KR20150085447A - Power cable and manufacturing method of conductor of power cable - Google Patents

Abstract

The present invention relates to a power cable and a method of making a conductor of the power cable. The power cable according to the present invention is a power cable comprising a flat conductor formed of a plurality of flat stranded stranded layers in which a flat stranded strand layer in which a center strand and a plurality of flat stranded strands are stranded in order on a central strand, an inner semi- conductor layer surrounding the outside of the flat- Wherein at least one of the rectangular parallelepiped strands constituting the rectangular conductor is formed by coating an insulating layer on the insulating layer, the insulating semiconducting layer surrounding the insulating semiconducting layer, and the outer semiconductive layer surrounding the insulating semiconducting layer, .

Description

Technical Field [0001] The present invention relates to a power cable and a method of manufacturing a conductor of a power cable,

The present invention relates to a method of manufacturing a conductor for a power cable and a power cable, and more particularly, to a method for manufacturing a conductor for a power cable and a power cable, And a method of manufacturing a conductor of a power cable.

The power cable has a tendency of increasing the diameter of a conductor provided inside the power cable so that a high voltage can be transmitted in response to an increase in power demand. In the past, in the case of a conductor in which a plurality of wires are connected together, a wire having a circular cross-sectional shape is mainly used as the conductor constituting the conductor. However, in the case of a conductor using such a circular wire, the spot rate is relatively low, and the area of the conductor for transmitting the actual power is lowered. In order to solve this problem, recently, a cable using a rectangular conductor having a square rectangular wire instead of a circular wire has been developed. The use of such a square wire allows the conductor to be increased to approximately 98% to 99% of the area, thereby increasing the area of the conductor that transmits power compared to the circular wire.

On the other hand, when AC (alternating current) is transmitted in the case of power transmission, a current flows along the surface of the conductor due to a skin effect, thereby limiting the transmission capacity. As the volume of the conductor increases, the power transmission amount does not increase and causes a loss. In order to reduce such AC loss, it has been attempted to use a so-called " stranded insulated conductor " in which insulated coated wires, which are round wires coated with an insulating film, are twisted (Korean Patent Laid-open Publication No. 10-2011-0024568). The use of such insulated coated wires reduces the AC resistance and reduces transmission losses, thereby increasing the transmission capacity relative to the same conductor cross-sectional area. As a method for coating an insulating film on such a circular wire, a method of coating a circular wire by passing it through an enamel solution is mainly used.

However, in the case of a quadrilateral rectangular parallelepipedal wire, when the insulating film is coated by passing the enamel solution, there is a problem that the angled corners are not coated properly. In the case where a part of the conductor wire is not coated, the insulating coating is not effective because it is electrically connected to the other wire through the uncoated portion. For this reason, a rectangular conductor made of an insulated coating wire is not currently used. Therefore, although the dot rate can be improved in the case of the rectangular conductor, in the case of AC transmission, the transmission capacity is limited by the surface effect.

Disclosure of Invention Technical Problem [8] In order to solve the above-described problems, the present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a power cable capable of improving the drop rate of a conductor of a power cable, And a manufacturing method thereof.

It is an object of the present invention to provide a semiconductor device comprising a flat conductor formed by a plurality of layers of flat stranded stranded layers each having a central strand and a plurality of flat stranded strands connected to each other on a central strand, An insulating layer surrounding the inner semiconductive layer, an outer semiconductive layer surrounding the insulating layer, And an outer sheath provided outside the outer semiconductive layer, wherein at least one of the square wire strands forming the flat conductor is coated with an insulating film.

Here, the flat rectangular wire coated with the insulating film is coated with an adhesive between the rectangular wire and the insulating film. The insulating film is coated by extrusion and may be made of nylon. On the other hand, the elementary wire coated with the insulating film and the elementary wire not coated with the insulating film can be twisted at a predetermined ratio.

In addition, any one elemental wire not coated with the insulating film is disposed so as not to be adjacent to another elementary wire not coated with the insulating film.

According to another aspect of the present invention, there is provided a method of manufacturing a conductor for a power cable, comprising the steps of: extruding a plurality of wires constituting the conductor into a rectangular shape; coating an insulating film on at least a part of the extruded wire; And a step of stranding the stranded wire coated with the insulating film and the stranded wire not coated with the insulating film.

The step of coating the insulating layer may include the steps of applying an adhesive to the surface of the elementary wire, heating the elementary wire to which the adhesive is applied to a predetermined temperature, passing the elementary wire coated with the adhesive through the insulating film extruder, And cooling the element wire to which the insulating film is adhered.

On the other hand, in the step of heating the wire coated with the adhesive, the wire is heated using a high frequency induction heater. The insulating layer may be formed of nylon.

Meanwhile, in the step of twisting the strands, the strands coated with the insulating layer and the strands not coated with the insulating layer may be stranded at a predetermined ratio. In this case, one of the elemental wires not coated with the insulating film is disposed so as not to be adjacent to another elementary wire not coated with the insulating film.

According to the present invention having the above-described configuration, it is possible to improve the drop rate of the power cable by producing a rectangular conductor by twisting a square wire element having a rectangular shape in comparison with a conventional circular wire element.

Further, in the case of fabricating the flat rectangular wire, the transmission loss due to the surface effect can be remarkably lowered when AC is transferred by coating at least a part of the rectangular wire with an insulating film. Particularly, a stranded wire coated with an insulating film and a stranded wire not coated with an insulating film are mixed and stranded at a predetermined ratio, thereby making it possible to prevent an increase in manufacturing cost and an increase in manufacturing time due to the coating cost of the stranded wire.

In addition, an insulating film composed of nylon is coated by extrusion after applying an adhesive to a square wire element, and the insulating film can be uniformly coated on the angled corners of the square wire.

1 is a perspective view illustrating an internal configuration of a power cable according to an embodiment,
Fig. 2 is a front view of Fig. 1,
3 is a front view showing a conductor according to another embodiment;
4 is a schematic view for explaining a surface effect,
5 is a flowchart illustrating a method of manufacturing a conductor of a power cable according to an embodiment of the present invention.
FIG. 6 is a schematic view schematically showing a process of extruding a rectangular wire,
7 is a cross-sectional view showing a state in which an adhesive is applied to the surface of a stranded wire,
8 is a schematic view schematically showing a step of adhering an insulating film to the surface of a wire wound with an adhesive,
9 is a front view showing a configuration of a conductor of a power cable according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described herein but may be embodied in other forms. Rather, the embodiments disclosed herein are provided so that the disclosure can be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals designate like elements throughout the specification.

FIG. 1 is a perspective view showing an internal configuration of a power cable 100, and FIG. 2 is a front view of FIG.

Referring to Figures 1 and 2, the power cable 100 has a conductor 10 along its center. The conductor 10 serves as a passage through which current flows, and is made of, for example, copper or aluminum. The conductor (10) is constituted by twining a plurality of element wires (11).

However, the surface of the conductor 10 is not smooth, so that the electric field may be uneven and corona discharge tends to occur partially. Further, when a gap is formed between the surface of the conductor 10 and the insulating layer 14 described later, the insulating performance is lowered. In order to solve such a problem, the outer surface of the conductor 10 is covered with a semiconductive material such as semiconductive carbon paper, and the layer formed by the semiconductive material is defined as the inner semiconductive layer 12.

The inner semiconductive layer 12 uniformizes the charge distribution on the conductive surface to make the electric field uniform, thereby improving the dielectric strength of the insulating layer 14 described later. Furthermore, the formation of a gap between the conductor 10 and the insulating layer 14 is prevented to prevent corona discharge and ionization. The inner semiconductive layer 12 also prevents penetration of the insulating layer 14 into the conductor 10 when the power cable 100 is manufactured.

An insulating layer 14 is provided on the outside of the inner semiconductive layer 12. The insulating layer 14 electrically insulates the conductor 10 from the outside. In general, the insulating layer 14 should have a high breakdown voltage, and the insulating performance must be stable for a long period of time. Furthermore, it should have low dielectric loss and resistance to heat such as heat resistance. Accordingly, the insulating layer 14 may be composed of, for example, XLPE (Cross-linked Polyethylene) or insulating paper impregnated with insulating oil or the like.

On the other hand, if not only the inside but also the outside of the insulating layer 14 is not shielded, a part of the electric field is absorbed by the insulating layer 14, but most of the electric field is discharged to the outside. In this case, if the electric field becomes larger than a predetermined value, the insulating layer 14 and the outer surface of the power cable 100 may be damaged by an electric field. Therefore, a semiconductive layer is provided on the outer side of the insulating layer 14 and is defined as an outer semiconductive layer 16 to distinguish it from the inner semiconductive layer 12 described above. As a result, the outer semiconductive layer 16 serves to improve the dielectric strength of the insulating layer 14 by making the distribution of the lines of electric force between the inner semiconductive layer 12 and the inner semiconductive layer 12 equal.

A shielding layer 18 made of a metal sheath or a neutral wire is provided outside the outer semiconductive layer 16 in accordance with the type of the cable. The shielding layer 18 is provided for electrical shielding and return of short-circuit current.

A jacket (20) is provided on the outer side of the power cable (100). The sheath 20 is provided on the outer side of the cable 100 to protect the internal structure of the cable 100. Therefore, the outer cover 20 is excellent in chemical resistance and mechanical strength to withstand chemicals such as weatherability, chemical substances, and the like that can withstand various environments such as light, weather, moisture, air and various climatic conditions. Generally, it is made of PVC (Polyvinyl Chloride) or PE (Polyethylene).

Meanwhile, the conductors of the power cable 100 according to FIGS. 1 and 2 are formed by forming the conductor 10 by forming the circular wires 11 in a stranded shape. Therefore, as shown in FIG. 2, even if twisting is performed by applying a predetermined pressure to the strands 11, a gap S is generated between neighboring strands 11 because of a sectional shape formed in a circular shape. The gap S acts as a factor for lowering the drop rate of the power cable 100. For example, in the case of the conductor 10 in which the circular strands 11 are stranded, the gap S is approximately 80% to 90% Lt; / RTI > When voids are generated between the strands 11 as described above, the proportion of the cross-sectional area of the strand 11 becomes smaller than the entire volume of the conductor 10, . This is a major factor in generating losses in power transmission even when the volume of the conductor is large. The configuration of the conductor for overcoming this problem is shown in Fig.

FIG. 3 shows a configuration of a square conductor 200 according to an embodiment of the present invention.

3, the rectangular conductor 200 according to the present embodiment includes a plurality of rectangular parallelepiped strands in which a central strand 210A and a plurality of parallel stranded strands 210B are stranded, Of the square wire strand. Here, the center strand 210A has a circular shape, and the rectangular parallelepiped 210B has a rectangular cross-section. That is, by using the square wire element 210B having a rectangular cross section in place of the conventional round wire element, voids between neighboring wires can be prevented and the drop rate can be improved.

For example, as shown in Fig. 3, a circular central core wire 210A is disposed at the center of the square conductor 200, and a plurality of square wires 210B, A plurality of layers can be formed. In the case of forming a plurality of flat stranded stranded layers, a plurality of flat stranded strands are stranded on the central strand to form a flat stranded stranded layer of the first layer, and a plurality of flat stranded strands are stranded on the flat stranded strand layer of the first layer Two rectangular parallelepiped strands are formed, and a plurality of rectangular parallelepiped strands are stranded on the square strand strand of the (n-1) th stratum in this manner to form a square stranded strand of the n-th layer. In this way, a square stranded wire layer can be formed in a desired number of layers (n layers). At this time, the twisted direction of the square wire element is selected from the S or Z direction, and the twisted directions of the respective layers may be different from each other.

In the drawing, three rectangular parallelepiped stranded strands each formed by connecting a rectangular square strand 210B along the periphery of the center strand 210A of the central portion are shown as being formed, but the number is not limited to this and the numbers of the strands, The height and the width of the wire 210B can be appropriately modified. When comparing the rectangular conductor 200 of Fig. 3 with the conductor 10 of Fig. 2 described above, the conductor 10 of Fig. 2 generates a gap S between neighboring wires due to the circular wire, The rectangular conductor 200 according to the present embodiment can arrange the element wires in close contact with each other while preventing the occurrence of voids in arranging the rectangular element wire 210B, and thus the dot rate can be remarkably improved. As shown in FIG. 3, when the small wire is formed into a square shape, its drop rate reaches approximately 98% to 99%. Here, rectangles should not be interpreted strictly in a strict sense. That is, the square used in the present specification means four corners but all sides connecting the corners should be interpreted as not being straight lines, and some surfaces may include curved shapes rather than straight lines.

On the other hand, in the case of transmitting power through an electric power cable, an AC (alternating current) transmission will reduce the area where electric power is actually transferred due to a skin effect. 4 is a schematic view for explaining the surface effect.

Referring to FIG. 4, when a current flows along the conductor 250, the current does not flow through the entire cross-sectional area of the conductor but flows along some surface in the cross-sectional area and defines it as a so-called 'skin effect' do. The surface effect refers to a phenomenon in which current flows only near the surface of a conductor when a high frequency current is applied to a conductor such as a metal. The reason why the surface effect occurs is that the direction of the current flowing along the conductor rapidly changes, such as AC, so that an induced electromotive force is generated inside the conductor and the current does not flow to the center of the conductor

For example, when a current is passed through one conductor 250 as shown in FIG. 4, a current flows along a region A where a current of a conductor having a predetermined thickness delta flows due to the surface effect , And a region (B) in which current does not flow is generated inside the conductor. Here, the thickness of the skin, through which the current penetrates into the conductor and the current can flow, is expressed by the following equation (1).

In Equation (1), δ is the thickness of the skin through which a current can flow, f is frequency, μ is permeability in vacuum, and ρ is the resistivity of the conductor. This surface effect limits the amount of current flowing along the conductor, which significantly reduces its efficiency when transmitting and receiving power.

For example, when a conductor is constituted by a plurality of square-shaped element wires 210B having a rectangular shape as shown in Fig. 3, since the elementary wires are arranged close to each other, the conductor can be assumed to be a conductor through which a current flows as a whole. However, when AC is transmitted as described above, a current does not flow through the entire cross-sectional area of the square conductor 200 due to the surface effect, but a part of the thickness along the surface, for example, And the current does not flow in the inner layer. This causes a decrease in transmission efficiency even when the diameter of the conductor is increased. Hereinafter, a power cable and a method of manufacturing the same will be described.

5 is a flowchart illustrating a method of manufacturing a conductor of a power cable according to an embodiment of the present invention.

Referring to FIG. 5, a method of manufacturing a conductor of a power cable includes the steps of (S100) extruding a plurality of wires constituting the conductor into a rectangular shape, a step (S130) of coating an insulating film on at least a part of the extruded wire, And a step (S150) of connecting the elementary wire coated with the insulating film and the elementary wire not coated with the insulating film.

That is, in this embodiment, when a rectangular wire is stranded to form a conductor, at least a part of the wire is coated with an insulating film. As described above, when a part of the strand is coated with an insulating film, the strand coated with the insulating layer may be electrically insulated from other strands to form a passage through which a current flows. Therefore, even if the surface effect is applied, the sectional area of the entire conductor is electrically divided by the elementary wire coated with the insulating film, and the surface effect is applied to each of the divided areas. The area where the current flows is relatively wider. Hereinafter, each step will be described in detail.

6 is a schematic view schematically showing a step of extruding a plurality of elemental wires into a rectangular shape.

Referring to FIG. 6, a circular strand 310 is extruded through a plurality of extrusion rollers 320 and 340 into a square wire having a rectangular shape.

For example, in the 'C ₁ ' region before passing through the plurality of extrusion rollers 320 and 340, the strand 310 has a circular cross-sectional area as shown in the figure. Subsequently, when a pair of first extruding rollers 320A and 320B arranged to form a predetermined slope on both sides of the strand 310 is passed, both sides of the strand 310 are squeezed to have a roughly rectangular shape. That is, in the 'C ₂ ' region passing through the first extrusion rollers 320A and 320B, the strands 310 are in a compressed state on both sides, but the top and bottom surfaces are still arc-shaped. Subsequently, when the pair of second extrusion rollers 340A and 340B vertically disposed on the upper and lower portions of the wire 310 are passed, the upper and lower surfaces of the wire 310 are pressed and flattened in the 'C ₃ ' region. As a result, when the first extrusion rollers 320A and 320B and the second extrusion rollers 340A and 340B are passed, the circular strand is converted into a rectangular strand. On the other hand, the above-described arrangement of the extrusion rollers is merely described as an example, and the arrangement and number of the extrusion rollers can be suitably modified.

After the rectangular wire 310 is extruded as described above, the insulating film is coated on the surface of the wire 310 (S130).

As shown in FIG. 5, the step of coating the insulating layer may include a step S132 of applying an adhesive to the surface of the strand 310, a step S134 of heating the strand coated with the adhesive to a predetermined temperature, The step (S136) of bonding the insulating film by passing the elementary wire coated with the adhesive through the insulating film extruder, and the step (S138) of cooling the elementary wire to which the insulating film is adhered. Hereinafter, each step will be described in detail.

The rectangular wire 310 is extruded and then the adhesive 400 is coated on the surface of the wire 310 as shown in FIG.

Referring to FIG. 7, since the insulating film is bonded to the surface of the elementary wire 310, the insulating film is easily adhered to the surface of the elementary wire 310, and further, The adhesive 400 is applied. In this case, the adhesive 400 may be determined depending on the kind of the insulating film coated on the surface of the strand. For example, the insulating layer may be composed of synthetic fibers such as nylon, etc. In this case, the adhesive may be determined depending on the kind of the synthetic fibers.

It is preferable to apply the adhesive 400 using a separate device capable of applying the adhesive rather than manually working the adhesive 400 along the surface of the wire 310. When the adhesive is applied by the application device, the time and cost required for application can be significantly reduced.

Subsequently, the wire to which the adhesive is applied is heated to a predetermined temperature (S134). This is to enable the insulating film to be more easily adhered in the step of activating the adhesive applied on the surface of the wire element 310 and bonding the subsequent insulating film. The method for heating the stranded wire can be variously implemented. For example, the stranded wire can be heated using a high frequency induction heater. The high-frequency induction heating is well known in the art to which the present invention pertains, so a detailed description thereof will be omitted.

After heating the element wire to a predetermined temperature, the elementary wire 310 coated with the adhesive is passed through the insulation film extruder 500 to bond the insulation film 510 as shown in FIG. 8 (S136).

8, the insulating film extruder 500 includes a frame 505 through which an insulating film 510 is drawn, and the strand 310 is moved along the inside of the frame 505. As shown in FIG. The insulating layer 510 is prepared to be drawn out to the frame 505 and is disposed to cover all the inner spaces of the frame 505. 8, the insulating layer 510 is drawn along the moving direction of the strand 310, and the strand 310 is pulled along the moving direction of the strand 310 as shown in FIG. 8, (Not shown). In this case, a plurality of adhesive rollers 520 may be provided to more firmly adhere the insulating layer 510. The adhesive roller 520 applies a predetermined pressure to the insulating layer 510 on the surface of the stranded wire so that the insulating layer 510 can more firmly adhere to the adhesive on the surface of the stranded wire 310.

After the insulating film 510 is adhered to the surface of the elementary wire 310 as described above, the elementary wire 310 to which the insulating film 510 is adhered is cooled (S138). During the heating step (S134) and / or the insulating film adhering step (S136), the stranded wire (310) is maintained in a relatively heated state, so that the stranded wire is subjected to a cooling step before stranding.

As described above, since the insulating film is formed by extrusion after applying the adhesive to the stranded wire, there is an effect that the insulating film is well formed at the corner of the stranded wire.

After cooling the strand 310, the strand is twisted (S150). In the case of constructing a conductor of a power cable by twisting the strands together, the strands coated with the insulating film 510 and strands not coated with an insulating film are stranded at a predetermined ratio.

For example, in order to reduce the loss due to the surface effect, all the wires constituting the conductor can be coated with an insulating film. However, if all the wires are coated with an insulating film, the coating cost and the coating time are increased, which increases the unit cost of the power cable. Therefore, in the present embodiment, not only all of the elemental wires are coated with the insulating film but also at least a part of the elemental wires constituting the conductor is coated with the insulating film.

However, when a part of the element wire is coated with an insulating film as described above, it is preferable to reduce the coating cost and the coating time, and to reduce the loss due to the surface effect. To this end, in this embodiment, as shown in FIG. 9, in the step of stranding the stranded wire, any one stranded wire not coated with the insulating film is disposed so as not to be adjacent to another stranded wire not coated with the insulated film.

Referring to FIG. 9, a wire 310A 'not coated with an insulating film in the conductor 500 is surrounded by a wire 310B coated with an insulating film, and a wire 310A' And is not adjacent to the other elemental wire 310A '. If the strand 310A having no insulating film is adjacent to the other strand 310A 'which is not coated with the insulating film, electric power is supplied only through a part of the surface of the two strands due to the surface effect. Therefore, in order to solve such a problem, in this embodiment, the elementary strand 310A 'not coated with an insulating film is arranged so as not to be adjacent to another elementary strand 310A' which is not coated with an insulating film. As a result, the elementary wire 310A, which is not coated with the insulating film, is enclosed by the elementary wire 310B coated with the insulating film, so that it has an insulating effect along the periphery of the elementary wire. Therefore, power loss due to the surface effect can be prevented.

FIG. 9 shows a conductor in which a wire is stranded according to the above description. The conductor can of course be applied to the power cable according to Fig. That is, the power cable 100 includes a conductor 500 in which a plurality of wire strands are stranded, an inner semiconductive layer 12 surrounding the outer surface of the conductor, an insulating layer 14 surrounding the inner semiconductive layer, The semiconductor device according to claim 1, wherein the conductive layer (500) comprises a plurality of rectangular parallelepiped strands connected in series, at least a part of the strand is an insulating layer Can be coated. Here, the conductor 500 has been described above with reference to FIGS. 3 to 8, and therefore, a repetitive description thereof will be omitted. Furthermore, the inner semiconductive layer 12, the insulating layer 14, the outer semiconductive layer 16, and the sheath 20 have been described in detail with reference to FIGS. 1 and 2, and will not be described here.

While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims. . It is therefore to be understood that the modified embodiments are included in the technical scope of the present invention if they basically include elements of the claims of the present invention.

10, 600 .. Conductor
11, 210,
12. Internal semiconducting layer
14. Insulation layer
16 .. outer semiconductive layer
18. Neutral line
20. The sheath
400 .. Adhesive
500 .. Insulating Film Extruder

Claims (12)

A square conductor formed of a plurality of layers of flat stranded wire strands each of which has a central strand and a plurality of flat stranded strands to which stranded stranded strands are successively formed on a central strand;
An inner semiconductive layer surrounding the outside of the rectangular conductor;
An insulating layer surrounding the inner semiconductive layer;
An outer semiconductive layer surrounding the insulating layer; And an outer sheath provided outside the outer semiconductive layer,
Wherein at least one of the square wire strands forming the flat conductor is coated with an insulating film. The method according to claim 1,
Wherein the flat rectangular wire coated with the insulating film has an adhesive applied between the rectangular wire and the insulating film. The method according to claim 1,
Wherein the insulating film is coated by extrusion. The method according to claim 1,
Wherein the insulating film is made of nylon. The method according to claim 1,
And a stranded wire coated with the insulating film and a stranded wire not coated with an insulating film are stranded in a predetermined ratio. The method according to claim 1,
Wherein one of the elemental wires not coated with the insulating film is disposed so as not to be adjacent to another elemental wire not coated with the insulating film. A method of manufacturing a conductor of a power cable,
Extruding a plurality of wires constituting the conductor into a rectangular shape;
Coating an insulating film on at least a part of the extruded wire; And
And stranding the stranded wire coated with the insulating film and the stranded wire not coated with the insulating film. 8. The method of claim 7,
The step of coating the insulating film
Applying an adhesive to the surface of the stranded wire;
Heating the wire coated with the adhesive to a predetermined temperature;
Bonding the insulating film by passing the elementary wire coated with the adhesive through an insulating film extruder; And
And cooling the stranded wire to which the insulating film is adhered. 9. The method of claim 8,
And heating the stranded wire using the high frequency induction heater in the step of heating the stranded wire to which the adhesive is applied. 8. The method of claim 7,
Wherein the insulating film is made of nylon. 8. The method of claim 7,
In the step of twisting the strands
And the stranded wire coated with the insulating film and the stranded wire not coated with the insulating film are stranded in a predetermined ratio. 8. The method of claim 7,
Wherein one of the elemental wires not coated with the insulating film is disposed so as not to be adjacent to another elemental wire not coated with the insulating film.

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