Ester based dielectric fluids have gained wide popularity for applications in high voltage apparatus. Synthetic and natural esters have been subjected to research for decades vis-à-vis mineral insulating oils across the globe. Despite many researchers are affirmative towards the application of ester fluids, utilities are still under a dilemma and application of these alternatives is still a contemporary challenge. The intent of this survey is to present the recent research progress and highlights the state of the art for the key aspects that need to be emphasized in future research. The contemporary research scenario pertaining to performance of ester fluids, miscibility, and retro filling of insulating fluids vis-à-vis mineral oils are discussed. Furthermore, the topic of pre-breakdown phenomena of ester fluids, usage of esters on load tap changers, environmental and fire properties, use of the esters in cold climatic regions are also discussed. Importantly, challenges and future aspects that are to be investigated to improve the existing knowledge of ester dielectric fluids for applications in transformer technology are emphasized.

The life of an electrical transformer is mainly determined by that of its cellulosic solid insulation. The analysis of the chemical markers of cellulose degradation dissolved in oil is a simple and economical way to indirectly characterize the insulating paper. Methanol, a marker that is intimately linked to the rupturing of 1,4-β-glycosidic bonds of cellulose, has been observed together with ethanol during laboratory ageing experiments. Regardless of the simulated ageing conditions (temperature, humidity, air), the ratio of methanol to ethanol concentration is always higher than one (unity). However, in approximately 10 % of transformer oil samples, the ethanol generation is higher than that of methanol. In this study, thermal degradation by pyrolysis is coupled with gas chromatography/mass spectrometry to assess the volatile by-products generated at high temperatures with emphasis on methanol/ethanol generation. Some cellulose model compounds were also pyrolyzed and thermally aged in oil. The results showed that the generation of ethanol from paper pyrolysis is always smaller than for methanol, but it only occurs at temperatures higher than 300 °C. However, thermal ageing of levoglucosan in oil generates a massive amount of ethanol compared to methanol regardless of the conditions (temperature, humidity, air, nitrogen, acidity). The hypothesis that ethanol is a by-product of cellulose degradation through levoglucosan as an intermediary in power transformers is proposed. The presence of ethanol during transformer oil analysis is of high interest because it can be related to a thermal fault or hot spot within the solid insulation.

This article presents a comparative study of two types of transformer solid insulation: standard Kraft and thermally upgraded Kraft papers. The paper samples were impregnated with two different fluids, namely a synthetic dielectric ester fluid, Midel 7131, and a synthesized mineral oil, Luminol Tri. The impregnated papers were submitted to, accelerated aging at 150°C, temperature higher than the temperatures found inside transformers, for extended periods. The results show that for the first hours of aging, the cellulose depolymerization was similar for all the four studied cases. At a specific point, the paper samples aged in the ester fluid exhibited a lower depolymerization exceeding the 8000 hours of aging without reaching the levelling-off degree of polymerization (LODP). A similar trend was observed through the decrease of mechanical properties of papers in the ester-based oil. However, the difference in performance of both fluids was amplified due to the temperature used; at lower temperatures the difference should be smaller. Additionally, a relationship between the degree of polymerization and the mechanical properties accessed by the tensile testing was obtained regardless of the type of paper or oil. The total acidity in the ester fluid was higher than in mineral oil. Methanol, a chemical marker that is closely linked to the rupture of 1,4-ß-glycosidic bonds of cellulose, showed a partial sensitivity to the cellulose aging in the ester fluid. Furan (C4H4O) content was monitored during the aging and its concentration was found to be sensitive to the depolymerization along with the decrease in the mechanical properties of cellulose papers regardless of the type of paper. More importantly, its concentration was found to be dependent on the type of oil. More investigations are needed to further confirm the importance of furan as an indirect indicator of the mechanical performance along with its correlation with the depolymerization of paper aged in ester fluids.