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The invention offers a superconducting cable easy to form a twisted structure even when a plurality of cable cores are used and a DC transmission system incorporating the superconducting cable. A superconducting cable 1 has a structure formed by twisting together two types of cable cores (two first

The invention offers a superconducting cable easy to form a twisted structure even when a plurality of cable cores are used and a DC transmission system incorporating the superconducting cable. A superconducting cable 1 has a structure formed by twisting together two types of cable cores (two first cores 2 and one second core 3) having different structures and then housing them in a heat-insulated pipe 7. The first cores 2 each have a first superconducting layer 2a, to be used either as an outward line or for the transmission for a pole in DC transmission, and have no superconducting layer other than the first superconducting layer 2a. The second core 3 has a second superconducting layer 3a, to be used as a return line or neutral line in DC transmission, and has no superconducting layer other than the second superconducting layer 3a. The second superconducting layer 3a has an inner diameter larger than the outer diameter of the first superconducting layer 2a.

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The invention claimed is: 1. A superconducting cable including a plurality of cable cores that are twisted together, each cable core having a superconducting layer and an insulating layer; the superconducting cable comprising: (a) a first core having a first superconducting layer; and (b) a second

The invention claimed is: 1. A superconducting cable including a plurality of cable cores that are twisted together, each cable core having a superconducting layer and an insulating layer; the superconducting cable comprising: (a) a first core having a first superconducting layer; and (b) a second core having a second superconducting layer that has an inner diameter larger than the outer diameter of the first superconducting layer, wherein the first and second superconducting layers each include helically wound tape-shaped wires having a structure in which a plurality of filaments made of a superconducting material are configured in a matrix such as a silver sheath, wherein the first core includes no additional superconducting layer made of a superconducting material other than the first superconducting layer, and wherein the second core includes no additional superconducting layer made of a superconducting material other than the second superconducting layer. 2. A superconducting cable as defined by claim 1, wherein the second core has a coolant-circulating pipe at the inside of the second superconducting layer. 3. A superconducting cable as defined by claim 2, wherein the coolant-circulating pipe is any one of a metallic pipe, a spiral steel tape, and a corrugated metallic pipe. 4. A superconducting cable as defined by claim 1, the superconducting cable being formed by twisting two first cores and one second core together. 5. A superconducting cable as defined by claim 1, wherein the first core has the same diameter as that of the second core. 6. A superconducting cable as defined by claim 1, the superconducting cable having a core-twisted structure that has an allowance for contraction at the time the cable is cooled. 7. A superconducting cable as defined by claim 1, wherein in order to flatten the radial distribution of a DC electric field in the insulating layer, the insulating layer is structured by using a ρ grading such that as the radial position moves toward the innermost portion of the insulating layer, the resistivity decreases, and as the radial position moves toward the outermost portion, the resistivity increases. 8. A superconducting cable as defined by claim 7, wherein the insulating layer has, in the vicinity of the superconducting layer, a high ∈ layer that has a dielectric constant higher than that of the other portion. 9. A superconducting cable as defined by claim 7, wherein the insulating layer is structured such that as its radial position moves toward the innermost portion, the dielectric constant, ∈, increases, and as the radial position moves toward the outermost portion, the dielectric constant, ∈, decreases. 10. A DC transmission system incorporating a superconducting cable as defined by claim 1, the DC transmission system performing a unipolar transmission by using: (a) the first superconducting layer provided in the first core as an outward line; and (b) the second superconducting layer provided in the second core as a return line. 11. A DC transmission system incorporating a superconducting cable as defined by claim 1, the DC transmission system performing a bipolar transmission by the specification of: (a) providing the superconducting cable with a plurality of first cores; (b) using the first superconducting layer provided in at least one first core to perform the transmission for one pole selected from the group consisting of the positive pole and the negative pole; (c) using the first superconducting layer provided in the remaining at least one first core to perform the transmission for the other pole; and (d) using the second superconducting layer provided in the second core as the neutral line. 12. A superconducting cable including a plurality of cable cores that are twisted together including two cable cores and one coolant-circulating pipe twisted together; each of the two cable cores comprising: (a) a superconducting conductor layer; (b) an insulating layer provided on the outer circumference of the superconducting conductor layer; and (c) an outer superconducting layer provided on the outer circumference of the insulating layer; the coolant-circulating pipe having the same diameter as that of the cable cores, wherein each superconducting conductor layer includes helically wound tape-shaped wires having a structure in which a plurality of filaments made of a superconducting material are configured in a matrix such as a silver sheath, and wherein one of the two cable cores or the one coolant-circulating pipe is configured so as to include no superconducting material, thereby facilitating the twisting operation of the overall superconducting cable arrangement. 13. A superconducting cable as defined by claim 12, the superconducting cable having a two-core-twisted structure that has an allowance for contraction when the cable is cooled. 14. A superconducting cable as defined by claim 12, wherein the coolant-circulating pipe: (a) has an expanding-and-contracting property that allows the coolant-circulating pipe to contract at the time the cable is cooled; and (b) is twisted together with the two cores without having a slack for contracting at the time the cable is cooled. 15. A superconducting cable as defined by claim 14, wherein the coolant-circulating pipe is a corrugated metallic pipe. 16. A superconducting cable as defined by claim 12, wherein the coolant-circulating pipe is provided with a protecting layer on its outer circumference. 17. A superconducting cable as defined by claim 12, wherein in order to flatten the radial distribution of a DC electric field in the insulating layer, the insulating layer is structured by using a ρ grading such that as the radial position moves toward the innermost portion of the insulating layer, the resistivity decreases, and as the radial position moves toward the outermost portion, the resistivity increases. 18. A superconducting cable as defined by claim 17, wherein the insulating layer has, in the vicinity of the superconducting conductor layer, a high ∈ layer that has a dielectric constant higher than that of the other portion. 19. A superconducting cable as defined by claim 17, wherein the insulating layer is structured such that as its radial position moves toward the innermost portion, the dielectric constant, ∈, increases, and as the radial position moves toward the outermost portion, the dielectric constant, ∈, decreases. 20. A DC transmission system incorporating a superconducting cable as defined by claim 12, the DC transmission system performing a unipolar transmission by using: (a) the superconducting conductor layers provided in both cores as an outward line; and (b) the outer superconducting layers provided in both cores as a return line. 21. A DC transmission system incorporating a superconducting cable as defined by claim 12, the DC transmission system performing a bipolar transmission by using: (a) the superconducting conductor layer provided in one of the two cores to perform the transmission for one pole selected from the group consisting of the positive pole and the negative pole; (b) the superconducting conductor layer provided in the other core to perform the transmission for the other pole; and (c) the outer superconducting layer provided in each of the two cores as the neutral line.