Oil treatment by three types of absorbents such as clay,
activated carbon and silica gel is found to have a significant
effect of decreasing the ECT of deteriorated oil. The duration of
this effect is investigated by additional thermal aging test and the
ECT increases to the same level as before the oil treatment. Oil
treatment can remove high ECT compounds. However,
originating compounds, from which the high ECT compounds
are generated, are not removed. Thus there is a possibility that
ECT will again return to high levels over the long term.
The results presented in this paper demonstrate the
effectiveness of typical additives for suppressing the increase in
ECT of insulating oil and the effectiveness of oil treatment by
typical absorbents for decreasing the ECT. The long-term effect
of these measures, however, is not ideal. When applying either of
these measures, insulating-oil characteristics such as ECT, sulfur
compounds and dissolved copper in the oil should be periodically
measured and the duration of its effect checked.