The reliability of oil-immersed power transformers depends on the insulation system to a large extent

and the lifetime decrease with the deterioration of the insulation system.Therefore

insulation diagnosis and lifetime evaluation for oil-immersed transformers are necessary.In this paper

diagnostics of transformer insulation are analyzed

and a new diagnostic technique based on frequency domain spectroscopy（FDS）is presented.The 750 kV transformer operated in Lanzhou East Substation is measured on-site with the FDS method.The experimental results show that the moisture content in transformer pressboard is eligible

and its insulation condition is well.And the experimental results are consistent with those of the conventional preventive test.The validity of FDS method is proved

and it is promising as a novel preventive test method.