Insulating oil consists mainly of hydrocarbon compounds, but
it also includes small amounts of sulfur, nitrogen and oxygen
compounds. Some sulfur compounds such as sulfide and
sulfoxide compounds were found to increase the ECT of
insulating oil by thermal aging tests. The ECT of insulating oil
was found to increase by the generation of sulfonium ions via
sulfoxide compounds due to the oxidation of sulfide compounds,
which are normally contained in new oil [7, 8].
The aim of this paper is to find out the effective measures for
suppressing the increase in ECT of insulating oil in aged power
transformer insulation. The effects of typical additives such as 1,
2, 3-benzotriazol (BTA) and 2, 6-ditertiary-butyl para-cresol
(DBPC) are investigated by thermal aging tests with insulating
oils containing either sulfide or sulfoxide compound. The
suppression effects of these additives are considered from the
separation process of these additives occurred in insulating oil
and the charge compensation process of decreasing the ECT. The
duration of suppression effect of BTA is investigated by
monitoring both the ECT and the concentration of BTA in oils.
The effects of oil treatment by absorbents such as clay, activated
carbon and silica gel are also investigated. The duration of oil
treatment is also investigated by additional thermal aging tests.