Flow electrification risk assessment of conventional and new alternative insulating liquids for power transformers

To prevent such risks for power transformers fleet, as end-users, the purpose of this study is to assess, in presence of paper and pressboard, the new insulating liquids recently developed, on which there is not enough return on experience from real operation, in comparison with conventional mineral oil in terms of flow electrification risks. The insulating liquids considered include re-refined mineral oil, bio-based hydrocarbons, ester-based liquid and conventional mineral oil from transformer. The solid materials used are Whatman paper and high-density pressboard frame.

Two types of experimental setups were used, the first one for the evaluation of electrostatic charging tendency (ECT) of the dielectric oils and the second, capacitive sensor, for the assessment of electrical charges generated on the pressboard/oil interface as well as the accumulated charges on the pressboard surface.

For conventional mineral oil, re-refined liquid and bio-based hydrocarbons, the results show that the accumulation of charges on the pressboard surface remains low with a limited hazard for transformer. This is probably related to the electrical conductivity of the oil, which increases with temperature and limits the potential rise within the pressboard.

For ester Oil-BE, the flow electrification behaviour presents a reverse polarization with higher generating currents compared to mineral oils. Positive leakage and accumulation currents are measured. This indicates an accumulation of positive charges in the pressboard, which corresponds to the opposite phenomenon observed in most of the cases studied, particularly for mineral oil/pressboard system. The accumulated charges in pressboard, despite the high generating currents, remain low.