中文摘要：

换流变压器绝缘状态与交直流复合比例具有密切关系，研究放电特征与交直流复合比例之间的关系对换流变压器的故障诊断具有实际的工程意义。以内部气隙缺陷为研究对象，根据换流变压器内部的实际运行环境，搭建了一套交直流复合电压作用下油纸绝缘局部放电测量平台，采用常规脉冲电流法着重对比了不同交直流复合比例下油纸绝缘内部气隙起始阶段、临近击穿阶段及整个发展过程的局部放电特征。试验结果表明：每个比例交直流复合电压作用下绝缘内部气隙缺陷放电具有其特有的特征；交直流复合电压作用下油纸绝缘内部气隙放电所产生的空间电荷在电场作用下所形成的反向电场会使总起始电压Ui和总击穿电压Ut随着交直流复合电压中直流成分的增加而逐渐增大；起始阶段、击穿阶段及整个发展过程的放电脉冲重复率及放电量降低，而且随着直流所占比例的增加，负半周期放电重复率逐渐高于正半周期，放电相位逐渐向负半周期集中。该结论为实现换流变压器故障诊断及状态监测提供了一定的依据。

英文摘要：

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.