Method and armoured cable for transporting high voltage alternate current

Abstract

Armoured cable (10) comprising: a plurality of cores (12) stranded together according to a core stranding direction; an armour (16) surrounding the plurality of cores (12) and comprising a layer of metal wires (16a) helically wound around the cores (12) according to an armour winding direction; wherein the at least one of core stranding direction (21) and the armour winding direction (22) is recurrently reversed along the cable length L so that the armoured cable (10) comprises unilay sections (102) along the cable length where the core stranding direction (21) and the armour winding direction (22) are the same. The invention also relates to a method for improving the performances of the armoured cable (10) and to a method for manufacturing the armoured cable (10).

Claims

1. An armoured cable having a cable length and comprising: a plurality of cores, each core comprising an electrical conductor surrounded by a respective insulating layer, stranded together according to a core stranding direction; an armour surrounding the plurality of cores and comprising a layer of metal wires helically wound around the cores according to an armour winding direction; wherein the plurality of cores are embedded in a polymeric filler and the armour surrounds the polymeric filler, and wherein at least one of the core stranding direction and the armour winding direction is recurrently reversed along the cable length L so that the armoured cable comprises unilay sections along the cable length where the core stranding direction and the armour winding direction are the same.

2. The armoured cable according to claim 1, wherein the at least one of core stranding direction and the armour winding direction is recurrently reversed along the cable length L so that unilay sections alternate along the cable length with contralay sections.

3. The armoured cable according to claim 1, wherein the unilay sections along the cable length L involve, as a whole, at least 40% of the cable length L.

4. The armoured cable according to claim 1, wherein a number N of consecutive turns of at least one of the core stranding and the armour winding in a first direction is the same or varies along the cable length L.

5. The armoured cable according to claim 4, wherein a number M of consecutive turns of at least one of the core stranding and the armour winding in a second direction, reversed with respect to the first direction, is the same or varies along the cable length L.

6. The armoured cable according to claim 5, wherein N is equal to or different from M.

7. The armoured cable according to claim 4, wherein N1.

8. The armoured cable according to claim 4, wherein N10.

9. The armoured cable according to claim 5, wherein M1.

10. The armoured cable according to claim 5, wherein M10.

11. The armoured cable according to claim 1, wherein the plurality of cores is stranded together according to a core stranding pitch A that, in modulus, is the same or varies along a cable length L.

12. The armoured cable according to claim 1, wherein the metal wires are wound around the plurality of cores according to an armour winding pitch B that, in modulus, is the same or varies along a cable length L.

13. The armoured cable according to claim 2, wherein the metal wires are wound around the plurality of cores according to an armour winding pitch B that, in the contralay sections, is greater, in modulus, than the armour winding pitch B in the unilay sections.

14. The armoured cable according to claim 1, wherein the core stranding direction is recurrently reversed along the cable length L and the armour winding direction is unchanged.

15. The armoured cable according to claim 1, wherein at least part of the armour metal wires are made of ferromagnetic material.

16. A method for improving the performances of an armoured cable having a cable length L and comprising a plurality of cores, each core comprising an electrical conductor surrounded by a respective insulating layer, stranded together according to a core stranding direction, each electric conductor having a cross section area X; and an armour surrounding the plurality of cores, the armour comprising a layer of metal wires helically wound around the cores according to an armour winding direction, the armoured cable having losses when an alternate current I is transported, said losses determining a maximum allowable working conductor temperature , the plurality of cores being embedded in a polymeric filler and the armour surrounds the polymeric filler, the method comprising the steps of: reducing the losses by building the armoured cable such that the at least one of core stranding direction and the armour winding direction is recurrently reversed along the cable length L so that the armoured cable comprises unilay sections along the cable length L where the core stranding direction and the armour winding direction are the same; building the armoured cable with a reduced value of the cross section area X of each electric conductor, as determined by the value of the reduced losses, and/or rating the armoured cable at the maximum allowable working conductor temperature to transport said alternate current I with an increased value, as determined by the value of the reduced losses.

17. A method for manufacturing an armoured cable with a cable length L having losses when an alternate current I is transported, said losses determining a rating of the cable at maximum allowable conductor temperature , comprising the steps of: stranding a plurality of cores, each core comprising an electrical conductor surrounded by a respective insulating layer, together according to a core stranding direction, each electric conductor having a cross section area X; embedding the plurality of cores in a polymeric filler and surrounding the plurality of cores by helically winding an armour comprising a layer of metal wires around the plurality of cores and the polymeric filler according to an armour winding direction; wherein at least one of the core stranding direction and the armour winding direction is recurrently reversed along the cable length L so that the armoured cable comprises unilay sections along the cable length where the core stranding direction and the armour winding direction are the same; and wherein the cross section area X of each electric conductor is reduced and/or the rating of the cable at the maximum allowable working conductor temperature is increased, compared to a cable wherein the core stranding direction and armour winding direction are contralay along the cable length L.

Description

(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 armoured cable according to an embodiment of the invention;

(3) FIG. 2 schematically shows an embodiment of the invention wherein the core stranding direction is regularly reversed along the cable length;

(4) FIG. 3 schematically shows an embodiment of the invention wherein the armour winding direction is regularly reversed along the cable length;

(5) FIG. 4 shows the armour losses computed for a three-core cable versus the armour winding pitch B, by considering the armour losses inversely proportional to crossing pitch C;

(6) FIG. 5 shows the armour losses versus the armour winding pitch B computed for the same cable of FIG. 4 by using a 3D FEM computation;

(7) FIG. 6 is a sketch of a submarine cable deployment.

(8) FIG. 1 schematically shows an AC cable 10 for submarine application comprising three-phase cores 12. Each core comprises a metal conductor 12a in form of a rod or of stranded wires. The metal conductor 12a can, for example, be made of copper, aluminium or both. Each metal conductor 12a is sequentially surrounded by an insulating system 12b made of an inner semiconducting layer, an insulating layer and an outer semiconducting layer, said three layers (not shown) being based on 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 further comprise each metal screen 12c. The metal screen 12c can be made of lead, generally in form of an extruded layer, or of copper, in form of a longitudinally wrapped foil or of braided wires.

(9) The three cores 12 are helically stranded together according to a core stranding pitch A and a core stranding direction.

(10) The three cores 12 are, as a whole, embedded in a polymeric filler 11 surrounded, in turn, by a tape 15 and by a cushioning layer 14. For example, the tape 15 is a polyester or non-woven tape, and the cushioning layer 14 is made of polypropylene yarns.

(11) Around the cushioning layer 14, an armour 16 comprising a single layer of metal wires 16a is provided. The wires 16a are helically wound around the cable 10 according to an armour winding pitch B and an armour winding direction.

(12) The armour 16 surrounds the three cores 12 together, as a whole.

(13) At least part or all the metal wires 16a are made of a ferromagnetic material, which is advantageous in terms of costs with respect to non-ferromagnetic metals like, for example, stainless steel.

(14) The ferromagnetic material can be, for example, carbon steel, construction steel or ferritic stainless steel, optionally galvanized.

(15) The conductor 12a has a cross section area X, wherein X=(d/2).sup.2, d being the diameter of the conductor 12a.

(16) According to the invention, at least one of the core stranding direction and the armour winding direction is recurrently reversed along the cable length so that the cable 10 comprises unilay sections along the cable length wherein the core stranding direction and the armour winding direction are the same.

(17) FIG. 2 schematically shows an embodiment wherein the core stranding direction 21 is regularly reversed along the cable length so that the cores are alternately stranded together according to a right-handed (or clockwise) direction Z (Z-lay) and a left-handed (or counterclockwise) direction S (S-lay). This alternated laying configuration is hereinafter called S/Z configuration. On the other side, the armour winding direction 22 is unchanged along the cable length. In particular, in the embodiment shown, the armour winding direction is left-handed S. In this way, the cable comprises unilay sections 102 along the cable length L wherein the core stranding direction and the armour winding direction are the same (in the embodiment shown, they are both S). The cable also comprises contralay sections 101 along the cable length L wherein the core stranding direction and the armour winding direction are the opposite. In particular, in the embodiment shown, the core stranding direction is Z while the armour winding direction is S.

(18) FIG. 3 schematically shows another embodiment wherein the armour winding direction 22 is regularly reversed along the cable length so that the armour metal wires are alternately stranded together according to a right-handed (or clockwise) direction Z and a left-handed (or counterclockwise) direction S. On the other side, the core stranding direction 21 is unchanged along the cable length L. In particular, in the embodiment shown, the core stranding direction is right-handed Z. In this way, the cable comprises unilay sections 102 along the cable length L wherein the core stranding direction and the armour winding direction are the same (that is, in the embodiment shown, they are both Z). The cable also comprises contralay sections 101 along the cable length L wherein the core stranding direction and the armour winding direction are the opposite. In particular, in the embodiment shown, the core stranding direction is Z while the armour winding direction is S.

(19) FIG. 2 shows an embodiment wherein the number N of turns 21a of the cores in a Z section (a section of the cable length L with a Z core stranding direction) and the number M of turns 21b of the cores in a S section (a section of the cable length with a S core stranding direction) are equal to each other (in the example, N=M=4).

(20) Analogously, FIG. 3 shows an embodiment wherein the number N of turns 22a of the armour metal wires in a Z section (a section of the cable length L with a Z armour winding direction) and the number M of turns 22b of the armour metal wires in a S section (a section of the cable length with a S armour winding direction) are equal to each other (in the example, N=M=4).

(21) The case on N=M can be advantageous in terms of mechanical construction of the cable.

(22) However, the invention also applies to the case wherein N is different from M.

(23) Moreover, N and M can be either integer or decimal numbers. N and/or M can be the same (i.e. unchanged) along the cable length L (as shown in FIGS. 2 and 3) or vary (when N has different values in different S sections and M has different values in different Z sections).

(24) N is preferably greater than 2.5 and lower than 4.

(25) M is preferably greater than 2.5 and lower than 4.

(26) FIGS. 2 and 3 schematically show examples wherein the core stranding pitch A and the armour winding pitch B are, in modulus, equal to each other and unchanged along the cable length. However, the core stranding pitch A and the armour winding pitch B are preferably different from each other (in sign and/or absolute value) in order to avoid drawbacks in terms of mechanical strength of the cable.

(27) Moreover, the core stranding pitch A and/or the armour winding pitch B can vary along the cable length.

(28) For example, in an embodiment (not shown) of the invention, the armour winding pitch B in the contralay sections 101 is preferably greater, in modulus, than the armour winding pitch B in the unilay sections 102. As shown in FIGS. 4-5 described below, a higher value of B, in modulus, advantageously enables to limit the armour losses in the contralay sections 101 (the armour losses in the unilay sections 102 being already reduced by the unilay configuration per se).

(29) Further details about the values of A and B are disclosed, for example, by U.S. Pat. No. 9,431,153, the disclosure of which is herein incorporated by reference.

(30) With reference to what disclosed by U.S. Pat. No. 9,431,153, FIG. 4 shows the percentage of armour losses (in ordinate) versus the armour winding pitch B (in abscissa; meters), as obtained by computing by assuming the armour losses as inversely proportional to crossing pitch C. The following conditions were considered: an AC three-core cable with the cores stranded together according to a core stranding pitch A, with A=2500 mm; only one armour wire, wound around the cable according to a variable armour winding pitch B; a current of 800 A into the conductors; a conductor cross section area X of 800 mm.sup.2. Negative value of the armour winding pitch B means contralay winding directions of the armouring wires with respect to the cores; positive value of the armour winding pitch B means unilay winding directions of the armouring wires with respect to the cores. The computation considered losses at 100% those empirically measured with a comparative contralay cable having three cores stranded together according to a core stranding pitch A of 2570 mm; an armour single layer of wires wound around the cable according to an armour winding pitch B contralay to the core stranding pitch A, B being 1890 mm, and crossing pitch C equal to about 1089 mm; a wire diameter d of 6 mm; a cross section area X of 800 mm.sup.2.

(31) With reference to the disclosure of U.S. Pat. No. 9,431,153, FIG. 5 shows the armour loss percentages (in ordinate) as a function of the armour winding pitch B (in abscissa, mm), as obtained by using a 3D FEM (Finite Element Method) computation, for verifying the hypothesis made in the computation of FIG. 4 Like in the case of the computation of FIG. 4, the FEM computation considered losses at 100% those empirically measured with the comparative contralay cable.

(32) Both figures show that 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 the armour winding pitch B is contralay to the core stranding pitch A. The armour losses have a minimum when core stranding pitch A and armour winding pitch B are equal (unilay cable with cores and armour wire with the same pitch) while they are very high when B is close to zero (positive or negative). In addition, an increase of armour winding pitch Beither unilay or contralay with respect to core stranding pitch Abrings to reduction of the armouring losses. In order to reduce losses, the armour winding pitch B is preferably higher than 0.4 A.

(33) During development activities performed by the Applicant in order to investigate the losses (in particular, armour and metal screen losses) in an AC armoured cable, the Applicant analyzed an AC cable having: three cores stranded together according to a S/Z configuration (of the type shown in FIG. 2) with a core stranding pitch A of 3000 mm in absolute value (A being equal to +3000 mm in the Z sections and to 3000 mm in the S sections); a single layer of ninety-five (95) wires of galvanized ferritic steel wound around the cable according to a S armour winding direction and an armour winding pitch B of 2000 mm; a crossing pitch C equal to 1200 mm in the contralay sections; a crossing pitch C equal to 6000 mm in the unilay sections; an external wire diameter d of 7 mm; a cross section area X of 1000 mm.sup.2 for a rated voltage of 150 KV; an overall external diameter of the cable of 246 mm; a metal screen of lead with an electrical resistivity of 21.4.Math.10.sup.8 Ohm.Math.m and relative magnetic permeability .sub.r=1; and armour wires with an electrical resistivity of 20.8.Math.10.sup.8 Ohm.Math.m and relative magnetic permeability .sub.r=300.

(34) Results of the Applicant's activities are given in the examples 1-3 below.

EXAMPLE 1

(35) A first sample of the cable has been cut in order to obtain a single contralay section of the cable (named S-Z sample), with S armour winding direction and Z core stranding direction.

(36) A second sample (named S-Z/S sample) of the cable has been cut in order to obtain a first half of the sample in contralay condition (with a single contralay section having S armour winding direction and Z core stranding direction) and the remaining half of the sample in unilay condition (with a single unilay section having S armour winding direction and S core stranding direction).

(37) A third sample of the cable has been cut in order to obtain a single unilay section of the cable (named S-S sample), with S armour winding direction and S core stranding direction.

(38) All of the threes samples had the same length.

(39) The three samples have been tested in order to experimentally measure a value of the ratio between the eddy currents in the metal screens (I.sub.screen) and the current in the conductors (I.sub.conductor). The following Table 1 shows the measured values.

(40) TABLE-US-00001 TABLE 1 Sample I.sub.screen/I.sub.conductor S-Z sample 0.219 S-Z/S sample 0.203 S-S sample 0.192

(41) The experimental measures show that the S-Z/S sample enables to reduce the eddy currents in the metal screens and thus, the cable losses, with respect to a contralay configuration (S-Z sample).

(42) The unilay configuration (S-S sample) has the best performances in terms of reduction of eddy currents in the metal screens and, thus, of screen losses. However, as said above, a whole unilay configuration is disadvantageous in terms of mechanical performances of the cable, especially in terms of torsional stability of the cable during laying operations.

(43) On the other side, the contralay configuration (S-Z sample) has the worst performances in terms of reduction of eddy currents in the metal screens and, thus, of screen losses.

(44) The configuration according to the invention, wherein contralay sections alternate with unilay sections, enables, on the one side, to reduce cable losses with respect to a whole contralay configuration and, on the other side, to improve the mechanical performances of the cable, especially during laying operations, with respect to a whole unilay configuration.

(45) FIG. 6 sketches a laying operation of a submarine cable 62. The cable 62 is connected to an anchoring point 61 on a deposition vessel 60, and a tensile strain is exerted on the cable 62 between the anchoring point 61 and a point T where the cable 62 touches the seabed 63, the point T substantially corresponding to the deposition depth. During deployment, the tensile strain tends to straighten the lay of the cable cores and of the armour wires. In case of unilay configuration at least between anchoring point 61 and point T, and especially in deep or extra-deep water deployment, the drop of tensile strain on the cable, possibly occurring during laying operation or when the cable reaches the seabed (point T), could result in a cable buckling up to a bending radius which could compress the cores and result in potential harms. According to the configuration of the invention, this phenomenon is counterbalanced by contralay sections so that the torsional stability of the cable as a whole is not affected.

(46) Similar results can be obtained in an embodiment (not shown) of the invention wherein both the core stranding direction and the armour winding direction are regularly reversed along the cable length so that the armoured cable comprises unilay sections alternating with unilay sections having opposite sign of the core stranding direction and the armour winding direction.

EXAMPLE 2

(47) The permissible current ratings of the above mentioned cables were computed with various combinations of unilay and contralay sections.

(48) The permissible current ratings were computed by using a numerical model of the cable and according to IEC 60287 for the following conditions: laying depth 0.8 m at top of the cable, ambient temperature of 15 C., soil thermal resistivity 0.7 K.Math.m/W, and steady state conditions.

(49) In particular, the permissible current rating has been computed according to the above mentioned formula (1) of IEC 60287 wherein, however, the armour losses and screen losses have been computed, taking into account, in said numerical model, that the cable comprises cores (in the example, three cores) helically stranded together with a core stranding pitch A and armour metal wires (in the example, 95 galvanized ferritic steel wires) helically wound around the cores with a armour winding pitch B.

(50) The following Table 2 shows the computed values.

(51) TABLE-US-00002 TABLE 2 % contralay % unilay % (I-Ic)/Ic % (L-Lc)/Lc 100 0 0.00% 0.00% 90 10 0.44% 4.53% 80 20 0.88% 9.01% 70 30 1.32% 13.45% 60 40 1.87% 17.86% 50 50 2.31% 22.22% 40 60 2.75% 26.55% 30 70 3.19% 30.84% 20 80 3.63% 35.09% 10 90 4.07% 39.30% 0 100 4.51% 43.47%

(52) Table 2 shows the permissible current ratings I and the cable losses L (in particular, armour and screen losses) computed in cables having increasing percentages of length in unilay configuration with respect to the permissible current rating Ic and the cable losses Lc, respectively, computed in a whole contralay cable (100% contralay configuration).

(53) The computed values show that the permissible current rating I increases as the percentage of length in unilay configuration increases. On the other side, the cable losses (due to armour and metal screen losses) decrease in value as the percentage of length in unilay configuration increases.

(54) As stated above, the rise of permissible current rating (and, accordingly, the reduction of cable losses) 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 X. 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.

(55) The armoured cable of the invention is thus built with a reduced value of the cross section area X of the electric conductor, as determined by the value of the reduced losses.

(56) In alternative or in addition, the armoured cable of the invention is rated at the maximum allowable working conductor temperature to transport an alternate current I with an increased value, as determined by the value of the reduced losses. In particular, the armoured cable of the invention can be operated at the maximum allowable working conductor temperature so as to transport an alternate current I with an increased value, as determined by the value of the reduced losses.

(57) The armoured cable of the invention can be operated with an increased value of the transported current and/or can be built with a reduced cross section area X, with respect to what calculated on the basis of the IEC 60287 recommendations.

(58) In order to guarantee a good compromise between the two conflicting needs of increasing the permissible current rating I (and reducing the cable losses) and improving the mechanical stability of the cable, an armoured cable according to the invention preferably has 20-80% of unilay sections, more preferably 30-70%, even more preferably 40-60%, along the cable length. These values advantageously enable to obtain an increase in permissible current rating I, with respect to a whole contralay cable, of 0.88%-3.63%, 1.32%-3.19%, 1.87%-2.75%, respectively.

(59) Moreover, in the armoured cable according to the invention, the preferred percentage of unilay sections is preferably attained by regularly arranging the unilay sections along the cable length L (regularly alternated with contralay sections) in order to avoid a cable configuration having a too long contralay section (e.g. covering a first half of the cable) followed by a too long unilay section (e.g. covering the second half of the cable). This latter solution would be disadvantageous both in mechanical terms (because the advantage of having alternating contralay and unilay sections is reduced) and electrical terms (because a potentially harmful voltage of a significant level can build up at the end of a long section that may be dangerous in submarine cables in case of water seepage).

(60) Regarding total losses for capitalisation, in the cable of the invention they are computed as an average value of dissipated power per length unit (W/m) due to armour and screen losses in the contralay sections and unilay sections, weighted over the length covered by the contralay sections and the unilay sections. As the (armour and screen) losses in the unilay sections are lower than in the contralay sections, the total losses for capitalisation in the cable of the invention are reduced with respect to that of a whole contralay cable.

(61) Moreover, the total losses for capitalisation in the cable of the invention are reduced with respect to what calculated on the basis of the IEC 60287 recommendations.

EXAMPLE 3

(62) The permissible current ratings and the cable losses of the above mentioned cable as in the example 2 were computed with the difference that 48 (forty-eight) armour wires of galvanized ferritic steel were considered, instead of 95. The results are set forth in Table 3.

(63) TABLE-US-00003 TABLE 3 % contralay % unilay % (I-Ic)/Ic % (L-Lc)/Lc 100 0 0.00% 0.00% 90 10 0.21% 3.83% 80 20 0.43% 7.65% 70 30 0.64% 11.46% 60 40 0.85% 15.26% 50 50 1.07% 19.06% 40 60 1.28% 22.84% 30 70 1.49% 26.61% 20 80 1.71% 30.38% 10 90 1.92% 34.13% 0 100 2.13% 37.88%

(64) Also in this example, the computed values show that the permissible current rating I increases as the percentage of length of unilay sections increases. On the other side, the cable losses L (armour and metal screen losses) decrease in value as the percentage of length of unilay sections increases.