1. Introduction
Sulfur hexafluoride (SF6) is a colorless, orderless, nontoxic, and non-flammable gas. The gas is strongly electronegative and tends to attract free electrons. SF6 is widely used as insulative gas in electric transmission and distribution equipment and its insulation property is about 2.5 times to those of air, which was traditionally used as isolative materials in electric equipment. The SF6 electric equipment takes less area, has less operation noise and has no danger of fire, so SF6 electric equipment elevates operation security.
In the occurrence of arc discharges, spark discharges and corona discharges, SF6 would be reacted with little water, electrode and solid insulated material. SF6 were decomposed to complicated gas and solid byproducts. Gas byproducts include carbon tetrafluoride (CF4), thionyl sulfide (SOF2), sulfuryl fluoride (SO2F2), and sulfur dioxide (SO2), and solid byproducts include aluminum fluoride, tungsten fluoride, and so on [1-9].
It is difficult to judge the internal operation condition of electric equipment, while analyzing the concentrations of SF6 byproducts is an effective method to judge the internal operation condition of electric equipment. There are many cases these years about how to judge faults by analyzing SF6 byproducts, such as CF4 and SO2, in Guangdong, and many similar examples were reported in the literature as well [10,11]. In past successful examples, we can only judge the faults of electric equipment and the position of the faults in the electric equipment by analyzing SF6 byproducts. There is still little experience about characters of SF6 byproducts in different condition of different types of faults in the electric equipment. A series of overheated faults about different temperature of electric equipment were simulated. The paper reports different types of SF6 byproducts and their concentrations about different condition of overheated faults in electric equipment.
2. Method
The experiments were simulated in a section of bushing of a breaker, which can be found in Figure 1 in detail. The moisture of the SF6 was between 3616 μL/L and 5189 μL/L, and the temperature was 200˚C, 250˚C, 300˚C, 350˚C, 400˚C, 450˚C and 500˚C, respectively. The pressure of SF6 in the simulator was 0.3 Mpa. There was a contact, which can simulate overheated faults through 900A current, but the heat was not enough for simulation requirements. There was a heating rod and a controller that was binding with the contact as well. The device can strictly control the temperature of simulation. In each simulated temperature, SF6 was taken by a 1.25 L steel bottle from the simulator every hour to analyze SF6 byproducts, such as fluoride 1 (probable SF4), fluoride 2 (probable S2F10), SO2, H2S, HF and CO. The analysis method can refer to the literature [12-17]. The chromatograph used in the simulation was Agilent 7890 N with a flame photometric detector and a Gaspro capillary column(0.32 mm × 30 m).
3. Results and Discussion
Table 1-7 show SF6 byproducts and their concentrations of overheated faults simulation at 200, 250, 300, 350, 400, 450 and 500˚C in high humidity. Moisture of the SF6 was 4049, 4098, 4218, 3616, 3642, 5789 and 4064 μL/L respectively.
At 200, 250 and 300˚C, the concentrations of fluoride 1 and fluoride 2 (probable SF4 and S2F10) were not increasing significantly. The concentrations of SO2, H2S, HF and CO were below detection limits. SF6 is stable and not decomposed significantly at 200, 250 and 300˚C. The concentrations of fluoride 1 (probable SF4), fluoride 2 (probable S2F10) and SO2 are not significantly increased with the time of heating.
Table 1. SF6 byproducts of overheated faults simulation at 200˚C (unit: µL/L).
Table 2. SF6 byproducts of overheated faults simulation at 250˚C (unit: µL/L).
Table 3. SF6 byproducts of overheated faults simulation at 300˚C (unit: µL/L).
Table 4. SF6 byproducts of overheated faults simulation at 350˚C (unit: µL/L).
At 350˚C, the concentrations of fluoride 1, and fluoride 2 (probable SF4 and S2F10) were not increasing significantly. The concentration of SO2, H2S and HF was 7.2, 1.6 and 1.9 μL/L after heating for 5 hours, and it was increased to 23.0, 3.0 and 1.2 μL/L 3 hours later. The concentration of CO was below detection limits. SF6 will decompose and produce 0.3 μL/L H2S and 0.5 μL/L HF after heating for 1 hour, and will produce 7.2 μL/L SO2, 1.6 μL/L H2S and 1.9 μL/L HF after heating for 5 hours. The concentration of HF decreased when its concentration increases up to 1.9 μL/L. It may be because that HF was strongly corrosive and its corrosion to inner equipment made the concentration of HF decreasing. The
Table 5. SF6 byproducts of overheated faults simulation at 400˚C (unit: µL/L).
Table 6. SF6 byproducts of overheated faults simulation at 450˚C (unit: µL/L).
Table 7. SF6 byproducts of overheated faults simulation at 500˚C (unit: µL/L).
conntrations of fluoride 1 (probable SF4) and fluoride 2 (probable S2F10) are not significantly increased with the time of heating, while the concentration of SO2 is significantly increased.
At 400˚C, the concentrations of fluoride 1, and fluoride 2 (probable SF4 and S2F10) were not increasing significantly. The concentration of SO2 was 17.6 μL/L after heating for 5 hours, and it was increased to 26.1 μL/L 3 hours later. The concentration of H2S was 1.6 μL/L after heating for 2.5 hours, and it was increased to 5.3 μL/L 5.5 hours later. The concentration of HF was 2.4 μL/L after heating for 5 hours and it was decreased to 1.6 μL/L 3 hours later, the tendency of which was the same with that at 350˚C. The concentration of CO was below detection limits. SF6 will decompose and produce 0.6 μL/L H2S and 0.9 μL/L HF after heating for 1 hour at 400˚C, and will produce 17.6 μL/L SO2, 3.3 μL/L H2S and 2.4 μL/L HF after heating for 5 hours. The concentrations of fluoride 1 (probable SF4) and fluoride 2 (probable S2F10) are not significantly increased with the time of heating, while the concentration of SO2 is significantly increased.
At 450˚C, the concentrations of fluoride 1 and fluoride 2 (probable SF4 and S2F10) were not increasing significantly. The concentration of SO2 and H2S was 13.3 and 1.5 μL/L after heating for 2 hours, and it was increased to 40.6 and 8.4 μL/L 6 hours later. The concentration of HF was 2.7 μL/L after heating for 5 hours, and it was decreased to 2.4 μL/L 3 hours later, the tendency of which was the same with that at 400˚C. The concentration of CO was below detection limits. SF6 will decompose and produce 0.8 μL/L H2S and 1.0 μL/L HF after heating for 1 hour, and will produce 13.3 μL/L SO2, 1.5 μL/L H2S and 1.5 μL/L HF after heating for 2 hours. The concentrations of fluoride 1 (probable SF4) and fluoride 2 (probable S2F10) are not significantly increased with the time of heating, while the concentration of SO2 is significantly increased.
At 500˚C, the concentrations of fluoride 1 and fluoride 2 (probable SF4 and S2F10) were not increasing significantly. The concentration of SO2 was 33.4 μL/L after heating for 2 hours, and it was increased rapidly to 91.4 μL/L 6 hours later. The concentration of H2S was 4.6 μL/L after heating for 1 hour, and it was increased to 25.2 μL/L 7 hours later. The concentration of HF was 3.7 μL/L after heating for 2 hours and it was decreased to 2.3 μL/L 6 hours later, the tendency of which was the same with that at 400˚C and 450˚C. The concentration of CO was below detection limits. SF6 will decompose and produce 4.6 μL/L H2S and 3.4 μL/L HF significantly after heating for 1 hour, and will produce 33.4 μL/L SO2 significantly after heating for 2 hours. The concentrations of fluoride 1 (probable SF4) and fluoride 2 (probable S2F10) are not significantly increased with the time of heating, while the concentration of SO2 is significantly increased.
4. Conclusions
SF6 is very stable and not significantly decomposed at 200˚C, 250˚C and 300˚C. SF6 will decompose and produce 0.3 μL/L H2S and 0.5 μL/L HF after heating for 1 hour at 350˚C, and it will produce 7.2 μL/L SO2, 1.6 μL/L H2S and 1.9 μL/L HF after heating for 5 hours. At 400˚C, SF6 will decompose and produce 0.6 μL/L H2S and 0.9 μL/L HF after heating for 1 hour, and it will produce 17.6 μL/L SO2, 3.3 μL/L H2S and 2.4 μL/L HF after heating for 5 hours. At 450˚C, SF6 will decompose and produce 0.8 μL/L H2S and 1.0 μL/L HF after heating for 1 hour, and it will produce 13.3 μL/L SO2, 1.5 μL/L H2S and 1.5 μL/L HF after heating for 2 hours. At 500˚C, SF6 will decompose and produce 4.6 μL/L H2S and 3.4 μL/L HF after heating for 1 hour, and it will produce 33.4 μL/L SO2, 7.0 μL/L H2S and 3.7 μL/L HF significantly after heating for 2 hours. According to above simulated experiments, SF6 is beginning to format significant SF6 byproducts at 350˚C, and it is more easily to be decomposed at higher temperature. SF6 will be decomposed to format HF, which is strongly corrosive and whose concentration is likely to decrease when it is above a certain concentration.
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