and inadequate inspections have contributed to a growing number of failures. Most of the U.S. electric grid has been in operation for more than a century [2]. This has made the customers more vulnerable to power collapses due to the aging of the grid. Conductor cracking is one of the main causes of failure, which is typically caused by aging. Considering the age of the grid, there is a high probability of a large failure due to conductor damage and / or failure. Hence, monitoring the status and health of the nation s electrical grid conductors is an important process required to predict defects before its complete failure. This is the main objective of this thesis Overhead Power Lines Overhead power lines are a reliable, cost effective, easily maintainable, and established method of transporting bulk electrical energy across long distances. Since the lines are supported by towers or wooden poles, air provides the majority of the insulation. Hence, it is considered as the most cost effective method to transfer bulk electrical energy. In general, the conductors on the overhead power lines are either all aluminum, all aluminum-alloy, or aluminum-conductor-steel-reinforced or aluminum conductor aluminum-alloy reinforced. Copper wires are rarely used in medium-voltage distribution and low voltage connections to customer premises. In addition to being a cost effective way of power transportation, overhead power lines also provide suitable clearance of 3

15 energized lines from the ground. The overall structure also provides a reliable support for the conductors and minimizes the possibility of dangerous line contacts [8]. A sample overhead power line is presented in Figure 1.2. There were around 160,000 miles of 230 KV or more prominent high voltage transmission lines in the United States in 2006 [3]. A gradual increase was shown in subsequent years, and it is projected to grow from about 370,000 miles in 2009 to 406,000 miles by 2019 [50]. Although overhead power lines are ordinarily more efficient, they are vulnerable to harm brought about by phenomena such as lightning, high winds, tornadoes and hurricanes, rain and flooding, etc. Other causes, for instance, vehicle and development obstacles, human and animal interruptions, and different elements likewise add to the problems on the transmission networks. Hence, real time health condition monitoring is certainly a necessity to sort out some of these problems on time. Figure 1.2: Overhead power lines Overhead Line Conductor Today, steel-reinforced aluminum conductor (ACSR) is the most common conductor in use for overhead transmission lines. It is essentially utilized for medium to 4

16 high voltage lines and may likewise be utilized for overhead facilities. Aluminum conductors are utilized because they have the advantage of better resistivity and weight than copper, and also being less expensive. Copper is nonetheless in use, particularly for lower voltages and grounding as their energy losses are smaller [1, 5]. Although conductors having a large cross sectional area might lose small amounts of energy because of lower resistance, their cost is higher than conductors having a smaller cross sectional area. Steel-reinforced aluminum conductors are available in several specific sizes, with one or multiple center steel wires, and correspondingly large quantities. The ratio of aluminum to steel can be chosen so as to obtain the desired compromise between the current carrying level and mechanical quality most appropriate to a particular application. An example of ACSR is shown in Figure 1.3. Figure 1.3: Aluminum conductor reinforced with steel (ACRS). A phenomenon related with high-voltage transmission lines is corona. Basically, when the electric field strength near a power line conductor exceeds a specific value, then the air molecules around the conductors will be ionized. The flow of the ions results in an electrical discharge called corona. Corona causes power loss and interference with 5

17 communication channels. Corona expresses itself with a hissing sound and ozone release. For extra high voltage lines, corona causes a major issue if only one conductor per phase is used. The impact of corona is significantly diminished by having two or more conductors per phase in close vicinity to each other. This is called conductor bundling. The bundled conductors are grouped into two, three, or four as shown in Figure 1.4. The bundled conductors are isolated from each other every so often with spacer dampers that keep them from coming together and being affected by wind. In general, bundled conductors are used for overhead transmission lines having voltages over 200 KV [5, 6, 7]. Figure 1.4: Conductor bundling Electric Power Line Monitoring Although overhead power transmission lines are normally cost-effective, they are exposed to several causes of damages that can potentially harm the system. Ordinary harmful situations include conductor cracking, conductor breakage, tower base splitting, and insulator breakage. These damages may result from lightning strikes, wind-borne tree branches, earth-quakes, ice-loading, severe weather conditions, and aging [2]. If the power 6

18 lines are not continuously monitored, the majority of these harmful situations could go unnoticed or be identified too late. Most of the time, damage to the power lines leads to immediate power outages of small areas, however enormous power outages, which can influence a great many individuals, are likewise conceivable. While most of the power interruptions last just a couple of hours, a few power outages can continue for days or even weeks, totally closing down generators at corporations, financial institutions, water supplies and the health sector. For example, on August 14, 2003, a widespread power outage struck large parts of the Midwest and Northeast of the United States and Ontario, Canada, affecting an estimated 50 million people and 61,800 megawatts (MW) of electricity. The costs associated with this massive power outrage were estimated at about 6 billion US Dollars in the United States alone, while Canada s gross domestic product was down by 0.7%, and about 19 million work hours were lost [8]. Most of the US power lines have been in operation for many years and are approaching the limits of their original design lifetime. To avoid failure of the lines, continuous line monitoring is needed. Even though the electrical power line damage detection is turning out to be progressively more challenging due to the nature of power energy resources [9, 10, 11], continuous health monitoring of the power lines is very important. This helps detect damages at an early stage of development, allowing them to be repaired properly before complete power failures occur. Knowledge about the health condition of power lines may also help to predict possible future fault occurrences due to aging or conductor damages. To accurately detect power line damages and reduce the number of failures that could occur, different methods have been developed [9, 12, 13, 21, 7

19 26, and 27]. However, still a lot of research work has to be done to improve the existing methods and develop new ones. In wireless monitoring networks, the electromagnetic field produced from the AC current and voltage can be utilized to estimate the damage level on the power line conductors [9]. For the significant parts of power plants, the temperature and pressure variations of the power lines are used to relate and predict the condition of the power lines [12, 13]. Ultrasonic waves can be used to detect defects, such as broken wires, in overhead power transmission lines. This method uses longitudinal waves for diagnosis purposes [21]. Further, the average height of overhead power line conductors can be found by relating sag with measured changes in magnitude of signals having frequency of 50 to 500 KHz propagating through the lines [26, 27] Proposed Method for Power Line Impedance Characteristics and Health Condition Monitoring Overhead power line conductor damage is the main cause of power failures. The conductors are exposed to severe ambient conditions such as extreme temperature variations, icing, pollution, and mechanical stress due to galloping, Aeolian vibration, sway oscillation, and unbalanced loading. Mechanical stress and temperature variations are the main reasons for power line failures. If the current flowing over the line heats the electrical conductor up to a temperature over its design limit, the conductor begins to deteriorate, and break easily. Depending on the working condition of the electrical conductors, the most extreme allowable continuous conductor temperature fluctuates from 50 o C to 100 o C [39]. 8

20 To avoid the huge costs that may result from long periods of power interruption, the power lines should be monitored continuously, which is an important process required to predict defects on the power line before it fails. Thus, the reliability of the power system can be improved and the maintenance cost can be reduced significantly. Monitoring the damage levels continuously is a crucial step towards preventing system failures. Many fault prediction techniques have been developed to date [15-27] with some limitations. Therefore, either a new method of fault prediction needs to be developed or the existing methods need to be improved to scan the entire power network. In this thesis, the high frequency impedance characteristics of the power lines are studied for health monitoring purposes. The power lines are modeled and simulated at various test frequencies. Then, real-time high frequency impedance measurements are conducted, analyzed, and correlated to the health condition of the power lines. The physical characteristics of the power grid can be represented by its high frequency impedance. By tracking the variations of the grid impedance at high frequency, the health condition of the power system can be monitored and faults can be detected. Here, a high-frequency impedance-behavior-based analysis to identify damaged and faulty overhead conductors is conducted. Figure 1.5 shows a single conductor transmission line and the ground, which acts as a reference conductor for the voltage and as a return path for the current. Figure 1.6 represents the equivalent circuit of a transmission line (of infinitesimally longitudinal length dz). This circuital representation requires knowledge of the parameters that describe the line using a per-unit-length (p.u.l) series impedance Z = R + jωl and shunt admittance Y = G + jωc. This representation, called the Transmission Line (TL) approach, is used here to represent a single-phase system. A multi-phase system 9

21 consists of N line conductors plus the reference, which is either the ground or a perfectly conducting plane. Figure 1.5: Single phase line with reference conductor. Figure 1.6: Line parameters in a single-phase line. In this research, a high-frequency impedance-behavior-based analysis is performed for overhead conductors. As the conductor degrades, its impedance at high frequency varies accordingly. Any unusual variation in the high frequency impedance of the line can be detected to determine if the power line needs replacement due to damage. This kind of analysis is very useful to predict future power line failures due to conductor degradation and act accordingly to prevent power grid failures. In this research, the experimental tests are performed on three different overhead line conductors with different degree of damage. The sample conductors are classified as 10

22 good conductor, bad conductor-1, and bad conductor-2. The good conductor has no damage at all and bad conductor-1 and bad conductor-2 were damaged intentionally, with bad conductor-2 being more heavily damaged than bad-conductor-1. It is worthwhile mentioning that the damage inflicted upon these conductors was done by the manufacturer for these testing purposes. As such, it is not readily possible to determine the type, location, or extent of the damage. In this set up, a perfectly conducting metal plate was used as the reference Thesis Outline This thesis is organized in five chapters. Chapter I provides a brief overview of the generation, transmission, and distribution of electric power, overhead power lines, power conductors, existing power grid monitoring techniques, and the motivation for and objectives of this thesis. The various types and causes of conductor damages are presented in Chapter II. A literature review of various health monitoring systems is also presented. Chapter III takes an in-depth look at high-frequency impedance modeling of overhead power lines. Various damage scenarios are prepared and simulated using Matlab TM. Chapter IV provides a detailed explanation of the high-frequency impedance characteristics and health condition of overhead power line conductors. In this chapter, the experimental setup of the method and experimental results are presented. Here, plots of the various types of measurements and comparisons that were carried out are presented. In Chapter V the conclusion and recommendation for future work are presented. 11

23 CHAPTER II LITERATURE REVIEW 2.1. Introduction As today s systems cannot tolerate power interruptions, even for short periods of time, electric power systems should be highly reliable. Reliable operation of power systems requires continuous monitoring of their health condition. By continuously monitoring power grids, critical states can be detected in the early stage of development, and appropriate countermeasures can be applied on time. Hence the chances of power failure can be considerably reduced. To date, significant research work to monitor power systems for both overhead and underground lines has been done. In this chapter the main types and causes of overhead power line conductor damages are reviewed. Then, some of the research work which has been carried out for overhead lines is presented. Finally, the motivation for conducting this research is presented Causes and Types of Overhead Power Line Damages As demand for electrical energy continues to rise, multiple alternatives with various capacity and efficiency for the overhead transmission line conductors have been developed. Those alternatives have come due to the rapid changes in the price of raw materials for conductor manufacturing, changes in tensile strength, weight requirements, and upgrades in assembling innovation. Traditional transmission lines used the cost ineffective copperbased bare conductors for both the inner and outer conductors. 12

24 Today, the more economical aluminum and aluminum alloy are utilized widely for power line conductors as it is a legitimate alternative to copper. As overhead transmission lines are a vital component of a power system, the power line conductors contribute a substantial amount of money towards the overall cost of the system. It is estimated that the overall cost of the power line conductors, including their installation fee, accounts for about 40% of the entire principal costs of the system [1, 38]. Even though the introduction of aluminum conductors reduced the overall cost of the power system, the exposure of the overhead power lines to damage has been increased due to its simply bare surface. The conductors of overhead power lines are exposed to severe ambient conditions such as extreme temperature variations, icing, pollution, mechanical stress due to galloping, Aeolian vibrations, sway oscillations, and unbalanced loading. Mechanical stress and temperature variations increase the possibility of power line conductor failure. If the current flowing over the line heats the electrical conductor up to a temperature over its design limit, the conductor begins to deteriorate, and break easily. Depending on the working condition of the electrical conductors, the most extreme allowable continuous conductor temperature fluctuates from 50 o C to 100 o C [39]. To analyze the condition of the conductors, the various types and causes of conductor damage need to be studied in detail. This is the main focus of this section Causes of Conductor Damage Overhead power line conductors can be damaged for many reasons. Some of the reasons are due to the motion occurring on the overhead power lines such as Aeolian vibration, galloping, sway oscillation, and unbalanced loading. All of these terms have different causes and effects which can damage the overhead power lines and create 13

25 complete failure of the system. High-wind speeds are the main causes of conductor oscillations, galloping, and sway oscillations. For galloping to happen, some amount of ice covering is needed to start motion. These movements are for the most part described as low frequency, high amplitude oscillations. Aeolian vibration is related to wind velocities in the range of 2 mph to 15 mph. As opposed to galloping, Aeolian vibration is described as high frequency, low amplitude movement. Studying each term individually is very important in order to understand the kind of damage they cause on the power line conductors [40-43] Aeolian Vibration Aeolian vibration of overhead power transmission lines is one of the most significant issues damaging power lines. It is characterized as the main cause of fatigue damage of conductor strands, specifically at the suspension points. Aeolian vibration is the periodic motion of a conductor induced by a wind, mainly in a vertical plane, of relatively high frequency in the range of ten Hz to tens of Hz (3 to 100 Hz) and small amplitude, comparable to the line diameter (millimeters to centimeters). Aeolian vibration is created by wind speeds below 15 miles per hour (MPH) [40-44]. Figure 2.1: Aeolian vibration - Amplitude vs. time [41]. 14

26 The presence of Aeolian vibration on overhead power transmission line conductors may not be problematic. However, if the size of the aeolian-vibration movement is excessively large, disastrous overhead power line failure can occur due to fatique breakage of the conductor strands. Damages caused by Aeolian vibrations can be categorized as: i) Conductor fatigue breakage at: a) Suspension points b) Splices and end points c) Damaged wire leads ii) Sever abrasion of conductors at: a) Loose links b) Loose hardware iii) Sever abrasion of tie wires iv) Loosening related to hardware [40] Conductor Galloping Conductor galloping is a wind induced motion of the power line conductors. The galloping frequency, which depends on the power line infrastructure, normally varies from 0.15 HZ to 1 Hz. The oscillation has huge vertical amplitude, and usually reaches several meters. The power line conductors usually move in a vertical plane, although that is not always the case. Sometimes, the conductors move in a horizontal or rotational direction. The galloping amplitude may exceed the distance between the power line phases, and could short the phase lines - damaging the power line conductors due to a power arc. Most often galloping occurs when there are ice deposits on the power lines, but on rare occasions it can happen without ice deposits. For galloping to occur, wind velocities in excess of

27 miles per hour (MPH) are required. A sample galloping of overhead power line is shown in Figure 2.2. Figure 2.2: Galloping of overhead power line [51]. Once galloping has started on a transmission line, the conductor supporting devices are exposed to high tensions that cause damage to the system. In general, the most common types of damages that occur from galloping are: 1. Conductor fatigue 2. Loose tower bolts 3. Landing bolt wear 4. Conductor damage due phase-phase contact 5. Excessive sag from overstressed conductors 6. Complete conductor failure [40, 42] Sway Oscillation Sway oscillation, which is created by gusting wind forces, is the most widely recognized movement on electric conductors. It is the typical conductor movement experienced by power distribution lines, and it tends to abrade the surface of the conductor [40]. In general, sway has a low frequency of oscillation in the power lines. The amplitude 16

28 of oscillation depends on conductor sag, and the frequency can reach up to 3 Hz irrespective of the wind speed. The frequency of oscillation can increase if the conductor tension is low. High amplitude swaying affects the supporting infrastructures, hand-wrapped ties, and spreader rods which ultimately damage the conductor [43] Unbalanced Loading Unbalanced loading affects the power line conductors by creating a longitudinal motion on the conductor at the supporting points. This movement can be started for different reasons, such as ice dropping from the power line conductors, wind-impelled movement, and temperature changes in uneven ranges. Unbalanced loading, which is typically experienced by power distribution lines, can deteriorate the surface of a conductor Types of Conductor Damages When the electric power lines are not monitored continuously, the first sign of conductor failure/damage might not be recognized on time. This prevents one from applying basic restorative measures before a complete failure occurs. Hence, continuous examination of the power line conductors and equipment should be conducted in order to eliminate concerns about damaged conductors. Before implementing a system to monitor the health condition of the line, it is worthy to study the most common types of conductor damages. Conductor damage occurs at points where cyclic movement of the conductors is confined or where connecting equipment touches the surface of the conductor. Common occurrences of damage are found at supporting points, strain joints, and spacers [43]. In this section, different types of conductor damages are presented. 17

29 Abrasion Abrasion damage is the scraping of the surface of a conductor which is caused by the relative motion between loose conductor fittings or conductor connecting devices and the conductor. The loose connection that permits the scraped spot to happen is frequently the outcome of extrem Aeolian vibration [41]. When high Aeolian vibration occurs on the overhead power lines, a relative motion between the power line conductors and the connecting equipment is created. This causes the connection between the conductor and connecting devices to loosen. Indeed, even well-made connections can become loose if they are exposed to extended and serious movement. Because abrasion damage occurs on the surface of a conductor, it can be easily distinguished by dark deposits. Aluminum deposits on the conductor insulator also help to identify abrasion damage [45]. Figure 2.3 shows a sample of abrasion damage of an overhead power line conductor. Figure 2.3: Abrasion damage [45]. 18

30 Abrasion damage can happen at various locations of the overhead power line conductors such as spacers, dampers, or at supporting points. Figures 2.4 and 2.5 illustrate abrasion damage at a spacer and a loose hand tie, respectively. Figure 2.4: Abrasion damage at spacer [45]. Figure 2.5: Abrasion damage at loose hand tie [41]. 19

31 Figure 2.6: Severed strands due to abrasion [43]. Figure 2.6 shows that about half of the conductor s surface area has been damaged at two different locations because the conductor damage was not identified at an early stage. In this case, to restore the power system, the damaged section has to be removed and replaced by a mid-span joint [43]. Figure 2.7 shows a restored section of a power line conductor. Figure 2.7: Restoration of abraded conductor wear from the spreader rod clip [43]. 20

32 Fretting Wear Fretting wear of power line conductors is a damage on the surface of the conductors. Cyclic movement between two contacting power line conductors is the main cause of all kinds of fretting wear. The small amplitude cyclic movement creates a slight tangential displacement between the conductors. Because the conductors are under tension, a presure is produced on the layers under them, which creates friction during oscillation. This friction removes the oxide film usually found on the surface of the conductor, exposing the metal to additional oxidation processes. The oxidation process between the scraped surface conductor and the surroundings air creates particle debris. The particle debris accumulate and the damaged surfaces become rough. As shown in Figure 2.8, the damaged surfaces are usually black, and the debris looks like black dust because of the aluminum oxide Al2O3 product. The oxidation process happens at a fast rate because aluminum is an active metal in terms of chemical reaction [40, 46, 47]. Figure 2.8: Damaged conductor - Presence of blackish debris due to the presence of aluminum oxides (Al 2O 3) [46]. 21

33 Figure 2.9: Fretting wear [40]. As shown in Figure 2.9, fretting damage is usually hidden in the inner parts of the multi-layer conductor, and thus it can be identified by spreading the outer parts. Fretting wear happens at locations where the movement of the conductor is limited, mainly at supporting structures, conductor clamps, or dampers [40, 47] Fatigue Breaks Fatigue breaks of a power line conductor are caused by rapidly bending the conductor back and forth with limited motion. In this case, the conductor strands are entirely damaged. The existence of fatigue is directly related to the stiffness of the restriction. Aeolian vibration, sway oscillation, galloping, and unballanced loading are the main causes of conductor damage. Fatique failures occur when the bending stress of the power line conductor exceeds its strength limit. The extent of the bending pressures and the number of bending cycles determine the time of failure [40, 43]. Figure 2.10 shows a sample of conductor strand fatigue breaks. 22

34 Figure 2.10: Fatigue breaks [40]. For a conductor being exposed to Aeolian vibration, the highest bending pressure occurs at places where the conductor movement is limited. Such limits occur mainly at supporting structures, conductor clamps, or dampers. Nevertheless, the extent of restraint and so the extent of bending pressure, is normally maximum at the supporting points [40, 41] Tensile Break Power line conductors are exposed to several external loadings, such as high speed winds, severe icing, or large temperature variations. If the coordination of loadings and conductor strength is not designed properly, the internal stress in the conductor material may increase beyond the damage [47]. If the damage limit is reached, the conductor will be permanently deformed as shown in Figure Deformation leads to inadmissible increase of sags leading to a decrease of clearances between the phase conductor and the ground, excessive creep, and undesirable permanent phenomena, such as bird caging [47]. Conductor break down can also happen if the load exceeds the strength of the material break limit [40]. 23

35 Figure 2.11: Tensile break [40] Overhead Power Line Monitoring Overhead power lines are occasionally monitored using both on-ground as well as helicopter-aided visual examination apparatus. Components including sun glare, overcast spread, closeness to electrical power line conductors, and quickly altering visual situations make airborne examination of electrical power line conductors an especially risky assignment. In this study, a review of continuous, on-line monitoring of power lines is presented. Theoretical examinations of electrical and mechanical characteristics of power lines indicate that power line conductor temperature is one of the greatest significant limiting factors for safe operation of the overall power system [16]. The sag of the power line conductor is affected by the temperature and age of the conductor. These parameters may extend the sag to a critical state under which the height of the power line conductor from objects below in the surrounding area reaches the minimum allowable safety limit. In addition to conductor temperature and aging, icing on a power line conductor creates extra weight that causes conductor sagging to increase further [17], 18]. In [15], [16], and [24] a continuous health monitoring system incorporating several widely distributed 24

36 sensors to measure conductor temperature, the distance to the ground, and the degree of icing is presented. The conductor temperature is not consistent over the entire conductor segment, and surface temperature is impacted by current environment or weather conditions. Since these effects can vary from one location to another, the conductor temperature needs to be measured at several sites. Then, a comparison between the measured values and computed values is made based on the current weather conditions. The sensor devices are directly connected to the conductors, and the measured raw data are wirelessly transmitted to a control station. To supply the sensors, the electromagnetic and electrostatic fields created from the AC current and voltage [19, 25] are used. The principle of the wireless monitoring system for this method is shown in Figure Figure 2.12: Principle of wireless monitoring system for overhead power lines [16]. In [20], overhead power line condition monitoring and fault location based on voltage and current transients is presented. Phase voltage and current changes, on each phase, are monitored at many monitoring sites on the network. If a suspicious condition is 25

37 found on the line, the system gathers waveforms from related locations. Analyzing the voltage and current transients allows the fault to be located. The online status monitoring and fault location system consists of a Wireless Overhead Monitor (WOM), a Wireless Overhead Communicator (WOC), and a Wireless Overhead System Software (WOS) as shown in Figure Each monitoring location consists of three WOMs and one WOC. One WOM is installed on each conductor of the overhead power line. A local radio having a frequency of 470MHz is used to communicate with the WOM and WOC. The WOC and WOS are connected through an internet connection. Figure 2.13: Monitoring point configuration [20]. Monitoring the condition of the power lines and fault detection, involves the following steps: The WOM continuously monitors a single phase voltage and current signals to see if there is a significant increase or decrease in amplitude. If a large change 26

38 is observed, the WOM triggers the WOC indicating that there may be a suspected condition. After being triggered, the WOC collects signals of the three phases using the WOMs. The zero sequence of the signals is then calculated. The WOS integrates the signals collected from all locations. Another method, continuous on-line health monitoring of power lines using ultrasonic waves is presented in [21]. This method is used to detect defects, such as broken wires, in overhead power transmission lines. Longitudinal waves are utilized for analysis purposes. In this method, a transducer is utilized to generate/detect ultrasonic waves in the power line. First, the transducer generates and sends ultrasonic waves. If there is a defect in the line, such as a broken conductor, a portion of the transmitted waves is reflected back. It is then received by the receiving transducer. By comparing the reflected wave with a predefined threshold value, the defect can be identified. A wireless transmitter is integrated with the transducer to send data to a central location. Figure 2.14 shows the basic idea of the overhead power line defect detection. Figure 2.14: Overhead power line defect detection using ultrasonic pulses [21]. 27

39 The power transmission line defect detection system uses a pulser-receiver circuit to energize a piezoelectric ring. The ring is attached to the power transmission line as shown in Figure In order to receive reflected waves from a broken line, the pulserreceiver circuit repeatedly switches from send to receive state. Any discontinuities in the power transmission line conductor cause the waves to be reflected back, and the reflected wave is received by the piezoelectric ring. Since the received signal has very small amplitude, it is amplified by the pulser-receiver circuit [21, 22]. The experimental set up and the attached piezoelectric ring transducer of the system are shown in Figures 2.15 and 2.16, respectively. Figure 2.15: Set up for defect detection of Figure 2.16: Piezoelectric ring transducer transmission lines [8]. attached to a transmission line [8]. In [26], a real-time sag monitoring system for high-voltage overhead transmission lines based on power-line carrier signal behavior is presented. According to power line regulations, electrical power transmission lines should always work safely. Hence, for safe operation, the maximum allowable line sag should not be exceeded. The average height of the high-voltage overhead transmission-line conductor changes for many reasons. Some of the reasons are conductor heating due to current flowing though the conductor and 28

40 weather conditions. These factors determine the characteristics of the conductor to correlate with the overhead power transmission line sag. The authors of [26, 27] propose a new method of determining the average height of the overhead power line conductors or conductor sag. This method correlates conductor sag with measured signal amplitude variations between power line stations. The real time sag monitoring system is called power-line carrier sag (PLC-SAG), and it injects multifrequency PLC-SAG signals of 50 to 500 KHz into the PLC system, while the line is still energized. By using existing PLC infrastructure and additional PLC hybrid, the continuous-wave signals for sag monitoring are injected into the overhead power lines. The injected signals are received at the remote PLC stations and analyzed to determine the conductor sag height. The overall sag monitoring system is presented in Figure Figure 2.17: Sag monitoring system with transmit and receive PLC-SAG units [26]. The PLC signal is attenuated in proportion to the height of the conductor because of the ground resistivity effect. Hence, the magnitude of the signal is sensitive to the height of the power line conductor. An important linear relationship between the received signal amplitude, in decibels, and the overhead power line conductor average height can be 29

41 determined as shown in Figure In this method, the frequencies of the monitoring signals should be selected carefully to avoid any interfere with the functioning PLC system. Figure 2.18: Sample calibration curve for the principal PLC-SAG signal taken from [26]. 30

42 CHAPTER III OVERHEAD POWER LINE IMPEDANCE MODELING AND PARAMETER ESTIMATION 3.1. Introduction All analyses in engineering sciences start with the formulation of an appropriate mathematical model to describe the physical system. Hence, depending on the purpose of the analysis, different models of the same physical system can be derived. In the case of power transmission lines, a general model was given by the telegraph equations, which describe the voltage and current signal propagation along electrical transmission lines with distance and time. Solving these equations yields the lumped-circuit line model, also referred to as the π-model. These equations were derived by Oliver Heaviside in 1880 [1]. Since this theory applies to transmission lines at all frequencies, such model is used in this work. All transmission lines in a power system exhibit the electrical properties of resistance, inductance, capacitance, and conductance. The resistance, R, is due to losses in the conductor. The inductance, L, is due to the current in the conductors and the magnetic flux linking the current path. The capacitance, C, is due to the time varying electric field between the two conductors. The amount of the capacitance is a function of the conductor size, spacing, and height above the ground plane. The inductance and capacitance parameters are essential for the development of the transmission line models used in power 31

43 system analysis. The shunt conductance accounts for leakage currents flowing across insulators and ionized pathways in the air. The leakage currents are negligible compared to the current flowing in the transmission lines and may be ignored. These parameters are distributed along the entire line and are used to model the behavior of the voltage and current signals as they travel throughout the line Power Line Analysis In the following analysis, a single conductor power line and ground are used as a power circuit and are approximated as a closed-form version of the two-wire transmission line, as shown in Figure 3.1. The ground acts as a reference conductor for the voltage and as a return path conductor for the current. Based on the above considerations, the singlephase conductor line, with ground return, is regarded as a distributed parameter network, where voltages and currents can vary in magnitude and phase over its length. Therefore, it can be described by circuit parameters that are distributed over its length. Figure 3.1: Single phase line with reference conductor. Figure 3.2 shows the equivalent circuit of a power line, of infinitesimally longitudinal length dz; an element so small that we can assume its distributed properties as 32

44 lumped into four components. As dz 0, our lumped model becomes a distributed model. This circuit-like representation of the line requires knowledge of the parameters that describe the line, as represented in the figure; namely, its p.u.l. series impedance, Z = R + jω L, and its p.u.l. shunt admittance, Y = G + jω C, where R defines the p.u.l. resistance of both the conductor and the earth (in Ω/m), L defines the p.u.l. inductance of both the conductor and the earth (in H/m), G is the p.u.l. conductance (in S/m), and C is the p.u.l. capacitance (in F/m). The quantities v(z) and v(z + dz) denote the instantaneous voltages at locations z and z + dz, respectively. Similarly, i(z) and i(z + dz) denote the instantaneous currents at the respective locations. These electrical parameters are determined from the material type, the power line geometry, the bundling configuration of the conductor, as well as the distance of the conductor from the earth, and the ground earth characteristics. The reference conductor, in real systems, could be either the ground or a perfectly conducting plane as is the case in this research work. Figure 3.2: Schematic representation of an elemental length (dz) of a transmission line. Applying Kirchhoff s voltage and current laws to the circuit of Figure 3.2 yields the following two equations: 33

45 v(z) Rdz i(z) Ldz. i(z) t v(z + dz) = 0 (3.1) i(z) Gdz v(z + dz) Cdz v(z+dz) i(z + dz) = 0 (3.2) t Dividing these equations by dz gives: v(z) v(z+dz) dz = R. i(z) + L. i(z) t (3.3) i(z) i(z+dz) dz = G. v(z + dz) + C. v(z) t (3.4) Expressing v and i in phasor form; that is, v(z) = Re[V(z)e jωt ] and i(z) = Re[I(z)e jωt ] and letting dz 0, the line voltage V(z) and the line current I(z) can be written, in the frequency domain, from Equations (3.3) and (3.4) as: dv(z) dz = (R + jωl)i(z) (3.5) di(z) dz = (G + jωc)v(z) (3.6) Differentiating and combining Equations (3.5) and (3.6), allow the line voltage and current to be expressed, in terms of the position z, as: d2 V(z) dz 2 = (R + jωl) di(z) dz = (R + jωl)(g + jωc)v(z) (3.7) d2 I(z) dz 2 = (G + jωc) dv(z) dz = (G + jωc)(r + jωl)i(z) (3.8) Equations (3.7) and (3.8) can be rewritten as (Telegrapher s equations): d2 V(z) dz 2 = γ 2 V(z) (3.9) 34

46 d2 I(z) dz 2 = γ 2 I(z) (3.10) where, γ = α(ω) + jβ(ω) = (R + jωl) (G + jωc) (3.11) γ is the propagation constant whose real and imaginary parts, α and β, are the attenuation constant (Np/m) and phase constant (rad/m) of the line, respectively. ω is the angular frequency (rad/sec) that is, f is the frequency (Hz) of the propagated signal, where ω=2πf. The complex propagation constant is also given as: γ(ω) = Z Y (3.12) where Z = R + jωl is the p.u.l. series impedance and Y = G + jωc is the p.u.l. shunt admittance of the line. The characteristic impedance of the line is given by: Z o = R+jωL G+jωC = Z Y (3.13) Notice that the propagation constant γ and characteristic impedance Zo are characteristic properties of a line whether the line is infinitely long or not. They depend on R, L, G, C, and ω but not on the length of the line High Frequency Modeling of Overhead Power Line Conductors For a direct current flowing through a conductor, the resistance of the conductor is constant due to the uniform distribution of the current. However, the distribution of an alternating electric current flowing through a conductor is not uniform, a phenomenon called skin effect. The current density at the center of the conductor is lower than at any 35

47 part of the conductor. The density at the surface of the conductor increases with frequency. This causes the resistance of the conductor to increase. Hence, the series impedance model of the power line conductor is dependent on the frequency of the current passing through it. The Skin depth, which is a function of frequency (f), resistivity (ρ), and relative permeability (µr), can be calculated as: 2ρ Skin Depth = δ s = (3.14) 2πfµ o µ R where ρ is resistivity (Ohm-meters), f is frequency (Hertz), µo is the permeability constant 4πx10-7 (Henries/meter), and µr is relative permeability (unit-less). As can be seen from Equation (3.14), skin depth shrinks with frequency Power Line Impedance and Admittance In order to prepare a lumped parameter model of a single-phase power transmission line, the impedance and admittance of the line segment, as well as the impedance and admittance effects of the earth return path are used. When a signal travels along the power line segment of a certain length, it will be subjected to attenuation losses which depend on the physical properties of the material. These losses can be determined by the transmission line parameters (i.e. R, L, G, and C) and the frequency of the propagated signal ω. The line model per unit length consists of 3 series impedances; namely, internal impedance Zi, external impedance Ze, and ground return path impedance Zg; and 2 shunt admittances; namely, external admittance Ye, and ground return path admittance Yg. All of these parameters, shown in Figure 3.3, are functions of the frequency of propagation. 36

48 Figure 3.3: Equivalent electrical model at high frequency. For a uniform conductor, the overall equivalent impedance of the line can be approximated by the combination of all standard impedances (Z c ) and admittances (Y e, Y g ) times n (n being the length of the conductor to the nearest integer number of the per-unitlength blocks). The expression is given as: Z equi = Z c. n + 1 Y eg.n (3.15) The total series impedance of the power line conductor Z c is found as: Z c = Z i + Z e + Z g (3.16) The total parallel admittance of the power line conductor Y eg is calculated as: Y eg = 1 1 Ye + 1 Y g (3.17) The internal impedance Z i of the single conductor above the earth is expressed as: Z i = 1 ωµ ω I 0 (jk ω r C ) 2πr C k ω I 1 (jk ω r C ) (3.18) 37

49 where I 0 and I 1 are the first kind Bessel functions of zero and first order, respectively, and k o and k w are the propagation constant of the electric field in the air and the wire. Their expressions are given by: k o = ω (µ o ε o ) (3.19) k ω = k o ( ε ω ε o jσ ω ωε o ) (3.20) where µ o is the permeability of free space, ε o is the permittivity of free space, ε ω is the permittivity of the conductor, and σ ω is the conductivity the conductor. The external impedance Z e, due to the geometrical inductance of the line at a height H above the ground level, is given by: Z e = jωµ o 2π ln (2H r c ) (3.21) where r c is the radius of the conductor. Similarly, the external admittance is also expressed as: Y e = jωε o2π ln ( 2H rc ) (3.22) Ground Return Path Impedance In order to gain a complete understanding of the impedance characteristics of overhead power lines, a detailed model that properly accounts for all of the electromagnetic characteristics of all involved media is necessary. The modeling must also take into account the effect of the imperfect earth, as it acts as a return path conductor for the high frequency current used for line health monitoring. Earth is practically a non-homogeneous 38

50 medium, and this behavior is usually modeled by means of horizontal layers with different earth resistivity. For the sake of simplicity, earth is considered as a homogenous medium in this work. Accurate representation of the imperfect earth to calculate electrical parameters of a conductor has been a research topic for many years. Significant contributions for the modeling of the earth return impedance have been made by Pollaczek [35] and Sundle [36]. In 1926 Pollaczek attempted to calculate the electromagnetic field distribution of the ground. However, his solution was stated in the form of an infinite integral with a highly oscillatory behavior. In 1968, Sundle proposed an expression similar to that of Pollaczek, but the effect of the displacement current was included. As shown in Equation 3.23, it involves an integral over an infinitely long interval, and hence it is not suitable for numerical evaluation. Since this expression has been derived, several analytical and numerical approaches to approximate the solution to this integral have emerged. Here, a simple form expression proposed by Sundle is given by the logarithmic function; Equation 3.24; [34]. The ground impedance expression is given by: Z g = jωµ o exp[ 2Hx] 2π 0 x 2 +γ 2 g +x dx (3.23) Equation (3.28) maybe approximated by the following equation: Z g = jωµ o ln 2π (1+γ gh ) (3.24) γ g H where γ g can be calculated as: γ g = jωµ o (σ g + jωε o ε rg ) (3.25) 39

51 in which σ g is the ground conductivity in S/m, ε o is the permittivity of free space, and ε rg is the relative permittivity of the ground. H is the height of the power line conductor above the ground level Estimation of Transmission Line Parameters from Measurements Accurate knowledge of transmission line impedance and admittance parameters is very important to determine the condition of the line. These parameters can be calculated using synchronized measurements at two end points. To estimate the impedance and admittance of the transmission line, a Matlab/Simulink model is prepared as shown in Figure 3.4. The line is modeled using the distributed line model. The first subsystem shows the distributed line model of the line, whose detailed implementation is shown in Figure 3.5. In this section, to simulate long transmission lines, simulation parameters are taken from Matlab TM default values as this one is only for demonstration purposes. In the next section, the simulation will be conducted based on the parameter values taken from the laboratory setup measurements. Hence, two long distributed parameter lines of 100 km long each are used here. Voltage and current values are measured at both end points of the transmission line using the three-phase V-I measurements. From these measurements, the magnitude and phase values are extracted using the Magnitude-theta block. Then, the impedance and admittance values are estimated from the measured values. To validate the estimated values, the actual values of the line impedance and admittance are calculated using the given per unit length parameters. As it can be seen from the Display Block of Figure 3.4, the estimated values match the actual values well. High frequency simulations of the power line conductors used in the lab are presented in the following section. To match 40

52 with the practical measurements done in the lab, the simulations were conducted over the frequency range of 100 KHz to 500 KHz. Figure 3.4: Matlab/Simulink model to estimate parameters of transmission line. Figure 3.5: Distributed line model for the transmission line parameter estimation. 41

53 3.5. Comparison between Simulated and Measured Power Line Conductor Impedance Power line conductor impedance simulations and measurements were conducted. To simulate the power line conductors, the high-frequency impedance model presented in Section 3.3 were used. In order to make a comparison between the simulated and measured impedance values, parameters of the power line conductor were taken from the experimental setup prepared for this purpose. A detailed explanation of the laboratory setup is presented in Chapter IV. The parameters of the conductor are listed in Table 3.1 below. Table 3.1: Power Line Conductor Parameters. Parameter Conductor Diameter Conductor Length Conductor Height from ground Conductor Conductivity, σ Permeability of free space, µ o Permittivity of free space, ε o Resistivity of earth, ρ Value in or m ft or m 22 in or m 3.5*10 7 S/m 4π*10-7 H/m 8.854*10-12 F/m Ohm/m The impedances of the power line conductor have been measured and are presented here, with the physical results shown in Figure 3.6 and the simulated results shown in Figure 3.7. For both figures the impedance has been measured from line to the ground plane in both good and distressed conditions. Figure 3.6 was prepared by measuring the impedances of the good-conductor (a conductor having no damage) and bad conductor-2 (a conductor having cracks). An Agilent E4980A Precision LCR Meter controlled by LabVIEW was used to measure the impedances. Figure 3.7 was prepared by simulating the overhead power line conductor model presented in Section 3.3. In this simulation, the parameters of the overhead power line conductors; listed in Table 3.1; were used. To 42

54 simulate conductor damage, the power line parameters were altered slightly. However, it should be noted that the simulated damage and the conductor damage at the lab are not the same as conductor crack simulation is not easy. Here, conductor abrasion damage is simulated to show that other types of damages can be simulated easily. Simulations of various conductor damage scenarios are presented in the next section. As it can be seen from Figures 3.6 and 3.7, the physical and simulated results are similar. Figure 3.6: Line to earth impedance magnitude (physical measurements result). Figure 3.7: Line to earth impedance magnitude (Simulated result). 43

55 3.6. High Frequency Simulation of Power Line Conductor damages Under normal operating conditions, the total impedance of the power transmission line is constant due to the fixed nature of the power line structure and geometry. Load variations have no effect on the total impedance of the line. Hence, by tracking any change on the total impedance of the line, the health condition of the transmission line can be assessed. As the different types of faults or damages have their own electrical characteristics, their effects on the line impedance can be investigated at high frequencies. To model these effects, a Matlab TM model was developed, various faults or damage types were applied to the line, and the changes in the impedance characteristics were observed. A threshold value can be set to determine if the power line needs replacement due to excessive damage. Although the various conductor damages are simulated here, the conductor damage in the experimental setup is not simulated as conductor crack damage is not easily simulated. The experimental setup to conduct high-frequency impedance measurement using LCR meter is presented in Chapter IV. To simulate the effects of various types of conductor damage on the high frequency impedance, several typical fault or damage conditions were applied to the model. Then, the total impedance of the conductor, from the line to a conducting earth plane at the surface of the earth, was measured. The overhead conductor line to earth plane impedance model is shown in Figure 3.8. For accurate modeling of the transmission line, the model used is a distributed line model where each section of impedance and admittance represents a per unit length block of the conductor. For a uniform conductor, the total impedance and admittance of the line are found by multiplying the stranded series impedance per unit length (Ω/m) and parallel admittance per unit length (S/m) by the total length of the line. 44

56 To calculate the overall equivalent impedance of the line, all the stranded impedances and admittances of the conductor are combined. Y g1 Y g2 Y gn Y c1 Y c2 Y cn Z c1 Z c2 Z cn Figure 3.8: Line to Earth Plane Impedance Model. When the overhead conductor is damaged due to any reason, its electrical parameters change. Hence, several typical fault or damage conditions may be simulated by properly altering the parameters of the conductor model. For example, conductor sagging may be simulated by reducing the height of the conductor above the earth level. The various types of conductor damage explained in the previous chapter cause the conductivity σ al of the line to decrease. This situation can be simulated by decreasing the conductivity of the conductor. As explained in the power line conductor modeling section, the effect of homogenous earth is included in the model. In this fashion, any change in the parameters of the earth affects the total impedance of the line. The electrical characteristics of the earth can be described by its permeability µ, permittivity ε, and conductivity σ. The actual values of these quantities are found by the nature of the soil, moisture content of the soil, and temperature. Permittivity and conductivity significantly increase with high moisture content [49]. Temperature changes of the ground decreases with depth. Hence, the effect of temperature is significant only at high-frequencies (small penetration). At 45

57 high temperatures, the conductivity is increased while the permittivity change is negligible. At the freezing point, both permittivity and conductivity are highly decreased [48]. To simulate these effects, approximate electrical properties of the earth can be chosen from a known moisture content, material composition, and temperature. The impedances of a conductor with length of ten meters have been simulated with different damage scenarios and the results are presented here. For all cases, the impedances have been taken from line to the ground plane for conductors in both good and distressed conditions. In the first scenario, conductor sagging was simulated by reducing the height of the conductor above the earth level. As shown in Figure 3.9, some amount of disparity between the impedance magnitudes of the two sets of simulated conductors was observed. In order to compare the two sets of measurements quantitatively, the relative change of impedance magnitude between good and bad conductors was calculated as: D mag = 100 ( Z good Z bad Z good ) (3.26) As it can be seen from Figure 3.10, the impedance magnitude difference between the two sets of simulated conductors was about 7 percent in the given frequency range; that is 100 KHz to 500 KHz. This difference is enough to distinguish damaged conductors from good conductors. 46

58 1.8 x Good Bad Magnitude (Ohm) Frequency (KHz) Figure 3.9: Line to earth impedance magnitude for conductor sagging Percentage difference Frequency (KHz) Figure 3.10: Impedance percentage difference for conductor sagging. To represent conductor abrasion, the radius of the simulated power line conductor was reduced from its normal value, since the radius decreases as the surface of the conductor wears away. The total impedance magnitude for this case is presented in Figure A large disparity between the impedance magnitudes of the two sets of simulated conductors was observed. To make the comparison easier, the relative change of impedance magnitudes between them was calculated. As it can be seen from Figure 3.12, 47

59 the relative percentage difference was found to be about nine (9%) in the given frequency range, which is good enough to identify damaged conductors. These plots provide clear evidence of the potential of the high-frequency impedance measurement method in detecting damaged conductors, given the large difference between the magnitudes of the impedance of healthy versus damaged conductors. 2 x 104 Good Bad 1.5 Magnitude (Ohm) Frequency (KHz) Figure 3.11: Line to earth impedance magnitude for conductor abrasion Percentage difference Frequency (KHz) Figure 3.12: Impedance percentage difference for conductor abrasion. 48

60 CHAPTER IV HIGH FREQUENCY IMPEDANCE CHARACTERISTICS AND HEALTH CONDITION MONITORING OF OVERHEAD POWER LINES 4.1. Introduction Continuous monitoring is an important process required to predict defects on power lines before they may lead to complete failure and thus power outages. Also, the reliability of the power system can be improved and its maintenance cost can be reduced significantly. One way of monitoring the condition of the power lines is through its high frequency impedance characteristics. Any unusual variation in the high frequency impedance of the power line can be detected and used to determine if the power line needs replacement due to aging, or other conditions. The objective of this thesis work is to observe the effects that damages on conductors have on the high frequency impedance of the power line in order to develop a high frequency impedance-based analysis to identify damaged and faulty overhead conductors. Overhead transmission lines are an intricate part of a power system, and their health condition is very important for the reliable operation of the overall system. Hence, in this research a high frequency impedance analysis of overhead power lines is conducted in order to identify faulty and damaged conductors. This method of analysis also helps to predict the life of the power line conductors by setting a threshold value, and take adequate action before complete line failure occurs. 49

61 4.2. Experimental Setup and Results of Overhead Power Line Conductor Impedance Measurements One of the main objectives of this thesis work is to identify damaged and faulty overhead power line conductors using the high-frequency impedance measurement technique. The validation of the proposed method through experimental analysis is an important step. Hence, to conduct the necessary set of experiments, the experimental set up shown in Figure 4.1 was constructed in the lab. The experimental set up consists of three different overhead line conductors having different degrees of damage. The provided sample conductors were classified as good conductor, bad conductor-1, and bad conductor-2, where the good conductor had no damage at all whereas the two bad conductors were damaged intentionally with more damage inflicted upon bad conductor-2 than on bad conductor-1. Conductor cracking was the damage inflicted upon the two bad conductors. In this setup, a perfectly conducting plate, a metal plate, was used as a reference ground. Figure 4.1: Experimental set-up for overhead conductors. 50

62 The three overhead power line conductors had equal lengths (31 feet) and thicknesses. The impedance measurements were done between one end of the conductor and the metal plate reference ground. An Agilent E4980A Precision LCR Meter controlled by LabVIEW was used to measure the impedances of the conductors. LabVIEW software was used to sweep the test frequency of interest and record the impedance magnitude and phase of the conductor at specific frequencies to observe the impedance behavior over a frequency range. In this experimental test, a frequency sweep from 100 KHz to 500 KHz, in steps of 5 KHz, was used. This is because a higher percentage difference between the good and bad conductors was found in that specific frequency range Impedance Measurement Test Plan To perform the high frequency impedance measurement of the overhead power line conductors, the test plan presented in Table 4.1 was prepared. As it can be seen from Figure 4.2, the high frequency impedance was measured at different points along the conductors. The impedance was first measured at the first end points of the three conductors; that is, at point A for the good conductor, and at point C for both bad conductor-1 and bad conductor-2. To observe the effects of measurement points on the total impedance of the line, the same measurements were conducted at the opposite end of the conductors. For extra high voltage lines, corona causes power loss and interference with communication channels. To reduce the effect of corona, two or more conductors per phase in a close proximity are bundled together. Hence, to consider the bundling effect, two conductors at a time were shorted first at point EE, and then at point FF, and the impedances of the conductors were measured on the same side. 51

63 Figure 4.2: Measurement plan for Good and Bad conductors. Table 4.1: Overhead power line impedance measurement test plan. Test # Point A Point B Point C Point D Point E Point E' Point F Point F' 1 X Y 2 X Y 3 X Y 4 X Y 5 X S S' Y 6 X S S' Y 7 X S S' Y 8 X S S' Y 9 X Y Point G Impedance Measurements of Overhead Power Lines Experimental Results As presented in Table 4.1, a total of nine different high frequency impedance measurements of the overhead power line conductors were conducted. The impedance measurements were done between one end of the conductor and the metal plate reference ground. In the first set of measurements, an Agilent E4980A Precision LCR Meter controlled by LabVIEW was used. The LCR Meter injects high frequency signals that are swept from 0 to 500 KHz in steps of 5 KHz, and measures the impedances at the corresponding frequencies. In the second set of measurements, a signal generator that 52