Difference of dielectric properties between AC and DC XLPE cable used insulation materials

In order to understand the difference of dielectric properties and their mechanisms between AC and DC high voltage XLPE cables used insulation materials, Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), DC electrical conductivity and thermal stimulated currents were measured to compare the dielectric properties and analysis the contact between DC electrical conductivity and trap characteristics. The results showed that the DC cable used XLPE (DC-XLPE) had a relative high DC conductivity than that of the AC cable used XLPE (AC-XLPE) at the test temperatures and this was governed by the shallow trap densities. The activation energies of the conductivity were 0.89 eV and 1.09 eV for DC-XLPE and AC-XLPE respectively and this were same with the energy depths of the bulk deep traps. The results of the FTIR proved that the higher energy depth of the bulk deep traps of AC-XLPE was caused by its higher content of polar groups such as carbanyl groups. The crystallinities of the samples measured by XRD had no obvious difference and they were 49% and 50% for DC-XLPE and AC-XLPE respectively. As a result, it should be paid attention to reduce the content of polar groups in the design of HVDC used XLPE to get a better DC insulation properties.