Dielectric Spectroscopy of Very Low Loss Model Power Cables

This research study focuses on the dielectric response of XLPE model power cables that have combinations of homo- and co-polymer insulation with furnace and acetylene carbon black semicon shields. Three dielectric spectroscopy techniques, which are frequency response analyzer and transformer ratio bridge in both frequency domain, and charging/discharging current system in time domain, were jointly used to measure the low loss XLPE cables in the frequency range from 10-4Hz to 104Hz at temperatures from 20°C to 80°C. Degassing effects and thermal ageing effects have also been studied with the spectroscopy techniques. Thermal-electric behaviour and maximum voltages for thermal breakdown have been theoretically simulated for the model cables. Three loss origins of the XLPE cables have been found with different loss mechanisms. Conduction loss due to thermally activated electron/hole hopping dominates the lower frequency range from 10-4Hz to 1Hz; Semicon loss due to its in series resistance with the insulation layer in cable equivalent circuit dominates the higher frequency range from 102Hz to 104Hz; intrinsic polarization loss of the XLPE insulation has dominant flat loss spectra in the mid-frequency range from 1Hz to 102Hz. Degassing was found to decrease the conductivity of the model cables, while thermal ageing greatly increased the conductivity. Thermal-electric simulation results with FEMLAB have shown that the position of maximum field changes from inner to outer insulation boundary under higher applied voltages. A loss mechanism model with mathematical expression for dielectric loss spectrum calculation is finally proposed to explain the total dielectric loss of polymer power cables.