Dissolved gases generated of partial discharges and electrical breakdown in oil-paper insulation under AC-DC combined voltages

One of the major insulations of high voltage direct current (HVDC) converter transformers is oil-paper insulation. Valve-side windings withstand complex stresses combined with AC, DC, and pulse voltages. Partial discharge in oil-paper insulation is a major incipient fault of an HVDC converter transformer inside insulation. There are few studies on the development characteristics of partial discharge in oil-paper insulation and dissolved gases in oils of an HVDC converter transformer. Based on artificial insulation defect models of oil-paper insulation, the present study investigates the variation characteristics of dissolved gases in the discharge development process. Experiment and analysis results show that the characteristics of dissolved gases in oils under AC-DC combined voltages are appreciably different with those under an AC voltage. These results provide theoretical support for further research on partial discharge and on dissolved gases of other internal insulating defects and the fault diagnosis of HVDC converter transformers.