ELECTRIC CABLES

CABLES ELECTRIQUES

English Abstract

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ABSTRACT
ELECTRIC CABLES

An electric cable having a current carrying
conductor provided with an extruded polymeric insulating
layer, has a layer of a high permittivity material
extruded over the whole length of the insulating layer,
which layer is covered with semi-conducting tape in
contact therewith.

Claims

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

1. An electric cable having at least one current
carrying conductor provided with an extruded polymer
insulating layer, over the whole length of which is an
extruded layer of material having a relative permittivity
of at least 15, said latter layer being covered with
semi-conducting tape in contact therewith.

2. A cable as claimed in claim 1, wherein said semi-
conducting tape includes a desiccant.

3. A cable as claimed in claim 1, wherein said relative
permittivity is at least 20.

4. A cable as claimed in claim 3, wherein said semi-
conducting tape includes a desiccant.

5. A cable as claimed in claim 1, wherein said
polymeric insulating layer is extruded on to a layer of
semi-conducting material which is extruded over said at
least one current carrying conductor.

6. A cable as claimed in any one of claims 2, 3 and 4,
wherein said polymeric insulating layer is extruded onto
a layer of semi-conducting material which is extruded
over said at least one current carrying conductor.

7. A method of making a joint or termination for a
cable as claimed in claim 1, including stripping back the
semi-conducting tape over said layer of material having a
relative permittivity of at least 15 along an end portion
of the cable but not stripping said latter layer from the
insulation layer of the cable.




8. A method of making a joint or termination for a
cable as claimed in claim 2, including stripping back the
semi-conducting tape over said layer of material having a
relative permittivity of at least 15 along an end portion
of the cable but not stripping said latter layer from the
insulation layer of the cable.

9. A method of making a joint or termination for a
cable as claimed in claim 3, including stripping back the
semi-conducting tape over said layer of material having a
relative permittivity of at least 15 along an end portion
of the cable but not stripping said latter layer from the
insulation layer of the cable.

10. A method of making a joint or termination for a
cable as claimed in claim 4, including stripping back the
semi-conducting tape over said layer of material having a
relative permittivity of at least 15 along an end portion
of the cable but not stripping said latter layer from the
insulation layer of the cable.

11. A method of making a joint or termination for a
cable as claimed in claim 5, including stripping back the
semi-conducting tape over said layer of material having a
relative permittivity of at least 15 along an end portion
of the cable but not stripping said latter layer from the
insulation layer of the cable.

12. A method of making a joint or termination for a
cable as claimed in claim 2, 3 or 4, including stripping
back the semi-conducting tape over said layer of material
having a relative permittivity of at least 15 along an
end portion of the cable but not stripping said latter
layer from the insulation layer of the cable, and
wherein said polymeric insulating layer is extruded onto




a layer of semi-conducting material which is extruded
over said at least one current carrying conductor.

Description

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ELECTRIC CABLES
This in~ention relates to electric cables, and
more particularly to electric cables in which the
current carrying conductors are provided with extruded
polymeric insulating layers.
In known high voltage cables in which the current
carrying conductors are provided with extruded polymeric
insulation, a layer of polymeric insulation is disposed
over the conductor with a first semi-conducting layer
between the conductor and insulation and a second semi-
conducting layer over the insulation, and a conducting
sheath is provided over the second semi-conducting layer
to provide an earth screen. It is conventional to co-
extrude the semi-conducting layers with the insulating
layer whereby the semi-conducting layers are bonded to
the insulating layer. The purpose of the semi- conduct-
ing layer is to prevent electrical discharges occurring
due to voids bein present between the inner surface of
the insulating layer and the conductor and between the
outer surface of the insulating layer and the conducting
sheath-
The semi-conducting layer over the insulating
layer has to be removed during jointing or termination
of the cable in the region of the joint or termination
as it normally has a resistance such that it would carry
unacceptable high currents if it were allvwed to form a

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path bet~en the ~rrent ~arr~in~ conductor and earth
in the joint or t~rmination. Remo a~ of the ~emi-
condurtin~ ~a~er o~er the insu]atin~ ]a~r ~herefrom is
a diffjcu]t operation ~hich in ol~es th~ ris~ of damage
b~ing caused to the insu~ation ]a~er.
~ o a~oid this prob]em it has been prop~sed to
pro~id~ o ~r the ~ho~e length of the insu~ation la~er a
~a~er of hi~hl~ resisti e materia] ~lose resistance is
~o~ta~e depend~nt instead of a se~i-conductin~ ~ayer
'~ ~ince ~u~h a ~o~ta~e dependent resisti~ la~er does not
ha~e to be remo~ed d~rin~ jointin~ or termination.
Ho~ er this soluti~n suffers from the disad antage
that material ~ith the appropriate ~oltage dependent re-
sisti~it~ is not suitab~e for application by con ention-
'S a~ extrusion equipment.
~ he present in ention seeks an a3ternati~e solu-
tion in ~hich the semi-conducting la~er o er the insu~-
ating ~a~er is rep~aced b~ an alternati-~e la~er hhich
can be readil~ extruded o er the insulating la~er but
hich does not ha-e t-o be remo ed during jointing or
terminating.
~ he pres~nt in~entio~ provides an electric cable
ha ing at least one current carr~in~ conductor pro~ided
hith an extrud~d pol~meric insulating la~er o~er the
2~ ~hole ~en~th of hhich is an extruded layer. of materia~
having a re]at.ive permitt.ivity of at least 15 said ~atter
~ayer heing co~ered with semi-conducting tape in
contact. there~ith.
The present inverl~ion in another aspect resides
3 in a meth~ ~r making a joint or termination for a cable
as described above including stripping back the semi-
conducting tape over said layer of material having a
relative permittivity of at least 15 along an end portion
of the cable but not stripping said latter layer from the
insulation layer of the cable.

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ln order that the invention may be well understood
an embodimen~ t~hereof, hhich is given by h~ay of example
only,h~ no~ be described ~ith reference to the accom-
pan~ing drawing in ~hich the single figure schematica]ly
shohs in radial cross-section a high ~oltage electric
cable. Referring to the figure, the high voltage
~lectric cable ~ has a sing~e current carrying, stranded




.

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conduct.or 2. A ~ayer 3 of semi-cond~cting material i5 ex-
truded o~er the conductor Z. A layer 4 of polymeric in-
sulating material is extruded over the layer 3 of semi-
conducting material and a layer 5 of high permittivity mat-
5 erial, that is a material with a relative permittivity ofat least. 15 and preferably for example 20 to 30, or even
more, is extruded over the layer 4 of insulating material.
All three extruded layers 3,4 and 5 extend the whole length
of the conduct.or 2 and t.he insulating layer 4 is bonded to
1~ the layers 3 and j on each side of it. The three layers
3,4 and 5 may be co-extruded or extruded in separate oper-
ations on t.he cable length.
In order to fulfil its functions of preventing elec-
trical discharges the high permittivity layer 5 is covered
~5 with semi-conducting tape 6 which is lapped or longitudin-
ally wrapped around the high permittivity layer 5 in con-
tact with the outer surface thereof. The semi-conducting
tape 6 may include a desi~cant. Whilst it is preferable
that the entire surface of the high permittivity layer be
~0 contacted by the semi-conducting tape small breaks in
such contact will not give rise to electrical discharges
because of the high permittivity of the layer. A convent-
ional metallic earth screen 7 is provided over the semi-
conducting tape and an outer plastics sheath 8 is provided
over the screen 7.
~ 'hen making a Joint or termination the semi-conduc-
ting tape 6 over the high permittivity layer 5 is
stripped back by simple unwinding or unwrapping thereof
to the termination of the earth screen 7, but since the
high permittivity layer 5 will carry negligble current if
left in a position at a joint or termination, it does not
have to be stripped from the insulation layer 4. This not
only simplifies jointing and terminating operations and
eliminates the possibility of damage to the insulation
layer's outer surface but also assists stress distribution

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within the joint or termination as the high permittivity
layer tends t,o reduce the surface gradient at the earth
screen terminat,ion.
An example of a suitable high permittivity material
j is nitrile rubber containing anatase.

Representative Drawing