Effects of palm-based trimethylolpropane ester/mineral oil blending on dielectric properties and oxidative stability of transformer insulating liquid

Blending of palm-based ester can effectively increase the flash point and thermal-oxidative stability of mineral transformer oil especially in the event of high loading and high temperature. Inversely, owing to the excellent dielectric properties of mineral oil, the dielectric properties of the esters can also be improved. Thus, high oleic palm-based trimethylolpropane (TMP) ester was blended and homogenized with 20 to 50 % of mineral oil, and the physico-chemical and electrical properties of each base fluid was compared with that of pure TMP ester and pure mineral oil. The samples were also subjected to oxidative stability test conducted at 95°C for 500 hours, with the presence of water, oxygen and copper/steel catalyst. During ester and mineral oil degradations, the oxidized oils take on dark brown color with the increase in oxidation products concentration and the sludge formation due to the reaction between the oil and the metal catalyst. XRF measurements performed on the samples showed an increase in iron and sulfur contents, which can be explained by the formation of superficial surface layer of iron (II) sulfide or ferrous sulfide. From the investigations, it was found that the 80/20 of ester/mineral oil blending sample have properties near to the standard values for transformer liquid insulation; thus, it can potentially replace traditional mineral oil as an alternate liquid insulation.