Note: Descriptions are shown in the official language in which they were submitted.
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1 ~ BACKC,~OUND OF THE INVEN~'IOl~
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3 The present invention relates to a moisture-proof,
4 electrical cable for hiyh voltages.
6 Cables of the type to which the invention pertains
7 include plural conductors, preferably in a stranded configu-
8 ration and being enveloped by and in an insulating, synthetic
9 material. For many years, damages -to such a type of cable
have been observed which can be traced to the ingress
11 and accumulation of water. The water causes formation of
12 so-called water trees in -the insulation. Recently, great
13 effort has been devoted to discover details about these
14 water trees and to provide Eor adequate remedies. Many
publications exist describing the growth of these water trees,
16 and many suggestions have been made concerning their avoidance
17 to, thereby, increase the life of such type of cable.
~8
19 All approaches to solving the problem begin with
the point that the mois-ture, to do any damage, must first
21 enter into and penetrate -the insulation. Emphasis here is
22 placed on the point of preventing moisture penetration after
23 the cable has been installed, e.g., in wet soil, or the like.
24 It has -to be observed, however, -that moisture may diffuse alreaa
into the insulation during cable manufacture. For example,
26 if the insulation is cured (cross-linking, vulcanization) in
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1 ~ steam, water is indeed directly applied. Some water rnay readily
2 I be trapped, particularly if subsequen-tly a me-tal jacke-t is
3 I formed around the insulation. Moreover, such a jac]~et may
a I be damaged and water or water vapor may pene-trate direc-tly
5 I the otherwise undamaged insulation.
7 The water trees themselves are believed to be formed
8 I when the insulation is subjected to an electric field sur-
I rounding the conductor. The water trees grow until they cause
an electric breakthrough, which, in turn, destroys the cable
11 locally. The cable des-truction can be fur-thered by water
12 in the conductor core which corrodes. A]so, water in the
13 ~ interface between the conduc-tors proper and a smoo-thing la~er
14 I surrounding the conductor bundle may tend -to form an electro-
voltaic cell resulting in electrocorrosion. Particularly
16 I prone to this kind of damage are aluminum conductors sur-
17 rounded by a carbon-black-containing layer.
18
19 Numerous attempts have been made to avoid water damages
20 ¦ as described. These attempts follow a common approach; the
21 ingress of water is to be stopped. For example, United States
22 Patent 4,145,567 suggests winding a metal ribbon around the
23 insulation; German Printed Patent Applica-tion 27 54 336 pro-
2~ poses the generation of a diffusion gradient, directed in
radial outward direction; Germany Printed Paten-t Applica-
26 tion 27 37 108 proposes the use of a water-absorbing, outer
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1 i coating. All of these approaches may have merit and impede,
2 I or even prevent, the diffusion of water into the insulation
3 ¦ from the outside of the cable. They do not, however, prevent
4 I the migration of water inside the conductor bundle.
6 The migration of water in the conductor bundle is
actually a kind of conduction of water through the voids and
8 gaps between and alongside the conductors of the bundle. One
9 has stopped this conduction by applying a technique which
resembles bùlkheads. This approach has been used in particular
11 I for communication cables, but has also been applied to power
12 ¦ cables. German Printed Patent Application 21 54 749 discloses
13 I metallic, transverse walls, placed at regular intervals in
14 the cable core. German Printed Patent Application 28 08 438
discloses such "bulkheads" consisting of cured rubber-elastic
16 material. These plug-like elements are r axially, short~as
17 compared with the distance from bulkhead to bulkhead. Indeed,
18 these bulkhead-like plugs prevent axial migration of moisture
19 through the conductor bundle or core.
2~
21 However, we have discovered that water, which has
22 remained in the'cable since its production, such as conden- ¦
sated water, will remain, e.g., inbetween two plugs. ~his
24 water can penetrate the synthetic, semiconductlve smoothing
layer and will reach the insulation. After a high voltage
26 has been applied and is being applied to the cable, water trees
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will grow, quite rapidly so in the immediate vicinity of the
conductors because the electric field streng~h is quite high.
According to the present invention there is provided
a moisture-proof, electrical cable, comprising a core composed
of a plurality of stranded-together wires or conductors, there
being inherently voids between the conductors or wires, and a
metal tube made of a longitudinally folded and welded strip,
made of the same material as the wires or conductors and being
waterproof and directly disposed around the wires or conductors
in metal-to-metal contact with outer ones of the wires or
conductors so as to serve additionally as an outermost conduct-
ive layer; and an insulation of synthetic material, being prone
to exhibit watertreeing and being disposed around the metal
tube of the core, the metal tube sealing the insulation from
the interior of the core containing the stranded-together
wires or conductors.
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Application of the invention in this instance is of particular
significance ~ecause such a smoothing layer is usually made
of an e~truded, electrically conductive synthetic (polymer
with carbon black as an additive~. These types of layers
are particularly sensitive to moisture and have to be most
prominently protected to ensure trouble-free cable use.
Aside from waterproofing the insulation, this tube
has the added advantage of preventing the stranded conductors
or wires from untwisting during production. The insulation
can be protected against ingress of water from the outside
in any conventional manner; but see also Canadian Application
Serial No, 359,36~ filed August 31, 1980 of common assignee.
However, internal, bulkhead like protection may not be
needed.
The primary function of the tube as formed about
the conductor or wlre bundle is to prevent moisture which is
lodged or flows through the voids between the conductors
from penetrating the insulation to, thereby, avoid the
formation of water trees. In order to avoid the formation
of electrovoltaic cells as between wires and tubes, they
should be made of electrochemically similar material; i.e.,
they should have a substantially similar position in the
electrovoltaic series. For practical reasons, the same kind
of metal should be used; a copper tube for copper wires,
and an aluminum tube for aluminum wires (or aluminum alloy),
these being the most
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1 , commonly used metals. In the case of copper-plated aluminum
2 I wires (low weight), the tube should also be made of copper.
3 I The tightly fitting and drawn tube has also a mechanically
4 ~ stabilizing effect on -the cable, while kinking of the tube
is prevented due to its tight fit on the conductor bundle
6 serving physically as internal tube support. The -tube,
7 particularly when made of the same metal as the conductor,
8 becomes an integral part of the conducting assembly. This,
g in turn, leads to an overall reduction in cross section and
diameter of the cable which, in turn, enhances its flexibility. ¦
11 ¦ The overall layer configuration conduc-tor-insulation-shield-
12 jacket remains the same. The tube, moreover, functions already
13 to some extent as a smoothing layer, and the electric field
14 of the conductor assembly becomes more uniform as compared
to just a wire bundle. In some cases, an additional conduc-tive
1~ smoothing layer may not be needed. The invention can be
17 applied to all those cables in which the cable core is com-
1~ posed of a multitude of wires, filaments, and conductors,
19 even those with special profiles (circle segments), in which
2Cl moisture can propagate lengthwise.
21
22 It should be mentioned that encasing of wire bundles of
23 uninsulated cables is known per se (British Patent 757,745),
2~ but that aspect has no bearing upon the problem of water -trees.
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; D-6939(Kab)
l DESCRIPTION OF 'I'l~E DRAW_NGS
2 ~
3 I While the specification concludes with claims, parti-
4 ¦ cularly pointing out and distinctly claiming the subject
5 ¦ matter which is regarded as the invention, it is believed
6 ~ that the invention, the objects and features of the inven-
tion, and fur-ther objects, features and advantages thereof,
8 will be bet-ter understood from the following descri.pt.ion taken
in connection with the accompanying drawings, in which:
11 ~ Figure 1 is a cross section through a cable improved
12 in accordance with the preEerred errlbodiment of t:he inven-tion;
13 and
1~
Figures 2 and 3 are respectively cross sections through
16 differentt types of cable, also i.mproved in accordance with
1~ ¦ the preferred embodiment.
18
19 Proceeding now to the detailed description of the drawings,
the conductor core 1 in Figure 1 is comprised of a bundle
21 of stranded wires 2, e.g~, copper wires. This bundle of
22 conduc-tors 2 is jacketed in a copper -tube or sleeve 3. This
23 sleeve or tube has been made :Erom a copper strip which is
2~ folded longitudinally around the s-tranded bundle of wires or
conductors. Next, the edges of the strip have been seam-
26 welded to form a water--tight, well-sealed enclosure around
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D-6939(Kab)
1 the wires. Thereafter, the tube is drawn onto the wires
2 11 or conductors, into intimate metal-to~metal con-tact with
3 I the outer layer of conductors or wires of the stranded bundle.
The tube may also be sized to have a circular outer periphery
and contour so tha-t the effective outer contour of the con-
6 ductor assembly 2, 3 is already quite smooth, and the elec-
7 ` tric field developed by this assembly is quite uniform.
9 The tube or sleeve 3 is, thus, actually made part of
10 I the conductoI core. The strip of which the sleeve is made
11 should, for this reason, be of the same kind of metal as the
12 conductor so that, upon sitting tightly on the bundle, a
13 uniformly conductive assembly is established therewith~
14 Sleeve or tube 3 is covered by a conductive layer 4 which
serves as a supplemental smoothing layer. This is particularly
16 needed i~ the tube, as drawn, tends to match to some extent
17 the uneven outer contour of the wire bundle underneath.
18 Moreover, the layer (any layer) extruded onto tube 3 will
19 adhere better when the surface of tube ~ is not quite even.
Indents or bosses may even intentionally be provided in order
21 to enhance adhesion of any layer that is extruded thereon;
22 for this reason, a first layer should be a conductive layer
23 (e.g., layer ~) being extruded onto the tube. The water-proof
2~ and water-proofing tube 3 serves already as a protection
against ingress of water from the interior of the bundle
26 into the sensitive layer 4.
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1 I The conductor assembly is insulated by a suitable
2 ¦ layer 5, e.g., of polyethylene or the like. The tube 3
3 serves as a moisture barrier to preven-t any water that may
4 reside in the voids between the condllctors 2 of the b~lndle
from penetrating the insulation from the inside.
7 Insulation 5 is conventionally covered by an outer, con-
ductive layer 6 which, in turn, carries a shield 7 (e.g.,
9 plural wires, and so forth). The cable is protected by an
outer jac~et 8, such as a polyvinylchloride or rubber jacket,
11 possibly including flame-retarding additives.
12
13 The conductors and tubes do no-t have to be made of
1~ copper; one may use copper-plated aluminum wires wi-th a
copper -tube for reasons of weight, or one may use an all~
16 aluminum assembly. For reasons of uniform conduction
17 the metals should be the same; at least to the extent that
18 they contact each other. As far as the specific purpose
19 of the invention is concerned, the metals should be at least
electrochemically similar so that they will not undergo
21 electro--corrosion if moisture migrates through the interior
22 of the core.
23
2~ As far as the conduc-tor assembly is concerned, the
ratio of diameter d of the s-trandecl core or wire bundle
26 to -thic~ness s of tube 3 is rather cri-tical.
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l I ear1ier, the conductive tube 3 becomes a part of -the con-
2 I ductive core proper, particularly so when made of -the same
3 I material as the stranded wires 2. From tha-t point of view, I
it appears -to make li.ttle difEerence how many wires may
actually be replaced by the tube for a given total of solid
6 ¦ material and conductive cross section and cross sectional
7 ¦ area F. However, there are other points to be considered.
8 ¦ The larger the ratio d/s, the more likely it becomes that
tube 3 will kink when the cable is bent during conventional
installation. On the other hand, when the tube's wall is too
11 -thick, the cab].e will loose its flexibility and become quite
12 stiff.
13
1~ The total effec~ive cross section for this assembly is
the sum of the cross section of all conductors in the bundle
16 to which the cross section of tube 3 is added. Thus, one
17 does not diminish the conductive performance of the cable
18 by choosing the thickness s of tube 3 sufficiently thick
19 and by choosing a smaller bundle. In the case of copper,
it was found that d/s = 25, which is an optimized or nearly
21 optimized ratio. It was found, moreover, that for this relation,
22 -the tube's thickness s equals one-twentieth of the square root
23 of the total desired solid material (metal) cross sectional
24 area F available for conduction. In the case of aluminum,
the ratio should be d/s = 18, and the wall's thickness s
26 can be calculated to be equal to 17 ~F.
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Followillg these rules will resul-t in a -tube that is,
2 11 even after being drawn onto -the conductor bundle, quite
3l~ smooth,obviating thereby the need for a smoothing layer.
~¦l If, however, the insulation would not adequately adhere to
5 ¦ such a metal tube, intentional unevennesses should be provided
6 ¦ on tube 3; and that, in turn, renders advisable -the use of
7 ¦ a smoothing layer of conductive synthetics. The insula-
81~ tion (5) will adhere better to such a layer, even when
9 1! having a smoo-th surface. The larger the ratio d/s, -the more
likely it becomes that tube 3 will kink when the cable is
11 bent during conven-tional installation. On the other hand,
12 when the tube's wall is too thick, the cable will loose its
13 flexibility and become quite stiffo
1~
Figure 2 i]lustrates a modified conductor core. The
16 core is comprised here of a plurality of (four) sections or
17 segmented conductors 9 which are stranded together. Other
18 numbers, such as three or six, could be used. Each conduc- ¦
19 tox 9 may be solid or consist by itself of stranded wires.
~f the conductors 9 are made of aluminum, an aluminum tube 10
21 has been formed about the conductors, in the manner described
22 above (longitudinally folded, welded strip). This -tube 10
23 is also watertight and avoids migra-tion of water from the
24 voids between the conduc-tors 9 into an insulation layer on
26 -the outside.
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1 ll Fiyure 3 illustrates an example in which the conduc-
2 j tor core includes wires 11 stranded onto and around a support
3 I tube 12 whose in-terior is hollow. Tube 13 is provided as
a moisture barrier and wires 11 as well as -tube 13 are
made of the same material, e.g., aluminurn or copper. This
6 type of an assembly will be chosen if the skin effect causes
7 the interior of a conductor to not contribute -to -the con- ¦
8 duction. I
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10 I It should be mentioned that the invention may actually
1 render -the plugs, alluded to earlier, unnecessary, which
12 would facilitate installa-tion.
13
14 The inven-tion is no-t limited -to the embodiments
described above; but all changes and modifications thereof,
16 not constituting departures from the spirit and scope of
17 the invention, are intended to be included.
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