Understanding ethanol versus methanol formation from insulating paper in power transformers

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.