DE102009024149A1 - Magnetic shielding arrangement for single-conductor electrical cable in high voltage rotary current system, has metal shield formed with enlarged electrical conductance value to reduce electrical leakage at metal shield - Google Patents

Abstract

The arrangement has a metal shield (20) radially enclosing a single-conductor electrical cable and arranged above the cable. An uninterrupted tooth banding (22) made of high-permeable material encloses the metal shield. The metal shield comprises a contact unit at ends of the shield for contacting the shield. The metal shield is formed with enlarged electrical conductance value to reduce electrical leakage at the metal shield, so that shield current is induced. The shield current is same as conductor current of a cable conductor (12) during marginal case.

Description

The invention relates to a single-core electrical cable with integrated electromagnetic shielding for a high-voltage three-phase current system in a single-plane arrangement.

To reduce the external 50 Hz magnetic field of three-phase cables different shielding measures can be used. A summary of measures has been taken recently CIGRE (Paris, 2008): CIGRE TF C4.204; "Mitigation techniques of power-frequency magnetic fields originated from electric power systems" , technical brochure. Depending on the demand, such measures can be carried out with relatively little effort, such. B. the use of compensation ladders (eg. P. Maioli, E. Zaccone, "Passive Loops Technique for Electromagnetic Field Mitigation, Applications and Theoretical Considerations" in Jicable-Conf. 2007, Versailles, pp. 231-236 ). In some cases, however, the effort can also be described as significant - as with the shielding of cables with a steel pipe or in a steel box ( WO 2004/034539 ; EP 1598911 A1 ) - and with adverse effects on additional losses and ampacity.

It has already been proposed to provide a flexible cable with a braided copper screen, wherein - for the purpose of magnetic shielding - particles of high permeability are pressed into the braid or the particles are glued to the braid ( DE 19807527 A1 ). The construction is especially designed for drag chain cables, since the flexibility of the line is in the foreground. A quantitative estimate of the shielding effect is not stated in the document.

It is in the case of very high demands on the magnetic shielding of a power cable system, the object of the invention to provide a system variant, in which a highly effective magnetic shield is integrated into the construction of a single conductor cable.

The solution of the problem can be found in the features of the independent claim Advantageous developments are formulated in subclaims.

The essence of the invention is that instead of a screen of good conductive material with 'usual' cross-section a much larger cross-section is selected for the screen. The shield is thus designed in the form of a reinforcement and has a cross-section, or a corresponding conductance in the order of magnitude of the values of the conductor. The material used for the screen is copper or aluminum. The thus formed reinforcement is wrapped with a high-permeability harness.

For the Rebänderung is preferably used: overlapping wound, thin strips of a special steel, which have a relative permeability of some 10,000. In addition to their high shielding effect, these bands are characterized by low eddy current and magnetic reversal losses.

Irrespective of the axis spacing of the cables, this type of cable allows extremely high shielding factors to be achieved, with which all conceivable requirements for limiting the magnetic field can be met. This makes the use of such a cable in the case of extreme magnetic field restrictions (values around 0.2 μT) interesting and is superior to other solutions, such as shielding with steel pipes or steel channels, with very considerable advantages in terms of current carrying capacity.

In addition, in areas where it is imperative to guide the cables in a steel tube, such as underpassing railway systems, the single-conductor cable provides a means of laying the single cable core in a steel tube, which without this design is inadmissible due to the extreme current heat losses is.

The invention is illustrated in drawings, the figures show in detail

1 : Cable construction,

2 : Screen loss factor y ₁ as a function of the cross section of the copper screen;

3 : Dependence of the three screen currents on the cross section of the copper screens 'without' and 'with' magnetic harness;

4 : in addition to 3 : Screen factors depending on the screen cross section;

5 : horizontal distribution of magnetic induction above ground;

6 : maximum magnetic induction above the earth surface as a function of the screen cross section and

7 : Current carrying capacity as a function of the center distance of the cables in a single-level arrangement.

The 1 illustrates the cable structure and the two design features used for magnetic shielding: Instead of a normal screen (copper or aluminum) with cross-sections between about 50 mm ² and 200 mm ² is a much larger cross section - as a reinforcement 20 (made of copper or aluminum) - used. The reinforcement 20 is wrapped with a highly permeable vine change 22 , Such electrical tapes as used in transformer and electrical engineering are made of low silicon silicon steel sheet and are made in a thickness of 0.02 to 0.04 mm.

Three single conductor cables 10 are preferably laid in a horizontal single-plane arrangement for a high voltage three-phase system. The structure of a single conductor cable is as follows: over a plastic insulated cable conductor 12 is a foil 14 made of aluminum; above it lies a reinforcement 20 of copper, which with a harness 22 is covered by high permeability material. Outward, the construction is with a cable sheath 24 , especially with corrosion protection. The insulation is preferably made of VPE, wherein an inner and an outer conductive layer - as usual - is present.

With such a cable construction, regardless of the pitch of the cable cores, extremely high shielding factors can be achieved.

The influence of the screen, which has been enlarged with regard to the conductance, is replaced by three three-phase single-core cables 10 considered in a horizontal single-level arrangement. Be the three umbrellas 20 (made of copper) of the cable cores short-circuited at both ends to each other, currents are induced in them, which are (nearly) opposite to the respective conductor currents and the size of which approaches the conductor current with increasing screen cross-section. This results in a range of copper screen cross-sections up to about 500 mm ² to a dependence of the shielding losses (here: relative to screen loss factor y ₁ on the conductor losses), as shown in the 2 for a 110 kV XLPE cable with an aluminum conductor cross-section of 2500 mm ² . The starting point was a laying depth of 1.2 m and a horizontal center distance of the cable wires of s = 0.5 m.

• – bei einem Schirmquerschnitt A_S von etwa 100...200 mm² ein Maximum der Schirmverluste mit mehr als dem Sechsfachen der Leiterverluste auftritt und
• – dass jenseits dieses Maximums bei weiterer Steigerung des Schirmquerschnitts die Schirmverluste wieder abnehmen.

The 2 shows the screen loss factor y ₁ (middle cable core) as a function of the cross section A _{S of} the copper shields in a 110 kV single conductor cable with 2500 mm ² aluminum conductors and a horizontal wire spacing s = 0.5 m. With this figure it is clarified that in this laying arrangement

• - At a screen cross section A _S of about 100 ... 200 mm ^2, a maximum of screen losses with more than six times the conductor losses occurs and
• - Beyond this maximum, the screen losses decrease again as the screen cross section is further increased.

This behavior can be explained by the fact that initially - with small screen cross sections - with increasing cross section, the screen currents grow rapidly and approach the phase current. This is in the 3 with the three characteristics 'without' reproduced. The figure shows the dependence of the three screen currents on the cross section A _{S of} the copper screens 'without' and 'with' magnetic harness; in a horizontal single-level arrangement with wire spacing s = 0.5 m; the conductor current is I _c = 1108 A.

Out 3 It can also be seen that the three screen currents are different. In a screen cross-sectional area of 500... 1000 mm ² , the size of the conductor current is assumed to be approximately such that a further increase in cross-section no longer results in an increase in current, but only in a corresponding decrease in shielding losses.

For a cross-sectional area of A _p ≥ 400 m ^2, this screen with increasing cross-section A falling _s Screen loss factors y ₁ are given in the following table. In the present example, shielding loss factors SF of less than 40% can be achieved with correspondingly large screen cross-sections of ≥ 2500 mm ² .

The magnetic inductors that adjust for a symmetrical phase current of 1108 A are in the 5 horizontal distribution of the magnetic induction in 0.2 m above the earth surface 'without' and 'with' magnetic harness (x = 0 m is the coordinate of the middle cable core)

Of the 5 (a and b) shows the distribution of the magnetic induction given without shield currents: it reaches close (0.2 m) above the earth's surface and directly above the cables with approx. 120 μT noticeably above the statutory limit of 100 μT. Also, in 5 the distributions for screen cross-sections of A _s = 400 m ² (a) and for A _S = 2500 m ² (b) are given (note the axis labels differing by a factor of 30!).

In addition are in the 6 the maximum magnetic induction occurring directly above the cable is compiled in 0.2 m above the earth's surface as a function of the screen cross-section A _s 'without' and 'with' magnetic banding.

To 5 and 6 the highest magnetic induction for a screen cross section of 400 mm ^{2 is} still more than 25 μT and of 2500 mm ² more than 4 μT. In view of the high design complexity and the high additional losses, the shielding effect does not appear to be very satisfactory.

Used for the high-permeability harness, overlapping wound thin strips of special steel. Their relative permeability should be around 10,000. The effect of this applied above the copper screen bands are the 3 to 6 refer to:

To 3 The highly permeable layer in the cables already forces shield currents at small screen cross-sections, which almost reach the size of the conductor current. With sufficiently large screen cross-sections of> 400 mm ² , where this is also the case with the cables without harness, will be after 4 the losses of the cables with and without harness comparable. This is because the additional losses in the magnetic layer remain negligibly less than 1 W / m.

Unlike other cables with magnetic enclosure, z. In steel tube cables, for example, in the present design, the additional conductor losses due to field concentration within the magnetic shell are not increased.

To 5 and 6 is achieved with a screen cross-section of A _s = 400 mm ² an extremely high shielding effect; However, in such a screen cross-section also extremely high screen losses of more than twice the conductor losses occur, cf. 4 ,

If the screen cross-section is increased to A _S = 2500 mm ² , the magnetic induction directly above the cables becomes extremely low: after 5 and 6 For example, an extreme limit of 0.2 μT, as required for example in some regions of Italy, is maintained with great certainty just above the surface of the earth. The occurring screen loss factor is below 40%. Table: Screening factors SF for growing screen cross sections A _S 'without' magnetic harness 'with' magnetic harness Shield cross-section A _S in mm ² SF SF 400 5 280 1200 14 750 2500 28 1350

In the table, for the landing point at 0.2 m above the ground level, just above the horizontal cable system in a single plane arrangement, the screen factors are plotted as a function of screen cross section A _S , with and without a high permeability layer above the copper screen. Without magnetic harness, shielding factors up to around 30 are achieved, depending on the screen cross section. If, in addition, a highly permeable layer is applied above the copper screen, extremely high screen factors result, which reach up to about 1400.

Furthermore, it is discussed to what extent requirements for magnetic field limitation and the subsequent shielding measures according to the preceding considerations affect the current carrying capacity of the cables. For this purpose, the already discussed 110 kV VPE single-conductor cable in a laying depth of 1.2 m at a load factor of m = 0.85 is assumed again. The cable trench is thermally stabilized.

In the 7 the current carrying capacity is shown as a function of the wire spacing s of the cables in a single-level arrangement.
black: cable with crossed shields of 50 mm ²
Dashed line: Single conductor cable according to the invention with screen cross section 2500 mm ²
Parameter: magnetic field restriction (thin characteristic: without restriction)

In this figure, first the black characteristics show the current carrying capacity of the cable with a "normal" screen cross-section of 50 mm ² with cyclical cross-over of the screens (crossbonding), as a function of the wire spacing of the cables in the horizontal single-level arrangement. While the thin black characteristic does not take into account any magnetic field restriction, the thick dashed characteristic at distances of about s> 0.25 m shows the required current reduction in the event that a maximum magnetic induction of 100 μT is set at a height of 0.2 m above the earth's surface becomes. A favorable thermal expansion distance of the cable leads to a reduction of the maximum current.

The black circle in 7 returns the load capacity of this cable in the case of an extreme magnetic field restriction of 0.2 μT, assuming a tight laying of the cables in a steel pipe. In this case, not only the thermally unfavorable arrangement, but also the additional losses of the steel pipe in the order of 20% and the additional losses caused by the steel pipe in the cable have a decreasing load-bearing effect.

The thick characteristic in 7 shows the current-carrying capacity of the present single conductor cable, which depends on the wire spacing, here with a copper screen cross section of 2500 mm ² . To 5b This cable causes a maximum magnetic induction of less than 0.1 μT, even at large wire pitches, so that a case distinction with respect to the magnetic field restriction is unnecessary.

To 7 For a magnetic field restriction of 100 μT, the current carrying capacity of the single-core cable of the present application exceeds that of a 'normal' cable for wire distances of more than 1 m, however, the required effort (construction, trench width) is comparatively large.

LIST OF REFERENCE NUMBERS

10

single-core three-phase cable

12

cable conductor

14

Insulation (plastic, VPE)

18

Al-foil

20

Reinforcement of copper

22

Foil made of highly permeable material

24

Cable sheath (corrosion protection)

A _S

Cross section copper screen

y ₁

loss factor

s

Core distance

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2004/034539 [0002]
• EP 1598911 A1 [0002]
• DE 19807527 A1 [0003]

Cited non-patent literature

• CIGRE (Paris, 2008): CIGRE TF C4.204; "Mitigation techniques of power-frequency magnetic fields originated from electric power systems" [0002]
• P. Maioli, E. Zaccone, "Passive Loops Technique for Electromagnetic Field Mitigation, Applications and Theoretical Considerations" in Jicable-Conf. 2007, Versailles, pp. 231-236 [0002]

Claims (6)

Electric single-core cable for a high-voltage three-phase system, with the following structure: - over a plastic-insulated cable conductor ( 12 ) is a foil ( 14 ) made of aluminum, - above a reinforcement ( 20 ) of high conductivity material, - covered with a harness ( 22 ) made of highly permeable material, - closed to the outside with a cable sheath ( 24 ). Single conductor cable according to claim 1, characterized in that the reinforcement ( 20 ) has a cross section (A _S ) in the order of the cross section of the cable ladder ( 12 ) having. Single conductor cable according to claim 1 or 2, characterized in that the reinforcement ( 20 ) consists of copper or aluminum. Single conductor cable according to one of the preceding claims, characterized in that the reinforcement ( 20 ) consists of a network. Single conductor cable according to one of the preceding claims, characterized in that the harness ( 22 ) of overlapping wound thin strips is made of special steel with a relative permeability of some 10,000. High-voltage three-phase current system consisting of three single-core cables ( 10 ), preferably in a single-plane arrangement according to one of the preceding claims, characterized in that the three reinforcements ( 20 ) of the single conductor cable ( 10 ) are shorted together at both ends.

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