Dielectric response and electrical conductivity of mass impregnated HVDC cable insulation
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- Institutt for elkraftteknikk 
The main purpose of this thesis has been to gather data which facilitates design improvementsof oil-impregnated paper insulation used in HVDC cables. Conductivities and losstangents were gathered for this purpose. Several minor studies for improving the methodologywas conducted throughout the thesis. Samples of oil-impregnated paper and modified samples with oil pockets (called bulk oilsamples) and butt gaps were constructed and subjected to several voltages and temperaturesin order to obtain polarization and depolarization currents. These currents were thenutilized to calculate the conductivity of the various samples. Conductivities of paper wereobtained from literature while conductivities of oil were derived from measurements.The method Polarization Depolarization Current (PDC) was used to obtain the currentsby use of a laboratory setup. The loss tangent was obtained from the Hamon approximationwhere the goal was to verify the applicability in paper, bulk oil and butt gap samples.Results from the main study showed that the conductivity dependency of paper on electricfield strength and temperature was in accordance with the literature. Voltage across the oilin bulk oil samples differed significantly from zero, as opposed to previous assumptions ofzero voltage across the bulk oil. Steady state current in bulk oil samples were exponentialwith respect to voltage but turned linear with increasing oil volume. Conductivity of oiland paper were in the same range, as opposed to previous assumptions of a higher conductivityfor oil. Conductivity of oil versus electric field strength in oil showed an exponentialdependency. Calculated DC currents of a bulk oil sample compared to measured values showed equalmagnitudes at low voltages but different exponential rate of rise. A sensitivity analysisshowed that conductivity of paper dominates the calculated DC current. The Hamon approximation was applicable for paper samples but not for bulk oil and butt gap samples. Results from the minor studies showed that PDC is viable for monitoring the impregnationprocess. It showed that most of the absorption of oil into paper occurs during the first 12hours. Currents in dried paper were 10 times higher than impregnated paper. Insufficientpolarization durations significantly affects the DC currents at low voltage. The samplesrequired at least one day to reach thermal equilibrium after a temperature drop. Sampleswith at least 5 sheets of paper are applicable for PDC measurements.