Frequency domain spectroscopy analysis of superconducting cable during pre-cooling.

• During the 15-day pre-cooling of 1.2 km HTS cable, Frequency Domain Spectroscopy (FDS) test was carried out. • A special phenomenon is found in the tan δ -frequency curves at different temperatures. • The above phenomenon is explained according to the temperature shift theory of dielectric polarization. • An evaluation method of HTS cable pre-cooling process based on FDS is proposed. A 1.2 km, 35 kV superconducting cable project has been built in Shanghai, China. In order to monitor the internal condition of the cable in real time, during the 15-day pre-cooling process, the Frequency Domain Spectroscopy (FDS) test at 0.1 Hz–1000 Hz was carried out, and the dielectric loss angle tangent (tan δ)-frequency curves at different temperatures were obtained. A special phenomenon is found: in the early stage of pre-cooling, tanδ is the largest at the lowest frequency and decreases with the increase of frequency; in the late stage of pre-cooling, it becomes that tanδ is the largest at the highest frequency and decreases with the decrease of frequency. According to the Dielectric Polarization Theory in the field of dielectric physics, it is believed that the above phenomenon occurs because the tan δ -frequency curve shifts to the low frequency direction with the decrease of temperature. The evaluation method of the pre-cooling process based on FDS is proposed. In the early stage of cooling, it is suitable to use high frequency tan δ to monitor, and in the late cooling stage, low frequency tan δ is suitable. [ABSTRACT FROM AUTHOR]