换流变压器是直流输电的关键设备，其安全稳定运行关系到整个直流输电系统的稳定。换流变压器阀侧绕组承受的电压较为复杂，包括直流、交流、交直流叠加等多种电压形式，以及极性反转电压等复杂运行工况。油中气体分析是电力变压器故障诊断的重要手段，而目前换流变压器的产气机理尚不明确，针对换流变压器故障诊断的油中气体分析依然沿用传统交流电力变压器采用的方法。本文在构建交直流复合电压系统的基础上，基于典型针板电极模型，对变压器油在交流、直流和交直流复合电压条件下的放电特性进行研究，并对不同电压类型下油中电弧放电产气规律进行了探索。搭建了两种交直流叠加电压发生器，并针对各自的特点进行仿真和实验，从交直流叠加电压范围、时间常数等角度对两者进行了比较。分别采用升压法击穿试验和恒压法耐压试验，比较交流、直流和交直流叠加电压作用下油隙的绝缘耐受特性。升压法中直流电压、交直流叠加电压条件下击穿电压比交流电压下的低得多，交直流叠加电压下击穿电压随纹波因数增大而增大。恒压法中试品在交流电压下并未击穿，而交直流叠加电压和直流电压作用下击穿次数较多。因此在直流电压和交直流叠加电压作用下，油隙的短时击穿性能和长时耐压性能均比交流电压弱得多。从气体的组分比例、产气总量两方面比较了三种电压类型的产气规律。不管在升压法还是恒压法试验条件下，交流电压、直流电压和交直流叠加电压其油中电弧放电产生气体组份比例一致。而绝对产气量上直流电压最高，交直流叠加电压其次，交流电压最少。结合特征气体法和改良三比值法，认为在交直流叠加电压和直流电压下，针对电弧放电故障诊断的改良三比值法判据仍然适用。但是，换流变压器的油中溶解气体含量的注意值和气体增长率注意值应有别于传统交流变压器。

HVDC converter transformer is very important to the stability of entire DC transmission system. The valve winding of converter transformer withstands much more complex voltages, including AC, DC, and combined AC-DC voltage. Dissolved Gas Analysis (DGA) is an important fault diagnosis method of power transformers. By now the gas generation mechanism of converter transformer is not so clear. Traditional DGA method of transformer is now used for converter transformers.In this paper, a combined AC-DC voltage system was built and several experiment was done based on the needle-plate electrode. Researches were carried out on arcing properties of transformer oil under AC, DC, and combined AC-DC voltage, while the gas generation property was also studied. Two kinds of combined AC-DC voltage system was designed and was compared by simulation and experiment in many ways.Withstand characteristics of transformer oil under AC, DC, and combined AC-DC voltage was studied by breakdown test under boost and constant pressure test. In the boost pressure test the breakdown voltage of DC and combined AC-DC voltage was much lower than AC voltage, and the breakdown voltage of combined AC-DC voltage is related to the ripple factor. In constant pressure test, no breakdown happened under AC voltage, while much more happened under DC and combined AC-DC voltage. So oil gap is much easier to breakdown under DC voltage and combined AC-DC voltage than AC voltage.Gas generation characteristics of these three types of voltage was also studied from two sides: ratio of every gas and total gas production. In boost and constant pressure test, the ratio of gas was the same under AC, DC and combined AC-DC voltage. However, the amount of total gas generated under DC voltage was the most, with AC voltage the least.The diagnosis criterion for arcing fault in three-ratio method is still workable under DC and combined AC-DC voltage. But the noticing value of dissolved gas in oil and its growing speed for converter transformers should be different from that of AC transformers.