Armoured cable for transporting alternate current

Abstract

The present disclosure relates to an armoured cable (10) for transporting alternate current comprising: at least one core (12), each core comprising an electric conductor (121); at least one metallic screen (126) surrounding the at least one core (12); an armour (16), surrounding the at least one metallic screen, comprising an inner layer (16a) of armour wires and an outer layer (16b) of armour wires, at least part of the armour wires of the inner layer (16a) and at least part of the armour wires of outer layer (16b) comprising a ferromagnetic material; and a separating layer between the inner layer (16a) of armour wires and the outer layer (16b) of armour wires. The separating layer has a thickness of at least 1 mm. The present disclosure also relates to a method for reducing losses in said armoured cable and to a method for improving the performances of said armoured cable.

Claims

1. An armoured cable for transporting alternate current comprising: at least one core, each core comprising an electric conductor; at least one metallic screen surrounding the at least one core; an armour surrounding the metallic screen, comprising an inner layer of armour wires and an outer layer of armour wires, at least part of the armour wires of the inner layer and at least part of the armour wires of outer layer comprising a ferromagnetic material; and a separating layer between the inner layer of armour wires and the outer layer of armour wires, wherein the separating layer has a thickness greater than 1 mm.

2. The armoured cable according to claim 1, wherein the separating layer is based on a material having a thermal conductivity greater than 0.1 W/(m.Math.K).

3. The armoured cable according to claim 1, wherein the separating layer has a thickness of 3 mm at most.

4. The armoured cable according to claim 1, wherein the armour wires of the inner layer and of the outer layer have an elongated cross section.

5. The armoured cable according to claim 1, wherein the armour wires of the inner layer and of the outer layer have a circular cross section.

6. The armoured cable according to claim 1, wherein the armour wires of the inner layer are wound around at least two cores according to a inner helical armour winding lay and the armour wires of the outer layer are wound around the at least two cores according to an outer helical armour winding lay, the inner helical armour winding lay and the outer helical armour winding lay having opposite direction.

7. The armoured cable according to claim 1, wherein the armour wires of the inner layer are wound around at least two cores according to a inner helical armour winding lay and the least two cores are stranded together according to a core stranding lay, the inner helical armour winding lay having a same direction as the core stranding lay.

8. The armoured cable according to claim 1, wherein the separating layer is made of polymeric material or of natural fibres.

9. A method for improving the performances of an armoured AC cable having cable losses when an alternate current I is transported, wherein the i armoured AC cable comprises: at least one core, each comprising an electric conductor having a cross section area X sized for operating the armoured AC cable to transport the alternate current I at a maximum allowable working conductor temperature θ, as determined by the cable losses; at least one metallic screen surrounding the at least one core; an armour surrounding the at least one metallic screen, comprising an inner layer of armour wires and an outer layer of armour wires, at least part of the armour wires of the inner layer and at least part of the armour wires of outer layer comprising a ferromagnetic material, the cable losses including conductor losses, screen losses and armour losses; a separating layer between the inner layer of armour wires and the outer layer of armour wires; the method comprising the steps of: reducing combined losses in the at least one metallic screen and in the inner layer and outer layer of the armour by making the separating layer with a thickness greater than 1 mm; sizing the cross section area X of each electric conductor with a reduced value, this reduced value being determined and made possible by the reduced combined losses, and/or rating the armoured AC cable at the maximum allowable working conductor temperature θ to transport said alternate current I with an increased value, this increased value being determined and made possible by the reduced combined losses.

10. A method of reducing losses in an armoured AC cable wherein an alternate current I is transported, the armoured AC cable comprising: at least one core, each comprising an electric conductor; at least one metallic screen surrounding the at least one core; an armour surrounding the at least one metallic screen, comprising an inner layer of armour wires and an outer layer of armour wires, at least part of the armour wires of the inner layer and at least part of the armour wires of outer layer comprising a ferromagnetic material, the losses including conductor losses, screen losses and armour losses; a separating layer between the inner layer of armour wires and the outer layer of armour wires; the method comprising the steps of: reducing combined losses in the at least one metallic screen and in the inner layer and outer layer of the armour by making the separating layer with a thickness greater than 1 mm.

Description

(1) The features and advantages of the present disclosure 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 FIGURE. FIGURE schematically shows an exemplary armoured cable according an embodiment of the present disclosure.

(2) In particular, FIGURE schematically shows an exemplarily armoured AC cable 10 for submarine application comprising three cores 12.

(3) Each core 12 comprises a metal electric conductor 121, typically made of copper, aluminium or both, in form of a rod or of stranded wires. The conductor 121 is sequentially surrounded by an inner semiconducting layer 122, an insulation layer 123 and an outer semiconducting layer 124. The three layers 122, 123, 124 are made of polymeric material (for example, polyethylene), wrapped paper or paper/polypropylene laminate. In the case of the semiconducting layers 122, 124, the material thereof is charged with conductive filler such as carbon black.

(4) Each conductor 121 has a cross section area X, wherein X=π(d/2).sup.2, d being the conductor diameter.

(5) Outside the respective semiconducting layer 124, each core 12 also sequentially comprises a water blocking layer 125, a metallic screen 126 and a polymeric sheath 127 (the latter being made, for example, of MDPE, Medium Density Polyethylene).

(6) The metallic screen 126 can be made, for example, of lead, generally in form of an extruded layer, or of copper, in form of a longitudinally wrapped foil, of wounded tapes or of braided wires.

(7) The three cores 12 are helically stranded to one another according to a core stranding pitch A.

(8) The three cores 12 are kept in place by a filler 13, which can be in form of extruded polymeric material, polymeric or natural yarn or plastic shaped filler (as disclosed, for example in EP3244422). If needed, one or more optical cables 18 (two, in the present case) can be provided within the filler 13.

(9) The three cores 12 are surrounded, as a whole, by a bedding 14. The bedding 14 may be made from fabric tapes (e.g. Hessian tapes) or polypropylene (PP) textile.

(10) Around the bedding 14 an armour 16 is provided. The bedding 14 protects the underlying surface from undue located pressure from the armour 16.

(11) The armour 16 is surrounded by a textile protection layer 15 (made, for example, of PP textile) and a protective jacket 17 (made, for example, of MDPE).

(12) The armour 16 comprises an inner layer 16a of armour wires and an outer layer 16b of armour wires. In FIGURE, the armour wires are depicted as having a round cross-section but this is to be intended as merely an illustrative example as the wires can have, alternatively, an elongated cross section; in particular, the wires 16a, 16b can be flat rods with a substantially rectangular cross-section.

(13) A separating layer 19 is provided radially between the inner layer 16a of armour wires and the outer layer 16b of armour wires.

(14) According to the present disclosure, the separating layer has a thickness greater than 1 mm.

(15) In an embodiment, the separating layer has a thickness lower than or equal to 3 mm.

(16) All the armour wires (or at least part thereof) of the inner layer 16a and of the outer layer 16b comprise ferromagnetic material; in an embodiment all of them (or at part of them) consist of ferromagnetic material.

(17) The ferromagnetic material has, for example, a relative magnetic permeability r in absolute value equal to 300 (i.e. |μ.sub.r|=300) and may consist, for example, of construction steel.

(18) The metal wires of the inner layer 16a and of the outer layer 16b are wound around the cores 12 respectively according to a inner helical armour winding lay and an inner armour winding pitch B, and an outer helical armour winding lay and an outer armour winding pitch B′.

(19) The inner helical armour winding lay and the outer helical armour winding lay have opposite direction. This contralay configuration of the outer layer and inner layer is advantageous in terms of mechanical performances of the cable.

(20) During development activities performed to investigate the losses in an AC electric power cable, the Applicant tested an AC three single-phase HV cable having the following structural and operational features:

(21) TABLE-US-00001 TABLE 1 Diameter Element (mm) Material Notes Core Conductor 23.5 Copper Round strand Nominal section: 400 mm.sup.2 Rated current: 470 A.sub.rms Rated voltage: U = 132 KV Frequency of 50 Hz Insulation + 70.43 XLPE semiconductive layers Water blocking 72.75 Water- layer swellable tapes Metallic screen 73.78 Copper Polymeric sheath 84.48 MDPE Cable assembly Three-core 182.1 Pitch A = 1763.62 mm assembly Laying direction: Z (right-handed) Bedding 187.54 PP textile Inner layer of 194.14 Ferromagnetic Flat wires 3.3 × 12 mm armour galvanized No. of wires (Nw1): 48 steel Pitch B = 3858 mm Laying direction: Z Separating layer 195.14 PET tape Thickness as in Table 3 Outer layer of 201.74 Ferromagnetic Flat wires 3.3 × 12 mm armour galvanized No. of wires (Nw2): 50 steel Pitch B′ = 4012 mm Laying direction: S (left- handed) Textile 209.88 PP textile protection layer Polymeric jacket 220.88 MPDE

(22) The physical behaviour of metal materials in the metallic layers of the cable is described below in terms of electrical resistivity and relative magnetic permeability.

(23) TABLE-US-00002 TABLE 2 Conductor Metallic screen Armour wires Material copper copper Ferromagnetic galvanized steel Electrical resistivity 2.03*10.sup.−8 1.8*10.sup.−8 20.8*10.sup.−8 ρ.sub.20 at 20° C. (Ω*m) Temperature 3.93*10.sup.−3 3.93*10.sup.−3 4.5*10.sup.−3 coefficient α (1/° C.) Working 85  77.8 73.7 temperature (° C.) Relative magnetic 1 1  μ.sub.r = |μ.sub.r| *e.sup.−iϕ permeability μ.sub.r |μ.sub.r| = 300 ϕ = π/3

(24) The electrical resistivity of metal materials can be evaluated at the working temperature T.sub.w with the known formula:
ρ(T.sub.w)=ρ.sub.20*[1+α*(T.sub.w−20)]

(25) Considering this cable as a starting point, the Applicant computed the armour losses in the inner layer and outer layer, the losses in the screens and the combined losses in the armour layers and screens, by using a 3D FEM (Finite Element Method) model that enables a comprehensive description of the electromagnetic phenomena that occur inside the cable. For example, the 3D FEM method may be that described in Sturm S. et al., “Estimating the Losses In Three-Core Submarine Power Cables Using 2D And 3D FEA Simulations”, Jicable'15, 2015.

(26) The computed losses are shown in Table 3 below.

(27) TABLE-US-00003 TABLE 3 Losses (W/m) Inner Outer Case layer of layer of Over- Δ** no. Short description screens armour armour all* (%) 1a Z-Z-S 16.23 1.02 1.72 18.97 — Separating layer thickness = 0.5 mm 1b Z-Z-S 16.15 0.65 1.83 18.63 −1.79 Separating layer thickness = 1.5 mm 1c Z-Z-S 16.10 0.56 1.84 18.50 −2.48 Separating layer thickness = 2.4 mm 2a Z-S-Z 16.84 1.86 0.46 19.16 — Separating layer thickness = 0.5 mm 2b Z-S-Z 16.56 2.04 0.38 18.98 −0.94 Separating layer thickness = 1.5 mm 2c Z-S-Z 16.44 2.13 0.35 18.92 −1.25 Separating layer thickness = 2.4 mm *Combined screens + armour layers losses **[Overall losses for case 1a(2a) − Overall losses for case 1b or 1c (2b or 2c)]/Overall losses for case 1a(2a)

(28) In Table 3 above, cases 1a and 2a relate to two comparative configurations of the cable structure of Table 1 and a separating layer thickness of 0.5 mm. Cases 1b, 1c, 2b, 2c relate to the cable structure of Table 1 wherein the separating layer thickness is equal to 1.5 mm or 2.4 mm, according to the present disclosure.

(29) Moreover, cases 2a, 2b, 2c reflect the cable structure of Table 1 apart from the fact that the laying direction of the inner layer of the armour is S (that is, contralay with respect to the stranding direction Z of the cores) while the laying direction of the outer layer of the armour is Z (that is, unilay with respect to the stranding direction Z of the cores).

(30) Compared to cases 1a and 2a, the increased thickness of the separating layers in cases 1b, 1c, 2b, 2c is accompanied by an increased number of wires in the armour outer layer (and by an increase of the outer diameter as well). Accordingly, a greater amount of ferromagnetic metallic material could potentially increase the losses. However, as from the results set forth in Table 3, a reduction of the combined losses is anyway achieved by increasing the thickness of the separating layer.

(31) The results of Table 3 also show that an increased thickness of the armour separating layer reduces the losses in the screens as well. This reduction in the screen losses is advantageous because, being the screens in a more central position in the cable, the heat losses induced therein are more difficult to dissipate with respect to the heat losses induced in the surrounding armour layers.

(32) In addition, the results of Table 3 show that, in general, the combined losses in the screens and armour layers are lower in a cable wherein the inner layer of armour is unilay with respect to the stranding direction of the cores (Cases 1a-1c), with respect to a cable wherein the inner layer of armour is contralay with respect to the stranding direction of the cores (Cases 2a-2c).

(33) As stated above, the reduction of the combined losses in the screens and armour layers, achieved thanks to the use of a separating layer of an increased thickness between the inner layer and outer layer of the armour, 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 X, the increase/reduction being considered with respect to the case wherein the armour losses are instead computed with a cable having a separating layer of a lower thickness.

(34) This 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.

(35) It is noted that, even if in the above description and FIGURE, cables comprising an armour with two layers of armour wires have been described, the present disclosure also applies to cables wherein the armour comprises more than two layers, radially superimposed.

(36) In such cables, the multiple-layer armour can comprise the inner layer of wires and the outer layer of wires, surrounding the inner layer, as described above, and one or more further layers surrounding the outer layer, with a separating layer between each layer and the radially external one.

(37) As to the further armour layer/s possibly surrounding the outer layer, the alternation unilay/contralay should be maintained, with a separating layer between each radially adjacent layers.