Return voltage measurement（RVM）method can effectively detect the transformer solid insulation moisture content

but it is difficult to get the definite diagnostic results from polarization spectrum with multi-ple local peaks. This paper analyzed influence law of circuit parameters change on polarization spectrum and CTC

Urmaxbased on the extended Debye model simulation

and put forward branch number of the extended De-bye model as a new characteristic

finally verified the feasibility of this characteristic. Simulation and ex-perimental results show that:The front of RVM polarization spectrum depends on oil dipole steering polar-ization of transformer; The middle of RVM polarization spectrum depends on oil-paper interfacial polariza-tion; The end of RVM polarization spectrum depends on dipole steering polarization and interfacial polariza-tion in insulation paper. Oil-paper insulation aging will increase polarization capacitance

decrease polarization resistance and time constant of each extended Debye model branch. There may be a partial polarization spectral peak when the transformer oil aging serious. Branch number of extended Debye model could be used as a new RVM characteristic and changing of branch number may produce local peaks of polarization spectrum. Moreover the front of the polarization spectrum ’s local peaks reflected the condition of the trans-former insulation oil and field test also verified the validity of the new characteristic.