Extraction and analysis on oil immersed paper’s obscured low frequency polarization based on Frequency Domain Spectrum

Frequency domain spectrum is regarded as a reflection of insulation material's dielectric process such as conductance and polarization which provides non-destructive insulation diagnosis on power equipment nowadays. Theories and explanations on this method are controversial with plenty of arguments, which limits its further application. Traditionally, low frequency range of oil-impregnated paper's curve was treated as a conductance-dominant band where polarization contributes little to the dielectric loss. However, based on the motion characteristics and classic models of charges' conductance and polarization process, this paper proposes for the first time that a polarization process with rather long relaxation time, which differs from the low frequency dispersion phenomenon, is supposed to exist in oil impregnated paper's broadband dielectric response but is mainly obscured by large conductance loss. Meanwhile, the dielectric losses caused by this type of polarization under series of temperatures are quantified, which indicates that the polarization loss is comparable with conductance loss under some certain frequencies. The temperature characteristic and its explanation theory for this extracted polarization are further presented, by which the polarization's barrier height, one of the polarization's intrinsic parameters, is able to be obtained.