LIGHTNING ARRESTOR INSULATOR AND METHOD OF PRODUCING THE SAME

Abstract: A lightning arrestor insulator is provided having a discharge gap portion and an arrestor ZnO element device (5) both in the body of the insulator, comprising opposed projecting discharge electrodes (3a,3b) arranged inside the insulator body. The discharge gap portion is formed of a heat resistant protrusion (2) inside the insulator body surrounding the discharge electrodes (3a,3b), and a pair of metal plates and/or electrically conductive ceramic plates (4a,4b) sandwiching the protrusion (2) from both sides thereof and electrically connected to the discharge electrodes. The pair of plates (4a,4b) are joined and airtightly sealed to the protrusion via an inorganic glass (10a,10b). The arrestor has a highly reliable airtight fixing and sealing structure so that accidental troubles in a power supply or distribution line at a normal working voltage can be substantially eliminated, and damage caused by hygromeration and lightning can be noticeably decreased.

Description: The present invention relates to a lightning arrestor insulator having a ZnO arrestor element and a method of making the same. It is known to use a lightning arrestor insulator having a lightning arrestor function of protecting a power supply line and decreasing risk of accidents in the power supply line at the time of direct hit by lightning. An example of such insulator and a method of producing the same is disclosed in the present applicants' JP-A-59-49178, wherein the ZnO element, which protects the insulator per se from an excessively large electric current at the time of a lightning strike, is integrally fixed and sealed in the inside of the insulator by means of an inorganic glass. The insulator has a characteristic feature of superior airtight sealing and electric insulation properties. However, in the method of producing the above insulator, the whole of the insulator is heated and retained in a large homogeneous heating furnace such as an electric furnace, while casting an inorganic glass thereinto, so that production efficiency is bad and an annealing process and other processes are necessary after the casting of the inorganic glass in the insulator. Therefore, the production method requires a large furnace and a long time for the sealing, and cannot produce insulators efficiently because the number of insulators that can be produced in the furnace in one sealing operation is restricted by the inner volume of the furnace. EP-A-196 370 illustrates a lightning arrestor insulator having a ZnO arrestor element and electrodes at each end, with an insulating cladding around the ZnO element and sealed to the electrodes. An object of the present invention is to obviate the above drawbacks. An other object of the present invention is to provide a lightning arrestor insulator having a high reliability and reducing accident risk in a power distribution line at a normal working voltage and hence reducing the trouble caused by lightning. The lightning arrestor insulator of the invention is set out in claim 1. The method of making it is set out in claim 4. In the method of producing a lightning arrestor insulator of the invention, the fixing and sealing of the arrestor ZnO element and electrically conductive covers acting as the electrodes by means of an inorganic glass can be put into effect simply by partial heating of the insulator. The present invention can thus permit production of a lightning arrestor insulator having a lightning arrestor function, an airtight sealing property, and an electrical insulative property promptly using a simple and economical apparatus, and which can, if desired, control freely an environmental atmosphere around the arrestor ZnO element built therein. These covers are made of an electrically conductive material and are induction heated by a high frequency induction heating, for example. For a better understanding of the present invention, reference is made to the accompanying drawings, in which: Fig. 1 is a schematic view partly in cross section of an example of the lightning arrestor insulator of the present invention; and Fig. 2 is a schematic view partly in cross section of another example of the lightning arrestor insulator of the present insulator. Referring to Fig. 1 showing an embodiment of a lightning arrestor insulator of the present invention, the insulator body 1 accommodates in its cavity a columnar arrestor ZnO element 5 consisting essentially of ZnO in airtight state. More particularly, the upper and the lower end portions 1a, 1b of the insulator body 1 are respectively sealed airtightly by metallic covers 17a, 17b acting as electrodes via inorganic glasses 10a, 10b. A ceramic cylinder 16 and inorganic fibers 20 are disposed as reinforcing members in a space between the side wall of the arrestor ZnO element 5 and the inner wall of the insulator body 1 for protecting the insulator body by mitigating an increase of the inner pressure caused by deteriation of the ZnO element when passing extraordinary large current due to direct hit of a lightning. Further, a resilient electrically conductive material 21 is disposed between the arrestor ZnO element 5 and the upper end cover 17a, in order to mitigate external stress which is always exerted on the lightning arrestor insulator from the exterior. In this embodiment, the covers 17a, 17b function as the electrodes. Referring to Fig. 2 showing another embodiment of a lightning arrestor insulator of the present invention, the upper and the lower end portions of the insulator body 1 are sealed airtightly by electrically conductive ceramic covers 17a, 17b via inorganic glass 10a, 10b, the covers acting as the electrodes. In the structure of either Fig. 1 or 2, the upper and lower end portions of the insulator body 1 are sealed airtightly to the metallic or electrically conductive ceramic covers 17a, 17b via the inorganic glass 10a, 10b. Therefore, inorganic glass has to be applied by various possible methods on the surfaces of the metallic covers and/or the ceramic covers which are to be contacted to each other. Illustrative examples of such application methods are known methods of directly applying a glass powder, a spray method, a paste method, and a tape method. After the application of the glass, the upper cover 17a and the lower cover 17b are mounted on the arrestor ZnO element 5 and the insulator body 1 from both sides thereof, pressed thereon, and induction heated to melt the inorganic glass 10a, 10b so as to form airtight seals between the upper metallic cover 17a and the upper end 1a of the insulator body 1 and between the lower metallic cover 17b and the lower end 1b of the insulator body 1 for the embodiment shown in Fig. 1. For the heating of the glass, a high frequency induction heating of the upper and the lower covers can be adopted, since the covers are made of an electrically conductive material. If the heating is effected by a high frequency induction heating, a heating apparatus of a large scale is not necessary, and partial heating of insulators solely at the covers can be effected, and an environmental atmosphere and the inner pressure of the atmosphere around the arrestor ZnO element 5 can be adjusted freely. Thus, the inner pressure can be adjusted to a desired pressure in the range of 1-10 atm, and a highly electrically insulative gas, such as SF₆, can be used and sealed in as the atmosphere. In this case, the portions to be heated of the insulator can be localized or restricted, so that a fiber reinforced plastics (FRP) can be used as the reinforcing member 16. In order to enhance the joining, preferably, the metallic covers are preliminarily heated up to 800-1,000°C in an oxidizing atmosphere to form a coating of an oxide on the surfaces thereof, more preferably, the portions of the covers to be joined are preliminarily coated with an inorganic glass and fired prior to the joining. Hereinafter, explanation will be made in more detail with reference to examples. Example Lightning arrestor insulators as shown in Fig. 1 are produced by preparing arrestor ZnO element devices of Test Nos. 1-6 of the following Table 1 by using inorganic glass and various sealing structures and structural conditions as shown in the following Table 1. As seen from the above Table 1, various sealing covers and reinforcing members can be used, and environmental atmosphere around the ZnO element can be adjusted. These sealing covers and reinforcing members can be sealed in a short time by high frequency induction heating of the electrically conductive sealing covers. The lightning arrestor insulator of the present invention thus has electrodes and an arrestor ZnO element and is formed by directly joining the inside of the insulator body and metallic covers and/or electrically conductive covers acting as the electrodes by means of an inorganic glass, so that lightning arrestor insulators having a highly reliable airtightly sealed arrestor ZnO element device can be obtained. As a result, accidents in a power service line at a normal working voltage can be substantially eliminated, and damage caused by lightning can be noticeably decreased, so that electric power can be supplied with widely improved reliability. Also, according to the method of the present invention, the arrestor ZnO element device can be formed and sealed airtightly by partial heating of the lightening arrestor insulator by means of an induction heating solely of the upper and lower electrically conductive covers sandwiching the arrestor ZnO element via an inorganic glass, so that a position of breakage of the insulator at the time of hit of a lightening can be restricted to the covers accommodating the arrestor ZnO element. As a result, a crack formed in the covers can be prevented from developing to the insulator body, and discharge characteristic properties of the insulator at the time of short-cut of an extraordinary excessive electric current can be improved. In addition, heating means in an apparatus for producing the lightning arrestor insulator can be minimized, and an environmental atmosphere around the arrestor ZnO element can be adjusted as desired. Though the contacting end surfaces of the upper and lower covers and the insulator body are shown as tapered surfaces in Fig. 1, the contacting end surfaces may have other shapes, such as shown in Fig. 2. The present invention is not limited to a suspension type lightning arrestor insulator, and clearly applicable to other shapes of lightning arrestor insulators.

Claims: A lightning arrestor insulator having an insulator body (1), a ZnO arrestor element (5) housed within the insulator body (1) and electrodes (17a,17b) for the ZnO arrestor element (5) at the opposite ends thereof and sealed to the insulator body (1), characterized in that the ZnO arrestor element (5) is spaced from the insulator body (1) and the electrodes (17a,17b) are sealed airtightly to the insulator body (1) by inorganic glass (10a, 10b). A lightning arrestor according to claim 1 wherein the electrodes (17a,17b) for the ZnO arrestor element (5) are made of a metallic material or an electrically conductive ceramic material. A lightning arrestor element according to claim 1 or claim 2 having at least one reinforcing member (16,20) around the ZnO arrestor element (5) between it and the insulator body (1). A method of making a lightning arrestor according to any one of the preceding claims including the steps of applying inorganic glass to the insulator body (1), mounting and pressing on the electrodes (17a,17b) for the ZnO arrestor element, and heating by induction heating to melt the inorganic glass, thereby to form an airtight fixing and sealing between the electrodes and the insulator body after solidification of the molten glass.

Inventors: MIMA, TOSHIYUKI 24, NGK KITA-KAZOKU APARTMENTS 9, NOZAKI, MASAYUKI 13-48, AZA-KAKIBA, SEIKE, SHOJI 167-1, TAISHI 2-CHOME
Applicants: NGK INSULATORS, LTD.
Application year: 1989
Main classification: H01C7
IPC codes: H01T1, H01C7