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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.

International audienceIn recent years, significant attention has been paid to biodegradable vegetable oils as an alternative to mineral insulating oils. This study presents the influence of electrical discharges and thermal stress on the variation of the properties of uninhibited olive oil. This latter constitutes a serious environmentally friendly and renewable substitute for currently used mineral oils. The investigated properties are: dielectric loss factor and permittivity, resistivity, acid number, cinematic viscosity, water content, breakdown voltage, flash point and colour index. To analyse the effect of thermal stress, the uninhibited olive oil was aged in sealed bottles at 110°C for durations up to 716 h. As concerns the electrical stress, series of 200 up to 1600 low energy discharges are applied under 50 Hz alternating divergent electric field. Frequency domain spectroscopy has also been performed in order to assess the oil behaviour in a large frequency range (20 Hz-1 MHz). The real part of the complex dielectric permittivity of oil tends to increase with ageing. The obtained results show an acceptable stability with regard to thermal ageing and even an improvement of the oil key properties when submitted to discharges of controlled low energy level

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