JPH0515014B2 - - 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) A terminal connection portion of a plastic insulated power cable having a plastic insulation layer made of polyethylene, cross-linked polyethylene, etc. has an internal semiconducting layer on its outer periphery, covering the conductor connection portion and the cable insulation layer as well as the cable body. 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, causing the breakdown voltage 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. It has been found that trees are more likely to form when used in a stretched and strained state, such as in a shrink tube, compared to unstrained polyethylene. As a result, by suppressing the trees caused by micron 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, which is mainly composed of an ethylene-propylene copolymer with a propylene polymerization ratio of 4% by weight and not exceeding 25% by weight. There is. By covering the conductor with this heat-shrinkable tube and forming an insulating layer, it is possible to suppress the generation of trees within the heat-shrinkable tube or from the interface between the heat-shrinkable tube and the conductor, compared to using a conventional polyethylene heat-shrinkable tube. It was possible to form an insulating layer that is thinner than the previous method and can withstand electrical damage for a long period of time. It is desirable for the polymerization ratio of propylene in the ethylene-propylene copolymer to be 4% by weight or more for tree suppression. That is, the breaking time ratio becomes 2.0 or more. Furthermore, if the polymerization ratio of propylene is too high, the tube becomes rubbery, cannot be heat set as required for a heat-shrinkable tube, and has poor tree resistance. Therefore, by controlling the polymerization ratio of propylene to not exceed 25% by weight, the problem of heat setting can be solved, and a preferable heat-shrinkable tube with a tearing time ratio of 2.0 or more can be obtained. At this time, it is of course possible to add anti-aging agents, processing aids, etc. that are usually added to heat-shrinkable tubes. FIG. 2 is a longitudinal cross-sectional view of an embodiment of the connection part of a power cable according to the present invention, in which 1 is a cable conductor, 2 is a cable insulation layer such as polyethylene or cross-linked polyethylene, 3 is an internal semiconducting layer of the cable, and 4 is a cable conductor. The cable sheath 5 is made of extruded plastic such as polyethylene or vinyl chloride resin, and the cable conductor 1 is connected to the cable sheath 6. An electric field relaxation cable layer 7 for relaxing the electric field at the conductor connection part 6 is provided on the connection part 6 of the cable conductor 1 and the outer periphery of the cable insulating layer 2, and an insulation layer 7 is provided on the cable layer 7 for alleviating the electric field at the conductor connection part 6. 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. (Examples) Example 1 A tube with an inner diameter of 3 mm and a wall thickness of 2 mm was produced by extrusion and had a composition having a polymerization ratio of propylene as shown in Table 1. After cross-linking this tube by electron beam irradiation, the tube was expanded using the usual method for making heat-shrinkable tubes, and the inner diameter was 2 mm.
A heat shrink tube was obtained. This heat-shrinkable tube is heated and shrunk into a wire with an outer diameter of 5mm, and then placed in water at 50℃.
An AC voltage of 3KV was applied. After applying electricity for 200 hours, the tube was cut to examine the growth status of the tree and to measure the time it took to break. The result is the first
As shown in the table.

[Table] Tree characteristics ×: Many trees occur
◎: No trees were generated.As can be seen from the results in the table above, polyethylene that was not copolymerized with propylene at all (Comparative Example 1) generated many trees and broke the insulation in a short period of time. In the embodiment of the present invention, no trees are generated and the tearing time is significantly improved.
Furthermore, even with propylene copolymerized, in those with a small polymerization ratio (Comparative Example 2) and those with a large polymerization ratio (Comparative Examples 3 and 4), many trees were generated and the breaking time was not improved. Example 2 A tube with an inner diameter of 20 mm and a wall thickness of 3 mm was extruded using a composition consisting of the polymerization ratio of propylene as shown in Table 2. After crosslinking by electron beam irradiation, the tube was expanded to an inner diameter of 40 mm. A heat shrink tube was obtained. Using this tube, a connection part of a 22 KV, 150 mm cross-linked polyethylene insulated polyvinyl chloride sheath cable was formed as shown in FIG. An AC voltage of 60 KV was applied to the above connection part in water at room temperature.
After applying electricity for 300 hours, the connection was disconnected and the tree formation status was examined, and the time taken for the connection to break was also measured for connections created in the same way. The results are shown in Table 2.

[Table] Tree characteristics ×: Number of tree occurrences
◎: No trees occur at all (effect of the invention) As described above, the heat shrinkable tube of the present invention and the power cable connection section using the same do not generate trees, which could be avoided with conventional heat shrinkable tubes, and are free from insulation damage. The characteristics are also significantly improved. Therefore, there is no need to use thick insulation to account for the reduction in insulation as in the past, and this has the effect of maintaining stable insulation characteristics for a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a heat-shrinkable tube according to the present invention, and FIG. 2 is a longitudinal 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 heat-shrinkable tube for electrical insulation, characterized in that the main composition is an ethylene-propylene copolymer in which the polymerization ratio of propylene is 4% by weight or more and does not exceed 25% by weight. 2. In a cable connection part in which an electric field relaxation tube layer, an insulating tube layer, and an external semiconducting layer are sequentially provided on the outer periphery of a cable conductor connection part and a cable insulating layer, the insulating tube layer has a propylene polymerization ratio of 4% by weight. A connection part for a power cable, characterized in that it is constructed by shrinking a heat-shrinkable tube whose main composition is an ethylene-propylene copolymer of at least 25% by weight.