Improving measurement techniques of power transformer insulation: A study of the intermolecular interactions between water and vegetable oil based dielectrics

The use of water activity probes immersed in the vegetable oil dielectric of a transformer requires a fresh look at the equations, and assumptions, behind the conversion of water activity to concentration of dissolved water. This is important because water negatively affects the effectiveness of insulation. A large volume of work has been performed on mineral oil based dielectrics to determine mathematical relationships. However, there is a lack of data on whether these relationships hold for vegetable oil. One example is the appropriateness of using the Henry´s law coefficient of proportionality to calculate the concentration of water dissolved in the fluid from its water activity. This paper presents research investigating the water activity-concentration relationship for one vegetable oil dielectric. It was found that, at room temperature under the prescribed conditions, using Henry´s law is appropriate.