Influences of Different Ratios of AC-DC Combined Voltage on Internal Gas Cavity Discharge in Oil-Pressboard Insulation

The degradation of insulation would be accelerated by the partial discharge (PD) incurred by an internal air-gap defect in oil-pressboard insulation. At different bridge rectifier locations in the converter transformer, the oil-pressboard insulation typically endures various ratios of combined ac and positive dc voltages. Therefore, the dielectric status of the converter transformer is closely correlated to the ratios of ac to dc voltages. It is practically significant to conduct the study of the correlation between discharge characteristics and ac-dc ratios. At the same time, it enhances the diagnosis of faults in converter transformers. According to the actual internal operation conditions of converter transformers, the study established a test platform. Furthermore, the conventional pulse current method was adopted to compare the discharge characteristics at the initiation and close-to-breakdown stages. Under these ratios, test results display distinctive PD dynamics. The PD of the air gap generates surface charges. These charges, in turn, establish an inverse electric field. As a result, the proportion of dc in the ac-dc combined voltage increases, and total initial voltage and total breakdown voltage increase continuously, but the recurrence rate of discharge pulse and discharge magnitude decrease at the initiation and close-to-breakdown stages. As the positive dc proportion increases, the discharge recurrence rate in the negative half cycle gradually exceeds that in the positive one. These research findings provide useful criteria for the fault diagnosis of converter transformers.