Abstract:
An insulative cover is utilized to protect an insulator supporting an electrical power line relative to a support structure in an electrical power transmission system. A first end of the cover is secured to the insulator adjacent the insulator&#39;s first end and a second end of the cover is secured to the insulator adjacent to the insulator&#39;s second end. The cover defines a volume substantially enclosing a central portion of the insulator.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to protective barrier structures for electrical insulators, and more particularly to particular cover structures for shed-type insulators and their use with power transmission lines. 
     (2) Description of the Related Art 
     In the field of electrical power transmission, high voltage power lines (typically operating in excess of 1 kV) are supported by structures such as utility poles. To prevent leakage of power from the lines into the supporting structure, the lines are advantageously held at one end of an insulator, which in turn is held at its other end by the supporting structure. Common insulators are formed of ceramic, porcelain, epoxy, or other electrically nonconductive materials. As such insulators are used in the open, they are exposed to rain (including acid rain), humidity, salt fog, acid fog, particulate pollutants, and other environmental contaminants. 
     If a continuous surface of electroconductive moisture existed between the ends of the insulator (e.g., deposited as rain), it would provide a conductive path between the line and the supporting structure. To avoid this, insulators are configured so that water does not accumulate over a continuous surface between the two ends. In “shed”-type insulators this may involve providing the insulator as the combination of: a body (also known as a core or stem) which is formed in a generally circular cylindrical or frustoconical shape; and a number of “sheds” formed as annular flanges projecting radially outward and longitudinally/vertically downward from the body. Both the upper surface of each shed and the lower surface (underside) of each shed along substantial portions thereof are inclined downward, leaving the underside of each shed largely protected from falling rain and preventing a continuous flow of water from end-to-end. 
     The surfaces of such insulators may be contaminated in other ways so as to compromise their insulative properties. For example, in coastal areas, wind-blown, salt-laden, mist may deposit salt over substantially the entire exposed surface of the insulator. Such salt can become embedded in the porous surface of the insulator and, eventually, provide a conductive pathway from the wire to the supporting structure. Wind-blown sand may abrade the surface of the insulator, increasing porosity and rendering the insulator more susceptible to later water or salt contamination. In industrial areas, chemical and particulate pollution may similarly compromise the insulator. To partially address these problems, it is known to periodically wash the insulators with a high pressure stream of water. Such a stream may be delivered from the ground or from the air such as from a helicopter. To avoid the risk of conducting electricity from the lines through the stream of water, power to the lines is advantageously shut off during cleaning. Although shutting off the power for cleaning may be optional, it is substantially required during replacement of the insulator. Furthermore, cleaning may not be complete and the service life of the insulator may be diminished. 
     It is thus desirable to reduce the required cleaning of insulators and extend their service lives. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, in a first aspect, the invention is directed to an insulative cover having a first end secured to the insulator adjacent the insulator&#39;s first end and a second end secured to the insulator adjacent the insulator&#39;s second end. The cover enshrouds a central portion of the insulator. The cover may define a substantially sealed volume surrounding the central portion. The insulative cover may have first and second pieces, each being the unitarily-formed combination of a body, first and second vertical flanges extending along first and second sides of the body, and first and second collar sections at first and second ends of the body. The bodies may form a series of reduced diameter regions alternating with a series of enhanced diameter regions, each reduced diameter region having a minimum diameter smaller than a maximum diameter of an adjacent enhanced diameter region. Each enhanced diameter region may accommodate an associated shed of the insulator. 
     The reduced and enhanced diameter regions may form a series of sheds on the cover. Each of the cover&#39;s sheds has an upper surface which substantially slopes downward in the outward radial direction and a lower surface which substantially slopes downward in the outward radial direction. An end surface joins the upper and lower surfaces. 
     The first and second pieces may be formed essentially from a fluorocarbon-vinylidene fluoride/hexifluoropropylene. The first and second insulator ends may be formed by first and second metallic end caps and first and second tie wraps may secure the respective first and second cover ends proximate the first and second insulator ends. 
     In a second aspect, the invention is directed to a combination for supporting a power line relative to a support structure in an electrical power transmission system. An insulator has a first end secured to the support structure and a second end carrying the power line. An insulative cover has a first end secured to the insulator adjacent the insulator&#39;s first end and a second end secured to the insulator adjacent the insulator&#39;s second end. The cover defines a volume substantially enclosing a central portion of the insulator. The insulator&#39;s first end may be an upper end and the insulator&#39;s second end may be a lower end. The volume may be a sealed volume or it may include at least one vent hole preferably no greater than 1 sq. mm in cross-section. The support structure may be a utility pole and the insulator&#39;s first end may comprise a metal fitting bolted to the pole. The insulator&#39;s second end may comprise a metal fitting into which at least one eyebolt is threaded for supporting the power line. 
     In a third aspect, the invention is directed to a method for protecting an insulator in an electrical transmission system from environmental contaminants. An insulative cover is provided. The insulator is surrounded with the cover. A first end of the cover is secured to the insulator adjacent to the insulator&#39;s first end. A second end of the cover is secured to the insulator adjacent to the insulator&#39;s second end. This defines a substantially sealed volume surrounding a central portion of the insulator which is effective to protect such central portion from environmental contaminants. The surrounding step may include placing a first cover half on a first side of a separation plane extending longitudinally through the insulator and placing a second cover half on a second side of the separation plane. The first half is then secured to the second half. The method may be performed in situ while the power line is supported by the insulator and carries an alternating current voltage (e.g., in excess of 1 kV). 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a utility pole. 
     FIG. 2 is an enlarged view of a covered insulator on the utility pole of FIG.  1 . 
     FIG. 3 is a partial cutaway view of the covered insulator of FIG.  2 . 
     FIG. 4 is a partial transverse sectional view of the covered insulator of FIGS. 2 and 3 taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is a partial exploded view of the covered insulator of FIG.  2 . 
     FIG. 6 is a partial sectional view of an insulator bearing an alternate cover. 
     FIG. 7 is a partial sectional view of an alternate flange construction for an insulator cover. 
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     FIG. 1 shows a utility pole  20  having a central mast or pole  22  extending vertically from a lower end embedded in the ground to an upper end proximate which a horizontally-extending crossarm  24  is mounted. Extending upward from opposite ends of the crossarm  24  are first and second covered insulators  26 A and  26 B. At their upper ends, the insulators carry medium tension distribution lines/wires  28 A- 28 D. 
     FIGS. 2 and 3 show the covered insulator  26 A in further detail. FIG. 3 shows the covered insulator  26 A with half of the cover  29  removed. Within the cover  29  the basic insulator  30  shown in FIG. 3 may be of any appropriate type. The insulator  30  extends an insulator axis  500  from a first end  32  to a second end  34 . In the illustrated use, the insulator axis  500  is substantially vertical with the first end  32  being an upper end and the second end  34  being a lower end. At the respective upper and lower ends  32  and  34 , the insulator  30  includes respective upper and lower metallic end caps or fittings  36  and  38 . The fittings  36  and  38  surround upper and lower ends of an insulative member  39  formed of ceramic, porcelain, resin, or other rigid electrically non-conductive material. 
     FIG. 3 shows that the insulator  30  has angular symmetry about the axis  500  with exceptions for various connective features such as threaded holes  40  and  44  (FIG. 2) in the respective upper and lower fittings  36  and  38 . The four threaded holes  40  receive a first pair of eyebolts  42 A and  42 B and a second pair of eyebolts  42 C and  42 D. The first wire  28 A passes through the eyes of the first pair of eyebolts and the second wire  28 B passes through the eyes of the second pair of eyebolts. The wires extend in a wire direction  502  generally transverse to the insulator axis  500 . The four threaded holes  44  (FIG. 2) in the lower fitting  38  (FIG. 3) may be identically formed to the threaded holes  40  and may receive bolts  46  (FIG. 2) extending through the crossarm  24  to secure the insulator  30  atop the upper surface  48  of the crossarm. 
     As shown in FIG. 3, the insulative member  39  includes the unitarily formed combination of a central core or body  49  and a plurality of annular sheds  50 A- 50 D extending radially outward and slightly downward from the core  49 . Each shed includes an upper surface  54 , a lower surface  56 , and an end surface  58  joining the upper and lower surfaces. In the exemplary embodiment, over a substantial portion of their radial extent the upper and lower surfaces  54  and  56  are parallel to each other and are angled downward in the outward radial direction (e.g., at an angle θ of about 10-30°). In FIG. 3, the angle θ is shown separating a central frustoconical median  504  of a shed from a horizontal plane  506 . 
     FIG. 4 shows the cover  29  assembled from first and second halves  60  and  62  which, with the exception of interlocking features described below, are substantially mirror images of each other about a vertical separation plane  508 . The halves  60  and  62  have respective central body portions  61  and  63  extending nearly 180° about the insulator axis  500 . Extending vertically along first and second sides of the body  61 , the first half  60  has flanges  64 A and  64 B, respectively, on a first side  510 A of the separation plane  508 . Along first and second sides of the body  63 , the second half  62  has flanges  66 A and  66 B, respectively, on the second side  510 B of the separation plane  508 . To secure the two halves together, the flanges have interlocking features formed as male projections  68  and associated female receptacles  70 . The projections  68  may be formed on the flanges  64 B and  66 A while the receptacles  70  are formed on the flanges  64 A and  66 B. In certain embodiments such a configuration allows two halves  60  and  62  to be manufactured as identical pieces. Alternatively, the projections may be on the flanges of one of the cover halves while the receptacles are on the flanges of the other, or each flange may have an alternating series of projections and receptacles mating with respective receptacles and projections of the associated flange of the other cover half. Between associated flanges, a rubber gasket  72 A,  72 B (e.g., silicone, butyl, or neoprene rubber) provides a seal between surfaces of the flanges facing the separation plane  508 . At their outboard edges, the flanges  64 B and  66 A of the first half  60  and second half  62  respectively bear a rib  74  which spans the separation plane  508  to cover outboard edges of the associated gasket  72 A,  72 B and adjacent flange  64 A and  66 B. 
     FIG. 5 shows the two cover halves  60  and  62  prior to being secured over the insulator  30 . The cover may be assembled in situ with the wires hot. The rubber gaskets  72 A,  72 B may initially be provided having adhesive on both surfaces with release tape (not shown) covering such adhesive. The gaskets  72 A and  72 B are preferably pre-installed on one or both of the halves  60  and  62  via removal of the release tape from one surface and the application of such gasket to the inboard surface of the flange of the associated cover half. FIG. 5 shows the gaskets  72 A and  72 B preinstalled on the flanges  64 A and  66 B. At its upper and lower ends, the first half  60  includes fitting-engaging collar portions  76 A and  76 B, respectively, extending from upper and lower ends of the body  61 . In the illustrated embodiment, these are formed as a nearly 180° sector of an annular sleeve. The second half  62  includes similar collar portions  78 A and  78 B, respectively, extending from upper and lower ends of the body  63 . The inboard surfaces of the collar portions  76 A,  76 B,  78 A,  78 B are configured to engage with the lateral surfaces  82  of the fittings  36  and  38 . Each inner surface  80  bears a gasket  84  which may be formed of a similar material as the gaskets  72 A and  72 B. The halves  60  and  62  are brought into proximity with the insulator  30  and the remaining release tape removed from the exposed surfaces of the gaskets  72 A and  72 B and the four gaskets  84 . The two halves are then assembled over the insulator with the projections  68  being locked into the associated receptacles  70 . The gaskets  72 A and  72 B provide a seal between the two halves and the gaskets  84  provide a seal between the assembled halves and the fittings  36  and  38  thus defining a sealed volume between the cover and insulator. To further supplement the adhesion and sealing provided by the gaskets  84 , a pair of upper and lower plastic tie wraps  86 A and  86 B are wrapped around and cinched over the assembled upper collar portions  76 A and  78 A and lower collar portions  76 B and  78 B, respectively, to firmly clamp such portions to the associated upper and lower fittings  36  and  38 . 
     As is shown in FIG. 3, the assembled cover  29  has an outer or exterior surface  90  and an interior surface  91  substantially parallel to the exterior surface and spaced apart therefrom by a cover thickness of from about 0.04 in to about 0.10 in. The outer surface is formed having a vertically-arrayed series of annular protuberances at substantially even level with the ends  58  of associated sheds  50 A- 50 D. The protuberances each define an enlarged diameter area of the cover, with reduced diameter areas being located between adjacent protuberances. The protuberances have convex outer surface portions  92  of the exterior surface  90 , each of which has an associated concave portion  93  of the interior surface  91 . Each protuberance has an upper surface portion  94  and lower surface portion  95  along the exterior surface  90 . 
     FIG. 6 shows a partial sectional view of an alternate cover  100  which more closely accommodates the profile of the sheds of the insulator. This configuration has the advantage of providing the cover with its own shed-like construction that shields the undersides  101  of the cover sheds  102  from rain, etc. Thus, both the upper surface  103  and underside  101  of each shed  102  predominately slope downward in the outward radial direction. However, this construction complicates manufacturing. Advantageously, the projections and receptacles of the cover  100  are located in phase with the cover&#39;s sheds (e.g. substantially along the frustoconical median of each such shed rather than closer to intermediate such medians). 
     FIG. 7 shows a partial sectional view of an alternate flange construction wherein the male projections  68  are supplemented by an elongate male protrusion  108  running between the projections  68  and projecting parallel therewith. A mating female channel  110  similarly connects the receptacles  70 . The protrusion  108  and channel  110  cooperate when the two halves are assembled to provide a seal running along the mated flanges. This seal may replace that provided by the elastomeric gaskets  72 A and  72 B or may complement seals formed by gaskets running inboard and/or outboard of the mated protrusion and channel. 
     The material chosen for the cover should have low electrical conductivity, low surface porosity (in particular, the surface should be highly hydrophobic), high physical robustness (including both strength and abrasion resistance) and high stability (including resistance to heat and deteriorative effects of UV light). High molecular density polymers are believed particularly advantageous. One particularly preferred polymer is sold under the trademark KYNAR FLEX 2850 by Elf Atochem North America, Philadelphia, Pa. This is a fluorocarbon-vinylidene fluoride/hexifluoropropylene thermoplastic (VDF/BFP (TP)). This material has a density of 1.78 g/cm 3 , a melting point of 138° C., a volume resistivity of 2×10 14  Ohm-cm, a dielectric constant of 7.8 at 1 MHz, a dissipation factor of 2×10 −2  at 1 MHz, a coefficient of linear thermal expansion of 9.9×10 −5  mm/mm/°K, and a water absorption of 0.04% at twenty-four hours. This exemplary material is believed to offer a particularly advantageous combination of properties and cost. Other suitable materials include polyvinylidene fluoride (PDVF), polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene (FEP), ethyltetrafluoroethylene (ETFE), ethylchlorotrifluoroethylene (ECTFE), polypropylene, polyvinyl chloride, polyethylene (Type 1), polyetherimide, and polyethersulfone. Advantageously, the material used for the cover should have a dielectric strength greater than about 10 kV/mm, a volume resistance of about 10 13 -10 16  Ohm-cm, a surface resistance of 10 15 -10 17  Ohm, a dielectric constant of 2.5-8, a dissipation factor of 10 −3 -10 −1 , a coefficient of linear thermal expansion of about 10 −6 -10 −4  in/in/°F. (10 −5 -10 −3  mm/mm/°K), and a twenty-four hour water absorption of about 0.01%-0.04% for 24 hours. An advantageous service temperature ranges from about −30° F. to 180° F. 
     The cover encloses a sealed volume surrounding at least a central portion of the insulator and, advantageously, that portion extending between the fittings  36  and  38 . The volume is substantially sealed to the extent that under an expected range of atmospheric conditions, there is substantially no infiltration of water or water vapor so as to affect performance of the insulator. Nevertheless, certain of the benefits of the invention may be obtained via enclosing the insulator with less than substantial sealing (e.g. providing a small opening of relatively small cross-sectional area which allows pressure equalization across the cover while still substantially protecting the insulator from rain and wind-borne moisture and contaminants). For example, one or more small vent holes  104  (FIGS. 3 &amp; 6) may optionally be provided for pressure equalization. Advantageously, the vent holes are laser-formed, having a circular section of between about 0.005 mm and about 0.01 mm in diameter. Holes of the resulting cross-sectional area are effective to allow pressure equalization while remaining small enough to prevent wind-blown infiltration of water and other contaminants. Even much larger holes (e.g., up to about 1 mm in diameter) may provide effective venting with insignificant compromise of the shielding. Larger ventilation holes in a cover which enshrouds the insulator are possible but not preferred. 
     A variety of manufacturing techniques such as molding and vacuum forming may be utilized. One preferred method is to vacuum form the halves  60  and  62  from sheet stock 0.06 in thick. This yields a wall thickness for the cover halves of approximately the same 0.06 in. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the cover may be configured for use with a variety of existing insulators. Additionally, new insulators may be designed for use specifically with the covers of the invention. Fasteners other than the illustrated interlocking projections  68  and receptacle  70   s  (e.g., plastic screws and nuts and/or plastic rivets) may be utilized or bypassed altogether in favor of fastening via adhesive, solvent bonding, heat bonding, etc. These may also replace the exemplary rubber gaskets  72 A and  72 B sealing the two halves of the cover to each other. Additionally, the gaskets  84  may be replaced or complemented by other sealing means such as caulk, silicone sealant/adhesive, or the like. Optionally, the two cover halves may be unitarily formed, separated by a flexible reduced-thickness hinge portion. When separately formed, the two halves may be hinged along associated flanges by a piano-type hinge or the like. Although the exemplary support structure is a utility pole, the invention may be used with other support structures including the towers used to support high tension lines. Although the exemplary use involves supporting wires from beneath, the inventive cover may be applied to insulators supporting wires from other directions (e.g. wherein the wire is suspended below the cross-arm and the insulator is under tension rather than compression). Accordingly, other embodiments are within the scope of the following claims.