Feasibility of free radical detection for condition assessment of oil/paper insulation of transformers

The aging mechanism of oil/paper insulation in transformers is the result of chemical and physical changes in the materials used which degrade the dielectric properties of the insulation system and leads to the eventual failure of the equipment. Some of the chemical changes are suspected to involve very reactive intermediates called free radicals. Therefore the detection of free radical species in oil/paper systems may in principle provide a powerful technique for monitoring inception of oil/paper system degradation. Electron spin resonance (ESR) is by far the best available technique for detection and characterization of free radicals. This paper presents the results of a study to determine the feasibility of using ESR for the detection of free radicals for the purpose of developing on-line diagnostic tools and field monitoring techniques to assess the condition of oil/paper insulation in high voltage equipment. ESR measurements were carried out on virgin Voltesso 35 oil and oil obtained from service aged transformers, with and without exposure to partial discharge activity. The measurements did not detect the formation of free radicals due to partial discharge activity. Based on the results of this study, it is concluded that any free radicals that may be generated due to partial discharge in oil have a very short lifetime (much shorter than the time scale of the experiment) and thus make the technique unsuitable for on-line monitoring of the condition of oil/paper insulation of high voltage equipment.