Measurement of the Relative Free Radical Content of Insulating Oils of Petroleum Origin

Abstract

Oil/paper insulation degradation in transformers involves chemical and physical changes in the materials. Some of the chemical reactions involve very reactive intermediates called free radicals. Free radicals play a major role in a wide variety of ageing processes. The detection of these reactive species in oil may, in principle, provide useful information for monitoring oil degradation. This manuscript details a laboratory technique, which determines the relative content of free radicals in insulating oils of petroleum origin by a spectrophotometric method. Free radicals may be formed in oils under operating or test conditions. The procedure enables the determination of the relative concentration of free radicals, which can act as the precursors of decay products such as charge carriers, oxidized molecules, as well as polymerization products. The technique involves using a reactive free radical reagent, 2,2-diphenyl-1-picrylhydrazyl (DPPH), added to oil to assess free radical concentration. This method is applicable to new, reclaimed, or used oils as well as naturally or artificially oxidized oil (the cause of aging can be chemical, physical, or electrical). In this contribution, free radicals were assessed following electrical discharge application in oil.

Stability of mineral oil and oil–ester mixtures under thermal ageing and electrical discharges

“…This alteration is attributed to the complex chemical processes which are accelerated mainly by temperature, catalysts, moisture, metal particles and other compounds produced by ageing insulation (mainly cellulose). This, in turn, changes the chemical and physical properties of the oil, accelerating the ageing of the oil and the cellulose paper [16,17].…”

Review of Physicochemical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers

“…Free radicals are very reactive and can adversely affect the chemical, physical, and dielectric properties of the insulating liquid [122,123]. The reactive free radical reagent, 2,2-diphenyl-1-picrylhydrazyl (DPPH), is added to a solution, the free radical concentration of which is to be determined [124]. The presence of free radicals in solution will increase the rate at which DPPH disappears from the background solution; the higher the free radical concentration in the test specimen, the faster DPPH disappears.…”

“…The presence of free radicals in solution will increase the rate at which DPPH disappears from the background solution; the higher the free radical concentration in the test specimen, the faster DPPH disappears. The relative free radical concentration of an insulating oil test specimen is determined as follows [124]: initially, the absorbance of the background solution of known concentration is recorded. Subsequently the decreasing absorbance of the oil specimen added to the background solution is plotted.…”

Comparative study of the degradation rate of new and regenerated mineral oils following electrical stress

“…The process of decomposition of insulating oils under electric stress begins with the breakdown of unstable mineral oil molecules covalent bonds. Oxygen, moisture or other chemical reactive radicals can spontaneously be generated; the process being catalysed by heat [5, 6]. Free radicals are very reactive and can adversely affect the physicochemical and dielectric properties of the insulating oil.…”