RU2299507C2 - Method for shielding power transmission line generated magnetic field and shielded power transmission line - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: proposed power transmission line 1 has at least one electric cable 3, ferromagnetic cable conduit 2 around cable 3 that incorporates base 5 and cover 6, as well as electric contact members 7 for interconnecting base 5 and cover 6; contact members 7 are chosen of group of molten metal connections and flexible components capable of penetrating into ferromagnetic material so that magnetic integrity between base 5 and cover 6 can be ensured.

EFFECT: improved magnetic shielding of cable conduit.

27 cl, 9 dwg, 4 tbl, 5 ex

Description

The invention relates to a method for shielding a magnetic field generated by a power transmission line and to a shielded power transmission line.

State of the art

A high power transmission line is designed to operate at medium voltages (usually at voltages from 10 kV to 60 kV) and high voltages (usually above 60 kV) and currents of the order of hundreds of amperes (usually from 500 A to 2000 A). The electric power transmitted along these lines can reach values of the order of hundreds of MVA, usually 400 MVA. Typically, the transmitted current is an alternating current of low frequency, i.e. has a frequency of less than 400 Hz and usually 50-60 Hz. Basically, these lines are used to transfer energy from power plants to urban centers over distances of the order of tens of kilometers (usually (10-100) km).

Typically, power transmission lines are three-phase current lines containing three cables laid in a trench at a depth of 1-1.5 m. In the space immediately near the cables, the magnetic field H can reach relatively high values, and at ground level (i.e. at a distance 1-1.5 m from the power line) magnetic induction can be of the order of 20-60 μtl (depending on the geometric arrangement of the cables with respect to each other).

To avoid possible biological effects due to irradiation with magnetic fields of this order generated by low-frequency (50 Hz) sources, the "safety threshold" is taken into account, below which the probability of biological damage is minimized or even eliminated. The threshold value of magnetic induction, which is the basis of some national laws, is 0.2 μtl, which is a value that is approximately 100 times less than the magnetic induction generated by the unshielded three-phase current line, as described above.

It is known that laying power cables in shielded conduits can reduce the generated magnetic field.

In publication P. Argaut, J.Y. Daurelle, F. Protat, K.Savina and CAWallaert "Shielding techniques to reduce magnetic fields from buried cables", journal JICABLE, A 10.5, 1999, the screening effect created by an open section screen, such as a sheet of ferromagnetic material placed above the cables, and a closed section screen, such as a rectangular conduit made of ferromagnetic material, placed around the cables, is examined and compared. The publication states that attenuation coefficients of about 5-7 can be obtained by open section screens, attenuation coefficients of about 15-20 can be obtained by closed section screens, and 30-50 attenuation coefficients can be obtained when the screen is closed the cross section is formed very close to the cables (for example, from a sheet of ferromagnetic material wound directly around three cables).

Although the enclosed conduits mentioned above are designed to achieve the best shielding effects, it has been observed that installing cables in enclosed conduits is a time-consuming and expensive operation due to the high pulling forces required, as well as the use of two-part conduits, in particular conduits, containing base and cover to facilitate installation. In the case of a two-part conduit, after placement in the trench of the base, the cables are laid in the base, a cover is placed on the base in order to close the conduit. Conduits consisting of two parts make it possible to lay long cables, in particular in winding trenches, where cable routing inside closed (one-part) conduits would be very laborious. Moreover, the two-part conduits provide the ability to inspect the cables during installation and after installation.

However, it was noticed that the two-piece conduit has an air gap at the junction between the base and the cover due to the inevitable roughness and waviness of the contact surfaces, and it was found that the presence of such an air gap can significantly affect the properties of the magnetic shielding of the conduit. Using numerical simulation, it was determined that a cylindrical shielded conduit having a longitudinal air gap of 1 cm wide determines the presence of a magnetic field at a distance of 2.4 m, which is approximately 6 times the measured field when using a fully enclosed conduit. The deterioration of the gap in the electrical and magnetic integrity of the screen is due to the high electrical resistivity and high magnetic resistance of the air gap. In other words, the screen locally loses its ability to hermetically hold the magnetic field and there is a significant leakage of magnetic field lines.

It was noted that in practice it is impossible to avoid the presence of such an air gap, not only because of the imperfect quality of the plane of the front surfaces of the base and cover, but also because of the presence of oxide, sand, dirt, bulk material, which are usually present during installation when laying the cable which can prevent or limit the formation of tight contact between the base and the cover.

Summary of the invention

The technical task of the present invention is to improve the magnetic shielding of the conduit, which consists of two parts, which, as described above, is a convenient solution when installing the cable.

Applicant has found that the magnetic shielding properties of two conduit parts can be controlled by providing acceptable magnetic integrity between the base and the cover.

In particular, such magnetic integrity can be obtained by providing an electrical connection between the base and the lid having a predetermined high active conductivity capable of providing a highly conductive path between the base and the lid.

The Applicant has discovered that the magnetic integrity of a two-part conduit can be improved by providing mechanical and electrical communication between the base and the cover, and noted that various communication solutions can provide a wide variety of results in improving the electrical and magnetic integrity of the screen as in relation to reducing the air gap, and in relation to creating a highly conductive path between the base and the cover.

In particular, the applicant noted that when an electrical connection is established through a flat surface contact, this electrical connection may be malfunctioning due to corrosive effects, as well as due to the accumulation of dust and dirt on the connecting joint.

The applicant has found that by connecting the cover and the base of the conduit by means of an elastic connection with the mutual penetration of the material or by local metal alloying, it is possible to obtain the electromagnetic integrity necessary for the effective shielding of the cables, commensurate with the efficiency of the completely closed screens.

In the present invention, the expression "mutual penetration of a material" means a state achieved when one body undergoes plastic deformation so as to accept, within its boundaries, the material of another body.

The term "local metal fusion" means metal fusion achieved by welding (in which the two-part bond is a metallurgical bond and requires significant diffusion, preferably with the addition of a welding aid), brazing (in which the brazing metal is molten, but the parts to be joined may not to melt, so that the bond is usually formed by limited solid-phase diffusion of the metal of the solder in the connected parts), soldering (in which there is no tacking, nor solid-phase diffusion of the parts to be joined, and here the bond is usually determined by the adhesion of the molten solder to each metal surface), or by a similar technique (such as soldering-welding, which is a welding method in which the joint is formed by heating above 427 ° C and using filler from non-ferrous metal having a melting point lower than the melting temperature of the base metals, while, unlike brazing, capillary attraction does not distribute the filler metal in the compound) .

An elastic connection with the mutual penetration of the material can be obtained, for example, by using connecting elements provided with sharp protruding parts, such as elastic clamps having penetrating teeth, superimposed on overlapping sections of the base and cover. Sharp protruding parts, in addition, determine the point of electrical contact, mainly perform a localized cleaning effect on the bonding surfaces during application of the connecting element by removing any oxide, dirt, sand or bulk material present on these surface points.

The elastic connection of elements with mutual penetration of the material, such as connection by clamps, is a simple and quick operation, and unlike metal fusion connection, it does not require a local power supply (which may not be available at the place of cable laying).

According to a first aspect of the present invention, there is provided a power transmission line comprising at least one electric cable, a conduit made of ferromagnetic material, covering said at least one cable and comprising a base and a cover, and electrical contact elements electrically connecting the base and the cover, the electric elements Contact selected from the group consisting of compounds by metal alloying and elastic elements capable of penetrating into the ferromagnetic material.

The conduit can be laid underground.

The power transmission line contains three cables connected in the form of a Gothic outlet (trefoil).

The base and cover have predominantly overlapping portions on both sides of the conduit, and electrical contact elements are preferably superimposed on the overlapping portions. The overlapping portions can be separated by an air gap and preferably have a width that is at least five times the thickness of said gap. The clearance is preferably a maximum of 3% of the perimeter of the cross section of said conduit. A material having a magnetic permeability greater than the magnetic permeability of air can be placed between overlapping portions of the base and cover.

The electric contact elements have a mutual longitudinal gap of preferably 50 cm maximum, and more preferably 25 cm. According to one embodiment of the invention, the electric contact elements can be elastic clamps provided with portions capable of penetrating into the ferromagnetic material, causing its plastic deformation.

The conduit mainly comprises a plurality of longitudinal sections partially overlapping each other, and each section comprises a base portion and a cover portion. The base portion preferably has a U-shaped cross section, and the cap portion is substantially flat.

Preferably, the longitudinal sections are electrically connected to each other. For example, the longitudinal sections may overlap by a length which preferably is at least 25% of the width of said conduit. The lid and base portions of each of the longitudinal sections can be offset relative to each other in the longitudinal direction. At least two of the longitudinal sections may extend in different directions, and the conduit may comprise a connecting element for connecting said two sections of the conduit, wherein the connecting element consists of two parts electrically connected by means of said electric contact elements.

According to a second aspect of the present invention, there is provided a method for shielding a magnetic field generated by a power transmission line comprising at least one electric cable, said method comprising the following steps of placing an electric cable in a conduit of ferromagnetic material comprising a base and a cover, providing an electrical connection between the base and the cover having active conductivity, calculated per meter length, of at least 150 S / m, and more preferably m at least 1500 S / m.

Preferably, the electrical connection is formed by metal alloying (for example, welding, soldering, brazing) or by elastic bonding with the mutual penetration of the material.

Preferably, at least one electric cable is placed in the conduit, for which the respective side sections of the base and the cover are overlapped, and the base and the cover are electrically connected by electrical connection of the side sections.

The electrical connection of the aforementioned side portions may include applying to the side portions a plurality of metallic elastic clips capable of penetrating through the surface of the side portions when they are elasticly applied.

Placing said at least one electric cable in a conduit preferably includes installing the base underground, laying at least one electric cable in the base, and applying a cover to the base to close the conduit.

The conduit mainly contains many longitudinal sections, and in the method additionally carry out electrical communication of the longitudinal sections.

Brief Description of the Drawings

The invention is further explained in the description of the preferred embodiments of the invention with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a portion of a power transmission line according to the invention.

FIG. 2a is a cross section in the plane II-II in FIG. 1.

FIG. 2b is an enlarged view of a portion indicated by a dashed line in FIG. 2a.

FIG. 2c is another embodiment of the portion shown in FIG. 2b according to the invention.

FIG. 3 is a perspective view of a two-piece conduit that is part of the power transmission line of FIG. 1 (the cover is laterally offset for simplicity) according to the invention.

Figa is a side view of a power transmission line.

FIG. 4b is an enlarged view of a portion indicated by a dashed line in FIG. 4a according to the invention.

FIG. 5 is a curved section of a power transmission line according to the invention.

FIG. 6 is a diagram of the results of experimental studies according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

An electric line 1 for transmitting medium or high power three-phase currents comprises a conduit 2 made of ferromagnetic material and three cables 3 inside the conduit 2. Cables 3 are used to transmit alternating current with a frequency of usually 50-60 Hz and are installed in contact with each other to form trefoil configuration (in other words, with their geometric centers located at the vertices of an essentially equilateral triangle. Alternatively, cables 3 may lie on the bottom of conduit 2, although such a solution can This may require a wider conduit and may increase the magnetic field around the cables.

Electric line 1 is, for example, a power electric line suitable for operation at a voltage of about 132 kV and a current of about 400 A, and capable of transmitting currents up to 860 A. Electric line 1, in particular, is intended for underground use, although it can be shielded apply above ground.

Each of the cables 3 may include, for example, an enamelled copper Milliken wire insulated with an extruded polymer, such as, for example, cross-linked polyethylene (XLPE). Millikenovy wire may have a cross section of 1600 mm ² . The outer surface of each cable may also be provided with a metal sheath for protection against moisture. The total outer diameter of each cable is usually 100 mm.

Cables 3 can be connected in a trefoil configuration by means of connecting elements. Alternatively, with proper sizing of conduit 2, one or more wedge elements, preferably wooden, can be installed between the cables 3 and the inner walls of the conduit 2 at appropriate distances to hold the cables 3 in a trefoil configuration along the line.

Moreover, the trefoil of cables 3 can directly contact the bottom of the conduit 2 or can be raised relative to it to a position closer to the geometric center of the cross section of the conduit 2. To raise the trefoil of the cables 3, gasket elements (not shown) between the conduit can be installed 2 and cables 3.

The space inside the conduit 2 not occupied by the cables 3 may remain empty, i.e. filled with air or filled with a material having a low thermal resistivity, for example an inert filler, described in the publication of international patent application WO 9964367 A1.

The conduit 2 is a modular conduit and contains many longitudinal sections 4 (only three are shown in FIG. 1) having a typical length of several meters and connected end-to-end to form an elongated cable duct.

The length of each section is selected for practical reasons, such as the weight of the section and the method of laying the conduit. For example, in the case where the laying operation has to be performed manually, the length of each section should be such as to provide a weight that can be easily lifted when laid by workers. In other cases, if a laying operation using mechanisms is contemplated, other selection criteria may be applied.

Each section 4 contains two separate elements, in particular the base 5 and the cover 6, which overlap and are connected to each other to form a substantially closed tubular channel. Sections 4 are preferably the same length, and each cover 6 has the same length and width of the corresponding base 5. In addition, the bases 5 and covers 6 have the same thickness, which is preferably a maximum of 10 mm, more preferably a maximum of 5 mm, and most preferably a maximum of 3 mm.

As can be seen in the cross section (Fig. 2a), the base 5 may have a substantially U-shaped cross section, and the cover 6 may be in the form of a flat rectangular valve. It should be noted that the base 5 may have a base wall 5a, two side walls 5b, obliquely extending outward from the base wall 5a, and two flanges 5c extending parallel to the base wall 5a and outward from the end portions of the side walls 5b to form corresponding “wings "the base 5. The base 5 can be made, for example, by folding a flat rectangular valve along four essentially parallel lines. In use, if the excavation and installation of the conduit 2 were performed correctly, the base wall 5a and the flanges 5c should preferably occupy a horizontal position.

The cover 6 is placed with the side edges aligned with the edges of the base 5, so that the side portions 6a of the cover 6 overlap the flanges 5c. Two overlapping areas are thus defined on both sides of the conduit 2 having a width substantially corresponding to the transverse dimensions of the flanges 5c. Said width is preferably 10-150 mm. In order to better limit the magnetic field of the conduit 2, the ratio between the width of the overlapping region and the possible air gap between the flange 5c and the side section 6 (indicated by w and g in Fig. 2b, respectively) should preferably be greater than 5. Moreover, the total air gap in the cross section of the conduit which is twice the gap g on each side of the conduit, is preferably less than 3%, and more preferably less than 1% of the perimeter of the conduit cross section (not including wings 5c and front sections 6a of the roof s) to provide a magnetic circulation.

As shown in an alternative embodiment of the invention (Fig. 2c), the gap between the base 5 and the cover 6 can be filled (at least partially) with a material 9 having a magnetic permeability greater than the magnetic permeability of air. For example, a magnetic rubber sealing element 9 can be sandwiched between the base 5 and the cover 6 on each side of the conduit 2. As a solution, a hard sealing element made of steel or other material with high magnetic permeability can be used. Alternatively, a resin or adhesive with a high magnetic permeability material such as iron powder may be used. By increasing the magnetic permeability between the base 5 and the cover 6, the magnetic integrity of the conduit 2 is improved. The material of the sealing element 9 is preferably selected from materials with low electrical resistivity in order to improve the electrical connection between the base 5 and the cover 6. As a solution, a material can be used, having high electrical conductivity and low magnetic permeability, such as copper. Any residual air gap between the base 5 and the cover 6 not filled with the sealing element 9 should preferably have the previously indicated maximum dimensions.

If the space inside the conduit 2 (not occupied by the trefoil of cables 3) contains a filler such as a concrete composition, care must be taken to prevent concrete or similar material from entering the flanges 5c before the cover 6 is laid on the base 5 during installation power lines.

The base 5 and the cover 6 are preferably made of the same ferromagnetic material, preferably steel. Relative magnetic permeability μmax. preferably more than 1000, and more preferably 4000, and most preferably more than 8000, and electrical conductivity is preferably more than 10 ⁶ S / m. The base 5 and the cover 6 can also be made of two different ferromagnetic materials, preferably two different types of steel having the above magnetic and electrical properties.

The surface of the base 5 and the cover 6 is preferably treated with electroplating and resin. Table 1 shows the composition, as well as the elastic and magnetic properties of two different types of steel, designated "B" and "Q", which can be used to manufacture conduit 2.

Table 1 Name "B" "Q" Type of Fe510 SNSU (ultra low carbon content) Items Iron 97,910 99,668 Carbon 0.180 0.010 Manganese 1,460 0.190 Silicon 0.270 0,030 Sulfur 0.018 0.011 Phosphorus 0.015 0.005 Chromium 0,050 0.013 Nickel 0,030 0,025 Molybdenum 0,020 0.005 Aluminum 0,027 0,027 Copper 0,020 0.016 Electrical / Magnetic Properties ρ [Ohm · m] 2.16 E-07 1.17 E-07 μmax 482 4900 Hμmax. [A / m] 1350 145

In Table 1, ρ is the electrical resistivity, μmax. - the maximum value of relative magnetic permeability, Hμmax. - magnetic field strength at μmax.

Due to the higher μmax. steel Q is preferable to steel B. By selecting a material with a higher μmax. It is possible to obtain acceptable shielding properties for a conduit having a smaller thickness and therefore less weight. The lighter conduit reduces installation costs since the conduit can be routed manually by operating personnel instead of using complex machines. The Applicant has found that the same shielding effect obtained with a 3-4 mm conduit made of Q steel could be obtained using a 5-8 mm conduit made of Steel B.

Other steels suitable for such an application are described, for example, in European patent application EP 870848 A1.

Adjacent sections 4 of the conduit 2 are preferably overlapped in the longitudinal direction (Figs. 3, 4a and 4b) in order to avoid breaks in the magnetic field. In particular, the bases 5 ', 5 "of two consecutive sections 4', 4" overlap to a length L. The application of the bases 5 ', 5 "is possible due to the shape of the truncated cone. The corresponding covers 6', 6" also overlap preferably on the same length L. However, each cover 6 is advantageously offset by a predetermined length with respect to the corresponding base 5, the offset length of which is equal to the overlap length L. The cover 6 'of section 4' is placed with its end opposite the end of base 5 'of the next section 4 "(Fig. 4b )

The overlay length L is preferably greater than 1/4 of the maximum inner width W of the conduit 2 (L> 1 / 4W), more preferably greater than 1/2 of this width (L> 1/2 W), and most preferably more than 3/4 of this width (L> 3 / 4W). In any case, the overlap length L is preferably 10-300 mm.

The base 5 and the cover 6 are connected to each other to increase the electrical and magnetic integrity at the junction and to improve the magnetic shielding of conduit 2. According to the present invention, the connection between the base 5 and the cover 6 is achieved by local metal fusion or elastic connection with the mutual penetration of the material. The conduit 2 is provided with communication means 7, which are either metal alloy compounds or applied elastic elements suitable for mutual penetration of the material. Communication means 7, in addition to fixing two parts and reducing any possible clearance at the junction, make it possible to form an electrical contact between the parts, which ensures the continuity of the flow of magnetic field lines.

Other types of communication that cannot guarantee continuity of flow due to gaps, dust, dirt and / or corrosion, which prevent the formation of good electrical contact, greatly reduce the ability of the magnetic shielding of the conduit. For example, if bolts with flat washers are used, electrical contact between the washers and the possibly dirty, oxide-coated conduit surface may be poor.

In one embodiment of the invention, the communication means 7 are (Figs. 2a and 2b) elastic clips of electrically conductive material, preferably steel. The clamps have a C-shaped elastic main body 7a for gripping and clamping the cover 5, and the base 6 due to the elastic force. The clamps are also provided with an adjusting flange 7b, which facilitates fixation. The two opposing arms 7d of the main body 7a are provided with one or more teeth 7c directed inward of the main body 7a, for surface penetration into the base material 5 and cover 6 when a clamp is applied. The ends of the arms 7d are bent predominantly outward to facilitate clamping. Penetration into the material is important to ensure a stable bond and to create a contact area that provides the required electromagnetic integrity between the cover 6 and the base 5. Moreover, by penetrating the surface of the base 5 and the cover 6, the clamps provide a more stable physical connection between the base 5 and the cover 6 .

Clips of the type described above are manufactured, for example, by ERICO Inc., 34600 Solon Road Solon, Ohio 44139 United States (clamps of registered models are protected by trademark CADDY H-Clips).

The clamps can be easily and quickly applied using a hammer, each clamp can be adjusted on a flange 7b placed so that the ends of the shoulders 7d face the side edges of the cover 6 and the base 5, and they are superimposed by impact of the hammer, causing the teeth 7c to penetrate into the material of the cover 6 and grounds 5.

Clips can be applied mainly in the form of equally spaced pairs (Fig. 1). As a rule, the value of the connection step has a lesser effect, the inductive coupling between the base 5 and the cover 6 is more effective. However, a compromise must be reached in order to avoid excessive costs. The Applicant has found that an improvement in shielding of the magnetic field of more than 30% (relative to the state of no connection) can be achieved with a clamp pitch of 50 cm in the longitudinal direction, and an improvement in shielding of more than 45% can be achieved with a clamp pitch of 25 cm

In the overlapping areas between adjacent sections 4 (Fig. 4b), the clamps will capture three overlapping parts (5 ', 5' ', 6' or 5 '', 6 ', 6' '), the total thickness of which is three times the thickness t separate part.

As indicated previously, the metal fusion compound is an alternative type of coupling means 7 suitable for implementation in the present invention. In particular, the base 5 and the cover 6 can be connected by continuous brazing, welding or brazing, extending over the entire length, or almost the entire length of the conduit, or spot brazing, welding or brazing. The distance between successive points of metal alloying may correspond to the distance previously specified for the clamps. Metal fusion bonding has the advantage of clamp bonding, which results in a longer and more uniform electrical contact between the two bodies to be bonded, but the disadvantage is that they are more susceptible to corrosion and require more time and energy to install .

The conduit 2 (Fig. 5) may include curved or curved sectional communication elements 8 to allow horizontal or vertical deviations in the direction of the transmission line 1. The communication element 8 is preferably a tubular element of the same material as the material of the base 5 and the cover 6, has a section bent or curved at an angle α, preferably 3-20 °. The coupling element 8 is dimensioned to provide coverage for the end of the two sections 4 to be connected. Alternatively, the coupling element 8 may be sized to be installed on the inside of the sections 4.

The communication element 8 may include two separate parts superimposed on each other having the same cross-sectional shape and the same dimensions of the base 5 and the cover 6, these two parts being identified as the base and the cover of the communication element 8.

Communication element 8 is superimposed on each of two sections 4 connected over a length of preferably 10-300 mm, while successive communication elements 8 must be spaced at least 1 meter apart to avoid bending due to excessive load on cables 3.

The base and the cover of the communication elements 8 can be connected by the same clamps used to connect the base 5 and the cover 6. In the overlap area between the communication element 8 and the section 4, the clamps must tie four parts together (base 5, cover 6, base and cover communication element 8) having a total thickness four times the thickness of an individual part.

To obtain satisfactory magnetic integrity of the screen, the communication means should be selected and spaced from each other so as to provide an electrical connection between the base 5 and the cover 6, having an active conductivity per meter length equal to at least 150 S / m, more preferably at least 500 S / m and most preferably at least 1500 S / m. For example, elastic clamps of the aforementioned type, spaced 50 cm apart, can provide active conductivity per meter of length greater than 1500 S / m. By decreasing the pitch of the communication means, a higher value of active conductivity can be achieved.

The base 5 and the cover 6 may have shapes other than those previously described. For example, the cover 6 may have a semicircular cross section or a polygonal cross section, for example triangular or trapezoidal, the base 5 may have a semicircular cross section or a polygonal cross section, for example a trapezoidal, the flanges 5c may lie on different planes, the trefoil of the cables 3 may be turned over when one cable is at the bottom and two cables are at the top, and the side walls 5b of the base 5 can be tilted to set up such a configuration. Communication elements 8, if required, should be in the appropriate form.

Moreover, the base 5, the cover 6 and the coupling elements 8, instead of being made entirely of ferromagnetic material, could include layers of various materials, at least one of which is a ferromagnetic material.

Regarding the connection between adjacent sections 4, an alternative solution to ensure the continuity of the magnetic screen 4 is to place the bases 5 and the covers 6 of the adjacent sections 4 butt and weld. Another alternative solution is to use direct coupling elements of ferromagnetic material (preferably of base material 5 and cover 6) containing a base and cover element, similar to communication element 8, and suitable for superimposing on each connection section 4 a length of preferably 10-300 mm Another solution is the use of transverse flanges at the ends of adjacent sections and their bonding, for example, with clamps.

These clamps are only a possible example of a means of communication suitable for elastic communication with the mutual penetration of the material. The communication means 7 can include any elastic element capable of penetrating through the material of the conduit (in particular, steel) under elastic action, the elastic element being provided with a sharp protrusion, for example, teeth 7c. You can also use industrially manufactured tools, such as self-tapping screws or bolts equipped with lock toothed washers and / or nuts (nuts with locking latches).

Example 1

The applicant has measured the magnetic field generated above the ground from an electric line laid in the ground, as described above, under various operating conditions.

The test line contained an electric current generator, three shamrock cables 3 connected to the generator, a 50 m long cylindrical steel conduit containing 3 cables from the generator outlet and six longitudinal sections containing cables from the end of the cylindrical conduit. At the output of six sections 4, cables 3 were short-circuited. The longitudinal sections 4 were superimposed with an overlap of 200 mm and placed at a depth of 1.4 m underground. Magnetic field was measured with a PLM-100WB Handheld ELF Model Magnetometer, MacIntyre Electronics Design Associates, Inc. (485 Spring Park Place, Herndon, VA, USA), at a point 1 m above the ground and between the third and fourth sections (i.e. in the middle of the line).

The base element 5 had the following dimensions:

- height: 215 mm; width: 370 mm; length: 3000 mm; the inclination of the side walls 5b: 8 °; width of base wall 5a: 230 mm; width of flanges 5c: 40 mm; thickness: 5 mm.

Cover element 6 was a flat panel 5 mm thick with dimensions (370x3200) mm. Both base 5 and cover 6 were made of Q steel.

Cable type 3, suitable for 132 kV lines, had a diameter of 105 mm. Cables 3 were powered by 400 A (RMS) 50 Hz balanced and balanced three-phase current.

The magnetic field was measured under various conditions:

a) cables 3 are directly laid in the ground (without shielding conduit);

b) the cables 3 inside the conduit 2 and the cover 6 are pressed against the base 5 with a force of 60 kg / m ² ;

c) the cables 3 inside the conduit 2 and the cover 6 are connected to the base 5 by means of clamps every 50 cm;

d) the cables 3 inside the conduit 2 and the cover 6 are connected to the base 5 by means of clamps every 25 cm.

Used clamps registered and trademarked by H-CADDY models from ERICO Inc., (34600 Solon Road Solon, Ohio 44139 United States).

Table 2 shows the maximum magnetic field values measured under the indicated conditions.

table 2 Condition Maximum magnetic field
[microtesla (mktl)] BUT) 1.8 B) 0.1 5 AT) 0.10 D) 0.08

When the conduit, in which the cover 6 is simply pressed against the base 5 (but not connected to it), can reduce the magnetic field above the ground by more than 10 times relative to the cables laid directly in the ground, a further reduction of 33% can be achieved by connecting the base 5 and covers with 6 clamps in increments of 50 cm. Another additional 20% improvement in performance can be obtained by reducing the pitch of the clamps to 25 cm. In practice, when switching from condition a) to condition d) the magnetic field is reduced by about 22 times.

Example 2

The applicant also discovered a change in the magnetic field when the measurement position changed along the transverse direction (i.e., the direction perpendicular to the conduit direction) at a height of 1 m above the ground. The shape, dimensions, and material of conduit 2 were the same as in Example 1, and the following conditions were considered:

a) there is no connection and perfect insulation between the base 5 and the cover 6, achieved by the introduction of a nylon sheet between the base 5 and the cover 6;

b) communication with clamps installed every 50 cm,

c) communication with clamps installed every 25 cm.

In FIG. 6 presents the results of this experiment. You can notice that the curves have a bell-shaped shape with a maximum value at the point of zero abscissa, that is, in the plane of the median of conduit 2. The improvement in performance achieved by using clamps remains essentially constant when moving away from electrical line 1.

Example 3

A new series of measurements has been performed to compare, using the ad hoc parameter, the effectiveness of various solutions. The parameter referred to herein as “electrical integrity” and designated by the symbol λ has the following expression.

where B _n is the maximum magnetic field measured at a height of 1 m above the ground (using the same device as in the first example) with a common galvanic isolation between the base 5 and the cover 6, achieved by the introduction of a nylon sheet between the base 5 and the cover 6; B _i - the maximum magnetic field measured at a height of 1 m above the ground under certain operating conditions. The parameter λ does not depend on the measurement distance and, therefore, is an absolute indicator of the growth of electrical integrity relative to the perfect isolation condition. The parameter λ, however, depends on the characteristics of the conduit material, in particular on the magnetic permeability.

The parameter λ was measured under the following conditions using two types of conduits having the same shape and dimensions as in Example 1, but made of two different steels, namely steel Q and steel B:

a) the cover 6 is superimposed on the base 5 with the introduction of an insulating nylon sheet;

b) the cover 6 is superimposed on the base 5 (without the introduction of the sheet);

c) the cover 6 is applied and pressed to the base 5 (60 kg / m ² ),

d) the base 5 and the cover 6 are bolted every 50 cm,

d) the base 5 and the cover 6 are bolted every 25 cm;

f) the base 5 and the cover 6 are connected by clamps 7 every 50 cm;

g) the base 5 and the cover 6 are connected by clamps 7 every 25 cm;

h) the base 5 and the cover 6 are connected by continuous soldering.

Table 3 shows the results of these measurements.

Table 3 Condition Q steel Steel B but) λ = 0% λ = 0% b) λ = 3% λ = 0% at) λ = 7% λ = 0% d) λ = 13% λ = 1% e) λ = 20% λ = 2% e) λ = 33% λ = 4% g) λ = 47% λ = 5% h) λ = 82% λ = 10%

From the results of table 3 we can see that

- binding using clamps 7, that is, an elastic connection with the mutual penetration of the material provides improved electrical integrity than a bolted connection,

- conduit made of a material with a greater magnetic permeability (steel Q), provides a more effective magnetic shielding.

Example 4

Other measurements were performed to verify the effect of overlapping between adjacent sections 4 on the measured magnetic field. The shape, dimensions and material of the conduit 2 were the same as in example 1, and the clamps were installed every 50 cm to connect the base 5 and the cover 6. The relative distance between adjacent sections was changed as follows:

a) overlay with an overlap of 200 mm;

b) end contact (without displacement in the longitudinal direction between the base and the cover);

c) the separation of adjacent sections from each other by 200 mm (without displacement in the longitudinal direction between the base and the cover).

Magnetic fields measured as in example 1 are shown in table 4.

Table 4 Condition Maximum magnetic field
[microtesla (mktl)] but) 0.10 b) 0.25 at) 0.40

These results show that overlapping between adjacent sections is important to ensure the integrity of the magnetic shielding.

Example 5

The applicant has also compared the electrical connection characteristics of the clamps with respect to conventional bolts.

Two flat sheets of Q steel having a thickness of 5.8 mm were superimposed overlapping and incorporating an insulating nylon sheet and electrically connected. Steel sheets were industrially manufactured sheets stored under ordinary conditions.

The electrical connection was obtained first by using one clamp of the above type (H-CADDY model), applied without surface preparation, and then using an industrially manufactured M4 bolt (tightened with a wrench) equipped with flat washers between the bolt head and the top sheet and the nut and bottom sheet . In the first test, the surfaces of the sheet in contact with the washers were left without preparation; in the second test, the surfaces mentioned were cleaned with glass skins. The resistance between the two plates was measured with a 4-pin technique using a DC generator and a millivoltmeter.

The detected resistance was 2.3 mOhm in the case of clamping and 4.2 mOhm in the case of a bolt with cleaned surfaces.

The presence of dirt and corrosion in the areas of bolt contact increased the measured resistance by almost a factor of two.

Claims (27)

1. Power line (1) power transmission containing at least one electric cable (3), conduit (2) of ferromagnetic material, covering the specified at least one cable (3) and containing the base (5) and the cover (6), electrical contact elements (7) for electrically connecting the base (5) and the cover (6), in which the electric contact elements (7) are selected from the group consisting of metal alloy compounds and elastic elements capable of penetrating into the ferromagnetic material. 2. The power line according to claim 1, characterized in that the base (5) and the cover (6) have overlapping sections (5c, 6a) on both sides of the conduit (2), and said electric contact elements (7) are superimposed on the overlapping sections (5c, 6a). 3. The power line according to claim 1, characterized in that the elastic elements (7) are clamps having sections capable of penetrating into the ferromagnetic material. 4. The power line according to claim 2, characterized in that the overlapping sections (5c, 6a) are separated by an air gap (g) and have a width (w) that is at least five times the thickness of the air gap (g). 5. Power line according to claim 4, characterized in that the air gap (g) is a maximum of 3% of the perimeter of the cross section of the conduit (2). 6. Power line according to claim 1, characterized in that the elements (7) of the electrical contact have a mutual longitudinal gap of a maximum of 50 cm 7. The power line according to claim 6, characterized in that the mutual longitudinal gap is a maximum of 25 cm 8. Power line according to claim 1, characterized in that the conduit (2) contains many longitudinal sections (4), partially overlapping each other, each of which contains a section (5) of the base and a section (6) of the cover. 9. Power line according to claim 8, characterized in that the longitudinal sections (4) are electrically connected to each other. 10. Power line according to claim 8, characterized in that the cover section (6) and the base section (5) of each of the longitudinal sections (4) are offset from each other in the longitudinal direction. 11. Power line according to claim 8, characterized in that the longitudinal sections (4) overlap a length (L), which is at least 25% of the width (W) of the conduit (2). 12. The power line according to claim 1, characterized in that the ferromagnetic material is steel. 13. Power line according to claim 1, characterized in that at least one cable (3) contains three cables forming a gothic outlet (trefoil). 14. Power line according to claim 8, characterized in that at least two of the longitudinal sections (4) extend in different directions, the conduit (2) comprising a connecting element (8) for connecting the two conduit sections (4), wherein the connecting element (8) consists of two parts electrically connected by means of the said electric contact elements (7). 15. Power line according to claim 8, characterized in that the section (5) has a U-shaped cross section. 16. The power line according to claim 1, characterized in that the section (6) of the cover is essentially flat. 17. The power line according to claim 1, characterized in that the conduit (2) is designed to be placed underground. 18. The power line according to claim 1, characterized in that the material having a magnetic permeability greater than the magnetic permeability of the air is placed between the overlapping sections (5c, 6a) of the base (5) and the cover (6). 19. A method of shielding a magnetic field generated by a power transmission line containing at least one electric cable, comprising placing an electric cable in a conduit of ferromagnetic material containing a base and a cover, forming an electrical connection between the base and the cover having active conductivity in terms of meter length of at least 150 S / m 20. The method according to claim 19, characterized in that the active conductivity is at least 500 S / m. 21. The method according to claim 20, characterized in that the active conductivity is at least 1500 S / m 22. The method according to claim 19, characterized in that form an electrical connection between the base and the cover by forming an elastic connection between the base and the cover with the mutual penetration of the material. 23. The method according to claim 19, characterized in that an electrical connection is formed between the base and the lid, for which metal fusion of the base and lid is performed. 24. The method according to claim 19, characterized in that when the electric cable is placed in the conduit, the corresponding side sections of the base and the cover are overlapped, and when the electrical connection between the base and the cover is formed, the side sections are electrically connected. 25. The method according to p. 24, characterized in that for the electrical connection of the side sections are applied to the side sections of a plurality of metal elastic clamps capable of penetrating through the surface of the side sections under elastic influence. 26. The method according to claim 19, characterized in that when placing the electric cable in the conduit, the base is installed underground, at least one cable is laid in the base and the lid is laid on the base to close the conduit. 27. The method according to claim 19, characterized in that the conduit contains many longitudinal sections that are electrically connected to each other.

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