Dielectric response of transformer insulation oils

Dielectric diagnosis with polarization and depolarization currents (PDC) which employs the dielectric response of system in time domain is applied to investigate differences between PDC patterns produced by two types of transformer insulating fluids, mineral and bio-degradable oil. The mineral oil used is Shell Diala BX, and the biodegradable one is Envirotemp FR3. The latter is ester-based, and a relatively new transformer insulating fluid introduced as an environmentally-friendly alternative to conventional mineral oil. The DC conductivity of the two oil types is obtained under the effects of various temperature and moisture levels. The experiment is conducted for two levels of moisture when temperature factor is changed from ambient to 90 °C. The oil water content measurement is conducted using Karl-FischerTitration. Also, an attempt was made to investigate the effects that different testing voltages and time durations have on the polarization and depolarization spectrum. A range of DC voltages up to 1000V, and time durations from 1000 to 10000 seconds have been applied. Finally, all the PDC patterns obtained under the effect of mentioned factors are evaluated.