Note: Descriptions are shown in the official language in which they were submitted.
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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
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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.
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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
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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.
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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.
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