The ECT of alkyl benzene with octyl sulfonic acid
exceeded 500 pC/ml with reverse polarity. Sulfide compounds
contained in mineral insulating oil in the brand-new condition
are transformed to produce sulfur compounds including
sulfoxide, sulfone and sulfonic acid compounds. The effects of
these compounds on the ECT are different with each other. It
has conventionally been believed that insulating oil tends to be
positively charged, while solid insulators such as pressboards to
be negatively when the flow electrification occurs in power
transformer insulation. However, when the octyl sulfonic acid
that is a strong acid is added, the opposite charging is observed
where the insulating oil becomes negatively charged and solid
insulators become positively charged.