Technical Field

[0001] The invention relates to the use of a steel wire for protection against

magnetic fields by absorbing magnetic fields originating from power cables, such as underground power cables, electrical appliances, or transformer housings, or for shielding information carriers from external, interfering magnetic fields.

Background Art

[0002] Electromagnetic fields are present wherever there is electricity.

Developments of residential housing with increasingly dense populations, have spurned new concerns about electromagnetic fields. In 2002 the International Agency for Research on Cancer stated that extremely low- frequency magnetic fields, i.e. fields in the range of 3 Hz to 3000 Hz, are possibly carcinogenic to humans. This was based on epidemiological studies showing statistical evidence between long time exposure and increased risk of childhood leukemia. Since then European governments have performed various studies on the risks for the population. For long term exposures at less than one meter distance and more than 0.4 micro- Tesla, a precaution principle is being applied.

[0003] The amount of electric power transmitted in an underground cable at any given time is determined by its voltage and current. While electric fields are determined by the voltage, magnetic fields are determined by the current. High current cables induce high magnetic fields. The fields are strongest close to the cables and rapidly reduce further away from them.

[0004] Prior art

[0005] US 7,622,669 describes a method for shielding the magnetic field

generated by an electrical power transmission line comprising a plurality of shielding elements made of ferromagnetic material arranged side by side as a base, comprising bottom and side walls, and a cover wherein the cabling is provided. US 7,365,269 describes a similar base and covering.

[0006] EP-A2-2 246 948 describes a shielding element for a cable in the form of a hollow cylinder. EP-A2-2 280 462 describes an arrangement with conductors surrounded by a shielding casing designed in a tube or box- shape and divided into individual elements in a longitudinal direction.

Shielding foils made from high-permeable material are arranged between the cable wires and the casing, and have gaps, such that the cable wires are surrounded by the casing only in a surface area of 70 to 95

percentages. A layer of electrically conductive material is attached to the shielding foils.

[0007] Such state of the art casings and tubes are cumbersome to install, not flexible and not very strong. In addition, they require quite some amount of space. Furthermore, the casing is expensive and needs to be

prefabricated as such.

Disclosure of Invention

[0008] It is a main object of the present invention to overcome the problems of the prior art.

[0009] It is another object of the present invention to reduce exposure to

electromagnetic radiation, especially to strong low-frequency (<3kHz) radiation.

[0010] It is still another object of the present invention to provide a magnetic

shielding with low magnetic losses.

[001 1] It is yet another object of the present invention to provide a magnetic

shielding which is flexible in design.

[0012] It is an additional object of the present invention to provide a magnetic shielding which provides sufficient strength.

[0013] It is a further object of the present invention to provide a magnetic

shielding which reduces the interference of external magnetic fields in signal transmitting cables. [0014] According to a first aspect of the present invention there is provided the use of a steel wire for magnetic shielding of for example a power cable such as an underground power cable, of welding equipment, of an electrical appliance or a transformer housing. The steel wire comprises the following steel composition:

- a carbon content ranging from 0.0 weight % up to 0.10 weight %, e.g. from 0.0 weight % to 0.08 weight %, e.g. from 0.005 weight to 0.06 weight

%;

- a silicon content ranging from 0.0 weight % up to 8.0 weight %, e.g. from 0.0 weight % up to 5.0 weight %; e.g. 1.5 weight % to 6.0 weight %, e.g. 2.0 weight % to 3.5 weight %;

- a maximum manganese content of 3.0 weight %, e.g. 2.5 weight %, e.g. 1.5 weight %;

- a maximum aluminium content of 2.0 weight %, e.g. 0.5 weight %; e.g. 0.2 weight %;

- a maximum copper content of 2.0 weight %, e.g. 0.5 weight %; e.g. 0.2 weight %;

- a maximum chromium content of 5.0 weight %, e.g. 1.0 weight %; e.g. 0.2 weight %;

- a maximum nickel content of 5.0 weight %, e.g. 1.0 weight %; e.g. 0.2 weight %;

- a maximum niobium content of 0.2 weight%

- maximum contents of sulphur, phosphorus, oxygen and nitrogen of 0.023 weight %, e.g. 0.010 weight % (individual maximum, not cumulated);

- the remainder being iron.

[0015] In the context of the present invention, the silicon content refers to the average bulk silicon content. Techniques are available to increase the silicon content close to the wire surface. The percentage mentioned here, however, refers to the average.

[0016] Such a magnetic shielding in the form of a low-carbon steel wire with

possibly a high silicon content has the advantage of providing the required strength, an enhanced degree of flexibility in design and low magnetic loss.

In a preferred embodiment the steel wire is used to shield a power cable, e.g. an underground power cable, a power cable for welding equipment or a power cable for an induction heater. The steel wire is wound around at least part of the power cable.

[0017] In one embodiment the steel wire is round and has a diameter ranging between 0.10 mm and 6.0 mm, e.g. between 1.0 mm and 6.0 mm, e.g. between 1.0 mm and 3.0 mm. In another embodiment the steel wire may be flattened, rectangular or square with a thickness varying between 0.10 mm and 6.0 mm and a width being equal or greater than the thickness.

[0018] The wire may be wound around the power cable to provide a compact winding, i.e. where one winding is in contact with an adjacent wire winding. This may be done by winding a single wire with a winding pitch which is substantially equal to the wire diameter. This is offering the highest level of shielding for three reasons. First of all the magnetic flux path formed by the thus wound wire is closed offering little space for magnetic flux lines to escape. Secondly, due to the small pitch the steel windings are more or less lying in the plane of the magnetic flux lines. Thirdly, this compact winding is offering a maximum of material mass per unit of length.

[0019] Tests, however, have shown that the winding is not necessarily closed or compact. Acceptable levels of shielding may be obtained by a winding which is not completely compact or closed. This may be achieved by selecting a winding pitch for a single wire where the pitch ranges between one wire diameter and two wire diameters.

[0020] The steel wire may be wound in one or more layers.

More than one wire may be wound around the cable. [0021] In another embodiment one or more steel wires is or are braided in a flexible tape or tube around the cable.

This flexible tape can be folded or wound around cables that carry large currents.

[0022] Other ways may be provided to incorporate steel wires of the invention for shielding.

[0023] A preferred steel wire according to the invention is an iron-silicon (FeSi) wire. In this preferred embodiment of the steel composition, the silicon content ranges from 1.5 weight % to 6.0 weight %, e.g. from 1.5 weight % to 5.0 weight %, e.g. from 2.0 weight % to 4.0 weight %.

[0024] In yet another embodiment, it can be considered to wind multiple layers of

FeSi wires around (parts of) cables.

[0025] The steel wire of the present invention combines low magnetic losses with a high strength and better (design) flexibility. The wire may have similar or better magnetic properties than electrical sheet.

[0026] Indeed, the steel wire may be subjected to a shearing treatment, such as disclosed in WO-A-201 1/1 10450. This shearing treatment further reduces the magnetic losses.

[0027] An invention steel wire wound around a power cable has a shielding factor higher than 10, preferably higher than 15, more preferably higher than 20, still more preferably higher than 30.

The shielding factor SF is defined as the ratio of the strength of the magnetic field without shielding over the strength of the magnetic field with shielding.

[0028] The steel wire has a relative magnetic permeability μ _Γ which is 3 to 30

times greater than the relative magnetic permeability μ _Γ of construction steel. [0029] Usually an underground power cable is buried at less than 2 m, e.g. less than 1.5 m, e.g. less than 1 m under the ground level. A power cable may also be used on ground level.

[0030] The power cable may be a high power cable of more than 100 A.

[0031] The power cable may be a low frequency cable operating with a base

frequency of about 50 Hz or 60 Hz.

[0032] The steel wire may be used particularly for shielding tri-phase cables, but its use for mono-phase cables is not excluded.

[0033] Although not really needed, in one embodiment the wire of the invention may have an insulating coating such as an iron oxide or a ceramic coating.

[0034] In a preferred embodiment the wire of the present invention is provided with a corrosion protection such as a polymer coating or a zinc or zinc alloy coating.

[0035] A polymer coating can be provided by means of an extrusion technique.

[0036] A zinc or zinc alloy coating can be provided by means of electroplating or by means of a hot dip bath or by a combination of both.

[0037] For this purpose the wire is preferably coated with a zinc aluminum coating since this provides a high resistance against corrosion.

The zinc aluminum coating has an aluminum content ranging from 2 % by weight to 12 % by weight, e.g. ranging from 3 % by weight to 1 1 % by weight, with a preferable composition around the eutectoid position: Al about 5 per cent. The zinc alloy coating further may have a wetting agent such as lanthanum or cerium in an amount less than 0.1 % of the zinc alloy. The remainder of the coating is zinc and unavoidable impurities. Coating compositions with about 10% aluminum are also common.

[0038] The cable may be wrapped beforehand, so that no extra casings are

needed to be built in underground. In addition, less underground space is needed and placing underground cables can be performed much faster and easier. Brief Description of Figures in the Drawings

[0039] Figure 1 is a cross-section of an underground cable with a magnetic

shielding in the form of steel wires.

[0040] Figure 2 is a graph representation of the magnetic field induced by an underground cable at ground level;

[0041] Figure 3 shows an underground power cable wound with a steel wire

according to the invention;

[0042] Figure 4 shows three embodiments of a wound power cable having

different winding densities;

[0043] Figure 5 represents a graph of the shielding factor in relation to a varying winding density.

Mode(s) for Carrying Out the Invention

[0044] Figure 1 is a cross-section of a tri-phase cable 10. Three electrical

conductors 12 of copper or aluminium are each surrounded by an electrical polyethylene based insulation 14. Low-ohmic conductors, e.g. of copper or aluminium, 16 are wound or wrapped around the cable. These conductors 16 form the electrical shielding and are designed for conducting the compensation currents. In case of a tri-phase cable, the cross-section is relatively small in comparison with the cross-section of the electrical conductors 12. However, in case of a mono-phase conductor, the cross-section of the electrical shielding 16 is about equal to the cross- section of the electrical conductors 12. An organic layer 18 is provided around the electrical shielding 16. This organic layer 18 may be

polyvinylchloride or a cross-linked polyethylene. The organic layer may be provided on the cable 10 by means of extrusion. An iron-silicon wire 19 is wrapped around the electrical shielding 16 but still separated from the electrical shielding 16 by the layer 18. The iron-silicon wire 19 is the magnetic shielding. The iron-silicon wire may be provided with a corrosion resistant coating such as zinc or a zinc alloy. A layer 18 of cross-linked polyethylene or of polyvinylchloride is provided around the magnetic shielding 19. [0045] Figure 2 is a graph 20 of the magnetic field B expressed in micro-Tesla (μΤ) induced by an underground cable. The abscissa is the distance d in meter. The magnetic induction B is inversely proportional to the square of the distance to the centre of the underground cable. This is typical for the magnetic field induced by a tri-phase cable.

[0046] Figure 3 illustrates a set-up 30 for test purposes of an underground power cable 32 provided with a steel wire 33 according to the invention. The distance between the level of the underground power cable 32 and the ground level 34 is one meter.

[0047] A first study was performed to test the magnetic shielding effect of FeSi wires underground cables.

[0048] The relative magnetic permeability μ _Γ of the FeSi wire 33 is 1500 for

currents varying from 10 A up to 1000 A with a frequency of 50 Hz. The electrical conductivity of the FeSi wire 2.5 10 ⁶ S/m.

[0049] The cable 32 has a typical diameter of 51 mm and a three phase current of

1000 A. More in general this diameter may range from 30 mm to 160 mm.

The three phase conductors are wound with an FeSi shielding wire 33.

The thickness of the shielding, or the diameter of the wire, has been varied from 1 mm up to 6 mm. In this first study a maximum density winding, whereby each winding perfectly connects to the next winding, was applied.

[0050] Table 1 represents the simulated shielding factor (SF) for a specific wire thickness. As mentioned, the shielding factor is the ratio of magnetic induction with and without shielding.

Table 1 [0051] The shielding is realised by the ferromagnetic properties of the FeSi wires. The influence of eddy currents in the material is negligible due to the high resistivity of the material.

[0052] Table 2 represents a comparison with classical types of shielding such as steel (different from iron-silicon) and aluminium (for a wire thickness of 2 mm as example). The FeSi wire is wound most favourably in highest density whereby all adjacent windings contact perfectly each other and without air gap in-between the windings.

Table 2

[0053] The results clearly show that the FeSi wire of the invention offers a higher shielding factor SF compared to the classic shielding by steel or aluminium.

[0054] To test different winding densities of FeSi wire of 4.0 mm diameter around the cable (Figure 4), a 3D simulation was performed. The winding density varies from zero (=no shielding) to 250 windings per meter (= full compact winding since 250x4.0 mm = 1.0 m).

- 42 is a cable with full compact winding 43;

- 44 is a cable with small openings between the windings 45;

- 46 is a cable with great openings between the windings 47.

[0055] Figure 5 represents the results 50 of the shielding factor SF (ordinate) in relation to the varying winding density (abscissa).

The shielding factor is 1 at a winding density of 0. This corresponds to the absence of any shielding. The shielding factor then increases more or less linearly with an increased winding density. At 200 to 250 windings per meter, a shielding factor of about 15 to 20 is feasible.

[0056] A winding density of 250 corresponds to an optimal shielding where no air gaps are left in between the windings.

[0057] One may notice that the results 50 show a saturation effect. The shielding factor SF at 200 windings per meter is not much less than the shielding factor SF at 250 windings per meter. This means that a full compact winding is not a strict necessity and that small openings between the windings, e.g. openings of about the size of a wire diameter, are allowed without drastically reducing the shielding factor SF.