Note: Descriptions are shown in the official language in which they were submitted.
37
DIRECT CURRENT ELECTRIC CA~LE~ HAVING A
COMPOUND-IMPREGNATED INSULATION
The present invention relates to direct current electric
cables having insulation impregnated with a compound and without
pressurized gas being present.
The electric cables of such type and according to the
invention comprise a conductor which is surrounded by a
stratiEied insulation ~ormed by the winding of tapes oE
insulating material. The stratified insulation is impregnated
with a viscous type of compound. A metallic sheath surrounds the
stratified insulation impregnated with said compound, and a
reinforcing structure, which is practically inextensible in the
radial sense and which is formed by windings of tapes of a
metallic material, surrounds the sheath.
The known D.C. electric cables of the type briefly described
are subjected, during use, to perforation risks due to the
presence in the stratified insulation of micro-cavities
containing low pressure gas which form during cable construction
and which, during use, continue to change their position and
their dimensions.
These micro-cavities are inevitably formed during the cable
construction because it is practically impossible to have a
complete impregnation of the strati~ied insulation with a
compound. In fact, the maximum impregnatlon that can be realized
is 99% in the known cables of the type being considered.
When a cable is functioning, the micro-cavities continue to
change their position and their dimensions, or the reasons set
forth hereinafter.
Dueing cable operation, the cable undergoes thermal cycles
of heating and cooling.
During heating cycles, the compound impregnating the
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stratified insulation of the cable becomes reduced in viscosity
and become subjected to a greater thermal expansion as compared
to that of the other cable components. The consequent volume
increase of the compound in the cable causes the micro-cavities
which were formed in the cable during cable construction, to
disappear.
During the cooling cycles, said micro-cavities reappear with
changes in their position and their dimensions due to the volume
contraction undergone by the compound.
It is known that any micro-cavities in a D.C. cable which is
impregnated with a compound are dangerous since they provide
places for electrical discharges which can give rise to
electrical perforations in the cable. Also, it is known that
greater perforation risks are to be encountered during the
cooling cycles when, due to the efEect of the contraction of the
compound, these micro-cavities re-form in the stratified
insulation of the cable.
For solving the problem of perforation risks which exists in
the cables having insulation impregnated with a compound, it has
been proposed to have said insulation aided by a high pressure
gas introduced into the cable and more precisely, a gas having a
pressure of not less than 1~ bar.
To date, this known solution has been considered to be the
best solution possible for facing the problem constituted by the
risk of perforations, but this solution is quite unsatiseactory
due to the Eact that it has the drawbacks mentioned hereinafter.
In the first place, the construction Oe the cable is rendered
complex due to the need for the presence of tanks holding
pressurized gas near the cable and for connections between these
tanks and cable to provide the pressurized gas in the cable.
Another drawback of the known solution is the considerable
limitation of the maximum length permissible for the cable.
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In fact, the known cables which have the insulation thereoe
fully impregnated with a compound (known as "mass impregnated
cables") and which have gas under pressure therein cannot be
produced in lengths greater than 5 Km in order to keep the
pressure losses of said gas within acceptable values during
the movement oE the gas along the cable.
Such limitations of length represent a very serious drawback
because, practically speaking, this Eact prevents the utilizing
oE such cables in the submarine field where very long cable
lengths are usually required.
One object of this invention is to provide D.C. cables which
have insulation impregnated with a compound in which the risks of
perforations occurring due to the presence of micro-cavities are
eliminated, which are not subject to limitations as to the
lengths of the cables and for which there is no need to provide
complicated cable constructions.
In accordance with the preferred embodiment, a D.C.
electric cable comprising a~ least one conductor, a stratified
insulation disposed around the conductor and impregnated with a
compound, a radially outermost metallic sheath and a reinforcing
structure formed by the winding oE at least one tape which
completely surrounds the sheath is characterized by the e act that
the said reinEorcing structure is subjected to a tension
sufficient to reduce the diameter of the sheath with the tapes
forming said reinorcing structure presenting mechanical
hysteretical cycles closed upon themselves, the diE~erence
between the maximum and minimum per cent deformation in said
cycles being in the range from 0.3% to 0.5%.
Other objects and advantages oE the present invention will
be apparent from the following detailed description of the
presently preferred embodiments thereoe, which description should
be considered in conjunction with the accompanying drawings in
~673~7
which:
Fig~ 1 is a perspective view oE a cable length,
according to the invention, with parts removed for
better showing its structure; and
Fig. 2 is a graph of a mechanical hysteretical
cycle, closed upon itself, showing the relation between
tension Eorces and the per cent deformations.
In Fig. 1, there is shown a cable having a diameter in the
range from 50 mm to 80 mm. The conductor 1 has a cross-section
in the range from 400 to 1200 mm2, and it is ormed by a
plurality of wires made, for example, of copper, layed-up
together, and surrounded by a semi-conductive screen 2 which
comprises at least one winding of a semi~conductive tape.
Around the semi-conductive tape 2, there is a stratified
insulation 3 impregnated with an insulating compound. The said
insulating compound with which the said stratified insulation is
impregnated can be any known compound of the type used for
impregnating cables. Said compound has a viscosity, at room
temperature (20C), of over 1000 cSt. and preferably, less than
50,000 cSt.
The stratified insulation 3 is covered outwardly by a semi-
conductive screen 4 having, Eor example, a structure identical to
the semi-conductive screen 2.
A metallic sheath 5 made, for example, oE lead or lead
alloy, adheres to the radially outer surface of the semi-
conductive screen or layer 4.
A reinforcing structure 6, the characteristics oE which Eor
the purpose oE the present invention will be given hereinafter,
entirely surrounds the metallic sheath 5.
In the cable shown in Fig. 1, the reinforcing structure 6 is
formed by helicoidally winding a single tape 7, but Eor forming said
reinforcing structure, several tapes can be used, either placed
i2~1~;737
adjacent to one another, or one overlapping the next. In the
latter case, the tapes can also have opposite winding directions.
The characteristics of the reinEorcing structure 6 Oe a
cable according to the invention, are as described hereinafter.
A Eirst characteristic is that the reinforcing structure 6
is tightened on the sheath 5 while being subjected to a tension
which causes as reduction of the diameter of said sheath 5 and
which exerts a compression orce upon the outermost layers of the
stratified insulation with an accompanying transmission of a
hydrostatic pressure to the impregnating compound. In this way,
with a compound having a viscosity of over 1000 cSt., the full
impregnation o the cable is had with a complete elimination of
the micro-cavities existing therein and without causing any
appreciable movements of the compound in the longitudinal
direction of the cable.
A second characteristic of the reinforcing structure 6 of a
cable according to the invention is that the component tapes must
have mechanical hysteretical cycles which are closed upon
themselves, like the one shown in Fig. 2 (and also defined
urther on in the text), wherein the difference between the
maximum a and minimum b per cent deformations is in the range
from 0.3% to 0.5%. The range o~ values just given corresponds
with the per cent variation o the outer circumference o~ the
sheath 5 which can be caused by the thermal expansion oE the
compound between the minimum and the maximum temperatures to
which the D.C. cable will be subjected.
As a consequence of the second characteristic, it results
that the full impregnation of the cable is maintained during the
use of the cable and hence, in the thermal cycles which occur
during cable operation, the ormation of any micro-cavities, is
practically prevented.
In this text, by the term "closed", as it re~Eers to
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"mechanical hysteretical cycles", is meant the curves drawn and
representing the tractional forces ~ and the per cent
deformations by means of an extensometer device, according to
the Standards ASTM E/83, such as, Eor example, an extensometer
INSTROM 2630036 which is used, in a manner known to those skilled
in the art, on a test-sample of tape which is subjected to
increasing and decreasing loads within a range of predetermined
deEormations while recording the data obtained.
Moreover, in this text, by the term "closed", as it refers
to "mechanical hysteretical cycles" is meant that in the graph of
tension e orces and per cent deformations, a curve "closed upon
itself" can be identiEied, such a graph being shown in Fig. 2
wherein both, for the forces and the deeormations, there exist a
maximum point c and a minimum point d, both said points remaining
substantially fixed, with the cycles passing throu~h these
points, repeatedly.
Tapes which permit realizing the cables according to the
invention, are nickel-chrome steel tapes, aromatic polyamide
tapes, fiber glass tapes, and polyester tapes. Other tapes which
are suitable are carbon steel tapes ~or tapes oE other metals,
the mechanical characteristics Oe which are comparable with those
oE carbon steel tapes) provided either with micro-undulations
disposed in the direction transverse to the tape, with micro-
drawings disposed in honeycomb Eashion or with rhombus shaped
slottings which are adapted for conferring on them elastic
elongations under tensile stresses which are higher than those
due to the material forming the tapes themselves.
The experimental tests to be described hereinaEter prove
that with cables according to this invention, any perforation
30 risks which can be attributed to micro-cavities are practically
eliminated without involving the drawback of any construction
complications in the cables, without the drawback Oe being
737
subjected to a limit of the cable lengths and without there being
any need for permanently feeding the cable with gas under high
pressure.
Cables "A" and "B" according to the invention which were
subjected to the experimental tests are described hereinafter.
CABLE "A" - ACCORDING TO THE INVENTION
This cable had a conductor formed by a copper rope having a
diameter oE 39 mm and covered with a semiconductive layer and by
a strati~ied insulation having a thickness of 18 mm.
The stratified insulation was Eormed by windings oE
cellulose tapes and was impregnated by a viscous compound having
a viscosity (at 20C) of 18000 cSt. and constituted by 98% of
mineral oil and by 2% of polyisobutylene.
The stratified insulation, covered externally by a
semiconductive layer, was enclosed with a lead sheath having a
thickness o 3.5 mm.
Around the lead sheath, there was a reinforcing structure
formed by two overlapped layers obtained by windings of nickel-
chrome steel tapes with a width of 25 mm and a thickness of 0.2
mm. In said tapes, closed mechanical hysteretical cycles are
obtainable with the difEerence between the maximum and the
minimum deformation being up to 0.45%.
At a temperature oE 10C, the tapes Eorming the reinforcing
structure of the cable are subjected to a tensile stress oE a
value of 9.6 Kg/mm2, and this value represents the tensile stress
existing in the reinforcing structure oE the cable which is
adapted to cause a compression e orce of 9 atm. upon the sheath
and which eliminates any micro-cavity existing in the cable.
CABLE "B" - ACCORDING TO THE INVENTION
This cable differs from the above-described CABLE "A" solely
by the fact that the reinEorcing structure, present around the
sheath, is formed by four overlapping layers obtained by windings
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of fiber-glass tapes having a width of 30 mm and a thickness of
0.20 mm. In said tapes, closed, mechanical hysteretical cycles
are obtainable with the differences existing between the maximum
and the minimum deEormation of up to 1%.
At a temperature of 10C, the tapes forming the eeinforcing
structure of cable B are subjected to a tensile stress of 5
Kg/mm2 and this value represents the tensile stress existing in
the reinEorcing structure of the cable which is adapted to cause
a compression force of 9 atm. on the sheath of said cable.
The experimental tests employed were those known a the
"Loading Cycle and Polarity Reversal Tests" recommended by the
"Working Group 21-10, ~tudy Committee No. 21 of the CIGRE" and
published in the review "ELECTRA", issue No. 72.
Following the methods established by this document, 30 meter
lengths of each cable under examination were subjected to thirty
thermal heating and cooling cycles, between room temperature and
the maximum working temperature, which is 65C for the cables
under examination as measured on the conductor, and with
increasing, after every said thirty cycles, the value oE the
continuous voltage applied to the cable itselE until the value of
the voltage at which the performation is found.
In addition, the following four types of cables, identified
as "C" - "D" - "F" - "G" which were not constructed in accordance
with the invention, were subjected to the above-described
experimental tests:
- A first type of cable, CABLE "C"~ having the identical
structure, materials and dimensions as that of the cable "A"
according to the invention and which difEers Erom the latter only
by the fact that, prior to the cable being put into service, the
resistant structure which surrounds the cable sheath does not, as
a practical matter, apply any compression force on the sheath.
- A second type of cable, CABLE "D", which diEfers Erom the
73'7
CABLE "C" solely by reason of the Eact that its impregnated
insulation has nitrogen at a pressure of 1~ atm. applied thereto.
The second type oE cable belongs to the category of known cables
which have their impregnated insulations assisted by the presence
of a pressurized gas.
- A third type of cable, CABLE "F", which differs from the
CABLE "B" according to the invention solely by reason o the fact
that the reinforcing structure disposed around the sheath is
formed by four overlapping layers o~ cotton tapes, i.e., tapes
which are devoid o-f any closed, mechanical hysteretical cycles.
Said cotton tapes are applied on the sheath with a tension of
5Kg/mm2 which is adapted to cause a compression force of 9 atm.
which guarantees the full impregnation of the cable prior to its
being put into service.
- A fourth type of cable, CABLE "G", which differs from the
cables according to the invention solely by reason of the fact
that the reinforcing structure disposed around the sheath is
formed by four overlapping layers of copper tapes, i.e., tapes
wherein a closed hysteretical cycle can only be obtained when the
20 difference between the maximum and the minimum deformation does
not exceed 0.12%. These copper tapes are applied over the
sheath, with a tension of 5 Kg/mm2 which is adapted to cause a
compression force of 9 atm. which guarantees the full
impregnation of the cable prior to its being put into service.
The experimental tests carried out on a plurality of cable
lengths according to the invention and on the other cable lengths
described hereinbefore have given the results which are set forth
in the following TABLE:
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Increment % oE the increase in
perforation voltage with respect
TYPE OF CABLE to CABLE C
CABLE A according to the invention 80%
CABLE B according to the invention 80
CABLE D - impregnated insulation with
48
nitrogen under pressure at 14 atm.
CABLE F 15%
CABLE G 20%
An examination of the results set forth in the TABLE
supports the following conclusions.
First and foremost, for the solution to the problem, it
appears to be essential that the cable have not only the Eirst
characteristic of the resistant structure which brings about the
full impregnation oE the cable but also the second characteristic
relative to the tapes with which the cable's resistant structure
~0 is formed and which allow for maintaining said ~ull impregnation
of the cable during use.
Moreover, with respect to known cables having the
insulations thereof impregnated with a compound and subjected to
high pressure gas, i.e., cables which to date were considered
capable of providing a solution sufficient-Eor overcoming
problems of perforation risks caused by the presence of micro-
cavities in the insulation, the cables according to the invention
provide an increase of 66% in the perforation voltage values.
What is more, the cables according to the invention solve the
problem in question without also involving any drawbacks. In
particular, the cables according to the invention do not require
any modifications to be ef~ected in the plants actually being
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utilized for cable manufacture, do not require any introduction
of complications in the structure of these cables themselves and
do not impose any limi~ations for the lengths of the cables.
Although preferred embodiments of the present invention have
been described and illustrated/ it will be apparent to those
skilled in the art that various modifications may be made without
departing from the principles of the invention.
