JPH0417208A - Rubber-plastic insulated electric power cable - Google Patents

Abstract

PURPOSE:To improve a dielectric breakdown characteristic at a high temperature and to reduce generation of an electric tree by forming an insulation layer of cross-linking polyolefine having a melting point of 105 deg.C or higher and a cross-linking degree by the xylene method of 88% or more. CONSTITUTION:An insulation layer is formed of cross-linking polyolefine having a melting point of 105 deg.C or higher and a cross-linking degree by the xylene method of 88% or more. Namely, the melting point and the cross-linking degree by the xylene method of the cross-linking polyolefine for forming the insulation layer are determined higher in comparison to conventional cross-linking polyolefine. Consequently, at an ordinary working temperature and in the vicinity thereof in an electric power cable, molting and thermal expansion of a crystalline part and amorphous part in the cross-linking polyolefine are suppressed. Accordingly, the electric power cable having an excellent dielectric breakdown characteristic at an ordinary working temperature and in the vicinity thereof in a high voltage electric power cable, in which generation of an electric tree is suppressed even in long time use, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rubber/plastic insulated power cable having excellent dielectric breakdown characteristics at high temperatures.

(Conventional technology) Conventionally, high voltage rubber/plastic insulated power cables (
A power cable (hereinafter referred to as an "electric cable") has a structure having an inner semiconducting layer, an insulating layer, and an outer semiconducting layer on a conductor. Such power cables are required to have an insulating layer that is difficult to cause dielectric breakdown at high temperatures (that is, to have excellent dielectric breakdown characteristics). Further, since the dielectric breakdown occurs due to the generation of electric trees, it is also necessary that electric trees are difficult to generate.

The insulating layer of conventional power cables has a melting point of 103 to 98
It is formed from a crosslinked polyolefin having a degree of crosslinking of 75 to 85% by the xylene method at .degree.

(Problems to be Solved by the Invention) When the insulating layer of the electric cable is formed from a crosslinked polyolefin having the above melting point and degree of crosslinking, the following problems occur.

The normal operating temperature of high-voltage power cables is about 90° C., but the problem is that the above-mentioned dielectric breakdown characteristics of the crosslinked polyolefin deteriorate near this temperature. Another problem is that as the operating electric field of the power cable increases, electric trees are more likely to occur, which causes a reduction in dielectric breakdown characteristics.

Therefore, the present invention has excellent dielectric breakdown characteristics even at the normal operating temperature of the elevated piezoelectric cable and at temperatures around it.
To provide an electric cable capable of suppressing the occurrence of electric trees even after long-term use.

(Means and effects for solving the problems) In order to achieve the above object, the present invention provides a rubber/plastic electric cable having an inner semiconducting layer, an insulating layer, and an outer semiconducting layer on a conductor. The present invention provides an electrical cable characterized in that the layer is formed from a crosslinked polyolefin having a melting point of 105°C or higher and a degree of crosslinking by a xylene method of 88% or higher.

The insulating layer of the electric cable of the present invention is formed by extrusion coating an uncrosslinked olefin resin containing a crosslinking agent etc. onto the internal semiconductive layer by a conventional method, and then heating to cause a crosslinking reaction.

Examples of uncrosslinked olefin resins include low density polyethylene, linear low density polyethylene, ultra low density polyethylene, ethylene-ethyl acrylate copolymer, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, Examples include ethylene-styrene copolymer.

These can be used alone or in an appropriate combination of two or more. Among these, uncrosslinked olefin resins having a melting point of 115°C or higher are preferable because they make the melting point of the crosslinked polyolefin 105°C or higher.

As a crosslinking agent, dicumyl peroxide, t-butylperoxycumene, α, α°-bis(t-butylperoxydiisopropyl)benzene, 2,5 dimethyl-2,5
-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane-
3, m-(t-butylperoxyisopropyl)isopropylbenzene, and the like. These can be used alone or in an appropriate combination of two or more.

The amount of crosslinking agent added to uncrosslinked olefin resin is as follows:
It is preferably 0.10 to 0.30 parts by weight in terms of active oxygen amount based on 100 parts by weight of the olefin resin. If this blending ratio is less than 0.10 parts by weight, the degree of crosslinking of the crosslinked polyolefin by the xylene method will be less than 88%, and the degree of crosslinking will be less than 0.30%.
If it exceeds 1 part by weight, the crosslinking reaction proceeds excessively and burns are likely to occur, which is not preferable. A particularly preferable blending amount is 0.17 to 0.2 parts by weight in terms of active oxygen amount.

In addition to the uncrosslinked olefin resin, an antioxidant can be further blended as a material for forming the insulator layer.

The antioxidants include 4,4'-thiobis(3-methyl-6-t-butylphenol), bis[2-methyl-
4-(3-n-alkylthiopropionyloxy)-5
-t-butylphenyl sulfide, 2,5-di-t-
Butylhydroquinone, 2.6-di-t-butyl-p-cresol, 2゜2'-thiodiethylenebis-[3-(3
, 5-di-t-butyl-4-hydroxyphenyl)propionate, dilaurylthiodipropionate, distearylthiodipropionate, and the like. Among these, 4,4° to thiobis(3-methyl-6-t-butylphenol) is preferred.

The blending amount of the antioxidant is 100% of the uncrosslinked olefin resin.
It is preferably 0.05 to 1.0 parts by weight. If the amount is less than 0.05 parts by weight, the antioxidant effect will be reduced, and if it exceeds 1.0 parts by weight, the crosslinking reaction will be inhibited and the degree of crosslinking will become low, which is not preferable.

In addition to the above-mentioned components, lubricants, various fillers, and the like can be blended with the uncrosslinked olefin resin as materials for forming the insulator layer.

In the electric cable of the present invention, the melting point of the crosslinked polyolefin forming the insulating layer and the degree of crosslinking by the xylene method are set higher than those of conventional crosslinked polyolefins. Therefore, the crystalline portion and the amorphous portion of the crosslinked polyolefin are prevented from melting or thermally expanding even at the normal use temperature of the electric cable and temperatures around it.

(Example) Examples 1 to 3 and Comparative Examples 1 to 3 An inner semiconducting layer, an insulating layer, and an outer semiconducting layer were coated on a conductor by a conventional method. The material for forming the insulator layer is the first
The olefin resin in the amount shown in the table (expressed in parts by weight) was used, and other known forming materials were used. Next, this was pressurized at a pressure of 10 kg/cm2 and a temperature of 270°C.
The mixture was heated to cause a crosslinking reaction. Thereafter, a metal shielding layer, an external semiconducting layer, and a sheath layer were formed by a conventional method to obtain each 6 kV power cable having an insulating layer with a thickness of 3 mm. The melting point and degree of crosslinking of these insulator layers are as follows:
An insulator layer was cut out from each electric cable, and the melting point was measured using a DSC (differential calorimeter) and the degree of crosslinking was measured using a xylene method. In addition, DSC was measured in a temperature range of 30 to 150°C under a temperature increasing condition of 5°C/sec, and the bottom value of the chart was taken as the melting point.

For each of these electric cables, an impulse breakdown test was conducted while the conductor was energized so that the conductor temperature was 90°C. In addition, AC voltage was applied at 90 kV at 50 Hz for 10 days, and then the insulator layer was observed with a microscope to find foreign particles.
The number of foreign objects in which electric trees were generated was counted among the 00 particles. These results are shown in Table 1.

(Effects of the Invention) The electric cable of the present invention has an insulating layer formed from a crosslinked polyolefin having a melting point of 105° C. or higher and a degree of crosslinking by a xylene method of 88% or higher.

The power cable of the present invention can be used even at normal usage temperatures.
The cross-linked polyolefin that forms the insulator layer melts,
Thermal expansion is suppressed. This improves dielectric breakdown characteristics at high temperatures. Further, even after long-term use, electrical damage is unlikely to occur, and excellent dielectric breakdown characteristics are maintained over a long period of time. Further, since the insulating layer of the power cable of the present invention has a high degree of crosslinking, it is also excellent in suppressing water trees, and there is no need to separately provide a water-blocking layer.

Claims (1)

[Claims] A rubber/plastic insulated power cable having an inner semiconducting layer, an insulating layer, and an outer semiconducting layer on a conductor, in which the insulating layer is crosslinked with a melting point of 105°C or higher and a degree of crosslinking by a xylene method of 88% or higher. A rubber/plastic insulated power cable characterized in that it is formed from polyolefin.