Patent Publication Number: US-6215075-B1

Title: Composite insulator

Description:
RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 08/230,417, filed Apr. 19, 1994 now abandoned which is a continuation of application Ser. No. 08/082,063, filed Jun. 23, 1993 now abandoned which is a continuation of Ser. No. 07/898,075 filed Jun. 12, 1992 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to electric power transmission line insulators. More particularly, it relates to an improved weathershed for a composite insulator. 
     BACKGROUND OF THE INVENTION 
     Insulators are used to prevent the loss of electric charge or current from conductors in electric power transmission lines. A typical insulator is made from a material which has a very high resistance to electric current so that current flow through it is negligible. One type of insulator is referred to as a suspension insulator. It suspends a transmission line from an overhead support. A particular type of suspension insulator is referred to as a composite insulator. 
     A composite insulator may include a coated fiberglass rod surrounded by weathersheds made from a highly insulating material (see, for example, Kuhl U.S. Pat. No. 4,217,466, or, Bauer U.S. Pat. No. 4,246,696). A common approach has been to cover the fiberglass rod with an insulating coating. The weathersheds are molded from rubber or other polymeric material and then bonded to the coated rod using a polymer based adhesive. Metal fittings are provided at each end of the rod for connecting one end to a support pylon and the other end to a power line. 
     The weathersheds and the fiberglass rod are made from different material in order to enhance the performance of the two components in their distinct functions. In general, the rod takes the mechanical stress and contributes to the insulation function, while the weathersheds provide the majority of the insulation. The weathersheds also increase the distance which leakage currents must travel from one metal fitting to the other. The weathersheds must be fitted in such a way that no electric conduction can occur through the seam between a weathershed and the coated rod. Each weathershed must have good stability in all weather conditions. Furthermore, it must be of sufficient thickness to prevent its electrical resistance from being overcome. Additional information regarding the operation and construction of composite insulators may be found in U.S. Pat. Nos. 3,898,372; 4,267,403; 4,331,833; and 4,355,200. 
     BRIEF SUMMARY OF THE INVENTION 
     A primary object of the present invention is to provide an improved weathershed for composite insulators. 
     It is another object to provide a weathershed which is lightweight and easy to manufacture. 
     It is a further object to provide a weathershed having a reduced electric field gradient. 
     It is still another object to provide a weathershed having a reduced tendency to trap contaminants such as dust, water and air pollutants. 
     It is yet another object to provide a weathershed which creates a maximum leakage current distance. 
     The foregoing and other objects are realized in accord with the present invention by providing a weathershed having a molded body with a hub and a skirt. The thickness of the skirt is substantially uniform from the hub to its annular outer edge. This maximizes leakage current distance while, at the same time, minimizing material usage. The annular outer edge is radially curved between upper and lower faces of the skirt to reduce the value of the electric field gradient. Other edges or external corners on the body are also curved. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     The invention, including its construction and method of operation, together with additional objects and advantages thereof, is illustrated in the following drawings, in which: 
     FIG. 1 illustrates a composite insulator embodying features of the present invention; 
     FIG. 2 is a vertical sectional view taken through a portion of the insulator shown in FIG. 1; and 
     FIG. 3 is an enlarged sectional view taken along line  3 — 3  of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a composite insulator  10  embodying the present invention. The insulator  10  may be used to insulate any of a variety of electric power transmission lines (not shown), including high tension lines (typically 100-400 KV) or distribution lines (typically 15-35 KV). A transmission line normally includes a set of conductors, usually bare, which are supported by a series of supporting structures such as wooden H-frames or steel towers, for example. With a distribution line, the supporting structures may be conventional wood poles. 
     The insulator  10  may be connected to the power line in variety of connection configurations, including “suspension,” “deadend” and “post” connections. The general structure of the insulator  10  is essentially the same, regardless of the manner in which it is connected to the power line or the support. 
     As shown in FIG. 1, the composite insulator  10  comprises a rod  20 , a shield layer  30  and a plurality of weathersheds  40  fastened to the shielded rod. Metal fittings  22  are attached to each end of the rod  20  for connecting one end of the rod to a support pylon (not shown) and the other end to a power line (also not shown). 
     Suitable materials and methods of construction for the rod  20  and the shield layer  30  are described in U.S. Pat. No. 4,217,466 to Kuhl, and U.S. Pat. No. 4,246,696 to Bauer et al. Suitable materials for the weathersheds  40 , along with adhesives and methods for attaching the weathersheds  40  to the shield layer  30 , are also disclosed in Kuhl and Bauer et al. The entire disclosures of both of these patents are incorporated herein by reference. 
     The rod  20  is generally elongated and may be made as a composite structure from glass fibers and a resin produced by pultrusion. Since the rod  20  must support the entire tension load to which the insulator  10  is subjected, it is important that the rod be of high mechanical strength as well as high dielectric strength. The glass fibers are resin bonded into an elongated, cylindrical rod  20  having a smooth exterior surface of substantially uniform diameter. The bonding resin must provide a good mechanical bond and have high dielectric strength to ensure that the completed rod will have the desired insulation capability. However, the tensile strength of the rod is imparted, in large measure, by the fibers. Accordingly, it is desirable that the fibers constitute a large part of the cross section of the rod. 
     The rod  20  has an extrusion applied shield layer  30  which consists of an envelope of silicone or other elastomer that is current peak-proof and weather resistant. The shield  30  provides an intermediate layer of material between the rod  20  and the weathersheds  40 . The shield  30  protects the rod  20  from deleterious material and may be made from a mono-functional or poly-functional polymer. Each weathershed  40  is mounted on the rod  20  by inserting the rod through an aperture  43  (see FIG. 3) in the weathershed  40 . The weathersheds  40  may be secured to the shield layer  30  with an adhesive. Alternatively, the weathersheds  40  and the shield layer  30  may be cross-linked together. 
     Turning now to FIGS. 2 and 3, the construction of a weathershed  40  is illustrated in greater detail. The weathershed  40  is preferably molded from a flexible, yet sturdy, synthetic material. Molding is accomplished by conventional injection and/or compression molding techniques. Suitable synthetic materials include EPDM rubber, silicone rubber, and other materials having similar properties. In the preferred embodiment shown, silicone rubber is employed. 
     The weathershed  40  has a disc-like body  42  with a slight bell shape. The body  42  includes a central hub  44  through which the aperture  43  extends, and an outwardly extending skirt  41 . Being disc-like, the hub  44  and skirt  41  together form a flattened round body  42 . An adhesive primer is applied to the internal face of the aperture  43  to bond the weathershed  40  to the shield layer  30  on the rod  20 . 
     The annular outer edge  48  of the skirt  41  is radially curved between the upper face  49  and lower face  50  of the skirt  41 . This continuously rounded edge  48  configuration reduces the value of the electric field gradient across the weathershed  40 . This edge  48  configuration also reduces the tendency to trap contaminants such as dust or water, for example, on the upper face  49  of the skirt  41 . Two other external edges or corners  51  and  52  on the hub  44  are also continuously curved for the same purpose. 
     The skirt  41  of the weathershed  40  has a substantially uniform thickness. To accomplish this the lower face  50  extends substantially parallel to the upper face  49  from the outer edge  48  of the skirt  41  to the hub  44  of the weathershed  40 . As a result, an annular depression  58  is formed around the hub  44  in the lower face  50 . The rounded corner  52  defines the lower extremity of an external face on the hub  44 . Uniform thickness is maintained throughout the skirt  41  of the weathershed  40  without compromising the vertical height of the hub  44 . The skirt  41  curves downwardly (as seen in cross-section in FIG. 2) and outwardly from the hub  44  at a decreasing radius of curvature so that an inner portion of the skirt adjacent the hub has the aforementioned bell shape and the outer portion of the skirt is shaped like a flattened frustum of a cone. Consequently, the surface area of the internal aperture  43  face is also maximized, resulting in an increased bonding surface for attaching the weathershed  40  to the shield layer  30  on the rod  20 . The distance from the internal aperture  43  face of the hub  44  to the outer edge  48  of the skirt  41  is also maximized, thus maximizing the travel distance for leakage current. As best seen in FIG. 2, the hub  44  is thickened along its entire height, which is three-quarters of the overall height of the body. The thickness of the hub  44 , from its external upper edge  51  to its external lower edge  52 , is as great or greater than the thickness of the skirt  41  throughout its height. 
     The weathershed  40  configuration which has been described minimizes the material needed to manufacture the weathershed. Thus, the weight of the weathershed  40  is minimized. The use of a reduced amount of material allows a faster process cycle to be utilized when curing the weathershed  40  during manufacture. In particular, the depression  53  allows better and faster penetration of curing heat into the body  42  of the weathershed  40 . Overall production quality is improved while providing a less expensive product. 
     While a preferred embodiment of the invention has been described, it should be understood that the invention is not limited to it. Modifications may be made without departing from the invention. The scope of the invention is defined by the appended claims, and all devices that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.