Amir Abbas Shayegani - University of Tehran, Department of Electrical Engineering, Iran
Hossein Mohseni - University of Tehran, Department of Electrical Engineering, Iran
Hossein Borsi - University of Hannover, Institute of Electric Power System (Schering-institute), Germany
Ernst Gockenbach - University of Hannover, Institute of Electric Power System (Schering-institute), Germany

In this paper the relaxation distribution function, which is obtained from measuring the polarization and depolarization current (PDC) on oil-pressboard samples, will be presented. The samples have been aged at 115 °C with Fe, Cu and Zn contents as catalyst in order to simulate the stress conditions. Four samples have been aged for 250, 500, 1000, 2000 hours. Tests have been made also on four moisturized samples. Depolarization current curves are decomposed to basic relaxation functions. The results obtained show classification of samples to dry and wet is possible. Also, the quality of oil can be determined from the relaxation distribution function. Aging state of the samples has an impact on the measuring result and the degree of aging of the insulation can be distinguished.

Transformer insulation, Diagnostics, Dielectric response, Moisture contents, Aging, Relaxation distribution