## 4 Effects of Different Sulfur Components on Oil Quality  
According to the above investigation, the corrosive sulfur in the transformer oil is mainly mercaptan and disulfide. The mercaptan content is lower than the detection value at environmental temperature. In addition, some non-corrosive sulfide may convert to corrosive mercaptan sulfide when partial overheating occurs in the transformer. According to the test results, the disulfide content in the oil is mainly DBDS. Therefore, the benzyl disulfide and dodecyl mercaptan were selected as typical corrosive sulfurs to add into the new transformer oil in this article. The influence of different sulfur components on oil quality was investigated by testing the breakdown voltage at power frequency, the moisture in oil, dielectric power factor, and volume resistivity of insulation oil.  
The test materials used in this paper included the following: new 25# transformer oil, DBDS, and DDM (dodecyl mercaptan). The concentration of sulfides added in oil samples is shown in Table 5. The principle of addition is to ensure that the total sulfur concentrations in the different groups of oil samples are equal.  
The 15 mL new 25# transformer oils were poured into each of the four prepared 20 mL empty bottles. Then the specified concentration of DBDS, DDM and a combination of the two were added into the bottles. Next, they were heated and stirred at 40 \({}^{\circ}\)C to ensure that the sulfide compounds are fully dissolved in the oil. The new 25# transformer oil was used as the blank control group. The above four prepared insulating oils were dehydrated and degassed at 40 \({}^{\circ}\)C/50 Pa for 24 h, and then the characteristic parameters were tested. The experimentally tested characteristic parameters of insulating oil include the followings details. (A) The breakdown voltage of insulation oil at power frequency  
Figure 6: The FT-ICR MS distribution of disulfide compounds: (**a**) WLT, (**b**) HCT, (**c**) QZT, and (**d**) HLT.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sulfide** & **New Oil** & **DBDS** & **DDM** & **DBDS + DDM** \\ \hline DBDS (ppm) & - & 200 & - & 100 \\ DDM (ppm) & - & - & 400 & 200 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Concentration of sulfides added in oil samples.  
refers to the IEC 60156 standard. The standard ball-plate electrode was used and the voltage was increased at a constant rate of 2 kV/s until breakdown. The breakdown voltage of insulating oil at power frequency was measured by the automatic oil dielectric strength tester. (B) The moisture in oil refers to the ASTMD 1533 standard. The moisture content in oil was tested by the coulomb method moisture meter. (C) The dielectric power factor and volume resistivity of insulation oil refer to the IEC 60247 standard. The dielectric power factor and volume resistivity of insulation oil were measured by the automatic insulation oil dielectric loss and volume resistivity tester.  
The test effect of different sulfur components on the electrical properties of insulating oil is shown in Figure 7. It can be seen from the figure that the addition of sulfide would increase the moisture content in oil. The addition of dodecyl mercaptan nearly doubled, and it could be concluded that the water absorption of the mercaptan is better. The impurity could reduce the breakdown voltage. Therefore, the breakdown voltage of all three groups was decreased, in which the DDM group decreased most. The increase of dielectric loss could be caused by the water inhalation. In addition, the decrease of volume resistivity could cause the increase of conductance loss and dielectric loss.