Oil-paper insulation

serving as the primary insulation system in oil-immersed transformers (OIT)

endures challenging and severe operational environments. Under long-term operation

the aging of partial insulation inside the transformer increases

and the insulation performance decreases

which seriously impairs the stable and safe operation of the transformer. To maintain the stability of power grids

it is crucial to develop an accurate method for assessing the aging of localized insulation in OIT. In the conditions of laboratory

we conducted frequency domain dielectric response assessments on both transformer oil and oil-immersed pressboard under various temperature conditions to observe the effects. The impacts of moisture content

aging and test temperature on AC conductivity were revealed. Meanwhile

based on the transformers’ main insulation structure

a calculation model of the distribution equivalent conductivity of the transformer main insulation is constructed. An evaluation method of local insulation aging of oil-immersed transformer based on adaptive expansion of dielectric response data is proposed. The multi-parameter coupled equations are solved and verify the validity of the approach through the uneven aging scale model tests. The study results can present a certain theoretical foundation and technical reference for the decision-making of operation and maintenance of power sector.