Development of a new spectrophotometric method for analysis of 2-furfuraldehyde in transformer oil as an indicator of paper degradation

摘要

Apart from abnormal faults detected by dissolved gas analysis (DGA), one of the most important factors in shortening the lifetime of a power transformer is the ageing of the paper insulation used in transformer construction. The analysis of furans in transformer oil is one of the best techniques when we talk about the assessment of remaining life of a used transformer. Once the furans content in the oil is quantified, the degree of polymerization (DP) in the solid insulation of the transformer can be estimated. The most commonly used solid insulation is formed by 90% by cellulose. This natural polymer degrades during the life of a transformer, resulting in oil soluble sub-products. The DP is the parameter that evaluates condition of the paper which decreases with ageing of the insulation. New insulation with a high degree of mechanical strength will typically have a DP from 1000 to 1300. "Middle-aged" paper is approximately at 500 and paper at less than 250 is "old-aged". It is important to consider that the severely degraded insulation with a DP of 150 or less will have very little mechanical strength, and may result in a transformer failure. In addition, degradation of solid insulation can lead to the end of transformer's life, because the paper cannot be altered or subjected to regeneration processes Although in the literature, some works for determination of 2-furfuraldehyde (2-FAL) in dielectric oils by a colorimetric method can be found, these methods can have interferences for aged oils, and some are subject to variation with temperature. These interferences have been studied in the development of this method. Therefore, several thermal degradations of dielectric oil in order to achieve the widest possible range of visual aspects have been considered, as well as thermal variations of the results. Calibration curves were made with standard solutions for each visual aspect to permit real sample analysis. Finally ultraviolet-visible spectrophotometry (UV-VIS) method data for real samples was compared with high-performance liquid chromatography (HPLC), method data with good results. Once demonstrated the capability of the UV-VIS for evaluating the content of 2-FAL dissolved in dielectric oil, the possible methodology to extract the 2-FAL was evaluated. Both liquid phase extraction and solid phase extraction were considered and tested, but finally solid phase extraction was chosen because it is the easiest extraction method for automation in the laboratory. To sum up, a brand new rapid, inexpensive and easy colourimetric method has been developed to determine the content of 2-FAL in transformer oils. We have obtained accurate and reproducible results. Using a small sample volume the limits of detection and quantification are consistent with the needs of the laboratory. We have also conducted a study of kinetics of the derivatisation reaction with aniline, concluding that with dimethyl sulfide we can vary the speed of the reaction and sensibility according to our needs.