Study of water clusters in insulating oils by Fourier transform infrared spectroscopy

The electrical parameters of insulating oils applied in Hv transformers strongly depend on the moisture content. The water/oil system was studied in the practical important concentration range of 0 to 130 mu g/g to explain the unusual decrease of the breakdown strength. The relationship between water concentration and oil breakdown strength was determined in the range 0 to 50 mu g/g,using almost 100 oil samples taken from functioning transformers from each part of Hungary. The plotted data can be approached by two straight lines having very different slope, from which conclude that the water/oil system has two different structures in this concentration range. The water in oil structure was examined by both theoretical and experimental methods. By a Hartree-Fock calculation the distance between water and hydrocarbon molecules was estimated. The water molecules arrangement and distance were calculated by a geometrical model, Fourier transform infrared spectroscopy was applied for experimental structure determination. Three oil samples were selected in the range 0 to 50 mu g/g. Assigning the ON vibration bands, revealed that water is present in low concentration as monomer and at higher concentration in cluster forms (dimers, tetramers etc.). Similar graphs were found of the total dipole moment of water clusters vs, the resistance of insulating oil and the water concentration vs, the breakdown strength. The latter curve can be explained with the different state of water in oil. Water is present in molecular disperse state at greater than or similar to 10 mu g/g. Above this value water clusters appear. Due to the similar total dipole moments of different water clusters there is practically no change of the breakdown strength in the range 10 to 50 mu g/g.