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Diagnosis on electrostatic charging tendency (ECT) of mineral insulating oil for power transformers has been performed. Increase in the ECT is derived from sulfonium ions, which are generated by sulfoxide compounds and hydrogen ions. Strong correlation between the ECT and the sulfoxide concentration is obtained in a series of thermal aging tests with oxygen and/or copper catalyst. The volume resistivity of oil is used as an alternative indicator for the concentration of hydrogen ion because hydrogen ions increase the electrical conductivity of oil and the sensitivity of conventional property such as total acid value is too low to evaluate the aging deterioration of closed-type transformers. Analysis of oils extracted from 98 actual transformers, whose operating years are between 3 and 46 years, are performed. High ECT values more than 200 pC/ml are detected in some oils and their volume resistivity values are less than 4 x 1014 Ωcm. Degree of the correlation between the ECT and the sulfoxide concentration is low compared to the result of the thermal aging tests. A diagnostic map with four areas classified using a pair of indicators of volume resistivity and sulfoxide concentration is useful to predict prospective increases in the ECT. Appropriate transformer operation and maintenance including air exposure avoidance of their mineral insulating oils is one of the effective solutions to prevent the ECT from increasing. [ABSTRACT FROM AUTHOR]

Additives to insulating oil and oil treatment by absorbents are investigated by thermal aging tests with the aim of suppressing flow electrification in power transformer insulation. 1, 2, 3-benzotriazol (BTA) is found to be effective for suppressing the increase of electrostatic charging tendency (ECT) in oils that contain either sulfide or sulfoxide compounds. Sulfide compounds are contained in new oil while sulfoxide compounds are generated by the oxidation of sulfide compounds. However, 2, 6-ditertiary-butyl paracresol (DBPC) is only effective for oils that contain sulfide compounds. The addition of sulfoxide compounds significantly increases the ECT of DBPC-added oil by the thermal aging tests. DBPC can suppress the conversion of sulfide to sulfoxide by blocking the generation of peroxide. However, DBPC has negligible effect on suppressing the increase in ECT by sulfonium ions, which are generated by the oxidation of sulfoxide. Negatively charged BTA ions, which are generated by the dissociation of hydrogen ions from BTA molecules, can mitigate the increase in ECT by sulfonium ions, which charge insulating oil positively. The effect of BTA is found to be lost when the concentration in oil decreases due to the adsorption on solid insulators. Oil treatment by absorbents such as clay, activated carbon and silica gel is found to remarkably decrease the ECT of deteriorated oil. However, additional thermal aging test increases the ECT to the same level as before the oil treatment. Although the additives and the oil treatment by absorbents are effective for suppressing and decreasing the ECT of insulating oils, their long-term effect is not assured. Thus periodical measurements of the ECT of insulating oils are crucial for the sake of preventive maintenance of aged transformers. [ABSTRACT FROM AUTHOR]

Electrostatic charging mechanism in aged oil-immersed transformers is investigated. Sulfonium ions, which are generated by sulfoxide compounds and hydrogen ions, are identified as the prime compounds that increase electrostatic charging tendency (ECT) of mineral insulating oil. Sulfoxide compounds are generated by the oxidation of sulfide compounds that are contained in new oil. Hydrogen ions are considered to be supplied by organic acids that are generated due to the oxidation of hydrocarbons. Negative ions with low mass are generated by the dissociation of organic acids during the generation of sulfonium ions. Because these negative ions are hydrophilic, the oil is charged positively. Water is also generated by the deterioration of insulating oils and has the same effect. On the other hand, negative charging of oil is detected with the addition of sulfonic acids, which reflects the generation of positive ions with low mass and hydrophility. [ABSTRACT FROM AUTHOR]