Lightning arrester insulator and method of making 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 discharge gap portion, and a method of making the same. A lightning arrestor insulator having a lightning absorber portion consisting of a ZnO element and a discharge gap portion both in a body of the insulator is known. The discharge gap portion performs the function of discharging at a voltage sufficiently lower than insulative capacity of a transformer or a so-called cut-out apparatus to be protected, to pass the lightning current to earth so as to protect the transformer or the like at the time of lightning strike, and the ZnO element functions to restore instantaneously the electrical insulation of the gap portion to interrupt the electric current flow after the discharge of the discharge gap portion. An example of such a lightning arrestor insulator is disclosed in Japanese Utility Model Application Publication No. 52-17,719, wherein the gap portion and the ZnO element are arranged in the insulator body, and the insulator body is capped by a ceramic cap by means of threading or an O-ring. However, in the lightning arrestor insulator of the Japanese Utility Model Application Publication No. 52-17,719 the inside arrangements are connected only by mechanical means, so that it has a drawback in that, if the air-tight sealing of the ceramic cap is broken, the inside of the insulator body is humidified, causing risk of accident in a power distribution line at a normal working voltage, particularly due to hygromeration (damage due to water access) of the discharge gap portion. 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 risk of accident in a power distribution line at a normal working voltage and hence reducing the trouble caused by lightning. The lightning arrestor of the invention is set out in claim 1. The method of making it is set out in claim 4. The heat-resistant protrusion may be separate or an integral part of the insulator body. By the present invention it is possible to provide a lightning arrestor insulator having an excellently fixed and airtightly sealed discharge gap portion. The present invention can also provide a lightning arrestor insulator having both an excellently fixed and airtightly sealed discharge gap portion and an excellently fixed and airtightly sealed arrestor ZnO element. The formed airtight sealing of the discharge gap portion has a high reliability in that the pair of plates having the discharge electrodes is directly joined to the protrusion by means of an inorganic glass. By the arrangement of the invention, the lightning arrestor insulator exhibits equivalent functions to those of the known lightning arrestor insulator, and still prevents an accidental trouble in a power distribution line at a normal working voltage as well as hygromeration of the discharge gap portion due to accidental deterioration of the airtight sealing of the discharge gap, because the discharge gap portion is integrally fixed and airtightly sealed to the insulator body. As a result, the lightning arrestor insulator of the present invention can decrease trouble caused by lightning and increase reliability of power supply. In case of joining the discharge gap portion and the insulator body via the pair of plates by means of an inorganic glass, the pair of plates is heated by an induction heating and the glass is substantially solely melted to airtightly seal the discharge gap portion, so that the temperature of the whole insulator need not be increased. Therefore, a known phenomenon can not occur that an inner pressure within the discharge gap is left reduced after solidification of the molten glass which is always seen in a conventional method of joining the discharge gap portion and the insulator body by heating the whole of the insulator, and the inner pressure within the discharge gap portion is substantially not reduced even after the formation of the airtightly sealed discharge gap portion. As a result, as compared with a necessity of increasing a distance between the discharge electrodes corresponding to a decrease of the inner pressure within the discharge gap portion in conventional methods for obtaining a constant discharge voltage can be obviated, so that the distance between the discharge electrodes can be made small, and the lightning protective insulators can be produced cheaply without requiring conventional post treatments of controlling the inner pressure within the discharge gap through a hole and sealing the hole. For a better understanding of the present invention, reference is made to the accompanying drawings, in which: Figs. 1a and 1b are a partial crosssectional view of an example of the lightning arrestor insulator of the present invention and an enlarged crosssectional view of the discharge gap portion thereof, respectively; Figs. 2a and 2b are a partial crosssectional view of another example of the lightning arrestor insulator of the present invention and an enlarged crosssectional view of the discharge gap portion thereof, respectively; and Figs. 3a and 3b are explanational views illustrating the method of producing the lightning arrestor insulator having a built in discharge gap portion of the present invention, respectively. Referring to Figs. 1a and 1b showing an embodiment of the present insulator, an insulator body 1 is provided with a cylindrical protrusion 2 integrally formed with the insulator body 1 at the inner upper portion thereof. The protrusion 2 is sandwiched by metal plates 4a, 4b carrying projecting discharge electrodes 3a, 3b and airtightly joined and sealed by inorganic glasses 10a, 10b, to form a discharge gap portion as shown in Fig. 1b. The discharge gap portion is provided with an arrestor ZnO element 5 thereabove, and an electrically conductive member 6 therebelow, arranged in this order, and the Zno element 5 and the electrically conductive member 6 are connected to the insulator body 1 via resilient members 7a, 7b by metallic caps 8a, 8b, to form a lightening arrestor insulator of the present invention. In the spaces formed between the insulator body 1 and the ZnO element 5 and between the insulator body 1 and the electrically conductive member 6 is filled a filler 9 such as inorganic fibers. As the metal plates 4a, 4b, at least one of Kovar, stainless steel, aluminum, nickel, nickel-iron alloy and silver is used. Preferably, those metals having thermal expansion coefficients approximately to that of the insulator body 1 are used. Referring to Figs. 2a and 2b showing another embodiment of the present insulator, the same elements as in Figs. 1a and 1b are numbered with the same reference numbers, and explanations thereof are omitted. In this embodiment, different from the embodiment shown in Figs. 1a and 1b, the protrusion 2 is made of tapered surfaces 11a, 11b separately made from the insulator body 1, and the tapered surfaces 11a, 11b are joined to electrically conductive ceramic plates 12a, 12b via inorganic glasses 10a, 10b, to form a discharge gap portion as shown in Fig. 2b. Further, in this embodiment, a ceramic cylinder 16 is disposed between the electrically conductive ceramic plates 12a, 12b to surround the discharge electrodes 12a, 12b so as to reinforce the strength of the discharge gap portion. In addition, the ZnO element 5 and the electrically conductive member 6 are arranged in different order in the cavity of the insulator body 1, however, this embodiment can achieve similar effects as those of the embodiment of Fig. 1. As the electrically conductive plates 12a, 12b, preferably use is made of at least one of zirconium boride, zinc oxide, stannous oxide, graphite, and silicon carbide. Referring to Figs. 3a and 3b each showing another embodiment of the present insulator, a metal plate 4a having a projected discharge electrode 3a is disposed on a protrusion 2 via an inorganic glass 10a in such a fashion that the discharge electrode 3a comes to face the protrusion 2, then an induction coil 13 is mounted on the metal plate 4a, and an electric current is passed through the induction coil 13 to heat the inorganic glass 10a by induction heating so as to join the metal plate 4a to the protrusion 2, as shown in Fig. 3a. After completion of the joining of the metal plate 4a, the metal plate 4b is joined to the protrusion 2 in the same way to form a discharge gap portion. In the embodiment shown in Fig. 3b, the metal plates 4a, 4b are joined to the protrusion 2 by using an auxiliary stainless steel rod 15 having a pressing portion 14 arranged through the cavity of the insulator body 2, in addition to the use of the induction coil 13. This embodiment is more preferable, because the metal plates 4a, 4b can be pressed by the pressing portion 14 of the stainless steel rod 15 at the time of induction heating. In either embodiment, the inorganic glass 10a, 10b can be applied in a powder form or a paste form on the metal plates 4a, 4b or the protrusion 2. Instead of the metal plates used in the above embodiments of induction heating, electrically conductive ceramic plates or a pair of a metal plate and an electrically conductive ceramic plate can be used in the similar way to achieve the airtight fixing and sealing of the discharged gap portion to the same extent of effect by means of the inorganic glass. Hereinafter, the explanations will be made in more detail with reference to examples. Example 1 Inorganic glasses having the compositions and the characteristic properties as shown in the following Table 1 are used in combination with various metallic plates as shown in the following Table 2, and induction heated to form discharge gap portions of the shapes as described in Table 2. Thus formed discharge gap portions theirselves, and those after subjected to a cooling and heating test of thrice reciprocal cooling at -20°C and heating at 80°C, are tested on an airtight sealness test by means of He gas leakage measurement. The results are shown also in Table 2. In Table 2, symbol ○ represents those insulators that did not show a leakage of He gas, and symbol × represents those insulators that show a leakage of He gas. A condition of the He gas leakage test is 1×10⁻⁹ atm. cc/sec or more. As seen clearly from the results of Table 2, the metallic plates are substantially completely joined and sealed by means of inorganic glasses. However, the combinations of the copper plate and the PbO·B₂O₃ series glass of type A, and the niobium plate and the B₂O₃·ZnO series glass of type I, are insufficiently sealed, showing a leakage of He gas. Example 2 The various inorganic glasses shown in the above Table 1 are used in combination with various electrically conductive ceramic plates as shown in the following Table 3, and induction heated to form discharge gap portions. Thus formed discharge gap portions theirselves, and those after the cooling and heating test, are tested on the same airtight sealness test as in Example 1. The results are shown in the following Table 3. As seen clearly from the results of the above Table 3, the electrically conductive ceramic plates are substantially completely joined and sealed by means of inorganic glasses. However, the combinations of the plate of molybdenum silicide, tungsten carbide, or chromium oxide and the glasses of Reference 3-6, are insufficiently sealed, showing a leakage of He gas. Example 3 In order to examine the state of the induction heating in the method of the present invention, the various inorganic glasses shown in the above Table 1 are disposed between the protrusions of the insulator bodies and metal plates or electrically conductive ceramic plates shown in the following Table 4 in the forms as described in Table 4, and induction heated in conditions as described also in Table 4 to form discharge gap portions. Thus formed discharge gap portions theirselves, and those after the cooling and heating test, are tested on the same airtight sealness test as in Example 1. The results are shown in the following Table 4. As seen from the results of Table 4, substantially completely joined and sealed discharge gap portions can be formed. However, in case where a stainless steel rod is not used and induction heating is effected for a short time using powdery inorganic glass, the formed discharge gap portions show some leakage of He gas in the airtight sealness test after the cooling and heating. As is apparent from the above foregoing explanations, the lightning arrestor insulator of the present invention has a discharge gap portion formed by directly joining a protrusion arranged in the inside of the insulator body and metal plates and/or electrically conductive ceramic plates having discharge electrodes by means of an inorganic glass, so that lightning arrestor insulators having a highly reliable airtightly sealed discharge gap portion 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 hygromeration can be noticeably decreased, so that electric power can be supplied with widely improved reliability. According to the method of the present invention, the discharge gap portion can be formed and sealed airtightly by partial heating of the lightning arrestor insulator by means of an induction heating, so that temperature rise of the whole insulator can be avoided. As a result, inner pressure within the discharge gap portion is not changed substantially after the airtight sealing, and lightening arrestor insulators of the desired properties can easily be obtained.

Claims: A lightning arrestor insulator having an insulator body (1) and, within the insulator body (1), a ZnO arrestor element (5) and a discharge gap portion provided by projecting discharge electrodes (3a,3b) carried by opposed electrically conductive plates (4a,4b) electrically connected to the respective discharge electrodes (3a,3b), characterized in that said discharge gap portion comprises a heat-resistant protrusion (2) in the inside of the insulator body (1) and surrounding the discharge electrodes (3a,3b), said plates (4a,4b) sandwiching the protrusion (2) from opposite sides thereof and being joined and airtightly sealed to the protrusion (2) by inorganic glass (10a,10b). A lightning arrestor insulator as defined in claim 1, wherein the protrusion (2) is formed in one piece with the insulator body (1). A lightning arrestor insulator as defined in claim 1 or 2, further comprising a ceramic cylinder (16) surrounding the discharge electrodes (3a,3b) between the plates (4a,4b) for supporting the pair of plates (4a,4b). A method of making a lightning arrestor insulator according to any one of claims 1 to 3, wherein the plates (4a,4b) are arranged to sandwich and contact the protrusion (2) through inorganic glass, and then the plates (4a,4b) are heated by induction heating to melt the inorganic glass so as to join the plates and the protrusion by the glass, thereby to form an airtight sealing of the discharge gap portion.

Inventors: NOZAKI MASAYUKI, SEIKE SHOJI, NOZAKI, MASAYUKI, SEIKE, SHOJI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1989
Main classification: H01T1
IPC codes: H01C7, H01T1