JPH08167331A - DC cable and its manufacturing method - Google Patents

Abstract

(57) [Summary] [Purpose] To obtain a high-power DC cable using a cross-linked polyethylene insulator. The cross-linked polyethylene insulation of a DC cable contains α-methylstyrene in the range of 0.1% by weight to 1.0% by weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct current cable, and more particularly to a cross-linked polyethylene insulated cable for electric power which has improved dielectric breakdown characteristics due to direct current.

[0002]

2. Description of the Related Art Conventionally, a so-called OF cable, in which a cable sheath is filled with insulating oil (Oil-Fill), has been used for transporting a large amount of DC electric power. This OF cable has a zinc-plated copper spiral on its axis, and conductors, carbon paper, insulating paper, carbon paper, and a shielding layer are sequentially formed around the spiral, and the insulating paper is insulated. It is impregnated with oil. In this OF cable, it is necessary to constantly adjust the hydraulic pressure of the insulating oil inside by an oil pressure adjustment tank connected to each fixed laying section. Further, since the insulating oil deteriorates with time, it requires a lot of labor and cost for maintenance, such as periodically measuring the degree of deterioration and replacing it with a new one when the deterioration becomes severe. Also, disaster prevention measures are required. For this reason, the use of cross-linked polyethylene insulated cables (CV cables) is now the mainstream for AC power cables.

[0003]

Since the CV cable does not use insulating oil, it does not require auxiliary equipment such as a hydraulic adjustment tank unlike the OF cable, and does not require labor or cost for maintenance. Although it is suitable as a cable, there is a problem in that when it is used for transporting a large amount of DC power, dielectric breakdown easily occurs and the cable cannot be used. It is known that this dielectric breakdown is caused by the following mechanism. That is, inside the cross-linked polyethylene used as the insulator of the cable, a decomposition product of dicumyl peroxide (hereinafter referred to as “DCP”) used as a cross-linking agent when the polyethylene is cross-linked remains. When a large amount of direct current is transported by, the DCP decomposition product is activated by the strongly polarized electric field and heat generation, and the activated decomposition product lowers the insulating property of the crosslinked polyethylene and acts on the crosslinked polyethylene. To break the chain. Therefore, dielectric breakdown occurs even at a relatively low voltage. Although a method of cross-linking polyethylene that does not use a cross-linking agent has also been studied, for example, electron beam irradiation is effective when the thickness of the insulator is extremely thin, but it is effective for the case of large capacity high-voltage cables. Since the wall thickness is large, the electron beam is difficult to penetrate into the inside of the layer and cannot be applied. The present invention has been made to solve the above problems, and therefore an object thereof is to provide a direct current cable which is a crosslinked polyethylene insulated cable and which is capable of transporting high direct current power.

[0004]

SUMMARY OF THE INVENTION The above problems can be solved by providing a direct current cable containing .alpha.-methylstyrene in the range of 0.1% to 1.0% by weight in a crosslinked polyethylene insulation. . This DC cable is D
The polyethylene insulator is cross-linked with CP as a cross-linking agent, and then the cross-linked polyethylene insulator is heated to generate α-methylstyrene by decomposing DCP. At this time, the amount of DCP added and the heating conditions are adjusted, The content of α-methylstyrene in the crosslinked polyethylene insulator is 0.1% by weight.
It can be manufactured by a manufacturing method in which the content is within the range of 1.0 to 1.0 wt%.

[0005]

[Function] α-methylstyrene has π of its benzene ring,
Since the π ^* orbital acts as a trap for electrons or holes like styrene, the above DCP decomposition product activated by polarization and heat generation by a strong electric field is deactivated by the presence of α-methylstyrene, and crosslinked polyethylene is obtained. It is considered that the dielectric breakdown of is reduced.

Although a required amount of α-methylstyrene can be added in advance to the polyethylene insulator during cable production, when the crosslinking agent DCP decomposes in the polyethylene insulator, it is decomposed and produced as follows depending on the conditions. Since α-methylstyrene is produced as a product, the α-methylstyrene produced here can be used for the purpose of the present invention. That is, DCP is thermally decomposed in a polyethylene insulator to generate a cumyloxy radical as shown in the following formula (I), and this removes hydrogen of polyethylene, whereby crosslinking proceeds. In this process, methane, acetophenone, cumyl alcohol, etc. are produced as decomposition products.

[0007]

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When the cumyl alcohol produced here is further heated, it undergoes a secondary decomposition reaction as shown in the following formula (II) to produce α-methylstyrene and water.

[0009]

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It is possible to select the amount of DCP added and the heating conditions of the cable so that the amount of α-methylstyrene produced here is in the range of 0.1% by weight to 1.0% by weight. By selecting the amount of DCP added and the heating conditions of the cable in this way, it is possible to obtain a DC cable having good dielectric breakdown characteristics without adding additional α-methylstyrene. The optimum amount of DCP added and the heating conditions for the cable can be determined by experiments. However, if the amount of DCP used to obtain the above amount of α-methylstyrene is too large for the crosslinking of polyethylene, the minimum amount of DCP necessary for crosslinking is used, and the shortage of α-methylstyrene is It can also be added in advance to polyethylene.

Α- contained in the crosslinked polyethylene insulator
As a result of experiments, it was found that when the amount of methylstyrene was less than 0.1% by weight, the effect of improving the DC breakdown characteristic was not observed. Also, the amount of α-methylstyrene is 1.0% by weight.
When it exceeds the above, not only is it meaningless for improving the DC breakdown characteristics, but it also has an undesired effect on the physical properties and water resistance of the crosslinked polyethylene insulator.

Of the by-products other than α-methylstyrene produced by the above decomposition reaction, methane and water are harmful because they are harmful to the characteristics of the DC cable and are preferably removed. Therefore, in the manufacturing process of the crosslinked polyethylene insulated cable, a heating and leaving process for removing methane and water is generally provided. Therefore, if the conditions such as the temperature and time of this heating and leaving step are set to the conditions for producing α-methylstyrene, the object can be achieved without adding a heating and leaving step for producing α-methylstyrene again. be able to.

[0013]

Example A CV cable sample having a cross-linked polyethylene insulator having a thickness of 3 mm was prepared, the amount of α-methylstyrene contained in the insulator was adjusted under the following conditions, and the temperature was 90 ° C.
The DC dielectric breakdown strength was measured. (Example 1) Amount of α-methylstyrene: 0.1% by weight Heating conditions: Hold at 80 ° C for 10 days (Example 2) Amount of α-methylstyrene: 0.3% by weight Heating conditions: Hold at 80 ° C for 15 days (Example 3) Amount of α-methylstyrene: 1.0 wt% Heating conditions: Hold at 80 ° C for 15 days The measurement results of these examples are shown in Table 1.

For comparison, a CV cable sample similar to that of the example was formed, except that α contained in the insulator under the following conditions.
-The amount of methylstyrene was adjusted and the DC breakdown strength at a temperature of 90 ° C was measured. (Comparative Example 1) Amount of α-methylstyrene: Not present Heating condition: Not heated (Comparative Example 2) Amount of α-methylstyrene: 0.05 wt% Heating condition: Holding at 80 ° C for 5 days (Comparative Example 3) Amount of α-methylstyrene: 1.5% by weight Heating conditions: Hold at 80 ° C. for 20 days The measurement results of these comparative examples are shown in Table 1 together with the examples.

[0015]

[Table 1]

From the above results, the DC cable sample of the embodiment containing 0.1% by weight or more and 1.0% by weight or less of α-methylstyrene has a higher DC breakdown strength than the samples of the comparative examples. Is clear.

[0017]

INDUSTRIAL APPLICABILITY Since the DC cable of the present invention contains .alpha.-methylstyrene in the range of 0.1% by weight to 1.0% by weight in the crosslinked polyethylene insulator, the dielectric breakdown characteristics by DC are improved. The cross-linked polyethylene insulated cable can be used for DC high power.

Claims (2)

[Claims] 1. A direct current cable containing α-methylstyrene in the range of 0.1% to 1.0% by weight in a crosslinked polyethylene insulation. 2. A polyethylene insulator is cross-linked using dicumyl peroxide as a cross-linking agent, and the cross-linked polyethylene insulator is heated to produce α-methylstyrene by decomposing dicumyl peroxide, in which case dicumyl peroxide is used. A method for producing a direct-current cable in which the content of α-methylstyrene in the crosslinked polyethylene insulator is within the range of 0.1 wt% to 1.0 wt% by adjusting the amount of oxide added and the heating conditions.