Determination of Remaining Mineral Oil in Synthetic-Ester Retrofilled Transformers Based on Fourier-Transform Infrared Spectroscopy

During the last decades, ester liquids have been increasingly used in power transformers as an alternative to mineral oil largely due to their biodegradability and much improved fire safety. As well as use in new transformers it is becoming common to retrofill lower voltage transformers, swapping the mineral oil for ester liquid in the transformer tank to gain the advantages. However, due to the presence of materials that absorb liquid in the transformer a simple drain and refill operation will not remove all the original mineral oil and a residual amount will mix with the ester liquid. Since several properties of the ester liquid particularly fire point is degraded by the presence of mineral oil at even quite low concentrations and since some properties are difficult to measure, having an effective method of determining the concentration of mineral oil in ester liquid would be useful. This paper proposes a practical methodology to assess and quantify the concentration of mineral oil in a synthetic ester liquid using Fourier-transform Infrared (FTIR) Spectroscopy. Sample concentrations ranging from 20% to 0.5% of mineral oil in synthetic ester were prepared and analyzed. Both unused and aged mineral oils were examined to determine if ageing byproducts will affect the measurement or not. The results indicate that FTIR can be used for measurement of residual mineral oil in transformers retrofilled with synthetic ester liquids with sufficient accuracy and certainty to assure that the properties of the ester liquid are not unduly degraded.