Solid Nitrogen Cryogenic Thermal Storage in the Terminations for Resilient Helium Gas Cooled HTS Cables Utilized in DC Power Distribution Network

The low heat capacity of helium gas limits overall cooling capacity of helium gas cooled high temperature superconducting power cable and makes them susceptible to large temperature rises during unexpected events such as electrical faults or cryogenic system failures. Solid nitrogen has been explored as an additional cooling agent in the HTS cable termination. Additional cryogenic thermal storage in the form of solid nitrogen in the superconducting cable terminations can mitigate the risk by keeping the system in superconducting state until the appropriate contingency is activated. An external tank filled with activated carbon is used as an adsorption buffer tank for gaseous nitrogen to minimize the operational pressure. The use of activated carbon minimizes the volume of the buffer tank and prevents pressure rise due to the phase change from solid to gas. In addition, when the HTS cable system is back to normal operational condition, the activated carbon can provide the system back with the required nitrogen to establish cryogenic thermal storage in the form of solid nitrogen as a reversible and self-contained system. Comprehensive experiments were performed to explore the feasibility of using the activated carbon and its performance. It was found that the proposed system performs as a cryogenic thermal buffer to enable the superconducting cable system to tolerate unexpected heat surge and recover in an autonomous fashion while keeping the pressure below the desired value. In addition to integrate the concept into real applications, the mass of solid nitrogen has been quantified based on the required protection time and possible emergency heat load.