Liquid Chromatographic Analysis of Phenolic Inhibitors in Ester Insulating Liquids

Large electrical equipment require routine analysis of the dielectric liquid to ensure acceptable performance over the lifetime of the equipment. If the parameters are outside the acceptable range, maintenance should occur. For mineral insulating liquids, the inhibitor content is routinely determined as there are standardized international test methods. However, for ester liquids, there are no standardized international test methods. A robust method utilizing reversed-phase (C-18 column) high performance liquid chromatography (HPLC) with an ultraviolet (UV) detector has been developed. Using an acetonitrile elution system, the inhibitors are readily separated in the chromatogram and quantitated. Commercially available phenolic inhibitors have been analyzed and quantitated using this methodology as well as formulated commercial unused ester insulating liquids and used natural ester insulating liquids. The analytical conditions and results will be described in detail.