| Summary: | Ageing assessment of transformer paper insulation is commonly conducted through measurements of 2-furfural (2-FAL) and carbon oxide gases in oil. In addition to these conventional ageing indicators, the use of methanol and ethanol in oil to indicate the ageing state of paper was proposed in 2007. Methanol has been used to indicate the early stage of paper ageing, whereas ethanol has been used to indicate abnormal paper ageing that occurs due to severe overheating. However, there is little experience on the applicability of these indicators to the insulation systems with alternative transformer liquids. In addition, research on other factors such as the partitioning, stability and generation from oil is still needed. An in-house method, which uses the headspace gas chromatography (GC) mass spectrometry (MS) technique was developed based on [1] to measure methanol and ethanol in transformer oils. Partitioning (20 °C and 60 °C), ageing (80 °C, 100 °C and 120 °C) and stability experiments (130 °C) were conducted on samples with a thermally non-upgraded kraft paper and a gas to liquid technology based (GTL) oil or a synthetic ester. In addition, an inhibited mineral oil was used as a benchmark. Under the investigated temperatures of 80 °C, 100 °C and 120 °C, methanol in both alternative oils increased linearly with paper ageing until degree of polymerisation (DP) reached 400. During this period concentration of methanol in oil was higher than that of 2-FAL, which proved the concept of using methanol to indicate early paper ageing in alternative oils. The relationship between methanol and DP of paper in the GTL oil was similar to that of the mineral oil. However, the rate of increase of methanol against the reduction of DP in the synthetic ester was about three times higher than that in both the GTL oil and the mineral oil. This is due to a difference in the partitioning of the indicator between oil and paper. Since the synthetic ester was more polar than the hydrocarbon oils (the GTL oil and the mineral oil), it attracted more methanol from the paper resulting in a higher concentration of methanol in oil for the same reduction of DP. In addition, a reduction in methanol was observed at the later stage of paper ageing at 120 °C in all three oil types, which could be due to either a change in the partitioning between oil and paper or an esterification reaction that consumes methanol in the insulation system. At the same temperature of 120 °C, methanol measured in oil samples aged without paper was less than 10% of that measured in the samples aged with paper, which confirmed that methanol was generated largely from the paper rather than the oil. However, the amount of ethanol measured in the samples aged without paper was similar or even higher than that measured in the samples aged with paper, which indicated that ethanol was generated mainly from the oil rather than the paper. The production of methanol and ethanol from oil only samples increased with the increase of the temperature or the amount of air in the system. A reduction trend of methanol and ethanol was also observed in oil only samples at high acidity of about 0.4 mg KOH/g oil. Stability experiments conducted at 130 °C implied that this behaviour is not due to the direct effect of temperature but the high acidic condition, which results in the esterification of the alcohols in oil.
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