Influence of absorbed moisture on the dielectric properties of epoxy resin impregnated paper composites

The absorbed moisture of epoxy resins (ER) and epoxy resin impregnated paper (ERIP) composites have a great influence on the insulation of ultra-high voltage direct current (UHVDC) bushings. The ER and ERIP samples were prepared according to a certain formulation. The moisture absorption behaviors of ER and ERIP in deionized water were determined by measuring periodically the change of weight. Through comparing the absorption behaviors, it is found that ERIP samples absorbed more moisture than ER samples, and the moisture diffusion in ERIP samples can´t be described by one dimensional Fickian or Langmuir-type model. The dielectric spectra of ER and ERIP samples were measured over the frequency range of 10^-2 to 10⁶ Hz at five different temperatures. As measurement temperatures were below the glass transition temperature of ER, the low frequency dispersion wasn´t observed in dielectric spectra of ER and ERIP samples within absorbed moisture in this work. The results show that the absorbed moisture has a much greater influence on dielectric properties of ERIP samples than ER samples. The absorbed moisture in ER samples only slightly increases imaginary parts of relative permittivity of ER samples and shift the spectra to right, while the absorbed moisture greatly increase both real and imaginary parts of relative permittivity of ERIP samples. The near-Debye relaxations occurred in middle frequency ranges in ERIP samples within absorbed moisture, which may be caused by Maxwell-Wagner interfacial polarization or dipole polarization of water molecular.