The polymerization degree of insulating paper is a key parameter characterizing its aging level. Traditional viscosity-average polymerization degree measurements require sample destruction

and complex operational procedures lead to high result variability. Previous studies have preliminarily demonstrated the potential of applying terahertz time-domain spectroscopy (Terahertz-TDS) to non-destructive detection of insulating paper aging. However

at poor signal-to-noise ratios

existing methods present significant challenges and lack universal applicability. To address this issue

this study employs THz-TMSPECT instruments with different hardware configurations to investigate the absorption spectra of insulating paper at various aging stages using both transmission and reflection testing methods. Results indicate that during the late stage of aging

significant shifts in characteristic peaks and changes in peak intensity and steepness occur compared with the initial stage. This is attributed to the degradation-induced breakage of long cellulose molecular chains

leading to the cracking of polymers into smaller molecules. Therefore

this paper proposes the “self-similarity” parameter for terahertz absorption spectra. This parameter calculates the similarity between absorption spectra at different aging states and the baseline spectrum representing the initial state. Validation results from different laboratories and field samples demonstrate that this parameter is simple to operate

exhibits low result dispersion

and demonstrates broad universality. By establishing self-similarity calibration curves for various types of insulating paper

non-destructive detection of the aging state of insulating paper can be achieved. This paper provides a reference for rapid

non-destructive field testing of the polymerization degree of insulating paper.