Development of a HTS Magnet System for Long-Run Operation Test in Sub-Cooled Nitrogen

This paper deals with the development of a 6.6 kV/200A HTS magnet for inductive superconducting fault current limiter (SFCL) that uses a sub-cooled nitrogen cooling system which is expected to be very useful to the applied HTS machines. The several characteristics of the HTS magnet such as critical current, electrical insulation and heat transfer can be enhanced in the sub-cooled nitrogen cooling system rather than in saturated liquid nitrogen
$( LN_2)$
cooling system. To obtain sub-cooled nitrogen,
$ LN_2$
was cooled down to 64 K with GM-cryocooler and its pressure was kept at 101 kPa with gaseous helium. In this investigation, the HTS magnet for a 6.6 kV/200 A SFCL was developed and its long-run operation characteristics were measured and analyzed. The characteristics of the sub-cooled nitrogen system like temperature, level of
$ LN_2$
and pressure of cryogenic system were not degraded in spite of the several over current tests, electrical insulation tests, and even long-run operation tests with rated transport current.