Abstract:
A high voltage insulator for use with optical fibers includes an insulative support rod with at least one optical fiber wrapped about the support rod. The optical fiber and support rod are covered by an outer elastomeric skirted insulative sleeve which squeezes the optical fibers against the support rod. A dielectric sealant, such as a silicone gel, is dispersed along the optical fiber to fill any voids which occur adjacent the optical fiber, thereby providing a void-free bond between the interior surfaces of the insulator. The elastomeric outer sleeve provides a resilient barrier against the ingress of moisture. In a preferred embodiment, an inner layer of elastomeric material is provided between the support rod and the optical fiber to provide additional cushioning of the fiber and to reduce the size of voids which may occur adjacent the fiber.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to optical fibers, and in particular to optical fibers which are installed in contact with or closely adjacent to high voltage electrical equipment, such as high voltage power conductors. 
     The number of applications of optical fibers on overhead high voltage conductors is growing at an increasing rate as the general use of fiber optic systems continues to increase at a rapid rate. High voltage applications are generally associated with fiber optic devices such as current, voltage and temperature sensors installed on an electrical network to monitor the network operating conditions. In addition, fiber optic cables are often attached to high voltage conductors and used for long distance voice, video and data communications. 
     Although optical fibers are inherently good dielectric materials, they still constitute a threat to the integrity of the high voltage power system on which they are installed if proper precautions are not taken at all transition points where the fiber optic system is connected to electrically grounded opto-electronic signal processing equipment. A particular problem encountered when taking a fiber optic system from a high voltage potential to a ground potential is dielectric tracking. Dielectric tracking occurs when electric leakage currents flowing on the surface of an insulating material cause surface temperatures to rise to a level at which degradation of the material occurs. Dielectric tracking problems are increased by environmental conditions such as rain, fog, salt spray, dust and numerous industrial pollutants. Once dielectric tracking has been initiated, it tends to perpetuate until the dielectric strength of the insulating system is sufficiently reduced to cause dielectric failure of the system, usually by line to ground flashover. 
     The coatings, buffers and jackets used on optical fibers are primarily formulated and designed to enhance the handling and physical performance characteristics of the optical fibers with little or no consideration given to the dielectric tracking resistance for high voltage applications. It is therefore necessary that insulators specifically designed to support and protect the optical fibers must be used for high voltage applications. 
     Although the primary function of an insulator for high voltage applications is to provide a surface resistant to dielectric tracking with sufficient extended creepage length to prevent line to ground flashovers under inclement and contaminated conditions, there are other important performance criteria which must be met. In particular, in high voltage transmission line applications, distances of 10 to 25 feet between the ground and the high voltage line are common. It is thus important that the optical fibers are properly supported and contained to prevent excessive motion due to environmentally induced vibrations of the high voltage conductors and of the optical fibers. 
     The use of optical fibers in association with high voltage power conductors is known in the prior art. For example, U.S. Pat. No. 4,772,090 discloses an arrangement whereby a fiber optic cable may be routed through or around equipment at different electrical potentials, including ground potential. U.S. Pat. No. 4,717,237 discloses an overhead electric and optical transmission system in which the overhead electric conductor is mechanically secured to a support structure by a tension insulator having a through-bore for an optical fiber. U.S. Pat. No. 5,124,634 discloses an optical current transducer which uses an insulator pedestal as an optical fiber raceway. 
     The insulator systems of the prior art have disadvantages which would be desirable to overcome. In particular, ingress of moisture is a severe problem which will ultimately cause failure of an insulating system. Existing systems that have been modified for use with optical fibers rely on mechanical sealing mechanisms at each end of the insulator to prevent the ingress of moisture. Such mechanical seals are susceptible to failure after repeated thermal cycling. It would therefore be desirable to devise an insulation system which does not rely on mechanical seals. Also, the prior art insulators are typically made of ceramic or other heavy materials, making such insulators difficult to use in some situations. Further, prior art insulators are not easily adapted to different application configurations, such as varying line to ground distances. It would therefore be desirable to provide an insulation system which is easily adapted for differing applications, and which is lightweight as well. 
     SUMMARY OF THE INVENTION 
     The present invention provides a high voltage insulator for use with optical fibers which is light weight, easily adapted to different applications, and resistant to the ingress of moisture over repeated thermal cycles and long periods of time. The insulator includes an insulative support rod about which is wrapped at least one optical fiber. The optical fiber and support rod are covered by an elastomeric skirted insulative sleeve which squeezes the optical fibers against the support rod. A dielectric sealant, such as a silicone gel, is dispersed along the optical fiber to fill any voids which occur adjacent the optical fiber, thereby providing a void-free bond between the interior surfaces of the insulator. The elastomeric insulative sleeve covering the optical fibers provides a resilient barrier against the ingress of moisture. In a preferred embodiment, a resilient elastomeric layer of material is provided between the support rod and the optical fiber to provide additional cushioning of the fiber and to reduce the size of any voids which may occur adjacent the fiber. The elastomeric insulating materials are preferably silicone. The insulator is easily adapted to different voltage requirements by simply changing the length of the insulator. 
     The insulator may incorporate additional sensing elements to create a “smart” insulator for monitoring line conditions such as icing, wind loading, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a section of the novel insulator, with a portion of the insulative cover removed. 
     FIG. 2 a  is a cross-sectional view of the novel insulator, taken along line  2 — 2  of FIG.  1 . 
     FIG. 2 b  is a greatly enlarged view of the circled portion,of FIG. 2 a.    
     FIG. 3 is an elevational view, partially in cross-section, illustrating the application of the sealant and insulating sleeve over the optical fibers. 
     FIG. 4 a  is a cross-sectional view of an alternate embodiment of the novel insulator. 
     FIG. 4 b  is a greatly enlarged view of the circled portion of FIG. 4 a.    
     FIG. 5 is an elevational view, partially in cross-section, illustrating the termination of the insulator of FIG. 4 a  and  4   b.    
     FIG. 6 is an elevational view, partially in cross-section, illustrating the incorporation of strain sensors and temperature sensors into the insulator system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The insulator system of the present invention is designed to protect and support optical fibers between a high voltage conductor and electrical ground. The insulator is designed to be anchored at both extremities, the high voltage end to a structurally sound high voltage conductor or conductor assembly and the other end to a structurally secure ground attachment. The inventive insulators are configured to support only a limited amount of load when applied in a free standing configuration. The insulator system construction is based around a solid rod of a high grade dielectric material. The external diameter of the rod can vary to meet specific application criteria, such as the length and number of optical fibers to be incorporated. The length of the rod depends primarily upon the voltage level of the system on which it used. The preferred embodiments of the insulator system construction are described below. 
     A first preferred construction of the inventive insulator system  10  is shown in FIG.  1 . Insulator system  10  includes a support rod  12  formed from a suitable material, such as fiberglass or PVC. Optical fibers  14 , which may be coated and unjacketed, are wrapped around the surface of support rod  12  in a helical fashion consisting of approximately one or more turns over the length of support rod  12 . The number of turns of optical fibers  14  about support rod  12  may be altered to suit the particular application of the user, and are in particular dependent on the overall length of the support rod  12 , which will vary according to the voltage requirements of the finished insulator system  10 . The purpose of the helical application of optical fibers  14  about support rod  12  is to minimize the physical forces exerted on optical fibers  14  during large temperature changes which cause differential rates of expansion or contraction between support rod  12  and optical fibers  14 . 
     An insulator sleeve  16  is applied over support rod  12  and optical fibers  14 . Insulator sleeve  16  preferably is formed from silicone material, such as dimethylpolysiloxane, although other insulative materials, such as ethylene-propylene rubber, could also be used. Additionally, it is preferred that insulator sleeve  16  have skirts  18  for increasing the dielectric tracking length of insulator system  10 . Insulator sleeve  16  is preferably applied to support rod  12  and optical fibers  14  by removing a support core  20  (shown in FIG. 3) which holds insulator sleeve  16  in an expanded state. As support core  20  is removed, insulator sleeve  16  shrinks onto support rod  12  and optical fibers  14  to secure fibers  14  between support rod  12  and insulator sleeve  16 . Such insulating sleeves and support cores are known in the art, and are described, for example, in U.S. Pat. No.3,515,798 to Sievert, assigned to the assignee of the present invention, and which is incorporated herein by reference. A preferred skirted silicone insulator sleeve is the QTM ColdShrink™ skirted silicone insulator available from Minnesota Mining and Manufacturing Company of St. Paul, Minn. 
     It is important that the integrity of the internal interfaces of insulator system  10  be maintained through repeated temperature cycling and over periods up to 30 years, which is the generally expected field service life of products in the high voltage electric industry. Even minute air voids, when subjected to high voltage electric fields such as those found in the vicinity of high voltage transmission lines, will ionize and break into corona discharges that will ultimately result in failure of the optical fibers  14  and the dielectric insulation system itself. It is therefor necessary to achieve a completely void free insulator system. 
     As can be seen from FIGS. 2 a  and  2   b , when insulator sleeve  16  is applied over support rod  12  and optical fibers  14 , wedge-shaped voids  22  are formed immediately adjacent optical fibers  14 . The method used to achieve a complete void-free fill of voids  22  is illustrated in FIG.  3 . Voids  22  are filled by pouring a low viscosity sealant  24 , such as Dow Corning Sylgard #527 Silicone Dielectric Gel, into the core of insulator sleeve  16  as soon as enough of support core has been removed to form a seal between support rod  12  and insulating sleeve  16 . A small reservoir of sealant  24  is maintained in the transition area between the end of support core  20  and insulator sleeve  16  as the core  20  is removed to allow insulator sleeve  16  to shrink down onto support rod  12 . Sealant  24  is dispersed into the interstitial voids  22  along optical fibers  14  by the “squeegee” action of insulator sleeve  16  as support core  20  is removed by pulling free end  25  of support core  20  in the direction of arrow A in FIG.  3 . This action also continually advances the reservoir of sealant  24  as support core  20  is removed and provides a very efficient application of sealant  24 . To achieve satisfactory filling of voids  22 , it is preferred that sealant  24  have a viscosity of  750  poise or less, and most preferably has a viscosity of approximately 300-350 centipoise. If the viscosity of sealant  24  is too high, it does not uniformly fill voids  22 , which ultimately leads to premature failure of the insulator. After support core  20  has been completely removed, the result is a completely sealed, void-free insulator assembly cable of meeting the most demanding high voltage application requirements. 
     A second preferred embodiment of a fiber optic insulator system  10 ′ is shown in FIGS. 4 a - 4   b  and  5 . 
     Insulator system  10 ′ is similar to that described above, except that support rod  12  is first covered with a continuous length of elastomeric tube  26  to provide a resilient base for optical fibers  14 . Optical fibers  14  are applied over tube  26  and sealed between tube  26  and insulator sleeve  16  using the same techniques as described above and shown in FIGS. 1-3. Elastomeric tube  26  is preferably formed of silicone, but could alternatively be formed from other insulating materials such as ethylene-propylene rubber. In the configuration shown in FIGS. 4 a  and  4   b , optical fibers  14  are sealed void-free and cushioned between two elastomeric tubes  26 ,  16 . The construction of insulator system  10 ′ provides the required void-free seal with less compressive force applied to optical fibers  14 . This is especially important if fibers  14  are of a type that is sensitive to external forces, such as polarizing or polarization maintaining fibers. 
     The construction of insulating system  10 ′ has the further advantage of creating smaller voids  22 ′ adjacent optical fibers  14 . 
     FIG. 5 shows a longitudinal view, partially in cross-section, of an end of insulator system  10 ′, illustrating the manner in which insulator system  10 ′ may be terminated and anchored. (Insulator system  10  could be terminated in a similar manner). As can be seen in FIG. 5, support rod  12  extends into a threaded terminal  30 , which is in turn anchored to either a portion of the high voltage network or to the grounded opto-electrical system. Protective buffer tubes  32  guide optical fibers  14  from inside terminal  30  through transition region  34  onto elastomeric tube  26 . Optical fibers  14  are covered with insulator sleeve  16  in the manner described above, and transition region  34  is covered with yet another insulating tube  36 . As insulating tube  36  is applied to cover transition region  34 , any voids adjacent buffer tubes  32  in transition area  34  are filled, void-free, with sealant  24 . The method used to fill voids in transition area  34  is the same as that described above with reference to FIG.  3 . 
     The insulator systems  10 ,  10 ′ described herein are much lighter and have lower installed costs than existing insulator systems and provide the design flexibility necessary to facilitate the installation of optical fiber devices on high voltage power lines. A particular application in which the inventive insulator systems are useful is with optical sensing and monitoring devices which are used to monitor the performance of high voltage networks. New compact and lightweight optical sensors permit the installation of sensing devices on existing power lines, rather than rerouting the power lines to the location of the sensor. However, installing a sensing device on an existing power line makes necessary an effective insulating and supporting device like that disclosed herein. 
     Ingress of moisture in any dielectric insulation system will ultimately cause failure of that system. Existing insulator systems that have been modified for applications of optical fibers relying on mechanical sealing mechanisms at both extremities of the insulator to prevent the ingress of moisture. Long term reliability can be compromised by a relatively minor leak in a mechanical seal during the numerous thermal cycles the insulator experiences during its operating lifetime. The above described use of elastomeric insulator materials in conjunction with a dielectric filling material applied in a unique way to form a continuous void-free bond over the entire length of the optical fibers is a unique advantage provided by the embodiments described herein. The natural flexibility and resilience of the materials used in the inventive insulator systems ensures the integrity of the system over long periods of exposure to conditions, such as thermal cycling and line vibration, which can have serious detrimental effects on insulators that rely only on mechanical sealing systems. 
     Those skilled in the art will recognize that modifications may be made to the basic configurations described herein to provide other types of innovative insulators. For example, the insulator systems described herein also provide the potential to produce an insulator that is itself a sensor system, i.e., a smart insulator. For example, an insulator that could monitor a high voltage transmission line for icing conditions, wind loading, galloping conductors and conductor temperature would only require an optical strain sensor, such as a fiber Bragg grating, attached to the support rod, a temperature sensor attached to the line connection clamp and a second temperature sensor to monitor the ambient temperature and provide a reference source for the other sensors. With this configuration, the strain sensor on the support rod would measure the weight of the attached conductor. Under icing conditions, the weight would gradually and continuously increase at a relatively uniform rate, whereas wind loading and galloping conductors would cause loading having a cyclic nature with rapid load changes. 
     To achieve an accurate measurement of the line loading conditions, it is important to be able to differentiate changes in the rod caused by changing temperature conditions from changes caused by other elements, previously mentioned, acting on the line. The very unique feature of the inventive insulator systems described herein that makes this possible is the ability to locate a strain sensor and a reference temperature sensor in very close proximity in the same assembly, and yet to maintain mechanical isolation between the two sensors. Construction of this system, illustrated in FIG. 6, involves securing the optical strain sensor  40  onto the support rod  12  so that it will detect all incremental changes in the length of the rod  12 . The continuous extruded elastomeric tube  26 , as described in embodiment  10 ′ above, is then placed over the support rod  12 , optical fiber  14  and strain sensor  40 . An optical temperature sensor  42 , such as a fiber Bragg grating similar to the strain sensor  40 , is applied on the surface of the elastomeric tube  26  and then covered by the outer insulator sleeve  16 . This provides a temperature reference that is cushioned between two elastomeric layers and mechanically isolated from the measured strain element. 
     While the invention has been particularly shown and described herein with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.