Abstract
The influence of various compounds on the increase of the electrostatic charging tendency (ECT) in insulating oil was investigated. Sulfide compounds, which are contained in new mineral oil, are identified as original materials to increase the ECT for the first time. This is verified by heating tests of alkylbenzene with the addition of various compounds. Coexistence with copper and injection of oxygen enhance the increase of the ECT. Some of the sulfide and sulfoxide compounds increase ECT remarkably. On the other hand, the influence of sulfone, which is an oxide of sulfoxide, is negligible. Although sulfonic acid, which is the oxide of sulfone, remarkably increases ECT, the polarity is reverse. Thus ECT is influenced by the kinds and the molecular structures of additive sulfur compounds.
The influence of nitrogen compounds and that of oxygen compounds are confirmed smaller than that of sulfur compounds. Since the amount of nitrogen compounds and that of oxygen compounds are much smaller than that of sulfur compound in mineral insulating oils, the influence of these compounds is considered negligible.
Since ECT of the oil with the addition of sulfide or sulfoxide increases after the heating, the other compounds should be created from these compounds that increase ECT directly. Thus the increase of ECT is mostly caused by the oxidation of sulfide, which creates sulfoxide. Then certain compounds, which directly increase ECT, are created from sulfoxide. If sulfone or sulfonic acid is created by the oxidation of sulfoxide, ECT decreases because of the influence of these compounds on ECT. It is most likely that a certain process other than oxidation occurs at the creation of the compounds that increase the ECT directly.
