Journal of Electrical Engineering and Technology

The Korean Institute of Electrical Engineers (대한전기학회)
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Experimence Study of Trace Water and Oxygen Impact on SF6 Decomposition Characteristics Under Partial Discharge

  • Zeng, Fuping (School of Electrical Engineering, Wuhan University) ;
  • Tang, Ju (School of Electrical Engineering, Wuhan University) ;
  • Xie, Yanbin (State Grid Chongqing Electric Power Company, Shiqu Power Supply Company) ;
  • Zhou, Qian (State Grid Chongqing Electric Power Company) ;
  • Zhang, Chaohai (Wuhan NARI Limited Company of State Grid Electric Power Research Institute)
  • Received : 2014.10.05
  • Accepted : 2015.03.31
  • Published : 2015.07.01
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Abstract

It is common practice to identify the insulation faults of GIS through monitor the contents of SF6 decomposed components. Partial discharges (PD) could lead to the decomposition of SF6 dielectric, so new reactions usually occur in the mixture of the newly decomposed components including traces of H2O and O2. The new reactions also cause the decomposed components to differ due to the different amounts of H2O and O2 even under the same strength of PD. Thus, the accuracy of assessing the insulation faults is definitely influenced when using the concentration and corresponding change of decomposed components. In the present research, a needle-plate electrode was employed to simulate the PD event of a metal protrusion insulation fault for two main characteristic components SO2F2 and SOF2, and to carry out influence analysis of trace H2O and O2 on the characteristic components. The research shows that trace H2O has the capability of catching an F atom, which inhibits low-sulfide SFx from recombining into high-sulfide SF6. Thus, the amount of SOF2 strongly correlates to the amount of trace H2O, whereas the amount of SO2F2 is weakly related to trace H2O. Furthermore, the dilution effect of trace O2 on SOF2 obviously exceeds that of SO2F2.

Keywords

1. Introduction

Different patterns and strengths of partial discharge (PD) always occur when SF6 electrical equipment have some earlier insulation faults. High local electromagnetic energy caused by PD would cause SF6 to decompose into several kinds of low-fluoride sulfide SFx [1-4]. If trace levels of H2O and O2 exist in the equipment, the decomposed components would have further reactions with them and produce new characteristic components, such as SO2F2, SOF2, SO2, and so on [5-11].The concentration and variation regularity of these characteristic components have close relationship with the patterns of insulation faults, as well as the trace levels of H2O and O2 in gaseous SF6, making it more difficult to recognize the internal insulation deficiency when using them. Although the new gas SF6 contains few impurities, trace levels of H2O and O2 would enter the gas chamber as they are released from internal material or by penetration from the outside air over time [12]. There would be extra-trace levels of H2O and O2 inevitably existing in the SF6 electrical equipment. Hence, when extra-trace levels of H2O and O2 exist in SF6 gas, learning about the decomposition mechanism from both theory and experiment under PD are necessary; obtaining the influence regularity and influence mechanism of trace H2O and O2 on decomposed components is urgent. Furthermore, it is imperative to offer a amendment method considering the impacts of trace H2O and O2 so that all of the aforementioned methods would lay a solid theoretical foundation for the correct identification and evaluation of the internal insulation faults of SF6 electrical equipment when making use of the decomposed components.

R. J. Van Brunt from the U.S. National Bureau of Standards conducted a systematic research about the SF6 decomposition mechanism under PD. He studied the main source [13] of the O atom in SO2F2, SOF2, and SOF4 using the isotopic tracer technique under the condition of needle-plate electrode of corona discharge. His study pointed out that the O atom of SO2F2 mainly comes from O2, the O atom of SOF2 mainly comes from H2O, and the O atom of SOF4 comes from both O2 and H2O. However, the paper also claimed that SO2F2 obtains the O atom from H2O and SOF2 obtains the O atom from O2. Nevertheless, the Van Brunt research used the same fixed concentration of H2O and O2 without considering their levels of variation. According to Arrhenius’ law of chemical reaction kinetics and mass action law [14], the chemical reaction rate depends on the reaction temperature, reactant concentration, and catalyst. Although Derdouri studied the impact of diverse concentrations of H2O on SF6 gas under PD, there is a lack of explanation of the process [15].

In this paper, the authors take advantage of the PD decomposition platform in the laboratory and study the concentration of decomposed SF6 components and their variation trends under PD when different trace levels of H2O and O2 are mixed with SF6. Moreover, the mechanism of how the various concentrations of trace water and oxygen act on the characteristic decomposed components from the angle of related chemical reaction rate is explained. Considering that random factors may lead to unfavorable results during the experiment, statistical inference using ANOVA is used to investigate the degree of impact of trace H2O and O2 on decomposed characteristic components of SF6.

 

2. Decomposition Experiment and Quantitative Measurement

2.1 Experiment

This work studies the degree of influence of H2O and O2 on decomposed characteristic components of SF6 under PD from the statistical perspective. Hence, repeating the experiment independently n (here n=4) times under the same trace levels of H2O and O2 (the level is Ai ) and making sure that each experiment group has only one variable. The procedure suggests that the concentration of O2 is controlled below 100ppm (the rate of oxygen analyzer is 100ppm) in the experiment gas sample when the experiment on the influence of different trace levels of H2O was conducted. Likewise, the concentration of H2O is controlled below 150 ppm when the experiment on the influence of different trace levels of O2 was conducted. Experimental factors A (H2O) and B (O2) were subjected into seven experimental levels, as shown in Table 1.

Table 1.Factors affecting the by-product yields

Experiment material: SF6 (purity: 99.99%, H2O ≤ 100 ppm, O2 ≤100ppm), H2O, and O2 were used as experiment materials. The experiment was conducted in the multi-function electrical decomposition of SF6 equipment designed by our group, which is shown in Fig. 1 [16]. The main body of gas chamber is cylinder and both ends are oval structure to guarantee its air tightness. The volume of the chamber is approximately 10L and the maximum tolerance of air pressure can reach 0.5Mpa. The material of gas chamber is made of stainless steel for its corrosion resistance since the corrosive decomposed compositions of SF6 may be produced during the experiment. Lead the HV conductor in the gas chamber through HV bushing and the model of insulation faults is positioned in the middle of the gas chamber so that it can connect with bottle of the HV conductor. Gas inlet and gas outlet is equipped to fill in SF6 required in the experiment and gather the mixed gases sample after PD experiment.

Fig. 1.SF6 decomposition gas chamber

The gas chamber was filled with 0.2 MPa of SF6. The experimental system diagram is shown in Fig. 2. The needle-plate electrode is needed to simulate the common insulation fault (metal protrusion insulation fault) in the equipment. Moreover, the experiment made use of non-inductive detected impedance to send the pulse current signal to the WavePro 7100XL oscilloscope (Analog band: 1 GHz; sampling rate: 20 GHz; memory depth: 48 MB), which can monitor whether the PD is stable.

Fig. 2.Experimental system diagram

2.2 Experiment methods

This experiment uses needle-plate electrode model: spacing d is 10mm, curvature radius of needle tip is 0.3mm, diameter of ground electrode is 120mm and its thickness is 10mm. All the experiments are conducted at the same condition: the laboratory temperature is controlled at 15℃ and relative humility at 50%, to avoid the impacts of different temperatures and humidity and ensure the experimental results are comparable. The specific experimental requisition and steps are listed as follows:

(1) Measurement of the initial voltage Us of the intrinsic PD of the equipment (without putting insulation faults model) and the initial voltage U0 of the PD of the equipment (after putting the needle-plate electrodes). The respective measurements are Us=45 kV and U0= 15 kV. (2) The gas chamber is vacuumized and then filled with new gas, SF6, and vacuumized again. This process is repeated two or three times for purification.(3) For the experimental procedure on the influence of H2O on the decomposed characteristic components of SF6, step (4) is used. Otherwise, for the experiment on the influence of O2 on the decomposed characteristic components of SF6, step (5) is used.(4) The gas chamber is filled with the required amount of H2O by gas-syringe when the chamber is in vacuum condition, and subsequently heated in the equipment for 15 minutes. Another 15 minutes is spent to permit the H2O to undergo gasification and uniform distribution in the gas chamber. Step (6) follows.(5) The gas chamber is filled with the required amount of O2 when the chamber is in vacuum condition. Another 15 minutes is spent to permit the full volume of O2 to be uniformly distributed in the chamber.(6) The gas chamber is filled with SF6 equivalent to a pressure of 0.25 MPa and put aside for 24 hours, so that H2O (or O2) and SF6 are fully mixed. (7) The concentration of H2O and O2 in the mixed gas is measured. If the concentration fails to meet the experimental standards, the procedure goes back to step (2). When the measured concentrations have satisfied the standards, the gas sample is collected and its intrinsic components are analyzed. The respective concentrations of the constituent gases are also measured. Afterward, the mixed gas pressure is adjusted to 0.2 MPa.(8) The electrical wiring is connected as shown in Fig. 2. The experimental voltage is then gradually raised to 1.5U0 (22.5 kV) and the PD decomposition experiment is conducted for 10 hours under this voltage. This part of the procedure ensures that the contents of the characteristic components are stable. The oscilloscope is used to monitor the electrical discharge of the needle-plate electrode. (9) After 10 hours, the concentration of the different decomposed components in the collected gas sample is analyzed using gas chromatograph (GC). (10) After measuring all the experimental parameters, the gas chamber is vacuumized and put aside for 1 hour to enable absorption of the decomposed components by the surface of the electrode. The time allowance is also aimed to fully extricate the decomposed components attached on the chamber wall and inhibit the impact of the remaining components of the on-going experiment to the next experiment. The procedure is then repeated from step (2) for the next experiment.

2.3 Quantitative measurement of decomposed components

In the aforementioned experiments, the gas chromatograph (Varian CP-3800) was used to quantitatively measure the sample gas components produced by the discharge. The GC used the packed column Porapak QS and special capillary column CP-Sil 5 CB in parallel to separate the components in the mixture. Moreover, the chromatograph used PDHID double detectors (detection precision can reach up to 0.01ppm) to quantitatively detect each separated component. The chromatographic column was operated in the He (purity: 99.999%) carrier gas and the working conditions were flow rate, 2 mL/min; constant column temperature, 40 °C; sample size, 1 mL; and split ratio, 10:1. Under these conditions, the packed column could separate air, CF4, and CO2 effectively, and the special capillary column could separate air, SF6, SO2F2, SOF2, H2S, and SO2 effectively. Fig. 3 shows the standard chromatograph.

Fig. 3.Standard chromatogram

This study used the external standard method combined with the standard chromatogram to qualitatively and quantitatively detect the decomposed components of SF6. Since the SO2F2 and SOF2 are the most important characteristic decomposed components of SF6 [1, 3, 6-8, 13, 17, 18], the present study conducted intensive research on both. The raw data of each experiment as show in Table 2, Figs. 4 and 5 are the results of production amounts of the SO2F2 and SOF2 yields under PD at different levels of H2O and O2 in 10 hours (Each result is the production average value of four times repeated experiments under the same level of trace H2O or O2 ).

Table 2.The raw data of each experiment (ppm)

Fig. 4.Yield of by-products influenced by H2O

Fig. 5.Yield of by-products influenced by O2

 

3. Influence of H2O and O2 on Characteristic Components

Fig. 4 and 5 show that different levels of H2O and O2 contribute to different concentrations of SO2F2 and SOF2 produced by SF6 even under the same strength and time length of PD. Besides, the inevitable random factors which exert impacts on the experiment results should be considered in the experiment. Hence, the authors used ANOVA to study the impact of the various levels of trace H2O and O2 on characteristic components SO2F2 and SOF2 and indentify the main influence factors on the production of SO2F2 and SOF2.

3.1 Analysis of variance

ANOVA was introduced by the American statistician Fisher in an agricultural experiment [19]. Subsequently, the method has been widely used in other areas, especially in data analysis of industrial experiments where the method ANOVA shows that the total variance in the sample data can be divided into two parts: variance between groups and variance within groups . is caused by controllable influential factors of different levels and is caused by all random errors, that is . The size of the difference between groups and the size of the difference within groups are compared to identify the degree of impact of each level on the experimental results, where , , and can be achieved from Equ. (1) to (3):

In the above equations, xij is the j-th independent experimental result under the i-th concentration level of trace H2O and O2 (Ai), which means it is the result of the concentration of SOF2 or SOF2 when conducting the j-th experiment independently under the i-th concentration level of trace H2O and O2(Ai); represents the group mean of the product of SO2F2 or SOF2 under the i-th concentration level of trace H2O and O2 (Ai ) when conducting experiment n times independently; represents the mean of all the products of SO2F2 or SOF2 under the same influence factor (trace H2O or O2) in r experimental levels; and r is the number of influence factor (trace H2O and O2) concentration level. There are seven concentration levels in the present study, hence, r =7. n is the number of times the experiment is repeated under the same condition, and in the current study, the experiment is repeated 4 times under the same concentration of trace H2O and O2, thus, n =4.

If all the experimental factors (trace H2O and O2) have no significant influence in the experimental results, is almost equal to and statistics can prove that

In the equation, ( r−1), r(n−1) are the degrees of freedom of and , respectively. Furthermore, let and call them mean variance, so that equation (4) can be simplified as

ANOVA merely makes use of equation (5) to identify the degree of impact of each level (trace H2O or O2) on the experimental results by comparing the differences between groups and the differences within groups. Given the significance level α, when the calculated value of F is above the critical value F1−α(r − 1, rn − r ), the influence factor (trace H2O or O2) has significant influence on the experimental index (generation amount of SO2F2 and/or SOF2). Furthermore, the bigger the F value of the sample, the more significant influence the factor has on the experimental index. Hence, specific attention has to be accorded on such influence factor, and additionally such influence factors should be controlled during practical production.

3.2 Analysis of the significance level of influence factors

Significance level α is a critical probable value that represents the possibility to commit the fallacy of refusing the ‘assumption’ in a ‘statistical hypothesis test’ when using the sample information to draw conclusion. The smaller the value of α, the lesser the possibility of making the mistake of refusing the ‘assumption’. When analyzing data in the field of general industry, α = 0.05 ; in the field of biology and medicine, α = 0.01 . In the present study which examines the degree of influence of trace H2O and O2 on the main characteristic components of SF6 (SO2F2 and SOF2) under PD, high precision is required. Hence, a significance level of α = 0.01 is adopted. The table of F-distribution critical values shows that F0.99 (6, 21) = 3.81. The result is presented in Table 3.

Table 3.Analysis of variance result

By collating and analyzing the results from Fig. 4, Fig. 5, and Table 3, the findings indicate that both H2O and O2 exert an influence on the main characteristic components SO2F2 and SOF2. However, the products and the degree of influence of H2O and O2 are different. The differences include the fact that H2O has an obvious influence on SOF2 as the F value reaches 87.94, which is much larger than its influence on SO2F2. The formation of SOF2 is linearly proportional to the concentration of H2O, but the formation of SO2F2 has almost no relationship with the concentration of H2O. O2 has a significant effect on both SO2F2 and SOF2, but the impact on SOF2 is significantly higher than the impact on SO2F2.

The existence of H2O has an effect on the decomposition, as shown by Van Brunt. However, the effect caused by O2 I is different, which may be due to the fact that Van Brunt did his experiment under the same concentration of O2 and H2O without taking into account the concentration of the reactants on the relevant reaction when exploring the sources of O in SOF2 and SO2F2.

 

4. The influence Mechanism of H2O and O2 on the Characteristic Decomposed Components

Under PD, a series of characteristic components are produced by the reaction between the low-fluoride sulfide caused by the decomposition of SF6 and the trace levels of H2O and O2 mixed in the gas. Van Brunt carried out a more detailed study of the SF6 decomposition mechanism under PD with a needle-plate electrode mode. He proposed using the Plasma Chemical Model to explain the SF6 decomposition mechanism under PD [8]. He pointed out that under the effect of the high-energy electrons generated by the PD, the following reaction will occur in SF6:

The high-energy electrons lead to the decomposition of SF6 to produce low-fluoride sulfide SFx (x = 1 to 5). When no other impurities exist in SF6, SFx will recover quickly with the following reaction:

Here, k is the rate constants of the reaction. However, during the long-term operation of the SF6 gas-insulated equipment, it is inevitable that different amounts of ultra-impurity gases, such as H2O and O2, will appear in the chamber, released by the internal material of the device and the penetration of external H2O and O2 into the equipment. The impurities will lead to a series of more complex chemical reactions with SFx and generate SO2F2, SOF2, HF, SO2, and other compounds. Therefore, trace amounts of H2O and O2 play a key role in the production of SO2F2 and SOF2.

4.1 Analysis of the characteristic decomposed component with the impact of H2O

H2O will undergo the following reaction under PD when H2O exists:

Meanwhile, the following reactions will occur among H2O and SF6 decompositions:

The reaction rate constants k of the reaction (10), (11) and reaction (7), are in the same order of magnitude. On the other hand, the mass action law [14] tells us that the chemical reaction rate r depends on the reactant concentration, Ci , the stoichiometry number, bi , rate and the constant k, and the relationship is as follows:

Fortunately, under PD, for all of the chemical reactions where SF6 and H2O are involved, the stoichiometry number bi is one. This finding suggests that the reaction rate r is proportional to the concentration of the reactants. Therefore, when traces of H2O exist, H2O has a capture function of F equivalent to the inhibition of the recovery reaction SFx + (6−x) F → SF6. H2O inhibits the low-fluoride sulfide SFx (x = 1, 2, 3, 4, 5) composite to SF6, so that the concentration of SF4, SF5, and other components are increased. Additionally, under PD, the trace amount of H2O has always been small compared with a variety of low-fluoride sulfide SFx. Thus, the rates of reaction above are mainly determined by the concentration of H2O. The higher the concentration of H2O, the more severe the reaction and the more obvious the inhibition, as explained by the following reactions:

Formulas (8) to (11) and (13) to (14) show that when SF6 is mixed with H2O, H2O plays a role in providing OH and O. Hence, the formation of SOF4 is promoted. Meanwhile, reactions (6) to (11) and (13) to (15) constitute a comprehensive reaction, which is the means by which SO2F2 is generated. The generation capacity for SO2F2 is determined by trot reaction (15), and with the increasing concentration of H2O, the amount of SO2F2 will slightly increase. However, with nearly 10 orders of magnitude of reaction rate in (15) than the rates of reaction in (10), (11), (13), and (14), and with increased concentration of H2O, the increase of SO2F2 is not obvious, as shown in Fig. 4(a).

On the other hand, the reaction between SF4 and H2O will occur as follows [8]:

The reactions in (16) and (12) show that when the concentration of H2O increases in SF6, it will promote the production of SOF2, as shown in Fig. 4 (b). However, the reaction rate constant k of reaction (16) is 2 orders of magnitude higher than reaction (15). Therefore, with the increased concentration of H2O in SF6, the rate of increase of the SOF2 produced is significantly higher than that of SO2F2, as shown in Fig. 4(b).

The effect of H2O on SF6 decomposition characteristics under PD can be summarized as follows:

(1) H2O has a capture function of F which inhibits the low-fluoride sulfide SFx (x = 1, 2, 3, 4, 5) composite to recombine with SF6, leading to the increase in the main low-fluoride sulfide SF5, SF4, and other components. The higher the concentration of H2O, the more severe the reaction and the more obvious the inhibition. (2) H2O provides OH and O for the generation of oxygen-containing-sulfur-fluoride compounds, and promotes the generation of the intermediate product SOF4. (3) H2O plays a role in promoting the generation of the final and stable oxygen-containing-sulfur- fluoride compounds, such as SO2F2 and SOF2.However, because the hydrolysis reaction rate of SF4 is nearly two orders of magnitude higher than SOF4, the chemical reaction rate r of SF6 and H2O under PD is proportional to the concentration of H2O. Thus, with the growth of the concentration of H2O in SF6, the growth rate of SOF2 is significantly higher than that of SO2F2. From the foregoing generalizations, the impact of H2O on SOF2 is significantly higher than the impact of SO2F2. Thus, the formation of SOF2 has a positive linear association with the concentration of H2O.

4.2 Analysis of the impact of O2 on the characteristic decomposed component

In the case of SF6 mixed with O2, under the impact of high-energy electrons produced by PD, in addition to reactions (6) and (7), the following reactions will occur:

Besides the fact that the free state O generated by reaction (17) will react with SF5 generated by PD and generate SOF4, the action below will happen and generate SOF4:

Then, both SOF4 and SF4 react with the H2O released by the electrodes and the internal wall of the decomposition equipment and generate SO2F2, and SOF2. While O2 exists, SF2 will be involved in the following reaction:

At present, the reaction rate constant of reaction (19) has not been found yet. Reference [8] has given the maximum rate constant k=5.0×10−16 cm3/s. Similarly, under PD, the stoichiometric number bi of reaction (14) and the chemical reactions SF6 and O2 are involved in are also equal to one, and O2 is always a small amount compared with a variety of low-fluoride sulfide SFx. Thus, the rate of reaction in (19) is proportional to the concentration of O2. The higher the concentration of O2, the more severe the reaction and the more SO2F2 is generated.

For SO2F2, it can be seen from Fig. 5 (a) that when the concentration of O2 mixed in SF6 is less than 460ppm, the formation of SO2F2 decreases with the increase of O2. When the concentration of O2 is higher than 460ppm, the concentration of SO2F2 is positively correlated with the concentration of O2 because an increase in the concentration of O2 in SF6 is equivalent to the dilution of SF2, SF4, SF5, and other low-sulfur and fluorine F. Thus, O2 plays an inhibitory effect on the reaction:

SFx (6 − x)F → SF6, x = 1 ~ 5

Although the concentrations of SF2, SF4, SF5, and other low-sulfur components increase with the discharge and promote the reaction in (18) ~ (19), with the increase in the concentration of O2, the concentration of H2O released by the electrodes and the internal wall of the decomposition equipment is diluted, making the rate of reactions in (8) to (11) and (15) decrease. Reaction (19) is at lower status when competing with reaction (14), (15) and (18), thus leading to the reduction in the amount of SO2F2 generated within 10 hours.

However, when the concentration of O2 is higher than 460ppm, with a further increase of O2, the rate of reaction in (18) undergoes a significant increase, and the rates of reaction in (8) to (11) and (15) are no longer significantly reduced. This time, since the concentration of O2 is high, reaction (19) plays a dominant role in the generation of SO2F2 when competing with reaction (14), (15), (18). Thus, when the concentration of O2 is above 460ppm, the yield of SO2F2 increases with the increase of O2. Hence, with a low concentration of O2 (the concentration of O2 < 460 ppm), the dilution of the inherent moisture in the device is the most important factor that affects the formation of SO2F2 and the stability of the decomposition under PD. Furthermore, the reactions in (8) to (11) and (15), (18) play leading roles in the formation of SO2F2. But at high concentration of O2 (the concentration of O2 > 460ppm), reaction (19) plays a leading role in the generation of SO2F2, as shown in Fig. 5(a).

For SOF2, its formation always decreases with the increase of O2, but the reduction is not obvious when the concentration of O2 is higher than 460ppm. The reason for the practically unobservable reduction is that with the increase of O2 mixed in SF6, SF2, SF4, SF5, and other low-sulfur, fluorine F undergoes a dilution process, thus playing an inhibitory effect on the reaction:

SFx (6 − x)F → SF6, x = 1 ~ 5

Although the concentrations of SF2, SF4, SF5, and other low-sulfur components increase with the discharge, as the concentration of O2 increases, the concentration of H2O released by the electrodes and the internal wall of the chamber is diluted at the same time. The rate of the reaction which plays a decisive role in the generation of SOF2 is shown in the following reaction:

SF4 + H2O → SOF2 + 2HF, k = 1.5 × 10−19 cm3/s

will decrease with the decrease in the concentration of H2O. This phenomenon is most prominent when H2O is diluted (the concentration of O2 < 460ppm). With further dilution of H2O (the concentration of O2 > 460ppm), the decrease in the reaction rate is not obvious, resulting in a significant decrease in the formation of SOF2 with the increase of O2 when O2 is at a low concentration. When O2 is at a high concentration, the decrease in the formation of SOF2 is not obvious with the increase of O2, as shown in Fig. 5(b).

In summary, the concentration of H2O in the reaction chamber decreases because of the dilution effect of O2, resulting in the reaction rate of a series of reactions in which H2O decreases and the yield of SOF2 decreases with the increase of O2. However, as O2 promotes the formation of SO2F2, at the same time, the formation of SO2F2 has a U-shaped relationship curve with the concentration of O2.

 

5. Conclusion

(1) Both H2O and O2 influence the main characterisitic components SO2F2 and SOF2 during PD, but their by-products and degrees of influence are different. The influence of H2O on SOF2 is the most significant and the formation of SOF2 has a positive linear relationship to the concentration of H2O while its influence on SO2F2 is not obvious. The concentration of O2 influences the formation of both SO2F2 and SOF2 while the influence is much more obvious on SOF2. (2) H2O has the ability to catch an F atom and to inhibit the low-fluoride sulfide SFx by recombining to SF6, which increases the concentration of SF5 and SF4. H2O offers OH and O for the formation of oxygenated-sulfur fluoride, which creates a favorable condition for the ultimate formation of SO2F2 and SOF2. However, the hydrolysis rate of SF4 is much higher than the hydrolysis rate of SOF4 (nearly two orders of magnitude higher), as a result, the increase in the rate of SOF2 is much higher than that of SO2F2 when the concentration of H2O increases. (3) When the concentration of O2 is low, the content of H2O in the equipment is the main factor which influences the fomation of SO2F2. When the concentration of O2 is high, the reaction SF2 + O2 → SO2F2 contributes mostly to the fomation of SO2F2, Thus, O2 is the main factor. As for SOF2, an increase of concentration would diminish the H2O concentration, in which case O2 becomes the most important factor in the decrease of SOF2. (4) The trace levels of H 2O and O2 play key roles on the formation of characteristic decomposed components of SF6 during PD and have significant influence on the products, so it is necessary to study the decomposition mechanism of SF6 under different concentrations of H2O and O2 under the long run PD, and research on different concentrations will help achieve sufficient knowledge on what influences the regularity in the reactions to propose correction methods accordingly. Acquiring sufficient knowledge on the decomposition mechanism and the factors that affect variation in the reactions under PD will lay a solid foundation in using decomposed components of SF6 to assess insulation status and will support related repair guidelines for gas insulated electrical equipment.

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