Abstract
Several high temperature superconducting (HTS) cable demonstration projects have begun operation on the electric grid in the last few years with the liquid nitrogen-cooled cable contained in one or more vacuum-insulated, flexible cryostats with lengths up to 600 meters. These grid demonstration projects are prototypes of the anticipated commercial market which will require superconducting cable lengths in the multiple kilometer range with the vacuum-jacketed cryostats in underground ducts providing acceptable thermal insulation for decades. The current state-of-the art for flexible cryostats (installation constraints, heat loads with a good and degraded vacuum, impact of cable bends, getter lifetime and reliability) is discussed. Further development needed to meet the challenging commercial HTS cable application is outlined.