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.