Assessing Water Solubility of Ester‐Based Oils and Unravelling Their Mechanisms by a Quantitative Model

Moisture is a sensitive issue for transformer oils, as it adversely affects the stability of insulation papers and dielectric strength of the oils. However, the lack of a quantitative model makes it challenging to evaluate the water solubility of ester‐based oils, which is an undesirable restriction for developing novel ester‐based oils. Herein, a quantitative model, which requires only the molecular structure as input, was developed to assess the water solubility. Experimental data revealed that both the molecular structure and temperature are the primary factors influencing the water saturation. Our model successfully reproduced the results with a deviation of < 10.0%. Next, we explored the mechanisms that govern the water saturation, focusing on the contribution of each group within molecular structures. It was found that the ester groups play a key role in determining the water saturation, and their contribution value varies from 180 mg/(kg·atom) (FR3) to 820 mg/(kg·atom) (palm fatty acid ester). This variation arises from the differences in the electrostatic potential (ESP), and a linear relationship between the maximum absolute ESP value and the group contribution value was revealed. Finally, we analysed the vapour–liquid equilibrium, revealing the change of water content in the oils when the operating conditions of transformers vary.