Dissolved gases characteristics induced by PD in oil-paper insulation under composite AC-DC voltage

Converter transformer used in HVDC system is subjected to composite AC-DC voltage. The DGA standard is formed in AC transformer, the question is whether it is suitable in converter transformer. In the paper, the ball-plate and column-plate discharge model are adopted to study the application of DGA under AC-DC voltage. During the discharge development process, the gas content and PD pulse are recorded under AC and AC-DC. At last, the cause of failure is diagnosed with a variety of DGA diagnostic methods. The test results show that the dissolved gases characteristics under composite AC-DC are different from AC voltage. Under AC-DC, gas contents are so small value that they cannot be diagnosed in a long period until pre-breakdown stage. Under AC, gas contents increase to a certain value and increase with time during the whole process. However, in the end of failure development process, the diagnoses based on DGA are the same under two kinds of voltage.