## 5 Conclusions  
(1) According to the investigation statistics, the naphthenic base oil samples account for a large part of the total corrosive samples in Fujian area. The corrosive oil samples under 500 kV voltage level are more than other voltage levels. Relatively speaking, Nynas oil sample is corroded seriously, while the oil composition and the voltage level are not directly related to the corrosiveness of the oil sample. (2) The qualitative testing methods of various sulfide compounds in transformer oil was established. On this basis, the sulfur composition distribution in operating oil was analyzed. The thiophene sulfide in the transformer oil mainly exist in the form of benzothiophene with an unsaturation of 6 and dibenzothiophene with an unsaturation of 9. The monosulfide sulfide with an unsaturation of 3 or an unsaturation of 6 has the highest content in transformer oil. The disulfide in transformer oil is basically in the form of DBDS. Figure 7: Effect of different sulfur components on the electrical properties of insulating oil: (**a**) Moisture in oil, (**b**) breakdown voltage at power frequency, (**c**) tan \(\delta\), and (**d**) volume resistivity. (3) The addition of different corrosive sulfurs will lead to the increase of moisture content in oil, indicating that corrosive sulfur has a certain amount of water absorption. The mercaptan has the strongest water absorption capacity. The increase of water content in oil will lead to the decrease of the breakdown voltage and the increase of the dielectric loss, which will seriously affect the quality of oil products. (4) In view of the sulfide hazard on transformer oil, it is necessary to prevent and control the sulfur corrosion. On one hand, it is essential to prevent the insulating oil with corrosive sulfur from entering the transformers. On the other hand, it is also necessary to conduct corrosive testing on the operating transformer insulating oil to ensure that corrosive sulfur is detected and effectively blocked as early as possible. Recently, the use of vegetable oil for large power transformers is proposed and there is no corrosive sulfur hazard. If the mineral insulating oil is replaced by vegetable oil in the future, it can effectively avoid transformer failure caused by corrosive sulfur. [PERSON] and [PERSON] conceived and designed the experiments, [PERSON] performed the experiments and analyzed the data, [PERSON] wrote the main manuscript text. All authors read and approved the final manuscript. F.F. * IEC (2008) IEC 62535. Insulating Liquids-Test Method for Detection of Potentially Corrosive Sulfurin Unused and Unused Insulating Oil, 2008. Available online: [[http://www.iec-standard.com](http://www.iec-standard.com)]([http://www.iec-standard.com](http://www.iec-standard.com)) (accessed on 8 October 2008). * ASTM (2015) ASTM D1275. Standard Test Method for Corrosive Sulfur in Electrical Insulating Liquids, 2015. Available online: [[https://www.astm.org/Standards/D1275.htm](https://www.astm.org/Standards/D1275.htm)]([https://www.astm.org/Standards/D1275.htm](https://www.astm.org/Standards/D1275.htm)) (accessed on 15 November 2015). * [PERSON] et al. (2017) [PERSON]; [PERSON]; [PERSON]; [PERSON] Experimental simulation of effects of high temperatures on paper oil insulation of transformers in presence of DBDS in mineral oil. _IEEE Trans. Dielectr. Electr. Instul._**2017**, _24_, 2819-2827. [CrossRef]
* [PERSON] et al. (2016) [PERSON]; [PERSON]; [PERSON] Inhibition method for the degradation of oil-paper insulation and corrosive sulphur in a transformer using adsorption treatment. _IET Gener. Transm. Distrib._**2016**, _10_, 1893-1900. [CrossRef]
* [PERSON] et al. (2016) [PERSON]; [PERSON]; [PERSON] Dibenzyl disulfide adsorption on Cu(111) surface: A DFT study. _Theor. Chem. Acc._**2016**, _135_, 1-9. * [PERSON] et al. (2012) [PERSON]; [PERSON]; [PERSON] IEC 62697-2012: State of the Art Methods for Quantification of DBDS and Other Corrosive Sulfur Compounds in Unused and Used Insulating Liquids. _IEEE Trans. Dielectr. Electr.