Effect of chemical-structure modification on physicochemical properties of unsaturated and saturated natural esters for power transformers

The physicochemical properties of natural esters are intrinsically linked to their fatty acid composition. However, double-bond unsaturation in these fatty acids results in poor oxidation stability, restricting their application as insulation liquids in free-breathing transformers. Remediation technique through epoxidation involves altering the chemical structure of fatty acids in the esters. In this study, neem and palm kernel oils which represent highly unsaturated and saturated fatty acid oils respectively were transesterified and epoxidatized to improve flow and oxidation stability properties. Changes in viscosity, density, pour point, specific heat, viscosity activation energy, and breakdown strength due to the modification were examined. The pour point of neem increased from 6.0°C to 10.0°C, and its breakdown voltage (BDV) was raised by 14.9 %, while the pour point of palm kernel oil decreased from 25.2 °C to −6.0 °C, with a 16.5 % decrease in BDV after transesterification. However, both oil esters showed equal percentage decrease in viscosity and density after transesterification independent of their fatty acid composition. Epoxidation significantly increased the viscosity of the neem (∼76 %) but only slightly in palm kernel oil (∼8 %). However, the superior fire safety and BDV of epoxy neem make it a more suitable high voltage insulation liquid candidate than palm kernel ester.