Moisture and electrical discharges effect on naphthenic mineral oil properties

The physicochemical properties of three transformer mineral oils of different levels of degradation, resulting from exposure to moisture and electrical discharges, are investigated. The first oil is new and untreated whereas the second is taken from a transformer after one year of normal operation; and the third oil is extracted from a 15-year-old operating transformer. The considered parameters are breakdown voltage, dielectric constant ε_r, dissipation factor (tan δ) and resistivity. Isothermal charging currents have also been measured under dc for long application times. It is shown that the physicochemical characteristics present major differences depending on the considered oil sample. An easy method enabling to determine the water solubility in oil is proposed; this method is based on the measurement of oil resistivity. To validate a part of the results obtained in laboratory, measurements on an on-load operating transformer have also been achieved. These show that the properties of oils with dissolved water in laboratory are similar to those of oils with really dissolved water inside operating transformers.