Coated steel wire as armouring wire for power cable

Abstract

A steel wire as an armoring wire for a power cable for transmitting electrical power, where the steel wire has a steel core and a non-magnetic coating. The coating has a thickness in the range of 0.2 mm to 3.0 mm and selected from metals or alloys having a melting point below 700 C.

Claims

1. A power cable configured to transmit electrical power comprising at least a steel wire as an armouring wire, wherein the steel wire has a steel core and a non-magnetic coating, said coating having a thickness in the range of 0.5 mm to 3.0 mm and selected from metals or alloys having a melting point below 700 C., said non-magnetic coating configured to interrupt or deviate a magnetic field to pass through said steel wire, wherein said power cable comprises a plurality of said steel wires, and said steel wires are wound around at least part of said power cable.

2. The power cable as in claim 1, wherein said non-magnetic coating is corrosion resistant.

3. The power cable as in claim 1, wherein said non-magnetic coating is any one of zinc, aluminium, magnesium or their alloys.

4. The power cable as in claim 1, wherein said non-magnetic coating is formed by cladding.

5. The power cable as in claim 4, wherein said non-magnetic coating has been drawn or welded on said steel core, or via diffusion during a heat treatment on said steel core.

6. The power cable as in claim 1, wherein the thickness of said non-magnetic coating is in the range of 0.5 mm to 2.0 mm.

7. The power cable as in claim 1, wherein the thickness of said non-magnetic coating is in the range of 1.0 mm to 2.0 mm.

8. The power cable as in claim 1, wherein the steel core of said steel wire is low carbon steel.

9. The power cable as in claim 1, wherein said steel wire has a round cross-section and a diameter ranging between 1.0 mm to 10.0 mm.

10. The power cable as in claim 1, wherein said steel wire has a tensile strength above 340 MPa.

11. The power cable as in claim 1, wherein said power cable has at least an annular armouring layer made of said steel wires.

12. The power cable as in claim 1, wherein said power cable has at least an annular armouring layer made by alternating two types of coated steel wires, and one type of coated steel wire has low carbon steel core and the other type of coated steel wire has austenitic stainless steel core.

13. The power cable as in claim 1, wherein said power cable is a tri-phase submarine power cable.

14. The power cable as in claim 1, wherein said power cable is a high voltage cable of more than 110 kV.

15. The power cable as in claim 1, wherein said coating has a thickness in the range of 1.0 mm to 3.0 mm.

16. A power cable configured to transmit electrical power comprising at least a steel wire as an armouring wire, wherein the steel wire has a steel core and a non-magnetic coating, said coating having a thickness in the range of 0.5 mm to 3.0 mm and selected from metals or alloys having a melting point below 700 C., said non-magnetic coating configured to interrupt or deviate a magnetic field to pass through said steel wire, wherein said power cable comprises a plurality of said steel wires, and said steel wires are wound around at least part of said power cable, and wherein said non-magnetic coating comprises zinc or zinc alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

(2) FIG. 1 is a high voltage power cable according to prior art.

(3) FIG. 2 schematically shows the magnetic flux lines in the armouring layer of a high voltage tri-phase power cable.

(4) FIG. 3 is a cross-section of a cladded steel wire according to the present invention.

(5) FIG. 4 is a cross-section of a tri-phase power cable having cladded armouring steel wires.

(6) FIG. 5 is a cross-section of a tri-phase power cable having two types of cladded armouring wires in alternation.

DETAILED DESCRIPTION OF THE INVENTION

(7) FIG. 3 is a cross-section of a cladded steel wire 30. Carbon steel wire 32 is covered by a non-magnetic corrosion resistant cladding 34. As an example, an intermediate layer 33, such as nickel or copper, may also be applied in-between the wire 32 and coating 34 to improve the adhesion.

(8) A low carbon steel wire of a diameter of 5 mm is used as the core of the invention wire.

(9) A low carbon steel grade is a steel grade wherepossibly with exception for silicon and manganeseall the elements have a content of less than 0.50% by weight, e.g. less than 0.20% by weight, e.g. less than 0.10% by weight. E.g. silicon is present in amounts of maximum 1.0% by weight, e.g. maximum 0.50% by weight, e.g. 0.30% by weight or 0.15% by weight. E.g. manganese is present in amount of maximum 2.0% by weight, e.g. maximum 1.0% by weight, e.g. 0.50% by weight or 0.30% by weight. Preferably for the invention, the carbon content ranges up to 0.20% by weight, e.g. ranging up to 0.06% by weight. The minimum carbon content can be about 0.02% by weight. In a more preferred embodiment, the minimum carbon content can be about 0.01% by weight. The steel wire is processed continuously on one or more lines depending on the capabilities of the production site.

(10) This steel wire is first degreased in a degreasing bath (containing phosphoric acid) at 30 C. to 80 C. for a few seconds. An ultrasonic generator is provided in the bath to assist the degreasing. Alternatively, the steel wire may be first degreased in an alkaline degreasing bath (containing NaOH) at 30 C. to 80 C. for a few seconds. Electrical assistance is applied in the bath to assist the degreasing.

(11) The steel wire may be processed additionally in an acidic pickling bath at 30 C. to 60 C. to increase surface cleanness. HCl, H.sub.2SO.sub.4 or other acids may be used for this purpose. Preferably electrolytic assistance is applied.

(12) A cladding process is provided wherein a metal coating of predefined composition and thickness is applied to the low carbon steel core wire.

(13) A strip of suitable non-magnetic material, e.g. zinc or zinc alloy, and predetermined thickness, e.g. 0.5 mm or 1 mm, can be formed into e.g. a tube form. The width of this strip is somewhat greater or equal to the circumference of the steel core to be covered. The strip is closed in a tube and welded around or on a steel core. After welding, Turks heads press the metal coating to the steel core.

(14) Preferably the process step of welding may be preceded or followed by a step of drawing the steel wire in order to provide a steel wire with increased hardness and tensile strength and improved adhesion with the cladding.

(15) Preferably the process step of welding may be followed by a step of pressing the coating against the steel core by means of Turks heads at a minimum temperature of 200 C.

(16) Alternatively or additionally, the process step of enclosing the steel core with a strip or foil of metal may be followed by a step of annealing the steel core with the non-magnetic cladding at a temperature, such as above 550 C., for a time period ranging from a few seconds to a few minutes.

(17) FIG. 4 represents a cross-section of a tri-phase submarine power cable armoured with the cladded steel wires according to the present invention.

(18) The tri-phase submarine power cable 40 is shown in the illustration. It includes a compact stranded, bare copper conductor 41, followed by a semi-conducting conductor shield 42. An insulation shield 43 is applied to ensure that the conductor do not contact with each other. The insulated conductors are cabled together with fillers 44 by a binder tape, followed by a lead-alloy sheath 45. Due to the severe environmental demands placed on submarine cables, the lead-alloy sheath 45 is often needed because of its compressibility, flexibility and resistance to moisture and corrosion. The sheath 45 is usually covered by an outer layer 46 comprising a polyethylene (PE) or polyvinyl chloride (PVC) jacket. This construction is armoured by steel wire armouring layer 48. The steel wires used herein are according to the invention, i.e. they are non-magnetic and corrosion resistant material, e.g. zinc or zinc alloy, cladded steel wires. An outer sheath 49, such as made of PVC, cross-linked polyethylene (XLPE), a combination of PVC and XLPE layers, or bitumen is preferably applied outside the armouring layer 48. Due to the application of a 0.5 mm zinc cladding on the steel armouring wire, the magnetic loss is eliminated by about 75%.

(19) FIG. 5 shows a cross-section of a tri-phase submarine power cable armoured with the cladded steel wires according to an alternative solution of the present invention. The armouring layer comprises two types of cladded steel wires 57, 58 in alternation. The steel core of one kind of armouring wire 57 is low carbon steel. The steel core of the other type of armouring wire 58 is austenitic stainless steel. The cladding for both types of armouring wires is zinc aluminium alloy and has a thickness of 0.5 mm. The magnetic loss for this alternative solution is eliminated by about 90%.

LIST OF REFERENCE NUMBERS

(20) 10 steel wire armoured cable 12 conductor 14 insulation 16 bedding 18 armour 19 sheath 20 power cable 22 conductor 24 armouring wire 30 cladded steel wire 32 steel core 33 intermediate layer 34 non-magnetic coating 40 power cable 41 copper conductor 42 semi-conducting conductor shield 43 insulation shield 44 fillers 45 lead-alloy sheath 46 outer layer 48 steel wire armouring layer 49 outer sheath 50 power cable 57 armouring wire with low carbon steel core 58 armouring wire with austenitic stainless steel core