Oil reclamation - just a question of moisture?

High voltage equipment (e.g. transformers and switchgear), use mineral oil and paper as a cost effective and well tried insulation medium. However, this insulation is known to age over time and usage. An obvious sign of ageing is the change in the colour of the oil which changes from pale yellow to reddish brown. This colour change can be measured by the technique of ultraviolet/visual (UV/Vis) spectroscopy. Other techniques such as acid number measurement and infrared (IR) spectroscopy can also be used as indicators of the age of the oil. In general, decisions can be taken by the operator, depending upon the results of these and other tests, as to when and how to maintain the equipment. Maintenance solutions include refurbishment of the transformer or the replacement of the mineral oil. However, these treatments are expensive and a more cost effective option is to use either oil purification or reclamation in order to cleanse the system before refurbishment becomes necessary. The question then becomes: which of these two processes is the more effective? There is little up-to-date research published on the effects of fuller's earth on transformer oil. This paper considers an experiment where aged oil is processed using fuller's earth and contrasts the results of selected ageing indicators against the known results of the oil purification process.