Temperature gradient inside oil-immersed transformers will cause non-uniform aging of a paper.In order to study the time domain dielectric property of oil-paper insulation under temperature gradient

we established a test platform in laboratory.We measured the polymerization degree and polarization/depolarization currents of test samples.Finally we identified its equivalent model parameters.The results indicate that polarization/depolarization currents of insulation paper near heat source move to the top left with higher conductivity and higher polarization response speed.The polarization resistance and time constant in isothermal zone near heat source have minimum values

and there exists a relation between parameters from the extended Debye model and modified Debye model.The extended Debye model can only assess average aging condition of the insulation paper

and there exist an aging overvalued area and aging undervalued area when using the extended Debye model for paper condition assessment with up nearly 50% deviation.The results provide a reference for the polarization/depolarization current method to more accurately be applied to condition assessment of transformer oil-paper insulation.