Composite insulators with built-in optical fibre, and methods of making them

Abstract: An optical fiber built-in type composite insulator is provided having a sealing portion (4) in a central axial hole (2) of the insulator for sealing the optical fiber (3). The sealing portions (4) are made of inorganic glass arranged at both ends of the hole (2) for sealing the optical fiber (3) therein. Axially recessed portions (10) e.g. annular grooves are arranged around the ends of the hole, and heat generating elements (6) arranged in the recessed portions (10) for heating and melting the inorganic glass to form the sealing portions (4) at both ends.

Description: The present invention relates to optical fiber built-in type composite insulators such as are used in detection systems for finding out fault points at electric power transmission lines and transformer substations, etc., and a method of producing the same. Heretofore, in order to automatically find out fault points in electric power systems, optical fiber built-in type composite insulators have been used which are capable of transmitting signals from optical sensors at power supply side to detectors at grounded side and maintaining electrical insulative property. Various types of optical fiber built-in type composite insulator are known. Among them, a technique is known wherein a penetration hole is bored in the axis of a ceramic insulator body, one or two optical fibres are inserted in the penetration hole, and a portion or the whole of the penetration hole is filled with an organic insulative material, such as silicone rubber or epoxy resin, etc., to seal the optical fiber in the penetration hole and prevent decrease of creepage distance of the insulator. Another technique is known wherein the whole of a ceramic insulator having a penetration hole and an optical fiber therein are heated and a molten glass is poured in the whole or a portion of the penetration hole to seal the optical fiber in the penetration hole. However, in the abovementioned sealing with an organic material, the organic sealing material and the ceramic insulator body have thermal expansion coefficients so different that problems occur, namely deterioration of the organic sealing material is accelerated and the optical fiber is occasionally broken by a thermal stress generated by temperature change in use environment. Moreover, the organic sealing material has a problem in that it is liable to incur a tracking, etc., during a long use, so that it has a poor reliability when used for a long time. Also, in the abovementioned sealing with the inorganic material or glass, the whole of the long ceramic insulator has to be heated, so that problems occur in that a large apparatus is required, increasing the investment cost, and a large electric power is consumed to increase the running cost. Moreover, when heating the whole of the insulator and the optical fiber for melting the glass, problems occur in that the coating of the optical fiber is scorched, so that the optical fiber is liable to break down and a structure extending the optical fiber from the ends of the insulator is difficult to obtain. For obviating the problems, surfaces of the optical fiber exposed at the end surfaces of the insulator after sealed by the glass have to be optically polished and adhered by means of Ferrule, etc., so that other problems of complicated and expensive production steps arise. In order to solve the above problems, the applicant proposed in their Japanese Patent Application Laid-open No. 1-246,724 (U.S. Patent 4,921,322) two sealing methods, as shown in the attached Figs. 4 and 5. In the sealing method as shown in Fig. 4, an insulator 31 and an optical fiber 3 are fixed by jigs 26A, 26B for fixing the insulator 31 and jigs 24A, 24B for fixing the optical fiber 3. These jigs are constructed in such a fashion that their vertical and horizontal spacings can be adjusted depending on the positions of the insulator 31 and the optical fiber 3. For the insulator 31, which has finished preliminary heating, there are arranged an induction heating furnace 21 for melting a glass, a hot air blower pipe 22, and a cooling pipe 23. Next, an upper end of the insulator 31 is heated by hot blow of a temperature of, for example, 550°C±20°C, from the hot air blower 22 for 5 min., and then filled with a sealing glass of a desired composition melted at, e.g., 500°C in the induction heating furnace 21 to a sealing portion in the penetration hole. After filling a desired amount of the sealing glass to finish the sealing operation at the end of the insulator 1, the insulator 1 is turned over and the same glass-filling operation as described above is performed on the lower end portion of the insulator 1 to complete the sealing process. The cooling pipe 23 is used for preventing heating of the jigs 24A, 24B which fix the optical fiber 3. However, even the method of Fig. 4 has the following drawbacks. Namely, when the inorganic glass is filled in the sealing portion after melting by heating, the neighboring portion of the ceramic insulator 1 around the inorganic glass can be heated below the temperature of the inorganic glass and insufficiently expanded, though the glass is preliminarily heated by the hot blow. As a result, when the inorganic glass is cooled and solidified, a tensile stress is exerted on the inorganic glass and the neighboring portion of the ceramic insulator 1, so that a crack is liable to form in the sealing inorganic glass. Moreover, the heated and melted inorganic glass is difficult to maintain in uniform state. Furthermore, when filling the heated and melted inorganic glass in the sealing portions of the penetration hole, a high risk of damaging the optical fiber 3, e.g. by scorching of the coating portion of the optical fiber 3, occurs. In the sealing method as shown in Fig. 5, a preliminary sealing member 41 is formed at first. That is, at a position of the optical fiber 3 corresponding to the end portion of the penetration hole 2 an electrically conductive ceramic or metallic tube 37 having an outer diameter capable of being inserted in the penetration hole 2 is provided, and a spacer 5 and a sealing glass 34 are provided in the tube 37 to form the preliminary sealing member 41 for sealing the optical fiber 3 therein. Next, the optical fiber 3 with the preliminary sealing member 41 therearound is inserted in the penetration hole 2 of the insulator 1 to locate or position the preliminary sealing member 41 at the end portion of the penetration hole 2, as shown in Fig. 5. At this time a sealing glass 34, preferably in a paste state, is introduced between the outer circumferential surface of the tube 37 of the preliminary sealing member 41 and the inner circumferential surface of the penetration hole 2. Thereafter, a high frequency induction heating device 42 is positioned at a position corresponding to the end portion of the penetration hole 2 and high frequency induction heating is effected. The electrically conductive ceramic or metallic tube 37 is induction heated by the heating, so that the sealing glass 34 arranged between the outer circumferential surface of the tube 37 of the preliminary sealing member 41 and the inner circumferential surface of the penetration hole 2 is melted to complete the sealing operation. Thereafter, a protecting member for protecting the sealed end portion, such as silicone rubber, etc., is provided on the sealed end portion around the optical fiber 3. However, even the method of Fig. 5 has drawbacks in that the portion of the ceramic insulator 1 around the inorganic glass can be heated below the temperature of the inorganic glass, so that a thermal stress is generated between the inorganic glass 34 and the neighboring portion of the ceramic insulator 1 to occasionally form a crack in the sealing inorganic glass 34 when the glass 34 is cooled and solidified. Moreover, the inorganic glass 34 used between the ceramic insulator 1 and the electrically conductive ceramic or metallic tube 37 is more likely to peel from the tube 37 at the bonded interface thereof than from the insulator 1 at the bonded interface thereof during a long use, so that it has poor reliability of the bonding portion. Furthermore, positioning of the tube 37 at a desired position in the penetration hole 2 is a difficult job with a serious danger of damaging the optical fiber 3, and the tube 37 can hardly be uniformly heated. Furthermore, the tube 37 has to be formed to a desired shape beforehand, a high frequency induction heating device has to be used for heating the tube 37, and the inorganic glass 34 has to be applied, calcined and baked on the tube 37, so that production steps are difficult, cumbersome and too many. The general aim herein is to provide a novel insulator incorporating a built-in optical fibre, and preferably an insulator which can eliminate crack formation in the inorganic glass sealing portion and the neighboring portion of the ceramic insulator with decreased number of production steps and simplified production apparatus. Another aspect of the present invention is to provide a production method of such composite insulator. A further object of the present invention is to obviate the abovementioned problems and drawbacks. EP-A-0339791 describes an optical fibre built-in type composite insulator, comprising: a ceramic insulator body having an axial penetration hole; at least one optical fibre inserted in the penetration hole; sealing portions made of inorganic glass arranged at both ends of the penetration hole for sealing the optical fibre therein; wherein the ceramic insulator has recessed portions arranged around the ends of the penetration hole, for receiving heating elements in the recessed portions, for heating and melting the inorganic glass to form sealing portions at both ends of the penetration hole. The present invention proposes that there is a portion of the ceramic insulator arranged between the recessed portions and the inorganic glass sealing portions. Thus the neighbouring portion of the ceramic insulator around the inorganic glass is always maintained at a higher temperature than the inorganic glass during the heating and melting of the glass. This reduces the tendency for a crack to form due to tensile stress generated by uneven expansion and cooling and improves the reliability of the sealing. The present invention also provides a method of producing an optical fibre built-in type composite insulator comprising the steps of: providing recessed portions in the ceramic insulator around the ends of an axial penetration hole; inserting an optical fibre in the axial penetration hole of the ceramic insulator; sealing both ends of the penetration hole by means of an inorganic glass to provide sealing portions at both ends of the penetration hole; arranging heat generating elements in the recessed portions, and heating and melting the inorganic glass to form the sealing portions at both ends of the penetration hole, wherein the recessed portions are provided in the ceramic insulator so that there is a portion of the ceramic insulator arranged between the recessed portions and the inorganic glass sealing portions. Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings in which: Fig. 1 is a schematic enlarged partial cross-sectional view of an embodiment of the present composite insulator showing a state of arranging an inorganic glass in a recessed portion of a ceramic insulator around an end of a penetration hole of the ceramic insulator; Fig. 2 is a plan view thereof; Fig. 3 is a plan view of another embodiment of the present composite insulator; Fig. 4 is a schematic front view of a prior apparatus for sealing an optical fiber in a penetration hole of a ceramic insulator at the end thereof by means of an inorganic glass; and Fig. 5 is a schematic partial cross-sectional view of another prior apparatus for sealing an optical fiber in a penetration hole of a ceramic insulator at the end thereof by means of an inorganic glass. Numberings in the Drawings 1,31 ... ceramic insulator 1a ... protrusion 1b ... end portion of the ceramic insulator 2 ... penetration hole for receiving the optical fiber 3 3 ... optical fiber 4 ... calcined inorganic glass 5 ... spacer for preventing flow of inorganic glass 6 ... ring shaped heat generating element 7 ... cement (adhesive layer) 8 ... flange type metallic fitting 10 ... ring shaped groove for receiving the heater 6 16 ... columnar heat generating element 20 ... round bore for receiving the heat generating element 16 Referring to Figs. 1 and 2, an enlarged cross-sectional view and an enlarged plan view of an end portion of the present composite insulator are shown. In Figs. 1 and 2, a ceramic insulator 1 has a central axial hole 2 for receiving an optical fiber 3, an inorganic glass 4 is arranged at an end of the penetration hole 2, and a spacer 5 is arranged below the inorganic glass 4 in the penetration hole 2 to prevent flow down of the inorganic glass 4. The inorganic glass 4 is preferably a calcined body having a hole for allowing penetration of the optical fiber 3 therethrough and a diameter corresponding to the inner diameter of the penetration hole 2. Around the upper end portion of the penetration hole 2 is provided a ring-shaped or circular groove 10 which receives and fixes a ring-shaped heat generating element 6 therein. The upper end portion 1b of the ceramic insulator 1 is held by a flange type metallic fitting 8 via an adhesive layer 7. The ring-shaped groove 10 may be formed at the time of forming the ceramic insulator 1 or it may be formed by working by grinding, etc., after the forming or firing of the ceramic insulator 1. The heat generating element 6 has a shape fitted to the shape of the groove 10 and is desirably a heater made of an iron-chromium-aluminum series alloy or a nickel-chromium series alloy having a high heat generation power. When providing a sealing portion in the penetration hole 2 at the upper end of the ceramic insulator 1, an electric current is applied to the heat generating element 6 to generate heat therefrom so as to heat and melt the inorganic glass 4 through the protrusion 1a, and thereafter the applicaiion of the electric current to the heat generation element 6 is turned off to cool and solidify the melted inorganic glass 4 whereby a sealed portion made of the inorganic glass 4 is formed. The same operation as described above is effected on the lower end of the ceramic insulator 1 to provide another sealing portion in the lower end of the penetration hole 2. The second operation may be effected sequentially after the first operation. According to the production method of this embodiment, the neighboring portion or the protrusion 1a of the ceramic insulator 1 around the inorganic glass 4 is always maintained at a higher temperature than the inorganic glass 4 during heating and melting the inorganic glass 4. Therefore, the protrusion 1a is expanded not less than the inorganic glass when the inorganic glass 4 is melted, and the inorganic glass 4 solidifies and shrinks with a shrinkage similar to the protrusion 1a after the sufficient expansion of the protrusion 1a. As a result, no tensile stress is exerted on the inorganic glass 4 and the surrounding protrusion 1a, so that a crack tends not to form. Therefore, reliability of the sealing property of the sealing portion can be remarkably improved, and fraction defective or percentage of rejected products can be reduced, so that yield of the products can be noticeably elevated. Moreover, the ceramic insulator 1 and the inorganic glass 4 can be directly bonded without an intervening metallic tube, etc., so that peeling off, etc., of the inorganic glass 4 from the ceramic insulator 1 at the interface thereof scarcely occurs to improve the reliability of the sealing portion made of the inorganic glass 4, particularly the reliability of the sealing portion for a long period of use. Furthermore, the inorganic glass 4 is arranged at a desired position in the end portion of the penetration hole 2 and heated by a heat generating element 6 arranged with an appropriate spacing from the inorganic glass 4 to heat and melt the calcined body of the inorganic glass 4, so that the number of requisite materials, members and devices necessary for the production or sealing are small and the production steps are reduced. Therefore, the sealing portions made of inorganic glass can be quite simply and economically formed. The spacing of the heat generating element 6 from the inorganic glass 4 is preferably 5-20 mm. In order not to decrease the mechanical strength of the ceramic insulator 1, a ratio of the outer diameter of the circular groove 10 to the diameter of the ceramic insulator 1 is preferably not more than 0.5, and a ratio of the depth of the groove 10 to the length of the flange type metallic fitting 8 is preferably not more than 0.75. After finishing the sealing step using the inorganic glass, the heating element 6 is preferably removed from the groove 10 which is then filled with a waterproof material to prevent invasion of water, etc. As the waterproof material, silicone rubber is preferably used, because it can prevent invasion of water, etc., without suffering from deterioration due to water, ozone, and ultraviolet ray, etc., and without forming an internal crack and peeling-off at the bonded surface due to expansion and shrinkage thereof caused by temperature change in use environment. Coefficient of thermal expansion of the inorganic glass 4 is preferably smaller than that of the ceramic insulator 1, so that a compression stress is always exerted on the sealed portion made of the inorganic glass 4 after the sealing. When heating and melting the inorganic glass 4, preferably the inorganic glass 4 arranged in the end of penetration hole 2 is burdened with a pressure from both the upper and/or lower sides, so that the level of the inorganic glass after the sealing can always be maintained constant and the bonding strength of the inorganic glass 4 to the ceramic insulator 1 can be improved. In order not to scorch the coating of the optical fiber 3 extruded from the end portion of the penetration hole 2 to the exterior, when heating and melting the inorganic glass 4, the extruded portion should always be cooled during heating. If an end-protective member made of e.g., silicone rubber, epoxy resin, etc., is formed on the outer periphery of the projecting portion of the optical fiber 3 after the sealing, the damage of the optical fiber by means of external force can be prevented. The spacer 5 for stopping the downward flow of the molten inorganic glass 4 is preferably made of Kovar, iron-nickel series alloy, ceramic material or inorganic fibers having a substantially same coefficient of thermal expansion as that of the sealing inorganic glass 4. In the embodiment shown in Fig. 3, a plural number (8 in the drawing) of columnar bores 20 are arranged around the penetration hole 4 instead of the circular groove 10. Each bore 20 receives a columnar heat generating element 16, respectively, which generates heat for heating and melting the calcined body of the inorganic glass 4. Hereinafter, the present invention will be explained in more detail with reference to specific examples with concrete numeral values. According to the methods as shown in Figs. 1, 4 and 5, an inorganic glass sealing portion is produced. In the method as shown in Fig. 1, the ceramic insulator 1 has a diameter of 105 mm, and a length of 1,100 mm, and the circular groove 10 has an outer diameter of 30 mm, a width of 7 mm, and a depth of 30 mm. The inorganic glass 4 is made of a lead borate glass having a low melting point and has a spacing of 5 mm from the heat generating element 6. The inorganic glass is melted at 500°C and left to cool and solidify. The insulators having the sealing portions of Figs. 1, 4 and 5 are evaluated on operation hours required for the sealing and reliability of the sealing property. The results are shown in the following Tables 1 and 2. Thermal shock tests shown in the above Table 2 are effected by repetition of a cycle of holding the insulator samples in a hot bath of 80°C for 30 min. and in a cold bath of -20°C for 30 min. Each value of the initial trouble rate % is shown by a percentage of cracked samples detected by a fluorescent damage-detecting test in the whole number of 20 insulator samples. Each value of trouble rate after repeated thermal shock test is shown by a percentage of He-leaked samples in the whole number of 20 insulator samples containing He gas sealed in the central portion of the penetration hole 2, after repetition of desired cycles of thermal shock test. As seen from the above Tables 1 and 2, using the embodiment of the invention, the sealing operation can considerably be simplified and shortened, the initial trouble rate and the trouble rate after repeated thermal shock can substantially be eliminated, and the durability and the reliability of the composite insulator for a long period of use can appreciably be improved by prevention of crack in the sealing portion. As described in detail in the foregoing explanations, an axially recessed portion is arranged in the end portion of the ceramic insulator around the penetration hole receiving the optical fiber and the inorganic glass, the heat generating element is received in the recessed portion, and the heat generating element is heated to melt the inorganic glass, so that the neighboring portion of the ceramic insulator around the inorganic glass can be held at a higher temperature than the temperature of the inorganic glass during heating and melting the inorganic glass. Hence, the neighboring portion of the ceramic insulator around the inorganic glass is expanded not less than the inorganic glass prior to the cooling and solidification of the inorganic glass, and shrinks with a substantially same extent simultaneously with the shrinkage and solidification of the inorganic glass. As a result, a tensile stress is not exerted on the inorganic glass nor on the neighboring portion of the ceramic insulator, so that a crack is scarcely formed. Therefore, the reliability of the sealing property of the sealing portion can be greatly improved, and the percentage of rejected products can be much reduced. Moreover, the inorganic glass arranged in the end portion of the penetration hole receiving the optical fiber is heated and melted to form an inorganic glass sealing portion, so that a metallic tube, etc., need not be interposed between the inorganic glass and the insulator and can be dispensed with. Therefore, the problems of peeling off of the inorganic glass from the metallic tube at the bonded interface thereof, etc., can be prevented to materially improve the reliability of the sealing property of the sealing portion, particularly the sealing property of the sealing portion during a long period of use. Furthermore, the simple structure of receiving the heat generating element in the recessed portion and heating it to melt the inorganic glass affords reduced materials, members and devices required for the sealing as well as small number of production steps. Therefore, the present invention can produce the inorganic glass sealing portion quite quickly, simply and economically. Although the present invention has been explained with specific examples and numeral values, it is of course apparent to those skilled in the art that various changes and modifications thereof are possible.

Claims: An optical fibre built-in type composite insulator, comprising: a ceramic insulator body (1) having an axial penetration hole (2); at least one optical fibre (3) inserted in the penetration hole (2); sealing portions made of inorganic glass (4) arranged at both ends of the penetration hole (2) for sealing the optical fibre (3) therein; wherein the ceramic insulator (1) has recessed portions (10,20) arranged around the ends of the penetration hole (2), for receiving heating elements (6,16) in the recessed portions (10,20), for heating and melting the inorganic glass (4) to form sealing portions at both ends of the penetration hole (2), characterised in that there is a portion (1a) of the ceramic insulator (1) arranged between the recessed portions (10,20) and the inorganic glass (4) sealing portions. An insulator according to claim 1, in which the recessed portion is at least one axially-open axial recess formed in an end surface of the insulator. An insulator according to claim 2, in which the recessed portion comprises an annular groove (10) around the opening of the hole (2). An insulator according to claim 2, in which the recessed portion comprises plural recesses (20) distributed around the opening of the hole (2). An insulator according to any one of claims 1 to 4, in which the recessed portion (10,20) is disposed not more than half-way out from the central insulator axis to the insulator periphery. A method of producing an optical fibre built-in type composite insulator comprising the steps of: providing recessed portions (10,20) in the ceramic insulator (1) around the ends of an axial penetration hole (2); inserting an optical fibre (3) in the axial penetration hole (2) of the ceramic insulator (1); sealing both ends of the penetration hole (2) by means of an inorganic glass (4) to provide sealing portions at both ends of the penetration hole (2); arranging heat generating elements (6,16) in the recessed portions (10,20), and heating and melting the inorganic glass (4) to form the sealing portions at both ends of the penetration hole (2), characterised in that the recessed portions (10,20) are provided in the ceramic insulator (1) so that there is a portion (1a) of the ceramic insulator (1) arranged between the recessed portions (10,20) and the inorganic glass (4) sealing portions. A method according to claim 6, in which the recessed portion is at least one axially-open axial recess formed in an end surface of the insulator. A method according to claim 7, in which the recessed portion comprises an annular groove (10) around the opening of the hole (2). A method according to claim 7, in which the recessed portion comprises plural recesses (20) distributed around the opening of the hole (2). A method according to any one of claims 6 to 9, in which the recessed portion is disposed not more than half-way out from the central insulator axis to the insulator periphery.

Inventors: IKEDA MITSUJI, NISHIOKA MASAO, NOZAKI MASAYUKI, IKEDA, MITSUJI, NISHIOKA, MASAO, NOZAKI, MASAYUKI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: H01B17
IPC codes: G02B6, H01B17, H01B19