Explosion-proof porcelain housings for gas-filled insulating apparatuses and process for producing such porcelain housings

Abstract: An explosion-proof porcelain housing for use in a gas-filled insulating apparatus,comprises a porcelain housing body, and first and second films. The first film is made of a first insulating material having low hardness and high elasticity, and is bonded to an inner surface of the porcelain housing body. The second film is made of a second insulating material having high hardness and high mechanical strength, and is bonded to an inner surface of the first film. A process for producing such an explosion-proof porcelain housing is also disclosed.

Description: The present invention relates to porcelain housings for gas-filled insulating apparatuses, and a process for producing such porcelain housings. The invention is concerned with resistance to explosion and/or its effects. More particularly, the invention relates to porcelain housings for gas-filled insulating apparatuses, which can avoid a secondary accident by preventing broken pieces from being scattered even if the porcelain housing is broken owing to the pressure of gas inside the gas-filled insulating apparatus, and the invention also relates to a process for producing such porcelain housings. For the above purpose, explosion-resistant porcelain housings in which a film made of an insulating material is formed on an inner surface of a porcelain housing body are already known from e.g. EP-A-0 350289. A typical example of such porcelain housings is a porcelain housing having a construction in which a single layer of a synthetic resin or an elastomer is bonded to an inner surface of the porcelain housing body. However, as to this explosion-resistant porcelain housing having a single film layer bonded thereto, as shown in Fig. 3 when the porcelain housing body 11 is cracked for some reason, an internal pressure is abruptly applied to circumferentially expand the film 12 at a cracked portion. That is, since the film 12 is bonded to the porcelain housing body 11, circumferential stresses are concentrated on the outer surface side of the film 12 at the cracked portion 13 of the porcelain housing body 11. The distribution of circumferential stresses is shown in Fig. 3. Since the film 12 is readily torn by this concentration of the stresses, a sufficient explosion-proof effect cannot be obtained. In order to solve the defects of such a conventional explosion-resistant porcelain housing having a single film integrated with the porcelain housing boby, NGK Insulators, Ltd. formerly developed an explosion-resistant porcelain housing in which films made of two kinds of materials, respectively, are formed on an inner surface of a porcelain housing body in a non-bonded state as shown in Japanese patent application Laid-open No. 61-264,612. However, if such a porcelain housing is cracked owing to some cause, since none of the films are bound to the porcelain housing body, the internal pressure acts upon all the films. As a result, the films expand in the form of a bolloon, so that the films are stretched and become thinner. Since intensity of stresses occurring in a film owing to the internal pressure are proportional to the diameter and inversely proportional to the thickness, the films are further expanded with the stresses and finally broken. In addition, since none of the films are bonded to the porcelain housing body, broken pieces of the porcelain housing body are scattered in all directions. Therefore, sufficient explosion-proof effect cannot be expected, either. EP-A-53363 discloses bushes for gas-filled insulated electrical equipment, intended not to damage other bushings when they shatter. These bushes are made by placing an inner insulated tube within a separate outer porcelain tube and filling the interspace with a deformable medium. The present invention aims at least in part to solve the above-mentioned problems possessed by the related art, and to provide an explosion-resistant porcelain housing for a gas-filled insulating apparatus, which porcelain housing can reduce or suppress scattering of broken pieces of the porcelain housing even if the porcelain housing is broken by some cause. The invention also seeks to provide a process for producing such a porcelain housing. The present invention relatives to a porcelain housing for use in a gas-filled insulating apparatus, comprising a porcelain housing body, and a first film bonded to the inner surface of the porcelain housing body, and a second film bonded to the inner surface of the first film, wherein the first film is made of a first insulating material having low hardness relative to the second film and high elasticity, and said second film is made of a second insulating material having high hardness relative to the first film and high mechanical strength. The present invention also relates to the process for producing such a porcelain housing for use in gas-filled insulating apparatus, comprising steps of: preparing a porcelain housing body, lining a first insulating material having low hardness and high elasticity onto an inner surface of the porcelain housing body under rotation of the porcelain housing body, and applying a lining of a second insulating material having high hardness and high mechanical strength onto an inner surface of the first insulating material, thereby forming two layers consisting of first and second films on the inner surface of the porcelain housing body. According to the present invention, it is preferable that JIS-A hardness and elongation of the first film are 55∼80 and not less than 400%, desirably 400% - 700% and JIS-A hardness and tensile strength of the second film are 85-95 and less than 150 kgf/cm², desirably 400-700 kgf/cm². Further, it is preferable that the hardness of the first film is lower than that of the second film by not less than about 20 to about 30 in terms of JIS-A hardness. Furthermore, the thickness of the first film is preferably about 1 mm to about 2 mm. Moreover, it is preferable that when the inner diameter of the porcelain housing body is as small as about 100∼150 mm, tensile strength of the second film is not less than 150 kgf/cm², desirably 400-700 kgf/cm², and a thickness of the second film is a few mm to dozens mm. In addition, it is preferable that when the inner diameter of the porcelain housing body is as large as about 400∼600 mm, tensile strength of the second film is 400 to 700 kgf/cm², and a thickness of the second film is a few mm to dozens mm. Further, it is preferable that the second film is made of an arc-resistive material or the inner surface of the second film is lined with an arc-resistive material. Furthermore, it is preferable that the first and second films are made of materials selected from polyurethane resin, natural rubber, silicon rubber, butyl rubber, ionomer resin, polypropylene, polyethylene, ethylene-vinyl acetate copolymer, styrene-butadiene resin, and glass fiber-reinforced materials thereof. The thus constituted explosion-proof (or, more accurately, explosion-resistant) porcelain housing according to the present invention is intended to be used in the state that the porcelain housing is attached to the gas-filled insulating apparatus, such as a gas bushing, in which an insulating gas is filled at high pressure. If the porcelain housing body is broken by some cause, the first film is torn along a crack of the porcelain housing. However, since hardness and strength of the second film are greater than those of the first film, progression of the tear is stopped by the second film. In this case, the lining layer consisting of the first and second films tends to be expanded with an internal pressure. However, since the porcelain housing body and the first film as well as the first film and the second film are bonded together, the lining layer is expanded mainly at a cracked portion of the porcelain housing body, while the lining does not expand at the remaining portion. Therefore, the porcelain housing will not be self-destructed, following expansion-increase in diameter-reduction in thickness of the lining-increase in stresses-further expansion of the lining as in the case of the conventional explosion-proof porcelain housings. Since the first film is made of the insulating material having high elasticity, stresses occurring in the second film are mitigated through expansion of the first film 2 at the cracked portion of the porcelain housing body. Consequently, maintenance of strength proportional to the initial thickness of the second film can be expected Further, since the second film is made of the insulating material having high hardness and high mechanical strength, a considerably high internal pressure is necessary for tearing the second film. Even if the second film is partially torn, the tear will be prevented from easily propagating through the mitigation of stresses acting upon the second film at the cracked portion of the porcelain housing body, by the first film bonded to the second film. Thus, since the gas inside the porcelain housing body is gradually discharged through the partial tear of the second film during the mitigation of the stresses, explosion or scattering of broken pieces of the porcelain housing body following the explosion can be prevented or ameliorated. These and other optional features and advantages of the invention will be appreciated upon reading of the following description of embodiments of the invention when taken in conjonction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a vertically sectional view of one embodiment of an explosion-proof porcelain housing of the present invention; Fig. 2 is a horizontally sectional view illustrating a cracked portion of the explosion-proof porcelain housing in Fig. 1; Fig. 3 is a horizontally sectional view illustrating a cracked portion of the conventional explosion-proof porcelain housing having a single film layer ; Fig. 4 is a graph showing the relationship between the hardness of the first film and the explosion-proof performance; Fig. 5 is a graph showing the relationship between the thickness of the film and the explosion-proof performance; and Fig. 6 is a graph showing the relationship between the thickness of the first film and the explosion-proof performance. The present invention will be explained in more detail with reference to Fig. 1. In Fig. 1, a first film 2 is formed on the inner surface of a porcelain housing body 1 made of a porcelain, and a second film 3 is formed on an inner surface of the first film. The first film 2 is made of a first insulating material having low hardness and high elasticity, and for example, a soft polyurethane resin is used as the first insulating material. "Soft" means "low hardness". The second film 3 is made of a second insulating material having higher hardness and higher mechanical strength as compared with the first film, and for example, a hard polyurethane resin is used as the second insulating film. "Hard" means "higher mechanical strength". The first film 2 is bonded to the inner surface of the porcelain housing body 1 with an appropriate adhesive, which can be easily selected by the skilled person in the art based on the kinds of the materials used for the porcelain housing body and the first film. The second film is directly bonded to the first film 2 without interposing an adhesive therebetween. In order to form these two film layers on the inner surface of the porcelain housing body 1, the first film is formed on the inner surface of the porcelain housing body having the adhesive coated thereon, by flowing down and lining the soft polyurethane resin along the inner surface of the porcelain housing 1 under rotation, and then the second film is formed by similarly flowing down and lining the hard polyurethane resin directly onto the inner surface of the first film in the state that the first film is in an active condition. In order to form the first films, a liquid mixture of a main liquid ingredient and a curing agent is made to flow down along the inner surface of the housing body through a pouring hose, and the housing body is rotated until the mixture loses flowability (is gelled) but still keeps its active condition. After the first layer is gelled, the second layer is similarly lined thereon. As to the material for the porcelain housing body, any appropriate ceramic material can be easily selected by the skilled person in the art based on the intended use, the size, etc. of the porcelain housing body. Now, the relationship between the explosion-resistant effect of the porcelain housing and the thickness or the hardness of the film will be explained based on specific examples. Fig. 4 shows results in explosion tests in which hardness of the first film was changed. The tests were conducted as follows: First and second films made of polyurethanes having various thicknesses and hardness shown in Table 1 were lined on the inner surface of a porcelain housing body made of a conventional porcelain and having an inner diameter of 110 mm and an entire length of 460 mm, and a compressed insulating gas was sealingly filled into the porcelain housing body. A part of the porcelain housing body was broken by hitting a barrel portion of the housing body with a hammer having an acute tip, and the state of the films and the scattered state of broken pieces of the porcelain housing body were observed. In Fig. 4, symbols ○, □, Δ and denote the following meanings: ○: The films were not torn, and no broken pieces of the porcelain housing body were scattered. □ : A part of the films was slightly torn, and no broken pieces were scattered, although gas was gradually discharged. Δ : A part of the films was largely torn, so that the gas was instantly discharged, and most of the broken pieces were scattered. : The films were greatly torn, so that the gas was instantly discharged, and a most of the broken pieces were scattered. According to Fig. 4, when the hardness of the second film was 90 and the hardness of the first film was set at 73, some effect was recognized. When the hardness of the first film was 55, a conspicuously improved effect could be recognized. Stress at low expansion (Kgf/cm²) Film Polyurethane Nos. Thickness (mm) JIS-A hardness (degree) Tensile strength (Kgf/cm² 100% expansion 300% expansion First Film 1 1.5 55 120 10 20 2 1.5 65 150 20 35 3 1.5 73 170 28 50 4 1.5 85 200 50 90 Second film 5 9.0 90 450 90 180 Fig. 5 is a graph showing results in explosion tests with respect to porcelain housings in which the thickness of the second film was changed. In the porcelain housings as examples of the present invention, a porcelain housing body was lined with two layers of the polyurethane Nos. 1 and 5 shown in Table 1 as first and second films, respectively, while the thickness of the second film was changed. The thickness of the first film was 1.5 mm. In the porcelain housings as comparative examples, the second film No. 5 shown in Table 1 was lined, while the thickness thereof was changed. The explosion tests were conducted in the same manner as mentioned before. In Fig. 5, symbols O, □, Δ and denote the same meanings as in Fig. 4 with respect to the porcelain housings with the two lining layers, and symbols , ▪, ▴ and have the same meanings as in Fig. 4 with respect to the porcelain housings with a single lining layer of higher mechanical strength. From those test results, it is seen that the explosion-proof performance of the porcelain housings with the two lining layers is improved substantially in proportion to increase in the thickness of the second film. On the other hand, with respect to the porcelain housings having a single lining layer, it is seen that the explosion-proof performance cannot be greatly improved even when the thickness of the film is increased. This is considered to be that stresses concentrated at the cracked portion as mentioned before. Fig. 6 is a graph showing results of tests in which a preferable thickness range of the first film was confirmed by varying the thickness of the first film. According to the results, it is seen that preferable effect could be attained when the thickness of the first film is at least about 1.5 mm. From the above experiments, the following are seen. When the hardness of the first film is lower than that of the second film by about 20 to about 30 in terms of JIS-A hardness and the thickness of the first film is 1 to 2 mm, the explosion-proof performance of the porcelain housing having the two lining layers can be greatly improved as compared with the porcelain housing having a single lining layer. The tensile strength of the second film can be appropriately set depending upon the diameter or the internal pressure of the porcelain housing body. For example, when the internal pressure of the porcelain housing body is set at 3 to 6 kgf/cm² ordinarily employed in the gas-filled insulating apparatus, the scattering of the broken pieces of the porcelain housing body can be prevented by using the second film having a thickness of a few mm to dozens mm and tensile strength of not less than 150 kgf/cm² (up to 700 kgf/cm² tensile strength was experimentally confirmed acceptable, although no upper limit is set) in the case of the diameter of the porcelain housing body being as small as 100-150 mm or tensile strength of not less than 400 kgf/cm² (The maximum tensile strength of actual materials is considered to be around 100 kgf/cm², although no upper limit is set) in the case of the diameter being as large as 400-600 mm. In this way, the present invention can be applied to the large diameter explosion-proof porcelain housing having high internal pressure by appropriately selecting the hardness, strength, etc. of the first and second films, whereby excellent explosion-proof effect can be obtained. Further, when the second film is made of an arc-resistive material, the porcelain housing having both explosion-proof performance and arc resistance can be obtained. The arc-resistive materials are well known to the skilled person in the art, and an appropriate one can be easily selected. For example, a polyester-based polyurethane elastomer may be used as an arc-resistive material. Inventor's experiment revealed that although an arc current of 6 to 21 KA was passed through a porcelain housing provided with first and second films made of the above polyurethane and a polyester-based polyurethane elastomer, respectively, for a duration of 0.1-0.5 sec., the porcelain housing was not damaged. The above porcelain housing had an inner diameter of 100 mm and a height of 460 mm. If a material having excellent arc-resistive material may not be used from the standpoint of the explosion-proof effect, the arc resistance may be improved through the formation of a third layer by lining a material having excellent arc-resistance on the inner side of the second layer. Further, although the present invention is directly to the explosion-proof porcelain housings for use in the gas-filled insulating apparatuses, they can be used for oil-insulated type insulating apparatuses by lining the porcelain housing body with a material having excellent oil-resistance. In this manner, the use ways and the use ranges of the present invention can be widened by employing the multilayer lining structure. The present invention can be modified in actual uses. (1) In the above examples, the polyurethane resins are used as the materials for forming the films. However, instead of them, various rubbery materials such as natural rubber, silicon rubber, and butyl rubber, or various resins such as ionomer resin, polypropylene, polyethylene, ethylene-vinyl acetate copolymer, and styrene-butadiene resin, and FR materials in which fibers are mixed into such rubbery materials or resins to raise strength may be used. (2) When a material having excellent bondability to the porcelain of the porcelain housing body is used for the first film, the first film may be directly lined onto the inner surface of the porcelain housing body without interposing any adhesive between the porcelain and the first film. On the other hand, if bonding strength between the first film and the second film is insufficient, an appropriate adhesive may be used. As having been explained above, even if the porcelain housing according to the present invention is broken, broken pieces of the porcelain housing can be prevented from being scattered by effectively combining the first and second films having different properties. Further, according to the process for producing the porcelain housing in the present invention, the above-mentioned explosion-proof porcelain housings can be easily produced. Therefore, the present invention can greatly contribute to the industrial development as the explosion-proof porcelain housings for the gas-filled insulating apparatus and the producing process thereof in that the invention solves the conventional problems.

Claims: A porcelain housing for use in a gas-filled insulating apparatus, said porcelain housing comprising an porcelain housing body (1), and first and second films (2,3), said first film (2) being made of a first insulating material having low hardness relative to the second film and high elasticity and bonded to an inner surface of the porcelain housing body, and said second film (3) being made of a second insulating material having high hardness and high mechanical strength relative to the first film and bonded to an inner surface of said first film. The porcelain housing according to Claim 1, wherein JIS-A hardness and elongation of the first film (2) are 55-80 and not less than 400%, respectively, and JIS-A hardness and tensile strength of the second film (3) are 85-95 and not less than 150 kgf/cm², respectively. The porcelain housing according to Claim 1, wherein the hardness of the first film (2) is lower than that of the second film (3) by about 20 to about 30 in terms of JIS-A hardness. The porcelain housing according to Claim 1, wherein a thickness of the first film (2) is about 1 mm to about 2 mm. The porcelain housing according to Claim 1, wherein an inner diameter of the porcelain housing body (1) is not more than about 150 mm, and tensile strength of the second film is not less than 150 kgf/cm². The porcelain housing according to Claim 1, wherein an inner diameter of the porcelain housing body (1) is not less than about 200 mm, tensile strength of the second film (3) is not less than 400 kgf/cm², and thickness of the second film (3) exceeds 3 mm. The porcelain housing according to Claim 1, wherein the second film (3) is made of an arc-resistive material. The porcelain housing according to Claim 1, wherein an inner surface of the second film (3) is lined with an arc-resistive material. The porcelain housing according to Claim 1, wherein the first and second films (2,3) are made of materials selected from polyurethane resin, natural rubber, silicon rubber, butyl rubber, ionomer resin, polypropylene, polyethylene, ethylene-vinyl acetate copolymer, styrene-butadiene resin, and fiber-reinforced materials thereof. A process for producing a porcelain housing for use in a gas-filled insulating apparatus, said process comprising steps of: preparing a porcelain housing body (1), applying a lining (2) of a first insulating material onto an inner surface of said porcelain housing body during rotation of the porcelain housing body, and applying a lining (3) of a second insulating material onto an inner surface of the first insulating material, thereby forming two lining layers consisting of first and second films on the inner surface of the porcelain housing, said first film being made of said first insulating material having low hardness relative to the second film and high elasticity, said second film being made of said second insulating material having high hardness and high mechanical strength relative to the first film. The process according to Claim 10, wherein JIS-A hardness and elongation of the first film (2) are 55-80 and not less than 400%, respectively, and JIS-A hardness and tensile strength of the second film (3) are 85-95 and not less than 150 kgf/cm², respectively. The process according to Claim 10, wherein the hardness of the first film (2) is lower than that of the second film (3) by about 20 to about 30 in terms of JIS-A hardness. The process according to Claim 10, wherein the thickness of the first film (2) is about 1 mm to about 2 mm. The process according to Claim 10, wherein an inner diameter of the porcelain housing body (1) is not more than about 150 mm, and tensile strength of the second film (3) is not less than 150 kgf/cm². The process according to claim 10, wherein an inner diameter of the porcelain housing body (1) is not less than about 200 mm, tensile strength of the second film (3) is not less than 400 kgf/cm², and a thickness of the second film (3) is a few mm to dozens mm. The process according to claim 10, wherein the second film (3) is made of an arc-resistive material. The process according to claim 10, wherein an inner surface of the second film is lined with an arc-resistive material. The process according to claim 10, wherein the first and second films (2,3) are made of materials selected from polyurethane resin, natural rubber, silicon rubber, butyl rubber, ionomer resin, polypropylene, polyethylene, ethylene-vinyl acetate copolymer, styrene-butadiene resin, and fiber-reinforced materials thereof. The process according to any one of claims 10 to 18 in which the first film (2) is formed from a liquid mixture of a main liquid ingredient and a curing agent by being made to flow through a pouring hose onto the inner surface of the housing body, the housing body being rotated until the mixture is gelled, the second film (3) being applied thereafter.

Inventors: ASAI KEIICHI, KAWANO NAGAHIRO, MATSUURA YASUNORI, SUGI MASAFUMI, WATANABE AKIHIRO, ASAI, KEIICHI, KAWANO, NAGAHIRO, MATSUURA, YASUNORI, SUGI, MASAFUMI, WATANABE, AKIHIRO
Applicants: MOON STAR CHEMICAL CORP, NGK INSULATORS LTD, MOON-STAR CHEMICAL CORPORATION, NGK INSULATORS, LTD.
Application year: 1991
Main classification: H01B17
IPC codes: H01B19, H01B17, B05D7