Electric power cable impregnated with insulating fluid

An electric power cable insulating fluid which is a substance corresponding to the following formula ##STR1## where R is chosen between the following two radicals: ##STR2## and an electric power cable with its insulation impregnated with such fluid.

The present invention relates to an electric cable impregnated with an 
insulating fluid and more specifically, to an electric submarine cable 
impregnated with an insulating fluid, and in particular, to an electric 
submarine cable impregnated with an insulating fluid that is particularly 
adapted to being laid at great depths. 
Moreover, the present invention also relates to an insulating fluid for 
electric cables and in particular, for electric submarine cables 
especially suited to be laid at great depths. 
With an underwater power cable which is oil filled, care must be taken to 
minimize stresses on the cable sheath, and hence, the armoring, when the 
cable is lowered into the water and when the cable transmits electric 
power. This means that the density of the insulating oil should be 
substantially equal to the density of the water, and the insulation should 
be permeable to the oil so that the oil can move radially of the 
insulation. Thus, during the transmission of electric power, there are 
thermal changes, or transients, within the cable, and to prevent stresses 
on the sheath, the oil, which has a greater thermal coefficient of 
expansion than the thermal coefficient of expansion of the other cable 
components, must be able to move radially of the insulation. For example, 
if the cable temperature increases, the volume of the oil increases more 
than the volume of the other cable components and if the insulation is oil 
permeable, the oil moves radially of the insulation toward the cable 
conduit and longitudinally of the conduit which leads exteriorly of the 
cable. In this way, overpressure in the cable with thermal changes is 
avoided. With such avoidance of overpressure, a reduction in the cable 
armoring is possible. 
An insulating fluid for electric cables, and more particularly, for 
electric submarine cables laid in the sea must, in fact, possess at least 
the following properties: 
(a) its density has to be substantially equal to that of the sea water, so 
as to prevent any differences arising in the hydrostatic pressure between 
the inside and the outside of the cable which, if existing, could cause 
stresses in the armoring and in the covering sheath of the cable itself. 
This problem increases when the depth at which the cable is laid 
increases; 
(b) the viscosity of the insulating fluid, at whatever temperature to which 
the cable may be subjected and in any position of the cable, must be such 
as to allow the movement of said insulating fluid along the cable; 
(c) the factor of dielectric dispersion or tan .delta. of the insulating 
fluid, must be small and stable with the passing of time, so as to allow a 
good efficiency of the power transmission; and 
(d) the gas adsorption capacity on the part of the insulating fluid, known 
in the electrical-cable field art as "gassing", has to be good, so as to 
prevent the risks of perforations. 
It is very difficult to find all these properties, together, in a single 
insulating fluid. For this reason, in the present state of the art, there 
exist very few known and used insulating fluids for electrical cables and 
only a few of these are useful for the insulating fluid for electrical 
submarine cables, especially, for submarine electric cables laid at great 
depths. In the latter, the density of the insulating fluid plays a 
fundamental role, said density having to be as near as possible to that of 
the sea water to provide the greatest reduction as possible in the 
mechanical stresses in the cable. Also, the factor of dielectric 
dispersion must have a value such as to allow the manufacture of cable 
tracts having long lengths. 
The known types of insulating fluids for electrical cables and, in 
particular, for electric submarine cables impregnated with insulating 
fluids, are reduced to mineral oils and to alkylbenzenes having aliphatic 
chains from 9 to 12 carbon atoms such as, for example, decylbenzene. The 
known insulating fluids have been adopted in practice since they are those 
which have the closest to the ideal characteristics that an insulating 
fluid for cables must possess, particularly for submarine electric cables, 
but the known insulating fluids do not have the required characteristics 
of a satisfactory value. 
One object of the present invention is an electric cable impregnated with 
an insulating fluid, and in particular, an electric submarine cable 
impregnated with an insulating fluid that is better than the prior art 
insulating fluids. 
Another object of the present invention is an insulating fluid for electric 
cables and, in particular, for electric submarine cables, which possess 
the necessary characteristics for rendering it suitable for being used in 
cables and, in particular, for submarine cables to be laid at great depths 
and which has properties closest to those which are optimum. 
The electric cable impregnated with insulating fluid of the invention 
comprises a conductor encircled by a solid, stratified insulation, and is 
provided with at least one longitudinal conduit capable of allowing the 
circulation of an insulating fluid, said solid, stratified insulation 
being impregnated by said insulating fluid. Said insulating fluid 
comprises a substance having the formula: 
##STR3## 
where R is a radical selected from the following two: 
##STR4## 
The insulating fluid should also have a density at 20.degree. C. in the 
range from about 0.97 gr/cm.sup.3 to about 1 gr/cm.sup.3 and a viscosity 
at 20.degree. C. in the range from about 8 to about 12 centipoises, the 
latter viscosity being from 40% to 60% greater than a conventional 
insulating fluid, such as decylbenzene. Any increase in internal cable 
pressure when the cable has a long length and when such a higher viscosity 
insulating fluid is used can be compensated for by increasing the armoring 
around the cable sheath.

The present invention will be better understood from the following detailed 
description, given solely by way of example, with reference to the single 
FIGURE of the accompanying drawings which is a longitudinal cross-section 
of a length of electric cable that is particularly suitable for use as a 
submarine cable. 
The electric cable, of the type impregnated with an insulating fluid which 
is particularly suitable for use as a submarine cable and which is shown 
in the drawing, has a structure which is known in the art. 
With reference to the drawing and proceeding from outside toward inside, 
the cable C has an armoring 1, superimposed on a sheath 2 of an 
elastomeric material. Beneath the sheath 2, there is present a tubular 
metallic sheath 3 that, in the case of submarine cables, is a lead sheath. 
Co-axial of the metallic sheath 3 and enclosed within it, is a copper 
electric conductor 4 that has, on its inside, a conduit 5 completely 
filled with an insulating fluid and, more specifically, by an insulating 
liquid 7. 
Between the conductor 4 and the sheath 3, there is interposed a layer of 
solid, stratified insulation 6 constituted by a plurality of tapes of 
insulating material which are at least radially permeable from the outside 
of insulation 6 to the inside thereof, and viceversa to the insulating 
fluid, or liquid, 7, such as, for example, insulating paper impregnated 
with the same insulating fluid 7 with which the conduit 5 is filled. 
The air resistance of the tapes should not exceed 15.times.10.sup.6 
cm.sup.-2 Emanueli units and preferably, is in the range from about 
4.times.10.sup.6 cm.sup.-2 to about 8.times.10.sup.6 cm.sup.-2 Emanueli 
units so that the insulation 6 will have the required permeability to the 
insulating fluid 7. 
The insulating liquid 7 is an organic substance having a density at 
20.degree. C. from about 0.97 gr/cm.sup.3 to about 1 gr/cm.sup.3 and a 
viscosity at 20.degree. C. from about 8 to about 12 centipoises and 
corresponding to the following chemical formula: 
##STR5## 
where R is chosen between the following two radicals: 
##STR6## 
Thus, the insulating liquid 7 can be constituted by a substance 
corresponding to the chemical formula: 
##STR7## 
called phenylxylylethane or by the substance corresponding to the chemical 
formula: 
##STR8## 
called isopropyldiphenyl, or else by mixtures of these two substances. 
In particular, the isopropyldiphenyl consists of para-isopropyldiphenyl, or 
of meta-isopropyldiphenyl, or of mixtures of these two isomers. 
Of the two substances described above (for which we shall give the 
chemical-physical characteristics further on) of particular interest is 
the m-isopropyldiphenyl, which possesses a very high resistance to aging, 
in the presence of metals such as copper or lead. 
The following TABLE I sets forth the chemical-physical characteristics of 
the two substances of the present invention and sets forth the 
chemical-physical characteristics of one of the known insulating fluids 
more commonly used for cables and, in particular, for submarine electric 
cables. 
TABLE I 
______________________________________ 
CHEMICAL- 
PHYSICAL Prior Art 
CHARACTER- Phenylxylyl- 
Isopropyl- DECYL- 
ISTICS ethane diphenyl BENZENE 
______________________________________ 
Density at 20.degree. C. 
0.9874 0.9841 0.8613 
in gr/cm.sup.2 
Inflammability 
153 151 124 
in open cup in .degree.C. 
Viscosity at 20.degree. C. 
8.3 9.5 5 
in centipoises 
Dielectric Loss 
0.0003 0.0003 0.0001 
Factor 
at 100.degree. C. tan 
______________________________________ 
When comparing the chemical-physical characteristics of the insulating 
fluids of the present invention, with the chemical-physical 
characteristics of the prior art insulating fluid that is most used for 
cables, it can be seen, first and foremost, that the density of the fluids 
of the invention are closer to that of sea water. This signifies that an 
electric submarine cable according to the present invention, which 
utilizes an insulating fluid of the present invention, has only small 
mechanical stresses since the difference between the external and the 
internal pressures, owing to the sinking of the cable, is less than that 
occurring in the known cables. Moreover, this advantage becomes even more 
important when the depth of cable laying is increased. This fact also 
makes it possible to reduce the armoring of a cable according to the 
invention with the same safety factor as cables of the known type which 
utilize the insulating fluids of the prior art. 
By comparing the chemical-physical characteristics of the insulating fluids 
of the present invention, with the characteristics of the prior art 
insulating fluid which is most used, it will be observed that the values 
of the dielectric loss factor are good and are practically equivalent to 
the dielectric loss factor of the prior art fluid. A low dielectric loss 
factor is, of course, a necessary characteristic in an electric cable. 
One advantage offered by a cable according to the present invention, which 
results from using insulating fluids of the invention, is the high 
absorption capacity for gas, which brings about a greater guarantee of 
safety with respect to electrical perforation risks. 
What is more, it is to be noted that the anti-flame characteristics for an 
insulating fluid of the invention, are better than the anti-flame 
characteristics of the prior art insulating fluids for cables. This fact 
makes it possible to simplify the manufacturing process for electrical 
cables in general, and for submarine cables, in particular, by giving a 
greater guarantee against fires at least during the manufacture of the 
cables themselves. 
Of course, the insulating fluid of the invention may include other 
materials which are conventionally used in the insulating fluids for 
electric cables and which do not adversely affect the desired 
characteristics. 
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.