Abstract: Ultra-high voltage and flexible DC transmission are effective measures for large-scale integration of renewable energy. The converter transformer is a key equipment in ultra-high voltage and flexible DC transmission lines. Under the complex DC and AC electric fields, the transformer oil may be broken down, leading to short-circuit faults and affecting the safe and reliable operation of the power transmission system. KVG10 and TGB20 transformer oils are taken as research objects, and the conductivity and breakdown characteristics of transformer oils are studied by experiments and simulations. The experimental results show that the conductivity at different temperatures and electric fields obeys the Poole-Frenkel conductivity model, and the scale effect of the breakdown strength with the length of the electrode gap obeys the inverse power function relationship. It is found that the resistivity of transformer oil and its activation energy are positively correlated with the breakdown strength. Then, considering the interaction between the charge migration and the molecular motion in the transformer oil, a cavity breakdown model based on the charge transport and molecular chain displacement is established. The simulation results show that the breakdown strength varies inversely with the electrode gap length, with the simulation results matching the experiments. The simulations found that under the DC voltage, the heterogeneous space charges accumulate in the transformer oil, leading to field concentration near the electrodes. The molecular chains follow charge migration to form cavities, and when the cavity length reaches a certain threshold, the transformer oil breaks down. The results illustrate the influence of the difficulty of charge and molecular chain migration on the scale effect of breakdown strength of transformer oils. It provides experimental data and simulation model support for optimizing the insulation coordination of converter transformers.