Second Cooling Test of 1000-m Superconducting DC Cable System in Ishikari

A 1000-m-long superconducting dc power transmission system was built in Ishikari City, Hokkaido, Japan, as a national project (“Ishikari project”). The objectives of the project are to design and verify the validity of a system for a practical use, and also to obtain the properties required to get a future prospect for longer length cable system, such as over 10 km. Until the end of 2016, the cooling test was carried out two times. Important issues for the cooling test are: release of thermal contraction/expansion force below the rapture strength of the cable and the reliability and life time of the system after several cooling/warming procedures, as well as long-time operation. For the former issue, two techniques (a movable terminal cryostat and the helical core deformation) were adopted and these techniques were found to release the axial tension of the cable core effectively. For the latter issue, several types of current loading test, such as Ic measurements, continuous rated current test (2.5 kA-3 hrs), load cycling test (1 kA-8 hrs + 0 A-16 hrs for 20 days), etc., were performed. All the tests showed no deterioration of the electrical properties of the cable core, after cable laying and thermal cycling process, and also the stability of the system.