The reply voltage method based on the theory of dielectric response is effective in non-destructive diagnosis of transformer insulation condition. The polarization equivalent circuit model of transformer oil-paper insulation

especially the calculation of its parameters

plays a key role in the process of diagnosis. However

the popular extended Debye model is not able to determine the number of polarization branches. In this paper

the extended Debye model is solved by setting different number of polarization branches. Because the worse the insulation

the more the polarized branches

the number of polarization branches of the transformer is assumed to be 7 for poor insulation condition

and 5 for good insulation condition. Subsequently

Explores the relationship among different number of polarization branch is obtained. Combining the heuristic approach of branch

a characteristic quantity

the time constant of polarization branch

is suggested for evaluating the aging condition of transformer oil-paper insulation.