High voltage direct current (HVDC) power transmission lines in Japan consist of Hokkaido-Honshu DC link and Kii Channel DC link that connects Shikoku with Honshu. As for HVDC power transmission in foreign countries, main applications have been for long-distance power transmission such as intercontinental links. However, in recent years, there has been a growing trend toward its application to offshore wind power generation, which is being actively introduced in Europe as a renewable natural energy source. As its introduction has progressed, the locations of the wind power generation facilities have been shifted from coastal areas to offshore areas due to space constraints. As the power transmission distance has increased, HVDC power transmission technology has drawn more attention. Previously oil-impregnated insulation cables, such as mass impregnated (MI) cable and oil-filled (OF) cable, have been applied to DC power transmission. In recent years, however, because of the increasing awareness for environmental protection, extruded insulation cables have come to be desired as they have no fear of oil leakage. On the other hand, cross-linked polyethylene insulation cable, which is currently widely applied to AC power transmission, is known to have a number of problems in insulation when used for DC usage, i.e., prominent accumulation of space charge in cross-linked polyethylene (XLPE) insulation material. Therefore, we have developed a DCXLPE insulation material that has excellent DC characteristics. We have also developed a DC-XLPE cable using the above mentioned material as insulator. This paper describes the excellent DC characteristics of the DC-XLPE insulation material developed for DC applications, and reports on the implementation status of type tests and pre-qualification (PQ) tests in accordance with the International Council on Large Electric Systems (CIGRE) Technical Brochure on actual cables and accessories. 2. Changes in DC Cable Technology and History of DC-XLPE Cable Development
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