Strong corrosive active sulfur in the mineral insulating oil has been considered to be the main culprit to induce the insulation failures of oil-immersed power equipment. In order to protect the oil-paper insulation from sulfur corrosion, active sulfur is cleared in the refining process of crude oil. Some inactive sulfur with high antioxidant is still retained in the mineral oil to improve the oxidation stability of oil. However, the activation of inactive sulfur under the operating condition of oil-immersed power equipment is not gained attention. This paper focuses on the reaction mechanism of winding corrosion induced by non-active thiophene sulfide in mineral insulating oil. The thermal pyrolysis products of thiophene sulfide are investigated by the material phase analysis method, and the activation energy changes of thiophene sulfide at different heating rates are analyzed in combination with the pyrolysis kinetics analysis method, and the experimental study of inactive thiophene sulfide under the action of oil paper insulation thermal field is carried out. The results of gas chromatography, mass spectrometry and Fourier transform infrared spectroscopy show that the main activation product of thiophene sulfide (thiophene, benzothiophene, dibenzothiophene) during pyrolysis is H2S, which is highly corrosive and volatile. The thermogravimetric and differential thermogravimetric curves of thiophene sulfides are basically the same at different pyrolysis rates, and thiophene is the most prone to pyrolysis, followed by benzothiophene and dibenzothiophene. Under the condition of low temperature superheating of oil paper insulation, due to the continuous accumulation of system energy, the inactive thiophene sulfide will be activated and then generate low molecular highly corrosive sulfide, which intensifies the corrosivity of oil products and leads to sulfur corrosion of oil paper insulation.
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