Changes in ECT and dielectric dissipation factor of insulating oils due to aging in oxygen

It is believed that the aging of insulating oils affects the electrostatic charging tendency (ECT) and dielectric dissipation factor (tan δ), but there is little hard evidence. We have investigated changes in ECT and tan δ of insulating oils aged with oxygen at elevated temperature. We found that ECT and tan δ increased with aging time. In some oils aged with a small amount of oxygen, ECT and tan δ increased much more than in other oils aged with a large amount of oxygen. In yet other insulating oils, ECT and tan δ showed a peak and then declined. Moreover, it was found that aged insulating oil close to the peak in tan δ is very unstable. This result suggests that the methods of measurements of tan δ of insulating oil prescribed in International Electrotechnical. Commission (IEC) 60247 and Japanese Industrial Standards (JIS) C2101 may be unsatisfactory. The reason is that since in these methods tan δ is measured at elevated temperature (IEC 60247, 90°C and JIS C2101, 80°C), the quality of the insulating oil, especially aged oil, is changed during the measurement, resulting in an incorrect reading