Investigation on activation of inactive sulfur in oil-immersed transformer based on molecular dynamics simulation

Some amounts of inactive sulfur with high antioxidant are still retained in the mineral oil to improve the oxidation stability of oil, such as thiophenic compounds. However, whether the inactive sulfur in the oil-immersed transformer will be activated under thermal field has been not gained attention. In this paper, the molecular model of thiophene was established based on ReaxFF (Reactive Force Field), the molecular dynamics simulation was conducted to analyze the pyrolysis process of thiophene molecule, the product formation was obtained and analyzed. In addition, the distribution of sulfur corrosion deposit on a 500 kV electric reactor was investigated and analyzed, and the sulfur types and content of oil of electric reactor was also measured. The simulation results indicated that thiophene was pyrolyzed to form low molecular sulfide with carbon atoms and some sulfur-containing inorganic product under molecular simulation pyrolysis condition, especially H
2
S with strong corrosion. It may lead to sulfur corrosion in the oilpaper insulation due to the strong corrosive of pyrolysis product. The analysis of the oil-paper insulation of 500 kV electric reactor indicated the corrosion pits and particles were observed on the sampling areas of the copper winding surface, the oil contained thiophene, but not any active sulfur. It is preliminarily confirmed that thiophene lead to sulfur corrosion in the oil-paper insulation from macroscopic and microscopic aspects.