A Method for the Quantification of the Residual Mineral Oil in Impregnated Kraft Paper After Retrofilling with Synthetic Ester

Retrofilling transformers with synthetic esters (SEs) has become a widely adopted method for refurbishing old mineral oil-immersed transformers, aiming to improve fire safety and environmental sustainability. However, the mineral oil (MO) remaining in the solid insulation is the key factor determining whether the thermal class can be increased from 105° C to 120° C. To evaluate the impregnation state of the oil-paper insulation after retrofilling, it is necessary to propose a method to predict the MO content in kraft paper. This paper tests the electrical and physicochemical properties of kraft paper impregnated with mixtures of SE and MO at different MO ratios. As the MO content increases, the imaginary permittivity decreases significantly in the low-frequency range of 0.01-10 ~Hz. Notably, a strong linear correlation between absorbance at the wave number of 1738 ~cm-1 and MO content suggests a method for predicting MO content in oil-impregnated paper. The variation of volume resistivity and oil absorption were also presented, but the low correlation is inadequate for predictive applications.