Dielectric Response of Cross-Linked Polyethylene (XLPE) Cable Insulation Material to Radiation and Thermal Aging

This work seeks dielectric indicators of the aged state of cross-linked polyethylene (XLPE) that has been simultaneously irradiated and thermally aged. The XLPE samples studied here were of the same material formulation as XLPE insulation found in RSCC I46-0021 instrumentation cable, formed into tapes with dimensions approximately 20 x 0.7 x 100 mm(3). Aging was conducted by suspending the samples from a rack within an oven and subsequently exposing oven and samples to gamma radiation. Samples were aged at 60, 90, and 115 degrees C, with gamma-radiation dose rates from 0 to 450 Gy/h with total received doses from 0 to 216 kGy. The dielectric spectra of these samples were measured over the frequency range 1 Hz to 1 MHz using custom electrodes and a Novocontrol dielectric spectrometer before dielectric breakdown strength was measured on the same samples. Dielectric loss tangent was observed to increase as a function of dose, dose rate and exposure time for XLPE samples exposed as described above. Dielectric loss tangent did not, however, show sensitivity to thermal aging for the range of temperatures studied. Dielectric breakdown strength initially increased as the material oxidized, then decreased for more extreme aging, due to chain scission. It is concluded that dielectric loss tangent is a promising indicator of XLPE aging due to gamma radiation exposure.