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Tools for Conductor Evaluation: State of the ArtReview and Promising Technologies

1002002


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Tools for Conductor Evaluation: State of the ArtReview and Promising Technologies

1002002

Technical Update, December 2003

EPRI Project Manager

J. Chan

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com


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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

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

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT ISSUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISDOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

EPRIsolutions, Inc.

This is an EPRI Technical Update report. A Technical Update report is intended as an informal report ofcontinuing research, a meeting, or a topical study. It is not a final EPRI technical report.

ORDERING INFORMATIONRequests for copies of this report should be directed to EPRI Orders and Conferences, 1355 WillowWay, Suite 278, Concord, CA 94520. Toll-free number: 800.313.3774, press 2, or internally x5379;voice: 925.609.9169; fax: 925.609.1310.

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric PowerResearch Institute, Inc.

Copyright 2003 Electric Power Research Institute, Inc. All rights reserved.


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CITATIONS

This document was prepared by

EPRIsolutions, Inc.

100 Research DriveHaslet, Texas 76052

Principal InvestigatorD. Cannon

This document describes research sponsored by EPRI.

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

New Tools for Conductor Evaluation: State of the Art Review and Promising Technologies,EPRI, Palo Alto, CA: 2003. 1002002.


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ABSTRACT

EPRI has investigated degradation modes and inspection and assessment methods for overheadlines. This report provides an overview of the findings of this investigation. Primarydegradation modes that have been identified include broken strands due to vandalism orconductor motion, corrosion of steel shield wires and the steel core and adjacent aluminumstrands on conductors, degradation of conductor joints from corrosion and high temperatureoperation, and loss of strength in conductors due to high temperature operation. Inspectionmethods that are most effective vary according to the type and location of the conductordegradation. In particular, visual inspection techniques are effective only for degradation that isreadily visible from the ground or air during a routine inspection. Therefore, more sophisticatednon-destructive inspection technologies are needed to inspect for degradation that is inside theconductor or attachments and therefore not readily visible. This report identifies several of thenon-destructive inspection technologies available and briefly discusses their application tooverhead line conductors.


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CONTENTS

1INTRODUCTION....................................................................................................................1-1

2CONDUCTOR CHARACTERISTICS

.....................................................................................2-1

General..................................................................................................................................2-1

Materials................................................................................................................................2-1

Construction...........................................................................................................................2-2

AAC................................................................................................................................2-2

ACSR..............................................................................................................................2-2

AAAC .............................................................................................................................2-3

ACAR .............................................................................................................................2-3

ACSS ..............................................................................................................................2-3

Copper.............................................................................................................................2-3

Steel ................................................................................................................................2-3

Alumaweld....................................................................................................................2-4

Copperweld...................................................................................................................2-4

Special Conductors ...........................................................................................................2-4

3DEGRADATION MODES .......................................................................................................3-1

General..................................................................................................................................3-1

Broken Strands.......................................................................................................................3-1

Corrosion...............................................................................................................................3-3

Bad Joints ..............................................................................................................................3-6

Strength Loss .........................................................................................................................3-7

4INSPECTION & ASSESSMENT METHODS ..........................................................................4-1

Inspection Methods.................................................................................................................4-1

Inspection Methods for Broken Strands ....................................................................................4-1

Visual Inspection ..............................................................................................................4-1

Corona Inspection.............................................................................................................4-2

EMAT Inspection .............................................................................................................4-2

Thermal Imaging ..............................................................................................................4-2

Radiographic Inspection ....................................................................................................4-3

Inspection Methods for Corrosion ............................................................................................4-3


Corrosion Detector Inspection............................................................................................4-4

Rotesco Inspection............................................................................................................4-6

EMAT Inspection .............................................................................................................4-6

Inspection Methods for Bad Joints............................................................................................4-6


Thermal Imaging ..............................................................................................................4-7


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Resistance Measurements ..................................................................................................4-7

Inspection Methods for Strength Loss .......................................................................................4-8

Lab Testing ......................................................................................................................4-9

Methods of Condition Assessment .........................................................................................4-10

Making Repair and Replacement Decisions.......................................................................4-11

5SUMMARY .............................................................................................................................5-1


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2CONDUCTOR CHARACTERISTICS

General

It is important to have a clear understanding of the characteristics of overhead line conductors inorder to better evaluate and select appropriate inspection and assessment methods. Thefollowing sections provide a discussion of the materials commonly used in bare overheadconductors and of the construction characteristics of the more commonly used conductors.

Materials

Bare phase conductors and overhead ground wires used in overhead transmission lines aregenerally constructed from three materials: copper, aluminum, and steel.

Many years ago copper was the primary choice for conductor wires due to superior conductivityand strength. It is still common to find copper conductors in older lines. However, the use ofcopper in newer lines is uncommon because it is heavier and generally more expensive thanaluminum conductor with similar electrical properties. In addition to its good conductivityproperties, copper also has good corrosion resistance properties.

Today, aluminum is the conducting material most prevalent in overhead line conductors becauseof its favorable price and low weight relative to copper. Aluminum also has good conductivity

and while it is subject to corrosion, the resulting corrosion creates a protective layer that tends toprotect the conductor from further deterioration. However, the stiffness and strength ofaluminum is lower than copper. Therefore, for longer spans it becomes necessary to strengthenthe conductor either by using a stronger aluminum alloy or by adding material with a greaterstrength and stiffness.

Traditionally, the material added to aluminum to achieve greater strength has been steel. Steelhas high strength and low conductivity. But by combining the aluminum with steel we are ableto draw on the relative strengths of each material to obtain a conductor that has both theelectrical conductivity and the mechanical strength necessary to perform effectively under thearray of conditions to which it is subjected. Steel is also highly susceptible to corrosion attackand is therefore typically galvanized for protection.

An alternative to using zinc (galvanizing) as a protective layer for steel is to cover the steel with

a conductive layer that is more corrosion resistant than steel. Copperweldhas a thick layer of

copper bonded to the steel and Alumoweldhas a thick layer of Aluminum bonded to the steel.The result is wires that have both increased corrosion resistance and increased conductivityrelative to galvanized steel. These wires are more commonly used for overhead ground wires,although they may be used as components of phase conductors as well.


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Construction

Overhead phase conductors and shield wires can be constructed of either single solid wires or ofa stranded group of smaller wires. At transmission voltages it is most common to see strandedconductors on modern construction (Figure 1). These stranded conductors are classified as

concentric-lay-stranded conductor, meaning that they consist of a straight center core wiresurrounded by one or more layers of helically wound wires. While most stranded conductorsconsist of a number of round strands, there are special conductors that include layers oftrapezoidal strands. The number and size of strands depends of the type and size of theconductor. Following is a summary of more common conductor types.

Figure 1 - Cross-section of a Typical Stranded Aluminum Conductor, Steel Reinforced (ACSR)

AAC

All-aluminum conductor (AAC) is a low cost conductor that offers good conductivity andcorrosion resistance, but only moderate mechanical strength. Therefore it is most often used inapplications of short spans where maximum current transfer is required. The individual strandsof an AAC conductor are all the same diameter and layered to generate conductors having totalsof 1, 7, 19, 37, 91, or 128 strands (1, 2, 3, 4, 5, or 6 layers, respectively).

ACSR

Aluminum conductor, steel reinforced (ACSR) is probably the most common conductor foundon existing transmission lines. It consists of a steel core surrounded by one or more layers ofaluminum. The steel core may consist of 1, 7, or 19 galvanized steel strands. This steel core

may be surrounded by up to three layers of aluminum strands, which may be of the same ordifferent diameter from the steel strands in the core. The high tensile strength coupled with thegood conductivity of the ACSR conductor makes it the conductor of choice for manyapplications.


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AAAC

All-aluminum alloy conductor (AAAC) is similar in construction to the AAC except that thealuminum strands are replaced with an aluminum alloy that yields greater mechanical strengthwhile maintaining excellent conductivity and corrosion resistance. These conductors are

typically used in corrosive environments when the required strength is greater than that providedby an AAC conductor.

ACAR

Aluminum conductor, aluminum-alloy reinforced (ACAR) is similar in construction to theACSR except that the galvanized steel core is replaced with an aluminum alloy core that givesthe conductor higher mechanical strength that AAC conductor while maintaining corrosionresistance properties in the core.

ACSS

Aluminum conductor, steel-supported (ACSS) is similar to the ACSR conductor except that thealuminum strands are fully annealed. This causes the entire load to be carried by the steel coreunder typical operating conditions. The major benefit of this conductor is that it can be operatedat temperatures above 200

oC without loss of strength. While not common in older lines, this

conductor is often being considered and used in construction of new lines and upgrading of oldlines to increase the thermal limits.

Copper

While rarely used in new construction of overhead transmission lines, copper conductor is fairlycommonly encountered in very old transmission lines. Copper conductor is available in a varietyof sizes ranging from a single strand up to as many as 61 strands. Copper has excellentconductivity and good corrosion resistance. However it is heavier and more expensive thanaluminum and is therefore used less frequently in modern overhead line construction.

Steel

High-strength and extra-high-strength steel conductor is commonly used for overhead groundwires in transmission line construction. These conductors consist of 7-strands of steel and rangein size from 5/16-in. to 5/8-in. in diameter. The steel is very susceptible to corrosion and is

therefore coated with zinc (galvanized) to improve its corrosion resistance. However, in aparticularly corrosive environment the zinc tends to deteriorate over time and corrosion of thesteel eventually becomes an issue once again.


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Alumaweld

Alumaweldconsists of steel strands with a thick aluminum cladding in place of the typicalgalvanizing. This both provides corrosion protection to the steel and improves the conductivityof the wire, which improves its performance when subjected to lightning strokes. For these

reasons it may be used in place of galvanized steel for the overhead ground wire in transmissionlines. Alumaweldwires are available in several sizes consisting of 3, 7, 19, and 37 strands.

Alumaweldwires can also be used instead of the normal galvanized steel core in ACSRconductors to create ACSR/AW conductors.

Copperweld

Copperweldconsists of steel strands with a thick copper cladding in place of the typicalgalvanizing. This both provides corrosion protection to the steel and improves the conductivityof the wire, which improves its performance when subjected to lightning strokes. For thesereasons it may be used in place of galvanized steel for the overhead ground wire in transmission

lines. Copperweldwires are available in several sizes consisting of 1, 3, 7, and 19 strands.

Copperweldand copper strands can also be combined into a stranded Copperweldcopperconductor.

Special Conductors

Some of the more commonly used special conductors replace round wire strands of previouslydiscussed conductors with trapezoidal aluminum strands. This places a larger volume ofaluminum within the same cross-sectional diameter, increasing the power transfer capabilitywithout increasing the projected wind area and resulting loads on the conductor and its support

structures. These trap wire designs are typically a variation on the AAC, ACSR, and ACSSconductors discussed previously. They are designated by adding /TW to the conductordesignation (e.g. ACSR/TW).

Self-damping conductor (SDC) has a steel core surrounded by one or more aluminum layersmuch like ACSR. However, with SDC conductor the first layer of aluminum, and sometimes theouter layers as well, consists of trapezoidal strands of such a size that a gap exists between thesteel core and the first layer of aluminum. The structural characteristics of the steel andaluminum layers give them different natural vibration frequencies, which lead to frequentimpacts between the layers. These impacts tend to damp any Aeolian vibration of the conductor.

GAP conductors are similar to the self-damping conductor in construction. The primarydifference is that the gap between the steel and aluminum is filled with special heat resistantgrease. This allows the aluminum to float on the outside of the steel and surrounding greaseand the conductor can be tensioned via the steel core without placing any tension on thealuminum. The result is that the amount of sag in the conductor is much less at high temperaturethan that for traditional ACSR conductors.

Several new conductor technologies are currently emerging that take advantage of compositematerial technology. These new technologies include aluminum conductor carbon fiber


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reinforced (ACFR), aluminum conductor composite reinforced (ACCR), aluminum conductorcomposite core (ACCC), and composite reinforced aluminum conductor (CRAC). Each of theseconductors uses composite material technology to engineer conductors with improved thermalperformance.


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3DEGRADATION MODES

General

Having a clear understanding of the degradation modes that can lead to failure of the conductoror shield wire can help us make better selections of appropriate inspection and assessmentmethods. Degradation modes that can eventually lead to failure include broken strands,corrosion, deterioration of joints, and loss of material strength.

Broken Strands

There are two primary sources of broken strands in conductors and shield wires, vandalism andwear or fatigue.

In areas where hunting and shooting are common pastimes, one may find locations where theconductor has been shot. This is the primary type of vandalism that can harm conductors andcontribute to their long-term degradation. Damage from gunshot could range from nickedstrands to a few broken strands to a bullet lodged into the conductor. Damage that is likely theresult of gunshot is shown in Figures 2 and 3.

Figure 2 - Example of Gunshot Damage - Bulging Strands


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Figure 3 - Example of Gunshot Damage - Broken Strands

Wear and fatigue of conductor strands due to wind induced conductor motion can also lead tobroken strands. Three types of wind-induced conductor motion can cause damage to conductors.Aeolian vibration is high frequency, low amplitude vibration of the conductor due to low-speedwind flow perpendicular to the conductor. Galloping is low frequency, high amplitude motion of

the conductor due to wind-on-ice loading of the conductor. Wake-induced oscillation is lowfrequency motion of bundled conductor due to shielding effects of leeward conductors bywindward conductors.

Aeolian vibration results in fatigue of conductor strands in the area of attachments to theconductor. Eventually this fatigue results in broken strands in the area of attachment as shown inFigure 4. Attachments that could contribute to fatigue damage include tower suspensionattachments, spacer attachments, and compression splices and dead ends. Aeolian vibration iscaused by low speed smooth winds perpendicular to the conductor. Therefore, lines that areperpendicular to prevailing wind and in areas where light laminar winds are common are mostsusceptible Design steps are normally taken to minimize Aeolian vibration including limitingconductor tension, use of armor rods at attachment points, and application of vibration damperson conductors.


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Figure 4 - Example of Broken Strands Under Suspension Attachment Due to Fatigue from AeolianVibration

Galloping results in very high dynamic loading of the conductor and can lead to conductordamage and/or failure. Galloping is normally caused by steady moderately strong windperpendicular to an asymmetrically iced conductor. In a worst-case situation, the amplitude ofconductor galloping can meet or even exceed the sag of the conductor. A very short duration of

extreme galloping can cause major damage to not only the conductor, but also to many other linecomponents. Damage to conductors would typically occur in two areas. Broken conductorstrands may occur in conductor attachment areas. The surface of the conductor could be scaredwith pitting and burn marks out in the span due to flashovers between adjacent phase conductorsduring the galloping event.

Wake-induced oscillation may occur in bundled conductors under the influence of moderate tohigh-speed steady wind. A variety of motion patterns can occur depending on bundlearrangements. The magnitude of motion can be very small or can be large enough to causeadjacent conductors in the bundle to clash together. Resulting damage can include acceleratedwearing of conductors and hardware at attachments and at points where adjacent conductorsclash together.

Corrosion

Corrosion is a primary means of deterioration for metals. Most metals corrode in the presence ofwater, acids, bases, salts, oils and other solid and liquid chemicals. They also corrode whenexposed to various gases including acid vapors, ammonia, and sulfur containing gases. The rateof corrosion is dependent of a variety of factors including the properties of the metal and the


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presence of water, oxygen and various contaminates. Regardless of the corrosion rate, theeventual result is loss of material and potentially reduced ability of the metal to perform itsintended function.

Corrosion is an electrochemical process that seeks to reduce the binding energy in metals. In the

presence of an electrolyte (e.g. water) a metal atom is oxidized, whereby it loses one or moreelectrons and leaves the metal surface. The lost electrons are conducted through the metal toanother site where they are reduced. The site where metal atoms lose electrons is called theanode, and the site where electrons are gained is called the cathode.

There are two primary types of corrosion that can attack overhead line conductors and shieldwires, atmospheric corrosion and galvanic corrosion.

Atmospheric corrosion is simply the gradual degradation of a metal by contact with substancespresent in the atmosphere, such as oxygen, carbon dioxide, water vapor, and sulfur and chlorinecompounds. In the case of atmospheric corrosion, the anode and cathode would be on the samebase metal. Atmospheric corrosion is primarily a concern with galvanized steel overhead ground

wires and the galvanized steel core of ACSR or ACSS conductors. Figure 5 shows a sample ofcorrosion on an overhead ground wire. The internal strand is exposed in the photograph to showthat the corrosion is primarily on the exterior surfaces of the wire. Figure 6 shows exposed steelcore strands from two identical conductor samples, one sample showing corrosion on the steelcore and the other showing a corrosion-free steel core. Note that the outer surface of thealuminum strands looks about the same for both conductors, which is an indication of thedifficulty in identifying steel core corrosion by field observation.

Figure 5 - Corrosion of Overhead Ground Wire


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Figure 6 - Corroded and Pristine Steel Core Examples

Galvanic corrosion occurs when two dissimilar metals are brought together in the presence ofmoisture and electric potential. This causes one metal to become an anode and the other tobecome a cathode. The result is that the corrosion of the anode will accelerate and the corrosionof the cathode will decelerate or stop. The anode will be the metal that can most easily give upan electron.

In the case of ACSR conductors, the zinc galvanizing would be the anode and the aluminumwould be the cathode as long as the galvanizing is intact. However, once the zinc coating hasbeen exhausted by the galvanic corrosion and the aluminum is now in contact with the steel, thealuminum becomes the sacrificial anode and begins to corrode at a higher rate while the rate ofsteel corrosion is reduced. Figure 7 shows an expanded conductor sample illustrating both steelcore corrosion and galvanic corrosion of the aluminum strands in contact with the steel core.Note the dark discoloration of the outer aluminum strands at the top and bottom of the photo.Also note that there is no galvanizing remaining on the steel core strands. The lighter coloredareas on the internal aluminum strands are locations where galvanic corrosion of the aluminumhas probably begun. The byproduct of galvanic corrosion of the aluminum is a white powderybyproduct that might work its way out to the conductor surface and be visible in extreme cases ofgalvanic corrosion.


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Figure 7 - Expanded Conductor Showing Steel Core Corrosion and Initial Galvanic Corrosion ofAluminum

Bad Joints

One of the primary issues for the integrity of a conductor is the degradation of the compressionjoints used to splice two conductor segments together or connect the conductor to a dead-endsupport.

The most common conductor joints are fittings made of aluminum and steel sleeves or cylindersthat are compressed on the conductor using a hydraulic press. This creates a friction connectionbetween the conductor strands and the associated compression sleeve. For all aluminumconductors (AAC, AAAC, etc.) these joints consist of a single aluminum sleeve that iscompressed onto the outside of the conductor. For conductors that include a steel core the jointsnormally consist of two sleeves, one smaller sleeve compressed on the steel core and a larger

aluminum sleeve over top of the steel sleeve and compressed on the aluminum strands.

Conductor joints are subjected to most of the same degradation modes as the full conductorincluding corrosion, excessive current, mechanical overload, and conductor motion. For properperformance of these joints it is important that correct construction methods be followed. Whenthey arent followed correctly the resulting construction defects tend to aggravate the effects ofthe various degradation modes that have been discussed. The most common problems result inhigh resistance in the joint, which in turn leads to elevated joint temperature during high currentoperations and eventually to mechanical failure of the joint.


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Strength Loss

Excessive current in the conductor leads to overheating, which in turn can lead to annealing andresulting loss of strength in the aluminum portions of the conductor. This excessive current cancome from either emergency operations or from current surges due to lightning or faults.

Repeated operation of aluminum conductors at temperatures of 100oC or greater will result in

annealing of the aluminum strands and loss of conductor strength for most standard conductors(AAC, ACSR, etc.). For this reason normal operating limits are usually set below thattemperature. However, for emergency operations utilities will allow thermal limits up to 125

oC

or higher for short durations. The resulting loss of strength in the aluminum depends on thecumulative duration at various temperatures above 100

oC.

For example, 100-hrs of operation at 125oC would result in approximately 7% reduction in

strength for the aluminum strands of a normal conductor1. For an AAC conductor this equates to

a 7% loss of strength for the conductor. However, for a representative 795-kcmil ACSRconductor (26/7 Drake) this may yield only a 3% loss of strength. For emergency operatingpurposes, a utility might limit a conductor to 24-hrs at an emergency temperature of 150oC eachyear. Over a 30-year conductor life, this would result in a 24% loss of strength for an AACconductor but only a 10% loss of strength for our representative Drake ACSR conductor. Longerservice life with more accumulated time above 100

oC would lead to greater strength loss.

Fault currents and lightning also contribute to annealing of aluminum and loss of strength in theconductor. Although the duration of loading is generally very short, the current is also very high.Therefore, heating due to faults and lightning has a cumulative effect with emergency operations.Because the duration and frequency of these fault currents is normally relatively small, this effectis normally neglected in predictions of remaining strength of conductors. However, it could be asignificant conductor degradation factor if a line has been subjected to excessive fault events.

In addition to loss of conductor strength due to aluminum annealing, high temperature operationof conductors also contributes to additional conductor creep and increased sag. This is a verycomplex phenomenon and beyond the scope of this report. However, this effect can result insignificant changes in the conductor sag.

Pitting and melting of the conductor surface is a second form of degradation that occurs in thecase of faults or lightning strokes. Aluminum is more resistant to this effect than other metalsand may only show a loss of sheen, a slight roughening, and a change of color on the conductorsurface. However, the existence of these symptoms is indicative of a conductor that hasexperienced current surges and may have experienced other damage as a result.

1Aluminum Electrical Conductor Handbook, Third Edition, The Aluminum Association,Washington, D.C. 20006, 1989.


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4INSPECTION & ASSESSMENT METHODS

Inspection Methods

The challenge is to identify critical degradation of conductors before the degradation leads to anyof the failure modes defined previously. Therefore, adequate inspection methods andfrequencies are important to maintain the integrity of the system. Unfortunately, there is nosingle inspection method that is effective at detecting all of the degradation modes identified. Asa result, a combination of techniques is required to obtain a complete assessment of the conditionof overhead line conductors and ground wires.

Inspection Methods for Broken Strands

The ease with which broken strands can be detected is primarily determined by the location ofthe broken strands within the conductor cross-section and within the length of the conductorspan.

Strands that are broken due to vandalism are generally visible on the surface of the conductorbetween conductor attachments. The ease of their detection may depend upon whether brokenstrands protrude from the conductor or remain in their normal lay position flush with theconductor surface.

On the other hand, strands that are broken due to conductor motion are more likely to be oninternal conductor layers or within the confines of conductor attachments and therefore hiddenfrom view. These hidden broken strands are much more of a challenge to the inspector.

Visual Inspection

Visual inspection is the most commonly used form of inspection for all transmission lineproblems. Normally, this visual inspection takes place from the ground or from a helicopter. Inthe case of overhead conductors and ground wires, there is a limit to what problems can bedetected using visual inspection techniques. But the most commonly visible problem is brokenouter strands of the conductor, often with protruding strands making the damage more easily

detected.

Unfortunately, broken strands that occur internal to the conductor or within the confines ofattachments to the conductors cannot be easily detected during a routine visual inspection.Broken strands inside the conductor and under attachment points can be detected visually byphysically exposing the damage, which requires contact and possibly de-energization of theconductor. But this is an expensive inspection method that is generally undertaken only in caseswhere a history of broken strands at conductor attachment points warrants the added expense.


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Corona Inspection

Oftentimes, broken strands on the surface of the conductor will be protruded from the conductor.This may make them easier to detect visually. Additionally, at higher voltages these protrudingstrands are likely to generate corona that can be detected by various techniques. The DayCor

camera is coming into common use for corona inspection of overhead transmission lines. Thistechnology can detect small amounts of corona during daylight conditions, making it an idealchoice for inspecting for broken conductor strands. In addition to protruding broken strands,conductor problems that can be detected by the DayCor camera include bird-caging and severesurface scaring.

Conditions that are detectable by DayCor are limited to those that cause corona. Therefore,conductor corrosion and broken strands internal to the conductor and/or conductor attachmentsare generally not detectable by this technology. EPRI has published an extensive guide forcorona detection using DayCor that includes examples of conductor problems that can bedetected

2.

EMAT Inspection

The Electromagnetic Acoustical Transducer (EMAT) technology has recently been developed byEPRI for stranded conductors and overhead ground wires. This technology was developed fordetection of broken strands internal to conductor and attachments and is currently undergoingfield trials for this application. As illustrated in Figure 14, this technology can be applied on anenergized line using bare-hand techniques. The technology is able to identify conditions inwhich several broken conductor strand exist within the region of attachment to the supportstructure. As with all non-destructive inspection technologies, it is critical that a valid calibrationof the EMAT be performed in order to assure reliable test results. Field trials are continuing togenerate additional calibration and usability data to be incorporated in final production versions

of the technology.

Thermal Imaging

Thermal imaging with an infrared camera is commonly used to detect conductor joints with highresistance and resulting elevated temperature. In theory thermal imaging can be used to detectany conductor degradation that yields an increase in conductor resistance. This would includebroken conductor strands as well as bad joints. However, while thermal imaging has been usedsuccessfully to detect broken strands in the laboratory, it has generally been found that fieldapplication of thermal imaging to detect broken conductor strands outside of joints is noteffective.

2Guide to Corona and Arcing Inspection of Overhead Transmission Lines, EPRI, Palo Alto, CA:2001. 1001910.


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Figure 8 - EMAT Unit Being Placed on Conductor

Radiographic Inspection

Radiographic inspection can also be used to detect broken strands internal to the conductor

and/or the attachment point. This involves placing either X-rays or gamma rays near theconductor region to be inspected. These rays pass through the inspection article and are capturedon film. When the film is processed the image is a series of grey shades between black andwhite. If properly applied and tuned this image will show any broken conductor strands insidethe attachment point. This method is expensive and must be applied with care. Therefore it isnot used frequently in practice.

Inspection Methods for Corrosion

Corrosion is typically an issue for both aluminum conductors with galvanized steel core andgalvanized steel shield wires. The shield wire is subject to atmospheric corrosion, which begins

with the exposed zinc coating on the steel and progresses to corrosion of the steel once it isexposed to the atmosphere. The galvanized steel core of the conductor is subject to bothatmospheric corrosion and galvanic corrosion under the proper conditions. Corrosion beginswith the zinc coating on the steel and progresses to the steel by atmospheric corrosion or thealuminum by galvanic corrosion once the zinc coating has been pierced.


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The quantity and location of spans to be sampled with corrosion detection technologies should beestablished based on conductor age and environmental conditions. Areas of greater corrosivitymay warrant sampling of a larger number of spans. Particular attention should be given toconductor spans in areas subjected to point sources of pollution such as power plants, chemicalplants, and other industrial facilities.

Rotesco Inspection

Since the EPRI evaluation of the OHLCD and Cross-Checker technologies were completed, athird commercial device has been identified for inspection of steel shield wires and steelreinforced aluminum conductors. Rotesco Inc. of Ontario has been manufacturing instrumentsfor over 30-years that non-destructively test steel wire ropes used in underground miningoperations. This technology is able to inspect wire ropes up to 2 inches in diameter andmeasure changes of 0.1% in the metallic cross-sectional area of the rope. They can detect asingle broken wire in a 2-inch diameter rope that is made up of 162 wires. They have nowadapted and enhanced this technology to develop an instrument that can non-destructively test

the steel core of overhead line conductors. The instrument is remote controlled and will travelalong the conductor much like the OHLCD and Cross-Checker technologies.

EMAT Inspection

The previously mentioned EMAT technology also has promise for assessment of corrosion inconductors and shield wires. Although the technology was developed and has been calibratedfor detection of broken strands, it also has been shown that corrosion generates a distinctivesignal on the device. Additional evaluation and calibration is necessary to validate theapplication of EMAT for corrosion detection and assessment.

Inspection Methods for Bad Joints

Degradation of the compression joints used to splice two conductor segments together or connectthe conductor to a dead-end support is a relatively common problem in conductors. Jointproblems are often the result of construction flaws that allow degradation of the joint due tocorrosion, high temperature operation, or mechanical loading. The most common problemsresult in high resistance in the joint, which in turn leads to elevated joint temperature during highcurrent operations and eventually to mechanical failure of the joint.

Visual Inspection

Visual inspection of conductor joints can give some indications that a problem might exist.However, many joint problems can exist without providing visual clues. Some visual indicatorsthat joint problems might exist or develop include discoloration of the joint due to hightemperatures or excessive joint deformation.


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Thermal Imaging

As mentioned previously thermal imaging with an infrared camera is commonly used to detectconductor joints with high resistance and resulting elevated temperature. The principle is thatmost joint problems lead to high electrical resistance in the joint, which in turn leads to high

operating temperatures in the joint when heavy electrical loading is applied. Therefore, aninfrared camera can be used to detect joints that are operating at high temperature due to jointproblems.

EPRI has conducted research on behavior of compression joints and the application of thermalimaging for inspection of compression joints in conductors

5,6. While thermal imaging can be an

effective method for detecting problem joints in conductors, it is very sensitive to a number ofparameters and can give misleading results if not applied carefully. Among the factors that areimportant considerations in obtaining effective results are electrical loading, ambienttemperature, wind speed and direction, and emissivity of the joint.

It is critical that the conductor be heavily loaded electrically at the time the thermal images are

collected. If the conductor is not operating with a high current load, even a bad joint may notgenerate enough heat to be detected by the infrared camera.

Ambient temperature and the wind speed and direction also have an important effect on theactual temperature of the joint. If the temperature is very cool and the wind speed is very highand perpendicular to the conductor, the cooling effects may dissipate the heat in the jointsufficiently that the infrared camera will be unable to reliably detect a problem in a bad joint.

Finally, the actual emissivity of the joint is important to obtaining accurate results from theinfrared camera. The camera must be set for a specific emissivity. The emissivity of a new jointis probably significantly different than that of an older joint. Therefore, to get accurate results

the camera must be adjusted properly for emissivity, focus and distance for each joint inspected,and the emissivity selected on the camera must match the actual emissivity of the joint in orderto get the most accurate temperature measurements.

Resistance Measurements

An alternative to thermal imaging is to measure the resistance of the joint directly. Two deviceshave been identified for making this measurement on an energized transmission line.

The OhmStik, pictured in Figure 11, is pressed against the joint and provides a directmeasurement of the line current and the resistance of the joint in micro-ohms. The resistance of

a new joint should be 30 to 70 percent of the resistance of the connected conductor. If theexpected or accepted resistance of the joint is not know, a resistance measurement of the adjacentconductor can be collected and then the OhmStik can be programmed to indicate good or bad

5Electrical, Mechanical, and Thermal Performance of Conductor Connections, EPRI, Palo Alto, CA: 2001.

1001913.6Infrared Inspection Application Guide: Overhead Transmission and Substation Components, EPRI, Palo Alto, CA:

2002. 1001915.


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Figure 11 - Ohmstik Joint Resistance Measurement Device

based on the ratio of the measured joint resistance to the measured conductor resistance. Theadvantage of this device is that it gives a direct measurement of joint resistance, eliminatingconcern about many of the external factors that impact thermal imaging measurements. Thedisadvantage is that it requires someone to place the OhmStik in contact with the conductorjoint.

The second resistance measurement device, shown in Figure 12, is called ROBHOT. This is a

helicopter-borne robot unit developed by SwedPower. The ROBHOTis lowered from ahelicopter and placed directly on the energized transmission line. Measuring probes are foldedout and the resistance is measured in a few seconds. Like the OhmStik, the advantage is thatthe unit measures resistance directly, avoiding the complications from the external factors thataffect the thermal imaging measurements. However, it must be applied from a helicopter, whichis an expensive platform to operate. However, SwedPower claims that they can measureresistances for up to 40 splices per hour on an energized line.

Inspection Methods for Strength Loss

Loss of strength in a conductor comes from several factors including broken strands, corrosionand resulting loss of cross-section, and annealing due to electrical overloading. Identification ofbroken strands and corrosion will provide some indication of strength loss in a conductor.However, it will not allow you to easily quantify the loss of strength. Other than havingexcellent records of conductor electrical loading over its lifetime, which most utilities dont seemto have, it is impossible to determine via a field inspection how much strength loss has occurreddue to annealing. Therefore, the only sure way to quantify the loss of strength in a conductor isto remove samples from the field and conduct laboratory tests.


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Figure 12 - ROBHOT Helicopter-Borne Robot Unit

Lab Testing

It is advisable to remove conductor samples from a few inspected spans and conduct laboratorytests on them to validate inspection results and to quantify the actual condition of the conductorat those locations. Sampling of the conductor or overhead ground wire for laboratory testing isperhaps the most reliable technique available to determine the general condition of a conductor

and make decisions regarding replacement. Samples removed for testing should be at least 3-ftlong. Longer samples are preferred as they provide sufficient length to allow tests to be repeatedif necessary. If a tensile test of the full conductor is desired, an additional length of 20-ft or moreshould be removed.

Samples should first be carefully inspected. A length of at least 18 inches should bedisassembled to document the condition of each layer of strands. In particular, evidence ofcorrosion and/or broken strands should be noted and photographed.

Each strand from the dissected piece of conductor should be tensile tested according to ASTMB498 for steel and B230 for aluminum. For each strand the strand diameter and the breakingstrength should be recorded. From these values the average tensile strength of the steel strandsand aluminum strands should be determined and compared with minimum ASTM requirements.Once the tensile strength of each strand is known, the rated breaking strength of the conductor asfound should be calculated to determine the remaining strength of the conductor

7.

7Overhead Conductor Manual, Southwire Company, Carrollton, GA, 1994.


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If desired, a tensile test of the full conductor may also be conducted. However it is very difficultto obtain reliable results from this test due to the stress concentration effects from the endconnections used to apply the tensile loads. Without special arrangements the failure will almostalways occur adjacent to the end connections and at loads well below the rated breaking strengthof the conductor. A method has been published that reportedly eliminates these end effectsresulting in an accurate measurement of the actual breaking strength of the full conductor

8.

Additional testing should be conducted to evaluate the ductility of the as-found conductors. Thiscan be accomplished using either a torsional test of the strands (ASTM A938) or an alternatebending test of the strands (ASTM A363). Ideally these tests will also be conducted for a newconductor of the same type to establish a baseline. A severe reduction in the number of turnsto failure is an indication of loss of ductility due to corrosion.

Methods of Condition Assessment

Once the available technologies have been applied to determine the state of degradation of

conductors and overhead ground wires, the results must be reviewed and an assessment of thecondition of the conductor must be made to determine whether repair or replacement is required.The degree of condition assessment that can be done depends on the type of inspection that wascompleted and the quality of data collected.

In the case of a visual inspection from the ground or air, little more than a subjective assessmentof the conductor or overhead ground wire condition can be performed. Based on the externalappearance of the conductor and any visible evidence of broken strands, one might hypothesizewhether the conductor requires replacement or not.

In the case of a significant number of visible broken strands, the conductor strength may be

calculated based on the remaining intact strands observed at that location, providing a moreobjective assessment of conductor condition. Likewise, if a visual inspection of conductorstrands within the tower attachment area can be accomplished, or a nondestructive technologysuch as EMAT can be effectively applied to determine the existence and extent of broken strandsin conductor attachment points, a conductor strength calculation is theoretically possible. But inthe case where armor rods are included in the tower attachment connection, the armor rods maybe sufficient to reinforce the conductor and maintain its strength with several existing brokenconductor strands. Many utilities have their own guidelines regarding how many broken strandsare acceptable, with and without armor rods, before conductor replacement or repair is required.However, the author is unaware of any specific industry standard that quantifies how manybroken strands are acceptable before repair or replacement should be made.

8Akhtar, A, "Localized Intrinsic Strengthening Approach (LISA): A Practical Method forDetermining the Tensile Strength of Multistrand Cables",ASTM Journal of Testing andEvaluation (JTEVA), Vol. 16, No. 2, March 1988, pp. 124 133.


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By adding inspection with one of the corrosion detection technologies we increase ourknowledge of the conductor condition. However, it is still somewhat subjective in terms ofremaining strength and remaining life. There isnt a lot of documented evidence indicating howone should interpret results from corrosion detection technologies and convert them intoremaining strength or remaining usable conductor life. Nevertheless, this gives us the ability ofprioritizing line replacements based on an objective indication of the relative conductor corrosiondamage between different lines.

While perhaps not the most economical method, by far the most effective method for assessingthe condition of overhead line conductors and shield wires is removing conductor samples andtesting them in the laboratory. By collecting a sufficient number of samples to give a goodrepresentation of a line, or the population of lines, and obtaining laboratory measurements ofstrength we can have a very objective indication of the current condition and capability of theconductor or shield wire.

Making Repair and Replacement Decisions

The most obvious measure that can be used for determining the need to replace a conductor orshield wire is to compare the as-found conductor strength with the maximum design loads for theline in question. The National Electric Safety Code (NESC) establishes a tension limit forconductors at 60% of the rated breaking strength of the conductor under maximum ice & windloads. Using laboratory test results for the as-found conductor one can calculate the current ratedbreaking strength of the conductor. Then if the maximum tension at final conditions formaximum ice and wind loads exceeds 60% of this measured breaking strength something mustbe done to bring the design back into compliance. Replacement of the conductor is the obvioussolution, but one could also theoretically reduce conductor tensions to meet requirements,assuming this could be done without violating clearances.

A second consideration for replacement decisions is the loss of ductility in the conductor. Forconductors that are subject to heavy ice loading and/or galloping, this is a particularly importantconsideration. In particularly cold weather and with dynamic loading from galloping, conductorsthat have lost much of their ductility are at a greater risk for failure.


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5SUMMARY

EPRI has been investigating degradation modes and inspection and assessment methods foroverhead line conductors for a few years. This report provides an overview of the findings todate and identifies newer inspection technologies that have not yet been thoroughly evaluated.

Primary modes of degradation that have been identified include broken strands due to vandalismor conductor motion, corrosion of steel shield wires and of the steel core and adjacent aluminumstrands on conductors, degradation of conductor joints from corrosion and high temperatureoperation, and loss of strength in the conductor material due to high temperature operation.

Inspection methods that are most effective vary according to the type and location of theconductor degradation. In very few cases, visual inspection techniques may be used effectively.More frequently, some type of non-destructive inspection device is needed to assess thecondition of the conductor.

In the case of broken strands, two non-destructive inspection technologies have been developedthat are effective. The first is the DayCor camera, which is an effective aid in locating brokenstrands in cases where the strands protrude form the conductor sufficiently to create corona. Thesecond is a new EMAT (electromagnetic acoustical transducer) technology developed by EPRIin a separately funded project. The EMAT device that has been developed is able to detectbroken strands internal to the conductor and internal to conductor attachment points. The unit issimply placed on the energized conductor adjacent to the conductor section to be inspected. The

output of the device is a simple good or bad indication for the inspected conductor. Calibrationand field-testing of this technology is continuing with the expectation of a commercially viabledevice within the next year.

In the case of conductor corrosion, several non-destructive inspection devices have beenidentified and some have been evaluated in field trials. The Overhead Line Corrosion Detector(OHLCD) determines how much galvanizing remains on the steel core of conductors and gradesthe conductors accordingly. The OHLCD is also able to identify significant losses of aluminumdue to galvanic corrosion. The Cross-Checker device uses a magnetic field to quantify loss ofcross-section in the steel core of the conductor, but is unable to identify losses of cross-section inthe aluminum. These two technologies were evaluated in field trials, which have been

documented in a previous report. Recently, a third technology has been identified from Rotesco,Inc., which uses a modified wire rope tester to identify loss of cross-section in the steel core ofconductors.

Degradation of conductor joints has been addressed only briefly in the current investigation,since past EPRI projects have addressed this topic. Inspection methods identified are all basedon the increased resistance that occurs as a conductor joint deteriorates and approaches failure.The most commonly used method is thermal imaging to identify joints that are operating at


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