Method and armoured power cable for transporting alternate current

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DESCRIPTION

The present invention relates to a method and an armoured power cable for transporting alternate current.

An armoured power cable is generally employed in applications where mechanical stresses are envisaged. In an armoured power cable, the cable core or cores (typically three stranded cores in the latter case) are surrounded by at least one armour layer in the form of armour wires, configured to strengthen the cable structure while maintaining a suitable flexibility.

US 2009/0194314 discloses an oilfield cable comprising an electrically conductive cable core for transmitting electrical power and at least one layer of a plurality of armour wires surrounding the cable core. At least one of the armour wires is a bimetallic armour wire having a coaxial inner portion and a surrounding outer portion, which provides a return path for the electrical power transmitted through the cable core. The inner portion includes at least one of copper material, aluminium material and beryllium copper material. The outer portion is formed of a metal alloy material which includes at least one of MP35N material, HC-265 material, Inconel material, Monel material, and Rene material.

When alternate current (AC) is transported into a cable, the temperature of electric conductors within the cable rises due to resistive losses, a phenomenon referred to as Joule effect. The transported current and the electric conductors are typically sized in order to guarantee that the maximum temperature in electric conductors is maintained below a prefixed threshold (e.g., below 90°C) that guarantees the integrity of the cable.

The international standard IEC 60287-1-1 (second edition 2006-12) provides methods for calculating permissible current rating of cables from details of permissible temperature rise, conductor resistance, losses and thermal resistivities. In particular, the calculation of the current rating in electric cables is applicable to the conditions of the steady-state operation at all alternating voltages. The term "steady state" is intended to mean a continuous constant current (100% load factor) just sufficient to produce asymptotically the maximum conductor temperature, the surrounding ambient conditions being assumed constant. Formulae for the calculation of losses are also given.

In IEC 60287-1 -1 , the permissible current rating of an AC cable is derived from the expression for the permissible conductor temperature rise ΔΘ above ambient temperature Ta, wherein Δθ= T-Ta, T being the conductor temperature when a current I is flowing into the conductor and Ta being the temperature of the surrounding medium under normal conditions, at a situation in which cables are installed, or are to be installed, including the effect of any local source of heat, but not the increase of temperature in the immediate neighbourhood of the cables due to heat arising therefrom. For example, the conductor temperature T should be kept lower than about 90°C.

For example, according to IEC 60287-1 -1 , in case of buried AC cables where drying out of the soil does not occur, or AC cables in air, the permissible current rating can be derived from the expression for the temperature rise above ambient temperature:

(1 ) where: I is the current flowing in one conductor (Ampere)

ΔΘ is the conductor temperature rise above the ambient temperature (Kelvin)

R is the alternating current resistance per unit length of the conductor at maximum operating temperature (Ω/m);

W _d is the dielectric loss per unit length for the insulation surrounding the conductor (W/m);

Ti is the thermal resistance per unit length between one conductor and the sheath (K.m/W);

T ₂ is the thermal resistance per unit length of the bedding between sheath and armour (K.m/W);

T ₃ is the thermal resistance per unit length of the external serving of the cable (K.m W); T is the thermal resistance per unit length between the cable surface and the surrounding medium (K.m/W); n is the number of load-carrying conductors in the cable (conductors of equal size and carrying the same load); λι is the ratio of losses in the metal sheath to total losses in all conductors in that cable; λ ₂ is the ratio of losses in the armouring to total losses in all conductors in the cable.

In case of three-core cables and steel wire armour, the ratio λ ₂ is given, in IEC 60287-1 -1 , by the following formula:

where RA is the AC resistance of armour at maximum armour temperature (Ω/m);

R is the alternating current resistance per unit length of conductor at maximum operating temperature (Ω/m); d _A is the mean diameter of armour (mm); c is the distance between the axis of a conductor and the cable centre (mm); ω is the angular frequency of the current in the conductors.

The Applicant observed that, in general, the reduction of losses in an armoured AC electric cable enables to reduce the cross-section of the conductor(s) (thus, the quantity of material necessary to make the cable) and/or to increase the permissible current rating (thus, to enable the cable to carry higher power).

The Applicant noted that armour losses could be reduced by using a non- ferromagnetic material, for example austenitic stainless steel, but this substantially increases the cost of the power cable.

The Applicant investigated the contribution of armour losses to the overall cable losses in an armoured AC power cable when the armour wires are made of ferromagnetic material, which is economically appealing.

During its development activities, the Applicant observed that armour losses can depend on hysteresis and eddy currents generated owing to the magnetization of the ferromagnetic wires of the armour, caused by the magnetic field generated by the AC current transported by the cable conductors.

WO 2013/174399 discloses that, in a three core cable, the armour losses can be highly reduced when the armouring pitch is unilay to the core pitch compared with the situation wherein the armouring pitch is instead contralay to the core pitch. WO 2013/174399 states that the armour wires may have a ferromagnetic core surrounded by a non-ferromagnetic material (e.g. plastic or stainless steel). The Applicant perceived that the provision of an electrically conductive non- ferromagnetic cladding having an electrical conductivity greater than that of the stainless steel onto a ferromagnetic armour wire can substantially reduce hysteresis and eddy current losses in the cable armour while maintaining the mechanical resistance and limiting the increase of the cable manufacturing cost. Without the aim of being bound to any theory, the Applicant believes that an electrical conductivity above a predetermined value in the armour wire cladding causes the eddy currents to concentrate in the armour wires periphery where they generate low resistive losses because of such enhanced electrical conductivity. At the same time, the concentration of eddy currents in the armour wire cladding induces a magnetic field of opposite sign with respect to that of the magnetic field generated by the AC current transported by the cable conductors, thereby increasing the effect of shielding the armour wires from this latter magnetic field. In addition, the reduction of armour loss due to said electrical conductivity of the armour wire cladding enhances the reduction of the armour losses due to the non- ferromagnetic property of the cladding. Such non-ferromagnetic property of the armour wire cladding enables to shield the ferromagnetic armour wires from the magnetic field generated by the AC current transported by the cable conductors, thereby reducing hysteresis losses.

As a result, the armour losses of an armour made of wires having a ferromagnetic core coated by a non-ferromagnetic cladding of sufficiently high electrical conductivity are reduced with respect to those of an armour having wires with the same cross-section but made of ferromagnetic material only, or made of a non-ferromagnetic and relatively low electrically conducting applied cladding over a ferromagnetic inner portion.

The Applicant found that, by using an armoured AC cable comprising an armour with a layer of armour wires having a ferromagnetic core and an external electrically conductive and non-ferromagnetic cladding of a predetermined thickness/cross-section, the performances of the armoured AC cable can be improved in terms of transmitted current and/or cable conductor cross-section area S. Indeed, it is possible to comply with IEC 60287-1-1 requirements for permissible current rating by transmitting into the cable conductor an increased current value and/or using cable conductors with a reduced value of the cross-section area S (the AC resistance per unit length R in the above formula (1) being proportional to p/S, wherein p is the conductor material electrical resistivity), without increasing the working temperature of the conductor.

In a first aspect the present invention thus relates to an armoured power cable for transporting alternate current comprising: - at least one core, comprising an electric conductor, and

- an armour, surrounding the at least one core, comprising a plurality of armour wires having a ferromagnetic inner portion and an electrically conductive cladding.

The ferromagnetic inner portion and an electrically conductive cladding of the armour wires of the invention are as hereinafter defined.

In a second aspect, the present invention relates to a method of transporting an alternate current I in an armoured power cable comprising at least one core and an armour surrounding the core, said core comprising an electric conductor and said armour comprising a plurality of wires, the method comprising:

- operating the cable at a maximum allowable working conductor temperature T while transporting said alternate current I into each electric conductor having a cross section area S, said alternate current I and said cross section area S being determined by overall cable losses including armour losses;

characterized in that the method further comprises:

- reducing the armour losses by providing the plurality of wires with a ferromagnetic inner portion and an electrically conductive cladding;

- increasing the value of said alternate current I and/or reducing the value of the cross section area S of the electric conductors, as determined by the reduced armour losses;

- performing the operating step with said increased value of alternate current I and/or said reduced value of the cross section area S.

In a third aspect, the present invention relates to a method of reducing armour losses in an armoured power cable comprising at least one core, comprising an electric conductor, and an armour surrounding the at least one core, the method comprising:

- building the armour with a plurality of wires having a ferromagnetic inner portion and an electrically conductive non-ferromagnetic cladding.

The cladding of the armour wires of the invention is advantageously made of an electrically conductive and non-ferromagnetic material.

For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

In the present description, the term "core" is used to indicate an electric conductor surrounded by an insulating layer and at least one semiconducting layer. Optionally, said core further comprises a metal screen surrounding the conductor, the insulating layer and the semiconducting layer/s.

In the present description, all indications of directions and the like, such as "radial" and "tangential" are made with reference to the longitudinal axis of the cable.

In particular, "radial" is used to indicate a direction intersecting the longitudinal axis of the cable and laying in a plane perpendicular to said longitudinal axis; and "tangential" is used to indicate a direction perpendicular to the "radial" direction and laying in a plane perpendicular to the longitudinal axis of the cable.

In the present description, the term "electrically conductive" is used to indicate a material, e.g. copper or aluminium, having an electrical resistivity lower than 10x10 ^"8 Ohnrm; preferably lower than 8x 0 ^"8 Ohrrnn; more preferably lower than 5x10 ^"8 Ohnrm. In the present description, the term "ferromagnetic" indicates a material that below a given temperature has a relative magnetic permeability significantly greater than 1 , preferably greater than 100.

In the present description, the term "non-ferromagnetic" indicates a material that below a given temperature has a relative magnetic permeability of about 1.

In the present description, the term "maximum allowable working conductor temperature" is used to indicate the highest temperature a conductor is allowed to reach in operation in a steady state condition, in order to guarantee integrity of the cable. The working conductor temperature substantially depends on the overall cable losses, including conductor losses due to the Joule effect and other additional dissipative phenomena. The armour losses are another significant component of the overall cable losses.

In the present description, the term "permissible current rating" is used to indicate the maximum current that can be transported in an electric conductor in order to guarantee that the electric conductor temperature does not exceed the maximum allowable working conductor temperature in steady state condition. Steady state is reached when the rate of heat generation in the cable is equal to the rate of heat dissipation from the surface of the cable, according to laying conditions.

In the present description, the term "elongated cross section" is used to indicate the shape of the transversal cross section perpendicular to the longitudinal axis of the armour wire, said shape being oblong, elongated in one dimension. According to the invention, the performances of the armoured power cable can be advantageously improved in terms of increased transported alternate current with respect to a cable having substantially the same electric conductor cross section area S and armour wires having substantially the same cross section area, but the latter being made of material/s with different electrical and/or magnetic features and/or having a different material arrangement. In alternative, the performances of the armoured power cable can be advantageously improved in terms of reduced electric conductor cross section area S with respect to a cable transporting substantially the same amount of alternate current and having armour wires having substantially the same cross section area, but the latter being made of material/s with different electrical and/or magnetic features and/or having a different material arrangement.

A combination of the above two alternatives can also be envisaged.

In the cable market, a cable is offered for sale or sold accompanied by indication relating to, inter alia, the amount of transported alternate current, the cross section area S of the electric conductor/s and the maximum allowable working conductor temperature. With respect to a known cable, a cable according to the invention will bring indication of a reduced cross section area of the electric conductor/s with substantially the same amount of transported alternate current and maximum allowable working conductor temperature, or an increased amount of transported alternate current with substantially the same cross section area of the electric conductor/s and maximum allowable working conductor temperature.

This is very advantageous because it enables to make a cable more powerful and/or to reduce the size of the conductors with consequent reduction of cable size, weight and cost.

The present invention in at least one of the aforementioned aspects can have at least one of the following preferred characteristics.

The alternate current I caused to flow into the cable and the cross section area S of the conductors advantageously comply with permissible current rating requirements according to IEC Standard 60287-1-1 , by reckoning armour losses equal to or lower than 40% of the overall cable losses.

The armour losses can be equal to or lower than 20% of the overall cable losses. By a proper selection of the armour construction according to the teaching of the invention, the armour losses can be equal to or lower than overall cable losses.

By a proper selection of the armour construction according to the teaching of the invention, the armour losses λ ₂ · can be significantly lower than those λ ₂ calculated by international standard IEC 60287-1-1 , second edition 2006-12. In particular, and advantageously, λ ₂ ·≤ 0.75λ ₂ . Preferably, λ ₂ · < 0.50λ ₂ . More preferably, λ ₂ ' < 0.25λ ₂ . Even more preferably, λ ₂ ·≤ 0.10λ ₂ .

Preferably, each electric conductor has a cross section area S sized for operating the cable to transport alternate current I at a maximum allowable working conductor temperature T, as determined by overall cable losses including armour losses. Preferably, the cross section area S of the electric conductor is sized by reckoning armour losses not higher than 40% of the overall cable losses.

The armour wire of the present invention can have a substantially circular or an elongated cross section. In the case of an elongated cross-section, the cross-section major axis is preferably oriented tangentially with respect to the cable circumference.

Preferably, in case of circular cross-section, the armour wires have an overall diameter (including the inner portion and the cladding) of from 2 mm to 8 mm, more preferably of from 3 mm to 7 mm.

Preferably, in case of armour wire with circular cross-section, the electrically conductive cladding has a thickness (that is, size in the radial direction) at least equal to 2.5% with respect to the total diameter of the armour wire. Preferably, in case of armour wire with circular cross-section, the electrically conductive cladding has a thickness (that is, size in the radial direction) not higher than 20% with respect to the total diameter of the armour wire; more preferably, not higher than 15%.

Preferably, for any shape of the armour wires, the electrically conductive cladding has a cross-section area (in a plane perpendicular to the longitudinal axis of the cable) at least equal to 10% with respect to the total cross-section area of the armour wire.

Preferably, for any shape of the armour wires, the electrically conductive cladding has a cross-section area (in a plane perpendicular to the longitudinal axis of the cable) not higher than 55% with respect to the total cross-section area of the armour wire; more preferably, not higher than 40%.

The Applicant found that the above stated lower and upper limits for the thickness and cross-section area of the electrically conductive cladding enable to achieve, for a cladding material having an electrical resistivity lower than 10x10 ^"8 Ohnrm, a good compromise between two conflicting requirements. In fact, on one side, values of cross-section area of the cladding lower than 10% (or thickness lower than 2.5%) can provide an armour loss reduction non particularly valuable to the aim of reducing the electric conductor cross section area S and/or of increasing the transported alternate current (e.g. at least 10% reduction). On the other side, a cladding cross-section area greater than 55% (or thickness greater than 20%) could cause the armour wire to lose the tensile strength suitable for providing the cable with the sought tension stability and mechanical protection. That is because materials for the electrically conductive cladding generally have a tensile strength substantially lower than that of materials of the ferromagnetic inner portion of the armour wire.

The selection of an armour wire in view of its tensile strength can be made by the skilled person on the basis of the cable dimensions, weight and intended environment of use. Preferably, the ferromagnetic material of the armour wire inner portion of the invention has a tensile strength of at least 350 MPa, more preferably of from 350 MPa to 800 MPa, a more preferable higher limit of such range being 750 MPa.

The electrically conductive cladding of the armour wires of the invention can be made of a material selected from: zinc, copper, silver, aluminium, alloys and composites thereof. Preferably the electrically conductive cladding is made of copper or aluminium or alloys and composites containing them, more preferably of copper alloys and composites thereof.

The ferromagnetic inner portion of the armour wires of the invention can be made of a material selected from construction steel, ferritic stainless steel, martensitic stainless steel and carbon steel.

Besides the plurality of armour wires having the ferromagnetic inner portion and the electrically conductive cladding, the armour can also comprise further armour wires made of material/s with different electrical and/or magnetic features and/or having a different material arrangement. For example, said further armour wires could be made of ferromagnetic material only.

The plurality of armour wires preferably defines an armour layer.

According to an embodiment, the plurality of armour wires defines an inner layer of the armour, the armour comprising an outer layer with a plurality of armour wires, surrounding said inner armour layer.

The armour wires of the outer layer are preferably metallic. The armour wires of the outer layer are preferably made of ferromagnetic metallic material only.

According to an alternative, the armour wires of the outer layer can comprise a ferromagnetic inner portion and an electrically conductive non- ferromagnetic cladding.

In a preferred embodiment, the armour surrounds the at least one core along a circumference and the armour wires have an elongated cross section with major axis oriented tangentially with respect to said circumference.

The Applicant found that this feature advantageously contributes to further reduce armour losses.

The armour wires have elongated cross-section with a ratio between major axis length and minor axis length at least equal to 1.5; more preferably at least equal to 2. Advantageously, said ratio is not higher than 5 because armour wires with elongated cross-section having a too long major axis could give place to manufacturing problem during the step of winding the armour around the cable. Advantageously, the elongated cross section of the armour wires has smoothed edges. Besides being preferable from a manufacturing point of view, armour wires with smoothed edges avoid damages to the underlying cable layers and the risk of occurrence of electric field peaks.

The elongated cross section of the armour wires can have a substantially rectangular shape.

Alternatively, the elongated cross section is substantially shaped as an annulus portion.

In a further embodiment, the elongated cross section is provided with a notch and a protrusion at the two opposing ends along the major axis, so as to improve shape matching of adjacent wires. The notch/protrusion interlocking among wires makes the armour advantageously firm even in case of dynamic cable.

Preferably, the elongated cross section of the armour wires have a minor axis from about 1 mm to about 7 mm long, more preferably, from 2 mm to 5 mm long.

Preferably, the elongated cross section of the armour wires have a major axis from 3 mm to 20 mm long, more preferably from 4 mm to 10 mm long.

When the cable of the invention comprises at least two cores, these cores are stranded together according to a core stranding lay and a core stranding pitch A.

Preferably, the armour wires of the armour are wound around the at least two cores according to a helical armour winding lay having the same direction as the core stranding lay, and an armour winding pitch B which is of from 0.4 to 2.5 the core stranding pitch A and differs from the core stranding pitch A by at least 10%. The Applicant found that this feature advantageously enables to further reduce armour losses.

Preferably, pitch B > 0.5A. More preferably, pitch B > 0.6A. Preferably, pitch B < 2A. More preferably, pitch B < 1.8A.

Suitably, when the cable of the invention comprises two or more cores, the armour surrounds all of the said cores together, as a whole.

When the armour comprises an outer layer of armour wires, surrounding the inner layer of the armour, the armour wires of the outer layer are suitably wound around the cores according to an outer layer winding lay and an outer layer winding pitch B'. Preferably, the outer layer winding lay has an opposite direction with respect to the core stranding lay (that is, the outer layer winding lay is contralay with respect to the core stranding lay and with respect to the armour winding lay). This contralay configuration of the outer layer is advantageous in terms of mechanical performances of the cable. Preferably, the outer layer winding pitch B' is higher, in absolute value, than the armour winding pitch B. More preferably, the outer layer winding pitch B' is higher, in absolute value, of B by at least 10% of B.

Preferably, the armour wires of the outer layer of the armour have substantially the same cross section in shape and, optionally, in size as those of the layer radially internal thereto.

Preferably, when the cable of the invention comprises two or more cores, each of them is a single phase core. Preferably, the at least two cores are multi-phase cores.

The armoured power cable can comprise three cores. The three-phase cable advantageously comprises three single phase cores.

The armoured electric can be a low, medium or high voltage cable (LV, MV, HV, respectively). The term low voltage is used to indicate voltages lower than kV. The term medium voltage is used to indicate voltages of from 1 to 35 kV. The term high voltage is used to indicate voltages higher than 35 kV. The armoured power cable may be terrestrial or underwater. The terrestrial cable can be at least in part buried or positioned in tunnels.

The features and advantages of the present invention will be made apparent by the following detailed description of some exemplary embodiments thereof, provided merely by way of non-limiting examples, description that will be conducted by making reference to the attached drawings, wherein:

- figures 1a and 1 b schematically show two exemplary armoured power cables according to two embodiments of the invention; - figure 2 schematically shows a cross section of an armour wire that can be used in a cable according to an embodiment of the invention;

- figure 3 schematically shows the total armour losses generated in armour layers formed by cylindrical wires having a wire diameter of 5 mm, a steel inner portion and different thickness of copper cladding; - figure 4 schematically shows an example of armour wires with elongated cross sections that can be used in a cable according to an embodiment of the invention;

- figure 5 schematically illustrates stranded cores and wound armour wires, respectively with core stranding pitch A and armour winding pitch B, that can be used in a cable according to an embodiment of the invention.

Figure 1a schematically shows an exemplarily armoured AC power cable 10 for underwater application comprising three cores 12. Each core 12 comprises a metal electric conductor 12a typically made of copper, aluminium or both, in form of a rod or of stranded wires. The conductor 12a is sequentially surrounded by an inner semiconducting layer and insulation layer and an outer semiconducting layer, said three layers (collectively referred to as 12b) being made of polymeric material (for example, polyethylene), wrapped paper or paper/polypropylene laminate. In the case of the semiconducting layer/s, the material thereof is charged with conductive filler such as carbon black. The three cores 12 are helically stranded together according to a core stranding pitch A. The three cores 12 comprise each a metal sheath 13 (for example, made of lead or copper) and are embedded in a polymeric filler 1 1 surrounded, in turn, by a tape 15 and by a cushioning layer 14. Around the cushioning layer 14 an armour 16 comprising a layer of wires 16a is provided. The wires 16a are helically wound around the cushioning layer 14 according to an armour winding pitch B. The armour 16 is surrounded by a protective sheath 17.

Each conductor 12a has a cross section area S, wherein S= (d/2) ² , d being the conductor diameter.

Figure 1 b schematically shows an exemplarily armoured AC power cable 10 for underwater application differing from the cable of Figure 1 a in that it comprises a single core 12. Cable features analogous to those of cable of figure 1a are indicated by the same reference numbers. As schematically shown in figure 2, the wires 16a are bimetallic. In particular, they comprise each a ferromagnetic inner portion 162 and an electrically conductive and non-ferromagnetic cladding 164.

For example, the cladding 164 is made of copper (having an electrical resistivity of about 1.8x10 ^"8 Ohm m) or aluminium (having an electrical resistivity of about 2.8x10 ^"8 Ohnrm).

Preferably, the ferromagnetic inner portion 162 of the armour wires is made of ferromagnetic steel like material, for example, construction steel, ferritic stainless steel, martensitic stainless steel and carbon steel.

During development activities performed to investigate the armour losses in an AC electric power cable, the Applicant tested an AC three-phase power cable having: three cores stranded together according to a core stranding pitch A of 1442 mm; armour made of a single layer of 61 cylindrical armour wires wound around the three cores according to a helical armour winding lay and an armour winding pitch B of 1117 mm; an angle of armouring of 17.4 degrees; a total armour wire diameter of 5 mm; an electric conductor cross section area S of 500 mm ² ; an AC current in each conductor of 800A; a frequency of 50 Hz; phase to phase voltage of 18/30 KV.

Considering this cable, the Applicant computed, by using a 3D FEM (Finite Element Method) model, the armour losses for different armour wires materials.

Table 1 shows the hysteresis losses, the resistive losses (due to eddy currents) and the total armour losses (the sum of hysteresis losses and resistive losses) obtained for armour wires (each having a circular cross- section and an overall diameter of 5 mm) made of: 1) ferromagnetic steel only; 2) copper only; 3) copper cladding (thickness of 1.0 mm) and ferromagnetic steel inner portion; 4) copper cladding (thickness of 0.5 mm) and ferromagnetic steel inner portion; 5) ferromagnetic steel cladding (thickness of 1.0 mm) and copper inner portion; 6) aluminium cladding (thickness of 0.5 mm) and ferromagnetic steel inner portion; 7) plastic (polyethylene) cladding (thickness of 1.0 mm) and ferromagnetic steel inner portion; 8) non-ferromagnetic steel cladding (thickness of 1.0 mm) and ferromagnetic steel inner portion.

The ferromagnetic steel used in the present examples (f-steel) was a ferritic stainless steel with an electrical resistivity of 20.8x10 ^"8 Ohnrm, and relative magnetic permeability of 300.

The non-ferromagnetic steel used in the present examples (a-steel) was an austenitic stainless steel with an electrical resistivity of 20.8x10 ^"8 Ohnrm, and relative magnetic permeability of about 1.

Table 1

Hysteresis Resistive Total

Armour wire losses losses losses

(W/m) (W/m) (W/m)

1* f-steel only (5 mm diameter) 6.53 2.59 9.1 1

2* copper only (5 mm diameter) ~0 0.12 0.12 3 copper cladding (thickness of 1.0 0.31 1.70 2.01 mm) + f-steel inner portion (3.0 mm

diameter)

4 copper cladding (thickness of 0.71 2.96 3.67 0.5mm) + f-steel inner portion (4.0

mm diameter)

5* f-steel cladding (thickness of 1.0mm) 6.37 1.04 7.41 + copper inner portion (3.0 mm

diameter)

6 aluminium cladding (thickness of 1.28 3.35 4.63 0.5mm) + f-steel inner portion (4.0

mm diameter)

7* plastic cladding (thickness = 1.0 mm) 5.03 0.85 5.88 + f-steel inner portion (3.0 mm

diameter)

8 ^* a-steel cladding (thickness of 1.0mm) 3.36 1.85 5.21 + f-steel inner portion (3.0 mm

diameter)

^* Comparative examples

From the results shown in table 1 , it is evident that armours having the armour wires of the examples 3, 4 and 6 according to the invention - with ferromagnetic inner portion (f-steel) and an electrically conductive and non- ferromagnetic cladding (copper or aluminium) - have armour losses much lower than the comparative armours having wires made of ferromagnetic material only (f-steel of example 1 *), or of an electrically conductive and non-ferromagnetic inner portion and a ferromagnetic cladding (f-steel cladding and copper inner portion of example 5*) or of a non-ferromagnetic and non-electrically conductive cladding and ferromagnetic inner portion (plastic cladding and f-steel inner portion of example 7* and a-steel cladding and f-steel inner portion of example 8 ^* ). The results for the wires made of copper only (example 2*) are given for the sake of comparison, such wires being not suitable for providing armours with the required tensile strength.

Figure 3 shows the total armour losses (hysteresis plus resistive losses) generated in the cable above mentioned when considering armour layers made of wires having a ferromagnetic steel (f-steel) inner portion while adding increasing thickness of copper cladding (the total wire diameter being equal to 5 mm).

It is evident from the results shown in figure 3, that the armour losses decrease for increasing values of the copper cladding thickness.

However, for the reasons already set forth relating to the tensile strength required for the armour wires, 5 mm-diameter wires with a copper cladding 1 mm-thick at most (20% with respect to the total diameter of the armour wire) are preferably used as armour wires for power cables. In the present case, it is possible to achieve armour losses reductions higher than about 10% starting from a thickness value of about 0.1mm; a cladding thickness of 1 mm yielding a loss reduction greater than 70%.

The Applicant found that - for a cladding material having an electrical resistivity lower than 10x 0 ^"8 Ohm m and a ferromagnetic material with suitable mechanical resistance for the inner portion - a good compromise between the said two conflicting requirements can be achieved when the electrically conductive non-ferromagnetic cladding has a cross-section area comprised between 10% to 55%, preferably, 40%, with respect to the total cross-section area of the armour wire. In case of the armour wire with substantially circular cross-section, a good compromise can be achieved when the electrically conductive non- ferromagnetic cladding has a thickness between 2.5% to 20%, preferably 15%, with respect to the total diameter of the armour wire.

Taking into account the above formula (1) provided by IEC 60287-1-1 , the armour losses reduction, achieved thanks to the use of an electrically conductive and non-ferromagnetic cladding for the armour wires, advantageously enables to increase the permissible current rating of a cable. The rise of permissible current rating leads to two improvements in an AC transport system: increasing the current transported by a power cable and/or providing a power cable with a reduced electric conductor cross section area S, the increase/reduction being considered with respect to the case wherein the armour losses are instead computed with wires having substantially the same cross section area but being made of material/s with different electrical and/or magnetic features and/or having a different material arrangement.

This is very advantageous because it enables to make a cable more powerful and/or to reduce the size of the electric conductors with consequent reduction of cable size, weight and cost.

During the development activities performed by the Applicant in order to investigate the armour losses in an armoured AC power cable, the Applicant further found that the armour losses are further reduced when the armour wires according to the invention have an elongated cross section with the major axis oriented tangentially with respect to a cable circumference. Such a further reduction of the armour losses can amount from 5% to 25%.

Figure 4 schematically shows an example of armour 16 made of wires 16a with elongated cross section suitable for a preferred embodiment of the invention. The major axis of the wire cross section is indicated with A' and the minor axis with A".

For the sake of clarity, in this figure only the armour wires 16a of the cable, surrounding a circumference O (enclosing the cable core/s) is shown.

As clearly shown in figure 4, the major axis A' of the elongated cross section of the wires 16a is oriented according to a tangential direction Tn of the circumference O.

In the embodiment of figure 4 the elongated cross section of the wires 16a has a substantially rectangular shape, with smoothed angles. In a preferred embodiment of the invention, in case of a power cable 10 having at least two cores 12, in order to further reduce the armour losses, the helical winding lay of the armour wires 16a has the same direction as the stranding lay of the cores 12, as schematically shown in Figure 5.

This embodiment can be used in combination or in alternative with the embodiment described above, relating to the armour wires with elongated cross section.

It is noted that, even if in the above description and figures cables comprising an armour with a single layer of wires have been described, the invention also applies to cables wherein the armour comprises a plurality of layers, radially superimposed.

In such cables, the multiple-layer armour preferably comprises an inner layer of wires and an outer layer of wires, surrounding the inner layer.

As to the features of the wires of the inner layer, the same considerations made above with reference to an armour with a single layer of wires apply.

As to the outer layer, the armour wires are preferably made, at least in the greater part, of a single ferromagnetic metal, such as a ferromagnetic steel like material.

For cables comprising a multiple-layer armour, the same considerations made above with reference to the ratio λ ₂ · (losses in the armour to total losses in all conductors in the power cable) apply, wherein the losses in the armour are computed as the losses in the (inner) layer and the outer layer.