Vacuum Studies of a Prototype Composite Coil Dewar for HTSC Transformers

Oak Ridge National Laboratory (ORNL) is collaborating with Waukesha Electric Systems (WES) to develop a high‐temperature superconducting (HTSC) utility power transformer with primary and secondary coils cooled by liquid nitrogen. Since the vacuum‐insulated cryogenic coil dewar surrounds the magnetic core limb and cannot form a shorted turn, non‐conductive materials are required. Two test vessels and a small prototype dewar have been fabricated by Scorpius Space Launch Company with epoxy/fiberglass composites, using their proprietary PRESSURMAXX vessel technology. The effects of pumping time, bakeout temperature, and cryogenic vessel temperature on vacuum outgassing rates have been investigated. Outgassing rates of the individual materials used in vessel construction have also been measured. The results will be scaled up to determine the required pumping capacity for a full‐size 25‐MVA commercial transformer dewar.