Influences of AC and DC voltage component on typical insulation model breakdown characteristics in transformer oil

In the HVDC transmission system, the failure of converter transformer is the key problem for the electrical power system safety. Oil-paper insulation system in converter transformer bears AC and DC voltage in their lifetime. Since having no way to simulate its actual working condition, usually, AC and DC test voltage are separately imposed on the transformer during the factory test. Different influences of AC and DC voltage on the insulation characteristics are unknown. This paper designs a needle-plate model, and imposes different AC and DC voltage values to the oil surrounded needle-plate model to study the differences of breakdown characteristics. The test reveales the different effects of AC and DC voltages to the insulation model breakdown characteristics. The results indicate that under the combined AC and DC voltage, the AC breakdown voltage decline linearly as the pre-DC-voltage value increase and vice versa. The result also suggests the pre-stressed voltage type affects the test result obviously and for the oil surrounded needle-plate model, the breakdown voltage value of pre-stressed AC-voltage is higher than the condition of pre-stressed DC-voltage.