Dielectric spectra of epoxy resin impregnated crepe paper composites in different humidity

Abstract

Epoxy resin impregnated crepe paper (ERICP) composites is the main insulating material of the ultra-high voltage direct current (UHVDC) dry bushing. The moisture in the environment may intrude into the ERICP composites in the manufacturing process of the bushing, which would affect the quality of the bushing. In order to investigate the diffusion process of moisture in the ERICP composites during the manufacturing process, the ERICP samples were casted under a vacuum according to a certain formula. Five different artificial humidity environments were produced in sealed containers by different saturated salt solutions. The weights of ERICP samples under different humidity were periodically measured so as to determine the moisture absorption behaviors of the samples. The moisture diffusion behaviors in the samples showed that the one dimensional Fickian law can't exactly describe the moisture diffusions in ERICP samples at long time, especially under high relative humidity. The dielectric spectra of ERICP samples within saturated absorbed moisture were then measured at room temperature, which frequency changed from 10−3 Hz to 106 Hz. The low frequency dispersion (LFD) process and the mid-frequency dispersion process appeared in dielectric spectra of ERICP within moisture absorption. The absorbed moisture shifted the spectra to the up and right, which moved to higher frequencies. The LFD means that some clusters were formed in ERICP samples after absorbing moisture. The moisture absorbed by the epoxy resins of ERICP may exist only in ‘mobile’ phase, while the moisture in the crepe paper impregnated with epoxy resins may exist in both ‘mobile’ phase and ‘bound’ phase.