Tools for Conductor Evaluation: State of the ArtReview and Promising Technologies 1002002 Tools for Conductor Evaluation: State of the Art Review 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 • USA 800.313.3774 • 650.855.2121 •
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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 of continuing research, a meeting, or a topical study. It is not a final EPRI technical report. ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, 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 Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2003 Electric Power Research Institute, Inc. All rights reserved. Inc. 1002002. 100 Research Drive Haslet. iii . The publication is a corporate document that should be cited in the literature in the following manner: New Tools for Conductor Evaluation: State of the Art Review and Promising Technologies. CA: 2003. Texas 76052 Principal Investigator D. EPRI. Cannon This document describes research sponsored by EPRI.CITATIONS This document was prepared by EPRIsolutions. Palo Alto. iv . Therefore. degradation of conductor joints from corrosion and high temperature operation.ABSTRACT EPRI has investigated degradation modes and inspection and assessment methods for overhead lines. This report identifies several of the non-destructive inspection technologies available and briefly discusses their application to overhead line conductors. v . This report provides an overview of the findings of this investigation. more sophisticated non-destructive inspection technologies are needed to inspect for degradation that is inside the conductor or attachments and therefore not readily visible. corrosion of steel shield wires and the steel core and adjacent aluminum strands on conductors. In particular. visual inspection techniques are effective only for degradation that is readily visible from the ground or air during a routine inspection. and loss of strength in conductors due to high temperature operation. Inspection methods that are most effective vary according to the type and location of the conductor degradation. Primary degradation modes that have been identified include broken strands due to vandalism or conductor motion. vi . ............................................................................................................................................2-1 General......................3-7 4 INSPECTION & ASSESSMENT METHODS ....................................................................................................................................................................................................................................2-3 Steel .........................................................................2-4 Special Conductors ............................................2-2 ACSR............................................................................................................................................................................................................................................................2-4 Copperweld.........................................................................CONTENTS 1 INTRODUCTION.............................................................................................................................4-1 Inspection Methods for Broken Strands ........................2-1 Materials.................................................3-1 Broken Strands.................2-2 AAC.........1-1 2 CONDUCTOR CHARACTERISTICS .......2-4 3 DEGRADATION MODES..............................................................................................................................................................................................................................................4-1 Corona Inspection.................................................................................4-2 Thermal Imaging ..........................4-1 Inspection Methods.....................................................................4-2 Radiographic Inspection ................................................................................................................................................................................................................................3-3 Bad Joints ................................................4-3 Visual Inspection ...........................................................4-1 Visual Inspection ..........................................................................................................................................................................................................................................................................................................................................................................................................................................................................4-7 vii ...................................................................................................................................................3-6 Strength Loss .....3-1 General............................................3-1 Corrosion......2-1 Construction..........................4-6 EMAT Inspection ...4-4 Rotesco Inspection............................................................................................................................................................4-2 EMAT Inspection .................................4-6 Visual Inspection .................................................................................................................................................................................................................................................................................................2-2 AAAC ....................................................2-3 Alumaweld........................................................................................4-3 Inspection Methods for Corrosion ..............................................................................................................2-3 ACAR .........2-3 ACSS .........................4-4 Corrosion Detector Inspection............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................4-6 Thermal Imaging ................................................................................2-3 Copper........................................................................4-6 Inspection Methods for Bad Joints................................................................................................................................................................................................................................................................................... ......................................................................................4-11 5 SUMMARY .....................................................................................................................................4-7 Inspection Methods for Strength Loss.................................................................................................................5-1 viii ......................................................................4-8 Lab Testing .....................................................................................................4-10 Making Repair and Replacement Decisions............................................Resistance Measurements ...4-9 Methods of Condition Assessment ....................................... 1 INTRODUCTION For the past few years EPRI has been investigating methods for inspection and condition assessment of conductors on overhead transmission lines. They must also resist daily and extreme loading events without failure and must maintain sufficient clearance from the phase conductors that they shield. The primary function of overhead ground wires is to shield the phase conductors from lightning strokes. 1-1 . The investigation has included two types of bare overhead transmission conductors: (1) phase conductors and (2) overhead ground or shield wires. which makes it appropriate to address them simultaneously. This report provides an update on the findings of this investigation to date. The characteristics and behavior of phase conductors and overhead ground wires are similar in many ways. An effective inspection and assessment method must be able to detect degradation that might lead to a failure of the conductor or shield wire well before such a failure occurs. Overhead transmission line conductors provide the conductive path to carry electric power from the power generator to the end user. The primary function of these conductors is to carry electric current without excessive electrical losses. To do this they must resist daily and extreme electrical/mechanical loading events without failure and they must maintain sufficient clearances to avoid phase-to-ground or phase-to-phase flashovers. . Steel has high strength and low conductivity. Traditionally. and steel. the resulting corrosion creates a protective layer that tends to protect the conductor from further deterioration. 2-1 . However. although they may be used as components of phase conductors as well.2 CONDUCTOR CHARACTERISTICS General It is important to have a clear understanding of the characteristics of overhead line conductors in order to better evaluate and select appropriate inspection and assessment methods. The result is wires that have both increased corrosion resistance and increased conductivity relative to galvanized steel. Copperweld has a thick layer of copper bonded to the steel and Alumoweld has a thick layer of Aluminum bonded to the steel. 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. the material added to aluminum to achieve greater strength has been steel. It is still common to find copper conductors in older lines. The following sections provide a discussion of the materials commonly used in bare overhead conductors and of the construction characteristics of the more commonly used conductors. But by combining the aluminum with steel we are able to draw on the relative strengths of each material to obtain a conductor that has both the electrical conductivity and the mechanical strength necessary to perform effectively under the array of conditions to which it is subjected. aluminum. Therefore. Materials Bare phase conductors and overhead ground wires used in overhead transmission lines are generally constructed from three materials: copper. the stiffness and strength of aluminum is lower than copper. Today. Steel is also highly susceptible to corrosion attack and is therefore typically galvanized for protection. copper also has good corrosion resistance properties. aluminum is the conducting material most prevalent in overhead line conductors because of its favorable price and low weight relative to copper. the use of copper in newer lines is uncommon because it is heavier and generally more expensive than aluminum conductor with similar electrical properties. In addition to its good conductivity properties. These wires are more commonly used for overhead ground wires. for longer spans it becomes necessary to strengthen the conductor either by using a stronger aluminum alloy or by adding material with a greater strength and stiffness. Many years ago copper was the primary choice for conductor wires due to superior conductivity and strength. Aluminum also has good conductivity and while it is subject to corrosion. However. there are special conductors that include layers of trapezoidal strands. While most stranded conductors consist of a number of round strands. These stranded conductors are classified as concentric-lay-stranded conductor. 19.Cross-section of a Typical Stranded Aluminum Conductor. ACSR Aluminum conductor. 7. At transmission voltages it is most common to see stranded conductors on modern construction (Figure 1). 37. but only moderate mechanical strength. The high tensile strength coupled with the good conductivity of the ACSR conductor makes it the conductor of choice for many applications. Steel Reinforced (ACSR) AAC All-aluminum conductor (AAC) is a low cost conductor that offers good conductivity and corrosion resistance. or 6 layers. meaning that they consist of a straight center core wire surrounded by one or more layers of helically wound wires. 2-2 . steel reinforced (ACSR) is probably the most common conductor found on existing transmission lines. This steel core may be surrounded by up to three layers of aluminum strands. 5. or 128 strands (1. Following is a summary of more common conductor types. Figure 1 . respectively). Therefore it is most often used in applications of short spans where maximum current transfer is required. 91. It consists of a steel core surrounded by one or more layers of aluminum. 4. The individual strands of an AAC conductor are all the same diameter and layered to generate conductors having totals of 1. The steel core may consist of 1. 3. The number and size of strands depends of the type and size of the conductor. 2. 7.Construction Overhead phase conductors and shield wires can be constructed of either single solid wires or of a stranded group of smaller wires. or 19 galvanized steel strands. which may be of the same or different diameter from the steel strands in the core. AAAC All-aluminum alloy conductor (AAAC) is similar in construction to the AAC except that the aluminum strands are replaced with an aluminum alloy that yields greater mechanical strength while maintaining excellent conductivity and corrosion resistance. These conductors are typically used in corrosive environments when the required strength is greater than that provided by an AAC conductor. Steel High-strength and extra-high-strength steel conductor is commonly used for overhead ground wires in transmission line construction. Copper has excellent conductivity and good corrosion resistance. in a particularly corrosive environment the zinc tends to deteriorate over time and corrosion of the steel eventually becomes an issue once again. The major benefit of this conductor is that it can be operated at temperatures above 200oC without loss of strength. This causes the entire load to be carried by the steel core under typical operating conditions. in diameter. ACAR Aluminum conductor. this conductor is often being considered and used in construction of new lines and upgrading of old lines to increase the thermal limits. copper conductor is fairly commonly encountered in very old transmission lines. aluminum-alloy reinforced (ACAR) is similar in construction to the ACSR except that the galvanized steel core is replaced with an aluminum alloy core that gives the conductor higher mechanical strength that AAC conductor while maintaining corrosion resistance properties in the core. However. However it is heavier and more expensive than aluminum and is therefore used less frequently in modern overhead line construction. Copper conductor is available in a variety of sizes ranging from a single strand up to as many as 61 strands. steel-supported (ACSS) is similar to the ACSR conductor except that the aluminum strands are fully annealed. to 5/8-in. While not common in older lines. Copper While rarely used in new construction of overhead transmission lines. 2-3 . These conductors consist of 7-strands of steel and range in size from 5/16-in. ACSS Aluminum conductor. The steel is very susceptible to corrosion and is therefore coated with zinc (galvanized) to improve its corrosion resistance. and 19 strands. This both provides corrosion protection to the steel and improves the conductivity of the wire. and ACSS conductors discussed previously. Special Conductors Some of the more commonly used special conductors replace round wire strands of previously discussed conductors with trapezoidal aluminum strands. GAP conductors are similar to the self-damping conductor in construction. 19. This both provides corrosion protection to the steel and improves the conductivity of the wire. 3. This allows the aluminum to “float” on the outside of the steel and surrounding grease and the conductor can be tensioned via the steel core without placing any tension on the aluminum. They are designated by adding /TW to the conductor designation (e. These new technologies include aluminum conductor carbon fiber 2-4 . which improves its performance when subjected to lightning strokes. ACSR. Copperweld wires are available in several sizes consisting of 1. This places a larger volume of aluminum within the same cross-sectional diameter. For these reasons it may be used in place of galvanized steel for the overhead ground wire in transmission lines. Self-damping conductor (SDC) has a steel core surrounded by one or more aluminum layers much like ACSR. Alumaweld wires can also be used instead of the normal galvanized steel core in ACSR conductors to create ACSR/AW conductors. The primary difference is that the gap between the steel and aluminum is filled with special heat resistant grease. Alumaweld wires are available in several sizes consisting of 3.g. and sometimes the outer layers as well. The result is that the amount of sag in the conductor is much less at high temperature than that for traditional ACSR conductors. Copperweld and copper strands can also be combined into a stranded Copperweld copper conductor. with SDC conductor the first layer of aluminum. consists of trapezoidal strands of such a size that a gap exists between the steel core and the first layer of aluminum. and 37 strands. These “trap wire” designs are typically a variation on the AAC. which lead to frequent impacts between the layers. Several new conductor technologies are currently emerging that take advantage of composite material technology. Copperweld Copperweld consists of steel strands with a thick copper cladding in place of the typical galvanizing. ACSR/TW). 7. For these reasons it may be used in place of galvanized steel for the overhead ground wire in transmission lines. These impacts tend to damp any Aeolian vibration of the conductor. However. 7. The structural characteristics of the steel and aluminum layers give them different natural vibration frequencies. increasing the power transfer capability without increasing the projected wind area and resulting loads on the conductor and its support structures.Alumaweld Alumaweld consists of steel strands with a thick aluminum cladding in place of the typical galvanizing. which improves its performance when subjected to lightning strokes. Each of these conductors uses composite material technology to engineer conductors with improved thermal performance.reinforced (ACFR). 2-5 . aluminum conductor composite reinforced (ACCR). and composite reinforced aluminum conductor (CRAC). aluminum conductor composite core (ACCC). . This is the primary type of vandalism that can harm conductors and contribute to their long-term degradation. Damage that is likely the result of gunshot is shown in Figures 2 and 3. Degradation modes that can eventually lead to failure include broken strands. one may find locations where the conductor has been shot. corrosion. Broken Strands There are two primary sources of broken strands in conductors and shield wires.Example of Gunshot Damage . deterioration of joints.Bulging Strands 3-1 . In areas where hunting and shooting are common pastimes. Figure 2 .3 DEGRADATION MODES General Having a clear understanding of the degradation modes that can lead to failure of the conductor or shield wire can help us make better selections of appropriate inspection and assessment methods. and loss of material strength. Damage from gunshot could range from nicked strands to a few broken strands to a bullet lodged into the conductor. vandalism and wear or fatigue. Broken Strands Wear and fatigue of conductor strands due to wind induced conductor motion can also lead to broken strands. lines that are perpendicular to prevailing wind and in areas where light laminar winds are common are most susceptible Design steps are normally taken to minimize Aeolian vibration including limiting conductor tension. spacer attachments. high amplitude motion of the conductor due to wind-on-ice loading of the conductor. Attachments that could contribute to fatigue damage include tower suspension attachments. use of armor rods at attachment points.Figure 3 . Aeolian vibration results in fatigue of conductor strands in the area of attachments to the conductor. low amplitude vibration of the conductor due to low-speed wind flow perpendicular to the conductor. and compression splices and dead ends. Three types of wind-induced conductor motion can cause damage to conductors. Aeolian vibration is caused by low speed smooth winds perpendicular to the conductor. Therefore. Aeolian vibration is high frequency. Eventually this fatigue results in broken strands in the area of attachment as shown in Figure 4. Galloping is low frequency. and application of vibration dampers on conductors.Example of Gunshot Damage . Wake-induced oscillation is low frequency motion of bundled conductor due to shielding effects of leeward conductors by windward conductors. 3-2 . Figure 4 . The surface of the conductor could be scared with pitting and burn marks out in the span due to flashovers between adjacent phase conductors during the galloping event. bases. Galloping is normally caused by steady moderately strong wind perpendicular to an asymmetrically iced conductor. Corrosion Corrosion is a primary means of deterioration for metals. acids. They also corrode when exposed to various gases including acid vapors. Most metals corrode in the presence of water. In a worst-case situation. A very short duration of extreme galloping can cause major damage to not only the conductor. but also to many other line components. Resulting damage can include accelerated wearing of conductors and hardware at attachments and at points where adjacent conductors clash together. Wake-induced oscillation may occur in bundled conductors under the influence of moderate to high-speed steady wind. and sulfur containing gases.Example of Broken Strands Under Suspension Attachment Due to Fatigue from Aeolian Vibration Galloping results in very high dynamic loading of the conductor and can lead to conductor damage and/or failure. the amplitude of conductor galloping can meet or even exceed the sag of the conductor. The rate of corrosion is dependent of a variety of factors including the properties of the metal and the 3-3 . oils and other solid and liquid chemicals. The magnitude of motion can be very small or can be large enough to cause adjacent conductors in the bundle to clash together. A variety of motion patterns can occur depending on bundle arrangements. Broken conductor strands may occur in conductor attachment areas. Damage to conductors would typically occur in two areas. ammonia. salts. There are two primary types of corrosion that can attack overhead line conductors and shield wires. The site where metal atoms lose electrons is called the anode. which is an indication of the difficulty in identifying steel core corrosion by field observation. Regardless of the corrosion rate. and sulfur and chlorine compounds. carbon dioxide.presence of water. water) a metal atom is oxidized. In the case of atmospheric corrosion. Figure 5 . The internal strand is exposed in the photograph to show that the corrosion is primarily on the exterior surfaces of the wire. Atmospheric corrosion is simply the gradual degradation of a metal by contact with substances present in the atmosphere. atmospheric corrosion and galvanic corrosion. oxygen and various contaminates. Note that the outer surface of the aluminum strands looks about the same for both conductors. In the presence of an electrolyte (e.g. water vapor. whereby it loses one or more electrons and leaves the metal surface. Corrosion is an electrochemical process that seeks to reduce the binding energy in metals. the eventual result is loss of material and potentially reduced ability of the metal to perform its intended function. the anode and cathode would be on the same base metal. The lost electrons are conducted through the metal to another site where they are reduced. Atmospheric corrosion is primarily a concern with galvanized steel overhead ground wires and the galvanized steel core of ACSR or ACSS conductors. and the site where electrons are gained is called the cathode. Figure 6 shows exposed steel core strands from two identical conductor samples.Corrosion of Overhead Ground Wire 3-4 . one sample showing corrosion on the steel core and the other showing a corrosion-free steel core. Figure 5 shows a sample of corrosion on an overhead ground wire. such as oxygen. The anode will be the metal that can most easily give up an electron. However. The result is that the corrosion of the anode will accelerate and the corrosion of the cathode will decelerate or stop. The byproduct of galvanic corrosion of the aluminum is a white powdery byproduct that might work its way out to the conductor surface and be visible in extreme cases of galvanic corrosion. The lighter colored areas on the internal aluminum strands are locations where galvanic corrosion of the aluminum has probably begun.Figure 6 . the zinc galvanizing would be the anode and the aluminum would be the cathode as long as the galvanizing is intact. once the zinc coating has been exhausted by the galvanic corrosion and the aluminum is now in contact with the steel. In the case of ACSR conductors. 3-5 . Figure 7 shows an expanded conductor sample illustrating both steel core 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.Corroded and Pristine Steel Core Examples Galvanic corrosion occurs when two dissimilar metals are brought together in the presence of moisture and electric potential. This causes one metal to become an anode and the other to become a cathode. the aluminum becomes the sacrificial anode and begins to corrode at a higher rate while the rate of steel corrosion is reduced. 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. For conductors that include a steel core the joints normally consist of two sleeves. For all aluminum conductors (AAC. This creates a friction connection between the conductor strands and the associated compression sleeve. 3-6 . AAAC. For proper performance of these joints it is important that correct construction methods be followed.Expanded Conductor Showing Steel Core Corrosion and Initial Galvanic Corrosion of Aluminum Bad Joints One of the primary issues for the integrity of a conductor is the degradation of the compression joints used to splice two conductor segments together or connect the conductor to a dead-end support. etc. which in turn leads to elevated joint temperature during high current operations and eventually to mechanical failure of the joint. The most common problems result in high resistance in the joint. and conductor motion. Conductor joints are subjected to most of the same degradation modes as the full conductor including corrosion. When they aren’t followed correctly the resulting construction defects tend to aggravate the effects of the various degradation modes that have been discussed. The most common conductor joints are fittings made of aluminum and steel sleeves or cylinders that are compressed on the conductor using a hydraulic press.) these joints consist of a single aluminum sleeve that is compressed onto the outside of the conductor. mechanical overload. excessive current.Figure 7 . which in turn can lead to annealing and resulting loss of strength in the aluminum portions of the conductor. However. this effect can result in significant changes in the conductor sag. for emergency operations utilities will allow thermal limits up to 125oC or higher for short durations. D. it could be a significant conductor degradation factor if a line has been subjected to excessive fault events. Aluminum is more resistant to this effect than other metals and may only show a loss of sheen. 1989. 1 Aluminum Electrical Conductor Handbook.). the existence of these symptoms is indicative of a conductor that has experienced current surges and may have experienced other damage as a result.C. Therefore. and a change of color on the conductor surface. 100-hrs of operation at 125oC would result in approximately 7% reduction in strength for the aluminum strands of a normal conductor1.Strength Loss Excessive current in the conductor leads to overheating. Washington. However. The resulting loss of strength in the aluminum depends on the o cumulative duration at various temperatures above 100 C. This is a very complex phenomenon and beyond the scope of this report. In addition to loss of conductor strength due to aluminum annealing. Third Edition. For an AAC conductor this equates to a 7% loss of strength for the conductor. etc. Longer service life with more accumulated time above 100oC would lead to greater strength loss. o Repeated operation of aluminum conductors at temperatures of 100 C or greater will result in annealing of the aluminum strands and loss of conductor strength for most standard conductors (AAC. this would result in a 24% loss of strength for an AAC conductor but only a 10% loss of strength for our representative Drake ACSR conductor. a utility might limit a conductor to 24-hrs at an emergency temperature of 150 C each year. a slight roughening. heating due to faults and lightning has a cumulative effect with emergency operations. Fault currents and lightning also contribute to annealing of aluminum and loss of strength in the conductor. For example. 3-7 . For emergency operating o purposes. For this reason normal operating limits are usually set below that temperature. ACSR. high temperature operation of conductors also contributes to additional conductor creep and increased sag. 20006. This excessive current can come from either emergency operations or from current surges due to lightning or faults. the current is also very high. However. Over a 30-year conductor life. this effect is normally neglected in predictions of remaining strength of conductors. The Aluminum Association. for a representative 795-kcmil ACSR conductor (26/7 Drake) this may yield only a 3% loss of strength. However. However. Because the duration and frequency of these fault currents is normally relatively small. Although the duration of loading is generally very short. Pitting and melting of the conductor surface is a second form of degradation that occurs in the case of faults or lightning strokes. . this visual inspection takes place from the ground or from a helicopter. Broken strands inside the conductor and under attachment points can be detected visually by physically exposing the damage. The ease of their detection may depend upon whether broken strands protrude from the conductor or remain in their normal lay position flush with the conductor surface. there is no single inspection method that is effective at detecting all of the degradation modes identified. broken strands that occur internal to the conductor or within the confines of attachments to the conductors cannot be easily detected during a routine visual inspection. a combination of techniques is required to obtain a complete assessment of the condition of overhead line conductors and ground wires. On the other hand. adequate inspection methods and frequencies are important to maintain the integrity of the system. Therefore. These hidden broken strands are much more of a challenge to the inspector.4 INSPECTION & ASSESSMENT METHODS Inspection Methods The challenge is to identify critical degradation of conductors before the degradation leads to any of the failure modes defined previously. But the most commonly visible problem is broken outer strands of the conductor. But this is an expensive inspection method that is generally undertaken only in cases where a history of broken strands at conductor attachment points warrants the added expense. which requires contact and possibly de-energization of the conductor. strands that are broken due to conductor motion are more likely to be on internal conductor layers or within the confines of conductor attachments and therefore hidden from view. As a result. Strands that are broken due to vandalism are generally visible on the surface of the conductor between conductor attachments. Visual Inspection Visual inspection is the most commonly used form of inspection for all transmission line problems. Inspection Methods for Broken Strands The ease with which broken strands can be detected is primarily determined by the location of the broken strands within the conductor cross-section and within the length of the conductor span. Normally. In the case of overhead conductors and ground wires. Unfortunately. there is a limit to what problems can be detected using visual inspection techniques. Unfortunately. often with protruding strands making the damage more easily detected. 4-1 . Conditions that are detectable by DayCor™ are limited to those that cause corona. it has generally been found that field application of thermal imaging to detect broken conductor strands outside of joints is not effective. making it an ideal choice for inspecting for broken conductor strands. at higher voltages these protruding strands are likely to generate corona that can be detected by various techniques. 2 Guide to Corona and Arcing Inspection of Overhead Transmission Lines. Field trials are continuing to generate additional calibration and usability data to be incorporated in final production versions of the technology. This may make them easier to detect visually. Additionally. broken strands on the surface of the conductor will be protruded from the conductor. while thermal imaging has been used successfully to detect broken strands in the laboratory. 4-2 . EMAT Inspection The Electromagnetic Acoustical Transducer (EMAT) technology has recently been developed by EPRI for stranded conductors and overhead ground wires. Palo Alto. conductor problems that can be detected by the DayCor™ camera include bird-caging and severe surface scaring. As illustrated in Figure 14. This technology can detect small amounts of corona during daylight conditions. In addition to protruding broken strands. CA: 2001. This technology was developed for detection of broken strands internal to conductor and attachments and is currently undergoing field trials for this application. The DayCor™ camera is coming into common use for corona inspection of overhead transmission lines. EPRI has published an extensive guide for corona detection using DayCor™ that includes examples of conductor problems that can be detected2. Thermal Imaging Thermal imaging with an infrared camera is commonly used to detect conductor joints with high resistance and resulting elevated temperature. conductor corrosion and broken strands internal to the conductor and/or conductor attachments are generally not detectable by this technology. 1001910. The technology is able to identify conditions in which several broken conductor strand exist within the region of attachment to the support structure. This would include broken conductor strands as well as bad joints. Therefore. However. In theory thermal imaging can be used to detect any conductor degradation that yields an increase in conductor resistance.Corona Inspection Oftentimes. EPRI. it is critical that a valid calibration of the EMAT be performed in order to assure reliable test results. As with all non-destructive inspection technologies. this technology can be applied on an energized line using bare-hand techniques. which begins with the exposed zinc coating on the steel and progresses to corrosion of the steel once it is exposed to the atmosphere. This involves placing either X-rays or gamma rays near the conductor region to be inspected. 4-3 . Therefore it is not used frequently in practice. If properly applied and tuned this image will show any broken conductor strands inside the attachment point. Corrosion begins with the zinc coating on the steel and progresses to the steel by atmospheric corrosion or the aluminum by galvanic corrosion once the zinc coating has been pierced.Figure 8 . These rays pass through the inspection article and are captured on film. Inspection Methods for Corrosion Corrosion is typically an issue for both aluminum conductors with galvanized steel core and galvanized steel shield wires.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. When the film is processed the image is a series of grey shades between black and white. This method is expensive and must be applied with care. The galvanized steel core of the conductor is subject to both atmospheric corrosion and galvanic corrosion under the proper conditions. The shield wire is subject to atmospheric corrosion. However.. shown in Figure 10. “Detection of Corrosion in ACSR Overhead Line Conductors. called the Cross-Checker (CC). 1985. Each of these technologies uses a motorized trolley to carry the sensors over the length of a conductor span. 4-4 . Corrosion Detector Inspection Since internal corrosion cannot be reliably detected or evaluated visually. there can be some exceptions. Depending on the severity of the corrosion and the configuration of the conductor. For example. This technology uses Eddy currents to assess the loss of zinc galvanizing on the steel core of ACSR conductors. However. A second corrosion detector has been developed in the U. they still may exist and provide an opportunity to develop a sensor that would detect them reliably. severe atmospheric corrosion of the steel core could also generate enough corrosion by product that it would become visible on the surface of the conductor. several technologies have been developed that can be used for assessment of corrosion on overhead line conductors. in general there is very little visual indication of corrosion on conductors. EPRI is in the process of conducting a feasibility study for just such a sensor.Visual Inspection Since this corrosion begins on the outside surface of the shield wire it can be detected visually. K. J. It does not assess the condition of the galvanizing directly. Although corrosion by products on the surface of conductors may be difficult to detect by the human eye. for severe galvanic corrosion of the aluminum strands in the conductor a white powdery corrosion by product may work its way to the surface of the conductor where it can be detected visually. For the steel reinforced aluminum conductors the corrosion activity occurs on and adjacent to the core of the conductor and is generally not readily detectible by visual techniques. visual inspection may not be adequate to quantify the severity of the corrosion on the steel shield wire. and must be moved from span to span and wire to wire 3 Lewis.S. if the feasibility study is successful it may be possible to identify the existence of corrosion on conductors with a simple field test that could be done without contacting the conductor.G.” Distribution Developments. nor does it assess the condition of the aluminum strands. assesses the loss of cross-section in the steel core of ACSR conductors. and Sutton. The most well known is the Overhead Line Corrosion Detector (OHLCD) developed by the Central Electricity Generating Board (CEGB) in the United Kingdom3 and shown in Figure 9. It is also able to assess the loss of aluminum once galvanic corrosion begins attack the aluminum strands. This device. They are not currently able to traverse around any obstructions encountered. Likewise. The details of this new technology cannot be released at this time due to possible patent implications. However. such as tower attachments and spacers. The OHLCD and CC inspection technologies have been evaluated previously by EPRI4. Therefore the application of the technologies becomes a matter of sampling a reasonable number of conductor spans on a line to obtain a general picture of the corrosion condition of the line. They were found to be reasonable predictors of conductor condition when adequately calibrated and used in conjunction with laboratory testing of conductor samples removed from inspected conductors.Research Version of Cross-Checker manually using either a bucket truck or helicopter.Overhead Line Corrosion Detector Figure 10 . They are designed for use on an energized line if necessary. 4 Application Guidelines for Existing Conductor Inspection Technologies.Figure 9 . 2002. at this time it is neither practical. 1002657 4-5 . nor economical to inspect the full length of all conductors on a transmission line using these technologies. EPRI. However. CA. Palo Alto. high temperature operation. EMAT Inspection The previously mentioned EMAT technology also has promise for assessment of corrosion in conductors and shield wires. it also has been shown that corrosion generates a distinctive signal on the device. of Ontario has been manufacturing instruments for over 30-years that non-destructively test steel wire ropes used in underground mining operations. They can detect a single broken wire in a 2-inch diameter rope that is made up of 162 wires. The instrument is remote controlled and will travel along the conductor much like the OHLCD and Cross-Checker technologies. Some visual indicators that joint problems might exist or develop include discoloration of the joint due to high temperatures or excessive joint deformation. The most common problems result in high resistance in the joint. Areas of greater corrosivity may warrant sampling of a larger number of spans. Rotesco Inspection Since the EPRI evaluation of the OHLCD and Cross-Checker technologies were completed. or mechanical loading.1% in the metallic cross-sectional area of the rope. 4-6 . Although the technology was developed and has been calibrated for detection of broken strands. This technology is able to inspect wire ropes up to 2 ¼” inches in diameter and measure changes of 0. many joint problems can exist without providing visual clues. chemical plants. which in turn leads to elevated joint temperature during high current operations and eventually to mechanical failure of the joint. a third commercial device has been identified for inspection of steel shield wires and steel reinforced aluminum conductors. Rotesco Inc. Particular attention should be given to conductor spans in areas subjected to point sources of pollution such as power plants. Visual Inspection Visual inspection of conductor joints can give some indications that a problem might exist.The quantity and location of spans to be sampled with corrosion detection technologies should be established based on conductor age and environmental conditions. Inspection Methods for Bad Joints Degradation of the compression joints used to splice two conductor segments together or connect the conductor to a dead-end support is a relatively common problem in conductors. However. They have now adapted and enhanced this technology to develop an instrument that can non-destructively test the steel core of overhead line conductors. and other industrial facilities. Additional evaluation and calibration is necessary to validate the application of EMAT for corrosion detection and assessment. Joint problems are often the result of construction flaws that allow degradation of the joint due to corrosion. Palo Alto. If the expected or accepted resistance of the joint is not know. it is very sensitive to a number of parameters and can give misleading results if not applied carefully. EPRI has conducted research on behavior of compression joints and the application of thermal imaging for inspection of compression joints in conductors5. 6 Infrared Inspection Application Guide: Overhead Transmission and Substation Components. 1001915. Therefore. even a bad joint may not generate enough heat to be detected by the infrared camera. which in turn leads to high operating temperatures in the joint when heavy electrical loading is applied. to get accurate results the camera must be adjusted properly for emissivity. The principle is that most joint problems lead to high electrical resistance in the joint. wind speed and direction. 1001913. Palo Alto. The resistance of a new joint should be 30 to 70 percent of the resistance of the connected conductor. CA: 2002. 4-7 . ambient temperature. EPRI. Finally. pictured in Figure 11. Resistance Measurements An alternative to thermal imaging is to measure the resistance of the joint directly. a resistance measurement of the adjacent conductor can be collected and then the OhmStik™ can be programmed to indicate good or bad 5 Electrical. an infrared camera can be used to detect joints that are operating at high temperature due to joint problems. It is critical that the conductor be heavily loaded electrically at the time the thermal images are collected. Ambient temperature and the wind speed and direction also have an important effect on the actual temperature of the joint. Mechanical. the cooling effects may dissipate the heat in the joint sufficiently that the infrared camera will be unable to reliably detect a problem in a bad joint. CA: 2001. The emissivity of a new joint is probably significantly different than that of an older joint. Among the factors that are important considerations in obtaining effective results are electrical loading.6. The camera must be set for a specific emissivity. the actual emissivity of the joint is important to obtaining accurate results from the infrared camera. is pressed against the joint and provides a direct measurement of the line current and the resistance of the joint in micro-ohms. The OhmStik™. If the temperature is very cool and the wind speed is very high and perpendicular to the conductor. and Thermal Performance of Conductor Connections. and emissivity of the joint. EPRI. While thermal imaging can be an effective method for detecting problem joints in conductors. Two devices have been identified for making this measurement on an energized transmission line.Thermal Imaging As mentioned previously thermal imaging with an infrared camera is commonly used to detect conductor joints with high resistance and resulting elevated temperature. focus and distance for each joint inspected. and the emissivity selected on the camera must match the actual emissivity of the joint in order to get the most accurate temperature measurements. Therefore. If the conductor is not operating with a high current load. Therefore. eliminating concern about many of the external factors that impact thermal imaging measurements. The disadvantage is that it requires someone to place the OhmStik™ in contact with the conductor joint. the advantage is that the unit measures resistance directly. which is an expensive platform to operate. the only sure way to quantify the loss of strength in a conductor is to remove samples from the field and conduct laboratory tests. However.Figure 11 . it must be applied from a helicopter. and annealing due to electrical overloading. is called ROBHOT®. The ROBHOT is lowered from a helicopter and placed directly on the energized transmission line.Ohmstik Joint Resistance Measurement Device based on the ratio of the measured joint resistance to the measured conductor resistance. which most utilities don’t seem to have. Identification of broken strands and corrosion will provide some indication of strength loss in a conductor. However. shown in Figure 12. avoiding the complications from the external factors that affect the thermal imaging measurements. The second resistance measurement device. This is a ® helicopter-borne robot unit developed by SwedPower. Like the OhmStik™. it is impossible to determine via a field inspection how much strength loss has occurred due to annealing. 4-8 . Measuring probes are folded out and the resistance is measured in a few seconds. SwedPower claims that they can measure resistances for up to 40 splices per hour on an energized line. However. Inspection Methods for Strength Loss Loss of strength in a conductor comes from several factors including broken strands. Other than having excellent records of conductor electrical loading over its lifetime. it will not allow you to easily quantify the loss of strength. corrosion and resulting loss of cross-section. The advantage of this device is that it gives a direct measurement of joint resistance. Figure 12 . the rated breaking strength of the conductor as 7 found should be calculated to determine the remaining strength of the conductor . 1994. Once the tensile strength of each strand is known. an additional length of 20-ft or more should be removed. 4-9 . evidence of corrosion and/or broken strands should be noted and photographed. Southwire Company. 7 Overhead Conductor Manual. Samples removed for testing should be at least 3-ft long. Sampling of the conductor or overhead ground wire for laboratory testing is perhaps the most reliable technique available to determine the general condition of a conductor and make decisions regarding replacement.ROBHOT Helicopter-Borne Robot Unit Lab Testing It is advisable to remove conductor samples from a few inspected spans and conduct laboratory tests on them to validate inspection results and to quantify the actual condition of the conductor at those locations. GA. Longer samples are preferred as they provide sufficient length to allow tests to be repeated if necessary. A length of at least 18 inches should be disassembled to document the condition of each layer of strands. Samples should first be carefully inspected. Each strand from the dissected piece of conductor should be tensile tested according to ASTM B498 for steel and B230 for aluminum. For each strand the strand diameter and the breaking strength should be recorded. If a tensile test of the full conductor is desired. From these values the average tensile strength of the steel strands and aluminum strands should be determined and compared with minimum ASTM requirements. In particular. Carrollton. No. But in the case where armor rods are included in the tower attachment connection. In the case of a visual inspection from the ground or air. Ideally these tests will also be conducted for a new conductor of the same type to establish a baseline. ASTM Journal of Testing and Evaluation (JTEVA).If desired. if a visual inspection of conductor strands within the tower attachment area can be accomplished. 8 Akhtar. Likewise. This can be accomplished using either a torsional test of the strands (ASTM A938) or an alternate bending test of the strands (ASTM A363). "Localized Intrinsic Strengthening Approach (LISA): A Practical Method for Determining the Tensile Strength of Multistrand Cables". a conductor strength calculation is theoretically possible. 4-10 . Additional testing should be conducted to evaluate the ductility of the as-found conductors. before conductor replacement or repair is required. A method has been published that reportedly eliminates these end effects resulting in an accurate measurement of the actual breaking strength of the full conductor8. March 1988. pp. little more than a subjective assessment of the conductor or overhead ground wire condition can be performed. The degree of condition assessment that can be done depends on the type of inspection that was completed and the quality of data collected. one might hypothesize whether the conductor requires replacement or not. the results must be reviewed and an assessment of the condition of the conductor must be made to determine whether repair or replacement is required. A severe reduction in the “number of turns” to failure is an indication of loss of ductility due to corrosion. or a nondestructive technology such as EMAT can be effectively applied to determine the existence and extent of broken strands in conductor attachment points. the author is unaware of any specific industry standard that quantifies how many broken strands are acceptable before repair or replacement should be made. the conductor strength may be calculated based on the remaining intact strands observed at that location. 124 – 133. with and without armor rods. However. Methods of Condition Assessment Once the available technologies have been applied to determine the state of degradation of conductors and overhead ground wires. In the case of a significant number of visible broken strands. 2. Without special arrangements the failure will almost always occur adjacent to the end connections and at loads well below the rated breaking strength of the conductor. However it is very difficult to obtain reliable results from this test due to the stress concentration effects from the end connections used to apply the tensile loads. the armor rods may be sufficient to reinforce the conductor and maintain its strength with several existing broken conductor strands. Vol. Based on the external appearance of the conductor and any visible evidence of broken strands. A. a tensile test of the full conductor may also be conducted. Many utilities have their own guidelines regarding how many broken strands are acceptable. providing a more objective assessment of conductor condition. 16. Replacement of the conductor is the obvious solution. By collecting a sufficient number of samples to give a good representation of a line. A second consideration for replacement decisions is the loss of ductility in the conductor.By adding inspection with one of the corrosion detection technologies we increase our knowledge of the conductor condition. 4-11 . The National Electric Safety Code (NESC) establishes a tension limit for conductors at 60% of the rated breaking strength of the conductor under maximum ice & wind loads. it is still somewhat subjective in terms of remaining strength and remaining life. Then if the maximum tension at final conditions for maximum ice and wind loads exceeds 60% of this measured breaking strength something must be done to bring the design back into compliance. conductors that have lost much of their ductility are at a greater risk for failure. In particularly cold weather and with dynamic loading from galloping. Using laboratory test results for the as-found conductor one can calculate the current rated breaking strength of the conductor. However. this gives us the ability of prioritizing line replacements based on an objective indication of the relative conductor corrosion damage between different lines. or the population of lines. assuming this could be done without violating clearances. There isn’t a lot of documented evidence indicating how one should interpret results from corrosion detection technologies and convert them into remaining strength or remaining usable conductor life. Making Repair and Replacement Decisions The most obvious measure that can be used for determining the need to replace a conductor or shield wire is to compare the as-found conductor strength with the maximum design loads for the line in question. this is a particularly important consideration. Nevertheless. While perhaps not the most economical method. by far the most effective method for assessing the condition of overhead line conductors and shield wires is removing conductor samples and testing them in the laboratory. but one could also theoretically reduce conductor tensions to meet requirements. For conductors that are subject to heavy ice loading and/or galloping. and obtaining laboratory measurements of strength we can have a very objective indication of the current condition and capability of the conductor or shield wire. . Degradation of conductor joints has been addressed only briefly in the current investigation. several non-destructive inspection devices have been identified and some have been evaluated in field trials. The most commonly used method is thermal imaging to identify joints that are operating at . The EMAT device that has been developed is able to detect broken strands internal to the conductor and internal to conductor attachment points. More frequently. The output of the device is a simple good or bad indication for the inspected conductor. In the case of broken strands.5 SUMMARY EPRI has been investigating degradation modes and inspection and assessment methods for overhead line conductors for a few years. which uses a modified wire rope tester to identify loss of cross-section in the steel core of conductors. Inspection methods identified are all based on the increased resistance that occurs as a conductor joint deteriorates and approaches failure. a third technology has been identified from Rotesco. two non-destructive inspection technologies have been developed that are effective. degradation of conductor joints from corrosion and high temperature operation. Calibration and field-testing of this technology is continuing with the expectation of a commercially viable device within the next year. The first is the DayCor™ camera. some type of non-destructive inspection device is needed to assess the condition of the conductor. Inc. corrosion of steel shield wires and of the steel core and adjacent aluminum strands on conductors. The unit is simply placed on the energized conductor adjacent to the conductor section to be inspected. The second is a new EMAT (electromagnetic acoustical transducer) technology developed by EPRI in a separately funded project. In the case of conductor corrosion. since past EPRI projects have addressed this topic. Inspection methods that are most effective vary according to the type and location of the conductor degradation. The Overhead Line Corrosion Detector (OHLCD) determines how much galvanizing remains on the steel core of conductors and grades the conductors accordingly. These two technologies were evaluated in field trials. The Cross-Checker device uses a magnetic field to quantify loss of cross-section in the steel core of the conductor. and loss of strength in the conductor material due to high temperature operation. but is unable to identify losses of cross-section in the aluminum. In very few cases. Recently. visual inspection techniques may be used effectively. Primary modes of degradation that have been identified include broken strands due to vandalism or conductor motion. This report provides an overview of the findings to date and identifies newer inspection technologies that have not yet been thoroughly evaluated. The OHLCD is also able to identify significant losses of aluminum due to galvanic corrosion. which is an effective aid in locating broken strands in cases where the strands protrude form the conductor sufficiently to create corona.. which have been documented in a previous report. were identified that make direct measurements of the resistance in a joint to identify joints that are degraded. The most reliable method to determine remaining strength and remaining life of a conductor continues to be removal of samples from the field and testing for strength and ductility in the laboratory. there are numerous external factors that must be carefully considered to ensure good results. This method is effective when properly applied. However. . the Ohmstik and ROBHOT. Finally. Two other technologies. a field method to quantify loss of strength in a conductor due to overheating or other factors has not been identified.excessive temperatures. . . . 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