The use of gaseous helium (GHe) as cryogenic coolant in high-temperature superconducting (HTS) transmission cables has proven to be advantageous to traditional coolants such as liquid nitrogen (LN2). Basic driving forces behind high-temperature superconducting cable design include&nbsp;maximizing the current carrying capacity of conductors, while limiting the resulting weight and footprint of cable systems. Since the critical current of a superconductor is temperature dependent, using gaseous helium to cool superconductors to lower temperatures than the 62 K limit of liquid nitrogen systems can yield significantly greater power densities in the cable format. At a fixed current carrying requirement, this directly results in increases in efficiency and a reduction of cable system footprint. In addition to the limitations associated with minimum cooling temperature, the hazards related to using liquid nitrogen in confined spaces make it unfavorable for naval and aviation applications, where a leak of lower density gaseous helium coolant would be significantly safer to personnel. However, realizing the benefits that result from implementing gaseous helium coolant&nbsp;presents challenges in locating effective dielectric insulation materials and systems for the sub-62&nbsp; K cryogenic environment. Traditionally, LN2-cooled systems utilized lapped polymer tape dielectric layers of high-performance materials such as Kapton polyimide. The lapped tape construction effectively reduced failure of brittle polymers at cryogenic temperatures by increasing their degrees of freedom in the dielectric jacket. These systems permitted the permeation of LN2 into the dielectric jacket around superconductors without concern, because the relatively high dielectric strength of the fluid obstructed shorts through the cryostat, even at high voltages. However, when gaseous helium is introduced as coolant, its significantly lower dielectric strength does not allow permeation into the dielectric layers without risking shorts through the fluid from superconductors at high voltages. Thus, the need exists for a suitable dielectric in the cryogenic operating environment that envelops conductive layers, while sealing against gaseous helium permeation.The capacity for nanocomposite fillers to make considerable improvements in the physical properties of host polymers, in this case, the coefficient of thermal expansion (CTE) of the polymer, makes them exceptional candidates for impregnation in cryogenic dielectric layers. Here, a polyamide (PA) host polymer has been selected that demonstrates comparable DC dielectric breakdown strength to polyimide, with lower temperature processing that does not exceed delamination temperatures of HTS windings in cables. Thus, the PA/SiO2&nbsp;nanocomposites presented are a viable option for a dipped-coated thermoset polymer insulation which meets the requirements for improved mechanical properties in the cryogenic environment and for effective sealing against permeation of gaseous helium in HTS cable systems. In addition to improvement of mechanical properties, dielectric breakdown testing has shown that nanocomposite samples have increased dielectric strength at cryogenic temperatures, when compared to room temperature tests. Results of the dielectric and thermomechanical analyses&nbsp;conducted here demonstrate a promising solution to providing effective dielectric materials for GHe-cooled HTS cables with enhanced mechanical properties suited to their challenging operating environment.

Addressing Cryogenic Dielectric Material Challenges in High-Temperature Superconducting Cables With Polyamide/silica Nanocomposite Thin Films

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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