This project plans to deeply study the breakdown mechanism of oil-paper insulation under complex electric field, propose the calculation method of the electrical field distribution in oil-paper insulation stucture of HVDC equapment, and thus provide a powerful support for the domestic manufacturing of EHVDC converter transformers. In the early stage of research, following achivements have been obtained. (1)The space charge distribution and the distortion of electric field in the single and double layers of oil-impregnated papers were studied. Moreover, a numerical simulation model was built to describe the change injection, migration and accumulation, and the distributions of charge and electric field in oil-impregnated papers were sucessfully simulated. (2)The electric field distribution in the bushing-end structure in converter transformers was analized. The effects of barrier stucture (e.g. the depth, position, oil gap, paper thickness and layer numbers etc.) on field distribution under AC/DC combined voltage and polarity reversal were investigated. A 400 kV bushing-end structure was preliminarily optimized and designed. (3)The breakdown properties of oil-paper stucture under AC/DC combined voltage and the effect of pre-stressing on breakdown voltage were studied. The results showed that the breakdwon strength was greatly influenced by the space charge behavior. Moreover, the partial discharge characteristics under AC/DC combined voltage was studied.%该研究针对油纸绝缘结构在复合电场作用下的击穿机理进行深入的研究，目标是提出特高压油纸绝缘结构复合电场的计算方法，对实现我国特高压换流变压器的国产化提供有力的支撑。在前期工作中，取得了一下阶段性成果：(1)研究了单层及双层油纸介质在常温及温度梯度下的空间电荷分布以及电场畸变特性。同时，建立了空间电荷注入、迁移和聚集的数值仿真模型，可以成功地模拟油纸结构中的电荷和电场分布特性。(2)针对换流变出线结构的电场分布进行了计算分析。研究了交直流及极性反转下的电场分布特性，屏障结构（包括屏障的罩入深度、位置、油隙间距、油浸纸筒厚度及层数等）对电场分布的影响。对出线结构进行了优化，完成了400 kV出线结构的初步设计。(3)测量了油纸结构在交、直流复合电压以及预加电压作用下的击穿特性，结果表明空间电荷特性可以对击穿强度产生显著的影响。同时，掌握了交直流复合电压下油纸绝缘结构的局部放电特性。
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