Flow electrification in transformers: correlation between winding leakage current and pressboard charge accumulation

As far as static electrification transformer diagnosis is concerned, the leakage current measurement is recommended on apparatus declared suspect following on the Electrical Charging Tendency values and, to a smaller amount, the continuous aging test for tan δ test. As part of the research program of Electricite de France in the field of static electrification in power transformers, an experiment has been carried out, at the University of Poitiers to investigate the correlation between the leakage current and the charge accumulation in some insulated pressboards of the transformer. On one hand, the leakage current has been observed as the result of flow electrification in a duct composed of pressboards and wrapped conductor standing for the oil channels between windings in transformer. On the other hand, the charge accumulation is computed from capacitive currents on insulated electrodes laying on the pressboards of a second duct in the loop. These two currents have been analyzed in terms of flow velocity, oil temperature, conductivity and moisture content. It finally turns out that to define an alarm threshold based on leakage current seems rather difficult as great values can mean an important charge generation but a weak accumulation when small values can lead to a dangerous accumulation in the pressboard