Abstract -- With increasing attention to environmentally friendly characteristics of power transformer insulating liquid, natural ester, synthetic ester and modified ester have gradually become alternative insulating liquids for transformers. Owing to the lack of data on dissolved gases in natural and modified esters for transformer, this study presents experimental results on dissolved gases in natural and modified esters under typical thermal faults. The experimental results show that the content and proportion of dissolved gases in different types of insulating liquids under the same thermal fault simulation are different due to the differences in their chemical compositions. Furthermore, to better explain the law of gas generation and the process of degradation, gassing behavior, neutralization titration, ultraviolet-visible spectroscopy, and infrared spectroscopy have been used to analyze the influence of thermal faults on insulating liquids. It can be concluded that the natural and modified ester produce acids, aldehydes, and other compounds under thermal faults, and the decomposition pathways of these products are different for natural and modified esters, For the diagnosis of power transformer thermal faults, different criteria are needed.

Decomposition pathway, dissolved gas analysis, natural ester, modified ester, thermal faults