Time–Frequency-Based Insulation Diagnostic Technique of High-Temperature Superconducting Cable Systems

For the electrical insulation of a high-temperature superconducting (HTS) cable, wrapped polypropylene laminated paper (PPLP) tape is typically used. Unfortunately, it is possible that unexpected faults at insulation layers will be present in the cables as a result of either a problematic manufacturing process or an incomplete installation procedure. In order to protect against operational failures of grid-connected HTS cable systems, this paper proposes a nondestructive diagnostic technique, i.e., time-frequency domain reflectometry (TFDR), and focuses on the characteristic of HTS cable that caused the local insulation defects . To verify the performance of the proposed method, detection and localization of local insulation failure via TFDR are compared with traditional time-domain reflectometry. The experiments are conducted at room temperature and under liquid nitrogen in order to check the efficacy of the proposed method in varieties of HTS cable's conditions. In addition, to improve the accuracy of detection and localization, a methodology to analyze incident signals, which are composed of upchirp and downchirp signals, is presented.