Terminal voltage and current measurements due to flow electrification

Electrical breakdown due to charge accumulation from liquid dielectric flow continues to cause failures of power transformers. The origin of the problem is the entrainment of charge from the diffuse double layer on the liquid side of the pressboard/oil interfaces. This mobile charge and concomitant image charge left behind on the pressboard surface can accumulate, causing the electric potential to rise, similar to the voltage build-up in a Van de Graaff generator. The voltage builds until the charge accumulation rate balances the charge leakage rate or until spark discharges occur. This paper describes part of an on-going effort to develop a more complete understanding of the electrification processes at work in a power transformer. A rotating cylindrical electrode experiment has been used to simulate flow electrification in an electric power transformer. A previously developed simple model relating the terminal open-circuit voltage and short circuit current to the volume charge density is summarised and quantitatively applied to several experiments. Open-circuit measurements are used to determine effective electrification current densities of the order of 1-100 nA/m ². Model calculations are found to be in reasonable agreement with the short-circuit current during, rotation rate driven transients