Effects of Thiophene Degradation on the Corrosiveness of Oil and the Properties of Oil–Paper Insulation in the Oil-Immersed Transformers

As a non-corrosive inactive sulfur, thiophene is one of the most common sulfides in mineral oil, which are retained in oil and used for improving the oxidation stability of the oil. However, thiophene may be decomposed and form low-molecular sulfide with strong corrosiveness because of rapid energy accumulation under operating conditions of the oil-immersed power transformer. Thus, this study explored the effect of thiophene degradation on the corrosiveness and properties of insulating oil under thermal and electrical aging conditions, including a case investigation of sulfur corrosion of a 500-kV electric reactor and the accelerated aging experiments of oil–paper insulation. The results showed that the degradation and pyrolysis of thiophene depend on energy accumulation induced by thermal and electric fields in the oil-immersed transformers. Under the premise of thiophene without degradation and transformation, thiophene can still delay the deterioration of insulating oil due to its high oxidation resistance. The continuous energy accumulation induced by thermal and electric fields may cause the molecular chains of thiophene molecules to break and form some corrosive low-molecular sulfides, thereby resulting in the increase of the corrosiveness of oil, the decrease of the properties of oil–paper, and posing a huge corrosion threat to the oil-immersed transformer.