Condition diagnosis of oil-paper insulation during the accelerated electrical aging test based on polarization and depolarization current

The oil-paper insulation is the main insulation for oil-impressed transformers. Its insulation condition can affect the secure and stable operation of transformers. In this paper, condition assessment of oil-paper insulation during accelerated electrical aging is investigated based on polarization and depolarization relaxation current (PDC) method. The samples of oil and paper were selected from the same manufacturer and pretreated before the test. The electric field applied to the samples of electrical aging was 10kV/mm and the aging time was set as 0h, 20h, 40h, 60h, 80h and 100h. The aging degree of samples is evaluated with PDC method. Firstly, the polarization and depolarization relaxation currents were measured during the aging process, respectively. The relaxation currents increased after accelerated electrical aging. The traps distribution of the oil-paper insulation was evaluated based on the analysis of relaxation current. The result shows that the trap density and depth will increase during the aging process, and it causes an increase in the amount of charge in both polarization and depolarization periods. Then, a relaxation model in time domain is built based on the information acquired from the experiment.