The solid insulator is normally charged negatively in the
insulating oil. On the other hand, BTA molecules adsorb the
surface of solid insulator and positive charges in the insulating
oil are adsorbed by BTA. This eliminates the unbalance at the
surface of the solid insulator and suppresses the flow
electrification. In addition, BTA reacts with the copper
material and suppresses the dissolution of copper in the
insulating oil, which suppresses the deterioration of insulating
oil due to the catalyst effect of copper. Thus the adsorption of
BTA on solid insulator has a positive effect on suppressing
the static electrification of solid insulators. However, the
decrease of BTA concentration in oil has a negative effect on
the increase in the ECT of insulating oil. The concentrations
of BTA in actual transformers decrease over time. Thus
insulating oil, even if BTA has been added, should be
periodically checked. Insulating-oil characteristics such as
ECT as well as the concentrations of BTA and dissolved
copper should be measured and their trends should also be
monitored.