Inspection & Assessment of Overhead Line Conductors

Inspection & Assessment of Overhead Line Conductors

A State-of-the-Science Report

1000258

Inspection & Assessment of Overhead Line Conductors

A State-of-the-Science Report

1000258

Technical Progress, November 2000

EPRI Project Manager

R. Lings

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION (S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION (S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

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ORGANIZATION (S) THAT PREPARED THIS DOCUMENT.

EPRIsolutions, Inc.

Electricite de France

J. A. Jones Applied Research Co.

Colorado School of Mines

This is an EPRI Level 2 report. A Level 2 report is intended as an informal report of continuing research, a meeting, or a topical study. It is not a final EPRI technical report.

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Copyright © 2000 Electric Power Research Institute, Inc. All rights reserved.

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CITATIONS

This document was prepared by

EPRIsolutions, Inc. 100 Research Drive Haslet, Texas 76052

Principal Investigator D. Cannon

Author L. Hill

Electricite de France Site des Renardieres – Route de Sens - Ecuelles 77818 Moret-Sur-Loing Cedex, France

Author M. Gaudry

J. A. Jones Applied Research Co. 8320 University Executive Park Drive, Suite 110 Charlotte, North Carolina 28262

Author R. Stone

Colorado School of Mines 1500 Illinois Street Golden, Colorado 80401

Author R. Shoureshi

This document describes research sponsored by EPRI.

The publication is a corporate document that should be cited in the literature in the following manner:

Inspection & Assessment of Overhead Line Conductors: A State-of-the-Science Report, EPRI, Palo Alto, CA: 2000 1000258.

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ABSTRACT

Without timely detection and mitigation, conductor deterioration can lead to expensive failures and line outages. Many utilities in moist and aggressive environments experience moderate to severe corrosion of steel core wire ACSR conductors. Conductor vibration due to wind can cause broken conductor strands in and around conductor attachment points and hardware. Both types of deterioration are very difficult and expensive to detect and correct. This project seeks to develop more reliable and cost-effective technologies for conductor inspection and assessment. The first step in this development is the establishment of the current state-of-the-science for conductor inspection and assessment, which is the subject of this report. The report includes the results of a survey that was conducted with twenty-three utilities to document their recent experience with conductor failures and their standard practices for line inspection and conductor maintenance. This survey indicated that conductor failures are a growing problem among utilities and that a more economical and reliable inspection technique is needed. A literature search was conducted on general non-destructive evaluation (NDE) methods, with emphasis on methods that are either in use, in development, or appear promising for future development for application to transmission line conductors. A detailed overview is provided for the one NDE technology that is currently available commercially, as well as two other technologies that are currently under development and field-testing. The combination of these three technologies looks promising as an approach to provide a complete conductor inspection package. The conclusion is that continued efforts in the near future should be focused on bringing development of these new technologies to completion and on combining the three technologies into a single inspection offering.

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CONTENTS

1 INTRODUCTION .....................................................................................1 Background................................................................................................................ 1 Objectives .................................................................................................................. 1 Approach ................................................................................................................... 2

2 UTILITY SURVEY....................................................................................3 Utility Survey Results ................................................................................................. 3 Failure Experiences ................................................................................................... 3 Inspection Methods.................................................................................................... 3 Need for NDE Technique........................................................................................... 4 Conclusion ................................................................................................................. 4

3 CURRENT AND POTENTIAL CONDUCTOR INSPECTION TECHNOLOGIES .......................................................................................6

Introduction to NDE Technologies ............................................................................. 6 Cormon OHLCD Technology ..................................................................................... 7 Current Research & Development Projects ............................................................... 8 Magnetostrictive Sensor ............................................................................................ 9 Neutron Radiography................................................................................................. 9 Ultrasonic Guided Lamb Waves............................................................................... 10 Electromagnetic Wire Rope Tester .......................................................................... 10 Time Domain Reflectometry (TDR).......................................................................... 10 BIBLIOGRAPHY ...................................................................................................... 11

4 CORMON OVERHEAD LINE CORROSION DETECTOR.....................14 Background.............................................................................................................. 14 Aging of ACSR Conductors ..................................................................................... 14 Cormon OHLCD....................................................................................................... 16

Measurement principle ....................................................................................... 16 Description.......................................................................................................... 16 The Sensing Head.............................................................................................. 16 The Trolley.......................................................................................................... 18 Calibration and Limits ......................................................................................... 18 General Results.................................................................................................. 19

Cormon Results Validation ...................................................................................... 19 Correlation Cormon results and life duration of the conductor ................................. 21

Tensile Strength Test on Aluminium Strands...................................................... 22

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Alternate Bending Test ....................................................................................... 22 Torsional Ductility Tests ..................................................................................... 23

EDF experience with CORMON OHLCD................................................................. 24 Scope of use....................................................................................................... 24 Choice of spans for inspection............................................................................ 25 Use and interpretation of results......................................................................... 25 Feedback............................................................................................................ 25 Evolution of the corrosion ................................................................................... 29

Decision Tree for Application of Cormon OHLCD.................................................... 31 Conclusion ............................................................................................................... 32

5 THE CROSS-CHECKER TRANSMISSION CONDUCTOR INSPECTION DEVICE ....................................................................................................33

Background.............................................................................................................. 33 Principles of Operation............................................................................................. 33 Preproject Testing.................................................................................................... 35 Plans For Development of the CC ........................................................................... 37 Results From Initial Project Testing ......................................................................... 37

Bench Testing of New, Deliberately Defected Conductor................................... 37 Controlled Speed Testing of Tensioned, Undefected Conductor........................ 40 Controlled Speed Testing of Tensioned, Defected Conductor............................ 42

Conclusion To Date ................................................................................................. 43 Future Development Activities and Schedule .......................................................... 43

6 ELECTRO-MAGNETIC ACOUSTIC TRANSDUCERS (EMAT) FUNDAMENTALS AND APPLICATIONS................................................44

Introduction .............................................................................................................. 44 Acoustic Transducers .............................................................................................. 45

Longitudinal Wave.............................................................................................. 45 Lamb Wave ........................................................................................................ 46 Shear Wave........................................................................................................ 46

Shortcomings of Ultrasonic Measurements ............................................................. 47 Fundamentals of EMATs ......................................................................................... 47

Basic Principles of EMATs [5]............................................................................. 48 EMAT Design Considerations.................................................................................. 51

System Configuration ......................................................................................... 51 EMAT Coils......................................................................................................... 53

Magnetic Field Source ............................................................................................. 56 EMAT Applications................................................................................................... 56

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MagnaScan™..................................................................................................... 57 Projectile Inspection: .......................................................................................... 57

Electric Power System Applications of EMATs ........................................................ 59 Detailed Description of the Sensor System:............................................................. 60 Summary and Conclusions ...................................................................................... 68 References .............................................................................................................. 69

7 SUMMARY AND CONCLUSIONS ........................................................71 Summary ................................................................................................................. 71 Conclusions and Recommendations........................................................................ 72

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1 INTRODUCTION

Background

The majority of overhead transmission lines is constructed of multiple strands of Aluminum Conductor Steel Reinforced (ACSR) conductor. These are composite conductors consisting of mostly aluminum because of its favorable resistance to weight ratio, and with a core made of steel because of its favorable strength to weight ratio. There are two major problems that decrease the integrity of ACSR conductors. They are corrosion and broken strands. Corrosion is a phenomenon that causes deterioration and breakage of the aluminum and steel strands through various mechanical and electrical methods. The two types of corrosion resulting from this kind of behavior are external and internal corrosion. Broken strands are the result of severe damage to the aluminum layer of the conductor.

There are two principal corrosion mechanisms. One is pitting corrosion, which results from corrosion of the outer strands due to a heavily polluted environment. The other is galvanic corrosion of the inner steel strands, which results in strand breakage and consequential line failure. It is caused by aqueous solution containing chloride ions that penetrate between aluminum strands of the conductor and attack the galvanizing of the steel core.

There are three methods to measure corrosion damage. The mechanical method measures the corrosion that causes deterioration to the mechanical strength of a conductor by determining how much of its ultimate strength has been reduced. The electrical resistance method measures the change in resistance of an element as it corrodes in a process environment. This action of corrosion tends to decrease the cross sectional area, therefore increasing the electrical resistance [1]. Finally, the weight loss method involves exposing a material to an environment for a given duration, and measuring the resultant weight loss.

Several environmental factors influence aluminum strand breakage. Weather conditions such as wind loading, ice loading, and conductor vibration may result in strain and stress exceeding the material strength of the conductor. The stresses induced by wind result from a number of different modes of excitation. First, aeolian vibration results from relatively mild winds incurring vortex shedding on the conductor at frequencies near the natural frequency of the line [2]. This results in fretting which can lead to wire failure or conductor breakage at suspension clamps. Aeolian vibration is generally dealt with by using vibration dampers. Next, galloping is an infrequent natural phenomenon in which a bluff body reaches aerodynamic instability, resulting in high-amplitude, low-frequency oscillations [3]. Anti-galloping devices exist and have been successfully deployed in a number of cases. Finally, strong turbulent winds may cause waked-induced oscillations. For all of these cases, the oscillations and movement may cause enough stress to break the strands of the conductor, or may result in fatigue over time. In extreme cases, this could result in catastrophic failure.

Objectives

The ultimate goal of this project is to provide utilities with dependable and economical technologies for assessment the condition of their in-service conductors. The first step to reaching this goal is to

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establish the state-of-the-science. The first objective is to document the recent experience of utilities with damage or failures of transmission line conductors and the techniques currently being used by utilities to detect and repair broken or damaged conductors. These objectives have been addressed through a utility. Other objectives are to identify currently available non-destructive and non-invasive technologies for inspecting conductors and detecting broken/damages aluminum or steel strands. Also, current available non-destructive technologies from other industries that have potential application for the inspection of transmission line conductors will be identified. These objectives have been addressed through a literature search and input from industry consultants. Finally, a formal report documenting and a proposed research and development plan for advancing the state-of-science for conductor inspection, assessment, and mitigation will be prepared.

Approach

To accomplish the objectives for the project, a survey was first sent out. The purpose of this utility survey was to document recent experience of utilities with damage or failures of transmission line conductors and techniques currently being used by utilities to detect and repair broken or damaged conductors, splices, and shield wires. It was also used to document the current inspection techniques used by utilities for the inspection of overhead transmission line conductors. Next, an extensive literature search was done to identify current non-destructive technologies for accessing ACSR conductor damage, current R&D activities, and current technologies from other industries that could be modified for ACSR applications. This research was done using the Internet and other library resources. Information was also gathered from technical experts who are engaged in research and development in particular areas of interest to this project.

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2 UTILITY SURVEY

Utility Survey Results

In order to measure the overall need for a non-destructive device for inspecting conductors and detecting broken aluminum strands or corroded steel strands, a survey was sent to various utilities. The purpose of this utility survey was to document recent experiences of utilities with damage or failures of transmission line conductors and the techniques currently being used by utilities to detect and repair broken or damaged conductors, splices, and shield wires. It was also used to document the current inspection techniques used by utilities for the inspection of overhead transmission line conductors. There were twenty-three utilities that responded to the survey. Detailed survey results are given in Appendix A.

Failure Experiences

The results from the survey revealed that within the past 5 to 10 years most utilities experienced an average of four conductor failures, three splice/connector failures, and seven shield wire failures. The majority of conductor failures resulted from broken aluminum strands of older transmission lines. Vibrations in the line caused most broken strand damages. The failures that corrosion damage caused were the result of rust, brittleness, and deterioration. It was also discovered that overheating due to improper insulation of splices was also the major cause of most splice failures. Most utilities agreed that in these cases, the grease was not properly inserted or the steel sleeve was somewhat off-centered. The majority of the shield wire failures were the result of lightning damage. Other factors influencing failures included gunshots, post insulator failure, mid span fatigue, trees, airplanes, helicopters, ice loading, and bad weather-thunderstorms, tornadoes, and cyclones. There appeared to be no obvious trend in comparing the conductor size and type to the frequency of these failures. However, conductor failures in 69kV lines were very common, perhaps because of the large quantity of lines at that voltage class.

Inspection Methods

It was discovered from the survey that the most common inspection techniques used by utilities for overhead transmission line inspections are aerial patrol, infrared, visual, and climbing when required. Helicopter inspections are normally done twice per year, infrared inspections are done once per year, and foot inspections are done once every three years. These inspection frequencies do vary with the type of conductor and the line voltage of the conductors being inspected. The present cost for these inspections vary from approximately $20 to $100 per mile. Utility personnel, contractors, or a combination of both do these inspections. Some utilities do have written procedures that are used as a guideline for the inspectors of their overhead transmission lines. These procedures specify how to identify defects such as burns, rust, discoloration, broken or missing strands, and any other signs of damage for the conductors, splice/connectors, and shield wires.

Depending on the severity of the damage, repairs are normally made. To repair damaged conductors that may led to severe failures utilities typically install armor rods over areas with broken aluminum strands, extend dead-end assembly to capture damaged areas within jumpers, use repair sleeves,

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insert line guards and full tension splices, or install vibration dampers. In addition, preformed strand splices in shield wires are sometimes replaced with compression crimp connectors. Some other scenarios may also occur once damage is detected. If a large number of broken aluminum or steel core strands exist, then a mid-span joint may be installed. Damaged strands of sleeves or conductors are cut and replaced with a splice. Hardware that shows wear and damaged splices are also replaced.

There is no consistent computer software used by utilities for the storing of inspection information. Computer software presently being used by utilities include Microsoft Word, MP2, PLT, Maximo, EPRI’s Transmission Inspection & Maintenance (TIM) System, Microsoft Excel, PASSPORT, Microsoft Access, SAP, and Oracle databases. The usage of these software packages varies greatly among individual utility companies.

Need for NDE Technique

Many utilities agree that conductor, splice/connector, and shield wire failures are a current and impending problem. However, they are concerned about the cost associated with the development of a new non-destructive technology for conductor inspection. Several utilities agree that they are seeking to get the “biggest bang” for their buck. Their primary goals are to reduce their repetitive manual activities and maintain a high level of availability while returning the highest return possible to their investors. Although present inspection methods are highly subjective and costly, utilities are still under great pressure to reduce operating costs. And conductor failures increase operating costs substantially. Therefore, cost effective and dependable methods of inspection, assessment, and repair are needed.

Utilities also fear that increasing operating temperatures may cause some severe failures in their older transmission lines because most fittings were not manufactured to withstand high temperatures. This is a valid concern because reliability indexes must be maintained to avoid economic penalties from regulatory agencies. As a precautionary measure, circuits that have been upgraded must also be checked to ensure conductor integrity. Based on these concerns, there is clearly a need for a diagnostic technology that can be used to give an assessment of conductor damage.

Most utilities agree that the costly infrared technique has identified some potential failures in overhead transmission lines. However, it has not always given them reliable information. In several cases where connections have shown up hot, an explanations of why could not be made. The reliability of IR inspections depends solely on the experience of the thermographer. Some feel that there is a need for a non-destructive corrosion and broken strand detector because current inspection techniques are difficult to coordinate due to the need to de-energize the line to inspect under suspension clamps. There is also a need for a device that could give information to predict imminent hardware failures in clamps, dead-ends, and joints, and predict the remaining life of the conductor. This kind of a non-destructive device would be very useful to improve the overall productivity of the inspection maintenance program.

Conclusion

A non-destructive device for conductor inspection and condition assessment would have to be very cost effective. For this technology to be most useful to utilities, it would have to detect broken aluminum and steel strands, identify damage to splices/connections and shield wires, evaluate shoe and dead end fittings, assess damage under spacers and marker balls, show operating temperature changes affects, and be user friendly. The accuracy of such a device is also a major concern because this technology must completely satisfy utilities present inspection requirements. This technology

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would have to give similar or better results than present techniques used, such as, helicopter, visual, and infrared inspections. Infrared is the only non-destructive technique being used now to effectively assess damage. Most utilities are somewhat satisfied with the results it gives, but many are concerned about inaccuracies they’ve observed with it. Large utilities also have different problems and economies than smaller ones. Long transmission lines, through varied climatic conditions have more potential problems than shorter length lines. The ultimate non-destructive device would be one that could detect damaged or broken aluminum and steel strands, predict the remaining life of the conductor, and economically do this for the entire length of a transmission line.

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3 CURRENT AND POTENTIAL CONDUCTOR INSPECTION TECHNOLOGIES

Introduction to NDE Technologies

Non-destructive evaluation is a descriptive term used for the examination of materials and components in such a way that allows materials to be examined without changing or destroying their usefulness to assure safety and reliability. NDE requires an understanding of various methods available, their capabilities and limitations, knowledge of the relevant standards, and specifications for performing the tests. Products and equipment that fail to achieve their design requirements or projected life due to undetected defects may require expensive repair or early replacement. The method used for non-destructive evaluation depends on the physical properties of the material. NDE is used for accident prevention, cost reduction, improvement of product reliability, to determine acceptance for a given requirement, and to give information on repair criteria [4].

Some common NDE methods that will be discussed in this report include visual inspection, liquid penetration inspection, acoustic emission, magnetic particle inspection, eddy current inspection, ultrasonic inspection, infrared thermography, and radiography. These are considered fundamental NDE techniques. Below is a brief description of each.

Visual inspection is the one NDE method used extensively to evaluate the condition or quality of a component. It is easily carried out, inexpensive and usually doesn’t require special equipment. It requires good vision, good lighting, and the knowledge of what to look for. Visual inspection can identify where a failure is most likely to occur and identify when failure has commenced. It is often enhanced by other surface methods of inspection that can identify defects that are not easily seen by the eye.

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a fluorescent dye from the flaw [4]. This technique is based on the ability of a liquid to be drawn into a clean surface-breaking flaw by capillary action. The fundamental purpose of penetrant testing is to increase the visible contrast between a discontinuity and its background.

Magnetic particle inspection is methods can be used to find surface and near surface flaws in ferromagnetic materials such as steel and iron [4]. This technique uses the principle that the magnetic lines of force will be distorted by the presence of a flaw in a manner that will reveal the location of the flaw. The flaw is located because the flux leakage follows the application of fine iron particles to the area under examination.

Acoustic emission monitoring involves listening to sounds that are usually inaudible to the human ear [4]. This technique involves attaching one or more ultrasonic microphones to the object and analyzing the sounds using computer based instruments. Noises may arise from friction, crack growth, turbulence, and material changes such as corrosion. Applications include testing pipelines and storage tanks, fiberglass structures, rotating machinery, weld monitoring, and biological and chemical changes.

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Eddy current testing is an electromagnetic technique that can only be used on conductive materials. Eddy currents are induced into a conductor by reacting with alternating magnetic fields that cause them to be circulated and oriented perpendicular to the direction of the applied magnetic field [5]. When eddy currents are distorted by the presence of the flaws or material variations, the impedance in the coil is altered. This change is measured and displayed in a manner that indicates the type of flaw or material condition. Its application ranges from crack detection to the rapid sorting of small components for flaws, size variations, or material evaluation

Ultrasonic inspection uses sound waves of short wavelengths and high frequencies to detect flaws or measure material thickness [4]. It is used on aircraft, in power generating plants, and on welds in pressure vessels at oil refineries or paper mills. Usually, a hand-held transducer that is placed on the specimen displays these pulsed beams of high frequency ultrasound. Any reflected sound returns to the transducer, like an echo, and is then shown on a screen that gives the amplitude of the pulse and the time taken to return to the transducer. The distance of the acoustical impedance flaw can be determined if the velocity of the sound in the test material, and the time taken for the sound to reach and return from the flaw.

Infrared and thermal methods for nondestructive evaluation are based on the principle that heat flow in a material is altered by the presence of some types of anomaly [5]. These changes in heat flow cause localized temperature differences in the material. Infrared denotes the radiation between the visible and microwave regions of the electromagnetic spectrum.

Radiographic inspection is a photographic record produced by the passage of electromagnetic radiation such as x-rays or gamma rays through an object unto a film [5]. When film is exposed to x-rays, gamma rays, or light, an invisible change called a latent image is produced in film emulsion. The areas exposed become darker when the film is immersed into a developing solution. Radiography is used to inspect a variety of nonmetallic parts for porosity, water entrapment, crushed core, cracks, and to inspect metal products such as welds, castings and corrosion, inclusions, debris, loose fittings, and rivets.

The NDE techniques discussed above are the fundamental techniques used in the development of nondestructive technologies from other industries. More specific applied technologies will be discussed in the remainder of this chapter. This discussion will begin with the a few technologies that have already been applied to transmission line conductors. Then we will discuss some other technologies that were given some significant consideration for future conductor inspection development.

Cormon OHLCD Technology

Presently, there is only one device that has been proven to give a nondestructive assessment of ACSR steel corrosion damage. The Overhead Line Corrosion Detector (OHLCD) is a corrosion detection device developed by the National Grid Company (NGC) in the United Kingdom to measure the amount of corrosion of the steel core by using eddy currents.

OHLCD consists of a split hollow-cylinder, sensing-heads that clamp around the conductor and is powered by a battery-powered electronic unit. When the head is closed, it forms two pairs of coils around the conductor. The first coil is fed with a high-frequency (77 kHz) current to produce a magnetic field that penetrates the conductor and induces eddy current around the individual strands. The second pair of coils senses the high-frequency flux within it by inducing a voltage into the pick-up coil. If the cross section of the aluminum has been reduced, more flux will thread this sensing coil. The behavior of the galvanized steel is more subtle because of the magnetic permeability of the

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steel. While intact, the zinc layer effectively hides the underlying steel. However, if some of the zinc is removed, marked changes occur in the phase as well as the amplitude of the output of the sensing coil [39]. Therefore, the loss of galvanizing can be detected by inducing eddy currents into the conductor and measuring the induced voltage in the pick-up coil. The electronic unit splits the signal into its in-phase and quadrature components so that loss of galvanizing is readily detected even though the zinc accounts for only a very small fraction of the cross section of the conductor.

Early goals of the detector were to provide early warning of corrosion damage, enable planned conductor replacement, minimize unexpected failures due to corrosion, reduce revenue loss, reduce emergency repair cost, reduce power outages, increase customer satisfaction, operate on any size ACSR conductor, and operate up to 225 kV. Several factors prevented the NGC from completely reaching all of their goals. For example, internal corrosion is a major factor limiting the life of steel reinforced aluminum conductor (ACSR) and a crucial stage in the corrosion process in the loss of zinc from the central galvanized steel strands. Once this galvanizing is lost, the aluminum strands are subject to galvanic corrosion and the conductor deteriorates rapidly. The effects of this form of internal corrosion are not visible or detectable until the conductor is near failure stage. The major problem with this device was that the loss of galvanizing of the steel detected was at late stages of the conductor’s remaining life. This has a devastating effect because the loss of galvanizing is normally followed by rapid corrosion of the aluminum strands. On the other hand, the measurement only indicates condition of the zinc coating on the steel strands and is not a direct measurement of the remaining steel core cross-section.

The original intention of the device was beneficial to utilities because the real cost benefit of this condition assessment strategy derives from the ability to plan maintenance and replacement. Unplanned outages and emergency repair lose revenue and eat into maintenance budgets. The hard data on conductor condition produced by OHLCD survey allows one to evaluate the condition of the network, allocate priorities for replacement, or repair and implement work to a planned timetable, reducing cost and enhancing quality.

A more detailed review of this technology is given in Chapter 4 of this report.

Current Research & Development Projects

While the OHLCD is the only currently available commercial technology for conductor inspection, there are two other technologies currently under development. These technologies are both being advance through EPRI Tailored Collaboration (TC) projects with various utility sponsors. Each will be briefly summarized here and in more detail in a later chapter.

The first technology under development is called Cross-Checker. The Cross-Checker (CC) is a device that works in a similar fashion to the Cormon OHLCD, but rather than detecting loss of galvanizing on the steel core it detects actual loss of steel core cross-section or broken steel strands. This is a patented technology that was developed by a former utility employee based on his experience with the problem of conductor core corrosion at his utility. A currently active TC project with one utility sponsor is funding the advancement of the Cross-Checker to a first field-deployable prototype and the initial field evaluation of this technology on the utility’s transmission lines. This technology and project is discussed in detail in Chapter 5 of this report.

The second technology under development is the application of Electromagnetic Acoustical Transducers (EMAT) to the inspection of conductors. EMAT is a technology that is a mature application in many other areas of non-destructive evaluation, particularly in power plants. A past EPRI project developed an application of this technology to determine the integrity of stranded

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copper ground mat risers in substations. Now this application is being expanded in an attempt to determine the condition of overhead transmission line conductors through a new TC project with five utility sponsors. This project is currently focused on the problem of non-destructively and non-invasively detecting broken and damaged aluminum strands within the zone around the attachment of a conductor to a support tower, including within the armor rods and suspension clamp. This technology and project is discussed in detail in Chapter 6 of this report.

Magnetostrictive Sensor

The magnetostrictive sensor is a non-destructive evaluation tool developed and patented by Hegeon Kwun at the NDE Science and Technology Division of Southwest Research Institute in San Antonio, TX, to detect fractured wires in ferromagnetic steel cables. By using low frequency mechanical waves, from approximately 10 to 100 kHz, physical assessments of the individual wires of a cable can be made without using direct physical contact [6]. This method is widely used in the civil construction industry to evaluate steel cables, which are the critical load-carrying members of these structures and also for detection of defects in pipelines.

With the magnetostrictive sensor, mechanical or elastic waves can be transmitted and detected in ferrous materials such as steel cables without physical contact. These generated waves propagate in either direction along the length of the cable. When the elastic wave passes through the area encircled by the receiving coil, the magnetic induction of the material changes. These changes in magnetic induction induce an electric voltage signal in the receiving coil via the Faraday effect. These defects are then amplified, filtered, and digitized to be later processed, displayed, and stored using a personal computer.

Magnetostrictive sensing techniques are very sensitive to corrosion-type defects can inspect a long segment of piping from a single sensor location [7]. Its wide-band frequency response characteristics make it possible to use the technology a low frequencies to cover a long range or at high frequency to achieve better resolution over a small area.

Magnetostrictive sensors provide at least some promise as a potential inspection technology for ACSR conductors. However, since it is a magnetic technique its value will likely be limited to inspection of the steel core. We currently have two existing conductor inspection technologies for the steel core in the OHLCD and the CC. However, each of these technologies currently inspects only the conductor zone that is positioned within the immediate vicinity of the transducer. The magnetostrictive sensor offers the additional potential for inspecting physically inaccessible areas such as splices, deadends, and joints.

Neutron Radiography

Neutron radiography is an analogy to x-ray radiography in that a beam of radiation is used to create images of objects. This technique uses a beam of penetrating radiation to interrogate an object and generate an image that allows visualization of different areas. The Phoenix Memorial Laboratory at the University of Michigan, is the world leader in both neutron radioscopy (for moving objects) and high-resolution neutron radiography (for static objects). Neutron radiography is commonly used today in the aircraft industry on honeycomb parts bonded to aluminum surface skins that are susceptible to corrosion. Since this technology requires very expensive equipment and materials (e.g. film), and would likely be limited to single images of short conductor segments, its practical application to conductors is doubtful.

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Ultrasonic Guided Lamb Waves

Ultrasonic Guided Lamb Waves is a non-destructive inspection technique used to detect corrosion on aircraft structures. Ultrasonic guided Lamb waves have demonstrated an attractive solution for disbonding and corrosion detection in relatively thin plates due to their guided behavior. Lamb waves work very well for nondestructive evaluation of adhesively bonded plate structures. The velocities of Lamb waves are a function of the frequency-thickness product [28]. Therefore, this nondestructive ultrasonic technique is based on velocity change, attenuation, and back scattering due to anomalies in the thickness of the material. The transducer emits a short ultrasonic pulse and listens for the echo. If the part has no flaws, echoes are returned only from the near and far surfaces. If a part has a defect, and additional defect echo is returned, which is received by the transducer as a signal between the echoes produced by the part surfaces. Since the primary application of this technology seems to be for plate structures, it’s application to stranded conductors may be a major development project.

Electromagnetic Wire Rope Tester

Electromagnetic Wire Rope tester is a computer-controlled dual function device used for NDE of wire ropes. It operates by subjecting the rope with magnetic flux and then measuring three values: (1) the leakage flux around a local fault, (2) the change in the rope’s magnetic impedance due to a loss in magnetic cross sectional area, and (3) the change in the value of the magnetic circuit of the sensor head [30]. The wire rope is suitable for rope sizes from 13-16 mm [30]. It has a test head that encircles the rope. A constant flux magnetizes a length of rope as it passes through the test head. Changes in the metallic cross-section cause variations in the constant magnetic field, which are sensed and electronically processed to produce an output voltage that is proportional to those changes in the cross-section [32]. Changes in the magnetic flux leakage created by a discontinuity in the rope (a broken wire or a pit in a wire from corrosion) are sensed, processed, and displayed. This wire rope tester provides at least some promise as a potential inspection technology for ACSR conductors. However, since it is a magnetic technique its value will likely be limited to inspection of the steel core. We currently have two existing conductor inspection technologies for the steel core in the OHLCD and the CC.

Time Domain Reflectometry (TDR)

Time-Domain Reflectometry (TDR) is a nondestructive pulse-echo method used for the detection of faults in transmission lines. This technique could be used to monitor, over time, the resistance degradation of transmission line splices and conductors. Such data could be useful for locating critical lines that may need extra attention or detailed inspection.

TDR is a simple technique for impedance measurement and for testing cables and connectors. It operates on the principle of determining the characteristic impedance of a transmission line or quantifying reflections caused by discontinuities along or at the termination of a transmission line. Whenever energy is transmitted through a medium, it encounters a change in impedance, and some of the energy is reflected back toward the source. The amount of energy reflected is a function of the transmitted energy and the magnitude of the impedance change. TDR is performed using a pulse generator to generate a signal and an oscilloscope to observe the characteristics of the pulse as it leaves the generator, as well as those that are reflected back from the circuit being tested. TDR measurements are based on the ratio of incident to reflected voltage. If any discontinuities are present in the line, either faults or permanent discontinuities, a voltage wave reflected at the discontinuity is found in the line response.

11

TDR may be an effective way to get an overall indication of the condition of a conductor on a circuit, or at least an indication of the relative condition of two adjacent conductors. However, it is doubtful that the technology would be effective at finding detailed damage to conductors, such as broken strands at a specific location. There are many discontinuities (suspension clamps, armor rods, splices, dampers, spacers, marker balls, etc.) along the length of a transmission line conductor that are likely to cause anomalies in the signal of greater magnitude that a broken or damaged strand at this or that specific location. Nevertheless, there is probably value in exploring this technology as a general assessment of the overall or relative conductor condition.

BIBLIOGRAPHY

[1]. Lewis, K.G. and Sutton, J., “Detection of Corrosion in ACSR Overhead Line Conductors,” Distribution Developments, 1985.

[2]. Conductor Fatigue Life Research, Report EL-4744, Electric Power Research Institute, Palo Alto, CA, 1987.

[3]. R. Zdero, and D. G. Harvard, “Toward Understanding Galloping: Near-Wake Studying of Oscillating Smooth and Stranded Circular Cylinders in Forced Motion,” Experimental Thermal and Fluid Science, pp. 28-43, 1995.

[4]. Non-destructive Testing, NDT Homepage, http://www.winzurf.co.nz/ndta/ndtint05.htm.

[5]. Alahi Uddin Khan, “Non-destructive Testing Applications in Commercial Aircraft Maintenance,” ECNDT, Vol. 4, No. 6, June 1999.

[6]. Kwun, Hegeon and Teller, Cecil, “Detection of Fractured Wires in Steel Cables Using Magnetostrictive Sensors”, Materials Evaluation, 1994, pp. 503-507.

[7]. Kwun, H., Hanley J., and Holt A., “Detection of Corrosion in Pipe Using Magnetostrictive Sensor Technique”, SPIE-The International Society for Optical Engineering, Vol. 2459, pp. 140-148.

[8]. Kwun, H., Hanley, J., and Bartels, K., “Recent Developments in Nondestructive Evaluation of Steel Strands and Cables Using Magnetostrictive Sensors”, IEEE Transaction on Power Delivery, pp. 144-148.

[9]. Kwun, H., and Bartels, K., “Magnetostrictive Sensor Technology and Its Applications”, Ultrasonics, 1998, pp. 171-178.

[10]. Kwun, Hegeon, “Back in Style: Magnetostrictive Sensors”, Technology Today, March, 95, pp. 2-7.

[11]. T.R. Schmidt, “The Remote Field Eddy Current Inspection Technique”, Materials Evaluation, Vol. 42, pp. 225-230, 1984.

[12]. D.L. Atherton, D. D Macintosh, S.P. Sullivan, and T.R. Schmidt, “Remote Field Eddy Current Signal Representation”, Materials Evaluation, Vol. 51, pp 782-789, 1993.

[13]. Applied Magnetics Home Page, Queen’s University, http://physerver.phy.queensu.ca/wwwhome/atherton/applied_magnetics.html.

12

[14]. S. Mitra, P. Urali, and J.C. Moulder: “Eddy Current Measurements of Corrosion-Related Thinning in Aluminum Lap Splices”, Review of Progress in Quantitative NDE, Vol. 12, pp. 2003, New York, 1992.

[15]. A.O. Solumsmo, “Oil Major Evaluates FSM Technology”, CorrOcean News, No. 3, 1999.

[16]. E. Breakey, “Apache Energy Quantify FSM Savings”, CorrOcean News, No. 2, 1999.

[17]. The Field Signature Method, CorrOcean Home Page, http://nettvik.no/naeringsparken/fsm/fsm.html.

[18]. “Ultrasonic Inspection of Thin Walled Composite Tubes,” by T.E. Michaels, T.M. Krafchak, and B.D. Davidson, Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12, 1993.

[19]. “Ultrasonic Imaging of Impact Damaged Composite Panels,” by B.D. Davidson, Acoustical Imaging, Vol. 19, 1992.

[20]. Nondestructive Testing Division, Panametrics Home Page, http://www.panametrics.com/div_ndt/pages/theory/index.shtml.

[21]. Ailton F., Arnaldo de A., and Verginia R., “Aluminum Corrosion Detection by Using a Neutron Radiographic Image Analyzer”, IEEE International Conference On Image Processing, 1, 1994.

[22]. Neutron Radiography, Michigan Memorial Phoenix Project Home Page, http://www.umich.edu/mmpp/services/neutradiography/nrad.htm1.

[23]. A. Pilarski, “Ultrasonic Evaluation of the Adhesion Degree in Layered Joints,” Material Evaluation, Vol. 43, No. 5, May 1985 pp. 765-770.

[24]. C.C.H. Guyott, P. Cawley, and R.D. Adams, “The Nondestructive Testing of Adhesive Bonded Structures: A Review”, Journal of Adhension, Vol. 20, No. 2, 1986, pp. 129-159.

[25]. J.L. Rose, JJ. Ditri, and A. Pilarski, “Lamb Waves for Aircraft Bond Inspection,” Italian Journal of Nondestructive Testing, Vol. 15, No. 4, Dec. 1994.

[26]. Meeker T.R. and A.H. Meitzler, “Guided Wave Propagation Elongated Cylinders and Plates,” Physical Acoustics, Vol. 1 Part A, 1964, pp. 111-167.

[27]. J.L. Rose, Dale Jiao, and Jack Spanner, Jr., “Ultrasonic Guided Wave NDE for Piping,” Material Evaluation, Nov. 1996.

[28]. V. Mustafa, A. Chahbaz, D.R. Hay, M. Brassard, and S. Dubois, “Imaging of Disbonds in Adhesive Joints with Lamb Waves,” Nondestructive Evaluation of Materials and Composites, SPIE Vol. 2944, Dec. 1996, pp. 87-97.

[29]. D.A. Hutchins, D.P. Jansen, and C. Edwards, “Lamb-wave Tomography Using Non-contact Transduction,” Ultrasonics, Vol. 31, No. 2, June 1993, pp. 97-103.

[30]. Geller, L.B., D. Poffenroth, J.E. Udd, and D. Hutchinson, “Evaluation of Electromagnetic Rope Testers: Joint Canadian-US Work”, Materials Evaluation, 1992, Vol. 50, No. 1, pp. 56-63.

[31]. Geller, L.B., K. Leung, and Kitzinger, “Computerized Operational Control of an Electro-magnetic Wire Rope Tester,” Materials Evaluation, 1995, Vol. 53, No. 9, pp. 1002-1006.

13

[32]. Poffenroth, Dennis N., “Nondestructive Testing of Elevator Suspension and Governor Ropes,” Elevator World, 1996, pp. 77-88.

[33]. H.R. Weischedel, “The Inspection of Wire Ropes in Service: a Critical Review,” Material Evaluation, 1985, Vol. 43, pp. 1592-1605.

[34]. “Mathematical Aspect of TDR-Based Discontinuity-Location in Transmission Lines,” by Tomo Bogataj, 17th International Conference ’95, June 13-16, 1995, Pula, Croatia.

[35]. “Controlled Impedance and Time Domain Reflectometry (TDR),” by Bob Neves, Circuitree, September 1998, pp. 32-36.

[36]. “TDR for Component Measurement,” by Simon Harpham, Electronics World+Wireless World, June 1990, pp. 497-500.

[37]. “Measuring Parasitic Capacitance and Inductance Using TDR,” by David Dascher, Hewlett-Packard Journal, April 1996, pp. 83-96.

[38]. “Measuring Controlled-Impedance Boards with TDR,” by Mark D. Tilden, PC FAB, February, 1992, pp. 98-104.

[39]. “Some Investigations of the Aging of Overhead Lines,” by B.J. Maddock, J.G. Allnutt, J.M. Ferguson, and K.G. Lewis, Internal Conference on Large High Voltage Electric Systems, August 27-September 4, 1986.

[40]. “Overhead Transmission Lines-Refurbishment and Developments,” by J.M. Ferguson and R.R. Gibbon, Power Engineering Journal, June 1994, pp. 109-118.

14

4 CORMON OVERHEAD LINE CORROSION DETECTOR

Background

In 1988, a major incident on a 225 kV line near Nantes whose conductors (ACSR, 301 and 294 mm² sectional area) dated from 1936 led Electricité de France (EDF) to ask questions about the condition of the ACSR cables on its network. Although visual inspection revealed nothing, it became evident that the conductors were subject to internal corrosion that caused gradual loss of their mechanical and electrical characteristics. EDF then looked for non-destructive methods, without dismantling, for preventive detection of internal corrosion of ACSR conductors in order to get a better picture of its network and be able to schedule the necessary refurbishment before incidents occur.

EDF thus contacted Cormon, the company in charge of marketing such a tool developed by the United Kingdom’s CEGB (Central Electricity Generating Board) in the early 1980s. This detector will be referred to as the Cormon OHLCD (Overhead Line Corrosion Detector) in what follows.

The experiments conducted with the detector on the French network in 1989 and 1990 were conclusive, and the detector was adopted in 1990. Since then it has been used regularly on a large part of the national grid.

After briefly describing the process of ACSR conductor aging, this paper presents the Cormon Overhead Line Corrosion Detector, the validation of the results obtained with it, and an analysis of nine years of use in France.

Aging of ACSR Conductors

Corrosion is an important factor in the reduction of the service lifetime of conductors. It is principally due to industrial pollution or marine environments.

When industrial pollution is severe, it can sometimes attack the outside of the conductors and rust can be detected at visual inspection, but the loss of cross-sectional area is generally insignificant. More often, internal corrosion attacks ACSR conductors, giving no external signs of damage until the conductor fails.

Due to ambient humidity, corrosive molecules penetrate the conductor to attack the protective coating of zinc on the steel strands. The gradual disappearance of the galvanization sets up contact electromagnetic field (e.m.f.) between the steel and the aluminum, which is gradually eaten away. As the cross-sectional area of aluminum decreases, there is first an increase in power transmission in the other aluminum strands, and eventually current passes through the steel strands, which accelerates the phenomenon of corrosion and considerably shortens the service lifetime of the conductor.

This corrosion also diminishes the strength of the conductors. The loss of strength gets greater as the phenomenon progresses and generally leads to conductor failure.

The following two diagrams give a simple illustration of how the phenomenon works.

15

Moisture + Pollution + Salt

Surface corrosion

Aluminium strands

AlCOH

AlCOH+AlCl3

Grease Degraded grease +AlCOH

Galvanizing Steel corrosion

Steel strand

Figure 4-1 Corrosion process for ACSR conductor

Loss of tensile strength

0%

5%

10%

15%

20%

25%

30%

35 y

ears

30 y

ears

25 y

ears

20 y

ears

15 y

ears

10 y

ears

5 ye

ars 0

conductor age

% lo

ss o

f m

ech

anic

al s

tren

gth

grea

se d

egra

datio

n

chlo

rides

agr

essi

on

galv

aniz

ing

corr

osio

n

Aluminium corrosion

Current migating to steel

____

____

____

____

____

____

____

__

____

____

____

____

____

____

____

__

____

____

____

____

____

____

____

__

____

____

____

____

Figure 4-2 Loss of tensile strength versus time

Experience shows that the degradation can be slowed down substantially if the conductors are greased. When talking about constants for the time of progression of corrosion, it is therefore important to know the exact composition of the conductors.

In France, for example, conductors are made of steel strands coated with zinc and aluminum or Almelec strands, all layers being coated with grease. Since 1955 the grease itself has evolved, and different grease qualities often lead to different evolution of the corrosion process.

In the United States and Canada, conductors are made of zinc-coated steel strands and aluminum strands. If they are greased at all, only the steel part is greased.

16

For a large part, this difference in make-up explains why, in terms of external aggression, a French conductor does not behave in the same way as an American conductor and does not require the same analyses in respect of characterization of corrosion and aging.

The best way to detect the existence and progression of corrosion is to measure the residual layer of galvanization on the steel strands.

Cormon OHLCD

We have just seen that internal corrosion and degradation of ACSR conductors begins as soon as the galvanizing of steel strands is lost. In order to detect this loss of zinc on internal steel strands, an electromagnetic non-destructive method has been developed at the Central Electricity Research Laboratories (NGC, England) in 1981.

Measurement principle

The loss of galvanizing is detected by inducing eddy currents into the conductor from a coil, which encloses the conductor. Alternative flux induces a magnetic field in the coil that is the resultant of an in-phase and a quadrature output voltage. The magnitude of this field is closely linked to the quality of the zinc layer as the magnetic properties are different for zinc and steel. It also varies with aluminum losses.

Description

The detector consists of a sensing head clipped over the conductor and connected to a trolley housing the distance measurement circuitry encoder. The trolley is moved along the conductor using a radio-controlled tug. A separate digital radio transmits the measurements to a ground station where it is recorded. The ground station is connected to a computer for data processing. This can be done instantaneously or later.

The Sensing Head

The sensing head is a hollow cylinder split in two along its length and clipped around the conductor. When closed, one pair of windings within the head forms a field around the conductor while the second forms a pick-up coil. An electronic unit is connected to these windings. High frequency current generates a magnetic field that penetrates the conductor and induces eddy currents around each strand of the conductor. The magnitude and phase of those fields are representative of the corrosion of the strand. In phase and quadrature components of this field are registered and represented as can be seen in Figure 4-3.

17

Figure 4-3 Example of CORMON responses for two conductors : Lynx and Zebra

18

The Trolley

Several kinds of trolleys have been developed for different uses. Powered or unpowered trolleys are used on single conductors, manned trolley on bundle conductors. Figure 4-4 shows a photo of the detector complete with trolley and sensor head.

Figure 4-4 OHLCD Detector

Calibration and Limits

The detector cannot determine exactly how much corrosion has occurred but can compare the zinc thickness on the inspected conductor with that on a new conductor. For each conductor, you need for calibration, the signal for a new conductor and the signal for a fully corroded conductor (no remaining zinc). An extrapolation between those two points is done for interpreting measurements. The detector is recalibrated each day with a short test-sample piece of conductor.

The detector can only be used on ACSR and AACSR conductors or earthwires (when an optical cable is not wrapped on the conductor).

Theoretically, there is no limit for the size of the conductor. Usual conductor diameters range from 10 to 56 mm. The operation is usually done dead line, but Ontario Hydro (Canada) adapted the

19

system for use live line. Survey rates of 8 to 12 500m spans can be achieved in a working day, depending on access condition and time of the year.

The choice of spans to be inspected has to be made on evidence for sources of external pollution (industrial or coastal area for example).

General Results

CERL studies have shown that a decrease in zinc thickness induces a decrease in the phase component response and an increase in the quadrature component.

For each conductor, curves similar to those shown previously in Figure 4-3 can be obtained.

The software with which one can interpret the results has been simplified and for each span of conductor, it presents to you the percentage of length where corrosion is severe, partial or possible.

Cormon Results Validation

In order to confirm Cormon results, measures have been done on 8 lines with various conductors, in size and age. Twenty-five conductor samples have then been removed from the field and analyzed. Samples have been chosen to represent various degrees of corrosion.

Figure 4-5 shows the results obtained for six samples named a, b1, b2, c, d and e. For each sample, visual examination of the cable first and of strands after has been made. Metallographic exams of the structure have been done and zinc thickness has been measured on steel strands. Before destranding, just small points of corrosion are identified.

Sample a: All measured points are located outside the area showing loss of galvanization. It means that the zinc thickness is still correct. Currents between strands are important, which means that aluminum strands are not corroded. This is confirmed by the tests. Grease is still existent and no internal corrosion has been detected. Zinc thickness is around 30 micrometer.

Sample b: Many points are close to the new cable point but some are in the area partial loss of galvanization. It means that the conductor is still protected. Analysis shows that grease is still present and some corrosion spots are detected. Zinc thickness is around 10 micrometer. Sample c: An important loss of galvanization is detected in some areas. Aluminum seems intact. Analysis shows that the conductor has no grease, and several areas of corrosion are detected. Zinc thickness is close to zero and the rust has already attacked aluminum. This conductor needs to be checked again in 5 years.

20

Figure 4-5 Cormon results for 6 samples of conductors, cut to be analyzed.

21

Sample d: Important losses of galvanization are detected but nothing is detected in aluminum. The analysis of this ungreased cable confirmed the results for the steel. About 25% of the internal surface is corroded. Zinc thickness varies from 0 to 25µm, but spots of corrosion are also seen on aluminum stands. Their length is more than 100 µm Sample e: Very important loss of galvanization is detected. Moreover, many points being outside the area indicating loss of zinc only, this means that corrosion also affects aluminum. This is confirmed by the analysis. External corrosion is also detected. Corrosion spot’s length is around 300µm. This conductor should to be changed.

The results of those tests show that there is a good correlation between Cormon results and the loss of galvanizing on steel strands. However, it is not able to detect external corrosion.

It is to be noticed that a correct interpretation of the measures is closely linked to a good calibration of the system with a same brand new conductor, which is sometimes difficult to find for old conductors that aren’t manufactured anymore.

After those tests, it has been decided to use Cormon device on our network, as part of maintenance. It was then useful to correlate Cormon results and life duration of the conductor.

Correlation Cormon results and life duration of the conductor

To confirm the results of Cormon OHLCD testing and to convert the measured values into residual service lifetimes, several mechanical tests were carried out in the EDF laboratories. The tests were carried out on accurately identified specimens on which Cormon readings had been made. These tests were chosen because they identify certain mechanical characteristics that are sensitive to corrosion.

Generally speaking, it is seen that the diminution in the mechanical characteristics of a steel or aluminium strand starts to become appreciable when the condition of the conductor enters the phase of “severe loss” of galvanization.

This loss of mechanical characteristics can then be observed at the interface where the two materials (aluminium and steel) give rise to contact e.m.f., which will be the cause of electrochemical corrosion. The mechanical tests are therefore concentrated on this interface.

Corrosion has two effects:

• it attacks the surface of the material, reducing the sectional area of strands;

• it penetrates deep into the material, causing cracks as much as 100-150 µm deep.

The mechanical testing for aluminium or steel strands recommended by French standard NF C 34-120 is as follows:

• tensile strength test on strands - this test can detect reduction in effective sectional area (at the surface or deep inside the material) due to cracking.

• torsional ductility test - this tests loads the peripheral part of the strand and all the incipient failure points (edges of microcracks);

• alternate bending test - this will put loading on cracks.

22

Tensile Strength Test on Aluminium Strands

This test gives very significant information on the state of conductors. The many measurements taken give the following correlation.

-3 30-2.8 30-1.5 29-0.8 29-0.5 29

0 300.2 290.5 28.50.7 28.5

1 281.2 281.3 27.51.5 271.8 26

2 222.2 202.3 16

2.35 15

Corrélation signal Cormon et résistance mécanique des brins d'aluminium en daN mm²

-4

1

6

11

16

21

26

31

-4 -2 0 2 4

signal CORMON (Q-P) en Volts

daN

mm

²

Green Yellow Red Black Pink

Figure 4-6 Correlation between Cormon signal and strength of aluminium strands (daN mm²)

The colours obtained with the Cormon OHLCD roughly correspond to the zinc thickness' shown below.

CORMON Green Yellow Red Black Pink

Zinc thickness 40 µm 30 µm 20 µm 10 µm 0 µm

Alternate Bending Test

Alternate bending tests are carried out solely on the aluminium strands of the layer in contact with the steel.

Principle

The alternate bending test consists of bending a specimen to a 90°angle several times, first in one direction, then in the other. The strand is secured at one end and is bent over a cylindrical support of a specified radius, the specified radius depending on the diameter of the strand. During the test the strand is tensioned at 3 daN.

The results are as shown in Figure 4-7. Alternate bending tests are very sensitive when the loss of galvanization is high.

Series 1

23

Corrélation entre diagnostic CORMON et essai de pliages alternés

0

5

10

15

Vert Jaune Rouge Noir Rose

Diagnostic CORMON

Nu

mb

er o

f al

tern

ate

ben

ds

Series1

Figure 4-7 Correlation between Cormon diagnosis and alternate bending test results

Torsional Ductility Tests

For each sample, this test is conducted on three strands of the outer layer of steel, in contact with aluminum. Tests are executed according to NFC standard 34113.

This test is difficult to perform because the influence of the steel quality is very important. In the specification, only the minimum required value is indicated and the real value for the new conductor is often unknown.

Many similar tests have been done in Canada and the results are summarized in Figure 4-8.

These tests are very pertinent for aluminum strands, a little less on steel strands, due to a poor knowledge of steel quality at the beginning.

As it is difficult to cut samples, it seems important to request mechanical tests only when important corrosion or loss of aluminum has been detected with Cormon OHLCD.

Taking into account risk factors for corrosion, temperature of the line, nature and age of conductor, we can determine how the corrosion will progress and define a residual life for the conductor.

Cormon diagnosis

–––_Series 1

Green Yellow Red Black Pink

24

Figure 4-8 Torsional ductility of outer layer of steelcore vs age for 46 conductor samples (Harvard, 1990)

EDF experience with CORMON OHLCD

Scope of use

The French power-distribution network currently consists of about 150,000 km of ACSR conductors (aluminium or Almelec alloy), the oldest of which date from the turn of the century.

The system is used to inspect ACSR cables of all cross-sectional areas, be they phase conductors or earth wires. However, for reasons of implementation, the equipment can only be used on single

25

conductors with no cut strands and not fitted with equipment which would prevent the trolley advancing (aircraft warning devices, counterweights, joints, or prefabricated repair units, etc.).

Choice of spans for inspection

To limit the cost of inspection, checks are not carried out from one end of the line to the other, but only on a few carefully selected spans considered to be representative of the general condition of the line.

• To start with, the lines to be inspected are chosen by age. Studies of conductor degradation have shown that internal corrosion is practically undetectable for the first thirty years in a line's lifetime. Lines less than 30 years old are therefore not inspected in this way.

• Since preliminary studies have shown that the main factors in corrosion are industrial pollution and saline atmospheres, the priority is to choose spans in areas where these conditions prevail, i.e.:

− near the coast or near roads where a lot of de-icing salts are used in winter,

− in industrial areas or near sources of release of large quantities of corrosive gas.

• Areas with sensitive points such as lines crossing motorways, urban areas, or strategic power supplies are inspected systematically.

Outside these areas, it is reasonable to choose one span every ten kilometres in order to appraise the aging of the cable (assuming that the cables are all of the same manufacture and were installed at the same time).

Measurements are taken on the spans on each side of the tower from which the system was launched. Different phase conductors are inspected on each span.

If the results for both spans differ, a few extra measurements may be made. The results are then extrapolated to the entire line.

Use and interpretation of results

The results obtained for each span inspected are presented in the form of diagrams like those in Figure 4-9 below.

Feedback

After ten years of intensive use of the system, more than 300 sections of line have been inspected. The results obtained have been used to show the condition of French ACSR (Figure 4-10 and Table 4-1) conductors and predict maintenance requirements.

As more measurements are made, each unit builds and refines its own diagram as a means of planning refurbishment.

26

Figure 4-9 Examples of Cormon OHLCD report

27

ACSR conductors

0

1000

2000

3000

4000

5000

6000

7000

85 -90 yrs

80 -85 yrs

75 -80 yrs

70 -75 yrs

65 -70 yrs

60 -65yrs

55 -60yrs

50 -55 yrs

45 -50 yrs

40 -45 yrs

35 -40 yrs

30 -35 yrs

25 -30 yrs

20 -25yrs

15 -20 yrs

conductor age

line

sect

ion

s

correct conductors

to be changed in 5 to 10 years

To be changed before 5 years

Figure 4-10 Repartition of ACSR conductors controlled with Cormon detector on French network

28

Table 4-1 – Table of French ACSR Conductor Condition

Schéma No loss Possible loss Partial losses Severe loss Loss of aluminum

1920 Building of the line 100% 0% 0% 0% 0%

1930 75% 25% 0% 0% 0%

1940 50% 50% 0% 0% 0%

1950 25% 50% 25% 0% 0%

1960 0% 50% 50% 0% 0%

1970 0% 25% 50% 25% 0%

1980 0% 0% 50% 50% 0%

1999 End of life 0% 0% 0% 75% 25%

first control

No loss

≥ 50% measured length

possible losses


partial losses


severe losses


Aluminum loss for less than 20% of the length

Aluminum loss for more than 20% of the length

second control

30 years later 30 years later 20 years later 10 years later Cond. To be changed within 5 years

Dangerous conductor. To be changed immediately

29

Evolution of the corrosion

In order to quantify evolution speed of the phenomenon and adapt our maintenance, measurements have been taken over several years on the same lines. Examples of the results are given in Figures 4-11 and 4-12.

62-6

1B

as

62-6

3B

as

62-6

3H

aut

62-6

1H

aut

0%

20%

40%

60%

80%

100%

% P

erte

62-6

1B

as

62-6

3B

as

62-6

3H

aut

62-6

1H

aut

Portée

MESURE CORMON JUIN 1992 LIHART MOHON 63 kV

Perte Alu

Sévère

Partielle

Possible

Pas de perte

62-6

1B

as

62-6

3B

as

62-6

3H

aut

62-6

1H

aut

0%

20%

40%

60%

80%

100%

% P

erte

62-6

1B

as

62-6

3B

as

62-6

3H

aut

62-6

1H

aut

Portée

MESURE CORMON SEPTEMBRE 1998 LIAHART MOHON 63 kV

Perte Alu

Sévère

Partielle

Possible

Pas de perte

Figure 4-11 Evolution of aluminum loss in 6 years on ACSR 288

30

Foncles Goncourt 150 kVportée 23-22

020406080

aucune possible partiel severe perte alu

Diagnostic CORMON

%

1991

1999


020406080


Diagnostic CORMON

%

1991

1999


020406080


Diagnostic CORMON

%

1991

1999


0

50

100


Diagnostic CORMON

%

1991

1999

Figure 4-12 Cormon measurement on five different spans of the same line

31

Decision Tree for Application of Cormon OHLCD

Based on experience with the Cormon OHLCD over the past 10 years, EDF has developed a decision tree to assist in the application of the technology to their lines. This decision tree is shown in Figure 4-13. It assists in determining whether conductor samples should be taken, when the next inspections should occur, and whether the conductor should be changed out. It is important to note that this is based on French experience and may need modification for application to North American conductors.

Decision to use CORMON device

CORMON

Diagnosis OK?

No severe loss of zinc

Determination of the date of next controls

>10 years

Sample tobe cut

Diagnosis OK?

Change of conductor

Date of nextcontrol

(5 to 10 years)

yes

yes

no

no

Figure 4-13 Proposol for controlling conductors with Cormon OHLCD

32

Conclusion

EDF has a vast amount of experience with the Cormon OHLCD and has made it an integral part of their predictive maintenance program for transmission lines. Though their experience and resulting application guidelines may not be fully transportable to North America because of differences in conditions and practices, it does serve as a valid model for how a routine predictive maintenance program can be structured around the OHLCD. Their overall experience with the OHLCD has been good, though they have recognized the importance of a companion conductor sampling program to verify conclusions from the OHLCD inspections.

33

5 THE CROSS-CHECKER TRANSMISSION CONDUCTOR INSPECTION DEVICE

Background

Jim Booker, formerly with American Electric Power Company (AEP) and now president of the consulting company J.R. Booker Consulting, Inc., has developed a device called the Cross-Checker (CC). This device has shown promise in detecting and sizing damage that reduces the cross-section of steel core strands of aluminum conductor, steel reinforced (ACSR) transmission conductor. Mr. Booker owns US Patent No. 5,744,955 and has several other patents pending relating to the CC function and application.

Mr. Booker and J.A. Jones Applied Research Company (JAJARC) have a contractual agreement in which JAJARC is to take a CC device furnished by Mr. Booker and produce improvements, modifications and additions to convert the research device to a first field prototype capable of inspecting selected live ACSR.

The impetus for the device is the desire of utilities to be able to obtain a more accurate estimate of the remaining useful life of older conductors. There can be many years between the time the galvanizing is corroded off the steel core strands (an effect which can be measured qualitatively) and the time when failure is imminent, in a probability sense. It is known that the loss of steel core load carrying cross-section occurs significantly after most of the galvanizing is gone and significantly closer to the end of useful conductor life. It has been shown in early testing that the CC can distinguish between minor and major losses of core cross-section. This capability to measure progressive deterioration further reduces the uncertainty in predicting the remaining useful conductor life. This capability is believed to have great economic significance for any owner of old ACSR transmission lines.

JAJARC presented information on the capabilities of the CC to Southern Company (SC) and requested their support of the technology work to bring the CC to the state of a first field prototype device. In this state, the CC could be used to examine several older conductors designated by SC in an initial field trial. The first field prototype state will not have all the features of a model which had been modified several times based on extensive field experience. SC has decided to support this project using the EPRI Tailored Collaboration funding mechanism. Thus, EPRI and SC are jointly providing oversight of the JAJARC technology transfer work.

Principles of Operation

Figure 5-1 provides a schematic illustration of the CC that Mr. Booker is providing for the SC/EPRI project. The completed device will contain a defect detection sub-system, a support/transport sub-system and a data collection and treatment sub-system.

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The CC detection sub-system is essentially a simple dynamo, which includes a rotating magnet and a coil of wire. When the magnet is rotated, its field induces a voltage in the coil. This voltage is then conditioned, measured and displayed by the appropriate circuitry, as the output of the device. When a conductor is placed between the rotating magnet and the coil, the shape, size and extent of the magnetic field will be altered and the voltage induced in the coil will be reduced. This effect is greatly amplified when the material inserted between the magnet and the coil is ferromagnetic. Such ferromagnetic material (e.g. carbon steel, alloy steel, iron, etc.) provides a low-reluctance path for the magnetic flux before it is able to reach the coil, and therefore there is less of the flux available for the induction of a voltage into the coil. This results in a reduced voltage reading. Consequently, serving as an inspection device, the CC, properly calibrated and applied, has the capability for detecting significant changes in the amount and continuity of steel strands in an ACSR conductor.

The support/transport sub-system will provide the mechanism for supporting the weight of the entire CC, maintaining the correct position of the conductor in the magnetic field and moving the CC along the conductor. Figure 5-1 shows the transport/support sub-system in the same equipment housing as the detection and data collection and treatment sub-systems. Early investigations during the project may mandate a separate tug device linked appropriately to the other CC elements.

The data collection and treatment sub-assembly may be based in an on-board computer as shown in Figure 5-1 or may involve on-board elements, a telemetry system and ground based data recording, treatment and display equipment.

Figure 5-1 Schematic Illustration of Cross-Checker Elements

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Figure 5-2 Early Research Cross-Checker Prototype Figure 5-2 shows an early CC research prototype that had no motorized drive element but was manipulated manually in early test applications.

Preproject Testing

Mr. Booker has carried out various conductor tests over several years with evolving versions of the research CC. These tests have predominately involved manual manipulation of short, defected lengths of conductor through a stationery CC but also included limited testing of field conductors carried out by manually pulling the CC along de-energized conductors. These tests were complicated by the fact that it was not possible to maintain uniform speed of the CC with respect to the conductor and that the conductor could not always be maintained in a consistent, repeatable position in the CC magnetic field. Despite these challenges, some very encouraging fundamental results were produced.

Figure 5-3 illustrates the results from the test of a short length of 7 steel core strands of an ACSR conductor in which one of the strands is broken and the ends separated by a short interval. The broken condition is clearly detectable.

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Figure 5-3 Single Strand Break Defect Signal

Figure 5-4 illustrates the results from a test of a short length of complete ACSR conductor in which the steel core has separate areas of a single and three-strand break. Again, the conditions are clearly detectable and the larger break provides the larger signal. This provides an indication of some sizing capability.

Figure 5-4 Single and Three Strand Break Defect Signals

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Plans For Development of the CC

The plans for development consist of a phased, sequential program to produce the final inspection results on the lines selected by SC. The program phases are described in the remaining paragraphs of this section.

Phase 1 of the CC development is to involve conducting an initial meeting with the interested parties to establish key project parameters.

Phase 2 will involve a set of parametric studies with a bench version of the CC to establish key inspection parameters and the associated detection and defect sizing capabilities.

Phase 3 will involve testing of defected and undefected conductors with as many of the actual field-test characteristics (tensioned conductor, controlled speed of CC, etc.) duplicated as possible. This phase of the project will yield a defect signal catalogue and document typical noise patterns in undefected conductor.

Phase 4 will involve the design, acquisition and integration of the CC auxiliary equipment. This will include the equipment to provide the motive force as well as that needed to record, transmit and store the inspection data.

Phase 5 will evaluate the ability of the CC device from Phase 4 to operate on an energized line representative of what is expected in the field trial for SC.

In Phase 6, the JAJARC staff and Mr. Booker will work with SC to establish the actual live line inspection procedures. Training of the SC staff in the use of the CC will be part of this phase of effort.

Phase 7 is the field trial of the CC. It is planned to inspect 2 or 3 old ACSR spans as designated by SC. It is now projected that this inspection will be done on spans in the Lay Dam area near Leeds, Alabama.

A report summarizing the inspection results and the project specifics will constitute Phase 8.

The work on the CC development began early in September of 1999 and is scheduled for completion by the end 2000.

Results From Initial Project Testing

Bench Testing of New, Deliberately Defected Conductor

Mr. Booker furnished a parametric study version of the CC (PSCC) to do the Phase 2 and some of the Phase 3 project testing. Figure 5-5 shows the PSCC along with the short length of new conductor used for the initial Phase 2 testing.

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Figure 5-5 Parametric Study Cross-Checker

The conductor is LARK, which contains a set of seven steel core strands covered by two layers of aluminum strands wound in opposite directions. There are twelve aluminum strands in the first layer and eighteen in the second (outer) layer.

The procedure for implanting the reference single strand break defect involved a careful partial unwinding of both aluminum layers to expose the location on the steel core desired for defect implanting, filing almost through one of the steel strands with a small diameter file to represent previously observed service induced single strand breaks (SSB) and reassembling the two layers of aluminum strands.

The testing was then done by moving the SSB through the magnetic field starting with the defect at one edge of the guide block opening and ending at the other. During this single traverse of the magnetic field the defect was initially in a position that did not affect the coil response, moved to a position in the center of the field where it had maximum influence on the coil response, and then continued to the final position where the coil response was unaffected by the defect. This traverse represented about 3 inches of relative movement between the coil/magnet axis and the test conductor. This traverse of the defect and conductor through the test field was then reversed to produce a second defect signal and another back and forth traverse produced the third and fourth defect signals. This sequence of producing four defect signals was repeated with the defect at four different clock positions with respect to the magnet/coil axis. The clock position protocol has been established such that when the defect is directly under the magnet, the defect position has been designated 12 o’clock: when the defect is facing up from the mounting plate, 9 o’clock and so forth.

CoilMagnet and Drive

Motor

Flawed LARK Conductor RMS/DC Converter

Amplifier

Guide Block

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Figure 5-6 presents the four defect signals at four different clock positions. These results represent a smoothing of the raw data resulting from a positioning of the conductor in the effective center of the magnetic field.

Figure 5-6 Bench Test Results of Single Strand Break Defect

Some observations about the voltage signal generated when the undefected conductor is wholly in the test field are in order. For example, in the top data display, (12 o’clock position) the voltage signal recorded for two different undefected positions (Positions A and B) varies by only about 10-15 millivolts (mV). In the second data display (9 o’clock position), the voltage signal recorded for the two undefected positions (Positions C & D) varies by about 90 mV. A dotted line connecting the areas not influenced by the defect have been added to the data plots to simplify the measurement of the defect voltage signals. When these tests were first done the results were perplexing, but when the testing of the undefected longer conductor was done it became clear that certain characteristics of the new, undefected conductor resulted in voltage output signal variations of up to 150 mV. This is basically the “noise” signal from which defect signals must be extracted. This is discussed more completely in the next section.

NUMBER OF DATA POINTS

VOLTS

VOLTS

VOLTS

VOLTS

40

The defect signals of Figure 5-6 show clear detectability of the SSB defect signal superimposed on the more slowly varying noise. This observation is important since the challenge of picking SSB signals from the noise pattern from the conductor will depend, at least partly, in the much more rapid defect signal rise and fall time with respect to variations in the noise.

A careful measurement of the defect signal amplitude with respect to the existing noise yields the data below:

Defect Signals (Millivolts)

Defect Number

Clock Position 1 2 3 4 Average

12 92 92 85 87 89

9 79 88 87 81 84

6 87 79 77 71 79

3 87 83 92 90 88

Overall Average 85

Note that when the conductor is in the effective center of the magnetic field; there is only a small clock position effect. The average results at 9 and 3 o’clock are essentially the same while the 12 o’clock defect position produces a modestly higher signal than the defect when it is in the 6 o’clock position.

Overall the bench testing verified that defects as small as a single strand break were detectable but illustrated that there was a noise pattern variation resulting from the movement of the undefected conductor section through the magnetic field which would complicate the detection of small defects.

Controlled Speed Testing of Tensioned, Undefected Conductor

JAJARC has developed a test frame and rig to allow controlled speed testing of a tensioned conductor. A DC motor and chain drive provide the means to move the PSCC along the tensioned conductor. A come-along attached to the frame applies the tensile force to the conductor which is attached to the come-along and frame by a pair of metallic flex grips which can transfer more than a ton of force.

The first conductor tested was a section of new LARK furnished by SC. There are no defects in any part of the conductor. Figure 5-7 presents the results of the test. The data displayed represents the variation in output voltage along about 55 inches of undefected conductor. The system was calibrated in the same manner as for the bench testing discussed in the previous section. The conductor was tested with a reference mark at the 12, 9, 6 and 3 o’clock positions.

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Figure 5-7 System Output Voltage Along Undefected Conductor Test Length

Some observations about the data are in order. First, there is a considerable variation in the test voltage at positions along the conductor length for any clock position. Those variations are more pronounced at some clock positions than others. Similar noise patterns have been measured during the testing of other new, undefected lengths of LARK conductor.

These noise variations are believed to result from either two possible sources. The first is that the cross-section configuration of conductor does not precisely repeat itself with respect to the magnetic test field in any length that would be meaningful for a field inspection. In order to visualize this it is useful to note that the layer of steel strands surrounding the straight central steel strand, the first layer of aluminum strands and the outer layer of aluminum strands each have a different “lay” dimension and number of strands. The lay dimension is the axial length along the conductor before a given strand within a layer winds around the underlying layer and returns to its original clock position. Since there are several strands in any layer, the cross-sectional configuration of that layer repeats itself at an axial length equal to the layer lay dimension divided by the number of strands of wire in that layer. The three layers of strands each have a different axial length in which their cross-sectional configurations repeat. Because the effective cross-sectional repeat lengths of the three strand layers are not equal, the overall cross-section of the conductor only repeats when there are specific

VOLTS

VOLTS

VOLTS

VOLTS


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mathematical relationships between the repeat length for each layer, which are not satisfied in this conductor. Because of this, each overall conductor cross-sectional configuration along the length of the conductor is unique and non-repeating.

The second possible cause of the noise variations could be variations in the amount of initial galvanizing on the steel strands along their length. In Mr. Booker’s limited testing of lengths of old, field removed conductor, he has not seen noise variations of the same magnitude as have been recorded during the test program for the new, undefected conductor. SC will be furnishing some old field removed LARK conductor to the program for testing during the Phase 3 work. At that time, it will be possible to determine the level of the noise variation that remains after substantial service.

Either source of predicted noise will tend to be random, and random noise can be reduced by several available signal-processing techniques. Even with no reduction in the random noise both single steel strand breaks and substantial losses of steel cross-section will be able to be detected among the background noise. As shown in the next section, the single strand breaks can be removed from the noise using their much more rapid rise and fall time which is characteristic of this defect signal. Substantial losses of steel core cross-section will be detected by identifying those test voltages that fall above the normal range of noise signal.

Controlled Speed Testing of Tensioned, Defected Conductor

Using the test frame and rig described previously, testing was next done on a new length of LARK conductor which had deliberate defects implanted along its’ length. The defects fell into two separate groups.

The first defect group included a SSB, a defect about two thirds of a SSB and a one-third SSB defect. These defects were spaced about 4 inches apart. The purpose of this defect group was to determine the threshold for detection prior to the use of any noise reduction signal processing techniques.

The second group of defects were all SSB’s with variable spacing. With the orientation of the flawed conductor and the direction of inspection travel for the PSCC, the SSB spacing varied from one-quarter inch to two inches in increasing length increments.

Figure 5-8 presents the inspection results for the defected conductor. The SSB defects were identified by an average rise and fall time greater than 65 Mv per inch along the conductor axis.

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Figure 5-8 System Output Voltage Along Defected Conductor Test Length

As previously mentioned, the first defect in the first group is a full SSB. The A on the inspection record designates this defect signal. The remaining partial SSB’s were not clearly detectable in the noise signal.

The second group of full SSB’s became fully detectable only when the axial spacing between defects reached one, one and one half and two inches. The same rise and fall time criterion previously described was used to extract the defect signal from the noise. The four defects at the above spacing are designated B, C, D and E on the record.

Conclusion To Date

The bench and frame testing done to-date show the ability to detect defects as small as single strand breaks in new conductors tested in a manner representative of the expected field situation (conductor tensioned and maintained at consistent position in test field). Tests of new conductor sections also indicates an amount of noise signal of the same or greater magnitude than SSB defect signals. Characteristics of the defect and noise are different; however, allowing the separation of the SSB defect signal from the noise based on a rise and fall time discriminator.

Some of the most important near-term tests will determine if the level of test noise from the undefected conductor is related in any way to its’ new condition. Receipt of lengths of old, field removed LARK conductor is expected from SC very soon. This will be tested in both the as received condition and after the addition of reference defects.

Future Development Activities and Schedule

Work continues on Phases 3 through 8, with the activities as described previously. Plans for field inspection and a final report by the end of 2000 are currently on track. A proposal for future developments to equip the CC for energized line operations, traversing conductor splices, and inspecting steel core within the splices has been made. Provided results of field tests are positive and adequate support for this future work can be garnered, continued development of the CC in 2001 is expected.


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6 ELECTRO-MAGNETIC ACOUSTIC TRANSDUCERS (EMAT) FUNDAMENTALS AND APPLICATIONS

Introduction

Acoustic transducers have been used as a non-destructive evaluation (NDE) technique for several decades. Sonar, the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects, was introduced prior to World War II. This technique inspired the development of ultrasound diagnostic devices for engineering and medical applications. In 1931, Mulhauser obtained a patent for using ultrasonic waves, using two transducers to detect flaws in solids. Firestone, in 1940, and Simons, in 1945, developed pulsed ultrasonic testing using a pulse-echo technique. In late 1940’s, researchers in Japan began applications of ultrasound in medical diagnostics. Japan’s work in ultrasound was relatively unknown in the United States and Europe until the 1950s. Then researchers presented their findings on the use of ultrasound to detect gallstones, breast masses, and tumors to the international medical community. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation.

Non-destructive evaluation (NDE) has been practiced for many decades, with initial rapid developments in instrumentation spurred by the technological advances that occurred during World War II and the subsequent defense effort. During the earlier days, the primary purpose was the detection of defects. As a part of “safe life” design, it was intended that a structure should not develop macroscopic defects during its life, with the detection of such defects being a cause for removal of the component from service. In response to this need, increasingly sophisticated techniques using ultrasonics, eddy currents, x-rays, dye penetrants, magnetic particles, and other forms of interrogating energy emerged.

Advances in the field of fracture mechanics enabled one to predict whether a crack of a given size would fail under a particular load if a material property and fracture toughness were known. Other laws were developed to predict the rate of growth of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for new a philosophy of “fail safe” or “damage tolerant” design. Components having known defects could continue in service as long as it could be established that those defects would not grow to a critical, failure producing size.

A new challenge was thus presented to the NDE testing community. Detection was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics based predictions of remaining life. These concerns, which were felt particularly strongly in the defense and nuclear power industries, led to the creation of a number of research programs around the world and the emergence of quantitative non-destructive evaluation (QNDE) as a new discipline.

An important character for NDE devices is the speed of operation in hostile environments and quantitative accept/reject criteria. This high-speed challenge is met by ultrasonic techniques, because the probing energy can propagate through solids at the speed of sound and the wavelength can be made comparable with the dimensions for flaws that must be detected.

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This chapter provides foundations of acoustic transducers and their shortcomings. Next, there is an introduction of electro-magnetic acoustic transducers (EMAT), as a technique to overcome disadvantages of the acoustic transducers. Key mathematical formulations for application of EMAT are presented. An overview of the EMAT applications in the engineering field is provided. Two application areas of EMAT in electric power transmission and distribution, as part of the author’s current research, are then presented. The chapter ends with a list of useful references.

Acoustic Transducers

Acousto-elasticity utilizes the stress dependence of ultrasonic velocities in solid bodies and provides a unique means for non-destructive stress determination [1]. The measurement of stress is a generic problem, which constantly occurs in NDE applications. X-ray diffraction provides the most widely used approach, but suffers from the limitation that only a very near surface layer is sensed. Neutron diffraction measurements overcome these limitations, but can only be made at specialized neutron sources. Ultrasonic measurements, based on the stress dependence of the wave speed, also have the capability of sampling bulk material with the advantage of being able to be applied with relatively inexpensive, portable instrumentation. However, ultrasonic measurements suffer from some practical problems, including degradations in accuracy due to competing sources of wave speed shifts and difficulties of making measurements in complex geometries. While many other techniques are available to evaluate the surface stresses, the alternative to acousto-elasticity is only the neutron diffraction method, which is rarely accessible. Many instances of the successful industrial and field applications are emerging, such as those for welded parts, railroad wheels, rails, bolts, and water pipes. However, the wide acceptance of acousto-elasticity as a standard technique has been limited for several reasons. They include the high accuracy needed to cope with the low sensitivity to stress, the prolonged preparation, etc. It needs a high grade of surface finishing to assure the thinnest coupling layer between the piezoelectric transducer and the object surface, thereby achieving the required accuracy of velocity determination. Experimental difficulties arise when the path length is not long enough as will be encountered with the thin-walled structures like aircraft and fuel-storage vessels. The relative accuracy decreases with decreasing thickness. Eventually, the successive echoes are overlapping each other even if high frequency, broadband signals are employed.

Ultrasonic transducers operating based on the acousto-elasticity may excite different types of waves in a solid. In the following, a brief description of these waves is given.

Longitudinal Wave

An important wave motion is the propagation of longitudinal waves, often encountered in solid bars, and at low frequencies in gas-filled tubes. As a longitudinal wave moves along the bar, the displacement of particles of the bar is essentially parallel to its axis. When lateral dimensions of the bar are small compared with its length, each cross-sectional plane of the bar may be considered to move as a unit.

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Lamb Wave

Lamb waves are similar to longitudinal waves, with compression and rarefaction, but bonded by the heet or plate surface causing a uniform guide effect, as shown in Figure 6-1.

Figure 6-1 Lamb Waves

Shear Wave

Shear waves have an inherent polarization direction depending on how they are generated. Pictured below are horizontally polarized shear waves propagating along the length of a plate, as shown in Figure 6-2

Figure 6-2 Shear Waves

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Shortcomings of Ultrasonic Measurements

One of the essential features of ultrasonic measurements is the mechanical coupling between the transducer, generally a piezoelectric disk, and the solid whose properties or structure are to be studied. This coupling is generally achieved in either “immersion” or “contact”. In the case of immersion, energy is coupled between the transducer and the sample by placing them in a tank filled with fluid. In the case of contact measurements, the transducer is pressed directly against the sample, and coupling is enhanced by the presence of a thin fluid layer inserted between the two. When shear waves are to be transmitted, the fluid is generally selected to be very viscous.

In normal ultrasonic testing, vertically polarized, transverse or longitudinal sound waves are used. Usually, this type of wave is generated by means of a piezoelectric crystal. In the case of ultrasonic testing of objects of carbon steel or of austenitic material, the result is generally satisfactory.

In the field of nuclear energy several material combinations occur which may cause difficulties from the point of view of testing technique. An example is the testing of a dissimilar joint between clad carbon steel and an austenitic stainless steel.

Among other things, the difficulties in testing dissimilar joints are caused by a large deflection and distortion of the sound due to the structural anisotropy of the weld metal. Wave transformation at interfaces is another factor making evaluation more difficult.

In principle, a piezoelectric crystal can generate horizontally polarized sound waves in the same way as by a conventional ultrasonic probe. The problem is that there is no suitable coupling medium because a medium with a very high viscosity is required. Therefore the sound waves are generated by electromagnetic means [2].

The advantages of horizontally polarized sound waves may be summarized as follows:

• Small deflection of the waves

• A good transmission at interfaces

• No loss of energy at reflection against a free surface because there is no wave transformation

• A variable angle of refraction (20˚-90°)

• A total edge effect independent of the angle of refraction

• A high sensitivity to defects perpendicularly oriented to the probe surface

• No coupling medium

• A simplified evaluation because only one type of wave mode is used

As discussed in the next section, the EMAT probe can generate sound waves in all conducting materials by Lorentz forces or by effects of magnetostriction, or by a combination of both. Both phenomena affect the atomic lattice in the material. Therefore no coupling medium is required.

Fundamentals of EMATs

Electromagnetic acoustic transducers (EMATs) derive their name from the fact that they can excite and detect ultrasonic vibrations in metals by an electromagnetic induction process across an air gap [3,4], thus they operate well without any water tanks or viscous couplant. This feature makes EMAT attractive for a variety of applications, such as inspections at high speed or at high temperatures that

48

are not suitable for conventional piezoelectric-based transducers. EMATs are much more amenable to inspections of parts with complex geometries than conventional transducers.

EMATs overcome many of the common problems encountered with traditional ultrasonic techniques. In addition to being non-contact, EMATs will work at high temperatures and pass through various coatings while scanning at high speeds. Because there is no incidence angle or crystal required, EMATs are less prone to operator error and will produce more repeatable variety of ultrasonic wave modes. Besides the familiar longitudinal and SV (shear vertical) waves, EMATs can also generate SH (shear horizontal) waves and Lamb waves.

Basic Principles of EMATs [5]

The electromagnetic acoustic transducer (EMAT) couples ultrasonic energy into conductive materials. The simplest EMAT is a wire loop held near a conductive material under test with a magnet placed above the wire. The transmitter using physics similar to those acting in an electric motor develops the acoustic forces. A copper coil is placed as close to the test medium as possible and an alternating current is injected (40 – 500A). This current produces a dynamic magnetic field ΗΗΗΗ, which varies in time and space.

The eddy current density produced in the test medium when the coil is placed near the test medium is given by

From Maxwell’s equations for quasi-static conditions, this eddy current flows in the medium. The current follows the path of the coil and is smaller than the driving current because of the air gap. The relationship between the eddy current and the transmitter driving current, I d ,is

J H= ∇ ×

I I eeddy d

G

D≈−2π

49

NSNSNS

I

F

IB

NS

F I

B

c o i l

m a g n e ts

Figure 6-3 Lorentz Force Configuration

where G is the size of the air gap and D is the distance between loops in the wire. From this, it is obvious that the air gap should be minimized. To create a force in the metal, permanent magnets with high magnetization are placed directly over the coil to flood the medium with magnetic flux. The eddy current interacts with the external magnetic induction B0 to produce a force density as given below

coupled to the lattice of the metal sample. This force is called a Lorentz force and acts in a direction indicated in Figure 6-3 [4]. An elastic disturbance involving particle displacement u and velocities du/dt in the test specimen then propagates to the receiver.

The receiving EMAT works similar to an electric generator. When the acoustic wave passes under the receiver, the surface of the material is displaced in the magnetic field. An electric field arises because of the term

where du dttot / is the total particle velocity, which incorporates reflected, as well as incident elastic waves at the surface. With the resulting conduction current density, JR is a dynamic magnetic field in the metal given by

F J B= × 0

Edu

dtBtot= × 0

∇× =H JR R

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A related field, RH , can be generated as

where A is the vector electric potential outside the test specimen and around the receiver coil and µ0 is the permeability of free space. The resulting electric field

E j A= ω

produces the receiver voltage, which is sensed. The current induced in the coil by the acoustic wave is also related by the eddy current equation except now the driving current is replaced by the eddy current that is attenuated by the exponential factor. The resulting current is the received current rather than the eddy current [4].

The efficiency of an EMAT pair is measured by a ratio of the received voltage and the driving current. Assuming the ideal case with no acoustic attenuation when the wave transverses from the transmitter to the receiver, the efficiency is

where N is the number of turns in each coil, and A is the area covered by the coil. The magnetic field B and the air gap factor are squared (factor of 4 rather than 2 in the air gap factor) because these factors influence the efficiency equation; the design of EMATs becomes versatile and powerful for solving numerous problems.

The vibrational mode, excited and detected by an EMAT, is controlled by applying surface tractions that match surface displacements and frequency of the mode of interest. The vibrational mode selectivity provided by EMATs allows sensitivity to defect with specific resolution [5]. In addition, the excitation of the transducer is contactless and may be used for applications with harsh environments or where firm contact with the test surface is not possible. With such flexibility, the EMAT has been designed and used for numerous NDE testing applications.

V

I

N B Aereceived

driving

G

D=−

22 2 4

ρν

π

H AR= ∇×( / )1 0µ

51

Therefore, the principles of operation of an EMAT can be summarized as follows:

EMATs make use of a magnetic field as their couplant. A magnetic field is applied to the test piece and the EMAT circuit is placed in the magnetic field. A high power AC toneburst is then driven through the EMAT. The toneburst passes into the test piece as eddy currents. The eddy currents interact with the static magnetic field to produce stresses in the metal leading to the generation of ultrasonic waves. The frequency of the current, the shape of the EMAT transducer, its position in the magnetic field, and the thickness of the test piece, combine to determine the mode of ultrasound generated. The wave mode and frequency determine the type of analysis to be performed.

EMAT Design Considerations

The design engineer can utilize many of the variables associated with the EMAT principles of operation as flexibilities to apply this technology to specific problems. Some of the key design considerations are:

• System configuration

• EMAT Coils

• Magnetic field source

Following is a brief discussion on the impact of these design components on the overall operation of the EMAT.

System Configuration

Figure 6-4 shows a simple schematic of an EMAT system, where port “I” represents a transmitter unit and port “II” is the receiving unit, Vt represents the generator voltage

Figure 6-4 Schematic of Simple EMAT

I II

yVt x

Rt

Rr

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of the transmitter side and Rt and Rv are the equivalent resistances in the transmitter and receiver circuits, respectively. The following relationships between the applied voltage and current can be derived.

I V Z e et tj kx

tj kx= −−

0 ( )Γ

Where Zt is the transmitter side impedance, V0 is the applied voltage amplitude, χ is the spatial coordinate between the generator and the unit “I”, and Γt is the voltage reflection coefficient at the

unit “I”. The following equations can be derived for the receiver side, where ΓR is the voltage transmission coefficient

The presence of flaws or failures is detected by EMAT through changes in the reflection and transmission coefficients.

Where zsf is the surface containing the flaw, n is the surface normal, V and T are the material

displacement velocity and stress fields of the ultrasonic waves radiated from the units “I” and “II”, and P is the electrical power exciting either at the transmitter or the receiver unit derived from the following [6,7].

V V e etjkx

tjkx= +−

0( )Γ

V V er rjky= −

0 ( )ΓI

V

Zer

rr

jky= −0 ( )Γ

∇ =− z ⋅ ⋅ ⋅ ⋅Γ 1

4PV TV T n dss t r r tf( )

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( )P V Z= 0

2 2

Since it is easier to measure electrical variables than the voltage reflection and transmission coefficients ( , )Γ Γ1 2 , then EMAT performance is defined in terms of transfer impedance, relating open-circuit received voltage to the current passed through the transmitter EMAT.

Depending on the geometry of the solid that the EMAT system is attached to, namely, a half-space, a plane wave, a finite beam, etc., different formulations can be obtained for Zr t .

EMAT Coils

Most of the contents of this section are taken directly from Alers and Burns [4]. Consider the three coil configurations shown in Figure 6-5 [5]. In this figure, the (a) configuration shows a uniformly polarized coil, (b)shows a meander coil, and (c) represents a periodic, permanent magnet case.

( )a ( )c( )b

N N NS SS

Figure 6-5 Common Coil Configurations

ZZ Z

r tr t r t

t

= +−

( ) ΓΓ1

54

Figure 6-6 shows two coil shapes that are particularly well suited for generating waves that enter or leave the metal perpendicular to the surface in a direct analog to the conventional piezoelectric transducer [8]. If the round spiral coil is placed under a single pole piece of a magnet, a shear wave is generated whose polarization is in the radius direction of the spiral; i.e., a circular polarized ultrasonic wave. The rectangular spiral will generate a shear wave that is polarized linearly in the direction perpendicular to the long axis of the rectangle. The magnetic field must be configured so that a north pole is over one side on the long-axis centerline and a

Figure 6-6: Specially Shaped Coils

south pole is on the other side. This is easily accomplished by using two permanent magnets stacked side by side with opposite pole orientation and by placing the magnet-to-magnet boundary on the long-axis centerline.

Figure 6-7 also shows examples of the so-called meander, or serpentine, coils that are wound in such a way that the direction of current flow (and hence the surface force), switches direction

55

Figure 6-7: Meander and Serpentine Coils

under adjacent wires [9]. This imposes a periodic force on the metal surface and establishes a wavelength for the wave launched by this coil. If the frequency of the drive current is chosen to satisfy the equation

where D is the spacing between adjacent wires, then a wave with a phase velocity v will be launched. In most cases, the phase velocity is a uniquely defined number for a given material and part geometry, so this equation defines the frequency at which a meander-type EMAT coil must be operated to achieve efficient transduction of the wave with that phase velocity. A simple shear or longitudinal wave propagating at an angle θ relative to the surface normal can be made to satisfy this equation if the angle θ satisfies

where Vc andVs are the usual propagation velocities of longitudinal and shear waves, respectively.

The right-hand side of Figure 6-7 shows a pair of curved meander coils that are designed to act as a transmitter and receiver of waves focused to a point at the center of the radius of curvature of the wire. Such a coil geometry is particularly well suited for plate and surface waves that need to be

F D= ν 2

s i n θ εc

V

D F=

2

56

focused to detect very small flaws. The center coil in Figure 6-7 is designed to fit between the north and south poles of a magnet, with the magnetic field parallel to the long dimension of each meander. When such a coil is placed over a magnetostrictive material such as steel, the magnetic interaction shears the surface to excite a shear wave whose polarization direction is parallel to the surface. These so-called, shear horizontal (SH) waves are not easily excited by piezoelectric transducers and can have very simple reflection properties from surfaces, corners, and flaws. Hence, they can by very useful for quantitative NDE requirements.

Magnetic Field Source

Several researchers have studied the impact of magnetic field on the performance of an EMAT system [4,10]. It is shown that the efficiency of an EMAT depends on the square of the magnetic field at the metal surface. Hence, the source of magnetic field plays a crucial role in the implementation of an EMAT inspection system. The largest fields are obtained by DC electromagnets that use iron cores with specially shaped pole pieces to focus the field to the area of the EMAT coil. Fields approaching 2T (20 kilogauss) over areas of a few millimeters on each side can be obtained in this way. Unfortunately, such electromagnets are bulky (about 0.1 m3 ) and massive (over 45kg) so they are usually applicable only for on-line inspections in pipe and bar mills where the material-handling machinery easily dwarfs the electromagnet and its power supply.

Small, handheld probes similar to the familiar piezoelectric transducer can be constructed out of carefully shaped rare earth permanent magnets such as samarium-cobalt or the new neodymium-iron-boron alloys. In these cases, fields in the range of 0.3 – 0.5 T (3-5 kilogauss) can be applied to coils whose range from a few millimeters on a side to several square centimeters.

An intermediate-size electromagnet can be constructed with a laminated iron core so that high fields can be obtained for a short time by driving a large pulse of current through the coil. [11] Because the drive current flows can be obtained for only a few milliseconds, the average power dissipated is quite managable and fields near 1T (10 kilogauss) can be achieved over areas of a few square centimeters. This class of magnet is very appropriate for automatic inspection systems in which the transducer must be lifted on an off the workpiece or mechanically scanned over large areas.

It should be noted that electronic circuit design for driving an EMAT plays an important role in the overall system operation. Since EMAT is made of a coil, it introduces an inductance to the driving electronic circuitry. Therefore, EMAT is a low resistance and high current system. This is in contrast to a piezoelectric system that behaves more like a capacitance and thus requires high voltage, low current source.

EMAT Applications

EMATs have become a viable commercial inspection tool that are used in a variety of industry. Applications of EMAT range from process control to field inspection systems. Several industries, as well as research groups have been expanding the envelope of applications for EMAT. The following is a brief description of some of these applications. For more details, readers are encouraged to study the cited references.

Paul Davidson of WIS, Inc. has several publications on the subject of EMAT applications [12], that WIS has developed and put in use for several years. Most of these devices have been designed for inspection in process industries.

57

MagnaScan™

The MagnaScan™ device uses EMATs for high speed scanning of in-service pipe in the oil & gas and petrochemical industries. By careful design, the EMAT is constructed so that an ultrasonic wave that is sensitive to changes in the wall thickness is sent circumferentially around the pipe. If there is a flaw or wall thinning in the pipe the system detects those conditions.

This system has been in use in the field for over six years and has been applied to a number of piping problems. One of the most successful applications of the system has been the measurement of wall loss in piping at support contact points. The waves do not interact with the pipe support and this one would be able to measure corrosive wall loss at the support point without raising the pipe and with the pipe still in-service. This leads to substantial savings for the plant both in increased production time due to not having to shut down and in the costs saved by not having to raise the pipes off of the supports.

Projectile Inspection:

Another highly successful use of EMATs has been for the inspection of 25mm projectiles. This system was first installed in 1985 and updated in 1988 with a computer-controlled system. The system must meet a rigorous government specification for finding flaws on OD and ID of the projectiles in three different areas. Thus this is a three-channel inspection system. The flaws are on the order of 0.1mm wide and 0.25mm deep and can be on either the ID or the OD. Once an hour a set of calibrations standards must be run through the instrument and 10 inspections of each standard must be reproducible. The inspection of the projectile takes one second per piece. This device has inspected over 4 million projectiles since its upgrade in 1988.

EMATs are used for welds inspections including: Laser Welds, Girth Welds, and Butt Welds. In 1988, Dr. Alers developed a Butt Weld inspection device [13]. In 1989, a manually operated EMAT inspection system was installed on a Butt Welder at a major American steel producer’s Pickle Line. The results were so encouraging that the procedures were improved upon until the weld breakage rate was cut dramatically and substantial cost savings were being realized.

The basis of this technology is the use of Shear Horizontal (SH) ultrasonic waves. SH waves are a class of ultrasonic waves that are difficult to generate with piezo-electric generated ultrasonics, but are easily generated with EMAT transducers. In fact, there exist three methods of SH generation by EMATs. These methods are periodic permanent magnet generation (US Patent # 4,127,035), parallel generation (US Patent # 4,295,214) , and ‘Magic Angle’ generation (US Patent # 5,537,876)

58

Figure 6-8 Magic Angle EMATs

The NDE Center at the Iowa State University has investigated applications of EMATs. These applications include: evaluation of the properties of diamond thin films, metal fatigue, stress in inconel tubing, texture measurement, inspection of welded joints, and evaluation of adhesive bonds [14, 15]. The center research has also focused on the integration and application of the Laser EMAT ultrasonic transducers. Researchers at Johns Hopkins University have also investigated the combined EMAT/Laser system for crack sizing. In this project, laser generated ultrasound and EMAT detector are used for measuring crack sizes in steasdel, e.g., steel pressure vessels without shutting down the plant. Initial testing on 1” aluminum plates with artificial cracks has shown that laser generation using a single line source and detection with a 1” X 1” meaner line EMAT coil works well for detection of the crack. However, interference of various waves from mode conversion at the crack tip and back surfaces does not allow sizing of the crack. Current efforts are being directed towards designing a more appropriate system which eliminates the interfering wave arrivals.

Researchers at Purdue University have investigated applications of EMAT for Civil Engineer’s structure monitoring, offshore tension leg inspection, submarine weld inspection, etc. [16].

The hybrid Laser-EMAT system for ultrasonic weld inspection has also been developed in Europe [17]. Their system has been designed to address the specific concern that in some applications, EMATs do not have sufficient sensitivity to both generate and detect ultrasonic waves. Thus a high energy pulsed laser is used to generate relatively large amplitude ultrasonic waves in a sample by illuminating the sample surface with the laser beam [18, 19 ]. The major disadvantage of the hybrid laser/EMAT system is that the ultrasonic waveform detected by the EMAT is extremely complicated. The signal scattered from a defect will usually be masked by other large amplitude ultrasonic arrivals. There can be many different defect types within welded components such as lack of wall

59

fusion, porosity, cracking and slag inclusion, and it would not be expected that all defect types could be inspected using a single test procedure.

EMAT has also been used in steel mills to sense as-cast billet temperature. A collaborative effort between seven steel mills, Battelle’s Pacific Northwest Lab and NIST resulted in an EMAT-based temperature sensing of over 1130˚C [20].

A more recent application of EMATs is inspection of residual stress in railroad wheels [21]. Conventional ultrasonic NDT units determine the residual stress in a metallic material on the basis of acousto-elasticity, which is defined as the change in sound velocity through a material with a change in residual or applied stress. A major difficulty with this method is that the velocity change due to stress is extremely small (-0.01%). The system calculates a time differential caused by the change in velocity of sound moving through steel under residual or applied stress. This differential is on the order of nanoseconds.

On the other hand, the EMAT system calculates bi-refringence, or the relative change in sound velocity of two orthogonal, polarized ultrasonic waves that travel through the thickness of the wheel rim, where the hoop stress is found. To increase accuracy, the velocity of a signal with a radial polarization is used as the baseline. The stress in the radial direction is extremely small, and any velocity change can be considered negligible.

Sending a high current pulse through coils placed very close to the rim face generates sound waves, which induce eddy currents into the wheel surface. These currents in the wheel surface interact with an external magnetic field to generate an ultrasonic wave. The EMAT system features a 2 MHz electric current generated in perpendicularly stacked coils, which makes it possible to generate polarized ultrasonic waves in orthogonal directions without moving the transducer.

The difference in travel time for an ultrasonic sound wave from the front of the rim face through the cross section of the wheel rim and back, between the radial (diametral) and hoop (circumferential) directions, allows the calculation of the residual hoop stress.

Electric Power System Applications of EMATs

Since 1996, the author, his co-researchers, and his graduate students [5] have studied applications of EMATs in electric power systems. This is an application area that has not been considered by any of the prior investigators. In addition to the uniqueness of this application, we have extended the boundaries of EMATcapability beyond a single solid medium. Specifically, we have applied EMATs to:

i – Stranded Wires (Ground Mat Risers)

ii – Bi-metallic Stranded Wires (ACSR – Overhead Transmission Lines)

In the electric power industry, and other industries, there is a major need for devices that can easily provide information regarding integrity of electrical conductivity, mechanical connectivity and transmissibility. Application areas such as ground mat risers, transmission lines, etc. that are not easily accessible for monitoring and diagnostics require a device that can provide such information as: integrity of the ground connection, determination of the degree of oxidation in the portion of the riser which is inside the ground, integrity of connections between the conductor and the poser post, corroded and broken strands in conductors, and discontinuity and damage within shoes or marker balls of transmission lines. At the present time, such a device does not exist, and for example, in the case of the riser, each riser has to be dug out for examination. For overhead line conductors, the

60

conductor suspension assembly must be disassembled to inspect conductor within assembly. Thus it is very tedious, labor intensive and costly. In almost all cases, the equipment or transmission line can not be energized during this manual inspection process. Thus at the present time, it is very costly to assess the integrity of any type of conductor.

Our research has resulted in the development of a device for non-destructive evaluation (NDE), health assessment and diagnostics of conductive materials, e.g. copper ground risers and ACSR transmission line conductors. This electro-magnetic device integrates advances in electro-magneto-acoustic technology with artificial neural networks. This enables the field engineer or maintenance crew to loosely clamp the device to a bare section of the conductor, and transmit and receive a very high frequency acoustic signal. Then they can analyze the received signature and identify whether there exists any loss of conductivity, connectivity, corrosion, etc. and the location of such a loss.

As shown in the references, there has been prior art in developing electro-acoustic transducers for cylindrical shape materials. However, prior art does not include development of such devices that:

• Transmit and receive electro-magneto-acoustic signals through a twisted bundle of conductors

• Apply such advanced signal processing technique as a combine wavelet-artificial neural network to analyze reflected signatures and identify location and degree of the loss of integrity within the conductor

• Do not require special coupling and/or attachment to the conductor. Furthermore, the application of this technology to many systems and components in the electric power, steel, and copper, etc. industries is very unique.

The resulting device from this research is composed of four components:

• A transmitter

• A receiver

• A high frequency power amplifier

• A microprocessor controller with display

The transmitter/receiver pair is composed of highly magnetized permanent magnets, wound copper coils, etched copper coils, and capacitors. When the transmitter is clamped onto a cable and the appropriate input current is given to the etched copper coil, a torsional mode acoustic wave is produced which travels down the conductor. This wave is reflected from defects, e.g. breakage, broken strand, oxidation, and loose connection, in the conductor bundle and the reflected energy produces a current in the wound copper wire of the receiver. This sensed current can be used to determine the physical condition of the conductor strands.

The poser amplifier is used to create the very high frequency signal for the transmitter. The reflected signal from the receiver is sent to the microprocessor controller which houses the smart signature analysis system. This system utilized the power of wavelets and artificial neural networks to diagnose the presence of any defects, assess defect(s), identify the location(s), and provide the degree of severity of the problem.

Detailed Description of the Sensor System:

Design of the transmitter/receiver is based on the assumption that the acoustic waves in a cable act similar to those in a solid copper cylinder of the same diameter. The copper coils are places as close to the cable as possible. Clamping the magnets over the coils holds the coils close to the cable. Figure 6-9 shows how the transmitter and receiver are clamped on the cable. When a current is input

61

to the transmitter coil (etched), eddy currents are created in the cable. These currents follow the path of the coil. The presence of a magnetic field produces a force as shown in Figure 6-3. The direction of this force is dependent on the polarity of the magnet. In order to define the wavelength of the acoustic wave, the magnets are places so the polarity alternates along the longitudinal axis of the cable. A key variable for operation of this system is the frequency of its transmitter pulse. This frequency depends on the composition of the conductor's material(s). For a general case of bimetallic conductor such as ACSR transmission lines, the excitation frequency is obtained by performing the following steps:

• Derivation of the equations for propagation of time-harmonic torsional waves in composite elastic cylinders using the axial component of the vector potential function Ψ

∆ Ψ Ψ= ∂∂

12

2

2C t

where ∆ is the Laplace operator and C is the Shear Wave speed.

Then: Ψ Ψ( , , ) ( ) ( )r z t r ej kz rt= −

102 2

r

d

drr

ck( ') [( ) ]Ψ Ψ+ − =ω

This last equation is a Bessel function.

• Imperfect interfaces are considered where tractions

- are continuous across the interface - displacement jump is proportional to the stress acting on the interface

• Modeling results have been verified for the two limit cases:

- dispersion curves for a bimaterial rod with a perfect welded interface (F=0) - dispersion curves for a completely flexible interface (F→∞)

• Then elastic bimaterials rod has resulted in

- a frequency equation for the rod - dispersion curves of normalized frequency versus normalized wave number for variable interface (F)

For a torsional mode in a cylinder with zero nodes and a single layer of torsion, as shown in Figure 6-10, the driving frequency f is found to be 461 kHz. The wavelength is set by the thickness of the magnets and is twice this thickness. The desired wavelength to allow detection of large defects, i.e. nicks or cuts in strands, is at maximum the diameter of the cable (diameter of cylinder with assumption). Figure 6-11 demonstrates how torsion is created in the cable. When the copper coils are wrapped around the cable, this ideally creates a tangential force on the surface of the cable. The

62

sum of the tangential forces around the circumference of the cable creates a torque. The alternating magnetic fields then produce alternating torsions along the longitudinal axis of the cable. The receiver works in an inverse manner in that the torsion, which passes under the coil, develops a current within the coil in the presence of a magnetic field. The capacitors are used to tune the frequency response of the transmitter/receiver to match the driving current frequency.

Prototypes of this transmitter/receiver sensor have been built and tested. Figure 6-12 shows the setup of the prototype. Figure 6-13 shows one set of data from a section of copper cable. The nomenclature follows: a 10% perpendicular cut represents a cut perpendicular to the longitudinal axis of the cable which reduces the cross-sectional area by approximately 10%. As part of this figure, results from reflections of a 25% perpendicular cut are shown. As can be seen from this set of data, the current signal from the receiver drastically changes as a defect is introduced to the cable.

Another major unique and differentiating factor for the research results of our advanced signal processing and signature analysis software, embedded in the system. This software takes reflective signal, received by the receiver component of the EMAT, and performs a wavelet transformation of the data. Figure 6-14 shows wavelet transform of a normal ground mat riser. Several features are extracted from this image. These features include the time, the amplitude, the area, the frequency of greatest amplitude, etc. To extract these features a robust feature detector is developed. Figure 6-15 shows the results after this robust feature detector is applied. This figure is the signature of a thermoweld joint.

Therefore, in addition to the EMAT concept, the apparatus, and the new application for stranded conductors, the wavelet-based signal processing and robust feature extraction are new and unique characteristics of our research.

Other potential applications of this sensor in the power industries include any application in which cabling is tested for defects. Several examples include: ground riser cables in power stations, transmission line cables between towers, and tension cables in structures.

63

N

NS

S

ForwardCurrent

ReturnCurrent

NN NS S S

SN N NS S

Side View Cross Section

magnets

coils

Placement on Cable

Figure 6-9 Clamping of the EMAT to Cables

64

Figure 6-10 Single Torsion Layer

65

Figure 6-10: Creation of Torsion in Rods

NSNSNS

F

B

wavelength istwice the magnetthickness (1/4inch)

I

66

Experimental Setup

Magnasonics Tunable EMAT Pulser/Receiver

Oscilloscope Signal Generator

ReceiverFilter

Activation Pulse(sets number of cycles)

DrivingCycles

Driving Current(50 - 400 App)

ReceivedSignal

Signature

Figure 6-12 Prototype of EMAT Tool Diagnostic

67

Figure 6-12 Wavelet Transform of EMAT Data

Clean Sample

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

-0.0005 -0.0004 -0.0003 -0.0002 -0.0001 0 0.0001

tim e (se c)

Vo

lts

10% Cut Perpend icular to R iser Axis

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

-0.0005 -0.0004 -0.0003 -0.0002 -0.0001 0 0.0001

Tim e (se c)

Vo

lts

25% C ut Perpend icula r to R iser Axis

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

-0.0005 -0.0004 -0.0003 -0.0002 -0.0001 0 0.0001

Tim e (se c)

Vo

lts

Figure 6-11 Sample Data from EMAT

68

Summary and Conclusions

EMATs have overcome many of the common problems and limitations of the traditional ultrasonic techniques. A major advantage of EMAT over conventional systems is that it does not require a couplant between the transducer and the test material. The absence of a couplant eliminates errors caused couplant variations, and increases the accuracy of the instruments. In addition to being non-contract, EMATs work at high temperatures and pass through various coatings while scanning at high speeds. Because there is no incidence angle or crystal selection required, EMATs are less prone to operator error and will produce more repeatable results. Additionally, EMATs can generate a variety of ultrasonic wave modes. Besides the common longitudinal and shear vertical (SV) waves, EMATs can generate shear horizontal (SH) waves and Lamb waves.

EMATs have realized many industrial applications from process control to online inspections. Some of these were reviewed in this chapter. EMATs have enjoyed other commercial successes. For example, EMATs are being used to measure the surfaces of work rolls, for measuring wall loss in boiler tube banks, for testing heat exchanger tubing, for testing inflator bag welds, for testing viability of steel strips, and for inspecting hot billets for flaws. EMATs have also enjoyed commercial success in Europe where they are used in many process control applications. For example, one system uses an EMAT to measure the phase separation of liquid from single to dual phase of coolant liquids inside a nuclear power plant. The EMAT is on the outside wall of the pipe and measures changes in the UT wave as the substance inside of the pipe undergoes phase transition. Other applications of EMAT in Europe include pipe inspection at the factory, inspection of honeycomb structures in airplanes and for a variety of high temperature inspection needs.

The envelope of EMAT applications is being continually expanded. There are recent efforts in integration of Laser/EMAT systems to increase its capabilities. The author’s recent developments in applications of EMATs in electric power industry has opened a whole new horizon for EMATs utility.

Figure 6-13 Results of Feature Extractor

69

There are two areas in need of development for ease of use and further applications of EMATs: electronic drivers and signal processing of the EMAT output. The equipment required to perform NDE through EMAT technology is very hard to obtain. Most EMAT systems are the result of modifications to commercially available electronic equipment. Electronic drivers for EMATs require significant developments for offering a commercially viable system. Recent advances in signal processing, e.g., wavelet transforms and neural networks have introduced new techniques for signal processing and analysis of signatures from EMATs.

With the increased industrial interests in applications of EMATs and recent R&D efforts, EMATs will soon play a major role in many monitoring, diagnostic, and health assessment systems for commercial and industrial usage.

References

1. Y. H Pao, W. Sachse, and H. Fukuoba, “Physical Acoustics”, Academic, New York, 1984, vol.xvii, pp.61-143.

2. K. Hogberg, “Ultrasonic Testing Using the EMAT Technique”, Materials and Design, 1993 Butterworth-Heinemann, LTD., vol.14, No.4.

3. R. B.Thompson, “Noncontact Transducers”, proceeding of the 1977 Ultrasonic Symposium, IEEE Cat.No. 77, CH 1264 – ISU, pp.74-83.

4. G. A.Alers and L.R. Burns, “EMAT Designs for Special Applications”, Materials Evaluation, vol.45, Oct 1987.

5. M. Chenoworth, “EMAT-Based Diagnostic Technique with Applications to Substation Ground Mat Risers”, MS Thesis, Colorado School of Mines, R. Shoureshi; Thesis Advisor, Sept. 1998.

6. B. A.Auld, “Wave Motion”, vol.3, No.10, 1979

7. B. A.Auld, “Acoustic Fields and Waves in Solids”, Wiley, New York, 1973.

8. C. M. Fortunko, and R.B. Thompson, “Optimization of EMAT Parameters for Maximum Dynamic Range”, Proceedings of The 1976 Ultasonic Symposium, IEEE Cat.No. 76, CH 1120-SSU, p.12.

9- R. B. Thompson, “Method of Ultrasonic Inspection”, Pat. No.3-850-028, Nov. 1974.

10- R. B.Thompson, “Ultrasonic Shear Wave Transducer”, Pat.No.4-295-214, Oct.1981

11- W. Mohr, W. Repplinger, “EMA Excitation of Ultasonic Bulk Waves”, proceedings of 1978 Ultasonic Symposium, IEEE Cat.No. 78 – CH 1344 –ISU, p.126.

12- P. K. Davidson,”Buried Pipe Inspection Techniques Using EMATs”, WIS, Inc. Publications.

13- G. A. Alers,”Butt Weld Inspection Techniques Using EMATs”, US Patent No. 4-127-035; 4-295-214; and 5-537-876

14- R. B.Thompson,”Physical Principles of Measurements with EMAT”, NDE Research Center, Iowa State University, www.cnde.iastate.edu.

15- J. A. Johnson and N. M. Carlson, “1986 NDT International”, No. 19. p 190-196

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16- Division of Construction Engineering and Management, Purdue University, www.ech.purdue.edu/ECT/other/soundprint.htm.

17- R. M. White, “Generation of Elastic Waves by Transient Surface Heating”, Journal of Applied Physics, No.34, pp. 3359-3565, 1963

18- D. A. Hutchins, “Ultrasonic Generation by Pulsed Lasers”, Physical Acoustics VII, Academic Press, 1988

19- S. Dixon, C. Edwards, S. B. Palmer, “A Laser-EMAT System for Ultrasonic Weld Inspection”, proceedings of the SPIE Conference, Newport Beach, March 1999

20- J. R. Cook, J. F. Jackson, B. E. Droney, “Sensing -Cost Billet Temperatures with EMAT”, Iron and Steel Engineer, Sept 1989.

21- J. Kristan, G. Garcia, “ EMAT Evaluates Railroad Wheels”, Advanced Materials and Processes, Nov. 1998, pp25-27.

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7 SUMMARY AND CONCLUSIONS

Summary

There is a real need within the electric utility industry for a dependable, cost-effective method for inspection of overhead transmission line conductors. Some utilities have experienced very few failures in recent years, while others have experienced many. Causes of failures also vary greatly, but the two most common causes are corrosion of conductor core steel and broken aluminum strands in and around attachment points. Although conductor failure rates and causes vary significantly within the industry, virtually any conductor failure results in significant avoidable costs to the affected utility.

Non-destructive evaluation is a relatively mature field in general, though applications of the technologies to conductor inspection have been minimal to this point. Only one commercially available technology application for conductor inspection is currently in the market. This technology is the Cormon Overhead Line Corrosion Detector, which gives a measure of the loss of galvanizing on the steel core wires of ACSR conductors. It does not give a direct measurement of remaining conductor life, though at least one utility has done extensive work to quantify remaining service life for conductors on their system based on OHLCD results. However, this correlation on their system has been quite involved and is not likely to be completely valid for utilities in other areas.

Two other technologies are currently under development for ACSR conductor inspection. The first technology is the Cross-Checker, patented by Jim Booker of Booker Consulting. This technology seeks to quantify the loss of steel cross-section in steel core of ACSR conductors and thereby determine the remaining strength of the in-service conductor. The Cross-Checker is currently undergoing development and field evaluation and testing of a first field prototype. This project should be done by the end of 2000, and if successful should lead to continuing development beginning in 2001 to support applications to energized conductors and capabilities to deal with splices.

The second technology under current development is the application of Electromagnetic Acoustical Transducers (EMAT) to inspection of conductors. The primary focus of this development at this time is on detecting damaged and broken strands in and around the conductor attachment points, including underneath the armor rods and suspension clamps. A first field test of this technology is also planned for late 2000. A successful field test will likely lead to more development work in 2001 aimed at refining the technology for applications to energized lines and further calibration under different circumstances.

Finally, several other potential technologies were discovered in other industries that could be applicable to transmission line conductors with some additional development. The most notable of these are the Magnetostrictive Sensor and the Wire Rope Tester.

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Conclusions and Recommendations

There is significant development work already active for transmission line conductor inspection. The Cormon OHLCD has been available for many years and is being used effectively by at least one utility. Both the Cross-Checker and the EMAT technologies show good promise for their respective aspects of conductor inspection and assessment. Field demonstrations should be completed shortly that will give us an even better indication of their potential to provide reliable and cost-effective inspection data on conductors. It would be premature to pursue any new technology possibilities until it has been determined that these technologies will not meet our goal of a cost-effective and reliable inspection capability. Therefore, any additional development activities at this time should be directed toward those technologies.

Every effort should be made to evaluate the relative merits of the OHLCD and the CC. Each appears to provide valuable information for long term planning of maintenance activities. However, it appears that the Cross-Checker may have a better chance of giving an accurate indication of current condition and loss of strength within the conductor. Three separate opportunities have been identified to allow us to obtain OHLCD and CC inspection data on the same aged conductors. In each case, one or the other of the devices is already scheduled for an inspection of in-service lines of EPRI member utilities. This represents opportunities to get comparative data for the small incremental cost associated with conducting inspections with only one technology on each of those lines. Each of these opportunities should be pursued with the goal of quantifying the relative benefits, as well as synergies of the two technologies.

The EMAT technology addresses a totally different aspect of conductor condition from that addressed by the OHLCD and CC technologies. Furthermore, the EMAT may have the potential to include the conductor core condition in its assessment with some further development and testing. If field tests conducted as a part of the current project prove successful, this technology should also be pursued further in the interest of getting the “full package” inspection.

Finally, since the OHLCD, CC, and EMAT technologies all look at different aspects of conductor condition, some work should be initiated to look at how we can combine all of these technologies into one package. Currently each one is totally independent and provided by totally independent vendors. Therefore, to obtain the “full package” a utility would have to pay for three separate inspections, which is certainly not an economical approach since much of the cost will be in repeated trips to the line with different people. Combining the technologies in a single package could prove to be just the step that is needed to reach our goal of a reliable, cost-effective conductor inspection technology.

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A Utility Survey Results

1. Within the past 5 to 10 years, how many failures have your utility experienced because of:

a). Conductor failures? b). Splice/connector failures? c). Shield wire failures?

Utility Conductor Failures Splice Failures Shield Wire Failures

A 5 3 4

B 10 0 2

C 0 0 4

D 1 3 3

E 2 3 4

F 2 1 4

G 49 21 34

H 0 0 5

I 2 10 30

J 4 3 1

K 0 0 10

L 0 1 1

M 2 6 1

N 4 6 37

O 11 2 0

P 2 0 0

Q 2 5 0

R 0 3 4

S 1 0 0

T 0 0 0

U 0 2 3

V 0 0 3

W 0 2 3

AVER. = 4.409091 3.227273 6.652174

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2. In reference to your past failures, what percentages were due to:

a). Conductor corrosion?

Utility Responses A None B 100%-steel core of ACSR was rusted and brittle C There were signs of corrosion but no failures D 100% conductor failures E 50% F N/A G 40% due to deterioration H None I 5% J None K Some ACSR installed in 1920's has had bad core wires L 50% M 100% N 41% for conductor O P Q R None S None T U N/A V N/A W None

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b). Improper installation?

Utility Responses A 30% B None C None D Caused most of splice failures E F N/A G 60% of conductor sleeve failures were due to

improper installation. H The five shield wire failures were due to improper

installation. I 25% J 12.50% K At least one case where the steel sleeve was off-

centered; may have been cause of 1947 sleeves operating hot by

10-15 degrees C. L M Splices were improperly installed N N/A O 50% splice installation P Q 80% R Resulted from improperly installed splices S None T U One of the five failures were due to improper

installation. V N/A W One due to improper installation.

76

c). Existence of broken strands near or inside connectors?

Utility Responses A None B None C None D E F N/A G Two failures due to vibration H None I J None K None L M None N N/A O P Q 10% R None S None T U V N/A W One failure due to broken strands

77

d). Overheating?

Utility Responses A None B Could have had some effect on half of the failures as the

line was overloading

C None D E F N/A G 20% of the conductor sleeve failures overheated due to

improper installation. Some of these were 10 to 20 years old.

Some failed within 3 to 5 years.

H None I J 25% K Overheating due to improper installation of splices. L 50% M None N N/A O P Q R None S None T U V N/A W None

78

e). Broken strands in or adjacent to suspension clamps and conductor attachments?

Utility Responses A 30% B This is happening in some lines but not at failure modes. We are finding this in some of our rebuilds.

C Yes, there are some broken strands in some older lines, but they are not causing failures. Many are removed during rebuilds or reconductoring.

D E F N/A G Two cases were from vibration damage H All on shield wire I J 25% due to vibration K A few L M None N N/A O 50% broken strands of the conductor P 100% due to broken aluminum strands Q 10% R None S None T U V N/A W 1 OHG

79

f). Other factors? Please explain.

Utility Responses A 40%- weather and gun shots B Lightning seems to be the primary cause of shield wire

failures. C Lightning is believed to have caused 3 to 4 shield wire

failures. D Shield wire failures are a combination of both lightning

and corrosion damage.

E 25% due to lightning damage, 25% due to post insulator failure.

F Conductor breakage- Mid Span Fatigue problems in 100 year

conductor. Shield wire breakage- Failures occurred when larger

poles were installed or replaced a much smaller pole. (i.e.replacing

a failed 50 foot pole with a new 55 to 60 foot pole). The wire is

stressed and breaks under wind and ice loads. G Airplanes, Helicopter, Tornado, Lightning, and Trees H None I Lightning-25%, misapplication of using preformed strand

splices in overhead ground wires-45%. Line icing can also be a

problem with the preformed stranded splices. J 37% were the results of broken stranding due to gunshots

and lightning damage.

K Generally in wooded and hilly areas, breaks were found in 1978

during reconductoring. L M None N N/A O P Q R Impact of debris from cyclones S Thunder strom damage T U Ice loadings of static wires V Shield wire failures due to factory formed splices or dead-

end failures. W Ice loads exceeding design capacities. 1-2.5 inches

80

3. For each failure, what was the:

a). Line voltage? b). Conductor size/type? c). Line tension? d). Environmental conditions? Utility Line Voltage Conductor Size Line Tension Environmental Conditions

A 69, 138, & 345 kV

B 34.5 & 69 kV 2/0 ACSR N/A No unusual conditions C 115 & 345 kV 3/8" high strength N/A Metropolitan area steel

D 69 & 138 kV 3/0 ACSR Unknown Clean E 34.5 & 69 kV Small wire sizes Thounsands of lbs Snow/ice storms F 69 & 100 kV 2/0 hemp core, 3/8" All types of weather Siemens-Martin

G 46, 69, 115, 230, & Include different sizes Varies according to Rural, urban, agricultural, 500 kV of Cu & ACSR cond. requirements industrial, & some coastal

H 230 kV 10m Unknown Coastal fog & heavy exposure to salt contamination I 115 & 230 kV Various Unknown Plant sulfur contamination

J 138 kV 1033.5 ACSR 54/7, Various but more Minimal contamination or 4/0 ACSR 6/1, 3/0 than 20% rated pollution ACSR 6/1,159 ACSR breaking strength at 12/7, 2156 ACSR 84/ 0 deg F final 19, 477 ACSR 24/7

K 69 & 138 kV 477 ACSR 30/7-Hen, 9000# @ H.L.-650' 336.4 ACSR 18/1- R.S., 4000# @ H.L.- Merlin 350' R.S.

L 115 kV

M 60 & 138 kV 4\0 N/A Near salt water N 34.5, 46, 69, 115, N/A N/A N/A 138, 230, & 500 kV

O 69, 115, & 230 kV 3-stranded Cu, 4/0 2000#, 4500#, & Hilly with small trees ACSR, & 7.95 ACSR 10000#

P 69kV 397.5 KCM, 26/7 7800 # ACSR "IBIS"

Q 138 kV 266 & 4/0 ACSR 40% U.S. ultimate None strength rating

R 110 & 132 kV ACSR/GZ "Bear", 20% MCLB Urban, coastal, & tropical ACSR/GZ "Tiger"

S 115 kV 266.8 ACSR 26/7 3000# Heavy loading & rolling terrain T U

V 138 & 230 kV 7#6 AW, 12.5M AW 35% ultimate, & Unknown 5000 #

W 230 & 765 kV 500 Mcm Cu, 1351.5 Design 9K, & 14 K 1" radial ice, Sun 50. Calm Dipper sub conductor

81

4. What current technique(s) or devices are being used for inspection of overhead lines:

a). For conductors? b). For splices and connectors? c). For shield wires?

Utility Conductors Splices/Connectors Shield Wires

A Aerial patrol Thermovision Aerial patrol

B Infrared Infrared Infrared

C Infrared None None

D Magnetic field to Infrared Visual after it breaks

measure loss of

galvanizing

E Visual, Infrared & Visual & Infrared Visual & Infrared

climbing

F Visual Visual Visual

G Visual by ground, Infrared to detect hot Visual, working on

climbing, & air patrol

splices or connectors guideline for rating

inspections remaining life of OHGW

H Aerial patrol and Aerial patrol and Aerial patrol and

infrared infrared infrared

I Visual Visual Visual

J Visual and Infrared Visual and Infrared Visual

K Visual aerial Visual and aerial Visual aerial inspections

inspections themal survey

L Visual and Infrared Visual and Infrared Visual and Infrared

M Visual and Infrared infrared Visual

N Aerial patrol and Aerial patrol and Aerial patrol and

thermovision thermovision thermovision

O Visual infrared Visual

P Helicopter Visual Helicopter Visual Helicopter Visual

Q Visual Inspection Visual Inspection Visual Inspection

R Ground based or Infrared & ground Aerial ground based or

aerial helicopter based or aerial aerial helicopter visual

visual inspection helicopter visual inspection

S Visual Visual Visual

T Visual Visual Visual

U Visual Visual & Infrared Visual

V Helicopter Visual Helicopter Visual Helicopter Visual

W Infrared, helicopter, Infrared, x-rays Helicopter & ground

& ground patrols shakedown

shakedown

82

5. Do utility personnel, contractors, or a combination of both do these inspections?

Utility Responses

A Both

B Fixed wing is Contractor. Helicopter is a combination

C Fixed wing is contract, and helicopter is done by lineman

D Combination of both- Shannon Tech for conductors

E Visual and helicopter by utility personnel and infrared done by

Contractors

F Combination

G Mostly utility personnel

H Utility trained inspectors and one thermographer

I Combination

J Utility personnel for visual and contractor for infrared

K Combination-company personnel in hired "chopper"

L Combination

M Utility personnel

N Combination of both

O Utility personnel

P Utility personnel

Q Both

R Combination of utility personnel and helicopter maintenance

contractor staff

S Utility only

T Utility personnel

U Only utility for visual

V Combination of utility personnel and contractors

W Utility personnel and some contractors

83

6. Are there formal written procedures for conductor, splice/connectors, and shield wire inspections? If so, may we contact you to obtain a copy of those inspection procedures?

Utility Responses

A No

B No

C None

D No

E Yes, the procedures specify looking for burns, rust, discoloration,

broken or missing strands, and any other signs of damage for the

conductors, splice/connectors, and shield wire.

F No

G

H Not at this time

I No

J No

K No

L Use inspection sheet for in-house inspection

M Yes

N Yes

O No

P No

Q No

R No, a brief list of typical defects which may be observed are

Given to the staff

S No

T No

U No

V No

W No

84

7). What methods do you currently use to repair conductors, splices/connectors, and shield wires to mitigate potential failures?

Utility Responses A Install armor rods over areas with broken aluminum strands, extend dead-end assembly to capture damaged areas within jumper, and install vibration dampers

B Depending on severity of problem, (how many strands are broken) armor rod, repair sleeve, full tension sleeve

C Depending on the amount of strands that are broken or damaged we use armor rod, repair sleeves, line guard or full tension splices.

D Replace sleeves or conductors E We place repair sleeves over conductors and shield wires when damage is found that can be safely repaired with this method. If this is not suitable, we cut the wire and put in a splice. Some- times we need to splice in some wire. We replace hardware that shows wear. Damaged splices are replaced.

F Conductor replacement, repair sleeves, and shield wire removal G H Traditional repairs or replacements I Preformed strand splices in shield wires are replaced with compression crimp connectors J Armor rods, splice in new conductor K Install armor rods & splice rods-cut out section if necessary-used preformed shunt splices on 477 ACSR 30/7 Hen in 1970's

L Use repair sleeve or replace section of conductor or shield wire. Replace splices or connectors

M Replace N Aerial work platform or conventional line procedures O Full tension splices or repair rods depending on amount of damage

P Replacement-repair sleeves for conductor and shield wire damage Q Repair sleeves, repair armor rod R Conductors-if only strands in outer layer are damaged, preformed repair rods are wrapped over the break. If a large # of strands are damaged or the steel core strands of a ACSR conductor is damaged, then a compression midspan joint is installed. Shield wires-performed repair rods are used in most cases to either repair damaged strands or alternately the wire is cut and a preformed splice is used. Splices/Connectors-cut out joint and splice in a new section of conductor/wire approx. 30-m. 'Hot' bolted connections discovered are unbolted, cleaned/repaired and rebolted, with conductor de-engergized. All parallel grove clamps on strain tower jumpers have now been removed and replaced with compression joints.

S Conductors-preformed wire repair kits for mild to moderate damage. Splice in new conductor for severe damage.

T U Replace them V Replace all factory formed splices and dead-ends on shield wires. Repair gunshot conductors with repair rods, repair splices or full tension splices, depending on damage.

W Armor rod exterior wrap, and repair of splices is accomplished using elongated splices

85

8. Do you use a resistance measurement device to check the conditions of splices and connectors?

Utility Responses A No B Have begun using a device to check connections on deadend jumpers

C No D No E Not currently F No G Only in a laboratory H No I No J No K No, but am aware of a device L No M No N No O No P No Q No R Not yet. We are now preparing to check some 110kV circuits before the end of 1999. The work will be done live from a Elevated Work Platform

S No T No U No V No W No

86

9. What is the inspection frequency for your overhead lines?

69kV _____ 115kV _____ 161kV _____ 230kV _____ 345kV _____ 500kV _____ 765kV _____

Utility 69 kV 115 kV 161 kV 230 kV 345 kV 500 kV 765 kV A 2/yr 2/yr 2/yr B 3/yr 3/yr 3/yr 3/yr C D Don't understand? Lines in general-yearly. Conductor after 50 years sleeves only where high

temperature operation is expected. E 3/yr 3/yr N/A N/A 3/yr 3/yr N/A F 10 year inspection on all line voltages G H 2/yr 2/yr 3/yr I 1/yr 2/yr 2/yr 1/yr J 2/yr N/A N/A 2/yr 2/yr N/A N/A K 2/yr 2/yr 2/yr 2/yr L 2/yr 2/yr 2/yr M 3/yr 3/yr 3/yr 3/yr N 1/yr 1/yr 1/yr 1/yr 1/yr 1/yr O 4/yr 4/yr 4/yr 4/yr P Six month helicopter inspection on rural. 5 year inspection on metro lines

Q 1/ 5yrs 1/yr R N/A 1/yr N/A 1/yr N/A N/A N/A S N/A 4/yr N/A 4/yr N/A N/A N/A T 5/yr N/A 5/yr 5/yr N/A N/A U 1/yr 1/yr N/A N/A 1/yr N/A N/A V 1/yr 1/yr 1/yr 1/yr 1/yr 1/yr W 2 air patrols, one ground patrol per year. Shakedown wood-5 years, and shakedown steel-10 years.

87

10. What is your approximate cost per mile for inspection and analysis of your overhead transmission lines?

Utility Responses A $100/mile B $100/mile C $100/mile D Varies, random partial conductor inspections is $20,000/line and infrared is $5,000/line

E Rough estimate is $75/mile F $70/mile G H I don't have these costs I Not available J $100/mile K Less than $100/mile L Not available M Not available N Not available O $60-80/mile per inspection depending on terrain P $100,000 per year (75% is ground inspection) Q R $78/mile for developed areas, and $153/mile for remote areas

S Do not know T Unknown U Not available V Comprehensive inspection-$650/mile, general- $20/mile. W Unknown

88

11. Are you using computer software to store your inspection data information? If so, what software?

Utility Responses A MS Word B MP2 C None D PLT internal database E Only simple spreadsheets like MS Excel. We are Migrating to the EPRI's TIM product

F EPRI's TIM system G Yes, our own system H We use EPRI's TIM inspection software I Maximo work management system J Yes, in house database K Excel spreadsheet L Yes, in house database M Yes, PASSPORT, and in house database N No O No P Database in Microsoft Access Q No R Some use is made of field data recorders by some Groups. All defects are manually entered into our Central maintenance management system (SAP).

S Yes, in house software on Access database T Yes, in house using an Oracle database. This will be Replaced by SAP.

U No V Evaluating TIM 2.4 W Maximo, TIM

89

12. Are there a higher percentage of conductor failures near connectors and joints?

Utility Responses A Yes B No C No D Yes, at low point of sag E It seems to be higher F No G Yes H N/A I No, have very few conductor failures J No K Not aware that they are L Conductor failure was at connector M Yes N N/A O No P ?? Q Yes R No data S No T U Yes V Unknown W Yes

90

13. Are conductor failures a major problem or concern in:

a). Shoe and dead end fittings? b). Spacers and marker balls?

Utility Shoe and Dead Ends Spacer and Marker Balls A No No B Yes Yes C Yes Yes D Yes Haven't been but could be E Definitely a concern Can't recall any F No No G A large number occur Had one failure due to marker at jumpers and other Ball fittings

H N/A N/A I No No J Yes, concern No K Not aware of problem L No No M No Yes N N/A N/A O Yes No P More occur here Q R No No S Suspension clamps Some concern but not a big only Problem

T U No N/A V No No W No No

91

14. If an IR camera is used and a hot spot is detected, do you: (check all that apply)

___ Replace the splice or connector immediately ___ Replace it within three months ___ Replace it within a year ___ Conduct close-up inspections before removal ___ Leave it intact until the next inspection cycle

Utility Replace Immediately Replace in 3-months

Replace in 1-year Close-up Inspection

Leave Until Next Inspection

A X X

B X X

C X

D X

E X X X X X

F X X

G

H X

I X

J X

K X

L X X

M X X

N X

O X

P Don't use IR camera per previous comment

Q N/A

R X X

S N/A

T

U X

V N/A

W X X X

92

15. To increase power flow, utilities are raising the operating temperature of their overhead lines. Do you have any specific concerns regarding the performance of line components such as splices and connectors?

Utility Responses

A No B Yes C Yes D Yes E Yes, as operating temperatures rise, we expect more failures of our 30, 40, and 50 year old fittings. That's one reason we're interested in the performance other utilities are experiencing. We'd like to increase operating temperatures of some lines, but believe we need more data before taking this step. We also have to maintain reliability indexes or face economic penalties from our regulatory agency.

F None as of yet G Yes, we are concerned that the hardware may not hold up at some temperatures on a long-term basis. We are also concerned about decreasing the life of the conductor as well as the hardware

H I do not, but our engineering group I am sure does. I No J Yes, splices and oxide inhibiting compounds. K Yes, along with the installation errors, we have found considerable difference in size of fittings by different suppliers. Some suppliers are not considering high temperature operation!

L Yes, that is why we established a base infrared survey in the past few years and will redo every five years to determine how much base has changed.

M No N O No P No, temperatures are not high enough to be of concern (85 C type and 100 C max).

Q R Many 100/132kV transmission lines have been upgraded in the past with no reported problems. In the past, no condition monitoring of dead ends, splices or connectors had been carried out as part of the upgrading project. However, reports of problems with ACSR conductors joints elsewhere have caused a rethinking of our strategy. We have commenced this year to check circuits which are to be upgraded to ensure conductor intergrity. Inspections processes are still being developed.

S We are not operating any conductor above 100 C and are not that concerned about splices or connectors at these temperatures for short durations on a very rare occasion.

T U No V No, not raising temperatures at present time W Yes, also conductor clearance due to increase loading. Other concerns are creep, localized annealing, increased resistance, and increased risk of failure.

93

16. Are there any non-destructive or non-invasive methods presently being used by your company to detect conductor damage before failures occur? If so, what are they?

Utility Responses

A IR camera

B Just Visual

C Only visual line inspections

D Shannon Technology

E We're evaluating some methods. We're interested in acoustical technology and the Daytime Corona Viewing Technology, but haven't considered anything else.

F No

G Just methods mentioned previously

H No

I No

J No

K No

L Only visual and infrared

M Observation and infrared

N No

O No

P No

Q No

R No, the Remote Controlled Corrosion Dector originally developed by the CEGB in mid 1980's has been tried on a few sections of line with limited success.

S No

T

U IR

V Trial only. Conductor core corrosion assessment device- Shannon Technologies

W Infrared every 2 years, x-rays of in-service splices

94

17. How useful would a non-destructive or non-invasive device for detecting corrosion, fatigue damage, and broken strands be to your organization for inspection of conductors?

Utility Responses

A Moderate, we don't have enough failures to perform routine testing

of all our lines.

B Very useful

C Very useful, but what are the cost? Maintenance dollars are limited these days!

D Minimal

E We'd find it very useful, provided it offered us a cost benefit.

F We would look at the device to see the applicability of the device and the cost.

G There would be use for a device that could give information and predict imminent failures in important hardware. There would also be use for a device to predict remaining life of conductors. (We thought that there was a project for Cross-Checker to do this).

H We are not having the problems that would cause us to seek out these analysis tools.

I Very little

J We're interested

K Need to have a better understanding of extent of problem to judge.

L Depends on cost. Based on our experience, the infrared and visual inspections are more than adequate. Any device would have to be utilized during our helo patrol and would have to be similar to the infrared inspection procedures.

M Valuable

N Very useful if cost effective

O Limited

P Useful, but must find damage immediately adjacent to fittings (clamps, deadends, joints, etc.)

Q

R Some use. At present we are attempting to measure conductor deterioration on our transmission lines by evaluation of conductor samples recovered during normal maintenance and augmentation work.

S Such a device would be of value to us if it could detect broken strands under a suspension clamps.

T It would be very useful to improve productivity and find undetected problems from the visual inspections.

U Little because IR is all we need.

V Useful if cost-effective. Would only be used on older lines

W The results would be invaluable, but the use would probably be by contractors.

95

18. Do you have any knowledge of or experience using any non-destructive devices or technologies for conductor, splice/connector, or shield wire inspection? If so, please give details.

Utility Responses A We are aware of a remote controlled device that travels the conductor. It determines the degree of corrosion of the steel core of ACSR.

B No C No D Shannon Technology E Nothing other than those mentioned above F No G Devices commercially available are expensive to use and time consuming. They are limited in scope. Accuracy is also very questionable.

H EPRI I believe is looking into a resistance device for splice integrity I No J None K No L M No N No O No P No Q R No S T U V See 16 W Through AgRoters-we did some x-rays on 4-bundle 1351.5 splices in services

on 765 kV.

96

19. Do you think that failures of conductors or splices/connectors are a current or impending problem? Please explain.

Utility Responses A No B No C As I have indicated above, the most recent problem have been old Shield wires that have failed, not conductors in the last 10 years. But, as we push the limits on the older existing conductors, there is the potential for failures.

D Yes, I don't think utilities are getting adequate answers on Assurances from manufactures. I think current industry "ANSI" test standards are OBSOLETE!

E Current problem. We have many older lines that have been in Harsh environments for many years. It is likely that some wires and connectors are in poor condition, even if they appear visually to be acceptable.

F Do not know G H I No J Yes, aging lines and increased loading (heating) K We are not aware of these types of failures causing wide spread black-outs. Certainly, as lines are "pushed harder & harder", they could fail.

L On our system, conductors and splices/connectors are not a serious or impending problem because we have had a proactive inspection program for many years. The recent infrared survey did point out some connector hardware concerns and were replaced. However, we also have a program of replacing older shield wire that has deteriorated. These shield wires have been in service up to 75 years. It appears a useful life of 50 years is "normal" for the galvanized and Copperweld conductors.

M Current N Yes, based on the percentage failure rate we have documented. O No P Impending-as lines age and vibration damages accumulate. Q No, the majority of our problem has been experienced on one line in which several splices were improperly installed years ago. That line is presently in the process of being rebuilt.

R We are assuming that as our lines age and loads increase, it is likely that failures will increase. In addition, urban development near our lines make the consequences of any failure far more serious. These factors are pushing us to preventive maintenance practices to prevent falling conductors'.

S We are only concerned about older lines that were built with inadequate vibration control. We have a program to inspect, repair, and add vibration control to these lines. Once this is completed, we will be much less concerned.

T U No V Only in older (50-60 yrs) lines. We are not experiencing enough failures to be concerned

W Splice/connectors may prove to be an impending problem as the age of the hardware increases and loading is raised.

97

98

20. Do you think that current inspection and repair techniques are satisfactory? Please explain.

Utility Responses A Yes B Repair, yes. Inspection, no. And maybe it isn't the inspection technique, infrared, so much as the interpretation of the information and how it is gathered.

C The infrared inspections have not given us true information that we can rely on. We have found cases that a connection shows up hot but is unexplainable as to why. All parts look like they were installed correctly.

D Infrared has worked well for us to identify problems. There may be better ways to inspect? Sleeve replacement has also worked okay. If conductor has extensive corrosion, replacement seems like the only liable option.

E Current methods are highly subjective and costly. The industry needs more non-destructive and/or predictive methods that can be done with low cost. With the pressure to reduce/freeze rates, and the deregulation of the generations industry resulting in more suppliers vying to use our lines, we need low cost, highly effective methods of inspection/repair and better predictive methods.

F Yes, in our area we're are not seeing the conductor or splice problems. G H I Yes, we could do IR inspection on entire line, but it gets rather costly. J Marginally, not many techniques out there. K Aerial thermal surveys are fast & fairly low cost. However, there is a risk that due to light line loading, serious problems may not show up.

L Yes-if done. Also see answer #17. M No, a corrosion detecting device for steel core conductor is not needed by the utility industry.

N Satisfactory, however, better methods of detecting poor splices would be helpful.

O Yes, all broken strands were found under clamps that were removed as part of structure change outs. The 115kV damage was on a line that should have been built with dampers but was not. Now all of our lines are built with dampers. The 69kV lines copper conductor was #2-3 strand in a section where the conductor was installed at too high a tension. Because of our dry climate, conductor corrosion is not a problem. We have inspected conductors removed after years of service and found no problems.

P Today, for us, yes. However, as facilities age, we'll be looking for additional techniques.

Q Yes R We experienced a step change 2 years ago when changing to biannual aerial inspections. This change allowed us to identify more conductor, shield wire and insulator defects than had previously been identified. We are continuing to explore new condition monitoring techniques to further improve our asset maintenance processes.

S Current inspection techniques are expensive and difficult to coordinate due to the need to de-energize the line to inspect under the suspension clamp, but the results are reliable and easy to interpret. Our long range goal is to avoid the need to inspect and repair conductor damaged by vibration.

T The present inspection process is working, however we would like to improve it with a non-destructive testing device that will hopefully come out of the EPRI project mentioned above.

99

U Yes V Our present failure rate does not justify additional expenditures or a change in methods. We are presently evaluating the use of helicopter-born IR.

W Yes, at this location failure has not occurred due to inspection methods.

100

21. Additional Comments?

Utility Responses A B I believe the infrared technology is good and can be the method that is best utilized if there can be consistency and accuracy that is believable. My experience has been that some items such as bolted connections can be 100% accurate where splices and deadends become very ambiguous in determining if they are really bad or not. I believe the original testing done at EPRI confirmed this.

C D My first priority on this subject is simply better testing standards for new sleeves. This should be easy to do and could be completed quickly. Next, how to inspect shield wire? After its is on lthe ground is too late!

E This whole issue is highly complex and variable problem. All utilities are seeking to get the biggest bang for their buck, reduce the repetitive, manual activities, and maintain a high level of availability while returning the highest return to the investors as possible. Large utillities have different problems and economies than smaller ones. Long transmission lines, through varied climatic conditions have more potential problems than shorter length lines.

F G We are currently contacting research facilities to investigate state of the art technology on conductor deterioration and connector integrity. How to predict remaining life and impending failures.

H I J K Regarding repair techniques, stocking of repair devices should be considered on an industry-wide basis. Suppliers or distributors may have to be paid to make and stock devices for emergency response. Determining an acceptable way to collect this cost is the challenge.

L M N O P Q R S T U V W

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Documents

Inspection & Assessment of Overhead Line Conductors