EP2020008B1 - Container for screening magnetic fields of low frequency - Google Patents

Abstract

The invention relates to a container for screening magnetic fields of low frequency, wherein the container may be provided in particular for accommodating electrical apparatuses or component parts. The invention proposes a container which comprises at least one inner container consisting of insulating material and an outer container consisting of insulating material. The space between the inner container and the outer container is filled with a ferromagnetic material layer having a high magnetic permeability, and the outer container is supported with respect to the inner container(s) by spacer elements. Preferably, the container is tubular, in particular cylindrical. Particularly strong sources of magnetic fields having a low frequency are electrical cables and lines. The invention therefore also proposes a use for laying electrical supply lines, preferably medium-voltage and high-voltage cables.

Description

The invention relates to a container for shielding magnetic fields of low frequency, wherein the container may be provided in particular for receiving electrical devices or components.

For example, shielding containers with a highly permeable material layer are known, in which the magnetic material is inserted as a film material and in which openings remain between container parts ( EP 1274 103 B1 ). Shielding containers, which are not closed all around, should have only a limited shielding effect.

Particularly strong sources of low frequency magnetic fields (f <1 kHz) are electrical cables and wires. As demands for the reduction of alternating magnetic fields increase, more and more attention is being paid to measures which are suitable for avoiding such fields or for weakening alternating fields of existing field generators. So one of the points of discussion is the field strength at the earth's surface with buried three-phase cables. Magnetic shielding of buried three-phase cables can be achieved by pulling the three wires of the cable system into a high-permeability tube, such as a commercial steel tube.

Disadvantage of a steel tube for shielding is the fact that on the one hand for mechanical reasons and for the purpose of shielding a large wall thickness of a few millimeters (about 4 to 10 mm) must have and on the other hand it is therefore not flexible and must be welded together in short pipe sections , In addition, it must be protected against corrosion by the soil moisture by providing it on the outside with a plastic jacket and on the inside mostly with a concrete filling.

It is the object of the invention to propose an arrangement with high shielding effect against alternating magnetic fields, which is constructed easily and flexibly.

The solution is presented in the features of the main claim and the use claim. Advantageous embodiments can be found in the respective subclaims.

The core of the invention is a shielding system consisting of a two-shell container with high permeable intermediate layer and in which the outer container is supported relative to the inner container with spacer elements.

As the material for the container plastic is proposed, wherein the material is preferably to select from the consistency and / or its wall thickness so that the container, or the inner and / or the outer container is flexible. The choice of plastic is hardly limited. Synthetic resins or other polymers can be used. Plastics are advantageous with which inner and outer containers can be produced by extrusion or by injection molding.

Preferably, the container may be tubular, in particular cylindrical, so it would be understood as a two-sided open container. However, there are also other pipe sections, such as rectangular cross sections used for the desired purpose. In the case of the tubular embodiment, it makes sense to speak of a support tube in the inner container and of a jacket tube in the outer container. Although these terms are used hereinafter, this should not be construed as limiting the invention to tubular containers.

Preferably, the outer container may be formed as a corrugated tube, since a corrugated tube is also very flexible.

There are various possibilities for constructing the material layer.

The material layer is produced as a covering of strips of high-permeability material on the inner container in a continuous winding process. A running through a winding device inner container is wrapped with these bands in one or more layers. It is important that the material layer is as magnetically tight as possible, so that the magnetic penetration is minimal. There should be such a high coverage of the band edges that there is as little as possible a restriction of the flexibility.

The tape layers are - preferably in the tubular formation - helically edge-overlapping or wound with a gap (applied with a short blow). The bands should still be displaced against each other when bending the tube, so that the banded plastic tube remains flexible. It can be drummed on cable drums and laid in longer lengths.

As materials for the material layer are very well-known electrical steel strips of cold-rolled, grain-oriented silicon steel; Special, crystalline nickel-iron alloys (eg from the company Vacuumschmelze) can be used. The grain orientation in the ribbons should be longitudinal the bands are when the embodiment is provided for electric cable, because then the bands come to lie transversely to the longitudinal direction of the cable.

The aforementioned materials have excellent magnetic properties. Important is the high permeability of the strip material, so that only one or two or only a few layers of tape must be applied. Their costs appear perfectly acceptable for the purposes mentioned. For special applications, tapes made of metallic glass (Metglas) can be provided, but these are difficult to machine mechanically and have a relatively high market price.

The proposed composite structure (material layer between an inner and an outer container) has the particular advantage that the highly permeable material is protected against external influences. If, however, the material layer should be accessible to the environmental influences, a corrosion protection for the material layer should be provided.

The magnetic property of the material layer should have a relative permeability of more than 1,000, in particular at least 10,000, and preferably 20,000.

The wrapped or bandaged inner container is introduced into a second container, wherein a cylindrical inner container can be drawn into a plastic tube of larger diameter.

Spacer elements can be understood to mean an integral element, for example a plastic thread or a multiplicity of elements. A plastic thread may be arranged in a coiled or helical position between inner container and outer container. Other spacer elements can preferably be designed as tensionable on the inner container straps.

For use in the preferred embodiment of the container in tube form, the spacers may be formed as a ring body. Such annular bodies can then be arranged in more or less regular intervals in the space between the support and casing pipe.

Typical and usable for the invention spacers are made of HD polyethylene (or nylon) and offered by the company Franken Plastik (Fürth). Experts also refer to them as skid rings or knob rings.

Preferably, the spacer element (s) may be designed to be particularly slidable or provided with means for reducing friction. This should allow a relatively easy sliding or sliding of the support tube in the outer tube. The lubricity can be determined by the choice of Material can be determined, for example, nylon or similar plastics are used. The lubricity can also be promoted by the special design of the spacer elements. For this purpose, the formation of nubs or spines offers that provide a small area, preferably punctiform contact of the container.

Slide knobs or skids ensure easy insertion and centering of the inner tube in the casing. Also in this variant, the jacket tube may be a corrugated pipe. The application of known skid bands is provided, for example, to increase the slipperiness of supply pipes, which are drawn into existing protective pipes during burial.

Particularly suitable annular body can be equipped with skids, which protrude substantially radially and axially. The ring body may be integrally formed, which is wrapped around the support tube. However, a spacer element may also be formed as a band or consist of ring segments. The ends of a band or the ends of the ring segments are connected together and if necessary, the ring body can be subsequently with a clamping tool (for example with clamps with skids) or stretched out of its own elasticity without tools, so that the spacers stuck immovably.

Preferably, the spacer elements can be applied to the material layer of high magnetic permeability and be clamped immovably.

The remaining cavity between inner container and outer container can be filled with a flowable, hardening mass. This hardening mass should have a good mechanical strength and also a good thermal conductivity, in order to be able to dissipate the heat loss of the electrical devices and components arranged in the shielding container well to the environment. For this purpose, for example, so-called fluid concrete ("twilight") in question.

To achieve a particularly good heat transfer, filled with graphite particles flowable concrete can be filled, as it is sold for example by the company Heidelberger cement BUT as "ThermoCem". Such a concrete has a thermal conductivity of more than 2.0 W / (K m).

To achieve a particularly high heat capacity of the fluidized concrete can also be filled with microparticles (eg paraffin-based), which act as latent heat storage and thus delay the heating of the shielding. Such a heat storage material is sold for example by the company BASF and already used for the production of particularly good heat-retaining plasterboard in the construction sector.

As a particular embodiment, instead of a single inner container, a plurality of inner containers may be used. In three-phase cable systems, these are preferably three inner tubes, namely for each one-core cable an inner tube made of plastic. The material layer (preferably as a high permeability band material) is wound around the bundle of the plurality of tubes. The changed bundle remains flexible, as the bands can move against each other. The further processing can be carried out as previously described. Skid shoes are applied as spacers, and the bundle can be retracted into an outer casing of larger diameter plastic.

• Solche ferromagnetische Schichten verlieren bei höheren Frequenzen zunehmend ihre Schirmwirkung, da die Permeabilität mit der Frequenz sinkt.
• Die drei Ströme eines symmetrischen Drehstromsystems ergänzen sich in jedem Augenblick zu Null. Fließt hingegen innerhalb der Schirmhülle ein Nullstrom oder, einfacher ausgedrückt, ein resultierender Wechselstrom, so geht für diesen die Schirmwirkung verloren.

Very good shielding effects can be achieved with the high-permeability material layer according to the invention. However, there are two limitations when it comes to three-phase cable systems:

• Such ferromagnetic layers increasingly lose their shielding effect at higher frequencies because the permeability decreases with frequency.
• The three currents of a symmetrical three-phase system complement each other at zero moment. If, on the other hand, a zero current or, more simply, a resulting alternating current flows within the screen envelope, the shielding effect is lost for it.

Both problems are solved in the EMC technique by highly conductive sheaths, e.g. made of copper or aluminum. It is therefore additionally proposed to combine for such applications, the material layer with another layer of copper or aluminum bands. The best effect is achieved when the highly conductive bands lie inside and are enveloped by the magnetic material layer.

Such a construction would be required, for example, for shielding in the field of home installations, since it has to do with the low-voltage cables usually with unbalanced, often single-phase currents. Also, telecommunication cables (e.g., speaker cables, antenna cables) require such a construction because of their high frequency currents.

If conductive layers or additional compensation conductors are present in the cables laid in a shielding container, their ends are either to be connected through or grounded in the zones in which installed shielding containers are present in abutment, but with a gap or without a continuous material layer.

Shock or transition points of shielding containers, as they occur, for example in sleeve portions of cables in the longitudinal direction of a shield, are shielded by the transition points overlapping sheaths, which are constructed on the same principle as the inventive arrangement, so that minimized in these areas, the penetration of magnetic fields or eliminated.

The invention is particularly effective for magnetic shielding and loss reduction in cables when the cable construction is extended with outer magnetizable components.

Cables with outer magnetizable components (such as reinforcement or steel pipe / culvert) have high additional losses. In the case of three-phase three-wire cables, additional losses of e.g. 80% of the conductor losses occur. It is advisable to undertake to reduce the additional losses wrapping the vein bandage with electrical tape to reduce the field weakening the losses in these structural elements.

The shielding according to the invention has a high flexibility and is very suitable to be laid (in tube form) in long lengths in cable trenches or cable ducts. As part of this use of the shielding electrical supply lines, preferably medium and high voltage cables can be fed.

Claims (14)

1. A container for shielding low frequency magnetic fields, in particular magnetic fields generated by electrical cables and lines, the container being constructed as follows,

   - of at least one inner container of insulating material,

   - of an outer container of insulating material,

   - the space between at least one inner container and the outer container being filled with a ferromagnetic material layer of high magnetic permeability,

   characterised in that the outer container is supported relative to the at least one inner container by spacer elements.
2. A container according to claim 1, characterised in that the container is of tubular, preferably cylindrical construction.
3. A container according to claim 2, characterised in that three tubular inner containers are provided.
4. A container according to any one of the preceding claims, characterised in that the spacer elements are applied to the material layer and tightened so as to be immovable.
5. A container according to any one of the preceding claims, characterised in that the spacer elements take the form of strips tightenable onto the inner container(s).
6. A container according to any one of the preceding claims, characterised in that the spacer elements exhibit a low coefficient of friction relative to the outer container or are provided with means of reducing friction.
7. A container according to any one of the preceding claims, characterised in that the space between inner container(s) and outer container is filled with a free-flowing, hardenable composition.
8. A container according to any one of the preceding claims, characterised in that the space between inner container(s) and outer container is filled with a composition with high heat conductivity.
9. A container according to any one of the preceding claims, characterised in that the material layer consists of at least one layer of strips.
10. A container according to the preceding claim, characterised in that the strips exhibit grain orientation.
11. A container according to any one of the preceding claims, characterised in that the material layer is magnetically soft material.
12. A container according to any one of the preceding claims, characterised in that the material layer has a relative permeability of more than 1,000, in particular at least 10,000, and preferably 20,000.
13. A container according to any one of the preceding claims, characterised in that the outer container takes the form of corrugated tube.
14. Use of a container according to any one of the preceding claims, characterised in that electrical supply lines, preferably medium and high voltage cables, are inserted.