JOINTING ELECTRIC CABLES WITH A MECHANICALLY RIGID SET COMPOUND

ABOUTEMENT DE CABLES ELECTRIQUES A L'AIDE D'UNE COMPOSITION DURCISSABLE MECANIQUEMENT RIGIDE SOUS LOGEMENT

English Abstract

ABSTRACT

A cable joint comprises an inner casing
enclosing the jointed conductors of the cables being
jointed, this inner casing being filled with a com-
pound serving as electrical insulation and an outer
casing enclosing the inner casing and filled with a
set compound serving as a mechanical support for the
jointed conductors. Thus, the compound filling the
inner casing is chosen purely for its electrical pro-
perties and provides the electrical insulation of the
joint and the compound filling the outer casing is
chosen purely for its mechanical properties and prov-
ides the mechanical strength of the joint.

Claims

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

1. A cable joint comprising an inner casing enclosing
the jointed conductors of the cables being jointed, this inner
casing being filled with a compound serving as electrical insula-
tion, and an outer casing enclosing the inner casing and filled with
a set compound serving as a mechanical support for the jointed
conductors, the compound filling the inner casing having better
electrically insulating properties than the compound filling the
outer casing.
2. A cable joint as claimed in claim 1, in which the
compound filling the inner casing comprises a flexible poly-
urethane resin.
3. A cable joint as claimed in claim 1 or 2, in which
the compound filling the outer casing comprises a rigid silica-
filled polyester resin.
4. A cable joint as claimed in claim 1 or 2, in which
the compound filling the outer casing comprises a rigid silica-
filled polyurethane resin.
5. A cable joint as claimed in claim 1 in which the inner
casing is electrically conductive and is electrically connected to
metal sheaths or screens of the respective cables.
6. A cable joint as claimed in Claim 5, in which the
inner casing includes a pair of discs, closing the opposite ends
of the inner casing, which discs are provided with apertures
through which the respective cable conductors extend, the discs
being flexible so as to seal around the conductors against loss of
compound from the inner casing.




7. A cable joint as claimed in claim 6, in which
said discs are formed of semi-conducting material.
8. A cable joint as claimed in claim 7, in which said
discs are formed of carbon-loaded EPR.
9. A cable joint as claimed in Claim 6, 7 or 8
in which said inner casing further includes a metal sleeve and a
pair of semi-conducting rubber shrouds, each shroud embracing, at
its opposite ends, the metal sleeve and a respective said disc.
10. A cable joint as claimed in Claim 6, 7 or 8 in which
said discs embrace individual screening layers of the cable con-
ductors.
11. A cable joint as claimed in Claim 1, 2 or 6 in which
the outer casing comprises a vacuum-formed plastics sheet.

Description

\
This inventi~n relates to jointing ~lectric cables.
A method used in the jointing of low and high voltage elec-
tric cables i~ to enclo6e the jointed conductors in a casing or
shell and then pour a setting compound into the casing so as to
embed the jointed conductors. This compound, when set, serves
as the primary electrical insulation, but in addition it serves
as a rigid support for the cables cores, ais protection against
mechanical damage and as a barrier against the entry of water
into the joint. ~esearch has shown that a rigid compound which
willfully satisfy th~ important mechanical requirements is un-
likely to have an electrical performa~ce adequate for the joint-
~ng of high voltage cables rated at 7.2 kV and abo~e. In par-
ticular it has been found that the inability of rigid thermo-
setting compounds ( for exa~ple rigid silica-filled acrylic
resins)to deform in sympathy with the expansion and contraction
of polymeric core insulation during conductor load cycling can
lead to a path of electrical weakness along the core surface
between the conductor connector and the terminated core screen
at earth potential.
This invention provides a cable joint comprising an inner
casing enclosing the jointed conductors of the cables being
jointed, this inner casing being filled with a compound serving
as electrical insulation, and an outer casing enclosing the iDner
caising and filled with a set compound serving as ~ mechanical
support and Protection for the jointed conductors, the compound
filling the inner casing having better electrically insulating
properties than the compound filling the outer casing.



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An embodiment of the invention will now
be described, by way of example only, with reference
to the accompanying drawings in which:
Figure 1 is a longitudinal section through
a straight joint between two high voltage cables; and
Figure 2 is a cross-section through the
straight joint shown in Figure 1.
Referring to the drawings, there is shown
a straight joint between two three-core high voltage
cables. The individual conductors are insulated with
extruded polymeric insulation, e.g. ethylene-propylene
rubber (EPR) or cross-linked polyethylene, shown at 1.
Respective core conductors of the two cables are
jointed by three connectors, two of which are ~hown
in Figure 1 at 2. Individual screening layers for
the conductors are shown at 3.
The joint comprises an inner casing ~0
which encloses the Jointed conductors and which is

filled, by pouring through the inlet 11, with a
compound whose function is solely to provide
adequate electrical insulation over the conductor
connectors and the lengths of extruded conductor
insulation from which the cable screens have been
stripped. The inlet 11 is closed, as shown, after
the inner casing has been filled.
The inner casing includes a metal (e.g.
aluminiu~ sleeve 12 which is maintained at earth
potential b~ connections 13 to the metal sheaths
or screen~ of the cables themselves. Each end of the
3 inner casing is closed by a flexible moulded di~c 14

provided with three circular apertures for the
core conductors, holding the three cores at their
appropriate spacings. The disc 14 is moulded in
a semi-conducting material such as carbon~loaded
EPR and is made sufficiently flexible to fit over
non-circular (e.g. sectoral) cores and in any event
form a seal against loss of compound from the inner
casing. This sealing can be assisted by a clip or
binding around the periphery of the disc. Each
such disc provides electric stre~s relief at the
ends of the core screens by being positioned to embrace
these screens at their ends and by being contoured
as shown around the core apertures on its inwardly- -
facing side.
The inner casing i~ completed at each
end by a semi-conducting rubber shroud 15 which
embraces and seals onto both the sleeve 12 and disc
14.
The joint further comprises an outer casing
20, which may comprise a vacuum-formed plastics sheet.
This outer casing is filled, by pouring through a
subsequently-closed inlet 21, with a compound which
sets rigid and which is chosen for itq ability to
provide mechanical support and protection to the
jointed conductors. There i~ no electrical requirement
for this compound since the electric field i9
confined within the inner casing and within the
core screens 3.
A core spreader element 22 is placed between
the cores to maintain the correct spacings between the



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three phases and also to support $he metal sleeve 12. The
joint in accordance with the invention, whether for 3 core
or single core cables, allows substantial cost savings over
conventional joints using solidified resin as filling
compound. This is because the volume of electrical grade
compound is kept to a minimum (approximately 30% of the
total volume of the 3 core joint) and because neither
compound has to satisfy both electrical and mechanical
requirements.
By way of example, the compound filling the inner
casing may comprise a flexible polyurethane resin, and the
compound filling the outer casing may comprise a rigid
silica-filled polyester or polyurethane resin.




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