JPH0452562B2 - - 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, 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. 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 electrically insulating tube 1 according to the present invention, which is mainly composed of low-density polyethylene having a weight-average molecular weight to number-average molecular weight ratio of 5 or less. 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. Compared to conventional methods, we were able to form an insulating layer that can withstand electrical damage for a long time with a thinner pressure. At this time, anti-aging agents such as those usually added to heat shrink tubes,
Of course, processing aids and the like can be added. 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 sheathing layer formed by winding a metal table made of aluminum or the like; 5 a cable sheath made of extruded plastic such as polyethylene or vinyl chloride resin; 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 cable connection, it goes without saying that the cable termination portion can also be formed in the same manner. (Example) Example A tube with an inner diameter of 3 mm and a wall thickness of 2 mm was extruded using a material whose main composition was low-density polyethylene having a ratio of weight average molecular weight to number average molecular weight as shown in Table 1. After cross-linking this tube by electron beam irradiation, the tube is expanded by the usual method of making a heat-shrinkable tube.
A heat-shrinkable tube with an inner diameter of 8 mm was obtained. After heating and shrinking this heat-shrinkable tube into a wire with an outer diameter of 5mm, it is heated to 50°C.
An AC voltage of 3kV was applied in water. 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 results are shown in Table 1. The molecular weight was measured by "Measuring the degree of polymerization of polymers" (Polymer Experimental Chemistry Course, Kyoritsu Publishing).

[Table] Tree characteristics ×: Many trees occur
○: Almost no trees occur
◎: No trees were generated at all. As can be seen from the results in the table above, many trees were generated in the tubes with a large ratio of weight average molecular weight to number average molecular weight, but almost no trees were generated in the heat shrinkable tube of the present invention. The insulation failure time was 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 portion of a 22 kV, 150 mm ² 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 ×: Many trees occur
○: Almost no trees occur
◎: 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 adopt a thick insulation thickness that takes into account 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 thereof is low-density polyethylene having a ratio of weight average molecular weight to number average molecular weight of 5 or less. 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 insulation layer, the insulation tube layer has a weight average molecular weight and a number average molecular weight. A connection part for a power cable, characterized in that it is constructed by shrinking a heat-shrinkable tube mainly composed of low-density polyethylene having a ratio of 5 or less.