Quantitative Estimation of the Gelling in Thermally Aged Natural Ester Dielectric Liquid

One of the major issues with natural ester fluid as transformer insulation is encountered in the oxidation process, which forms a thin gel layer due to polymerization. This latter can hinder the electrical and heat transfer properties. In order to understand the process, methodical thermal aging of natural esters is carried out for a 500-h duration under atmospheric conditions at 160 °C. The characteristic variation of fluid-forming gel is investigated by nuclear magnetic resonance (NMR) with thermal aging, through which molecular weight degradation and iodine index of thermally aged oil are estimated for transformer applications. The molecular weight and iodine index of thermally aged oil are reduced to 8.31% and 43.016%, respectively, after 500 h of aging. Thermogravimetric analysis (TGA) is carried out to further comprehend this, and heating calorific value is estimated through bomb calorimetric studies. Heat calorific value (HV) for 500-h aged oil and gel is nearly similar. The electrical discharge performance of the oil is estimated through corona inception voltage (CIV) studies under nonuniform electric field configuration adopting ultrahigh frequency (UHF) sensor techniques. The iodine index is in direct correlation with the CIV measurement.