Research Progress on Synergistic Effect between Insulation Gas Mixtures

Synergetic effect is a special gas discharge phenomenon among insulating gas mixtures, which has important reference value for gas selection of future gas-insulated power equipment. The research progress and investigation methods of synergistic effect and insulation characteristics of different gas mixtures at home and abroad are reviewed in this chapter. The synergistic effect between different kinds of gas mixtures including SF 6 gas mixtures and some new insulation gases such as c-C 4 F 8 , CF 3 I, and C 4 F 7 N is presented. Combined with the results of multiple studies, it can be seen that the synergistic effect of the gas mixture has a certain relationship with the electronic transport parameters and discharge patterns. Besides, the synergistic effect of the same gas mixture may change with the change of external conditions such as gas pressure, voltage type, and electrode distance. of a This reviews the research status of the synergistic effect of insulating gas mixtures including SF 6 gas. It has been found that some achieve-ments have been made in the process and mechanism of synergistic effect, the types of synergistic effects under different conditions, and their influencing factors:


Introduction
With the continuous development of power industry, gas-insulated power equipment has been used more and more widely in electrical transmission system. Gas-insulated power equipment mainly includes gas-insulated transformer (GIT), gas-insulated transmission line (GIL), and gas-insulated switchgear (GIS). These gasinsulated equipment are noncombustible, nonexplosive, safe, and stable and have long maintenance period [1]. At present, SF 6 gas is the most widely used gas in power equipment, which has excellent insulation performance and arc extinguishing ability. As early as the 1940s, SF 6 gas has been used as insulation gas in power equipment [2]. Since the 1970s, researchers have studied the use of gas mixture including SF 6 and some buffer gases such as N 2 , CO 2 , Ar, He, air, etc. to replace pure SF 6 as the insulation medium [3][4][5]. With further study, researchers found that the insulation characteristics of gas mixtures containing SF 6 gas did not increase linearly according to the mixing ratio of SF 6 , and the electrical strength of the mixture was higher than the weighted average of the electrical strength of the two gas components. In 1980, Wootton and Chantry first used synergism to describe this phenomenon [6]. In the study of synergistic effect, scholars have found that there are also super synergistic effect and negative synergistic effect, which are shown in Figure 1.
Suppose that the composition of gas mixture is gas 1 and gas 2, the breakdown voltage of gas 1 is U 1 , the breakdown voltage of gas 2 is U 2 , and U 1 > U 2 , the breakdown voltage of gas mixture is U m , and K is the mixing ratio of gas 1 in the gas mixture. For super synergistic effect, U m > U 1 and U m > U 2 exist in some mixing ratios, as shown in curve a; for synergistic effect, U m > U 1 + U 2 , as shown in curve b; for linear relationship, U m = U 1 + U 2 , as shown in curve c; for negative synergistic effect, U m < U 1 + U 2 , as shown in curve d in Figure 1.
At first, researchers studied the synergistic effect of SF 6 gas mixture in order to solve the problems of high liquefaction temperature, sensitivity to electric field, and the high price of SF 6 gas [7]. However, with the deepening understanding of SF 6 , scholars have found that SF 6 is a strong greenhouse gas. It is estimated that the global annual production of SF 6 gas is more than 20,000 tons and 80% of SF 6 gas produced globally each year is used in the power industry. Although SF 6 has many advantages, its greenhouse effect on the earth cannot be ignored. The global warming potential (GWP) value of SF 6 gas is 23,900; it means that the emission of 1 kg SF 6 is equivalent to the emission of 23,900 kg of CO 2 . What is more serious is that SF 6 has a very stable chemical property, which is difficult to decompose after it spreads to the outside environment, and can exist for up to 3200 years. The environmental impact and greenhouse effect generated by SF 6 will continue to accumulate [8][9][10].
Affected by climate change, more and more international cooperation has been carried out to reduce greenhouse gas emissions, so as to curb global climate change and maintain the sustainable development of the environment. In the Kyoto protocol of the United Nations framework convention on climate change signed in Kyoto, Japan, in 1997, SF 6 has been clearly regulated as one of the six greenhouse gases and requires developed countries to freeze and reduce the total greenhouse gas emissions [11]. It means that the use of SF 6 in the industrial field will be increasingly restricted and pressured. Therefore, it is an urgent task to study a new gas insulation scheme to replace SF 6 .
Although at present there have been many studies about alternative SF 6 gas, no gas can thoroughly replace SF 6 gas in the form of a single gas; they all have to be mixed with buffer gas for industrial application. The reasonable use of synergistic effect can effectively improve dielectric strength of gas mixtures and reduce the use of insulation gas, so in this paper, the research progress and methods of synergistic effect with gas mixtures are introduced; the prospect and the difficulties in the field were also discussed. This paper is expected to provide help and reference for future research on synergistic effect.

Synergistic effect in SF 6 gas mixtures
In view of the synergistic effect and insulation strength of SF 6 mixture, scholars studied and analyzed it through theoretical calculation and experimental research. Figure 2a is a simple and intuitive calculation method proposed by Wieland et al. for the insulation strength of SF 6 gas mixture, and Figure 2b is the comparison of the calculation results with the actual values and weighted values [12].
Christophorou et al. thought that the preferred gas mixture should include effective electron-attaching gas and/or electron-slowing down gas [13]. The attachment cross section of electron-attaching components should be as wide as possible, or the attachment cross section of different gases is in different energy interval, so the attachment cross section of gas mixtures is also wide. The effect of electron-slowing down gas is to slow down the free electrons, making them easier to attach and reducing secondary ionization. Based on this theory, they believed that when an electronegative gas and a gas with a large dipole moment are mixed, the synergistic effect and insulation strength of the gas mixture will be better. Figure 3 shows the experimental results of SF 6 gas mixtures with CF 4 , CHF 3 , and 1,1,1-CH 3 CF 3 gas, and the electric dipole moments of these three gases are 0, 1.65D, and 2.32D, respectively.
From the figure, it can be seen that the breakdown voltage curve of SF 6 -CF 4 gas mixtures shows almost a linear trend, and the electric dipole moments of CF 4 is 0. When it comes to SF 6 -CHF 3 gas mixture, as the content of SF 6 gas increases, the breakdown voltage of the gas mixtures does not increase in a straight line, and there is synergistic effect that occurs. The electric dipole moments of 1,1,1-CH 3 CF 3 gas is 2.32D, which is the one with the largest electric dipole moment among the three gases; from Figure 3c it can be seen that the synergistic effect of this gas mixture is the most pronounced.
Okubo et al. investigated the partial discharge (PD) and breakdown characteristics of SF 6 -N 2 gas mixtures in order to analyze the relationship between electronegativity, additive gases, and the insulation strength [14]. They believed that the synergistic effect of gas mixture is related to the change of discharge form. Figure 4 shows the impulse PD of SF 6 -N 2 gas mixture at different pressure. The needle-plate electrode was used in the experiment, and the content of SF 6 was 10% and N 2 was 90%.
It can be seen that with the change of gas pressure, the development of impulse PD has changed. When the gas pressure is 0.1 MPa, as shown in Figure 5a, the brushlike partial discharge occurred around the needle electrode, and it can be thought of as streamer discharge. When the pressure increases to 0.2 MPa, it can be seen from Figure 5b that the development of discharge process becomes longer; streamer discharge turns into leader discharge. As the pressure continues to increase, when the pressure is 0.3 MPa, the PD type of the gas mixture is still the leader discharge, and the path of discharge development becomes shorter with the increase of the pressure.
Yamada et al. studied the insulation properties of a kind of gas mixture containing ultra-dilute SF 6 gas [15]. It has been found that trace SF 6 has a significant effect on the streamer discharge of the gas mixture. As the SF 6 content increases, the number of the discharge channels decreased significantly, and the number of channels that can reach the plane electrode also reduced, as shown in Figure 4. Except for the effect on discharge characteristics, the results show that the breakdown voltage and the PD voltage of SF 6 /N 2 gas mixture have a significant synergistic effect. Yamada T thought that the addition of trace SF 6 inhibits the development of streamer discharge process, which leads to synergistic effect.
Osmokrovic et al. conducted an in-depth study on the synergistic effect of SF 6 /N 2 under impulse voltage. The experimental results show that the synergistic  effect of SF 6 /N 2 gas mixture related to the rising rate of the impulse voltage. As the impulse voltage rise rate increases, the synergistic effect is gradually weakened. The synergistic effect of SF 6 gas mixture is very weak and almost completely disappears under some impulse voltage with very high rise rate [16]. The insulation characteristics of SF 6 and SF 6 /N 2 gas mixture under impulse voltage with different rise rate are shown in Figure 5. The rise rates of shock voltage in Figure 6a-c are 1, 50 and 800 kv/ms, respectively.
Based on the above phenomenon, Osmokrovic proposed that after adding N 2 to the SF 6 gas, electrons can make N 2 vibration and rotation dynamics excited or dissociated, this process will make electrons lose energy, and the effective temperature decreases, to realize the modulation of electron energy spectrum and increase the probability that the electrons are captured. The rise rate of impulse voltage has influence on the modulation of the electron energy spectrum, which in turn affects the synergistic effect of SF 6 gas mixture.
Hayakawa et al. studied the generation and development of PD characteristics of SF 6 /N 2 gas mixture under positive lightning impulse [17]. PD and breakdown characteristics with different SF 6 content are shown in Figure 7. Hayakawa proved through the film of the streak camera and ICCD that the discharge process in SF 6 / N 2 gas mixture did change with the increase of gas pressure. Streamer discharge and leader discharge are the two types of PD process in SF 6 /N 2 gas mixture, which all have relationship with gas pressure and SF 6 content. With the increase of gas pressure and the content of SF 6 gas, the leader discharge process gradually takes the leading position, and the streamer discharge process of gradually weakens.
Chen studied the discharge characteristics of SF 6 /N 2 gas mixture under DC voltage and lightning impulse in extremely uneven electric field. The experimental   results show that there was a significant decrease of breakdown voltage with the increase of gas pressure under DC voltage and the abnormal discharge characteristics pressure range of SF 6 /N 2 gas mixture is larger than SF 6 gas. This phenomenon leads to the breakdown voltage of SF 6 /N 2 gas mixture is higher than that of SF 6 gas. So under this abnormal range, positive synergistic effect occurs. However, under lighting impulse voltage, no abnormal discharge phenomenon was found, but positive synergistic effect still existed, as shown in Figure 8 [18].
Tagashira et al. believe that synergistic effects can be divided into three categories: a-synergistic effect (SF 6 + SiH 4 ), η-synergistic effect (SF 6 + c-C 4 F 8 ), and γ-synergistic effect (N 2 + CH 4 ). The research found that the curves of SF 6 /SiH 4 , SF 6 /c-C 4 F 8 , SF 6 /C 3 F 6 gas mixtures with SF 6 gas content all decreased first and then increased, that is, the curve had a minimum point. For the curve of SF 6 /SiH 4 gas mixture, the falling part of the curve is due to the decrease of ionization coefficient α, that is, α-synergistic effect, and for the curve of SF 6 /c-C 4 F 8 gas mixture, the rising part is caused by the increase of attachment coefficient η and that is η-synergistic effect. In addition to these two synergistic effects, they also proposed a synergistic effect of γ on the secondary ionization coefficient of N 2 + CH 4 gas mixture [19]. Takuma et al. studied the synergistic effect of gas mixtures such as SF 6 /N 2 , CCl 2 F 2 /N 2 , etc. They assumed that the effective ionization coefficient of the gas mixture is equal to the sum of the coefficients of the two component gases multiplied by their respective partial pressure ratios, suggesting an empirical formula for breakdown voltage of SF 6 /N 2 gas mixture under slight uneven electric field [20]: where U 1 and U 2 are the breakdown voltage of component gas 1 and gas 2, U m is the breakdown voltage of gas mixture (U 1 > U 2 ), k is the partial pressure ratio of component gas 1, and C is the synergistic effect coefficient, which is independent of the partial pressure ratio k. When Constant C = 0.08, the calculated and experimental values of the SF 6 /N 2 gas mixture by this formula are shown in Figure 5. The breakdown voltage values are basically the same, which can reflect the synergistic effect of SF 6 gas mixture.
According to formula (1), the formula for calculating the synergy coefficient proposed by Takuma is As can be seen from Eq. (2), when C = 1, the breakdown voltage of the gas mixture is equal to the weighting value of breakdown voltage of the two components according to the mixing ratio. That is, the breakdown voltage of the gas mixture exhibits a linear relationship that increases as the mixing ratio of the component gas increases. When 0 < C < 1, the breakdown voltage of the gas mixture reflects synergistic effect phenomenon, and the smaller the value of C, the more significant the nonlinear increase of the breakdown voltage of the gas mixture, which means synergistic effect becomes more significant. When C = 0, the breakdown voltage of the gas mixture equals to the breakdown voltage of gas 1, which means U M = U 1 .
If all the situations of synergistic effect, i.e., positive synergistic effect, synergistic effect, and negative synergistic, are considered at the same time, then the above formula is no longer applicable. Assuming a positive synergistic effect of the gas mixture, it can be seen from the calculation that the coefficient C < 0 under positive synergistic. When the breakdown voltage of the gas mixture is lower than all the breakdown voltage of the component gas, that is, the phenomenon of "super  negative synergistic effect" appears, then value of the coefficient C is still less than 0, so the various synergistic effects of the gas mixture cannot be clearly distinguished by formula (2).
Guo et al. introduced a normalization coefficient h to investigate the synergistic effect of SF 6 /N 2 gas mixture under lightning impulse. The definition of coefficient h is as follows: where U 1 , U 2 , U m , and k in Eq. 3 have the same meaning as those in Eq. 2. When h > 0, it means that gas mixture has synergistic effect, and when h < 0, it represents that the gas mixture has negative synergistic effect. The relationship between the   coefficient h of SF 6 /N 2 and k under lightning impulse with different needle-plane electrodes is shown in Figure 9 [21].
The analysis results show that under the action of negative lightning impulse voltage, the negative synergistic effect increases with the increase of gas pressure. The synergistic effect under the positive impact voltage decreases with the decrease of gas pressure. With the increase of the electric field inhomogeneity coefficient, the synergistic effect has a negative synergistic effect. The analysis of the development process of the flow discharge shows that there are three reasons for the negative synergistic effect: similar flow corona starting voltage, different space charge effects, and different N 2 and SF 6 /N 2 mixed gas discharge forms. The difference of breakdown voltage between N 2 and SF 6 /N 2 gas mixture is shown in Figure 10. r sp represents the range of space charge, ΔU SP is the influence of space charge on breakdown voltage, U st is the streamer corona onset voltage, and U b = U st + ΔU SP .

c-C 4 F 8
c-C 4 F 8 is colorless, odorless, nonflammable, and nonexplosive; the GWP of c-C 4 F 8 is about 8700, and the molecular structure of c-C 4 F 8 is shown in Figure 11. The insulation performance of pure c-C 4 F 8 gas is better than SF 6 , and the gas mixture of c-C 4 F 8 gas has similar insulation characteristics to SF 6 , which can meet the requirements of practical application. The liquefaction temperature of c-C 4 F 8 is about −6°C, higher than that of SF 6 , which is −63.8°C [22]. Therefore, c-C 4 F 8 can only be mixed with the gas in a certain proportion to reduce the overall liquefaction temperature of the gas mixture for application.
While studying the synergistic effect of SF 6 gas mixture, Christophorou et al. also conducted a comparative study on the synergistic effect of c-C 4 F 8 gas mixtures. The insulation characteristics and synergistic effect of c-C 4 F 8 /CF 4 and c-C 4 F 8 /CHF 3 gas mixture are shown in Figure 12 [13].
It can be seen from the figure that the addition of a small amount of c-C 4 F 8 can greatly improve the insulation strength of gas mixture. Mixing 20% c-C 4 F 8 in CHF 3 can double the breakdown voltage. The insulation strength of c-C 4 F 8 /1,1,1-CH 3 CF 3 gas with the same mixing ratio is the same as that of pure SF 6 gas. By comparison, it can be seen that the synergistic effect phenomenon will also occur when c-C 4 F 8 gas and the gas with large dipole moment are mixed.
Xing et al. discussed the feasibility of replacing SF 6 gas with c-C 4 F 8 /N 2 mixture for gas insulation equipment from the perspective of PD performance. The initial PD voltage of c-C 4 F 8 /N 2 gas mixture was measured under different pressure, different mixing ratio, and different electrode distance. The influence of these three factors on the PD performance of the gas mixture was obtained and compared with the initial PD voltage of pure SF 6 gas. The results show that the initial PD voltage of pure c-C 4 F 8 gas is 1.3 times that of pure SF 6 gas [23]. c-C 4 F 8 and N 2 have synergistic effect, and the synergistic coefficient calculated by Takuma's equation under different electrode distance and gas pressure is shown in Table 1.
Yamamoto et al. examined the insulation characteristics of c-C 4 F 8 gas mixtures such as c-C 4 F 8 /N 2 , c-C 4 F 8 /air, and c-C 4 F 8 /CO 2 under different electric field [24]. The experimental results show that c-C 4 F 8 /CO 2 gas mixture has better synergistic effect than the other two mixtures, and improve gas pressure or gap distance can greatly  enhance the synergistic effect of c-C 4 F 8 gas mixture. The relationship of synergistic effect coefficient C with Pd is shown in Figure 13.

CF 3 I
CF 3 I gas is odorless, nonflammable, chemically stable, and material compatible. The molecular structure of CF 3 I is shown in Figure 14. CF 3 I is considered as one of the ideal alternatives to conventional Freon refrigerant. In terms of environmental characteristics, CF 3 I is an extremely environmental friendly gas, and its GWP value is about 1-5, much lower than SF 6 gas. At the same time, the C-I chemical bond in the molecular structure of CF 3 I is easy to photolysis under solar radiation, resulting in the very short existence time of CF 3 I in the atmosphere, and the ozone destruction potential of the gas can also be ignored. The boiling point of CF 3 I at normal pressure is −22.5°C, which indicates that CF 3 I gas will transform from gaseous to liquid when the temperature is lower than −22.5°C [25]. Therefore, CF 3 I gas must be mixed with buffer gas before it can be used in power equipment.
Urquijo et al. experimentally measured the electron transport parameters of CF 3 I and CF 3 I/N 2 gas mixture. They analyzed and studied several parameters including electron drift velocity, diffusion coefficient, electron ionization coefficient, and attachment coefficient by means of pulsed Townsend method. Experimental results show that the critical electric field intensity (E/N) lim of CF 3 I was 437Td, and the insulation strength of the gas is about 1.2 times that of SF 6 gas. When the content of CF 3 I is 70%, the dielectric strength of CF 3 I/N 2 gas mixture is basically the same with pure SF 6 gas [26]. The insulation characteristic comparison of CF 3 I/N 2 and SF 6 /N 2 gas mixtures IS shown in Figure 15. From the figure it can be seen that CF 3 I/N 2 and SF 6 /N 2 gas mixture both have synergistic effect phenomenon, but the phenomenon of CF 3 I/N 2 gas mixture is weak when compared with SF 6 /N 2 .
In addition to CF 3 I/N 2 mixture, there is also a synergistic effect of CF 3 I/CO 2 mixture. Jiao et al. studied the gas mixture of CF 3 I/CO 2 with low content of CF 3 I, and conducted breakdown test on CF 3 I/CO 2 gas mixture with different mixing ratio, different gas pressure, and different discharge gap distance under extremely uneven electric field [27]. The experimental results show that the mixing of trace CF 3 I can significantly increase the breakdown voltage, and the breakdown voltage tends to be stable when the content of CF 3 I gas is over 6%. Although there is still a gap of the breakdown voltage between CF 3 I/CO 2 and SF 6 , the trace amount of CF 3 I has a good synergistic relationship with CO 2 mixture. The relationship between breakdown voltage and CF 3 I content under different electrode distances is shown in Figure 16, and the synergistic coefficient of CF 3 I/CO 2 gas mixture calculated by Takuma's equation is shown in Table 2. The author believes that the negative synergetic effect coefficient is caused by the saturation of the breakdown voltage and    does not continue to analyze the reason. In fact, this phenomenon may be related to the abnormal breakdown of the gas mixture containing CO 2 .
In addition to binary CF 3 I gas mixture, some scholars have studied ternary gas mixture containing CF 3 I gas. Xu et al. calculated the electron energy distribution function (EEDF) of CF 3 I ternary gas mixtures by solving the Boltzmann equation and propose a new equation to analyze synergistic effect. The results show that if buffer gases such as N 2 or CO 2 are concluded in the ternary mixture, the distribution of low-energy electrons in EEDF increases, leading to the synergistic effect, but there is no synergistic effect with CF4. The mixture of two strongly electronegative gases, CF 3 I/SF 6 , shows weak negative synergistic effect, while the addition of N 2 or CO 2 can reduce the negative synergistic effect [28]. The EEDF of CF 3 I ternary gas mixtures is shown in Figure 17.
The computational formula of synergistic effect coefficient proposed by Xu is as follows: where (E/N) lim, mix is the critical reduced field intensity of gas mixture and (E/N) lim, i is the critical reduced field intensity of single gas. x i is the molar fraction of the gas component. Fix the buffer mole fraction of gas to 0, 10, and 30%, and change the mixing ratio of CF 3 I and SF 6 gas; the (E/N) lim and synergistic effect coefficient of CF 3 I ternary gas mixtures are shown in Figure 18.

C 4 F 7 N
C 4 F 7 N is an SF 6 alternative gas jointly developed by Alstom of France and 3 M of the United States. The commodity name of this gas is Novec 4710, which is an organic compound containing four C atoms and seven F atoms. Its molecular structure is shown in Figure 19. The chemical characteristics of the gas are similar to those of fluoro organic gas, and the chemical characteristics are relatively stable, which can achieve good compatibility with other materials in electrical equipment. The relative molecular weight of C 4 F 7 N is 195.0, and it also has a high liquefaction temperature, which is −4.7°C [29]. Therefore, it also needs to be mixed with buffer gas. C 4 F 7 N gas mixture is a popular alternative to SF 6 . At present, most researches on SF 6 alternative gases are concentrated on this gas. In order to obtain the optimal buffer gas type and mixing ratio in C 4 F 7 N gas mixture, Hu et al. studied the power frequency breakdown performance and synergistic characteristics of C 4 F 7 N/CO 2 and C 4 F 7 N/N 2 gas mixture with uniform electric field [30]. The gas pressure is  0.1-0.7 MPa, and the C 4 F 7 N content ratio in gas mixture is 5-20%. The experimental and calculation results show that the C 4 F 7 N/CO 2 and C 4 F 7 N/N 2 gas mixture both have synergistic effects, and the synergistic effect of C 4 F 7 N/CO 2 is stronger than C 4 F 7 N/N 2 . The interaction between C 4 F 7 N and CO 2 bimolecular is stronger than that of C 4 F 7 N and N 2 , and the research indicates that there is a certain correlation between the synergistic effect of C 4 F 7 N gas mixture and the intermolecular interaction. This result presents the theoretical calculation and analysis direction for the qualitative judgment of the synergistic effect of C 4 F 7 N gas mixture. The synergistic coefficient h of C 4 F 7 N/CO 2 and C 4 F 7 N/N 2 calculated by Guo Can's equation is shown in Table 3.
Zheng et al. also studied the synergistic effect and insulation performance of C 4 F 7 N/CO 2 gas mixture and found that the synergistic coefficient was related to avalanche parameters of pure gas. They proposed a graphical method based on the Wieland approximation to calculate the critical electric field of gas mixture.  Based on the formula, the C 4 F 7 N content takes 9% in gas mixture tends to be the optimal mixing ratio [31]. Figure 20 shows the relationship of the critical electric field of gas mixture and C 4 F 7 N content.

Conclusions
With the development of the power system, gas-insulated equipment such as GIS will be more and more widely used. At present, SF 6 gas is still the most used insulating gas in power systems. However, due to the greenhouse effect of SF 6 gas and the   consensus of countries on the development of low-carbon clean energy systems, it is imperative to use new environmental friendly insulating gas in power equipment. Looking for SF 6 replacement gas will also be a continuing research hotspot in the field of electrical engineering. According to the current research status, the replacement of SF 6 by single gas and its application in gas insulation equipment is still in the laboratory research stage. The use of an electronegative gas mixed with a buffer gas is a well-feasible solution. This paper reviews the research status of the synergistic effect of insulating gas mixtures including SF 6 gas. It has been found that some achievements have been made in the process and mechanism of synergistic effect, the types of synergistic effects under different conditions, and their influencing factors: 1. SF 6 , c-C 4 F 8 , CF 3 I, and C 4 F 7 N gas all have synergistic effects when mixed with various buffer gases. SF 6 /N 2 gas mixture will have negative synergistic effect under the lightning impulse. The synergistic effect of the gas mixture composed of c-C 4 F 8 , CF 3 I and C 4 F 7 N and CO 2 is more significant than that after mixing with N 2 .
2. The synergistic effect of gas mixture is affected by the type of gas and external conditions such as gas pressure, electric field type, voltage type, etc. The synergistic effect of the same gas mixture may change with the change of external conditions, such as from synergistic effect to negative synergistic or from negative synergistic to synergistic effect.