Patent Application
20050034892
The present invention relates generally to insulators for power transmission lines, and more specifically to chemically-doped transmission and distribution components, such as composite (non-ceramic) insulators that provide improved identification of units with a high risk of failure due to environmental exposure of the fiberglass rod.
Power transmission and distribution systems include various insulating components that must maintain structural integrity to perform correctly in often extreme environmental and operational conditions. For example, overhead power transmission lines require insulators to isolate the electricity-conducting cables from the steel towers that support them. Traditional insulators are made of ceramics or glass, but because ceramic insulators are typically heavy and subject to fracturing, a number of new insulating materials have been developed. As an alternative to ceramics, composite materials were developed for use in insulators for transmission systems around the mid-1970's. Such composite insulators are also referred to as “non-ceramic insulators” (NCI) or polymer insulators, and usually employ insulator housings made of materials such as ethylene propylene rubber (EPR), polytetrofluoro ethylene (PTFE), silicone rubber, or other similar materials. The insulator housing is usually wrapped around a core or rod of fiberglass (alternatively, fiber-reinforced plastic or glass-reinforced plastic) that bears the mechanical load. The fiberglass rod is usually manufactured from glass fibers surrounded by a resin. The glass-fibers may be made of E-glass, or similar materials, and the resin maybe epoxy, vinyl-ester, polyester, or similar materials. The rod is usually connected to metal end-fittings or flanges that transmit tension to the cable and the transmission line towers.
Although composite insulators exhibit certain advantages over traditional ceramic and glass insulators, such as lighter weight and lower material and installation costs, composite insulators are vulnerable to certain failures modes due to stresses related to environmental or operating conditions. For example, insulators can suffer mechanical failure of the rod due to overheating or mishandling, or flashover due to contamination. A significant cause of failure of composite insulators is due to moisture penetrating the polymer insulator housing and coming into contact with the fiberglass rod. In general, there are three main failure modes associated with moisture ingress in a composite insulator. These are: stress corrosion cracking (brittle-fracture), flashunder, and destruction of the rod by discharge activity.
Stress corrosion cracking, also known as brittle fracture, is one of the most common failure modes associated with composite insulators. The term “brittle fracture” is generally used to describe the visual appearance of a failure produced by electrolytic corrosion combined with a tension mechanical load. The failure mechanisms associated with brittle fracture are generally attributable to either acid or water leaching of the metallic ions in the glass fibers resulting in stress corrosion cracking. Brittle fracture theories require the permeation of water through permeation pathways in the polymer housing and an accumulation of water within the rod. The water can be aided by acids to corrode the glass fiber within the rod. Such acids can either be resident within the glass fiber from hydrolysis of the epoxy hardener or from corona-created nitric acid.
Flashunder is an electrical failure mode, which typically occurs when moisture comes into contact with the fiberglass rod and tracks up the rod, or the interface between the rod and the insulator housing. When the moisture, and any by-products of discharge activity due to the moisture, extend a critical distance along the insulator, the insulator can no longer withstand the applied voltage and a flashunder condition occurs. This is often seen as splitting or puncturing of the insulator rod. When this happens, the insulator can no longer electrically isolate the electrical conductors from the transmission line structure.
Destruction of the rod by discharge activity is a mechanical failure mode. In this failure mode, moisture and other contaminants penetrate the weather-shed system and come into contact with the rod resulting in internal discharge activity. These internal discharges can destroy the fibers and resin matrix of the rod until the unit is unable to hold the applied load, at which point the rod usually separates. This destruction occurs due to the thermal, chemical, and kinetic forces associated with the discharge activity.
Because the three main failure modes can mean a loss of mechanical or electrical integrity, such failures can be quite serious when they occur in transmission line insulators. The strength and integrity of composite insulators depends largely on the intrinsic electrical and mechanical strength of the rod, the design and material of the end fittings and seals, the design and material of the rubber weather shed system, the attachment method of the rod, and other factors, including environmental and field deployment conditions. As stated above, many composite insulator failures have been linked to water ingress into the fiberglass material comprising the insulator rod. Since all three failure modes—brittle fractures, flashunder, and destruction of the rod by discharge activity, occur in the insulator rod, they are hidden by the housing and cannot easily be seen or perceived through casual inspection. For example, simple visual inspection of an insulator to detect failure due to moisture ingress requires close-up viewing that can be very time consuming, costly, and generally does not yield a definitive go or no-go rating. Additionally, in some cases, detection of rod failure through visual inspection techniques may simply be impossible. Other inspection techniques, such as daytime corona and infrared techniques can be used to identify conditions associated with discharge activity, which may be caused by one of the failure modes. Such tests can be performed some distance from the insulator, but are limited in that only a small number of failure modes can be detected, the discharge activity must be present at the time of inspection to be detected. Furthermore, for this type of inspection, a relatively high level of operator expertise and analysis is required.
It is desirable, therefore, to provide improved inspection techniques for composite insulators or any other type of composite system with external protective coverings that detect failure modes associated with exposure of the interior structure to moisture by yielding a migration path from the inside of the insulator to the exterior surface.
It is further desirable to provide composite insulators that provide early warning of impending failure due to stress corrosion, flashunder, or destruction of the rod by discharge activity, and that allow inspection from a distance and without the need for the actual manifestation of failure symptoms.
It is also desirable to provide an automated inspection of composite insulators in the field by instrument-based scanning and image processing.
A composite insulator or other polymer vessel, containing means for providing early warning of impending failure due to environmental exposure of the rod is described. A composite insulator comprising a fiberglass rod surrounded by a polymer housing and fitted with metal end fittings on either end of the rod is doped with a dye-based chemical dopant. The dopant is disposed around the vicinity of the outer surface of the fiberglass rod, such as in a coating between the rod and the housing or throughout the rod matrix, such as in the resin component of the fiberglass rod. The dopant is formulated to possess migration and diffusion characteristics correlating to those of water, and to be inert in dry conditions and compatible with the insulator components. The dopant is placed within the insulator such that upon the penetration of moisture through the housing to the rod through a permeation pathway in the outer surface of the insulator, the dopant will become activated and will leach out of the same permeation pathway. The activated dopant then creates a deposit on the outer surface of the insulator housing. The dopant comprises a dye or stain compound that can either be visually identified, or is sensitive to radiation at one or more specific wavelengths. Deposits of activated dopant on the outer surface of the insulator can be detected upon imaging of the outer surface of the insulator by appropriate imaging instruments or by the naked eye.
Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.
The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements, and in which:
A composite insulator or vessel containing chemical dopant for providing early warning of impending failure due to exposure of the fiberglass rod to the environment is described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one of ordinary skill in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form to facilitate explanation. The description of preferred embodiments is not intended to limit the scope of the claims appended hereto.
Lightweight composite insulators were developed in the late 1950s to replace ceramic insulators for use in 1,000 kilovolt power transmission lines. Such insulators featured great weight reduction, reduced breakage, lower installation costs, and various other advantages over traditional ceramic insulators. A composite insulator typically comprises a fiberglass rod fitted with two metal end-fittings, a polymer or rubber sheath or housing surrounds the rod. Typically the sheath has molded sheds that disperse water from the surface of the insulator and can be made of silicone or ethyl propylene diene monomer (EPDM) based rubber, or other similar materials.
Embodiments of the present invention may also be implemented in other types of transmission and distribution line and substation insulators. Moreover other types of transmission and distribution components may also be used to implement embodiments of the present invention. These include bushings, terminations, surge arrestors, and any other type of composite article that provides an insulative function and is comprised of an outer surface with a composite or fiber glass inner component that is meant to be protected from the environment.
The composite insulator 202 illustrated in
If moisture is allowed to come into contact with the fiberglass rod within the insulator, various failure modes may be triggered. One of the more common types of failures is a brittle fracture type of failure in which the glass fibers of the rod fracture due to stress corrosion cracking. Other types of failures that can be caused by moisture ingress into the fiberglass rod are flashunder, and destruction of the rod by discharge activity. A significant percentage, if not a majority of insulator failures are caused by moisture penetration rather than by mechanical failure or electrical overload conditions. Therefore, early detection of moisture ingress to the rod is very valuable in ensuring that corrective measures are taken prior to failure in the field.
Although insulators are designed and manufactured to be hermetically sealed, moisture can penetrate the housing of an insulator and come into contact with the fiberglass rod in a number of different ways. For example, moisture can enter through cracks, pores, or voids in the insulator housing itself, through defects in an end fitting, or through gaps that may be formed by imperfectly seals between the housing and end fittings. Such conditions may arise due to manufacturing defects or degradation due to time or mishandling by line-crews, and/or severe environmental conditions.
Current inspection techniques typically attempt to detect the presence of moisture and the onset of a failure mode due to cracks in the rod due to brittle fracture, electrical discharges that may be destroying the rod, or changes in electrical field due to carbonization. These techniques, however, generally require that moisture be present at the time of inspection, or that the damage due to discharge be readily visible for the given inspection technique, e.g., visual inspection, x-ray, and so on.
Dopant Configuration
In one embodiment of the present invention, a chemical dopant is placed in or on the surface of the insulator rod or within the resin fiber matrix. When moisture penetrates the insulator housing and comes into contact with the rod, the dopant is activated. In this context, the term “activated” refers to the hydrolization of the dopant due to the presence of moisture, which allows the dopant to migrate to the surface of the insulator. The activated dopant is formulated to possess similar diffusion characteristics to that of water, so that upon activation, it can migrate through the permeation pathway in the housing, e.g., crack or gap, which allowed the moisture to penetrate to the rod. Once on the outside surface of the insulator housing, the presence of the dopant can be perceived through detection means that are sensitive to the type of dopant that is used. For example, a fluorescent-dyed dopant can be perceived visually using an ultraviolet (UV) lamp. The detection of dopant on the outside of the insulator indicates the prior presence of moisture in contact with the core of the rod, even though moisture may not be present on or in the insulator, or the crack or gap may not be readily visible at the time of inspection.
Aspects of the invention utilize the fact that in the failure of a composite insulator, water migrates through the rubber housing and attacks the glass fibers by chemical corrosion. The water is essentially inert to the housing and the resin surrounding the glass fibers. The water typically reaches the fibers by permeation through cracks in the housing and/or rod resin as well as seal failures between the housing and end-fittings. If a water-soluble dye within the dopant is in the pathway of the water, the dye will hydrolize and be dissolved in the water. Since the pathways or cracks likely contain residual molecules of water, the dye will migrate back to the exterior surface of the insulator housing. This dye migration is driven by a concentration gradient. Since chemical equilibrium is the lowest energy state, the dye will attempt to become a uniform concentration wherever water is present, and will thus move away from the interior high concentration of dye to the exterior zero or lower concentration of dye. In addition, many dyes have high osmotic pressures when solubilized in water, so migration to the exterior of the housing may be aided by osmosis.
The dopant 308 can be disposed around the surface of the rod or within the structure of the fiberglass rod in various other configurations than that shown in
The embodiments illustrated in
In a further alternative embodiment of the present invention, the dopant can distributed through the rubber or polymer material that comprises the insulator housing. For this embodiment, the dopant would preferably be placed in a deep layer of the insulator housing, close to the rod, so that it would be activated when moisture permeated the insulator close to the rod, rather than closer to the surface of the housing. Likewise, the dopant can be distributed through an upper layer of the fiberglass rod itself, rather than along the surface of the rod, as shown in
The dopant can be configured to be a liquid or semi-liquid (gel) composition that allows for coating on a surface of the rod, insulator housing, or end fitting or for flowing within the insulator; or for mixing with the fiberglass matrix for the embodiment in which the dopant is distributed throughout the rod. Alternatively, the dopant can be configured to be a powder substance (dry) or similar composition for placement within the insulator or rod. Depending upon the composition of the rod, and manufacturing techniques associated with the insulator, the dopant can also be made as a granular compound.
The mechanism for applying the dopant to the composite insulator, such as during the manufacturing process could include electrostatic attraction or van der Waals forces that adhere the solid particles to the surface of the road, end-fittings, and/or the interior surface of the housing. The dopant could also be covalently bonded to the resin or rubber surface, with the bond being weakened or broken by contact with moisture. Alternatively, the dopant can be incorporated in an adhesive layer, an extra coating of epoxy, or similar substance, on the rod, or intermingled in the rubber layer in contact with the fiberglass rod during vulcanization or curing process of the rubber housing.
For the microencapsulated embodiment, the dye could be coated with a water-soluble polymer that protects the dye from contaminating the manufacturing plant and minimizes the potential for surface contamination of the dye on the exterior of the insulator housing during manufacturing. Such a polymer coating could also help prevent hydrolization or activation of the dye through exposure to ambient moisture during manufacturing.
With regard to microencapsulation, an alternative embodiment would be to encapsulate the dye in a capsule that is itself capable of migrating out of the permeation pathway. In this case, the dye solution is contained in a clear (transparent to the observing medium) microcapsule coating. Upon moisture ingress, the dye containing capsule would migrate to the surface of the housing and be trapped by the surface texture of the housing. The dye would then be detectable at the appropriate wavelengths through the coating. For this embodiment, the dye solution can be entrapped in a cyclodextrin molecule. In general, cyclodextrin is mildly water soluble (e.g., 1.8 gm/100 ml), so exposure to heavy moisture may cause the coating to dissolve. An alternative form of such nanoencapsulation is the use of a buckyball molecule. For this embodiment, a fullerene (buckyball) can contain another small molecule inside of it, thus acting as a nanocapsule. The nanocapsule sizes should be chosen such that migration through the permeation pathways is possible.
It should be noted that the embodiments described above in reference to
Dopant Composition
For each of the embodiments described above, the dopant is a chemical substance that reacts with water or is transported by water that penetrates the insulator housing and comes into contact with the dopant on or in the proximity of the outer surface of the insulator rod. It is assumed that water penetrated the insulator housing or rubber seal through cracks, gaps, or other voids in the housing or seal, or in any of the interfaces between the end fittings, seal, and housing. The dopant comprises a substance that is able to leach out of the permeation pathway that allowed the water to penetrate to the rod, and migrate along the outside surface of the insulator housing. Embodiments of the present invention take advantage of the fact that if water migrates to the inside of the insulator, then compounds of similar size and polarity should be able to migrate out as well. The dopant is composed of elements that are not readily found in the environment so that a concentration gradient will favor outward movement of the dopant through the two-way diffusion or permeation path.
In one embodiment of the present invention, the dopant, e.g., dopant 308, is a water-soluble laser dye. One example of such a dopant is Rhodamine 590 Chloride (also called Rhodamine 6G). This compound has an absorption maximum at 479 nm and for a laser dye is used in a 5×10E-5 molar concentration. This dye is also available as a perchlorate (C1O4) and a tetrafluoroborate (BF4). Another suitable compound is Disodium Fluorescein (also called Uranin). This has an absorption max at 412 nm, used as a laser dye at 4×10E-3 molar concentration, and a fluorescence range of 536-568. A groundwater tracing dye could be also used for the dopant. Groundwater tracing dyes have fluorescent characteristics similar to laser dyes, but can also be visible to the naked eye.
In an alternative embodiment of the present invention, the dopant can be an infrared absorbing dye. An example of such dyes include Cyanine dyes, such as Heptamethinecyanine, Phthalocyanine and Naphthalocyanine Dyes. Other examples include Quinone and Metal Complex dyes, among others. Some of these exemplary dyes are sometimes referred to as “water-insoluble” dyes since their solubilities can be less than one part per two thousand parts water. In general, water solutions on the order of parts per million are sufficient to provide a detectable electromagnetic change. Dyes with greater water solubilities can also be employed.
In general, the characteristics of the dopant used for the present invention include the lack of migration of the dopant from within a non-penetrated or damaged insulator, as well as a dopant that remains stable and chemically inert within the insulator for a long period of time (e.g., tens of years) and under numerous environmental stresses, such as temperature cycles, corona discharges, wind loads, and so on. Other characteristics desirable for the dopant are strong detector response, migration/diffusion characteristics correlating with water, stability in the environment once activated for at long period of time (e.g., least one year) to allow detection long after moisture ingress in the insulator.
In one embodiment, the dopant can be enhanced by the addition of a permanent stain, such as methylene blue. This would provide a lasting impression of the presence of the dopant on the surface of the insulator, even if the dopant itself does not persist outside of the insulator. The dye may be provided in a microencapsulated form that effectively dissolves when in contact with moisture. Such microencapsulation helps to increase the longevity of the dye and minimize any possible effect on the performance of the insulator.
Also suitable for use as dopants are some materials that are not technically known as dyes. For example, polystyrene can be used as a dopant. Polystyrene has a peak absorption excitation at about 260 nm and its peak fluorescence at approximately 330 nm. For this embodiment, polystyrene can be encapsulated in nanospheres that are coated to adhere to the insulator outside surface. Upon migration to the insulator exterior, mercury light could be used as an excitation source to excite the polystyrene spheres and enable detection through a suitable detector, such as a daytime corona (e.g., DayCor™) camera that can detect the radiation in the 240-280 nm range, which is within the UV solar blind band (corona discharges typically emit UV radiation from 230 nm to 405 nm).
The polystyrene spheres could be coated with or made of a material with a surface energy lower than that of weathered rubber, but higher than virgin rubber. In this manner, the spheres would not wet the rubber on the inside surface of the insulator, but would wet and adhere to the weathered exterior surface. Physical entrapment from the roughened weathered rubber surface would help to keep the nanospheres from washing off of the housing. Alternatively, a “solar glue” that is inactive within the insulator, but becomes active upon exposure to sunlight could be used to help adhere the nanospheres to the insulator surface.
The dopant could also be comprised of water insoluble dyes for which the strongest signal is for a non-aqueous solution. An example of such a compound is polyalphaolefin (PAO) which is typically used as a non-conducting fluid for electronics cooling. PAO is a liquid, and can be used as a solvent for lipophilic dye. For this embodiment, a dye could be dissolved in PAO and added as a liquid layer between the rod and housing. Upon exposure to moisture through a permeation pathway, the PAO-dye solution would preferentially wet the exposed rubber in the housing and then migrate to the exterior of the housing by capillary action. As a related alternative, an organic solvent or PAO can be microencapsulated into a water soluble coating. The water solvent microcapsules could be dry blended with a water insoluble dye, and the mixed powder could then be placed within the insulator. Upon contact with penetrating moisture, the solvent capsules will dissolve which would then cause the released organic solvent to dissolve the dye. The organic solvent-dye solution would then wet the rubber and migrate out of the insulator housing.
Depending on the dopant composition and the detection means, a very small amount of dye may only need to be required to generate a detectable signal. For example one part per million (1 ppm) of dye on the surface of the insulator may be sufficient for certain dopant/dye compositions to produce a signal using UV, IR, laser, or other similar detection means. The dopant distribution and packaging within the insulator also depends on the type of dopant utilized. For example, a one kilogram section of fiberglass rod may contain (or be coated with) about 10 grams of dye.
Previously discussed embodiments described a dopant that contains a dye that migrates out of the housing upon hydrolization by penetrating moisture. Alternatively, the dopant could comprise an activating agent that works in conjunction with a substance present on the surface of the housing. Upon migration of the dopant to the surface, a chemical reaction occurs to “develop” a dye that can be seen or otherwise detected on the surface of the housing. In a related embodiment, the housing can include a wicking agent that helps spread the dopant or dye along the exterior surface of the housing and thereby increase the stained area. The wicking agent should be hydrophobic to maintain the functionality of the waterproof housing, thus for this embodiment, a lipophilic dye should be used.
In one embodiment of the present invention, an automated inspection system is provided. For this embodiment, the non-composite insulator is scanned periodically using appropriate imaging apparatus, such as a digital still camera or video camera. The images are collected and then analyzed in real-time to detect the presence of leached dye on the surface of the insulator. A database stores a number of images corresponding to insulators with varying amounts of dopant. The captured image is compared to the stored images with reference to contrast, color, or other indicia. If the captured image matches that of an image with no dopant present, the test returns a “good” reading. If the captured image matches that of an image with some dopant present, the test returns a “bad” reading, and either sets a flag or sends a message to an operator, or further processes the image to determine the level of dopant present or the indication of a false positive. Further processing could include filtering the captured image to determine if any surface contrast is due to environmental, lighting, shadows, differences in material, or other reasons unrelated to the actual presence of leached dopant.
Aspects of the present invention can also be applied to any other composite system or polymer article with external protective coverings in which failure of the system can be induced by water penetration through the housing. Composite pressure vessels are illustrative of such a class of items. For example, compressed natural gas (CNG) tanks for use in vehicles or for storage are often made of fiberglass and can fail due to stress corrosion cracking or related defects, as described above. Such tanks are typically covered by a waterproof liner or impermeable sealer to prevent moisture penetration. The composite overwraps used in these tanks or vessels often do not have a sufficiently good external barrier to moisture ingress, and are vulnerable to water penetration. The fiberglass material comprising the tank can be embedded or chemically doped with a dye as shown in
In certain applications, exposure to acid rather than water moisture can lead to potential failures. Depending upon the actual implementation, the dopant could be configured to react only to acid release (e.g., pH of 5 and below), rather than to water exposure. Microencapsulation techniques or the use of pharmaceutical reverse enteric coatings, such as those that do not dissolve at a pH of greater than 6 or so, can be used to activate the dopant in the presence of an acid. Alternatively, a pH sensitive dye that is clear at neutral pH but develops color at an acidic level, can be used.
In the foregoing, a composite insulator including means for providing early warning of failure conditions due to exposure of the rod to the environment has been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.
