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 tanδ-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.