Synthetic ester insulating oils are renewable, biodegradable, and possess high flash and ignition points, making them promising alternatives to mineral oils in power transformers. However, their acid value increases significantly during aging, compromising insulation performance. This study synthesized two ester oils—trimethylolpropane caprylic-nonanoic acid ester (TCNE) and pentaerythritol caprylic-nonanoic acid ester (PCNE)—via vacuum esterification, and investigated the effects of three representative acids (formic acid [C1], octanoic acid [C8], and nonanoic acid [C9]) on their oxidative stability. Results show that PCNE exhibits better thermal-oxidative resistance than TCNE. All acidic substances reduced the oxidation onset temperature and apparent activation energy, which accelerated degradation in insulation performance. C1 had the most significant impact, attributed to its strong polarity, acidity, and hydrophilicity.
 

synthetic ester, trimethylolpropane ester, pentaerythritol ester, thermal oxygen aging, thermal oxidation stability