Diagnosis on increase in electrostatic charging tendency of mineral insulating oil for power transformers due to aging

Abstract

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 10 14 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.

Electrical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers

“…It was reported also that the flow conditions-laminar or turbulent-also have a strong impact on the streaming current [21,22]. A direct relationship between ECT and aging was observed [23][24][25]. Sulfoxide compounds and hydrogen ions are identified as the prime compounds that increase ECT of mineral insulating oil [23].…”

Correlation between ageing condition and flow electrification charge in transformer mineral oil

“…These two layers, er in the insulating pposite signs. The phenomenon that al properties of the two mediums [8][9][10]. If the so (with low electrical conductivit charge density in the two laye interface, can reach important v of local electrical discharges [3] The development of EDL tak the consequence of the existe which crosses the contact zone b This current appears due to the charge carriers (resulted as reactions) at the interface.…”