JP4668003B2 - Coaxial cable for bipolar DC power transmission - Google Patents

Description

  The present invention relates to a single coaxial cable for direct current transmission in which a positive direct current power transmission path, a negative direct current power transmission path, and a neutral wire are coaxially integrated in a bipolar power transmission system.

  There are unipolar and bipolar transmission systems for direct current transmission. In addition to the advantage that the transmission capacity can be increased compared to the unipolar transmission system, the latter bipolar transmission system transmits 50% power using the transmission line of the other polarity when a failure occurs in the transmission line of one polarity. Therefore, there is an advantage that the power supply reliability is high.

  When power is transmitted through the other power transmission line at the time of one-line failure in the bipolar power transmission line, there are a method of flowing a return current through the ground or seawater and a method of flowing through a conductor of a neutral cable. In the case of the former earth return or seawater return, there are concerns about fluctuations in earth potential due to direct current, electrical corrosion of surrounding structures, ship compass errors, and adverse effects on ecosystems such as fish. For this reason, the neutral cable of the latter is often used in recent years. FIG. 3 shows a conventional bipolar power transmission system using neutral wires.

FIG. 3A shows a bipolar transmission system using three cables: a positive DC cable 11, a negative DC cable 13, and a neutral wire 12. In the figure, 14 is a forward converter and 15 is an inverse converter (hereinafter the same).
In FIG. 3B, for example, a DC coaxial cable 16 as shown in FIG. 1 of Patent Document 1 is used, and the main conductor 17 of the DC coaxial cable 16 is used for positive DC power transmission. The neutral wire conductor 18 provided on the outer periphery of the wire is used as a neutral wire, and another direct current cable 19 (or a direct current coaxial cable) may be used for negative direct current power transmission.
FIG. 3C shows a system using a DC coaxial cable for bipolar power transmission (see Patent Document 2), in which a positive direct current is connected to the central conductor 21 of the DC coaxial cable 20 for bipolar power transmission. A negative direct current is connected to the conductor 22, the lead sheath 23 is connected to the ground, and bipolar power transmission is performed, and the neutral wire circuit uses another neutral wire cable 24.

Japanese Patent Application Laid-Open No. 11-120837 JP 2001-189113 A

  The conventional bipolar power transmission system using neutral wires (Fig. 3A, Fig. 3B, Fig. 3C) requires two or three cables, which complicates cable installation and increases the cost of cable installation. It is a problem.

  Another problem is that the magnetic field leaking from the DC power transmission circuit has an impact on the environment, causing compass errors to ships, etc., adverse effects on ecosystems that act by sensing geomagnetism, and harmful electromagnetic waves. There are concerns about the occurrence of Therefore, in the method of FIG. 3A, it is necessary to lay the cables fairly close to each other in order to reduce the leakage magnetic field, but there is a problem that the cable laying becomes difficult. In the method shown in FIG. 3B using a DC coaxial cable, almost no current flows through the neutral wire (outer conductor) during normal operation (unbalanced current flows between the two electrodes, but less than a few percent of the rated current). The original effect of reducing the leakage magnetic field (the effect of canceling and reducing the leakage magnetic field by the current of the same current value in the reverse direction flowing in the main conductor and the return conductor) is not exhibited, and the DC coaxial cable and the other DC cable are also close to each other There are restrictions that must be laid.

  Further, in the method of FIG. 3C, the use of a bipolar coaxial cable for power transmission causes the leakage current to be reduced because currents having the same current value in opposite directions flow in the positive polarity and the negative polarity during normal operation. However, in the case of a unipolar failure, a DC transmission circuit is formed by the conductor of the DC coaxial cable for bipolar transmission of the other polarity and the conductor of the neutral cable, and in order to reduce the leakage magnetic field at this time, bipolar There is a restriction that a DC coaxial cable for power transmission and a neutral cable must be installed close to each other.

  Therefore, the object of the present invention is to reduce the cable laying cost and to suppress the generation of an external leakage magnetic field both in the case of bipolar DC power transmission (during normal operation) and unipolar DC power transmission (during unipolar failure). An object of the present invention is to provide a coaxial cable for bipolar DC power transmission.

The coaxial cable for bipolar DC power transmission according to the present invention is provided with a main conductor serving as a DC power transmission path having one of positive and negative polarities at the center, and a main insulator and a polarity opposite to that of the main conductor outside the main conductor. A return conductor, a return insulator, a neutral wire outer conductor, a neutral wire insulator, and a lead sheath , which serve as a direct current power transmission path, are sequentially provided coaxially.

  In the bipolar DC power transmission coaxial cable according to the present invention, the main insulator and the return insulator are preferably made of a polymer insulating material for DC resistance.

  Further, in the coaxial cable for bipolar DC power transmission according to the present invention, it is more preferable that the main insulator is made of DC-resistant cross-linked polyethylene and the return insulator is made of DC-resistant non-cross-linked polyethylene. The cable of the present invention may be an OF cable (Oil-filled Cable) or an oil-immersed paper solid cable (Mass Impregnated or Non Draining Cable) in addition to the above-described polymer material extruded insulated cable.

  According to the present invention, bipolar DC power transmission including a neutral wire can be realized with a single coaxial cable, compared to the conventional necessity of laying a plurality of (2 to 3) DC cables. Since the cable laying cost can be greatly reduced, the economic effect is great.

  In addition, by using the DC coaxial cable of the present invention, it is possible to suppress the generation of an external leakage magnetic field in both cases of bipolar DC power transmission (during normal operation) and unipolar DC power transmission (during unipolar failure). Therefore, it is possible to prevent compass errors during voyage, effects on ecosystems that act by sensing geomagnetism, and generation of harmful electromagnetic waves.

  When the DC coaxial cable according to the present invention is used in a bipolar DC transmission system, if the DC voltage of bipolar transmission is ± V0, a voltage of 2 × V0 is applied between the main insulators, and V0 is applied between the return insulators. Voltage is applied. Therefore, the main insulator and the return insulator are made of a polymer insulation material for direct current resistance (60 to 500 kV) (for example, a crosslinked polyethylene for direct current or a crosslinked polyethylene with a carbon filler) in order to withstand the above voltage. , Modified polyolefin, etc.).

  The DC coaxial cable according to the present invention has a difference in the maximum allowable temperature between the main conductor and the return conductor due to the structure of the cable. For example, the maximum allowable temperatures of the main conductor and the return conductor are about 90 ° C. and 75 ° C., respectively. It is. That is, the return insulator is not used at a higher temperature than the main insulator and is at most about 75 ° C. Therefore, it is preferable that the return insulator be formed of a polymer insulating material having a heat resistance lower than that of the main insulator, for example, non-crosslinked polyethylene. When non-crosslinked polyethylene is used for the return insulator, there is no need for crosslinking, so there is an advantage that the resin extrusion equipment for the return insulator can be simplified, and cable manufacturability can be improved.

  FIG. 1 shows an embodiment of a coaxial cable for bipolar DC power transmission according to the present invention. The coaxial cable 1 for bipolar DC power transmission has a main conductor 2 at the center and a main insulator 3 on the outer periphery thereof. The main insulator 3 is composed of an inner semiconductive layer, a crosslinked polyethylene insulator layer, and an outer semiconductive layer. Usually, the inner semiconductive layer, the cross-linked polyethylene insulator layer and the outer semiconductive layer are formed by a coextrusion method, and these three layers are integrated. As the insulating material of the main insulator 3, a polymer insulating material for DC resistance (60 to 500 kV) can be used. The main insulator 3 can be formed, for example, by extruding DC cross-linked polyethylene, cross-linked polyethylene with carbon filler, modified polyolefin, and the like.

  On the outside of the main insulator 3, a return conductor 4 in which a large number of copper wires are spirally wound is provided. Outside the return conductor 4, a return insulator 5 comprising an internal semiconductive layer, a polyethylene insulator layer, and an external semiconductive layer is provided. The return insulator 5 is formed by extruding a polymer insulating material for DC resistance (60 to 500 kV). As the insulating material of the return insulator 5, for example, a cross-linked polyethylene for direct current, a cross-linked polyethylene with a carbon filler, a non-cross-linked polyethylene, a modified polyolefin, or the like can be used.

  A neutral wire outer conductor 6 is provided outside the return insulator 5. The neutral wire outer conductor 6 can be formed by a method of spirally winding a large number of copper wires, a method of winding a metal tape, or a method of combining them.

  A neutral wire insulator 7 necessary for neutral wire insulation is provided outside the neutral wire outer conductor 6. The neutral wire insulator 7 includes, for example, an internal semiconductive layer, a polyethylene insulating layer, and an external semiconductive layer.

  A lead coat 8 is provided outside the neutral wire insulator 7, and an anticorrosion layer 9 is provided outside the lead coat. The lead coating 8 and the anticorrosion layer 9 are cable protection layers. In addition, when using this cable 1 as a submarine cable, a floor, an iron wire armor, and a serving layer are further provided on the outer periphery of the anticorrosion layer 9.

  FIG. 2 is a schematic diagram when the DC coaxial cable 1 having the cross section of FIG. 1 is used in a bipolar DC power transmission system. A positive direct current is connected to the main conductor 2, a negative direct current is connected to the return conductor 4, and a neutral point of the forward converter 14 and the reverse converter 15 is connected to the neutral conductor 6. Has been. It goes without saying that a negative direct current can be connected to the main conductor 2 and a positive direct current can be connected to the return conductor 4.

  In the bipolar DC power transmission system of FIG. 2, the reverse current flows in the same current value in the main conductor 2 and the return conductor 4 coaxially, so that the leakage magnetic field to the outside can theoretically be zero. During normal operation, an unbalanced current (several percent or less of the rated current) between the two poles flows through the outer conductor 6 for neutral wire, and a slight external leakage magnetic field is generated due to the current. For example, FIG. It is clear that the external leakage magnetic field can be reduced as compared with the case where a total of three cables, ie, a positive DC cable, a negative DC cable, and a neutral cable, are laid close to each other.

  In addition, in the bipolar DC power transmission system of FIG. 2, when a failure occurs in one polarity, 50% of normal operation is performed using the other polarity conductor (main conductor or return conductor) and the neutral conductor outer conductor. Electric power can be transmitted in a single pole direct current, and the reliability of power supply can be improved. In this case, the neutral wire outer conductor 6 is used as a ground-side return current circuit. However, since the opposite current flows coaxially in the other polarity conductor and the neutral wire outer conductor, The leakage magnetic field to the outside can theoretically be zero. At this time, since a DC voltage corresponding to the voltage drop of the DC current is generated in the neutral wire outer conductor 6, the neutral wire is interposed between the neutral wire outer conductor 6 and the outer lead sheath 8. Insulator 7 is provided. The DC voltage induced in the neutral wire outer conductor 6 depends on the current flowing through the neutral wire outer conductor, the conductor resistance (determined by the cross-sectional area of the neutral wire outer conductor and the power transmission distance), and the power transmission distance. Usually, it is about 10 kV at most. Therefore, the neutral wire insulator 7 can be formed, for example, by extruding non-crosslinked polyethylene.

  In addition, when it is assumed that unipolar DC power transmission is performed by using the neutral conductor 7 as a return current circuit on the ground side when a unipolar failure occurs, either during normal operation or during unipolar operation due to a unipolar failure In contrast, each conductor cross-sectional area may be selected so as not to exceed the maximum allowable temperature of the main conductor, the return conductor, and the neutral wire outer conductor.

Sectional drawing which shows one Embodiment of the coaxial cable for bipolar DC power transmission which concerns on this invention. The schematic block diagram at the time of using the cable of FIG. 1 for a bipolar DC power transmission system. (A)-(C) is a schematic block diagram of the conventional bipolar DC power transmission system, respectively.

Explanation of symbols

1: Coaxial cable for bipolar DC power transmission 2: Main conductor 3: Main insulator 4: Return conductor 5: Return insulator 6: Neutral wire outer conductor 7: Neutral wire insulator 8: Lead coating 9: Anticorrosion layer

Claims (3)

A main conductor serving as a positive or negative polarity DC power transmission path is provided at the center of the cable, and a main insulator, a return conductor serving as a DC power transmission path having a polarity opposite to that of the main conductor, and return insulation are provided outside the main conductor. A coaxial cable for bipolar DC power transmission, characterized in that a body, a neutral wire outer conductor, a neutral wire insulator, and a lead sheath are sequentially provided coaxially.   The coaxial cable for bipolar DC power transmission according to claim 1, wherein the main insulator and the return insulator are made of a polymer insulating material for DC resistance.   3. The coaxial cable for bipolar DC power transmission according to claim 2, wherein the main insulator is made of a cross-linked polyethylene for DC resistance and the return insulator is made of non-cross-linked polyethylene for DC resistance.

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