In this study, headspace gas chromatography/mass spectrometry has been used to assess the volatile by-products generated by the ageing of oilimpregnated paper insulation of power transformers. Sealed-glass ampoules were used to age under oxidative conditions 0.5-g specimens of insulating paper in 9 mL of inhibited mineral oil in a temperature range of 60-120 8C and moisture of 0.5, 1 and 2% (w/w). A linear relationship between one of the oil-soluble degradation by-products, i.e. methanol, and the number of ruptured 1,4-b-glycosidic bonds of cellulose, regardless of the type of paper (ordinary Kraft or thermally-upgraded (TU) Kraft paper), was established for the first time in this field. Ageing at 130 8C of model compounds of the Kraft paper constituents (a-cellulose, hemicellulose and lignin) and two cellulosic breakdown byproducts (D-(+)-glucose and 1,6-anhydro-b-D-glucopyranose) confirmed that the a-cellulose degradation was mostly responsible for the presence of this molecule in the system. Furthermore, additional 130 8C-tests with six different papers and pressboard samples under a tight control of initial moisture indicated that at least one molecule of methanol is formed for each rupture of 1,4-b-glucosidic bond of the molecular chains. Stability tests showed that the ageing indicator is stable under the oxygen and temperature conditions of open-breathing transformers. The presence of methanol was detected in 94% of oil samples collected from over than 900 in-service pieces of equipment, confirming the potential for this application. Lastly, the tests have shown that oiloxidation by-products and TU-nitrogenous agents modify the methanol partitioning coefficients in the paper/oil/air system, which makes their study essential over a range of field conditions encountered by power transformers. Results are presented and discussed in comparison with 2-furfuraldehyde, which is the current reference in the domain.

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Recently, the existence of a relation between the rupture of 1,4-b-glycosidic bonds in the cellulose during thermal-ageing of paper/oil systems and the detection of methanol in the oil has been reported for the first time in this journal (Jalbert et al. 2007). The present study addresses the rate constants of the reaction for standard wood kraft papers, two immersed in inhibited naphthenic oil under air (paper/oil weight-volume ratio of 1:18) and one in non-inhibited paraffinic oil under nitrogen (paper/oil weight-volume ratio of 1:30). The isotherms in the range of 60-130°C show that the initial rate of methanol production markedly increases with temperature and to a lesser extent with the moisture of the specimens (initially between 0.5 and 2.25% (w/w)), similarly to what is noted for the depolymerization through the Ekenstam's pseudozero order model. The Arrhenius expression of the rate constants reveals linear relationships that confirm the dominance of a given mechanism in both cases. A very good agreement is also noted for the activation energy over the entirely paper/oil systems studied (106.9 ± 4.3 and 103.5 ± 3.7 kJ mol -1 for methanol and scissions, respectively). Furthermore, a comparison of the rate constants k CH 3 OH =k scissions ð Þshows approximately constant values indicating an apparent yield for the methanol of about one-third molecule per every scission for the tests under air (0.27 ± 0.04 for Clupak HD75 and 0.37 ± 0.14 for Munksjö TH70) and even lower for the ones under N 2 (0.12 ± 0.03 for Munksjö E.G.). As expected from a pseudo-zero order model, these values were shown to be consistent with a similar comparison of the amount of CH 3 OH and chain-end groups produced under specific time-temperature ageing conditions (168 h at 120°C). Finally, an additional test carried out with unaged cellulose in contact with a fresh solution of methanol in oil (cellulose/oil weightvolume ratio of 1:18) shows that at equilibrium, over 58% of the species is lost from the solution due to penetration into the fibres. Such results reveal the importance of the species partitioning in establishing the true correspondence between the molecules of CH 3 OH produced and the scissions.

Ester-based dielectric fluids have gained widespread popularity for applications in high voltage apparatus. Synthetic and natural esters have been subjected to research for decades vis-à-vis mineral insulating oils around the world. Although many researchers favor the application of ester fluids, utilities are still uncertain and application of these alternatives remains a challenge. The intent of this survey is to present recent research progress and highlight the state of the art of key aspects that should be emphasized in future research. The contemporary research scenarios pertaining to the performance of ester fluids versus mineral oils, miscibility, and retrofilling of insulating fluids are discussed. In addition, pre-breakdown phenomena, usage of esters in on-load tap changers, environmental and fire resistance properties, and use of esters in cold climates are also discussed. Importantly, challenges and future aspects that should be investigated to improve the existing knowledge of ester dielectric fluids for applications in transformer technology are highlighted.

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