CA1046453A - Connection or terminal portions for cross-linked polyethylene cables - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In the connection or terminal portions of cross-linked polyethylene cables produced by winding irradiated tapes around the connection or terminal portions of cross-linked polyethylene insulated cables and heat molding the same, the irradiated tapes comprise polyethylene and/or an ethylene copolymer as a base resin plus an ethylene-vinyl acetate copolymer containing more than about 15% vinylacetate (VA) in amount which provides more than a 2.5% VA content in the irradiated tapes.

Description

; l ~

"~ 1 BACKGROUND OF THE INVENTION

-' 1. _Field of _he_Invent'ion . ' ` The present invention relates to molded connection or terminal portions of cross-linked polyethylene insulated cables.

- 2. Description of the Prior Art ~ - . .
~ To form molded connections for cross-linked poly-.,^ ,.
ethylene insulated cables, known methods generally comprise '~ subjecting a polyethylene tape to electron beam irradiation to 10 effect cross-linking, winding the irradiated polyethylene tape ~' ' ~- around a cable and then heat molding the resulting assembly.
, .; , .
''; The use of irradiated polyethylene which has been subjected to `
partial cross-linking permits satisfactory molding since no ,- eccentricity, deformation or the like is caused by heating, and the initial breaking or separation characteristics of the molds are superior as compared with the use of uncross-linked ~ - polyethylene. On the other handl complete fusion between -~' layers of the tape or between the tape and the cable insulators .~, :, . .. .
cannot be expected by this method. As a result, after long times - ' under high temperature, in particular after immersion in water for long times under an applied voltage, those portions which are not ~'i complete by fused some time produce water tree (or electrochemical ' .. . . .
trees as they are sometimes termed) to cause dielectr~c breakdown Most particularly, foreign'particles wound between ~'' layers of tape tend to form nuclei around which water ,~,, ~ . .
~ trees develop into the tapes remarkably.
~,.'~', 2`.; SUMMARY OF THE INVENTION

~ -- The object of the present invention is, therefore, to ~:..
overcome the a'bove defects and to provide good molded connection ' ~``' or terminal portions of cables.
:: .

." -, .

` ~46~53 - 1 The primary feature of the present invention lies in .
the use, as a molded dielectric layer, of a composition which comprises polyethylene and/or ethylene copolymer blended with an ethylene-vinyl acetate copolymer of more than about 15% by weight VA content the ethylene-vinyl acetate copolymer being present in such an amount as to provide a more than about 2.5% by weight VA content in the composition. The composition is shaped into a tape and then irradiated with electron beams.

. By winding the irradiated tape obtained as above around a connection or terminal portion of a cable, for example, ,.: .
~i as shown in the drawing, and heat molding, a molded dielectric ... .
-- connection layer 4 which is completely fused to cable conductor . :.
; 1, connection sleeve 2 and dielectric cross-linked polyethylene --~ layer 3 can be obtained in which no trees (such as electric . . .or water trees) are present or will develop to thereby produce a connection portion with excellent characteristics for long-.,.:~; ' term immersion in water under applied voltages.
~j DESCRIPTION OF THE ACCOM@ANYING DRAWIMG

The drawing is a schematic cross-sectional view of a ,....................................................................... .
~ 2~ connection portion of a cable according to the present invention~
....
. DETAILED DESCPIPTION OF THE INVENTION
The polyethylene used in the present invention is ~-~ not especially limited, but highly superior results are obtained `~ with polyethylene of a melt index of about 0.1 to about 40, .
preferably 1 to 2 and a density (p) of 0.91 to 0.96, preferably 0.91 to 0.94. In a similar manner, highly preferred ethylene-vinyl acetate copolymer used in the present invention have a melt -~ index of about 0.6 to about 70 and a density of 0.92 to 0.98.
- .
In the present invention, a vinyl acetate content in the ethylene-vinyl acetate copolymer is preferably about 15~ by ~ . .
~ weight to about 50% by weight and the vinyl acetate content in ,.." .:
.. .:
-~ - 2 -',.'s ~

.:, : .
.~- ~. ..
t, . ;.

~ 6~S3 ` 1 the finc~ blend is preferably about 2.5% by weight to about 13%
; by weight. The ratio of ethylene-vinyl acetate copolymer to polyethylene (EVA/PE) is preferably about 1/20 to about 6/1 on weight basis.
`~ The term "ethylene copolymer" as used herein as a base .. ...
~ resin means an ethylene copolymer comprising predominantly the .. ~ .
ethylene component, for example, more than 95% by weight ethylene, such as an ethylene-propylene copolymer, an ethylene-.
i. ~ acrylate copolymer, eg. ethylene-methyl acrylate copolymer, . :..
`~ 10 ethylene-ethyl acrylate copolymer or the like, an ethylene-.....
`;~ styrene copolymer and the like preferably an ethylene-acrylic ,,~,, . ' .
;~ acid copolymer.
There is no special limitation on tha comonomer used to form a copolymer with ethylene when high ethylene percentages are used, e.g., more than about 95~ by weight ethylene.
The composition of this invention can be compounded in ~-~ a conventional procedure, for example, using rolls which are ~-; well known in the art.
~~ The tape produced in accordance with the present 20 invention for most commonly encountered commercial applications ' generally has a thickness of about 0.1 to 10 mm, prefer~bly 0.1 to 0.2 mm. The tape is cross-linked by irradiation with, for example, electron beams at an intensity of about 1 to about 10 ;- ,, " , i` Mrad preferably 1 to 2 Mrad, to a degree of gelation of about 20 to about 50~.
... ~ ~, .
'i The resulting tape is then wound around a cable and -, ,. ;
~- can be heat molded in a conventional manner, for example, at a temperature of about 120C to about 200C for a period of =~ about 2 to about 8 hours under a pressure of more than about ~-; 30 3 ~g/cm . For example, in XLPE 3-6 KV cables, heat molding " ,;;::
~ is effected at a temperature of 130C for 2 hours and at a . . :;
.,~,r . ., 3 , .,~

"t';' ' `
,, ~''."' .: :'' ', ~ ' , . '' .' ' ' . ' ; 1 temperature of 200C for 4 hours under a pressure of 4.6 Kg/cm2.
PREFERRED EMBODIMENTS OF THE INVENTION
~-~`;; The present invention will now be described in detail by .; - .
~"- way of preferred embodiments thereof and with reference to several .~; comparison examples. All percentages in Table 1 are by weight.
c~' ~ ., .
,~ ; Examples 1 - 11 ....~
. . .
~- Polyethylene and ethylene-vinyl acetate copolymer as shown in Table 1 were compounded in the ratio specified in Table 1 using a roll at a temperature of 120C for 20 minutes ;:
and formed into tapes having a thickness of 0.2 mm and a width of ~;; 25 mm in a conventional manner. The resultant tapes were irradi-, ated with electron beams to a degree of gelation of 30~ in a con-ventional manner, wound around 3.3 Kv cables of a 100 mm2 diameter insulated with cross-linked polyethylene and molded under heating .,~ ~, ............... . .
at 130C for 2 hours plus at 200C for 4 hours u~der a pressure ~! of 3.4 Kg/cm2 to form connection portions having a total thickness (overall dimensions of the layer of one tape on the conductor) of 3~2 mm.
, ~ .
~ The cables were then immersed in water at normal ;,.................... .
i - ~ temperature for 1000 hours under an applied voltage of 60 Kv.

The cables were then disassembled, and thereafter the development ~-~ of trees examined, the results of which are shown in Table 1 ~" Compar~son Examples 1, 2 and 5.

The connection portions ~ cables were prepared in the ,;; same manner as in Example 1, except that polyethylene alone was `,~ substituted for the mixture of polyethylene and the ethylene-. -~ vinyl acetate copolymer as shown in Table 1, and the character-....
;,` istics of the resultant connection portions were measured as .~, -~, for Examples 1-11. The results are shown in Table 1.

~`.' .'':i ' ;, :s, ,. ..

,,~ .................. .
~,.~.,.~ , ,~; ~ . , ~'` l Comparison Examples 3 and 4 The connection portions of cables were prepared in the same manner as in the Example l, except that an ethylene-vinyl acetate copolymer alone was substituted for the mixture .: .
of polyethylene and ethylene-vinyl acetate copolymer as shown in Table 1, and the characteristics of the connection portions ' . :
;~ measured, the result of which are shown in Table l.
. ..................................................................... . .
.
.. .

, 10 ,, " . .

,.:.................................................................... : ' ,.~: , .

.
.', ` .

~' ~
,~. .

~ 20 :: :
~" :
~ ~ .
.. . . . .

, ~:
;.. .

, ~, .
: ,.,-: ~
.... ~,. ..
, .. . . . .

;,,,~. :
~,',,', .' . ' .
.: :~
.,;. ~ :

- 4a -,, .
,: .
~, ;,., :~ ~ ., .
.,;,., ,..

` ~046~S3 .''.' 1 ~D
. ,;. ~ ~
,: Q) a) x x o o o o o o o o X a - :. ~ a) .,,............ ~
:,: o '"; o ~
.. ~ ~ ~ _ O O O O O O o O O O o O
J
. ,, ; ~
.' :~.:
~ 11') 0 It') O O c:~O
.-. .i ~c i_ ~ o\O
'.;~ I¢ O rl ~ ~ u7 0 ~ ~ 1~ O~ ~ O 11 .~
t~ ' '.

.j,,........... O O O O O O OO
.¢ ~1 ~ ~ U~ ~ ~ N t~l w a) ~ O O O O O O Oo ~ W
~";',' ,~ .

'.'':~ . ' .,. g m u~
. . ~1 ~ ~ cs~ o a~ o ~ o cn ,,," ~ 4-~ rr) . t~ O
X-~l O 11 o 11 o11 o 11 o li c~
~ ~ ~ ~ _ H 11 H 11 H 11 H 11 H 11 ., ......... . ~a) ~ a~ a . ' . 1~ ~ H C~
~r 2 0 o\~ d1' oP oP oP
1~ 0 Il') L('~
N ~ ~ ~r .. ~ . o ~ O - _ =
. r~ . ~ g ~

~ -. . O ~) ;~-.,~ -' r i O ~YO N o\ o\
11 o a~ a) o It~
::'.'?.. ' Ul Ul~t IC ¦¦ O ¦¦ O
,~ .;",. "1~a)~-1 _ - -- - :
m ~ ~H 11 ,H 11 H 11 j¢ ~ g ~
~, '- p~ ~ Q p~ $ Q 14 ~ ~ >
N ~ N ~ ~r U~ ~D r o~
.;..."
:.`',;,: o - O
',, ~'': In m ~I h ~ r ~,~ 30 3~ w ,..;. ,.~,.. .
, ..
., ...;
... ,.,.. ~ , ......

~, ....:, ~ ..-:
., .
. ., ::
;, '"'.:~
~ ~:
, .: .,.:
i . . . . . , , ' . .
.: ` .............. . .

.~

. .
.
.- a) a `............... ~ ~ o o o ~C .
..,:.............. ~i~ . -.
. ; ~ . . .
,~.................. ~
~ ~ o .~ g ~
.;. ~
. . ~ ~ ~ -- o o o o U~ ~ ~
.~. .
' ..
~' .. n O In o o ~

3 ~ A I` U) U~
'i. .. ' 1 0 ~ O
.. . ~ , O .
.. " ~) l'C o o o r`
~: ~ ~ ~ K a) , ., ;`.`: ~ O ~ o o o I Q
., ~ ~ O ~ H ,1 ;3.''.' ,~ ~) ~ '' ' ,;,~,, ~ O '~
~.,, ~ ~

~ o~ O O ,-1 X 'rl lH ~ U ~
a~ u~ o o o o - ~ O ,_ ` ' ~ . m ,,~ ~:^ . . . i ~ O A
a H il H 11 H D ~d IU
~ Q ~ Q ~ Q

~, 20 ~

a) 0~o 0~o A a~ l: $ ~1 U~
..... ~ - ~ ~ ~ t`~ N ~ X P ~
~ . ~ H ~) S~ o ~: O O O ~O O aJ
.;-...................... o\ . ~ n h .-`,". U~ ~q 11 . o . o . . - Sl ~ o CC H H H o~ E~ i~ Z
iQ- H ~ ~
O X ~ O
O ~
O
~`,'j` ~ a) :

~' ;~
~s"~;
.~, "~, .I',ii~, ji /'`'i ~.i' `~ . ' . ' ' ' '. . ~

:
~ ~09~69~53 ~ 1 The connection portions produced as in Example 1 and ::~ as in Comparison Example 1 were lmmersed in water at 70C and ~ . , a voltage of 100 Kv applied thereto, and the time required for . breakdown observed. While the connection portions of Comparison xample 1 showed dielectric breakdown in 334 hours, the connec- -~- tion portions of Example 1 withstood the indicated voltage for . more than 1,000 hours.

- As is apparent from the foregoing results, while compo-: sition containing only polyethylene or only an ethylene-vinyl acetate copolymer provide unsatisfactory characteristics, those .. containing a blend of polyethylene and an ethylene-vinyl acetate - copolymer within the present invention pro~ide highly preferred results.

- A VA content in the ethylene-vinyl acetate copolymer : less than 15% does not provide the advantageous effects of . the present invention; the VA content preferably ranges between : .:
:~ about 20 and about 40%.

. .: , ,. . .
.~ The proportion of the ethylene~vinyl acetate to the .: polyethylene should not be too low in view of the VA content '~ ~o required to attain the objects of this invention, and the VA
.. content in the composition should be at least 2.5%, preferably, .. in the range of from 2.5 to 20% and most preferably from 10-12.5~.
Within the VA content range specified above, satisfactory cable connection or terminal portions having excellent electric characteristics as well as being free of the development of ., .-trees can be obtained.
" -, ,~............................................. . .
'.''`-,, .
.-. 30 - 7 -~,-, . , . ,~ .

,',.:~ ;;, ;`, .

~ .
:` ~04~i~S3 ,.. ,. ~
As is apparent from Comparison Examples 3 and 4, the '~ use of ethylene-vinyl acetate copolymer alone shows substantially no effect even with the same VA content as in the compositions - of the present invention comprising two components, and, - therefore, an ethylene-vinyl acetate copolymer of a higher VA conten-t is required to o~tai~ a similar effect. A higher - VA content, however, degrades other electrical properties of the connection portions such as dielectric tan ~, inpulse withstanding voltage and the like, rendering them impractical ... ,~ 10 ; for commercial use.
:
:.
. It is clear from Table l that the gelation rate has ~- no substantial effect on the characteristics, and further that high density polyethylene and ethylene-vinyl , acetate copolymers as well as low density polyethylene can be ,'i,f.`. used as the base resin.
.. :~. - ~ , Ethylene copolymers that can also be used in this invention as a substitute for polyethylene include ethylene-.~,................................................. .
acrylate copolymers, ethylene-propylene copolymers, ethylene-.. . .
styrene graft copolymers and the like.
-: 20 As will be apparent to those skilled in the art, the compositions used to form the tape can contain, if desired, :'~
- anti-oxidants such as 4,4'-thiobis(6-tert-butyl-m-cresol), .,.;............... . .
~ stabilizers, cross-linking agents, fillers, and the like, i; -,. .
~;~ generally in an amount of 0.1 to 0.~% by weight depending upon ,. ~,-....
- the characteristics desired.
;.-~.~ .
Although the foregoing description has been made for application to cable connection portions, it should be under-~. .
stood that the present invention can also be applied to the terminal portions of cables.

. i ~ - 8 -~. ' ". ~
~,. ,, ~
. . ............... .
~1'''`'.'' ;'z,,' i. ,: . .
: .~
~, .

~0~L6~3 .. ,~ ~.
.`...;
1 While the invention has been described in detail and `. with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and . ~ modifications can be made therei.n without departing from the spirit and scope thereof.

,...... .
: .
;'-'.;, .
,...... .
.~ 10 ., .
- . .
:x, .. '~
~;
.", ~ .
. ~ .
; .
. ,.
~.
. 20 ,, :.
.....
: .:
. . .

., :..
,................................................. .
. .
~.,`-.
, ; ~ :
:
. .". .
.j,~:, .
:, .~. -.~~ 30 ,.,, _ g _ i .
.~,.
.;
;. . :

, .
-~;, .`. . . .
., ,.. .. . . . , . , . ~ , .. .

Claims (8)

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

1. In the connection or terminal portions of cross-linked polyethylene-insulated cables produced by winding a tape around the connection or terminal portions of the cables and heat-molding the same, the improvement wherein said tapes comprise a blend of polyethylene and/or an ethylene copolymer plus an ethylene-vinyl acetate copolymer having a vinyl acetate of content of more than about 15% present in an amount as to provide a vinyl acetate content of more than about 2.5% in said tape, which tape has been electron beam irradiated to a degree sufficient to inhibit the formation of water trees.

2. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said blend comprises polyethylene and/or an etheylene copolymer having a melt index of about 0.1 to about 40 and a density of 0.91 to 0.98 plus an ethylene-vinyl acetate copolymer having a vinyl acetate content of about 15% to about 50% present in an amount as to provide a vinyl acetate content of about 2.5% to about 13% in said tapes, and said tape has a thickness of about 0.1 to about 1.0 mm and has been irradiated with electron beams at about 1 Mrad to about 10 Mrad.

3. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 2, wherein the wound tape is heat molded at a temperature of about 120°C to about 270°C.

4. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said ethylene-vinyl acetate copolymer has a melt index of about 0.6 to about 70 and a density of 0.92 to 0.98.

5. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 4, wherein the ratio of ethylene-vinyl acetate copolymer to polyethylene is from about 1/20 to about 6/1, weight basis.

6. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said ethylene copolymer comprises more than about 95% by weight ethylene.

7. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 7, wherein said ethylene copolymer is selected from the group consisting of ethylene-propylene copolymers, ethylene-methyl acrylate copolymers, ethylene-ethyl acrylate copolymers, ethylene-styrene copolymers and ethylene-acrylic acid copolymers.

8. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said tape has a degree of gelation of from about 20 to about 50