Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B7/00—Insulated conductors or cables characterised by their form
  • H01B7/17—Protection against damage caused by external factors, e.g. sheaths or armouring
  • H01B7/18—Protection against damage caused by wear, mechanical force or pressure; Sheaths; Armouring
  • H01B7/26—Reduction of losses in sheaths or armouring

Definitions

• 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.
• 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.
• 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.
• 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.
• the conductor temperature T should be kept lower than about 90°C.
• the permissible current rating can be derived from the expression for the temperature rise above ambient temperature:
• ⁇ 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 ( ⁇ /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); ⁇ is the ratio of losses in the metal sheath to total losses in all conductors in that cable; ⁇ 2 is the ratio of losses in the armouring to total losses in all conductors in the cable.
• RA is the AC resistance of armour at maximum armour temperature ( ⁇ /m);
• R is the alternating current resistance per unit length of conductor at maximum operating temperature ( ⁇ /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);
• ⁇ is the angular frequency of the current in the conductors.
• 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 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.
• 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.
• the armour wires may have a ferromagnetic core surrounded by a non-ferromagnetic material (e.g. plastic or stainless steel).
• a non-ferromagnetic material e.g. plastic or stainless steel.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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:
• 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:
• the cladding of the armour wires of the invention is advantageously made of an electrically conductive and non-ferromagnetic material.
• the term "core” is used to indicate an electric conductor surrounded by an insulating layer and at least one semiconducting layer.
• said core further comprises a metal screen surrounding the conductor, the insulating layer and the semiconducting layer/s.
• 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.
• 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.
• the term “ferromagnetic” indicates a material that below a given temperature has a relative magnetic permeability significantly greater than 1 , preferably greater than 100.
• non-ferromagnetic indicates a material that below a given temperature has a relative magnetic permeability of about 1.
• 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.
• 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.
• 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.
• 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.
• 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 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.
• 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.
• 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.
• the armour losses ⁇ 2 ⁇ can be significantly lower than those ⁇ 2 calculated by international standard IEC 60287-1-1 , second edition 2006-12.
• ⁇ 2 ⁇ 0.75 ⁇ 2 Preferably, ⁇ 2 ⁇ ⁇ 0.50 ⁇ 2 . More preferably, ⁇ 2 ' ⁇ 0.25 ⁇ 2 . Even more preferably, ⁇ 2 ⁇ 0.10 ⁇ 2 .
• 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.
• 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.
• the cross-section major axis is preferably oriented tangentially with respect to the cable circumference.
• 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.
• 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.
• 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%.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• 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.
• said further armour wires could be made of ferromagnetic material only.
• the plurality of armour wires preferably defines an armour layer.
• 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.
• the armour wires of the outer layer can comprise a ferromagnetic inner portion and an electrically conductive non- ferromagnetic cladding.
• 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 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.
• 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.
• 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.
• the elongated cross section is substantially shaped as an annulus portion.
• 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.
• 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.
• 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.
• 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.
• 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%.
• 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%.
• pitch B > 0.5A. More preferably, pitch B > 0.6A. Preferably, pitch B ⁇ 2A. More preferably, pitch B ⁇ 1.8A.
• the armour surrounds all of the said cores together, as a whole.
• 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'.
• 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.
• 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.
• 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.
• the cable of the invention comprises two or more cores
• each of them is a single phase core.
• 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.
• - 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;
• FIG. 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.
• FIG. 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.
• 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.
• a metal sheath 13 for example, made of lead or copper
• a polymeric filler 1 1 surrounded, in turn, by a tape 15 and by a cushioning layer 14.
• an armour 16 comprising a layer of wires 16a is provided around the cushioning layer 14 according to an armour winding pitch B.
• the armour 16 is surrounded by a protective sheath 17.
• 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.
• the wires 16a are bimetallic. In particular, they comprise each a ferromagnetic inner portion 162 and an electrically conductive and non-ferromagnetic cladding 164.
• 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).
• 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.
• 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.
• 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 ferromagne
• the ferromagnetic steel used in the present examples 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 was an austenitic stainless steel with an electrical resistivity of 20.8x10 "8 Ohnrm, and relative magnetic permeability of about 1.
• 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 wire
• 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).
• 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.
• 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.
• 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.
• 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".
• 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.
• the elongated cross section of the wires 16a has a substantially rectangular shape, with smoothed angles.
• the helical winding lay of the armour wires 16a in case of a power cable 10 having at least two cores 12, in order to further reduce the armour losses, 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.
• the multiple-layer armour preferably comprises an inner layer of wires and an outer layer of wires, surrounding the inner 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.

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• 2014
  • 2014-04-17 EP EP14720536.3A patent/EP3132453B1/en active Active
  • 2014-04-17 AU AU2014390753A patent/AU2014390753B2/en active Active
  • 2014-04-17 WO PCT/EP2014/057948 patent/WO2015158396A1/en active Application Filing
  • 2014-04-17 BR BR112016023937-7A patent/BR112016023937B1/pt active IP Right Grant
  • 2014-04-17 NZ NZ724935A patent/NZ724935A/en unknown
  • 2014-04-17 ES ES14720536T patent/ES2762150T3/es active Active
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