The polarization information contained in the frequency-domain spectroscopy (FDS) could be utilised for characterising the ageing degree of transformer oil-immersed insulation. However, the polarization information will be obscured by its conductance effect, it is thus hard to analyse the polarization behaviour by using the traditional FDS technique. Given this issue, an alternative approach is proposed to analyse the polarization information using FDS decoupled by logarithmic-derivative spectroscopy (LDS). Findings proved that the polarization loss information generated by polarization behaviour can be separated from the total dispersion loss. The proposed feature parameters are later used to analyse the polarization loss information under ageing impact. Also, the polarization loss surface is constructed by using these feature parameters to characterise the ageing degrees of oil-immersed samples. Finally, the potential possibility of using the proposed method to analyse the polarization loss and ageing degree are respectively discussed.

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