Impact of Thermal Aging on The Conductivity of Kraft Paper Impregnated with Mineral Oil and Synthetic Ester

Space charges dynamics in the insulation system inside HVDC converter transformers are affected by its aging condition. In this research, the influence of paper-aging on the electrical conductivity of the paper was investigated and correlated to the accumulation of space charges with aging. First, different Kraft paper samples were impregnated with mineral oil and synthetic ester liquid and then aged at 150°C for different durations. Then, the aged samples were de-oiled to remove the aged insulating liquids and then re-impregnated with fresh insulating liquids to investigate the impact of paper-aging. The degree of polymerization and the conductivity of paper samples were measured and correlated to the space charge accumulation in aged impregnated paper. It was found that there is strong correlation between the impact of aging on the conductivity and the space charge accumulation and thus, conductivity measurement is essential in investigating space charge dynamics.