CA1266956A - Cross-linked polyethylene cables - Google Patents

Abstract

CROSS-LINKED POLYETHYLENE CABLES
Abstract of the Disclosure:
Cross-linked polyethylene cable, prepared by extrusion-coating a conductor with a composition composed of polyethy-lene, 15% or more and less than 30% by weight of ethylene-vinyl acetate copolymer containing more than 25% and less than 35% by weight vinyl acetate, and an organic peroxide and then causing cross-linking, has excellent long service life charac-teristics particularly remarkable at elevated temperatures.

Description

~2~i6~35~i CROSS-LINKED POLYETHYLENE CABLES

Technical Field:
This invention relates to cross-linked polyethylene cables having excellent characteristics against degradation, particularly degradation at high temperaturesD
Backaround ~rt:
It is well known that cross-linked polyethylene cables, when they are placed under an applied voltage for a long time in a water environment, cause so called "penetrated water degradation" resulting in breakdown of an insulation. This degradation is believed to take place because the water which has penetrated into a cross-linked polyethylene insulation acts a~s a nuclei to form so-called "water trees" in the insulation.
Many approaches have been tried for improving the penetrated water degradation of cross-linked polyethylene cables. For example, improved cables or cable joints are described in Japanese Patent Publication No. 37668/1979, Japanese Utility Model Publication No. 2~008/1974 and Japanese Patent Publication No~ 51180/1981. In these cables or ~oints, a polyethylene composltion with an ethylene-vinyl acetate copolymer (hereinafter ref0rred to as EVA) compounded thereln i~ u~ed as an insulating material, whereby resistance again~t'the penetrated water degradation have been improvecl to a certain extent.
Recently, however, it has been discovered,that when cro~s linked polyethylene cables, particularly t~ose cables ..

5fi having a thick insulation are placed under an applied voltage for a long time at high temperatures such as 90C, they are degraded seriouslyO
The present inventors conducted various experiments and analysis to clarify the reasons for the above degradation.
As a results, it has been found that the cross-linked poly-ethylene cable of the present invention which is described later causes no degradation as mentioned above.
pisclosure of the Inventlon:
The present invention is intended to provide a cross-linked polyethylene cable having excellent characteristics against degradation at high temperatures. The gist of the present invention resides in a cross-linked polyethylene cable characterized by extrusion coating a polyethylene composition on a conductor, the polyethylene composition comprising polyethylene, 15% by weight or more to less than 30% ~y weight of an ethylene--vinyl acetate copolymer having a vinyl acetate (hereinafter refer to as VA) content of more than 25% by weight to less than 35% by weight and an organic peroxide and thereafter cross-linkiny the thus coated poly-ethylene composition.
The VA contenk oE the ethylene-vinyl acetate copolymer used in the present invention is preferably in an amount of

2~ to 33% on weight ba~is.
Bxie~ c~iption Q~ ~he ~xawi~:
The Figure i5 a cross-sectional view of a cross-linked polyethylene cable.

5~i Modes for Carrying out the Invention:
The present invention will hereinafter be explained in detail by reference to the following examples and comparative examples. 2 Cross-linked polyethylene cables (150 mm , 66 KV
class) were produced using twenty-three compositions shown in Table 1 as an insulating material. The Figure shows a cross-sectional view of each cross-linked polyethylene cable comprising a conductor 1, an insulating material 2, an inner semiconductive layer 3 and an outer semiconductive layer ~. Semiconductive layers 3 and ~ were each made of NUC 0580 (trade name, produced by Nippon ~nicar Co., Ltd.).
A current was circulated bringing the temperature of the conductor to 90C and a voltage of 160 KV was applied.
The time taken before breakdown occurred in the cable under application of voltage was measured to evaluate degradation oE the cable at high temperatures. These times are hereinaEter reEerred to as the "breakdown times", and are shown in Table 1 with the breakdown times (334 hours) oE Comparative Example 1 as a re~erence (1).
The following can be seen from Table 1. With cables produced using compositions in which 20~ by weight EV~ having a V~ content oE 15~ by weight or 2S~ by weight, respectively, was compounded with polyethylene ~Comparative Examples 2 and

3), the ~esistance to degradation at high temperatures was slightly improved as compared with the cable of Comparative Example 1, but it was not satisfactorily improved to the 9~

extent that a long service life (e.g., 30 years) could be guaranteed.
On the other hand, when EVA having a VA content of more than 25% by weight was used (Example 1), degradation at ' 5 high temperature was greatly improved. In particular, when the VA content was 30% by weight (Example 2), the cable showed a very high resistance to degradation in having break-down times four times that of the cable of Comparative Example 1. Also, at a VA content of 33% by weight (Example 3), excel-lent characteristics were obtained.
However, when the VA content was further increased, e.g., when EVA havi,ng a VA content of 35% by weight was used (Comparative Example 4), the characteristics of the resulting cables were slightly improved as compared with -the cable of lS Comparative Example 1, but they were still not satisfactory.

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The amount of EVA having a VA content falling within the above-described range is specified by the amount of the polyethylene to be compounded. In cables using polyethylene compositions comprising various levels of an EVA whose VA
content is 2~ by weight (the same VA content as in the poly-ethylene composition of Example 1), the respective breakdown times of polyethylene compositions (Comparative Examples 7, 10 and 11) each comprising more than 30~ by weight oE said EVA are only slightly superior to that of Comparative Example 1, but are not satisfactory; the breakdown times of a poly-ethylene composition (Example 6) comprising 28~ by weight of said EVA is three times that of Comparative Example l; the breakdown times of a polyethylene composition (Example 7) comprising 16~ by weight of said EVA is also superior; and the breakdown times of a polyethylene composition (Compara-tive Example 13) comprising 13% by weight of said EVA are only slightly superior to that of Comparative Example 1.
The same thing applies to cables using polyethylene compositions comprising various levels of an EVA whose VA
content is 33% by weight. That is, the breakdown times o:E
polyethylene composit.ions (Examples 8 and 9) comprising less than 30% by weight oE said EVA show excellent resistance to degradation, the breakdown times of a polyethylene coMposition (Comparative Example 5) comprising ~0~ by weight of said EVA
are only slightly superior to that oE Comparative Example l;
the breakdown times oE a polyethylene composition (Comparative Example 6) comprising 10~ by weight oE said EV~ are roughly the same as that o:E Comparative Example l; and the breakdown ~6gs~

times of a polyethylene composition (Example 10) comprising 16% by weight oE said EVA are excellent.
Cables using polyethylene compositions comprising, for example, 15% by weight (Example 4) or 25~ by weight (Example 5) of an EVA whose VA content is 30~ by weight have an excel-lent resistance to degradation.
In cables using polyethylene compositions comprising an EVA whose VA content is 25~ by weight, both a polyethylene composition (Comparative Example 9) comprising 15% by wei~ht of said EVA and a polyethylene composition (Comparative Example 8) comprising 30% by weight of said EVA are not much different in breakdown times from that of Comparative Example 1.
In cables using polyethylene compositions comprising an EVA whose VA content is ~5~ by weight, a polyethylene composition ~Comparative Example 12) comprising 28% by weight of said EVA (the same EVA content as Example 8) has unsatis-factory breakdown times.
As can be seen from the results shown in Table 1, 2~ only when the compositions of the invention comprising poly-ethylene and EVA are used as ins~llating materials are there obtained cables having excellent characteristics with break-down times which are three times that of Comparative Example 1. The reasons for the efect are not clear; however, the effect was not shown in any o~ Japanese Patent Publication No. 37668/1979, Japanese Utility Model Publication No.
290Q8/1974 and Japanese Patent Publication No. 51180/1981.

,. :. ~ .. .

~ ~ 6 ~ 9 Exploitation in Industry:
The cross-linked polyethylene cables of the present invention are useful for transmission of current in high voltages, and for factory wiring and house wiring of current in low voltages. The cables show greatly improved resistance to degradation under continuously applied voltages at high temperatures for extended periods of time. Thus, the cables have a long service life even when used under conditions of high temperature Eor extended periods of time.

_ g _

Claims (2)

The embodiments of the invention in which an exclusive property or privilege are claimed are defined as follows:

1. A cross-linked polyethylene cable which is prepared by extrusion coating a polyethylene composition on a conductor, the polyethylene composition comprising polyethylene, 15% or more to less than 30% by weight of an ethylene-vinyl acetate copolymer having a vinyl acetate content of more than 25% to less than 35% by weight and an organic peroxide, and there-after cross-linking the thus coated polyethylene composition.

2. The cross-linked polyethylene cable as claimed in Claim 1, wherein the vinyl acetate content of the ethylene-vinyl acetate copolymer is from 28 to 33% by weight.