Furfural Concentration in Transformer Oil as an Indicator of Paper Ageing, Part 1: A Review

The concentration of the chemical furfural in transformer oil is well known to be an indicator of the extent to which the paper insulation of the windings has deteriorated. However, technical papers on the relationship between the loss of mechanical strength of the paper insulation in transformers and the increase in the concentration of furfural in the oil are to be found scattered over a variety of journals. The intention of this communication is to bring together the main findings of the laboratory investigations, and the rather fewer reports of field experience, for the convenience of power engineers. The linear relationship between the tensile strength and the 'degree of polymerisation' (DP) of paper insulation has been demonstrated by several workers under laboratory conditions; and so has the negative linear relationship between the logarithm of the furfural concentration and the DP. The concentration of furfural in transformer oil is seen as a paper-specific indicator of degradation and a method for measuring it using high performance liquid chromatography has now been standardised (IEC 1198). The presence of water increases the degradation rate of the cellulose chains and so, because water is also a product of the degradation process, this is an auto-catalytic (or 'positive feedback') process. This makes the overall process very much more complex. In addition, the temperature varies across the transformer itself. However a significant change in the FC over a period of months would indicate the probable development of a fault. The value of regular measurements of furfural concentrations in transformers seems clear