EP3179485B1 - High-power coaxial cable - Google Patents

Description

The invention relates to a coaxial cable for the transmission of high currents in the range of several hundred to several thousand amps at high frequencies, especially in the kilohertz range, with a first electrical conductor for a first current phase and a coaxially arranged second electrical conductor for a second current phase, respectively are formed by a number of annularly arranged bundles of a plurality of mutually insulated individual wires, wherein between the first and second conductor is an elastic insulating layer to prevent a flashover. In particular, the invention relates to a cable for transmitting high electrical power up to the megawatt range.

Such Hochleistungskoaxialkabel are known for example from EP 0 823 766 A1 , and are used mainly for industrial induction heating, for example, in the melting, tempering or tempering of steel, in the high currents between 1,000-15,000 amps at high frequency usually from 4 kHz to 150 kHz, in extreme cases even up to 400 kHz be routed over the cable. This must be done with minimal power losses. Today's cables have both a high tendency to inductive and ohmic losses. In particular, the skin effect occurring at high frequencies leads to high voltage drops in the cable.

To minimize the skin effect suggests the DE 20 2010 006 735 U1 a ladder assembly comprising a plurality of wells consisting of a plurality of parallel strands of a plurality of mutually insulated Having conductor wires, wherein groups of wells intertwined into braids and these are stranded to form the conductor assembly.

It is an object of the present invention to provide a high performance coaxial cable for industrial applications which has lower inductive losses and Furthermore, it has an increased usable conductor cross-section in the relevant frequency band by reducing the skin effect.

This object is achieved by a high-performance cable with the features of claim 1. Advantageous developments are given in the dependent claims and are explained below.

According to the invention it is proposed to develop the generic coaxial cable such that the bundles of the first and / or second electrical conductor form a mesh. The bundles are thus blurred, i. regularly entwined. By using a mesh, the coaxial cable's inductive losses and skin effect are reduced, resulting in better power utilization of the coaxial cable, and ultimately, more power can be transported to the consumer.

The individual wires are preferably made of copper, since copper has a low resistivity. As a result, the ohmic losses of the coaxial cable are low.

Preferably, each bundle in the braid in its direction of extension alternately cross two other bundles and cross each other. This simplifies the production compared to a braid in which each bundle alternately only crosses and undercuts another bundle due to the plurality of bundles. Intersecting bundles can lie in a braid angle between 30 ° and 60 °.

The isolation of the individual wires can be formed for example by a lacquer. This isolation is crucial for reducing the skin effect. Since the insulation of the individual wires is not an insulation against flashovers in the event of potential differences, the paint used can be an electrical paint commonly used in the construction of coils and transformers. A varnish for insulation forms a very thin layer, so that the diameter of the individual wires is not appreciably increased.

It is also advantageous if the individual wires of each bundle are wound around the longitudinal axis of the respective bundle, as is the case with a cable. As a result, the bundles remain bundles during processing or braiding and the individual wires do not separate.

Suitably, the individual wires may have a diameter of 0.1mm to 0.5mm. As a result, the bundles can have a large number, for example 20 to 60 such individual wires. The smaller the diameter, the more individual wires can be used with the same conductor cross-section and the more single wires are used, the more the current can be split, and the lower the skin effect.

For example, to double the conductor cross section, for example, from 70mm ² to 140mm ² , two mesh layers may be used in each of the two conductors so that when both the first and second electrical conductors are formed by a mesh of bundles of individual wires, there are four in total such braids are present in the coaxial cable. Thus, therefore, the first and / or the second electrical conductor of bundles of a plurality of mutually insulated individual wires may be formed, which are arranged in two concentric circles, wherein the bundles of the two circles each form their own mesh.

Preferably, the elastic insulating layer is made of EPDM (ethylene-propylene-diene rubber). This material is characterized by high insulation properties, so that the thickness of the insulating layer with the same insulation performance as in the prior art, may be lower. This, in turn, causes the first and second electrical conductors to be closer together, thereby reducing inductive loss.

Due to the high performance that is able to transmit the coaxial cable according to the invention, the electrical conductors are hot. For cooling therefore be provided to cool the coaxial cable, for example, with water. For this purpose, the first and second electrical conductors preferably lie in a liquid-flowable chamber. In this case, the insulating layer can separate a first liquid-permeable chamber from a second liquid-permeable chamber.

Figur 1:

den prinzipiellen Aufbau eines wassergekühlten Koaxialkabels nach dem Stand der Technik

Figur 2:

das Koaxialkabel nach Figur 1 mit Skin-Effekt

Figur 3:

elektrisches Ersatzschaltbild eines Koaxialkabels

Figur 4:

Darstellung des Geflechts aus Bündeln des zweiten Leiters

Figur 5:

perspektivische Darstellung eines teilweise aufgeschnittenen Koaxialkabels gemäß der Erfindung

Figuren 5a, 5b:

Darstellung beispielhafter Geflechtstrukturen

Further features and advantages of the invention will be explained below with reference to an embodiment and the accompanying figures. Show it:

FIG. 1:

the basic structure of a water-cooled coaxial cable according to the prior art

FIG. 2:

the coaxial cable after FIG. 1 with skin effect

FIG. 3:

Electrical equivalent circuit diagram of a coaxial cable

FIG. 4:

Representation of the mesh from bundles of the second conductor

FIG. 5:

perspective view of a partially cut coaxial cable according to the invention

FIGS. 5a, 5b:

Representation of exemplary braid structures

FIG. 1 shows a coaxial cable 1 for transmitting high currents in the kiloampere range, in particular in the range of several hundred to several thousand amps, at high frequencies in the kilohertz range, with a first electrical conductor 4 for a first phase of current and a second coaxial to this arranged second electrical conductor 7 for a second Current phase, each formed by a number of annularly arranged bundles 4a, 7a of a plurality of individual wires, wherein between the first and second conductors 4, 7 is an elastic insulating layer 5, 6 for preventing a flashover. Such a coaxial cable is state of the art.

Water-cooled coaxial cables are mainly used in industrial induction heating, for example in melting, tempering or tempering steel). A particular requirement in this area is that high currents (1,000-15,000 amperes) with very high frequency (i.d.R 4-150 kHz, in extreme cases up to 400 kHz) must be routed to the induction heater. In comparison, household electricity has only a current of a few amperes and a frequency of 50 Hz.

The most important requirement for current-carrying cables in this area is a throughput without loss of power. Here are two types of loss to distinguish, since you can neglect capacitive losses. First, this is the ohmic loss. This is the loss of power due to the electrical resistance of a conductor and leads to heating of the conductor. The ohmic resistance is determined mainly by the material of the conductors. Second, there are still inductive losses in applications described above. This loss is due to the frequency of the current, in particular the fact that the two phases of the current mutually negatively influence ("displace the current") with increasing frequency.

Due to the high frequencies that are needed in the field of inductive heating, this effect in the field of industrial inductive heating is of particular importance. The reason for these power losses is the so-called skin effect, which ensures that, as the frequency increases, the current flows less strongly through the conductor or, viewed in cross-section, only flows on the surface of the conductor. Thus, only a small part of the conductor cross-section is used for power transmission. The consequence is that a large amount of electricity flows through a very small cross-section. It comes to increased temperature development up to the destruction of the ladder. This in turn increases the demands on conductors for high frequency currents.

For the coaxial cable 1 after FIG. 1 Inside there is a cavity 2, which is bounded by a mechanical support member 2, for example by an inner spiral 2, preferably made of bronze. The inner spiral 2 is formed by a thick wire which extends helically around the cable axis, as shown in FIG FIG. 5 you can see. It supports the first electrical conductor 4, hereinafter also called inner conductor 4, from. The inner conductor 4 consists of a plurality of circularly arranged around the inner spiral 3 bundles 4a. Each bundle 4a in turn consists of a plurality of individual wires made of copper, also referred to below as copper wires. Copper is used here because it has a very low resistance and is therefore a very good conductor. The diameter of each copper wire is 0.2mm. The copper wires are wound along the longitudinal extent of the respective bundle 4a, so that each bundle 4a is formed in the manner of a rope. Hereinafter, the bundles 4a are therefore also referred to as Kupferseilbündel.

The cavity 2 is not needed for electrical reasons, since it is not penetrated by the current. However, it serves as a cooling water pipe and thus ensures more efficient heat dissipation. The inner cavity 1 and the annular space in which the cable collars 4a of the inner conductor are located together form an inner cooling water chamber 9.

In the sequence of the layer structure further radially outward, follows the elastic insulating layer 5, which here comprises an inner cooling tube 5 and a radially outside of the cooling tube 5 applied layer 6. The cooling hose 5 surrounds the inner conductor 4 annular and limits the inner cooling water chamber 9 to the outside. Next, the second conductor 7, hereinafter referred to as the outer conductor 7, which consists of a plurality of circularly arranged bundles 7a, which in turn are formed from a plurality of individual wires. The bundles 7a of the second conductor 7 may be formed equal to those of the first conductor 4, so that it is also possible here to speak of copper wires and copper wire bundles 7a.

The outer conductor 7 is located in an outer cooling water chamber 10 in the form of an annular space. An outer cooling hose 8 bounds the outer cooling water chamber 10 radially outward. This forms the outermost layer of the coaxial cable 1. It thus isolates the cable electrically and protects against mechanical and chemical action. The insulating layer, i. the inner cooling tube 5 together with its externally applied layer, fulfill the task of a dielectric and separate the two current phases in the second conductor 7 and the first conductor 4 from each other. In addition, they separate the two cooling water chambers 9, 10 from each other.

Both the inner cooling water chamber 9, ie the inner cavity 2 together with the space around the inner conductor 4, as well as the outer cooling water chamber 10, ie the space around the outer conductor 7 are flowed through cooling water, due to the high current, the high frequency and the herein to be able to deduce the resulting heat from the skin effect. This cooling ability is also a decisive factor for the power efficiency of such a cable 1. Because the more heat can be dissipated, the more current can be passed through such a cable 1.

An important factor limiting the carrying capacity of the cable 1 is the skin effect. It ensures that the two conductors 4, 7 are only superficially flowed through and because of the electromagnetic interaction between the two conductors 4, 7, the outer part of the inner conductor 4 and the inner part of the outer conductor 7, which Figure 2 is shown. The consequence of this is that in each case only a small part of the actual cross section of the cable bundles 4a, 7a is current-flowed through. Thus, in a high-frequency current much current flows through a relatively small cross-section of the respective conductor 4, 7, which leads to a significantly increased heat development and thus load on the cable 1.

FIG. 3 shows the electrical standard circuit diagram for a coaxial cable 1, in which a consumer 11, for example in the form of an induction furnace, which is electrically substantially a coil, between the inner conductor 4 and the outer conductor 7 is connected. The inner conductor 4 and the outer conductor 7 can each lead a current phase of a three-phase voltage network, wherein the phases 120 ° may be shifted from each other. The illustrated prior art involves various disadvantages:

For example, the cables have 1 inductive losses, which today average about 10% voltage drop (based on the usual lengths used). This is partly due to the skin effect, which is always present, ie it is only a relatively small proportion of the actual conductor cross-sections used for the passage of electricity. It is estimated that this proportion is on average only around 20% in the relevant frequency band today. Conversely, this means that almost 80% of the cable cross section is actually not used for the current flow. On the other hand, too little insulation is to blame. Because due to little high-impedance insulating fabric inserts in the cooling hoses used for the insulation 5, 8 wide distances between the inner conductor 4 and the outer conductor 7 must be selected to prevent flashovers. However, if the two conductors 4, 7 were closer together, the electromagnetic fields generated by the respective conductor 4, 7 would be more extinguished, so that less energy flows into these fields. Thus, for maximum conduction efficiency, a minimum distance would be required.

For a sample cable with a diameter of 80 mm, a length of 15 m, a frequency of 5 kHz, at 40 ° C, an input current of 4700 A and an input voltage of 1000 V, the voltage drop due to the inductive loss is about 79 V and due to the ohmic resistance about 13V.

The coaxial cable 1 according to the invention forms the coaxial cable according to FIG. 1 Now further to the effect that the individual wires are insulated against each other and the bundles 4a, 7a of the first and second electrical conductors 4, 7 form a braid 12, which leads to advantages in various points. FIG. 4 shows a plan view of the braid 12, from which the second conductor 7 is formed.

On the one hand there is a high mechanical flexibility of the electrical conductors 4, 7, since the regularly crossing bundles 4a, 7a of a conductor 4, 7 are mutually movable. It is particularly noteworthy that they adapt to each Biegelage the coaxial cable 1 wrinkle-free. In contrast to conventional coaxial cables 1 to FIG. 1 It does not happen, in particular due to the compression of the cable inside a bend, that a bundle of the second electrical conductor lifts from the insulation layer, ie, a loop or fold throws. This would result in inhomogeneities in the electromagnetic field generated by the individual strands and would result locally in a larger inductive loss, so the affected bundle would be avoided by the current. By using a braid 12, however, a constant distance between the two electrical conductors 4, 7 is always maintained on average.

Electrotechnically, the braid 12 is of particular importance. For of the total and total number of bundles 4a, 7a run one half of the bundles 4a, 7a helically in a clockwise direction about the longitudinal axis of the coaxial cable 1, the other half of the bundles 4a, 7a helically counterclockwise about the longitudinal axis, so that between the intersecting bundles 4a, 7a is a braid angle, which in the case of the embodiment in FIG. 4 is about 50 °. This causes the electromagnetic fields around the individual bundles to weaken, which in turn contributes to the reduction of inductive losses.

The individual bundles 4a, 7a are not covered separately. This has the advantage that they are not fixed in their cross-sectional shape. Thus, the individual wires in the cross section of the bundles 4a, 7a viewed not circularly arranged but approximately oval. That that the bundles 4a, 7a are rather flat, which is an effect of the axial train on the coaxial cable or on the respective braid 12. As a result, the bundles 4a, 7a are close to each other, so that there are no open stitches. The braid 12 causes a total of an absolutely dense electromagnetic field with low scattering effect, which also reduces the inductive losses of the coaxial cable 1.

Finally, it is spatially still to be considered that the braids 12 lead to a locally higher degree of filling in the respective annulus of the coaxial cable 1, in which the first and second conductor 4, 7, because the cross section of the bundles 4a, 7a on the circumference of respective conductor 4, 7 is distributed. This can be easily understood FIG. 1 make clear, if one considers that in circular bundles 4a, 7a free spaces forming gussets between two adjacent bundles 4a, 7a and the insulating layer 5, 6 lie. For circular bundles 4a, 7a as in FIG. 1 are present, corresponds to the radial thickness of the electrical conductors 4, 7 respectively the diameter of the bundles 4a, 7a, wherein between two adjacent bundles 4a, 7a each of the electrical conductors 4, 7 are unused gussets.

Compared with such an arrangement, the individual individual wires of the bundles 4a, 7a of the first and second conductors 4, 7 are each homogeneously distributed on the corresponding electrical conductors 4, 7 bearing ring in the cross section of the coaxial cable 1, so that almost fully the entire radial thickness of respective annular space is filled with the individual wires. This in turn causes the radial thickness of the respective annular space with braided bundles 4a, 7a according to FIG FIG. 4 is smaller at the same conductor cross-section, as in the case of the round bundles 4a, 7a in FIG. 1 , This has the advantage that the first and the second electrical conductors 4, 7 are closer to each other, ie have a smaller radial distance from one another. The radial distance is decisive for the height of the inductive losses, so that the inductive losses are reduced to a minimum due to the braid-related smaller conductor spacing.

The braid 12 from bundles 4a, 7a of the first and / or second electrical conductor 4, 7 also leads to a reduction of the skin effect and consequently to an increase in the current-carrying cross-sectional area, so that the coaxial cable 1 lead more power / power to the load 11 can, as comparable cables according to the prior art. This can be explained by the fact that in the braid 12, the distance of each bundle 4a, 7a from the axis of the coaxial cable 1 varies approximately sinusoidally in the radial direction. If a bundle 4a, 7a crosses another bundle, it lies radially further outward, if it crosses another bundle, it lies radially further inside. As a result, the distance of each bundle 4a, 7a of the one electrical conductor 4, 5 in the direction of extension also changes relative to the other electrical conductor 7, 4.

As for re FIG. 4 explained, it comes, however, that the current flows only in the radially outer surface region of the first electrical conductor 4 and in the radially inner surface region of the second electrical conductor 7. The higher the frequency, the lower the immersion depth of the current. The fact that each individual wire in its direction of extension times more and sometimes less far away from the other conductor 4, 7, also also about the longitudinal axis of the respective bundle 4a, 7a winds, and also the current in each individual wire is "trapped", the immersion depth of the current is increased compared to non-insulated individual wires. Thus, the effective line cross-section that carries the current is greater. As a result, the load on the first and second electrical conductors 4, 7 is reduced or, with the same load, a higher current can be passed through the coaxial cable 1.

How to get by FIG. 4 can recognize, crosses and intersects each bundle 4a, 7a in the braid 12 in its direction of extension alternately two other bundles 4a, 7a.

The isolation of the individual wires in the individual bundles 4a, 7a against each other by means of electrical paint, such as polyethylene, and has special significance for the reduction of the skin effect. Because of the insulation, the individual wires do not act like a single electrical conductor in which the current can "everywhere" out. each Bundle is rather the sum of individual wires, from which a current-apart from the axial end-can not flow out. This means that the current at the high frequencies can not "collect" on the surface of a bundle, but at most on the surface of a wire. Overall, the effective current used by the cross section of the electrical conductors 4, 7 is thereby increased.

In the coaxial cable 1 according to the invention, the insulating layer 5, 6 between the first electrical conductor 4 and the second electrical conductor 7 by a tube made of EPDM (ethylene-propylene-diene rubber). This material is diffusion-tight and good electrical insulation, so that the radial distance of the two electrical conductors 4, 7 is smaller than in the prior art according to FIG. 1 ,

FIG. 5 shows an embodiment of a coaxial cable 1 according to the invention in a perspective view, with a partially schematic representation selected and a portion of the cable 1 is cut to recognize the internal structure, which is substantially the structure in the FIGS. 1 and 2 equivalent. The axial end of the coaxial cable 1 has a connection head 13 for connecting the cable 1 to a load 11 or a voltage source, for example a transformer. In the connection head, the current is fed into the individual bundles 4a, 7a of the first and second electrical conductors 4, 7.

Schematically are in FIG. 5 the layers forming the first and second electrical conductors 4, 7 are each formed by a number of annularly arranged bundles 4a, 7a of a plurality of mutually insulated individual wires. Compared to the prior art, however, the layers are formed according to the invention as a braid 12. An enlarged and schematic representation of two exemplary braid structures is shown in FIG FIGS. 5a and 5b shown. FIG. 5a shows a simple mesh structure in which each bundle 4a, 7a alternately covers and bridges a single crossing bundle 4a, 7a. In contrast, shows FIG. 5b the braid structure out FIG. 4 in which each bundle 4a, 7a alternately overlaps and bridges two crossing bundles 4a, 7a. It should be noted, however, that in the braid 12 in the FIGS. 5a and 5b a braid angle of about 90 °, whereas a smaller braid angle is preferred to facilitate easy connection of the bundles to the terminal head to reach. Here, purely schematically, each bundle is represented by three individual wires, although each bundle consists of a plurality of individual wires.

Another difference of the variant in FIG. 5 opposite the cable in FIGS. 1 and 2 is that both the inner conductor 4 and the outer conductor 7 each consist of two braids 12. The first and the second electrical conductors 4, 7 are each formed from bundles 4a, 7a, which are arranged in two concentric circles. In this case, the bundles 4, 7a of the two circles each form their own braid 12. Consequently, there are two braids 12 one above the other. Thus, the outer conductor 7 has an inner braid 7.1 and an outer braid 7.2 and the inner conductor 4, an inner braid 4.1 and an outer braid 4.2. While a single braid, for example, leads to a cross-section of 70mm ^2, the conductor cross section can be doubled by a second braid layer for example to 140mm. ²

With the high-performance coaxial cable according to the invention, a further skin-effect reduction is integrated for the first time for increasing the efficiency for industrial applications. The inductive losses are reduced by up to 50% and the usable conductor cross section in the relevant frequency band is increased from approx. 20% to up to 35%.

• wie viele Bündel in Umfangsrichtung vorhanden sind, wodurch u.a. der Kabeldurchmesser beeinflusst wird,
• wie viele Drähte jedes Bündel hat, wodurch letztendlich der Durchmesser der einzelnen Bündel beeinflusst wird,
• welchen Durchmesser die Einzeldrähte haben, und
• wie viele Geflechtlagen übereinander liegen.

Of particular advantage is the modular design of the coaxial cable 1, since there are several ways to make a corresponding adjustment of the conductor cross-section to a particular use. Thus, the cross section of the electrical conductors is defined primarily by

• how many bundles are present in the circumferential direction, which influences, among other things, the cable diameter,
• how many wires each bundle has, which ultimately affects the diameter of each bundle,
• which diameter the individual wires have, and
• how many layers of mesh lie on top of each other.

By appropriate choice of these parameters, the coaxial cable according to the invention can be adapted to any application, so that neither an expensive over-sizing nor a risky undersizing can take place.

LIST OF REFERENCE NUMBERS

1

coaxial

2

Inner cavity

3

interior spiral

4

inner conductor

4a

Rope bundle of the inner conductor

4.1

inner mesh of the inner conductor

4.2

outer mesh of the inner conductor

5

Inner cooling hose

6

insulation layer

7

outer conductor

7a

Rope bundle of the outer conductor

7.1

inner mesh of the leader

7.2

outer braid of the outer conductor

8th

Outer cooling hose

9

Inside cooling water chamber

10

Outer cooling water chamber

11

Coil, consumer, induction furnace

12

weave

13

connection head

Claims (9)

1. Coaxial cable (1) for the transmission of high currents in the range of several hundred to several thousand ampere in the high-frequency kilohertz range, with a first electrical conductor (4) for a first current phase and a second electrical conductor (7) arranged coaxially to the first for a second current phase, respectively formed by a number of bundles arranged in a ring shape (4a, 7a) made of numerous individual wires that are insulated against each other, with the first and second conductor (4, 7) being separated by an elastic insulating layer (5, 6) to prevent a voltage flash-over, characterised by the bundles (4, 7a) of the first and/or second electrical conductor (4, 7) forming a mesh (12).
2. Coaxial cable (1) according to claim 1, characterised by each bundle (4a, 7a) in the mesh (12) in its extension direction alternately crossing over and under two other bundles (4a, 7a) respectively.
3. Coaxial cable (1) according to claim 1 or 2, characterised by the insulation of the individual wires being formed by a lacquer, in particular electrical lacquer.
4. Coaxial cable (1) according to one of the preceding claims, characterised by the individual wires of each bundle (4a, 7a) being wound around the longitudinal axis of the respective bundle (4a, 7a).
5. Coaxial cable (1) according to one of the preceding claims, characterised by the individual wires having a diameter of 0.1 mm to 0.5 mm.
6. Coaxial cable (1) according to one of the preceding claims, characterised by two bundles that cross over each other (4a, 7s) positioned at a 30° and 60° angle of twist relative to each other.
7. Coaxial cable (1) according to one of the preceding claims, characterised by the elastic insulation layer being made of EPDM.
8. Coaxial cable (1) according to one of the preceding claims, characterised by the first and/or second electrical conductor (4, 7) of the bundles (4a, 7a) consisting of numerous individual wires that are insulated against each other, arranged in two concentric circles, with the bundles (4, 7a) of the two circles respectively forming their own mesh (12).
9. Coaxial cable (1) according to one of the preceding claims, characterised by the first and second electrical conductors (4, 7) lying in a chamber (9, 10) that liquid can flow through.

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