## 1 Introduction  
Currently, transformer oil is mostly mineral oil refined from petroleum. In addition to carbon and hydrogen, sulfur is the third most abundant element, ranging from a few parts per million to as much as five percent. The active sulfides are easy to react with metal parts of equipment to produce cuprous sulfide [1; 2], causing partial discharges in electrical equipment and eventually serious insulation accidents. According to current studies by scholars, the source of corrosive sulfurs in insulating oil exist mainly four forms: (1) Corrosive sulfur contained in crude oil, (2) sulfur contained in electrical equipment, (3) corrosive sulfur compounds generated by crude oil pyrolysis reaction, and (4) corrosive sulfur compounds produced by hydrogenation reactions [3; 4]. In the past few decades, the sulfide deposition phenomenon was discovered successively in transformer fault inspections of 220 kV and above voltage classes in areas within China Southern Power Grid, East China, and North China [5; 6]. Studies and reports on corrosive sulfur faults were also conducted in other countries such as Japan, Europe and other countries [7; 8]. The insulation fault caused by sulfur corrosion has become a major threat to the safe operation of oil-immersed power transformers.  
As the sulphide damage in transformer oil was not exposed until recently, there are relatively few studies on this problem. Previously, the corrosion of sulfides in crude oil on petroleum refining equipment and the corresponding detection methods have been studied extensively. With the outbreak of corrosive sulfur problems, the detection of total sulfur content in transformer oil has been taken as an indicator by the power grid company [9; 10; 11; 12]. However, the types and contents of sulfides in different transformer oils are different, so there is no direct relationship between the total sulfur content and corrosiveness. For the detection of corrosive sulfur in transformer oil, the methods prescribed in different standards are different [13; 14; 15]. In the quantitative analysis of corrosive sulfur in insulating oil, most scholars believe that Dibenzyl disulfide (DBDS) is the main corrosive sulfur in transformer insulating oil [16; 17]. Therefore, various methods for detecting the DBDS content in oils have been proposed by scholars [18; 19]. Solid phase micro extraction (SPE) and gas chromatography-mass analysis (GC-MS) have been combined with these methods, increasing the measurement accuracy to 0.1 ppm [20]. The corrosiveness of a variety sulfides that may exist in transformer oil were examined by [PERSON] [21], and the sulfur compounds such as mercaptans, bisulphid, and thioether were all found to have a certain degree of corrosion. Moreover, one sulfide may convert to another under certain conditions, so the study on sulfur corrosion requires further analysis of other sulfide existing in the insulating oil.  
In this paper, the sulfur distribution in the transformer oil and its effect on the oil quality were studied. By investigating the types, contents, and sources of sulfides in the transformer oil samples with different alkyl and voltage levels, the quantitative test methods for various sulfide in transformer oil were established. The distribution of sulfur in oil was detected by these methods, and the influence of sulfide on oil quality was analyzed.