Dielectric properties of multi-layer epoxy resinimpregnated crepe paper composites

Epoxy resin-impregnated crepe paper (RIP) composites have been used as the inner insulation of dry condenser bushing in the ultra-high voltage direct current (UHVDC) power transmission system. In order to study the dielectric properties of pure epoxy resin and RIP composites, four kinds of samples (pure epoxy resin, RIP composites with 1, 2, 3 layers of crepe paper respectively) were prepared via the vacuum casting process. The Fourier transform infrared spectroscopy (FTIR) as well as three typical dielectric properties, i.e. complex permittivity, dc conductivity, thermally stimulated depolarization current (TSDC) have been investigated. It shows that the addition of crepe paper nearly brings about larger values of relative permittivity as well as dielectric loss factor than the pure epoxy resin. Samples containing crepe paper also have larger dc conductivities and TSDC currents. These differences are related to the abundant hydroxyl groups in the cellulose paper. The hydroxyl groups are polar groups and easy to absorb water or other impurities which resulting in larger permittivity and dc conductivity. Moreover, the crosslinking degree of the epoxy resin matrix decreases with the introduction of crepe paper. Then, the polarized dipole moments are easier to relax, which is responsible for the decrease of the trap depth corresponding to the process of glass transition.