JPH0521284B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to a heat-shrinkable tube for electrical insulation and a connecting portion of a plastic insulated power cable using the same. (Background of the Invention) The connecting portion of a plastic insulated power cable having a plastic insulating layer made of polyethylene, cross-linked polyethylene, etc. has an internal semiconducting layer, an insulating layer, an outer semiconducting layer, and an outer protective layer are sequentially provided. When forming these layers, there are methods such as wrapping each layer with tape or inserting a rubber molded product that integrates each layer, but each method has its advantages and disadvantages. As a simple connection method, a heat shrink tube connection method is adopted in which an electric field relaxation layer is used instead of the internal semiconductive layer, and both layers are formed by heat shrinking using a heat shrink tube. However, a major technical problem with such cable connections is that when electricity is applied for many years, the insulation performance gradually deteriorates and the breakdown voltage begins to drop. Therefore, in order to account for the reduction in insulation due to deterioration, thick insulation tubes are used,
Measures have been taken such as increasing the number of insulating tubes. We, the inventors, have conducted various studies on this deterioration over time, and have found that this deterioration phenomenon is caused by microvoids occurring in the insulating tube or moisture existing between the conductor and the tube. It was found that this is due to water trees generated at the interface. In particular, it has been found that when used in a stretched and strained state, such as in a shrink tube, trees are more likely to form than in unstrained polyethylene. As a result, by suppressing the trees caused by the microscopic defects unique to shrink tubes, deterioration phenomena no longer occur, and there is no need to take into account deterioration due to aging, making it possible to reduce the wall thickness of shrink tubes. (Disclosure of the Invention) The present invention solves the above-mentioned problems, has a thin wall thickness,
The present invention also provides a heat-shrinkable electrically insulating tube that has stable insulation performance over a long period of time, and a power cable connection using the same. FIG. 1 is a longitudinal cross-sectional view of a heat-shrinkable tube 1 for electrical insulation according to the present invention.
-Content of vinyl acetate in olefin copolymer
The main composition is a composition containing 15% to 25% by weight of ethylene-vinyl acetate copolymer. As the α-olefins, butene-1, pentene-1, octene-1, and those having substituents such as methyl groups and ethyl groups on their main chains are used. The ethylene-vinyl acetate copolymer is preferably a copolymer with a low residual crystalline vinyl content, and a copolymer with a vinyl acetate content of 15% to 25% by weight can be used. At this time, it is of course possible to add anti-aging agents, processing aids, etc. that are usually added to heat-shrinkable tubes. When this heat-shrinkable tube is coated on a conductor and an insulating layer is formed, it is possible to suppress the generation of trees inside the heat-shrinkable tube or from the interface between the heat-shrinkable tube and the conductor, and when a conventional polyethylene heat-shrinkable tube is used. It was possible to form an insulating layer that was thinner and resistant to electrical damage for a long period of time compared to the case of using only the ethylene-α olefin copolymer or the ethylene-vinyl acetate copolymer alone. FIG. 2 is a longitudinal sectional view of an embodiment of the power cable connection part according to the present invention, in which 1 is a cable conductor, 2 is a cable insulation layer made of polyethylene, cross-linked polyethylene, etc., 3 is an internal semiconducting layer of the cable, and 4 is copper. , a metal shearing layer formed by winding a metal tape such as aluminum, 5 a cable sheath made of extruded plastic such as polyethylene or vinyl chloride resin, and 6 a connection portion of the cable conductor 1. An electric field relaxation tube layer 7 for relieving the electric field at the conductor connection portion 6 is provided on the connection portion 6 of the cable conductor 1 and the outer periphery of the cable insulation layer 2, and on the electric field relaxation tube layer 7 is provided an insulation layer 7 using the heat-shrinkable tube described above in the present invention. The tube layer A is further provided with an external semiconducting tube layer 8 and an external protective layer 9 in this order. The formation of these tube layers is no different from the method of forming cable connections using conventional heat-shrinkable tubes, and each tube may be provided separately and heat-shrinked. For example, an electric field relaxation tube and an insulating tube may be integrated in advance. Alternatively, the electric field relaxation tube, the insulating tube, and the external semiconducting tube may be integrated and formed by heat shrinking. Further, although the above description has been made regarding the cable connection portion, it goes without saying that the cable termination portion may also be formed in the same manner. (Example) Example Inner diameter 3 mm, wall thickness 2 mm, composition as shown in Table 1
A tube was made by extrusion. After crosslinking this tube by electron beam irradiation, the tube was expanded by a conventional method for making a heat-shrinkable tube to obtain a heat-shrinkable tube with an inner diameter of 8 mm. This heat-shrink tube was heat-shrinked into an electric wire with an outer diameter of 5 mm, and then an AC voltage of 3 KV was applied in water at 50°C. 300
After applying electricity for a certain period of time, the tube was cut to examine the state of tree growth and the time taken to break the tree was measured.
The results are shown in Table 1.

[Table] As can be seen from the above table, compared to the conventional polyethylene, in which many trees were generated, in the examples of the present invention, no trees were generated at all, and the tearing time was significantly improved. Furthermore, compared to the case of using α-olefin alone or ethylene-vinyl acetate copolymer alone, both tree properties and breaking time are significantly improved. Example A tube with an inner diameter of 20 mm and a wall thickness of 3 mm was extruded using the composition shown in Table 2, crosslinked by electron beam irradiation, and then expanded to obtain a heat-shrinkable tube with an inner diameter of 40 mm. . Using this tube, a connection part for a 22 KV, 150 mm cross-linked polyethylene insulated polyvinyl chloride sheath cable was formed as shown in Figure 2. An AC voltage of 60 KV was applied to the above connection part in water at room temperature. 400
After time energization, the connection was disconnected to examine the state of tree formation, and the time it took for similar connections to break was measured. The results are shown in Table 2.

[Table] (Effects of the Invention) As described above, the heat-shrinkable tube of the present invention and the power cable termination connection using the same do not generate trees that cannot be avoided with conventional heat-shrinkable tubes.
The insulation breaking properties are also significantly improved. Therefore, there is no need to use a thick insulating material that takes into account the inevitable drop as in the conventional method, and the present invention has the effect of maintaining stable insulating properties for a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of a heat-shrinkable tube according to the present invention, and FIG. 2 is a longitudinal cross-sectional view of an embodiment of a power cable connecting portion of the present invention using the above-mentioned heat-shrinkable tube. A: Heat-shrinkable tube of the present invention, 2: Cable insulating layer, 6: Conductor connection portion, 7: Electric field relaxation tube layer, 8: External semiconducting tube layer.

Claims (1)

[Scope of Claims] 1. A composition comprising a copolymer of ethylene and α-olefin having 4 to 8 carbon atoms mixed with an ethylene-vinyl acetate copolymer having a vinyl acetate content of 15% to 25% by weight. An electrical insulating heat-shrinkable tube characterized by comprising a composition. 2. The heat-shrinkable tube for electrical insulation according to claim 1, wherein the α-olefin is butene-1. 3. The heat-shrinkable tube for electrical insulation according to claim 1, wherein the α-olefin is pentene-1. 4. The heat-shrinkable tube for electrical insulation according to claim 1, wherein the α-olefin is octene-1. 5. In a cable connection part in which an electric field relaxation tube layer, an insulation tube layer, and an external semiconducting layer are sequentially provided on the outer periphery of the cable conductor connection part and the cable insulation layer, the insulation tube layer is formed of ethylene and a carbon number of 4 to 8. It was constructed by shrinking a heat-shrinkable tube whose main composition was a composition in which an ethylene-vinyl acetate copolymer with a vinyl acetate content of 15% to 25% by weight was blended with an α-olefin copolymer. A power cable connection part characterized by: 6. The connection portion of a power cable according to claim 5, characterized in that the connecting portion is made of α-olefin butene-1. 7. The power cable connection portion according to claim 5, wherein the α-olefin is pentene-1. 8. The power cable connection portion according to claim 5, wherein the α-olefin is octene-1.