The service life of an oil-immersed power transformer is determined by its insulation condition. In order to preferably apply the time domain dielectric response method and frequency domain dielectric response method to nondestructively assessing the insulation condition of transformer oil-paper insulation

the polarization and depolarization current (PDC) and frequency domain spectroscopy (FDS) of a 500 kV-transformer which is out of service are measured first

then the time domain dielectric characteristic (TDDC) and the frequency domain dielectric characteristic (FDDC)

which represent the insulation condition of oil-paper insulation

are extracted by the blending method of time domain dielectric response and FDS technique. Finally

the variation rules of TDDC and FDDC are illustrated in detail. Research results show that the TDDC and FDDC are sensitive to the variations of the insulation condition of power transformer

thus capable of qualitative evaluation of the insulation condition of transformer oil-paper insulation. This work is supported by National Natural Science Foundation of China (No. 51277187).