JP2006135115A - Magnetic shield structure for ac power cable - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic shield structure shielding a magnetic field generated from an AC cable wherein a distance d between the cable centers of respective phases is not shorter than 50 mm. <P>SOLUTION: Cables of all phases are surrounded in a plane perpendicular to the current direction by a soft magnetic body having a thickness of 0.2 mm or more over 180° or more about the center of gravity of the cable in that plane. Excellent magnetic shield effect is attained by setting the closest distance L from the cable center to the surface of the magnetic body within following range; 0.5×d<L≤4×d. Significant shield effect is attained when a non-oriented electromagnetic steel plate or an oriented electromagnetic steel plate is employed as the soft magnetic body. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic shield structure that shields magnetic noise generated from an AC power cable.

  A magnetic field is generated by the current around the power cable. However, when the cable is far away from the place where the exposure to the magnetic field is concerned, there is no serious problem because the distance attenuation of the magnetic field can be expected. In recent years, however, cables are often buried underground, and the depth of the cables tends to be shallow. Also, if the cables of each phase are close to each other or have a stranded wire structure, there is little problem because the distance attenuation of the magnetic field is large, but in the joint part under the manhole, etc., when each cable is separated The magnetic field from here can be a problem. This is because the distance attenuation of the magnetic field decreases as the distance between the cables increases. Recently, the influence of the magnetic field on the human body has begun to be concerned, and the allowable magnetic field strength tends to be reduced.

As one method for shielding a magnetic field from a power cable, a magnetic shielding cable (Patent Document 1) having a structure in which a core wire in a cable is covered with a magnetic steel sheet and a magnetic soft iron tape has been proposed. In addition, a magnetic shield pipe (Patent Document 2 and Patent Document 3) in which a power cable is surrounded by an electromagnetic steel sheet has been proposed. Furthermore, a magnetic shield pit (Patent Document 4) characterized by surrounding a cable group with a magnetic material has also been proposed.
Japanese Patent Laid-Open No. 6-1887846 JP 9-196300 A JP-A-10-117083 Japanese Utility Model Publication No. 4-18493

The features of the above Patent Documents 1 to 3 are to magnetically shield a conductor through which a current flows by concentrically covering it with a magnetic material. However, such a configuration has a problem that a magnetic shield effect cannot be obtained sufficiently. Further, when the distance between the cables is increased, the magnetic shield effect may be insufficient even if the technique of Patent Document 4 is used.
As described above, regarding the magnetic shield of the magnetic field of the AC cable, in particular when the distance between the cables of each phase is wide, no technical establishment has been made to obtain a shielding effect. An object of the present invention is to propose a magnetic shield structure having excellent magnetic shielding performance against such problems.

In the present invention, a magnetic field generated by an AC cable having a distance d between the cable centers of each phase of 50 mm or more is targeted for shielding. Cables of all phases in a plane perpendicular to the current direction are continuously surrounded by a soft magnetic material having a thickness of 0.2 mm or more, centered on the center of gravity of the cable in the plane, and from the center of the cable. The closest distance L to the magnetic surface is
0.5 × d <L ≦ 4 × d
By setting it as the range, an excellent magnetic shielding effect is obtained.
Furthermore, when a non-oriented electrical steel sheet or a directional electrical steel sheet is used as the soft magnetic material, the shielding effect becomes remarkable. In the case of a grain-oriented electrical steel sheet, by directing the rolling direction in a direction perpendicular to the current direction of the cable, the magnetic field and the easy magnetization direction coincide with each other, and a more excellent magnetic shielding effect can be obtained.

  By using the magnetic shield structure of the present invention, an unnecessary magnetic field from the power cable can be efficiently shielded.

The present invention is described in detail below.
As described above, when the distance d between the cable centers of each phase is small, the distance attenuation of the magnetic field can be expected and there are few problems. However, as the distance d increases, the magnetic field increases, and no countermeasure technology has been established. The present invention is directed to a magnetic field generated by an AC power cable having a distance d between the cable centers of 50 mm or more.

  In the present invention, first, in AC power transmission, it is necessary to enclose all the phase cables among a plurality of phase cables using a soft magnetic material. The reason is as follows. Maxwell's equations teach that integrating a magnetic field element around a current source results in an integral value equal to the internal current. By the way, in AC power transmission, if the currents of all phases are added at a certain moment, the current becomes zero. Therefore, if all AC phase cables are surrounded by a magnetic material and each element magnetic field can be ideally formed, the integral value of the magnetic field along the magnetic material will be 0, and the outside magnetic field will be 0. Can do. That is, it can be magnetically shielded. On the other hand, even if some of the phases are excluded and the cable is enclosed, the integral value of the magnetic field, that is, the internal current does not become zero, and it cannot be shielded in principle. From the above, it is necessary to surround all phases of the AC cable with one magnetic material.

  Next, according to the present invention, 180 ° or more must be continuously surrounded by a magnetic material with the center of gravity of each phase cable as the center of the angle. In this case, the direction in which the magnetic field is desired to be reduced is surrounded by a magnetic material. This is because if the angle is less than 180 °, the magnetic field also circulates on the surface to be shielded and leaks.

In the present invention, when the distance between the cable centers of each phase is d, and the closest distance from the cable of each phase to the magnetic surface is L,
0.5 × d <L ≦ 4 × d
And shall be. When L is 0.5 × d or less, sufficient magnetic shielding performance cannot be obtained. This is probably because the magnetic field at that position is very large and the magnetic substance approaches saturation, so that the magnetic permeability decreases. Also, when L is larger than 4 × d, a sufficient shielding effect cannot be obtained. This is probably because the magnetic field is too small and deviates from the high magnetic permeability range of the magnetic material. Furthermore, when larger than 4 * d, the magnetic body for enclosing an electric current becomes large, and it is not efficient from a viewpoint of effective use of space, and an economical viewpoint. Therefore, L must be in the above range.

  Next, the thickness of the magnetic body of the present invention needs to be 0.2 mm or more. Below this range, magnetization is likely to be saturated, and it is difficult to obtain a sufficient shielding effect. Furthermore, the magnetic body is a non-oriented electrical steel sheet or a directional electrical steel sheet, so that an inexpensive and excellent magnetic shielding effect can be obtained. In the case of a grain-oriented electrical steel sheet, by directing the rolling direction perpendicular to the current direction of the cable, the magnetic field and the easy magnetization direction coincide with each other, and a more excellent magnetic shielding effect can be obtained.

  An example of the magnetic shield structure of the present invention is shown in FIG. These show configurations in a plane perpendicular to the current direction. In addition, the shield structures shown in Examples 1 to 3 are different from each other in the surrounding form of the three-phase cable by the soft magnetic material, but in any case, all the conditions defined in the present invention are satisfied.

The Example of this invention is shown.
Using three cables, a three-phase, 50 Hz, 400 A current was passed, and the periphery was surrounded by a magnetic steel sheet, and the magnetic shielding performance was measured. The U, V, and W cables each have a diameter of 30 mm, and were arranged in parallel at intervals of d = 100 mm. A non-oriented electrical steel sheet (JIS 50A470) having a thickness of 0.5 mm and a directional electrical steel sheet having a thickness of 0.35 mm (JIS 35G155) are used as the electrical steel sheet, and the cross section is rectangular (internal width a, height b) and length. Is a 3 m cylindrical, one-layer shield that surrounds the cable 360 °. The rolling direction of the steel sheet was perpendicular to the current direction. By changing the size of the shield cross section, the distance L from the center of the cable to the surface of the magnetic body was changed. Moreover, the center of the shield body cross section was set to the position of the central cable. The cross-sectional configuration of this test is shown in FIG. The magnetic field was measured at a position 2 m from the center on the X and Y lines in the figure. The shield coefficient S is obtained by using the magnetic field H0 before shielding and the magnetic field H1 after shielding S = H0 / H1
Defined by

  Table 1 shows the evaluation results. Moreover, about the relationship between the distance L and the shield coefficient S, the case where a non-oriented electrical steel plate is used is shown in FIG. 3, and the case where a directional electrical steel plate is used is shown in FIG. From Table 1, FIG. 2, and FIG. 4, it can be seen that if L is within the above-described range of the present invention, an excellent shielding effect can be obtained.

Using a non-oriented electrical steel sheet (JIS 50A470) having a thickness of 0.5 mm, a bowl-shaped shield body having a cross section as shown in FIG. 5 was produced. By changing the cross-sectional height C, the angle surrounding the cable was changed and the shielding performance was investigated. The U, V, and W cables each had a diameter of 30 mm, and were arranged in parallel at an interval of d = 100 mm, and a three-phase, 50 Hz, 400 A current was passed.
The results are shown in Table 2. It can be seen that the shielding effect is excellent when the angle surrounding the cable center is 180 ° or more.

It is a figure which shows the example of this invention, and has shown the structure in the surface perpendicular | vertical to an electric current direction. The structure of the cable and shield body of Example 1 of this invention is represented. The relationship between the distance L at the time of using the non-oriented electrical steel plate in Example 1 and the shield coefficient S is shown. The relationship between the distance L at the time of using the grain-oriented electrical steel plate in Example 1 and the shield coefficient S is shown. The structure of the cable and shield body of Example 2 of this invention is represented.

Claims (3)

Each phase cable has a structure that shields a magnetic field generated from an AC cable having a center-to-center distance d of 50 mm or more, and all the phase cables are in a plane perpendicular to the current direction, and the cables in the plane 180 ° or more around the center of gravity position is continuously surrounded by a soft magnetic material having a thickness of 0.2 mm or more, and the nearest distance L from the cable center to the magnetic material surface is
0.5 × d <L ≦ 4 × d
Magnetic shield structure for AC power cable, characterized in that The magnetic shield structure according to claim 1, wherein a non-oriented electrical steel sheet is used as the soft magnetic material. 2. The magnetic shield structure according to claim 1, wherein a grain-oriented electrical steel sheet is used as the soft magnetic material.

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