Power-frequency breakdown characteristics of cryogenic composite insulating materials for high-temperature superconducting cables

High-temperature superconducting (HTS) cables exhibit compelling advantages in large-capacity power transmission because of their high critical current density, low transport losses, and narrow corridor footprint. Operating in cryogenic environments, however, imposes stringent demands on the selection of primary dielectrics and on the compatibility among multiple insulating components. In this study, polypropylene-laminated paper (PPLP) and kraft paper were systematically combined with polyimide (PI), aramid paper (Nomex), and polytetrafluoroethylene (PTFE) to fabricate double- and triple-layer insulation specimens. Their AC breakdown characteristics were evaluated under controlled cryogenic conditions. Results demonstrate that PTFE–PPLP and PTFE–kraft paper composites exhibit superior dielectric performance, attributable to excellent interfacial compatibility and effective mitigation of electric-field enhancement, yielding markedly increased overall breakdown strength. These findings provide both theoretical insights and empirical benchmarks for optimizing HTS cable insulation design, offering critical engineering guidance for the practical deployment of HTS transmission systems.