Effect of Insulating Oil Type on Charge Transportation at Oil-Paper Interface

Oil-paper insulation is the most frequently used insulation in transformers, including HVDC converter transformers. Mineral-insulating oil has traditionally been the dominant choice, but the environmentally friendly nature of vegetable-insulating oil positions it as a viable alternative. In converter transformers, a typical configuration involves multiple layers of insulating paper and insulating oil, resulting in the presence of oil-paper interfaces. Charges tend to accumulate at the oil-paper interface under applied dc voltages. The type of insulating oil has a significant influence on the charge accumulation at the oil-paper interface, and this distinction comes from the discrepancy in the migration capabilities of charges from different sources in insulating oil and oil-immersed paper. The initial equilibrium value of the interface charges is determined by the ionization charges in the insulating oil and the oil-immersed paper, and the final equilibrium value of the interface charges includes the contribution of the injected charges and the ionization charges. In the oil-paper sample composed of mineral-insulating oil, the initial equilibrium value of the interface charges is lower than the final equilibrium value, so charges gradually accumulate at the oil-paper interface under applied dc voltages and stabilize after reaching the maximum value. In the oil-paper sample composed of vegetable-insulating oil, since the initial equilibrium value of the interface charges is higher than the final equilibrium value, charges accumulate rapidly at the oil-paper interface, reaching the highest value within 2 min and then gradually decreasing. The stability value of interface charges in the oil-paper sample composed of vegetable oil under applied dc voltage is relatively low, which is beneficial to the operation stability of converter transformers. However, in the initial stage of voltage application, the interface charges can reach a high initial equilibrium value in a short time, which may further cause partial discharges.