To increase the insulation life of oil-immersed transformers

we proposed a novel method using hybrid insulation systems of cellulose paper and Nomex paper after analyzing the main factor of insulation life and winding temperature distribution in transformers. Firstly

based on theoretic calculation of insulation aging rate and the software calculation of winding temperature field

the temperature cut-off value and the proportion of insulating paper of hybrid insulation system were obtained. Secondly

to verify the performance of the hybrid insulation system

thermal aging tests were performed on the hybrid insulation system. The scanning electron microscope（SEM） images of the microcosmic appearance of insulation paper show that the fiber of cellulose paper is fractured

the cell wall of cellulose paper is disrupted and the mechanical property is reduced

while the fiber of Nomex paper has no fracture

indicating that Nomex paper can maintain mechanical property well. Last

a transformer using hybrid insulation system was manufactured and the temperature rising results obtained by the optical fiber temperature measurement system of temperature-rise test show that the temperature cut-off value in prototype is lower than the preset values. The theoretical analysis and experimental results verify that the life of oil-immersed transformers can be improved 50% by hybrid insulation system.