Based on the test results, the ECT in mineral insulating oil is considered to increase in the process as shown in Figure 16. Sulfide compounds contained in oil in the brand-new condition initiate the increase of ECT. The ECT increases when sulfide compounds are oxidized and transformed into high ECT compounds via sulfoxide compounds. Sulfone and sulfonic acid compounds are not considered high ECT compounds because the reverse polarity of ECT by these compounds does not coincide with the flow electrification occurred in power transformer insulation. Oxygen and copper catalyst produce peroxides and accelerate the transformation from sulfoxide compounds into high ECT compounds.