Discharge characteristics of internal gas-gap defect in oil-paper insulation under different proportion of AC and DC combined voltages

Studying on the partial discharge of oil-paper insulation under different AC and DC combined proportions is important for the condition monitoring and fault diagnosis of converter transformer. Internal gas-gap defect was taken as the study object. Based on the condition of converter transformer, a set of platform for oil-paper insulation partial discharge measurement under AC and DC combined voltage was built. The characteristics of conventional pulse current signal of internal gas-gap defect partial discharge of the initial stage, close to breakdown stage and the entire development stages under different AC-DC combined voltages were compared. The results show that, the characteristics of internal gas-gap defect under each proportional AC and DC voltage are unique. With the decrease in the proportion of AC and DC combined voltage, the total initial voltage Ui and the total breakdown voltage Ut increase. However, due to the DC composition increased, makes the discharge pulse repetition rate and discharge capacity of the initial stage, the breakdown stage and the entire development stages have decreased to some extent. Meanwhile, with a higher percentage of DC, the discharge repetition rate of negative half cycle becomes gradually higher than the positive half cycle, and the discharge pulses concentrate in the negative half cycle of concentration. ©2015 Chin. Soc. for Elec. Eng.