Observations from measurements of the furfural content of oil samples fromtransmission 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 wind ings has deteriorated. However, the majority of publications on this topic are based on small-scale laboratory investigations with very little on fiel d 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 tw o readings were taken several months apart, indicated that the deterioratio n in paper strength and the associated increase in FC, occur in occasional and discrete steps. The model proposed is that these step-changes are due e ither to internal faults causing extreme local overheating, or to overloadi ng of the transformer causing general overheating. A simulation based on th ese criteria gave a similar pattern of FC versus service age. Overall the r esults indicated an upward trend of log(FC) versus length of service but wi th very large scatter and with clustering by manufacturer. It was clear tha t the majority of transformers show negligible deterioration even after 20 years in service. This is in agreement with the above scenario. There was r eason to believe that transformers located in industrial areas were likely to have been overloaded during the 1980s, so an analysis by locality was do ne. This showed that the percentage of transformers with high (>4 mg/l) FC' s in nominally industrial areas was double that of the rest. This differenc e, although suggestive, proved not to be statistically significant. The mod el described indicates that, because of the auto-catalytic nature of the pr ocess, oil changes would be beneficial where the FC level exceeds 4 mg/l: l ess deterioration will occur when the transformer is overloaded. (C) 2001 E lsevier Science B.V. All rights reserved.