Determination of Mineral Oil Concentration in the Mixture with Synthetic Ester Using Near-Infrared Spectroscopy

Currently, synthetic ester is gaining a bigger share in the market. This type of insulating liquid is used both in new and operated transformers filled with mineral oil. In the case of transformers in operation, the synthetic ester is used in the retrofilling procedure, drying the cellulose insulation, or as a blend with oil, the properties of which are better than those of base liquids. In all these three cases, we are dealing with a mixture of synthetic ester and mineral oil. The concentration of both of these liquids in the mixture has a significant impact on its properties; therefore, methods are necessary to determine the content of individual mixture components. The article presents a method for determining the concentration of mineral oil in a mixture with synthetic ester using near-infrared spectroscopy. Based on the conducted tests, an absorption band was determined that can be used for this purpose. This band is centered at 2126 nm. The determined dependence of the absorbance on mineral oil concentration in the mixture with synthetic ester confirmed the linear nature of this relationship. The conducted research confirmed the possibility of using the method based on near-infrared spectroscopy to determine the concentration of individual components of a mixture of mineral oil and synthetic ester. The proposed method can be used both for a mixture of new liquids and mixtures of new synthetic ester with mineral oils of different degrees of aging. The method of determining the concentration of mineral oil in a mixture with synthetic ester based on near-infrared spectroscopy is new and is characterized by a higher accuracy in relation to the methods previously described in the literature.