In the past, using a superconducting cable as an electric power cable for an electric power supply line has been studied. In a typical structure of a superconducting cable, a cable core having a superconductor is housed in a thermal insulation pipe and the superconductor is maintained in a superconducting state by cooling with a coolant filled in the thermal insulation pipe. In recent years, a multicore cable having a plurality of cores housed in a thermal insulation pipe, as well as a single-core cable having one cable core housed in a thermal insulation pipe, has been developed.
The above-mentioned superconducting cable is limited in terms of the cable length, depending on manufacturing, transportation, installation, and other conditions. Therefore, in order to construct an electric power supply line over a long distance using superconducting cables, it is necessary to provide intermediate connections for connecting the cables along the line. For example, a normal joint structure for a three-core superconducting cable is disclosed in Patent document 1. The disclosed normal joint structure is structured such that the three cores pulled out from the end of one of two superconducting cables to be connected are joined with the corresponding three cores pulled out from the end of the other superconducting cable, respectively, and the ends of the cores and the coupled parts of these cores are housed altogether in the same joint box.
After the construction of the line, a coolant is introduced into the thermal insulation pipes of the superconducting cables and the above-mentioned joint boxes. If rapid cooling is performed in the initial stages of the cooling, cable components might be damaged by the steep temperature variation. Therefore, generally the cooling is performed gradually, while observing the temperature over the full length of the cable including the above-mentioned coupled parts of the cable cores, so as to cause the temperature to change in a degree that may not have an influence on the performance of the cable components. Then, the operation of the line is commenced after it has been confirmed that the cable cores, the coupled parts of the cable cores, etc. are sufficiently cooled. It is considered to arrange temperature sensors using an optical fiber along the cable cores in order to observe temperatures along the full length of the superconducting cable (See Non-patent document 1).
When a normal joint structure is formed in a superconducting cable in which a temperature sensor using an optical fiber is arranged, it is necessary to splice optical fibers in addition to splicing cable cores. A fusion splicer is generally used for splicing optical fibers. Therefore, in order to connect optical fibers in the case of forming a normal joint structure, the splicing work is performed, pulling out an optical fiber from the end of the respective superconducting cable so that the end of each optical fiber may be arranged in the space where it is possible to place a fusion splicer. The excess length of the optical fibers thus pulled out and the spliced parts of the optical fibers as well as the coupled parts of the cable cores are housed in a joint box.
[Patent document 1] Japanese Patent Application Publication No. 2000-340274 (FIG. 1)
[Non-patent document 1] SUPERCONDUCTIVITY COMMUNICATIONS, Vol. 11, No. 1, February 2002, Title: Test results of a high temperature superconducting cable are partly published. Kishio Laboratory, Department of Superconductivity, School of Engineering, The University of Tokyo.
[Searched on Jun. 22, 2005] The Internet URL <http://www.chem.t.u-tokyo.ac.jp/appchem/labs/kitazawa/SUPERCOM>