Furfural Concentration in Transformer Oil as an Indicator of Paper Ageing: Field Measurements

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, the majority of publications on this topic are based on small-scale laboratory investigations with very little on field experience with power transformers. This paper reports on the analysis of data on the concentration of furfural in oil samples from 310 transmission transformers. The data showed that while high furfural concentrations (FC´S) tended to occur in older transformers, there was no distinct trend. This, together with the lack of an upward tendency for the FC in cases where two readings were taken several months apart, indicated that the deterioration in paper strength and the associated increase in FC occur in occasional and discrete steps. These step-changes are presumed to be due either to internal faults causing extreme local overheating, or to overloading of the transformer causing general overheating. A simulation based on these criteria gave a similar pattern of FC versus service age. Analysis by substation locality showed that the percentage of transformers with high (iquest4 mg/l) FC´s in nominally industrial areas was double that of the rest. The difference was not, however, statistically significant