Background of the invention A problem with prior art electrical cables comprising a plurality of conductors is that the required materials tend to be polluting to some degree, and even though the actual pollution is somewhat limited according to some, the commercial market seems to require still better cables and environmentally compatible cables.

This problem is relevant with respect to e. g. cables having low viscosity oil/paper insulation between the conductor and the peripheral armor of the cable. Thus, the oil of the insulation material implies an environmental risk to some consumers.

A further problem with prior art high voltage cables is that cables used under extreme conditions with respect to e. g. temperature, pressure, water depth or humidity are often being subjected to extremely expensive and difficult installation.

Summary of the invention The invention relates to an electrical cable comprising at least two cable cores, each of the at least two cable cores being encapsulated within a water barrier (8), at least two cable cores being situated within a jacket (10), said jacket (10) being wholly or partly water permeable.

In this manner, a unique advantageous pressure adaptive cable structure has been obtained.

The open-system cable of the invention provides a unique possibility of adaptive pressure equalization when the high voltage cable is exposed to great pressure from the

environment such as that affecting a submarine-cable. Consequently, according to the invention, the permeable outer jacket ensures that water is pressed into the cable in the cavity between the lead-sheathed cable cores and the permeable armor ensuring that the high pressure on the armor will be equalized. On the other hand, if the cable is utilized as an ordinary underground-cable, the pressure on the cable will be kept relatively low, and no pressure equalizer is necessary.

In other words, if the surroundings generate high pressure, the cable will take in water which may pass relatively easily into the cable to even out the pressure. Otherwise, if the cable is utilized as an underground cable, there is no need for importing a pressure equalizing material.

Thus, a cable according to the invention may be regarded as a pressure adaptive cable.

The water barrier may e. g. comprise an extruded lead sheath, corrugated extruded aluminum, stainless steel or other suitable materials or composites of materials.

An advantage of individual coating of the cores is that the thickness of the necessary water barrier layer may be reduced, thereby obtaining a reduction in material and/or manufacturing costs.

Moreover, the support of the barrier, especially when the cable is under pressure, improves significantly due to the fact that the barrier may be arranged directly on a circular cable core.

Moreover, the choice of individual"lead coating"implies that a pressure equalizer may be filled into the cable which ensures that the desired shape of the cross-section will be maintained under high pressure and that the pressure equalizer does not destroy the insulation material.

A further advantage of the individual packing of the cable cores is that potential damage or leaks in the water barrier of one of the cores will only affect one core instead of all cores within one common water barrier.

When said jacket comprises permeable bedding and armor layers (10,11,12,13,14), a further advantageous embodiment of the invention has been obtained.

When least one of said cable cores comprises a conductor (1), and extruded insulation (2) and at least two semi-conducting screens (3,4), each of said at least two cable cores being enclosed within a water barrier layer (s) (8), a further advantageous embodiment of the invention has been obtained.

When said insulation (2) comprises XLPE, a further advantageous embodiment of the invention has been obtained.

When said insulation (2) comprises mass-impregnated paper, a further advantageous embodiment of the invention has been obtained.

Mass impregnated paper, well-known within the art, may be impregnated with high viscosity oil or gel.

Mass-impregnated cables, also called non-draining cables, benefit from the fact that the cable needs no oil feeding. Moreover, there is no oil leakage to the environment if the cable is damaged.

When said insulation (2) comprises an interpenetrating network, a further advantageous embodiment of the invention has been obtained.

Interpenetrating network comprises polymer and oil.

When at least one of the water barriers (8) comprises a metal, a further advantageous embodiment of the invention has been obtained.

Compared to other applicable water barrier materials, metals such as lead benefit from the fact that they may constitute a barrier which offers effective protection of the encapsulated insulation against humidity.

When said metal comprises lead (8), copper, aluminum or steel, a further advantageous embodiment of the invention has been obtained.

When at least one of said water barriers (8) comprises a heat conductor thermally interacting with said insulation (2), a further advantageous embodiment of the invention has been obtained.

The use of a heat conducting material enclosing e. g. a DC XLPE cable is very advantageous, as XLPE cables would suffer from non-homogenous heating of the XLPE insulation. A non-homogenous distribution of the heat over the circumference of the core would result in a non-homogeneous electrical field on the core and consequently a breakdown of the insulation.

It should be noted that non-homogeneous distribution of heat is a particular problem with multi-core high voltage cables due to the very close arrangement of the cores.

Areas of no or little spacing between the cores would result in high non-homogenous temperature distribution over the core.

This problem is particularly relevant when dealing with multi-core XLPE cables due to the fact that XLPE offers a higher operation temperature than that of equivalent materials.

Brief description of the drawings The invention will be described below with reference to the drawings in which

fig. 1 shows a cross-section of a preferred embodiment of the invention and fig. 2 shows a cross-section of a second embodiment of the invention.

Detailed description Fig. 1 shows a cross-section of a preferred embodiment of a cable according to the invention.

The shown flat-type cable is an example of a 150 kV XLPE DC high voltage cable. It should be noted that the invention, although particularly advantageous in relation to DC applications, is not restricted to DC use. AC may also be applicable. Evidently, a traditional cable would comprise three cores.

The shown cable comprises two copper cores 1. The cross-section of each stranded waterproof core 1 is approximately 500 mm2. The insulation of each of the copper conductors will be described in detail below. Obviously, other core materials may be applicable within the scope of the invention, such as aluminum. Likewise, other cross- sections may be applicable.

Each conductor 1 is surrounded by an extruded semi-conducting layer 2 such as polyethylene forming the conductor screen.

The next layer of the cable forms an extruded XLPE insulation 3.

The insulation layer may also comprise other suitable insulation layers such as mass- impregnated paper, interpenetrating network, i. e. oil and polymer, or other suitable materials. According to a preferred embodiment of the invention, the insulation is XLPE.

An outer semi-conducting layer, also called the insulation screen 4, is provided on top of the XLPE insulation layer. This layer is adaptive e. g. in order to eliminate ionization on the outer surface of the dielectric.

The next layer is a humidity barrier in the form of a lead sheath 8. The barrier 8 encloses the XLPE insulation completely against ambient humidity. The XLPE insulation is very sensitive to even small amounts of humidity, and intruding humidity results in a so-called treeing phenomenon in the insulation. In time, the treeing phenomenon can lead to a breakdown of the insulation.

The humidity barrier may advantageously comprise other metals such as aluminum, copper or stainless steel.

The lead sheath is covered with asphalt and subsequently wrapped in bituminized crepe paper 9 in order to eliminate the risk of corrosion of the lead sheath 8.

The two lead-sheathed cores are subsequently wrapped together with a common galvanized steel tape layer 10 and two layers of asphalt and polypropylene yarn 11, 13 enclosing a galvanized steel armor wire layer 12. The final layer of the cable is comprised by chalk 14.

The galvanized steel tape reduces the mechanical stress on the lead sheath, and the yarn acts as cushion.

The latter layer 10,11,12,13 and 14 forms an armor jacket protecting the inner cable cores and the corresponding insulation and barrier materials against ambient mechanical stress. Especially, the armor wires are adapted to receiving tensile forces inferred during installation or recovering of the cable.

The asphalt protects the armor wires against corrosion.

The outer dimension of the illustrated embodiment of the invention is approximately 78 mm x 135 mm. Other outer dimensions may be applicable within the scope of the invention.

The layers forming the armor jacket are water permeable.

Consequently, water may pass through the armor jacket and form a pressure equalizing filling 20 of the cable under high ambient pressure. This adaptive filling of the inner compartment 20 of the cable prevents the armor from collapsing even under high pressure.

It should be noted that pressure exercised on a submarine cable may exceed 100 bar or more and that pressure equalization is expected to be vital in such cables.

Moreover, it should be noted that the pressure equalizer of the present preferred embodiment is ambient water which has been forced into the inner compartment 20 by the pressure exercised on the cable. This automatic adaptive pressure equalization of the cable is evidently extremely convenient.

Hence, a cable according to the invention is pressure adaptive due to the fact that the inner compartment automatically equalizes the ambient pressure due to the water permeable armor jacket.

Fig. 2 illustrates a further embodiment of the invention comprising the aforementioned main components of fig. 1.

The main difference between the cable of fig. 1 and fig. 2 is that the cable of fig. 2 has been fitted with a solid pressure equalizer 10. The solid pressure equalizer may comprise extruded polyethylene. Evidently, other choices may be applicable within the scope of the invention.

Still, the jacket is water permeable.

It should be noted that a cable according to the invention may be of another type than a sub-marine cable, such as underground cables.