Method and armoured power cable for transporting alternate current

Abstract

A method and armored cable for transporting an alternate current at a maximum allowable working conductor temperature, as determined by the overall cable losses, the overall cable losses including conductor losses and armor losses. The cable includes at least one core, including an electric conductor having a cross section area, and an armor surrounding the core along a circumference. The method includes: causing the armor losses not higher than 40% of the overall cable losses by having the armor made with a layer of a plurality of metal wires having an elongated cross section with a major axis, the major axis being oriented tangentially with respect to the circumference; and transporting the alternate current at the maximum allowable working conductor temperature, in the electric conductor having cross section area sized on the overall cable losses including the armor losses not higher than 40% of the overall cable losses.

Claims

1. A method of transporting an alternate current at a maximum allowable working conductor temperature using an alternate current power cable comprising at least one core, wherein each core comprises an electric conductor having a cross section area and an armour surrounding said core along a circumference, said cable having overall cable losses comprising conductor losses and armour losses, the method comprising: selecting an armour with armour losses not higher than 10% of the overall cable losses, wherein said armour is made with a layer of a plurality of metal wires having an elongated cross section with a major axis, said major axis being oriented tangentially with respect to the circumference, wherein the layer of the plurality of metal wires includes one or more wires made of a ferromagnetic material, wherein one or more non-ferromagnetic wires are mixed with the one or more wires made of ferromagnetic material in a circumferential direction along the armour's entire length; modifying a permissible current rating to an increased value, the increased value being determined by the value of the armour losses being not higher than 10% of the overall cable losses; and transporting at said maximum allowable working conductor temperature in the electric conductor, the alternate current at the increased value of the permissible current rating.

2. The method according to claim 1, wherein the elongated cross section of the plurality of metal wires of said armour has a ratio between a major axis length and minor axis length at least equal to 1.5.

3. The method according to claim 1, wherein the elongated cross section of the plurality of metal wires of said armour has a ratio between a major axis length and minor axis length not higher than 5.

4. The method according to claim 1, wherein the elongated cross section of the plurality of metal wires of said armour has smoothed edges.

5. The method according to claim 1, wherein the elongated cross section of the plurality of metal wires of said armour has a minor axis from 1 mm to 7 mm long.

6. The method according to claim 1, wherein the elongated cross section of the plurality of metal wires of said armour has a major axis from 3 mm to 20 mm long.

7. The method according to claim 1, wherein the alternate current power cable comprises more than one core, and reducing armour losses to a value not higher than 10% of the overall cable losses comprises: stranding together the cores according to a core stranding lay and a core stranding pitch A; and winding the plurality of metal wires around the cores according to a helical armour winding lay and an armour winding pitch B, wherein the helical armour winding lay has a same direction as the core stranding lay, and the armour winding pitch B is from 0.4 A to 2.5 A and differs from A by at least 10%.

8. An alternate current power cable comprising: at least one core comprising an electric conductor; and an armour surrounding the at least one core along a circumference, in which each electric conductor has a cross section area sized for operating the cable to transport said alternate current at a maximum allowable working conductor temperature, as determined by overall cable losses including armour losses, wherein: the armour comprises a layer of a plurality of metal wires with an elongated cross section with a major axis, said plurality of metal wires being arranged with the major axis oriented tangentially with respect to the circumference, whereby the armour losses are reduced to a value not higher than 10% of the overall cable losses, wherein the layer of the plurality of metal wires includes one or more wires comprising a ferromagnetic material, wherein one or more non-ferromagnetic wires are mixed with the one or more wires made of ferromagnetic material in a circumferential direction along the armour's entire length; and further wherein: the electric conductor has a cross section area sized with a reduced value as determined by reckoning the value of the reduced armour losses not higher than 10% of the overall cable losses; and/or the alternate current, to be transported in the electric conductor at the maximum allowable working conductor temperature, is sized with an increased value as determined by reckoning the value of the reduced armour losses not higher than 10% of the overall cable losses.

9. The power cable according to claim 8, wherein the elongated cross section of the plurality of metal wires has a ratio between a major axis length and a minor axis length at least equal to 1.5.

10. The power cable according to claim 8, wherein the elongated cross section of the plurality of metal wires has a ratio between a major axis length and a minor axis length not higher than 5.

11. The power cable according to claim 8, wherein the elongated cross section of the plurality of metal wires has smoothed edges.

12. The power cable according to claim 8, wherein the elongated cross section of the plurality of metal wires has a minor axis from 1 mm to 7 mm long.

13. The power cable according to claim 8, wherein the elongated cross section of the plurality of metal wires has a major axis from 3 mm to 20 mm long.

14. The power cable according to claim 8, comprising at least two cores stranded together according to a core stranding lay and a core stranding pitch A, wherein the plurality of metal wires is wound around the at least two cores according to a helical armour winding lay and an armour winding pitch B, wherein the helical armour winding lay has a same direction as the core stranding lay, and the armour winding pitch B is from 0.4 A to 2.5 A and differs from A by at least 10%.

15. A method of transporting an alternate current at a maximum allowable working conductor temperature using an alternate current power cable comprising at least one core, wherein each core comprises an electric conductor having a cross section area and an armour surrounding said core along a circumference and the cable having overall cable losses comprising conductor losses and armour losses, the method comprising: selecting an armour with armour losses not higher than 10% of the overall cable losses, wherein the armour is made with a layer of a plurality of metal wires having an elongated cross section with a major axis, the major axis being oriented tangentially with respect to the circumference, wherein the layer of the plurality of metal wires includes one or more wires made of a ferromagnetic material, wherein one or more non-ferromagnetic wires are mixed with the one or more wires made of ferromagnetic material in a circumferential direction along the armour's entire length; sizing the electric conductor with a reduced conductor cross section area determined by the value of the armour losses being not higher than 10% of the overall cable losses; and transporting, at the maximum allowable working conductor temperature in the electric conductor, the alternate current.

16. A method of transporting an alternate current at a maximum allowable working conductor temperature using an alternate current power cable comprising at least one core, wherein each core comprises an electric conductor having a cross section area and an armour surrounding said core along a circumference, said cable having overall cable losses comprising conductor losses and armour losses, the method comprising: selecting an armour with armour losses not higher than 10% of the overall cable losses, wherein said armour is made with a layer of a plurality of metal wires having an elongated cross section with a major axis, said major axis being oriented tangentially with respect to the circumference, wherein the layer of the plurality of metal wires is made of a ferromagnetic material along the armour's entire length; modifying a permissible current rating to an increased value, the increased value being determined by the value of the armour losses being not higher than 10% of the overall cable losses; and transporting at said maximum allowable working conductor temperature in the electric conductor, the alternate current at the increased value of the permissible current rating.

17. The method according to claim 16, wherein the elongated cross section of the plurality of metal wires of said armour has a ratio between a major axis length and minor axis length at least equal to 1.5.

18. The method according to claim 16, wherein the elongated cross section of the plurality of metal wires of said armour has a ratio between a major axis length and minor axis length not higher than 5.

19. The method according to claim 16, wherein the elongated cross section of the plurality of metal wires of said armour has smoothed edges.

20. The method according to claim 16, wherein the elongated cross section of the plurality of metal wires of said armour has a minor axis from 1 mm to 7 mm long.

21. The method according to claim 16, wherein the elongated cross section of the plurality of metal wires of said armour has a major axis from 3 mm to 20 mm long.

22. The method according to claim 16, wherein the alternate current power cable comprises more than one core, and reducing armour losses to a value not higher than 10% of the overall cable losses comprises: stranding together the cores according to a core stranding lay and a core stranding pitch A; and winding the plurality of metal wires around the cores according to a helical armour winding lay and an armour winding pitch B, wherein the helical armour winding lay has a same direction as the core stranding lay, and the armour winding pitch B is from 0.4 A to 2.5 A and differs from A by at least 10%.

23. An alternate current power cable comprising: at least one core comprising an electric conductor; and an armour surrounding the at least one core along a circumference, in which each electric conductor has a cross section area sized for operating the cable to transport said alternate current at a maximum allowable working conductor temperature, as determined by overall cable losses including armour losses, wherein: the armour comprises a layer of a plurality of metal wires with an elongated cross section with a major axis, said plurality of metal wires being arranged with the major axis oriented tangentially with respect to the circumference, whereby the armour losses are reduced to a value not higher than 10% of the overall cable losses, wherein the layer of the plurality of metal wires is made of a ferromagnetic material along the armour's entire length; and further wherein: the electric conductor has a cross section area sized with a reduced value as determined by reckoning the value of the reduced armour losses not higher than 10% of the overall cable losses; and/or the alternate current, to be transported in the electric conductor at the maximum allowable working conductor temperature, is sized with an increased value as determined by reckoning the value of the reduced armour losses not higher than 10% of the overall cable losses.

24. The power cable according to claim 23, wherein the elongated cross section of the plurality of metal wires has a ratio between a major axis length and a minor axis length at least equal to 1.5.

25. The power cable according to claim 23, wherein the elongated cross section of the plurality of metal wires has a ratio between a major axis length and a minor axis length not higher than 5.

26. The power cable according to claim 23, wherein the elongated cross section of the plurality of metal wires has smoothed edges.

27. The power cable according to claim 23, wherein the elongated cross section of the plurality of metal wires has a minor axis from 1 mm to 7 mm long.

28. The power cable according to claim 23, wherein the elongated cross section of the plurality of metal wires has a major axis from 3 mm to 20 mm long.

29. The power cable according to claim 23, comprising at least two cores stranded together according to a core stranding lay and a core stranding pitch A, wherein the plurality of metal wires is wound around the at least two cores according to a helical armour winding lay and an armour winding pitch B, wherein the helical armour winding lay has a same direction as the core stranding lay, and the armour winding pitch B is from 0.4 A to 2.5 A and differs from A by at least 10%.

30. A method of transporting an alternate current at a maximum allowable working conductor temperature using an alternate current power cable comprising at least one core, wherein each core comprises an electric conductor having a cross section area and an armour surrounding said core along a circumference and the cable having overall cable losses comprising conductor losses and armour losses, the method comprising: selecting an armour with armour losses not higher than 10% of the overall cable losses, wherein the armour is made with a layer of a plurality of metal wires having an elongated cross section with a major axis, the major axis being oriented tangentially with respect to the circumference, wherein the layer of the plurality of metal wires is made of a ferromagnetic material along the armour's entire length; sizing the electric conductor with a reduced conductor cross section area determined by the value of the armour losses being not higher than 10% of the overall cable losses; and transporting, at the maximum allowable working conductor temperature in the electric conductor, the alternate current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 schematically shows an exemplary power cable according to an embodiment of the invention;

(3) FIGS. 2-4 schematically show three examples of elongated cross sections of armour metal wires that can be used in the cable of FIG. 1;

(4) FIG. 5 schematically shows the meaning of symbols Dw, and ;

(5) FIG. 6 schematically illustrates stranded cores and wound armour wires, respectively with core stranding pitch A and armour winding pitch B, of a power cable according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 schematically shows an exemplarily armoured AC power cable 10 for underwater application comprising three cores 12. Each core 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 (not shown) 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.

(7) The three cores 12 are helically stranded together according to a core stranding pitch A. The three cores are each enveloped by a metal sheath 13 (for example, made of lead) and embedded in a polymeric filler 11 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.

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

(9) The wires 16a are metallic and are preferably made of a ferromagnetic material such as carbon steel, construction steel, ferritic stainless steel.

(10) In armour 16, the number of ferromagnetic wires 16b is preferably reduced with respect to a situation wherein the armour ferromagnetic wires cover all the external perimeter of the cable 10.

(11) Number of wires in an armour layer can be, for example, computed as the number of wires that fill-in the perimeter of the cable and a void of about 5% of a wire diameter is left between two adjacent wires.

(12) In order to reduce the number of ferromagnetic wires 16b, the armour 16 can advantageously comprise ferromagnetic wires 16b alternating with non-ferromagnetic wires 16c (e.g., plastic or stainless steel).

(13) According to the invention, the wires 16a have an elongated cross section with a major axis oriented tangentially with respect to the cable 10.

(14) FIGS. 2-4 schematically show four examples of armour 16 made of wires 16a with different elongated cross sections suitable for the present invention. The cross-section areas of the three examples can be different from one another. The major axis of the wire cross section is indicated with A and the minor axis with A.

(15) For the sake of clarity, in these figures only the wires 16a surrounding a circumference O, enclosing the core/s 12 of the cable 10, are shown.

(16) In the embodiment of FIG. 2a the elongated cross section of the wires 16a has a substantially rectangular shape, with smoothed angles.

(17) In the embodiment of FIG. 2b, the wires of the armour 16 are mixed ferromagnetic wires 16b and non-ferromagnetic wires 16c.

(18) In the embodiment of FIG. 3, where only a portion of the armour 16 is shown, the elongated cross section has a notch and a protrusion at the two opposing ends along major axis A, so as to improve shape matching of adjacent wires 16a.

(19) In the embodiment of FIG. 4 the elongated cross section is substantially a circumferential portion of an annulus, with smoothed angles.

(20) As shown in FIG. 2a, 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.

(21) During development activities performed in order 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 pitch A of 1442 mm; an electric conductor cross section area S of 500 mm.sup.2; an AC current in each conductor of 800 A; a frequency of 50 Hz; phase to phase voltage of 18/30 KV; armour wires having an electrical resistivity of 20.8*10.sup.8 ohm*m, and relative magnetic permeability .sub.r=|.sub.r|e.sup.i with |.sub.r|=300 and =60.

(22) In a first investigation performed on a model based on said cable, the Applicant computed, by using a 3D model, the losses generated in a single straight armour wire having circular, square or rectangular cross section with smoothed edges, with different sizes.

(23) The results of the computations are shown in Table 1 below. The meaning of symbols Dw, and in case of square and rectangular cross section with smoothed edges is schematically shown in FIG. 5. In case of circular cross section, Dw is the wire diameter. The wire total losses indicate both resistive and hysteretic losses.

(24) TABLE-US-00001 TABLE 1 wire cross wire total Wire cross section shape section losses and size area (mm.sup.2) (W/m) circular Dw = 5 mm 1 19.6 0.272 circular Dw = 5.5 mm 1 23.8 0.309 square Dw = 5 mm; = 0.15 1 25.0 0.327 Rectangular Dw = 5 mm; = 0.15 2 50.0 0.548 Rectangular Dw = 5 mm; = 0.15 3 75.0 0.744 Rectangular Dw = 5 mm; = 0.15 4 100.0 0.919

(25) In case of a single straight armour wire, substantially parallel to the cable longitudinal axis, the armour wire having a circular or square cross section generally provides lower losses with respect to a wire having a rectangular cross section. In the single wires having rectangular cross-section, the losses increase proportionally to the ratio major axis/minor axis .

(26) In a further investigation performed on the same model as above, the Applicant computed, by using a 3D model, the armour losses generated in a layer of armour formed by straight wires having circular, square or rectangular cross section with smoothed edges and different sizes, the overall area of the armour cross section being substantially the same.

(27) The results of the computations are shown in table 2 below.

(28) TABLE-US-00002 TABLE 2 overall area armour number of armour total Wire cross section of cross losses shape and size wires section (mm.sup.2) (W/m) circular 1 66 1194.3 8.78 Dw = 4.8 mm circular 1 61 1197.7 9.11 Dw = 5 mm circular 1 50 1187.9 9.41 Dw = 5.5 mm square 1 48 1200.0 9.56 Dw = 5 mm; = 0.15 Rectangular 2 24 1200.0 8.64 Dw = 5 mm; = 0.15 Rectangular 3 16 1200.0 8.12 Dw = 5 mm; = 0.15 Rectangular 4 12 1200.0 7.75 Dw = 5 mm; = 0.15

(29) In case of armour with a plurality of straight armour wires, substantially parallel to the cable longitudinal axis, the losses have a behaviour which is just the opposite of the behaviour shown in Table 1. Indeed, in the present test the armours having wires with rectangular cross section have losses much lower than the armours having wires with circular or square cross section. In particular, the armour losses decrease by increasing the ratio major axis/minor axis . The Applicant also measured the losses in an armour made of a metallic tube having a cross-section area of 1200.0 mm.sup.2. The losses of this tube amounted to 11.44 W/m, considerably greater than any other armour configuration tested in Table 2.

(30) Taking into account the above formula (1) provided by IEC 60287-1-1, the armour losses reduction due to the use of elongated cross section wires 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 not elongated cross section, the overall area of the armour cross section being substantially the same.

(31) 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.

(32) Without the aim of being bound to any theory, the Applicant believes that his finding (that the armour losses are highly reduced when the armour wires have an elongated cross section with the major axis oriented tangentially with respect to the cable) is due to the fact that the use of armour wires having an elongated cross section enables to reduce the wire surface facing the magnetic field generated by the AC current transported by the cable conductors with respect to the volume of magnetic material of the wires, thereby reducing the eddy currents induced into the armour wires.

(33) It is observed that the above investigations have been performed by considering straight armour wires, in order to investigate the effects of wire cross section on the armour losses independently from any other effect on the armour losses due, for example, to wire winding.

(34) However, in the cable 10 the wires 16a are advantageously helically wound according to an armour winding pitch B.

(35) During the development activities performed by the Applicant in order to investigate the armour losses in an AC electric cable, the Applicant further found that the armour losses highly change depending on the fact that the armour winding pitch B is unilay or contralay to the core stranding pitch A. In particular, the armour losses are highly reduced when the armour winding pitch B is unilay to the core stranding pitch A, compared with the situation wherein the armour winding pitch B is contralay to the core stranding pitch A.

(36) In a preferred embodiment of the invention, in order to further reduce the armour losses, the helical armour winding lay has thus the same direction as the core stranding lay, as schematically shown in FIG. 6.

(37) Advantageously, the armour winding pitch B is higher than 0.4 A. Preferably, B0.5 A. More preferably, B0.6 A. Advantageously, the armour winding pitch B is smaller than 2.5 A. More preferably, the armour winding pitch B is smaller than 2 A. Even more preferably, the armour winding pitch B is smaller than 1.8 A.

(38) Advantageously, the armour winding pitch B is different from the core stranding pitch A (BA). Such a difference is at least equal to 10% of pitch A. Though seemingly favourable in term of armouring loss reduction, the configuration with B=A would be disadvantageous in terms of mechanical strength.

(39) Advantageously, the core stranding pitch A, in modulus, is of from 1000 to 3000 mm. More advantageously, the core stranding pitch A, in modulus, is of from 1500 to 2600 mm. Low values of A are economically disadvantageous as higher conductor length is necessary for a given cable length. On the other side, high values of A are disadvantageous in term of cable flexibility.

(40) Advantageously, crossing pitch C is preferably higher than the core stranding pitch A, in modulus. More preferably, C3 A, in modulus. Even more preferably, C10 A, in modulus.

(41) Without the aim of being bound to any theory, the Applicant believes that this further finding (that the armour losses are highly reduced when B is unilay to A) is due to the fact that when A and B are of the same sign (same direction) and, in particular, when A and B are equal or very similar to each other, the cores and the armour wires are parallel or nearly parallel to each other. This means that the magnetic field generated by the AC current transported by the conductors in the cores is perpendicular or nearly perpendicular to the armour wires. This cause the eddy currents induced into the armour wires to be parallel or nearly parallel to the armour wires longitudinal axis.

(42) On the other hand, when A and B are of opposite sign (contralay), the cores and the armour wires are perpendicular or nearly perpendicular to each other. This means that the magnetic field generated by the AC current transported by the conductors in the cores is parallel or nearly parallel to the armour wires. This cause the eddy currents induced into the armour wires to be perpendicular or nearly perpendicular with respect to the armour wires longitudinal axis.

(43) In the light of the above observations, the Applicant found that it is possible to further reduce the armour losses in an AC cable by using an armour winding pitch B unilay to the core stranding pitch A, with 0.4 AB2.5 A. In particular, the Applicant found that, by using an armour winding pitch B unilay to the core stranding pitch A, with 0.4 AB2.5 A, the ratio .sub.2 of losses in the armour to total losses in all conductors in the electric power cable is much smaller than the value .sub.2 as computed according to the above mentioned formula (2) of IEC Standard 60287-1-1.

(44) Taking into account the above formula (1) provided by IEC 60287-1-1, the unilay configuration of armour wires and cores enables to increase the permissible current rating of a cable. As stated above, the rise of permissible current rating leads to two improvements in an AC transport system: increasing the current transported by a cable and/or providing a cable with a reduced cross section area S, the increase/reduction being considered with respect to the case wherein the armour losses are instead computed according to formula (2) above mentioned.

(45) 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.

(46) In such cables, the multiple-layer armour preferably comprises a (inner) layer of wires with an armour winding lay and an armour winding pitch B, and an outer layer of wires, surrounding the (inner) layer, with an outer layer winding lay and an outer layer winding pitch B.

(47) As to the features of the (inner) layer, the armour winding lay, the armour winding pitch B, the core stranding lay and the core stranding pitch A, the same considerations made above with reference to an armour with a single layer of wires apply.

(48) In particular, the wires of the (inner) layer have an elongated cross section with a major axis oriented tangentially with respect to the cable 10. In addition, the armour winding lay of the (inner) layer is preferably unilay to the core stranding lay.

(49) As to the outer layer, the outer layer winding lay is preferably contralay with respect to the core stranding lay (and to the armour winding lay). This advantageously improves the mechanical performances of the cable.

(50) As explained in detail above, when the armour winding lay of the (inner) layer of wires is unilay to the core stranding lay, the losses in the armour are highly reduced as well as the magnetic field (as generated by the AC current transported by the cable conductors) outside the (inner) layer of the armour, which is shielded by the inner layer. In this way, the outer layer, surrounding the (inner) layer, experiences a reduced magnetic field and generates lower armour losses, even if used in a contralay configuration with respect to the core stranding lay.

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