Investigation on formation and solubility of formic acid, acetic acid and levulinic acid in insulating oil using COSMO-RS

• The accelerated thermal aging experiment of oil-paper insulation and BSTFA silanization derivatization method were applied. • The σ-profiles of the molecular models and the polarity characteristics of the molecules were analyzed. • The solubility of formic acid, acetic acid and levulinic acid in mineral insulating oil were calculated. • The differences in the solubility of formic acid, acetic acid and levulinic acid in mineral insulating oil were analyzed. During the operation of the transformer, the oil-paper insulation system will produce formic acid, acetic acid, and levulinic acid, which belong to low molecular weight carboxylic acids (LMWCA), so these three carboxylic acids are the potential aging indicators of oil-paper insulation. This paper designs an accelerated thermal aging experiment of oil-paper insulation. The detection of BSTFA silanization derivatization method shows that the aging of insulating paper will generate formic acid, acetic acid and levulinic acid. Smaller volume of LMWCA tends to be distributed in insulating paper, but the solubility of LMWCA in insulating oil will change with the change of environmental conditions. In this paper, molecular models of formic acid, acetic acid, levulinic acid and mineral insulating oil are constructed. The COSMO-RS method is used to analyze the σ-profile of the molecule, and calculate the solubility of the three carboxylic acids in insulating oil at the temperatures of 273.15 K - 423.15 K. The results show that the increase of temperature increases the solubility of the three carboxylic acids. At the same time, the influence of moisture on the solubility of the three carboxylic acids is explored, the results show that the moisture in insulating oil will increase the solubility of the three carboxylic acids. Under the same conditions, the solubility of acetic acid in mineral insulating oil is the largest, followed by the solubility of formic acid, and the solubility of levulinic acid is the smallest. The calculated results are basically consistent with the previous experimental results. The research on the solubility of the three carboxylic acids in insulating oil further verifies its feasibility as an aging indicator of oil-paper insulation.