Chemical Characterization of Transformer Mineral-Insulating Oils

The chemical composition of a group of 14 refined naphthenic and paraffinic transformer-insulating oils and refinery-treated oils by hydrocracking and hydrotreating are compared with the composition of the starting untreated North Sea naphthenic crude oil. The crude oil has a broad carbon distribution from C7 to C40 +, and the refined oils have a more restricted distribution from approximately C11 to C30, optimizing in a range from C16 to C22. Neither the naphthenic crude oil nor its refined products contain n-alkanes or alkylcyclohexanes, but they all contain various concentrations of iso-alkanes, bicyclanes (sesquiterpanes), pentacyclic terpanes, and polycyclic aromatic hydrocarbons (PAHs). The base mineral oil and the inhibited transformer oils also contained a suite of steranes, similar in distribution to those in the starting crude oil. By contrast, the four samples of paraffinic transformer oils have a very different distribution. Three of four of the samples analyzed contain n-alkanes, alkyl-cyclohexanes and bicyclanes, but no iso-alkanes, steranes or PAH. Two of these transformer oils contain pentacyclic terpanes. Two used transformer oils contain all the families of compounds described previously, but in different proportions. It is apparent from the data, that the chemical properties of transformer insulating oils are a function of the crude oil chosen for refining as well as the refining practices, such as pressure, temperature, and catalysts, used in individual refineries.