Aged oils reclamation: Facts and arguments based on laboratory studies

Lifetime extension of power transformers is a subject of high importance for electric power systems utilities. The decision of replacing, refurbishing or repairing a service aged power transformer requires considering several factors, especially the cost and time to execute the work. The lifetime of the power transformer being related to the condition of the insulation system; one way of improving the situation is to reclaim insulating oil by Fuller's Earth treatment. This procedure is economically attractive because of increasing prices for both mineral and synthetic transformer coolants, effective cost and environmentally sounds. Reclamation rejuvenates the transformer oil by eliminating contaminants. In this paper, a series of experiments has been performed with service aged oils reclaimed in laboratory conditions. Fast, inexpensive and reliable laboratory testing procedures developed by ASTM (D 6802 and D6181) were also used to monitor decay products as traces impurities. The results obtained in laboratory conditions, indicate that a large number of reclamation passes (around 15 passes) are required to regenerate inservice aged oil to a grade close to new oil. It is also shown that not only the reclamation improves the gassing tendency of oil, but also the type of Fuller's Earth is very important for reclamation process. By upgrading the analytical chemistry of oil, the predictive maintenance of this non-renewable resource is modernized, its cost diminished and the service reliability of transformers enhanced. Currently, Fuller's Earth is only used once. After depletion it must be disposed of in a land fill. By using two organic solvents that are recoverable by atmospheric distillation, it is shown, in laboratory conditions, that this mineral absorbent can be reactivated and successfully reused many times.