Direct contact between the copper wires of a transformer, which would lead to failure, is prevented by insulating paper. In order to assess the remaining service life of the transformer, a paper sample can be removed and analysed after the transformer is opened up. Furan determination of the used transformer oil now provides a much simpler and cheaper alternative.
Cellulose degradation in the insulating oil gives rise to oil-soluble decomposition products, in part due to high temperatures, and they can be determined using chromatography. Since these substances cannot be converted to the gaseous phase without decomposition, a gas chromatograph, as used for gas-in-oil analysis, is not suitable for determining paper ageing. For this reason, we have installed a new HPLC unit in the OELCHECK laboratory. The Perkin Elmer Flexar Pump uses the high performance liquid chromatography (HPLC) method. Using the DIN 61198 procedure, it determines furan derivatives dissolved in oil, which can originate from the insulting paper, in 10-ml oil samples. It distinguishes between 5-hydroxymethyl-2-furfural (5HMF), 2-furfuryl alcohol (2FOL), 2-furfural (2FAL), 2-acetylfuran (2ACF) and 5-methyl-2-furfural (5MEF).
Based on the concentrations of the individual derivatives in mg/kg (ppm), conclusions can be drawn about the condition of the insulating paper. Internal inspection of the transformer is only carried out if elevated furan values signal the need for action.
Furan determination is now available as a special test.