Observations from measurements of the furfural content of oil samples from transmission transformers

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 the analysis of data on the concentration of furfural in oil samples from 310 transmission transformers. The data indicated that there was no discernible steady increase in the furfural concentration (FC) but that occasionally a large increment in FC occurred which was assumed to be due to significant overloads. This was demonstrated by: repeat measurements separated by up to 2 years; by a tendency for high-FC transformers to be over 12 years old and located in districts known to have had a high probability of overload in the 1980s; and by a simulation based on this model which showed a similar pattern of occasional high FCs but most transformers lasting 20 years or more without significant deterioration. The beneficial effect of changing the oil on high-FC transformers to limit further deterioration is also discussed.