Title of Invention

A PREMOLDED UNIT FOR A POWER CABLE JOINT

Abstract

The present invention relates to a premolded unit for a power cable joint having an external semiconductive layer which extends from an outer circumference to end portions and an inside of which is filled with an electric insulating material, characterized in that said unit has a shield break of said external semiconductive layer at an end portion of said unit, and said insulating material is cylindrically extruded out from said shield break.

Full Text

The present invention relates to a premolded unit for a power cable joint and especially to a premolded unit to be used in a slip-on type joint for a power cable, such as a XLPE cable, which is made from rubber or plastics. In a conventional slip-on type joint, for example, as shown in Japanese Patent Unexamined Publication No. Hei-5-76124, a rubber mold unit having a shield break is mounted on a cable and a semiconductive tape is wrapped around the rubber mold unit with the cable to form a shielding structure. The conventional slip-on type joint, however, had a problem that a shielding structure was formed by wrapping a semiconductive tape on site so that the electric performance could not be confirmed before shipping. Further, it had a further problem that wripping of a semiconductive tape on site not only prolonged th installation time but also required skill in tape wrapping work so that scattering was apt to be caused in the electric performance. [0005] It is therefore a main object of the present invention to provide a premolded unit in which a shield break excellent in electric characteristics is formed by injection molding so that a cable joint can be easily formed. [OOOG] [Means for Solving Problem] A first feature of the premolded unit for a power cable joint according to the present invention is in that the premolded unit having an external semiconductive layer which extends from an outer circumference to end portions and an inside of which is filled with an insulating material, is characterized in that the unit has a shield break of the external semiconductive layer at an end portion of the unit, and the insulating material is cylindrically extruded out from the shield break. 10007] Here, the external semiconductive layer is separated by the shield break into an end portion inner cylinder and an outer cylinder. The inner cylinder has a first outer circumferential surface which abuts on the cylindrically projected insulating material, a second outer circumferential surface which forms an external step between the second outer circumferential surface and the insulating material, and an internal step being formed between the first outer circumferential surface and an inner end surface of the inner cylinder. Further, the outer cylinder has one end which abuts on a cable and the other end which forms the sheild break between the other end and the first outer circumferential surface with the insulating material interposed therebetween. [0000] A second feature of the present invention is in that when an axial length of the first outer, circumferential surface is represented by a, a distance of the external step is represented by b, and a distance of the internal step is represented by c, a value of a/bc is selected to be not larger than 6. If this condition is satisfied, the integral molding of the premolded unit is carried out well. A third feature of the present invention is in that the respective outer circumferential surfaces of the external semiconductive layer and the cylindrically projected insulating material are made even in one and the same plane. Accordingly, the present invention provides a premolded unit for a power cable joint having an external semiconductive layer which extends from an outer circumference to end portions and an inside of which is filled with an electric insulating material, characterized in that said unit has a shield break of said external semiconductive layer at an end portion of said unit, and said insulating material is cylindrically extruded out from said shield break. Modes for carrying out the present invention will be described below. Fig. 1 is a longitudinal sectional view showing a premolded unit 1 according to the present invention. This is a cylindrical unit in which a central portion has a diameter which is uniform and each of opposite end portions has a diameter which is reduced stepwise. An external semiconductive layer (hatched portion) 2^ 3 extends from an outer circumference to the end portions to form an insulation screen, and the inside of the layer 2, 3 is extruded with insulating rubber 4 to form an insulation. Further, an internal electrode 5 (such as H.V. electrode) is foinned in the inner circumferential side of the layer 2, 3 at the center portion. [0011] A shield break 6 of the semiconductive layer is formed in one end portion of the unit. The semiconductive layer is separated into an end portion inner cylinder 2 and an outer cylinder 3 through the shield break 6. A cylindrical portion 7 is extruded from the shield break portion at the same time of forming the insulating rubber 4. The cylindrical portion 7 is foimed by injection-molding the insulating rubber 4 as described later. By this cylindrical portion 7, the creepage distance of the shield break 6 can be elongated. ^0012] The end portion inner cylinder 2 constitutes an end portion of the unit, and has a first outer circumferential surface 2A which abuts on an inner circumferential surface of the cylindrical portion 7, and a second outer circumferential surface 2B which forms an external step between the second outer circumferential surface 2B and the inner circumferential surface of the cylindrical portion 7. An internal step is formed so as to extend from the first outer circumferential surface 2A to an inner end surface 2C. fOOlJ] A shield break extending from the first outer circumferential surface 2A to the inner end surface 2C may be formed to be an angled corner or a curved surface. When a part of the inner end surface 2C is formed into a flat surface 2D substantially perpendicular to the first outer circumferential surface 2A so that the flat surface 2D and the first outer circumferential surface 2A form an angled corner, the width of the flat surface 2D becomes the distance c. of the internal step as shown in Fig. 2(A). When the shield break extending from the first outer circximferential surface 2A to the inner end surface 2C forms a curved surface, on the other hand, the diameter of a circle along the curved surface becomes the distance c. of the internal step as shown in Fig. 2(B). [0014] Since the axial length a of the first outer circumferential surface 2A and the distance b of the external step relate to the creepage distance of the cylindrical portion 1, the values of the length a, and the distance b are selected so as to obtain sufficient electric performance. The relation among suitable values of those dimensions a, b, and c. will be explained on the basis of an example which will be described later. Further, the outer cylinder 3 has a junction surface 3A at its one end so that the junction surface 3A abuts on an external semiconductive layer of a cable (not shown), while the outer cylinder 3 extends at its other end to a position where the outer cylinder 3 radially faces the first outer circumferential surface 2A. This other end of the outer cylinder 3 forms the shield break 6 between the other end and the first outer circumferential surface 2A, with the insulating rubber 4 interposed therebetween, [0016] In this embodiment, since the unit is configured such that the diameter of the unit is reduced stepwise at its end portions, the sectional shape of the outer cylinder at its other end side is made to be like a hook which is bent at right angles toward the inner circumferential side. The present invention, however, is not limited to such a shape. For example, when the end portion of the unit is tapered so as to have a continuously-reduced diameter, the shield break portion may be formed in the tapered surface. The unit described above is integrally formed by injection-molding of the insulating rubber 4. In order to perform integral molding, a metal mold 10 and insert dies 11 are used as shown in Fig. 3. That is, the end portion inner cylinder 2, the outer cylinder 3, and the internal electrode (not shown) are arranged in the hollow cylindrical metal mold; the insert dies 11 are fitted into the opposite end portions; a central core like a round rod (not shown) is provided in the inner circumference; and the insulating rubber 4 is injected into a space surrounded by those parts. As a result, the end portion inner cylinder 2, the outer cylinder 3, and the internal electrode are integrated with each other and, at the same time, the cylindrical portion 7 and the sheild break portion are formed. Conventionally, it was considered difficult to form a shielding structure of the shield break portion by injection molding because of its shape. But, it has become possible to produce the unit according to the present invention by injection molding because of formation of the shield break in the end portion. Further, the creepage distance of the insulating rubber is elongated by the cylindrical portion 7 to thereby make it possible to secure the electric characteristics. »[0018]- There is Japanese Patent Application No. Hei-8-80920 which has been filed by the same Applicant as the present application but which has not been laid-open yet. The application shows a slip-on type joint configured such that a ring-like shielding unit is fitted in a shield break of a rubber unit. In this case, it is necessary to produce the shielding unit separately from the rubber unit and it is therefore necessary to mount the shielding unit onto the rubber unit in laying the joint. [0019] In this regard, in the premolded unit according to the present invention, since the shield break is formed by integral injection molding, it is not necessary to separately produce the shielding unit and it is therefore possible to perform the production more efficiently. Further, it is not necessary to assemble the shielding unit and the rubber mold each other even in the installing step of the cable joint and it is therefore possible to further reduce the time for installation with lower skill in the technique on site. (Embodiment 2) Although the sectional shape of the other end of the outer cylinder is made to be like a hook which is bent at right angles toward the inner circumferential side in Embodiment 1 shown in Fig. 1, the sectional shape may be made linear. That is, as shown in Fig. 4, configuration may be made such that the respective outer circumferential surfaces of an external semiconductive layer (an outer cylinder 3) and a cylindrical portion 7 are made even in one and the same plane. That is, the outer circumferential surface of the outer cylinder 3 is made equal in diameter to the outer circumferential surface of the cylindrical portion. In Fig, 4, portions common to those in Fig. 3 are correspondingly referenced. -[0031} In the thus configured premolded unit, the outer diameter of the unit can be reduced in comparison with the unit according to Embodiment 1. That is, the outer diameter can be reduced in comparison with the unit according to Embodiment 1 by a value corresponding to the thickness twice as large as the distance between the outer circumferential surface of the cylindrical portion 7 and the inner circumferential surface of the outer cylinder 3 in Fig. 1, without any fear of influence given onto the electric performance. Further, in the unit according to Embodiment 1, in the case of performing treatment such as wrapping the tape on the outer circumferential surface of the cylindrical portion 7 in order to prevent impulse flashover, it is difficult to perform tape wrapping treatment because of existence of a step between the other end of the outer cylinder and the cylindrical portion 7. In Embodiment 2, on the other hand, the respective outer circumferential surfaces of the outer cylinder 3 and the cylindrical portion 7 are made even in one and the same plane so that it is possible to easily form a tape-wrapped layer continuously and smoothly from the outer cylinder 3 to the cylindrical portion 7. Further, in the unit according to Embodiment 2, the structure of the metal mold 10 can be simplified in comparison with the unit according to Embodiment 1. r n n ^ *? 1-ixjyjzz J (Excimple) Rubber premolded units were molded according to Embodiment 1 under the condition that the axial length a of the first outer circumferential surface, the distance b of the external step, and the distance c. of the internal step were variously changed, and the optimum combination of those values a, b and c. was examined. The Examination was carried out about four patterns of the values a, b and c. as follows (the unit of all the values was mm). pattern 1: a= 15; b=0.5; c=5; a/bc =6.0 pattern 2:a=18;b=0.5;c=5; a/bc =7.2 pattern 3:a=15;b=0.3;c=5; a/bc = 10.0 pattern 4: a=30; b=1.0; c=5; a/bc =6.0 [0023]- As a result, molding could be performed with no problems in the cases of the patterns 1 and 4, but molding could not be performed in the cases of the patterns 2 and 3 because the conductive rubber of the semiconductive layer was deformed. On the basis of this fact, it is presumed that good molding can be performed in the case where the value of a/bc is not larger than 6.0. [Effects of the Invention] As described above, in the premolded unit for a power cable joint according to the present invention, the following effects can be obtained. (1) The producing cycle of a premolded unit can be reduced by integral injection molding- It is not particularly necessary to mold only the shield break separately. (2) The shield break is integrally molded with the premolded unit so that there is no fear that, for example, the insulation is damaged. (3) The electric performance of the shield break can be confirmed before shipping. (4) Since the joint can be formed simply by combining the previously molded premolded unit on site, it is possible to realize shortening of the installation time and lowering of the skill for the installation technique. (5) The respective outer circumferential surfaces of the external semiconductive layer and the cylindrically projected insulating material are made even in one and the same plane so that the outer diameter of the unit can be reduced, and tape for preventing impulse flashover can be wrapped easily on the outer circumference of the cylindrically extruded insulating material. [Brief Description of Drawings] [Fig. 1] Fig. 1 is a sectional view showing the structure of the joint according to the present invention. [Fig. 2] Fig. 2 is a view for explaining an internal step c.^ in which (A) and (B) show the cases where the outer circumferential surface and inner end surface of the end portion inner cylinder constitute an angled corner and a curved surface, respectively. [Fig. 3] Fig. 3 is a sectional view showing the unit according to the present invention and a mold for injection-molding the unit. [Fig. 4] Fig. 4 is a sectional view showing a unit different from that of Fig. 3 and a mold for injection-molding the unit. [Description of Reference Numerals] 1 premolded unit; 2 end portion inner cylinder (external semiconductive layer); 2A first outer circumferential surface; 2B second outer circumferential surface; 2C inner end surface; 2D flat surface; 3 outer cylinder (external semiconductive layer); 3A junction surface; 4 insulating rubber; 5 internal electrode; 6 shield break; 7 cylindrical portion; 10 metal mold; 11 insert die. WE CLAIM; 1. A premolded unit for a power cable joint having an external semiconductive layer which extends from an outer circumference to end portions and an inside of which is filled with an electric insulating material, characterized in that said unit has a shield break of said external semiconductive layer at an end portion of said unit, and said insulating material is cylindrically extruded out from said shield break. 2. The premolded unit for a power cable joint according to claim 1, wherein said external semiconductive layer is separated by said shield break into an end portion inner cylinder and an outer cylinder. 3. The premolded unit for a power cable joint according to claim 2, wherein said inner cylinder has a first outer circumferential surface which abuts on said cylindrically projected insulating material and a second outer circumferential surface which forms an external step between said second outer circumferential surface and said insulating material, an internal step being formed between said first outer circumferential surface and an inner end surface of said inner cylinder, said outer cylinder has one end which abuts on a cable and the other end which forms said shield break between said the other end and said first outer circumferential surface with said insulating material interposed therebetween. 4. The premolded unit for a power cable joint according to claim 3, wherein when an axial length of said first outer circumferential surface is represented by a, a distance of said external step is represented by b, and a distance of said internal step is represented by c, a value of a/bc is selected to be not larger than 6. 5. The premolded unit for a power cable joint according to any one of the claims 1 to 3, wherein respective outer circumferential surfaces of said external semiconductive layer and said cylindrically projected insulating material are made even in one and the same plane. 6. The premolded unit for a power cable joint according to claim 3, wherein an edge portion extending from said first outer circumferential to said inner end surface is formed to be an angled comer. 7. The premolded unit for a power cable joint according to claim 3, wherein an edge portion extending from said first outer circumferential to said inner end surface is formed to be a curved surface. 8. The premolded unit for a power cable joint according to claim 1, wherein said cylindrically projected insulating material is coaxially arranged with respect to said external semiconductive layer. 9. A premolded unit for a power cable joint, substantially as herein described, with reference to the accompanying drawings.

Documents:

430-mas-1998 abstract duplicate.pdf

430-mas-1998 claims duplicate.pdf

430-mas-1998 description (complete) duplicate.pdf

430-mas-1998 drawings duplicate.pdf

430-mas-1998-abstract.pdf

430-mas-1998-claims .pdf

430-mas-1998-correspondence others.pdf

430-mas-1998-correspondence po.pdf

430-mas-1998-description complete.pdf

430-mas-1998-drawings.pdf

430-mas-1998-form 1.pdf

430-mas-1998-form 26.pdf

430-mas-1998-form 3.pdf

430-mas-1998-form 5.pdf

430-mas-1998-other documents.pdf

430-mas-1998-pct.pdf