Decomposition Characteristics of SF 6 under Arc Discharge and the Effects of Trace H 2 O, O 2 , and PTFE Vapour on Its By-Products

: The research on decomposition characteristics of SF 6 and its by-products have great signiﬁcance to the operation, maintenance, condition assessment and fault diagnosis of power equipment. In this paper, the particle composition models of SF 6 , SF 6 /polytetraﬂuoroethylene (PTFE), SF 6 /PTFE/O 2 , SF 6 /PTFE/H 2 O, and SF 6 /PTFE/O 2 /H 2 O were established by using Gibbs free energy minimization method, and the effects of trace H 2 O and O 2 impurities and PTFE vapour on SF 6 by-products were studied by the models. In order to verify the correctness of the simulation results, a series of breaking experiments were carried out on a 40.5 kV SF 6 circuit breaker, and a gas chromatograph was used to detect and analyse the SF 6 by-products. It was found that when PTFE vapour is involved in the arc plasma, the main by-product after arc quenching is CF 4 , and the molar fractions of C 2 F 6 and C 3 F 8 are very low. When O 2 is involved, the main by-products are SOF 2 , SO 2 and SO 2 F 2 , and a small amount of CO and CO 2 was also produced. When H 2 O is involved, the main by-products in simulation are SOF 2 , SO 2 and HF, and a small amount of SO 2 , CO 2 , CO, SO 2 F 2 and H 2 was also produced. The experimental results are in good agreement with the above results.


Introduction
SF 6 is a greenhouse gas. It is one of the six gases included in the Kyoto Protocol aimed at reducing greenhouse gas emissions. The global warming potential (GWP) of SF 6 is 23,900 times greater than CO 2 according to the 2013 report of Intergovernmental Panel on Climate Change (IPCC), and it has a lifetime in the atmosphere of 3200 years. However, SF 6 has excellent insulation performance, arc quenching performance, and molecular stability. It is widely used in high voltage circuit breakers [1,2]. The high voltage SF 6 circuit breaker will inevitably produce a high temperature and high energy arc in the process of breaking currents. It will cause SF 6 to decompose and form SF 6 arc plasma. Many sulfur fluorides such as SF 5 , SF 4 , SF 3 , SF 2 , SF, S, and F will be produced in the arc plasma. As the arc extinguishes, the arc temperature will gradually decrease, and these species will recombine into SF 6 molecules [3,4]. However, there are trace H 2 O and O 2 impurities in the circuit breaker, and the nozzle made by polytetrafluoroethylene (PTFE) material will be ablated by arc to produce trace PTFE vapour during the breaking process. The H 2 O, O 2 , and PTFE impurities will participate in the reaction of SF 6 arc plasma to form various by-products such as SOF 2 , SO 2 , SO 2 F 2 , CF 4 , C 2 F 6 , C 3 F 8 , CO 2 , CO, and HF [5,6].
On the one hand, some by-products such as SOF 2 and HF are corrosive and toxic, thus posing considerable threat to the safe and stable operation of equipment and health of the operation and maintenance personnel [7]. On the other hand, there is a close relationship between some by-products and the discharge faults in the equipment, so the decomposition component analysis (DCA) technology can be used in fault detection and condition assessment of SF 6 power equipment [8,9]. Therefore, the decomposition characteristics of SF 6 under arc discharge and the effects of trace H 2 O, O 2 , and PTFE vapour on its by-products have attracted extensive attention.
Many scholars have carried out a lot of research on this problem and achieved rich results. In terms of the experiments, Boudene et al. studied the decomposition products in detail under the condition of voltage of 60 kV, a current of 4.5 kA and arcing time 40-80 ms. They found that the gas production rate of SOF 2 and SO 2 F 2 and arc energy is almost linear [10]. Belmadani and Casanvas examined the SF 6 byproducts of power arc discharges using gas chromatography [11,12]. The maximum arc current reached 8.3 kA, and the primary byproducts were SOF 2 , SO 2 F 2 , SO 2 , and CF 4 . The concentrations of these byproducts decreased in the following order: SOF 2 +SO 2 > CF 4 > SO 2 F 2 . Our previous work used a circuit breaker to study the influence of trace H 2 O and O 2 on SF 6 by-products [13]. We found that the increase of the concentration of H 2 O and O 2 will increase the production of SOF 2 +SO 2 , and the concentration of CF 4 is hardly affected by the concentration of H 2 O and O 2 . Andrzej pelc studied the generation of negative ions from SF 6 gas by means of hot surface ionization. He found eight ion species: SF 5 -, F -, SF 6 -, SF 4 -, SF 3 -, SF 2 -, SFand F 2 -, with ion current intensities ratios of 1000:200:100:10:5:0.5:0.5:0.05. He also found the optimal temperatures at which the maximum of the ion current intensity is observed were estimated in the 1830-2000 ± 10 • C range [14].
In terms of the simulations, Brand et al. studied the particle compositions of SF 6 arc plasma at standard atmospheric pressure [15]. Chervy and Gleizes considered the influence of copper vapour on the particle compositions and proposed a particle composition model of an SF 6 /Cu mixture [16]. Coufal studied the effect of PTFE ablation on particle compositions, and the Gibbs free energy minimization method was first used in the calculation [17]. Wang et al. studied the arc plasma particle compositions of an SF 6 /CF 4 gas mixture [18]. Up to now, there are few reports about SF 6  In order to solve this problem, the arc plasma models of SF 6 , SF 6 /PTFE, SF 6 /PTFE/H 2 O, and SF 6 /PTFE/O 2 are established by using the Gibbs free energy minimization method in this paper. The effects of trace H 2 O, O 2 , and PTFE vapour on SF 6 by-products was studied by these models. On this basis, a comprehensive model of SF 6 /PTFE/H 2 O/O 2 gas mixture was finally established. The research on the particle composition of SF 6 arc plasma is improved. In order to verify the correctness of the calculation results, a series of breaking experiments were carried out on a 40.5 kV SF 6 circuit breaker, and a gas chromatograph was used to detect and analyse the SF 6 by-products. It was found that the simulation results are in good agreement with the experimental results.

Methods
Under the assumption of local thermodynamic equilibrium (LTE), the physical parameters of the system are only functions of temperature and pressure. The minimization of Gibbs free energy means that the Gibbs function of the equilibrium state is the smallest when the temperature and pressure are constant [17]. In this paper, the particle composition of SF 6 arc plasma is studied by this method. In pure SF 6 arc plasma, 12 kinds of particles such as SF 6 , SF 5 , SF 4 , SF 3 , SF 2 , SF, S, S 2 , F, F 2 , S 2 F 10 , and FSSF were considered. In SF 6 /PTFE arc plasma, 12 kinds of particles such as C 3 F 8 , C 2 F 6 , C 2 F 5 , C 2 F 4 , C 2 F 3 , C 2 F 2 , C 2 F, C, CF, CF 2 , CF 3 , and CF 4 were added. In SF 6 /PTFE/O 2 arc plasma, 10 kinds of particles such as O, O 2 , SOF 2 , SO 2 F 2 , SO, SO 2 , CO 2 , CO, COF, and COF 2 were added. In SF 6 /PTFE/H 2 O arc plasma, 9 kinds of particles such as H 2 O, OH, H, HF, H 2 , CH 4 , CH 3 , CH 2 , and CH were added. In SF 6 /PTFE/H 2 O/O 2 arc plasma, all the 43 particles mentioned above were considered. The thermodynamic data of all particles, such as standard enthalpy of formation, entropy, and specific heat at constant pressure can be obtained in the JANAF database [19]. All calculations were completed in the Chemkin software. In order to verify the correctness of the calculation results, a series of breaking experiments were carried out on a 40.5 kV SF 6 circuit breaker. The rated voltage was 40.5 kV, the rated current was 2.5 kA, the rated frequency was 50 Hz, the rated air pressure was 0.6 MPa, the rated short circuit breaking current was 31.5 KA, and the volume of single-phase air chamber was 30.0 L. Figure 1 shows the experimental arrangement. Before the experiment, SF 6 gases with different concentrations of H 2 O and O 2 were filled into three chambers respectively. Sinusoidal current was generated by an L-C circuit with pre-charged capacitor banks. Arc current was about 10 kA. Arcing time with one current half-wave was 6.5 to 10.0 ms due to the mechanical dispersion of the actuator. Arc voltage and arc current were measured by a high voltage probe and Rogowski coil respectively, and recorded by oscilloscope. After each breaking experiment, the SF 6 by-products in the chamber were analysed by gas chromatograph.
added. In SF6/PTFE/H2O/O2 arc plasma, all the 43 particles mentioned above were considered. The thermodynamic data of all particles, such as standard enthalpy of formation, entropy, and specific heat at constant pressure can be obtained in the JANAF database [19]. All calculations were completed in the Chemkin software.
In order to verify the correctness of the calculation results, a series of breaking experiments were carried out on a 40.5 kV SF6 circuit breaker. The rated voltage was 40.5 kV, the rated current was 2.5 kA, the rated frequency was 50 Hz, the rated air pressure was 0.6 MPa, the rated short circuit breaking current was 31.5 KA, and the volume of singlephase air chamber was 30.0 L. Figure 1 shows the experimental arrangement. Before the experiment, SF6 gases with different concentrations of H2O and O2 were filled into three chambers respectively. Sinusoidal current was generated by an L-C circuit with precharged capacitor banks. Arc current was about 10 kA. Arcing time with one current halfwave was 6.5 to 10.0 ms due to the mechanical dispersion of the actuator. Arc voltage and arc current were measured by a high voltage probe and Rogowski coil respectively, and recorded by oscilloscope. After each breaking experiment, the SF6 by-products in the chamber were analysed by gas chromatograph. This gas chromatograph was equipped with a hydrogen ionisation detector (FID) and a pulsed flame photometric detector (PFPD) to detect SF6 by-products. The carrier gas was He with purity over 99.99%, the output pressure was 0.5-0.6 MPa, the injection temperature was 150 °C, the injection volume was 250 μL, the split ratio was 20:1, the column temperature was 50 °C for 5 min, and 10 °C/min is raised to 200 °C for 10 min. Figure 2 shows the test result of gas chromatograph. The detected gases in channel A were H2, O2, N2, CO, CH4, CO2, CF4, and C2F6. The detected gases in channel B were SF6, SOF2, SO2F2, SO2, COS, C3F8, and CS2. Trace H2O content was measured using a SF6 mirror dew point instrument. This instrument is often used in the field detection of trace H2O content in SF6 circuit breaker, with the advantages of fast response and high precision. This gas chromatograph was equipped with a hydrogen ionisation detector (FID) and a pulsed flame photometric detector (PFPD) to detect SF 6 by-products. The carrier gas was He with purity over 99.99%, the output pressure was 0.5-0.6 MPa, the injection temperature was 150 • C, the injection volume was 250 µL, the split ratio was 20:1, the column temperature was 50 • C for 5 min, and 10 • C/min is raised to 200 • C for 10 min. Figure 2 shows the test result of gas chromatograph. The detected gases in channel A were H 2 , O 2 , N 2 , CO, CH 4 , CO 2 , CF 4 , and C 2 F 6 . The detected gases in channel B were SF 6 , SOF 2 , SO 2 F 2 , SO 2 , COS, C 3 F 8 , and CS 2 . Trace H 2 O content was measured using a SF 6 mirror dew point instrument. This instrument is often used in the field detection of trace H 2 O content in SF 6 circuit breaker, with the advantages of fast response and high precision.
The specific experimental settings are shown in Tables 1 and 2. In order to research the effect of trace H 2 O on SF 6 by-products after the arc was extinguished and verify the correctness of the simulation results, the three gas chambers of the circuit breaker were filled with SF 6 gases with different H 2 O concentration. The concentration of H 2 O before the breaking experiment in A, B, and C chambers were 106 ppm, 748 ppm, and 1131 ppm, respectively. The breaking current was 10 kA and the times of breaking is 5. The setting of the breaking experiment with different O 2 concentration was similar to that of H 2 O. The specific experimental settings are shown in Tables 1 and 2. In order to research the effect of trace H2O on SF6 by-products after the arc was extinguished and verify the correctness of the simulation results, the three gas chambers of the circuit breaker were filled with SF6 gases with different H2O concentration. The concentration of H2O before the breaking experiment in A, B, and C chambers were 106 ppm, 748 ppm, and 1131 ppm, respectively. The breaking current was 10 kA and the times of breaking is 5. The setting of the breaking experiment with different O2 concentration was similar to that of H2O.  Figure 3 shows the particle composition of pure SF6 arc plasma at 0.6 MPa. It can be found that SF6 molecules begin to decompose at about 1000 K and forms SF5, SF4 and F atoms first. With the increase of arc temperature, SF3, SF2, and SF begin to form. It should be noted that the temperature required for the formation of SF5, SF4, SF3, SF2, and SF increases step-by-step, and the maximum mole fractions of SF4 and SF2 is significantly greater than that of SF5, SF3, and SF. The main reason is that the decomposition process of SF6 is a process of gradually breaking the S-F bond to form a lower-level low fluorine sulfide, and the S-F bond energy of SF4 and SF2 is higher than that of SF5, SF3, and SF. When the arc temperature is greater than 3000 K, the mole fractions of all sulfur fluorides begin to decrease, and gradually decompose into S and F atoms. It should be noted that the arc temperature decreases gradually during the arc decay process. When the arc temperature decreases below 1000 K, all sulfur fluorides and atoms recombine into SF6. The

Results and Discussion
3.1. Particle Composition of SF 6 and SF 6 /PTFE Figure 3 shows the particle composition of pure SF 6 arc plasma at 0.6 MPa. It can be found that SF 6 molecules begin to decompose at about 1000 K and forms SF 5 , SF 4 and F atoms first. With the increase of arc temperature, SF 3 , SF 2 , and SF begin to form. It should be noted that the temperature required for the formation of SF 5 , SF 4 , SF 3 , SF 2 , and SF increases step-by-step, and the maximum mole fractions of SF 4 and SF 2 is significantly greater than that of SF 5 , SF 3 , and SF. The main reason is that the decomposition process of SF 6 is a process of gradually breaking the S-F bond to form a lower-level low fluorine sulfide, and the S-F bond energy of SF 4 and SF 2 is higher than that of SF 5 , SF 3 , and SF. When the arc temperature is greater than 3000 K, the mole fractions of all sulfur fluorides begin to decrease, and gradually decompose into S and F atoms. It should be noted that the arc temperature decreases gradually during the arc decay process. When the arc temperature decreases below 1000 K, all sulfur fluorides and atoms recombine into SF 6 . The results of this part show good agreement with previous work [20], which proves the reliability of our calculation results. results of this part show good agreement with previous work [20], which proves the reliability of our calculation results.  Figure 4 shows the particle composition of SF6/PTFE arc plasma at 0.6 MPa. Compared with pure SF6 arc plasma, CF4, CF3, CF2, S2F10, etc. appear in particle composition. CF4 and CF3 are formed by the combination of CF2 radicals produced by the decomposition of PTFE and the F atom produced by SF6 decomposition. The formation of CF4 will lead to the lack of F atoms in the arc plasma, so some SF5 radicals cannot obtain the F atoms to form SF6, and these SF5 radicals will combine with each other to form S2F10. It should be noted that as the arc temperature gradually decreases to room temperature, almost all PTFE vapour is converted into CF4, and C2F6 and C3F8 are hardly generated. The molar fraction of C2F6 and C3F8 are both less than 10 −6 .     Figure 4 shows the particle composition of SF 6 /PTFE arc plasma at 0.6 MPa. Compared with pure SF 6 arc plasma, CF 4 , CF 3 , CF 2 , S 2 F 10 , etc. appear in particle composition. CF 4 and CF 3 are formed by the combination of CF 2 radicals produced by the decomposition of PTFE and the F atom produced by SF 6 decomposition. The formation of CF 4 will lead to the lack of F atoms in the arc plasma, so some SF 5 radicals cannot obtain the F atoms to form SF 6 , and these SF 5 radicals will combine with each other to form S 2 F 10 . It should be noted that as the arc temperature gradually decreases to room temperature, almost all PTFE vapour is converted into CF 4 , and C 2 F 6 and C 3 F 8 are hardly generated. The molar fraction of C 2 F 6 and C 3 F 8 are both less than 10 −6 .
Energies 2021, 14, x FOR PEER REVIEW 5 of 13 results of this part show good agreement with previous work [20], which proves the reliability of our calculation results.  Figure 4 shows the particle composition of SF6/PTFE arc plasma at 0.6 MPa. Compared with pure SF6 arc plasma, CF4, CF3, CF2, S2F10, etc. appear in particle composition. CF4 and CF3 are formed by the combination of CF2 radicals produced by the decomposition of PTFE and the F atom produced by SF6 decomposition. The formation of CF4 will lead to the lack of F atoms in the arc plasma, so some SF5 radicals cannot obtain the F atoms to form SF6, and these SF5 radicals will combine with each other to form S2F10. It should be noted that as the arc temperature gradually decreases to room temperature, almost all PTFE vapour is converted into CF4, and C2F6 and C3F8 are hardly generated. The molar fraction of C2F6 and C3F8 are both less than 10 −6 .     Figure 5 shows the change of CF 4 , C 2 F 6 , and C 3 F 8 concentrations with the breaking times. The breaking current is about 10 kA. It can be found that the concentration of CF 4 is much higher than that of C 2 F 6 and C 3 F 8 , and shows an obvious increasing trend with the breaking times, while C 2 F 6 and C 3 F 8 have no obvious change. After five breaking

Particle Composition of SF6 and SF6/PTFE/O2
Figures 6 and 7 show the particle composition of SF6/PTFE/O2 arc plasma with different O2 concentrations. Compared with SF6/PTFE arc plasma, a large number of particles containing O element such as SOF2, SO2F2, COF2, SO2, CO, and CO2 appear in arc plasma. CO appears in the temperature range of 2000 K-5000 K. CO2 and SO2 appear in the temperature range of >1500 K. COF2 appears in the temperature range of 1000 K-4500 K. As the arc temperature gradually decreases to room temperature, the molar fraction of these species will be reduced to <10 −6 . SOF2 and SO2F2 can appear in arc plasma when the temperature is less than 1000 K, and their molar fraction is much higher than that of CO2, SO2 and COF2. Therefore, it can be concluded that the participation of a small amount of O2 will promote the formation of SOF2, SO2F2, COF2, SO2, CO, and CO2. After the arc is extinguished, the concentration of SOF2 and SO2F2 will be higher than that of COF2, SO2, CO, and CO2.  Compared with SF 6 /PTFE arc plasma, a large number of particles containing O element such as SOF 2 , SO 2 F 2 , COF 2 , SO 2 , CO, and CO 2 appear in arc plasma. CO appears in the temperature range of 2000 K-5000 K. CO 2 and SO 2 appear in the temperature range of >1500 K. COF 2 appears in the temperature range of 1000 K-4500 K. As the arc temperature gradually decreases to room temperature, the molar fraction of these species will be reduced to <10 −6 . SOF 2 and SO 2 F 2 can appear in arc plasma when the temperature is less than 1000 K, and their molar fraction is much higher than that of CO 2 , SO 2 and COF 2 . Therefore, it can be concluded that the participation of a small amount of O 2 will promote the formation of SOF 2 , SO 2 F 2 , COF 2 , SO 2 , CO, and CO 2 . After the arc is extinguished, the concentration of SOF 2 and SO 2 F 2 will be higher than that of COF 2 , SO 2 , CO, and CO 2 . Figure 8 shows the changes of SOF 2 , SO 2 , SO 2 F 2 , and CO concentrations with the breaking times under different O 2 concentration. As shown in Figure 8a, the concentration of SOF 2 increases with the breaking times. When the O 2 concentrations are 59 ppm, 748 ppm and 1130 ppm, respectively, the concentrations of SOF 2 can reach 761 ppm, 983 ppm and 1121 ppm, respectively, after five breaking experiments. This indicates that the increase of O 2 concentration can promote the formation of SOF 2 . As shown in Figure 8b, the concentration of SO 2 increases with the breaking times. The concentrations of SO 2 were 3.1 ppm, 9.0 ppm and 9.7 ppm, respectively, after five breaking experiments. As shown in Figure 8c, the concentration of SO 2 F 2 increases with the breaking times. After five breaking experiments, the concentrations of SO 2 F 2 are 0.4 ppm, 7.1 ppm and 9.8 ppm, respectively. This indicates that the increase of O 2 concentration can significantly promote the formation of SO 2 F 2 . Figure 8d shows the change of CO with the breaking times. It can be seen that the concentration of CO is very low, and the highest is only 1.0 ppm. Due to the limitation of experimental conditions and the accuracy of testing equipment, when O 2 are 736 ppm and 1202 ppm, CO has no significant regularity. When O 2 is 59 ppm, the concentration of CO is obviously lower than the former two cases. This indicates that the increase of O 2 concentration can also promote the formation of CO, but this phenomenon is not obvious.   Figure 8a, the concentration of SOF2 increases with the breaking times. When the O2 concentrations are 59 ppm, 748 ppm and 1130 ppm, respectively, the concentrations of SOF2 can reach 761 ppm, 983 ppm and 1121 ppm, respectively, after five breaking experiments. This indicates that the increase of O2 concentration can promote the formation of SOF2. As shown in Figure 8b, the concentration of SO2 increases with the breaking times. The concentrations of SO2 were 3.1 ppm, 9.0 ppm and 9.7 ppm, respectively, after five breaking experiments. As shown in Figure 8c, the concentration of SO2F2 increases with the breaking times. After five breaking experiments, the concentrations of SO2F2 are 0.4 ppm, 7.1 ppm and 9.8 ppm, respectively. This indicates that the increase of O2 concentration can significantly promote the formation of SO2F2. Figure 8d shows the change of CO with the breaking times. It can be seen    Figure 8a, the concentration of SOF2 increases with the breaking times. When the O2 concentrations are 59 ppm, 748 ppm and 1130 ppm, respectively, the concentrations of SOF2 can reach 761 ppm, 983 ppm and 1121 ppm, respectively, after five breaking experiments. This indicates that the increase of O2 concentration can promote the formation of SOF2. As shown in Figure 8b, the concentration of SO2 increases with the breaking times. The concentrations of SO2 were 3.1 ppm, 9.0 ppm and 9.7 ppm, respectively, after five breaking experiments. As shown in Figure 8c, the concentration of SO2F2 increases with the breaking times. After five breaking experiments, the concentrations of SO2F2 are 0.4 ppm, 7.1 ppm and 9.8 ppm, respectively. This indicates that the increase of O2 concentration can significantly promote the formation of SO2F2. Figure 8d shows the change of CO with the breaking times. It can be seen In comparison, the concentration of SOF 2 is much higher than that of SO 2 , SO 2 F 2 and CO. After five breaking experiments, the concentration of SOF 2 can reach 1000 ppm, and the concentrations of SO 2 and SO 2 F 2 are both about 10 ppm, while the concentration of CO is lower than 2 ppm. This phenomenon indicates that when O 2 is involved in the arc plasma reaction, SOF 2 will be the main by-product after the arc is extinguished, and the concentrations of SO 2 and SO 2 F 2 will also increase significantly. In comparison, the concentration of SOF2 is much higher than that of SO2, SO2F2 and CO. After five breaking experiments, the concentration of SOF2 can reach 1000 ppm, and the concentrations of SO2 and SO2F2 are both about 10 ppm, while the concentration of CO is lower than 2 ppm. This phenomenon indicates that when O2 is involved in the arc plasma reaction, SOF2 will be the main by-product after the arc is extinguished, and the concentrations of SO2 and SO2F2 will also increase significantly.   , CH, and H appear in arc plasma, and the molar fractions of SOF 2 and SO 2 F 2 were significantly affected. H 2 appears in the temperature range of 3500 K-5000 K. H appears in the temperature range of 2500 K-5000 K. HF can appear in the whole temperature range and its molar fraction is always greater than 10 −1 . SOF 2 can appear in the temperature range of 300 K-5000 K, and its molar fraction increases with the decrease of arc temperature. When the temperature decreases to room temperature, the molar fraction of SOF 2 can reach 10 −1 . SO 2 F 2 can appear in the temperature range of 300 K-3000 K, and its molar fraction relatively low. It can be concluded that the participation of a small amount of H 2 O will promote the formation of HF, H 2 , SOF 2 , SO 2 F 2 , COF 2 , Energies 2021, 14, 414 9 of 13 SO 2 , CO, and CO 2 . After the arc is extinguished, the concentration of HF and SOF 2 will be significantly higher than that of SO 2 F 2 , COF 2 , SO 2 , CO, and CO 2 .
K-5000 K. H appears in the temperature range of 2500 K-5000 K. HF can appear in the whole temperature range and its molar fraction is always greater than 10 −1 . SOF2 can appear in the temperature range of 300 K-5000 K, and its molar fraction increases with the decrease of arc temperature. When the temperature decreases to room temperature, the molar fraction of SOF2 can reach 10 −1 . SO2F2 can appear in the temperature range of 300 K-3000 K, and its molar fraction relatively low. It can be concluded that the participation of a small amount of H2O will promote the formation of HF, H2, SOF2, SO2F2, COF2, SO2, CO, and CO2. After the arc is extinguished, the concentration of HF and SOF2 will be significantly higher than that of SO2F2, COF2, SO2, CO, and CO2. SOF2 and SO2F2 were significantly affected. H2 appears in the temperature range of 3500 K-5000 K. H appears in the temperature range of 2500 K-5000 K. HF can appear in the whole temperature range and its molar fraction is always greater than 10 −1 . SOF2 can appear in the temperature range of 300 K-5000 K, and its molar fraction increases with the decrease of arc temperature. When the temperature decreases to room temperature, the molar fraction of SOF2 can reach 10 −1 . SO2F2 can appear in the temperature range of 300 K-3000 K, and its molar fraction relatively low. It can be concluded that the participation of a small amount of H2O will promote the formation of HF, H2, SOF2, SO2F2, COF2, SO2, CO, and CO2. After the arc is extinguished, the concentration of HF and SOF2 will be significantly higher than that of SO2F2, COF2, SO2, CO, and CO2.
(a) (b)   Figure 11 shows the changes of SOF 2 , SO 2 , CO 2 , and CO concentrations with the breaking times under different H 2 O concentration. It can be found that the concentration of SOF 2 , SO 2 , CO 2 and CO increase with the breaking times. The increase of H 2 O concentration can promote the production of SOF 2 , SO 2 , CO 2 , and CO. For example, when the H 2 O concentrations are 106 ppm, 748 ppm and 1130 ppm, respectively, the concentrations of SOF 2 are 1097 ppm, 1130 ppm and 1253 ppm, respectively, after five breaking experi-ments. This conclusion is consistent with the calculation results of particle composition of SF 6 /PTFE/H 2 O arc plasma.
In comparison, the concentration of SOF 2 is much higher than that of SO 2 , CO 2 and CO. After five breaking experiments, the concentration of SOF 2 can reach 1000 ppm, while the concentration of SO 2 and CO 2 is lower than 20 ppm, and the concentration of CO is lower than 2 ppm. This phenomenon indicates that when H 2 O is involved in the arc plasma reaction, SOF 2 will be the main by-product after the arc is extinguished. This conclusion also shows good agreement with the calculation result.
Contrary to the simulation results, SO 2 F 2 and HF are hardly detected in this experiment. We believe the main reason is that H 2 O does not promote the formation of SO 2 F 2 obviously, and HF can react with the metal materials and solid insulation materials in the circuit breaker quickly, which makes it hardly detected. The particle composition model of arc plasma needs to be further improved. Figure 11 shows the changes of SOF2, SO2, CO2, and CO concentrations with the breaking times under different H2O concentration. It can be found that the concentration of SOF2, SO2, CO2 and CO increase with the breaking times. The increase of H2O concentration can promote the production of SOF2, SO2, CO2, and CO. For example, when the H2O concentrations are 106 ppm, 748 ppm and 1130 ppm, respectively, the concentrations of SOF2 are 1097 ppm, 1130 ppm and 1253 ppm, respectively, after five breaking experiments. This conclusion is consistent with the calculation results of particle composition of SF6/PTFE/H2O arc plasma.
In comparison, the concentration of SOF2 is much higher than that of SO2, CO2 and CO. After five breaking experiments, the concentration of SOF2 can reach 1000 ppm, while the concentration of SO2 and CO2 is lower than 20 ppm, and the concentration of CO is lower than 2 ppm. This phenomenon indicates that when H2O is involved in the arc plasma reaction, SOF2 will be the main by-product after the arc is extinguished. This conclusion also shows good agreement with the calculation result.
Contrary to the simulation results, SO2F2 and HF are hardly detected in this experiment. We believe the main reason is that H2O does not promote the formation of SO2F2 obviously, and HF can react with the metal materials and solid insulation materials in the circuit breaker quickly, which makes it hardly detected. The particle composition model of arc plasma needs to be further improved. It can be found by comparing the breaking experiments under different O2 and H2O concentrations that both H2O and O2 can promote the formation of SOF2. Under similar experimental conditions, the promotion effect of H2O on the formation of SOF2 is more obvious than that of O2. For example, when the concentration of H2O is 748 ppm, the concentration of SOF2 can reach 1130 ppm after five breaking experiments, with an average   Figure 12 shows the particle composition of 60% SF 6 + 20% CF 2 + 10% H 2 O + 10% O 2 arc plasma at 0.6 MPa. A total of 43 kinds of particles are considered in this model. The molar fractions of 33 kinds of particles are more than 10 −6 . The SF 6 by-products detected after the breaking experiment all appeared in this model, and the molar fractions of these species are relatively high. CO appears in the temperature range of 2000 K-5000 K, and the maximum molar fraction is 4.6%. CO 2 and SO 2 appear in the temperature range of 1250 K-5000 K, and the maximum molar fraction is 0.27% and 0.19%, respectively. CF 4 appears in the temperature range of 300 K-3500 K, and the maximum molar fraction is 20.0%. SOF 2 appears in the temperature range of 500 K-4500 K, and the maximum molar fraction is 13.8%. SO 2 F 2 appears in the temperature range of 300 K-2750 K, and the maximum molar fraction is 15.0%. HF appears in the whole temperature range, and the maximum molar fraction is 20.2%. It should be noted that SOF 2 , SO 2 F 2 , CF 4 , HF, O 2 , and SF 6 are the main particles when the arc temperature drops below 1000 K.

Conclusions
This paper established the arc plasma models of SF6 (12 particles are considered), SF6/PTFE (24 particles are considered), SF6/PTFE/O2 (34 particles are considered), SF6/PTFE/H2O (43 particles are considered), and SF6/PTFE/O2/H2O (43 particles are considered) by using the Gibbs free energy minimization method. The effects of trace H2O and O2 impurities and PTFE vapour on the SF6 by-products were studied. In order to verify the correctness of the simulation results, a series of breaking experiments were carried out on a 40.5 kV SF6 circuit breaker, and a gas chromatograph was used to detect and analyse the SF6 by-products. It was found that the experimental results are in good agreement with the simulation results. The primary conclusions are summarized below: (1) SF6 molecules began to decompose at about 1000 K and form SF5, SF4, SF3, SF2, SF, S,

Conclusions
This paper established the arc plasma models of SF 6 (12 particles are considered), SF 6 /PTFE (24 particles are considered), SF 6  impurities and PTFE vapour on the SF 6 by-products were studied. In order to verify the correctness of the simulation results, a series of breaking experiments were carried out on a 40.5 kV SF 6 circuit breaker, and a gas chromatograph was used to detect and analyse the SF 6 by-products. It was found that the experimental results are in good agreement with the simulation results. The primary conclusions are summarized below: (1) SF 6 molecules began to decompose at about 1000 K and form SF 5 , SF 4 , SF 3 , SF 2 , SF, S, and F with the increase of arc temperature. The maximum molar fractions of SF 4 and SF 2 were higher than SF 5 , SF 3 , and SF. As the arc temperature gradually decreases to room temperature, all low fluorine sulphides recombine into SF 6 . (2) When PTFE vapour was involved in the arc plasma, the main by-product after arc quenching was CF 4 , and the molar fractions of C 2 F 6 and C 3 F 8 were very low. After five breaking experiments, the concentration of CF 4 can reach 289.6 ppm, while the concentration of C 2 F 6 and C 3 F 8 was only 2.1 ppm and 0.2 ppm, respectively. The simulation results were in good agreement with the experimental results. (3) When O 2 was involved in the arc plasma, the main by-products were SOF 2 , SO 2 , and SO 2 F 2 . At the same time, a small amount of CO, and CO 2 was produced. After five breaking experiments, the concentration of SOF 2 can reach 1100 ppm, and the concentrations of SO 2 and SO 2 F 2 were both about 10 ppm, while the concentration of CO was lower than 2 ppm. (4) When H 2 O was involved in the arc plasma, the main by-products were SOF 2 , SO 2 , SO 2 F 2 , and HF. At the same time, a small amount of CO 2 , CO, and H 2 was produced.
After five breaking experiments, the concentration of SOF 2 can reach 1200 ppm, and the concentration of SO 2 and CO 2 was lower than 20 ppm, while the concentration of CO was lower than 2 ppm. Contrary to the simulation results, SO 2 F 2 and HF were hardly detected in this experiment. (5) When H 2 O and O 2 impurities and PTFE vapour were involved in the arc plasma together, the main by-products were SOF 2 , CF 4 , SO 2 F 2 , and HF. At the same time, a small amount of SO 2 , CO 2 , CO, and H 2 was be produced.