The design and manufacturing of UHVDC equipment is still in the low stage in our country. It is significant to improve the manufacture level of UHVDC converter transformer which is the core component of UHVDC systems. According to the fundamental problem in the design process of ±1 100 kV converter transformer

this paper analyzesthe insulation properties of oilbasing on the aging experimental results. Three kinds of oilareselected and then the thermal aging experiment is conducted under 130 degree centigrade. According to the experiment results such as the degree of polymerization（DP）

acid

moisture content

dielectric constant

dielectric loss

resistivity

viscosity and the AC breakdown voltage

this paper finally gets the optimum kind of oil. Moreover

taking the complicated operating condition of UHVDC converter transformer and the oil requirement for converter transformer into consideration

this paper analyzes the comprehensive performance of three insulation oil and finally gets a better choice of oil. In the end

this paper provides the significant technical support to the manufacture of UHVDC converter transformer.