A study of parameters affecting the ageing of transformer oil in distribution transformers

Abstract

The aim of this paper is the study of various parameters affecting the ageing of transformer oil in distribution transformers of 20/0.4 kV. 69 samples of oil were taken from such transformers. The transformers function in the major Athens area, Greece. Parameters, such as breakdown strength (measured with a Foster test cell, according to IEC 156/95), oil colour (with a chromometer than meets the specifications DIN 51517-ASTM 155), humidity (measured with the Karl-Fischer method, according to IEC 814), interfacial tension (which is of importance especially in the early stages of ageing, measured according to ASTM D971-91) and tanδ (with a system counting tanδ values with maximum accuracy from 0.00001 to 4.0, measured according to IEC 247) were taken into account. Transformer ageing is strictly related to the rate of ageing of the whole insulating system, and mainly of the oil.