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.
Abstract. This work is devoted to an accurate investigation on the variations of the physicochemical properties of three transformer mineral oils of different levels of degradation submitted to moisture and electric discharges. The first one is a non treated new oil while the second is taken from a one year normal operating transformer and the third is extracted from a 15 years old operating transformer. The investigated parameters are breakdown voltage, relative permittivity r , dissipation factor (Tan) and resistivity. Each oil peculiarities are determined through these parameters. Measurements on an on load operating transformer are reported to validate some laboratory obtained results.
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