Measurement of Ionization and Attachment Coefficients in C4F7N/CO2 Gas Mixture as Substitute Gas to SF6

As one of the alternative insulation gases, C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> gas mixtures have attracted much attention recently. In this study, the normalized Townsend first ionization coefficient <inline-formula> <tex-math notation="LaTeX">$\alpha /N$ </tex-math></inline-formula> and the normalized attachment coefficient <inline-formula> <tex-math notation="LaTeX">$\eta /N$ </tex-math></inline-formula> were measured in C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> gas mixtures by using the Steady-State Townsend (SST) method over a range of electric fields <inline-formula> <tex-math notation="LaTeX">$E/N$ </tex-math></inline-formula> from 100 to 550 Td. The concentrations of C<sub>4</sub>F<sub>7</sub>N studied were 4.99, 7.00, 9.00, 13.13 and 20.02%, and the gas pressure were 500 Pa at <inline-formula> <tex-math notation="LaTeX">$20^{\circ }\text{C}$ </tex-math></inline-formula>. The effective ionization coefficients and the critical electric field were obtained at 500, 1000 and 2000 Pa to investigate the effect of gas pressure on dielectric strength in 9.00%C<sub>4</sub>F<sub>7</sub>N/91.00%CO<sub>2</sub> respectively. The results indicated that the <inline-formula> <tex-math notation="LaTeX">$(E/N)_{\mathrm {lim}}$ </tex-math></inline-formula> have not significant change on the SST experimental conditions. Meanwhile, a comparison with SF<sub>6</sub> gas indicates that 20.02% C<sub>4</sub>F<sub>7</sub>N/79.98% CO<sub>2</sub> has a similar insulation performance with <inline-formula> <tex-math notation="LaTeX">$(E/N)_{\mathrm {lim}}$ </tex-math></inline-formula> as 358.82 Td. However, when the liquefaction temperature is considered, the C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> gas mixtures with 9.00% C<sub>4</sub>F<sub>7</sub>N could be considered as an appropriate substitute gas for SF<sub>6</sub>, because the critical electric field of this gas mixture is over 70% of SF<sub>6</sub> gas and could be used under −15°C at 0.7 MPa.


I. INTRODUCTION
Due to the great insulation ability, toxic safety and the heat transfer properties, sulfur hexafluoride (SF 6 ) is widely used in high voltage applications, such as gas insulted switchgear (GIS), gas insulted transmission line (GIL) and so on [1]- [3]. However, as one of the six restricted greenhouse gases listed in the Kyoto Protocol in 1997, SF 6 has an extremely high value of global warming potential (GWP 100 ), which is 22800 times that of CO 2 [4], [5]. Therefore, the search for an environment-friendly insulation gas to reduce even replace the SF 6 in power systems has reached the urgent stage.
The associate editor coordinating the review of this manuscript and approving it for publication was Jenny Mahoney.
Recently, as a newly potential candidate gas, Fluoronitrile (C 4 F 7 N), whose value of GWP is about 2100, is attracting much attention. [2], [6]- [11]. However, pure C 4 F 7 N have to be diluted with the buffer gas such as N 2 , CO 2 or dry air for electrical applications due to its high boiling point as −4.7 • C [6]. Then, C 4 F 7 N/CO 2 gas mixtures could be considered as the alternative gases to SF 6 . So far, most of the researches focus on the breakdown characteristics and other insulation properties at high gas pressure in C 4 F 7 N/CO 2 gas mixtures. As far as we know, there is little reports about the transport parameters in C 4 F 7 N/CO 2 mixtures. Nechmi et al. [7] used steady-state Townsend (SST) method to obtained the normalized effective ionization coefficient (α−η)/N (per unit gas density N ) of C 4 F 7 N/CO 2 gas mixture with the concentration of C 4 F 7 N as 3.7, 6.7 and 20 %, within a small range of reduced electric fields E/N as 200 ∼ 430 Td.
Chachereau et al. [10], [11] used Puled Townsend (PT) method to obtained the electron rate and transport coefficients such as effective ionization rate coefficient and electron drift velocity in C 4 F 7 N/CO 2 mixtures with the concentration of C 4 F 7 N varying from 1% to 40 %. Nevertheless, there is few reports about the normalized ionization coefficient α/N and the normalized attachment coefficient η/N measured by using the SST method in C 4 F 7 N/CO 2 with different ratio of C 4 F 7 N.
In present work, the normalized ionization coefficients α/N , the normalized attachment coefficients η/N , the normalized effective ionization coefficient (α − η)/N were measured by using the SST method. The critical electric fields (E/N ) lim (for α − η = 0) of C 4 F 7 N/CO 2 , which can be used as references for evaluating the dielectric strength [12], were obtained as well. Moreover, in order to investigate the effect of gas pressure on the SST experimental conditions, the (α −η)/N and (E/N ) lim were obtained at 500 Pa, 1000 Pa and 2000 Pa respectively. According to the requirements of the funding program, the C 4 F 7 N/CO 2 mixture should achieve 80% of the insulation abilities of SF 6 and could be used under -15 • C. Then, the liquefaction temperature of gas mixtures with different contents of C 4 F 7 N were calculated according to the Antoine equation. Finally, several C 4 F 7 N/CO 2 mixtures were suggested as potential alternative gases when considering the critical electric fields and boiling points.

II. EXPERIMENTAL
The measurements of the C 4 F 7 N/CO 2 mixtures were performed on the SST apparatus setup in Wuhan University [13], [14]. The accuracy of the measurement is less than 2%. All equipment of the SST experimental platform have been verified by the Hubei Institute of Metrology and Testing Technology. As described in the previous work [13], the apparatus consist 5 parts, and the one of the important parts is the vacuum system. The background vacuum degree in the ionization chamber can reach approximately 1.0 × 10 −5 Pa at 20 • C. The low leak rate of pressure as 0.07 Pa/h guarantee the stable gas pressure as 500 ∼ 2000 Pa during the SST experiments. Additionally, the precision of the measurements of discharge currents is less than 1.5%. The purity of the C 4 F 7 N gas (produced by Beijing Yuji Science and Technology Co., Ltd.) used in this study is higher than 99.6%, and the purity of CO 2 gas is 99.99%. The partial pressure method is used to compound all the gas mixtures with C 4 F 7 N concentrations (molar fractions) of 4.99, 7.00, 9.00, 13.13 and 20.02% respectively. The gas pressure for these mixtures was 500 Pa at 20 • C during the SST experiments. The reduced electric fields (E/N ) applied in the experiments varied from 100 to 550 Td (where 1 Td = 10 −21 Vm 2 ).
As reported [7], [11], [15], the secondary ionization coefficient γ can be considered to be negligible at the low gas pressure. Then, the theoretical equation, which is widely used in SST experiments, for the current I of picoampere (pA) order in the non-self-sustained discharge stage could be expressed  as follows, where I 0 is the initial electron current, d is the electrode separation distance. In this study, I 0 is controlled to 4 pA, and d could be adjusted in the range of 0 ∼ 15 mm with 1.5 mm as step size.

A. IONIZATION AND ATTACHMENT COEFFICIENTS
The normalized ionization coefficient α/N and the attachment coefficient η/N of C 4 F 7 N/CO 2 mixtures were measured for 100 Td < E/N < 550 Td, and the results with the comparison to SF 6 gas [3] are shown in Figures 1 and 2. It was found that with increasing E/N , the value of α/N in each kind of C 4 F 7 N/CO 2 mixture shows a growth trend, while η/N shows a downward trend obviously, which is similar to the trends of SF 6 . Moreover, with the concentration of C 4 F 7 N increases, the values of α/N decrease and the values of η/N increase for the same E/N , which indicates that C 4 F 7 N is a strongly electronegative gas [6]. Moreover, it can be noted that both the values of α/N and η/N for same E/N in the C 4 F 7 N/CO 2 mixtures are smaller than that of SF 6 .

B. EFFECTIVE IONIZATION COEFFICIENT
The results of the normalized effective ionization coefficient (α−η)/N in the C 4 F 7 N/CO 2 mixtures were obtained and plotted in Figure 3, which were compared with the (α−η)/N coefficients for SF 6 [3] and CO 2 [16]. It could be found that comparing to pure CO 2 gas, there are negative values of (α−η)/N when the C 4 F 7 N was added before 200 Td. Obviously, with increasing E/N , the value of (α−η)/N increases. Moreover, for the same E/N , the (α − η)/N values for the mixtures with higher concentrations of C 4 F 7 N gas are smaller. Significantly, over the experimental scale of E/N , the value of (α-η)/N of the 20.02%C 4 F 7 N/79.98%CO 2 mixture is most similar to that of SF 6 , which indicates that 20.02%C 4 F 7 N/79.98%CO 2 mixture has the similar insulation performance to pure SF 6 near the critical electric field (E/N ) lim . Meanwhile, the other four mixtures show higher coefficients than 20.02%C 4 F 7 N/79.98%CO 2 and SF 6 gas for the same E/N . It is apparent that a higher (α − η)/N leads to easier ionization processes, which means the insulation performance is weaker for the gas with higher (α-η)/N . Therefore, the insulation ability of C 4 F 7 N/CO 2 mixtures with C 4 F 7 N concentrations less than 20% is weaker than that of SF 6 .

C. CRITICAL ELECTRIC FIELD (E/N) lim
In order to provide a comparison and evaluation of the insulation performance of these gases quantitatively, the results  of critical electric fields (E/N ) lim in C 4 F 7 N/CO 2 mixtures are obtained by linear fitting due to the linear variation of (α−η)/N with E/N near (α−η)/N = 0, and the comparisons with that of C 4 F 7 N/N 2 [13] gas mixtures as well as SF 6 [13] have been sorted into Table 1 and plotted in Figure 4. The strength of (E/N ) lim could indicate the insulation ability of insulation gases since when E/N is larger than (E/N ) lim , the gas shows the ionization characteristics of electrons. Therefore, the higher (E/N ) lim means the gas would keep attachment characteristics under higher E/N, which indicates that gas have better insulation performance. It could be found from table 1 that the 20.02%C 4 F 7 N/79.98%CO 2 mixture provides a similar gas insulation performance to pure SF 6 gas since the value of (E/N ) lim of this mixture is 358.82 Td, and the value of (E/N ) lim is 356.77 Td in SF 6 gas [13]. Meanwhile, it is worth noting that the value of (E/N ) lim in 13.13%C 4 F 7 N/86.87%CO 2 could achieve approximately 80% of that in SF 6 , and 9.00%C 4 F 7 N/91.00%CO 2 could achieve more than 70% of that in SF 6 . Moreover, it should be noticed that there are several researches which showed the dielectric strength in 9.00%C 4 F 7 N/91.00CO 2 mixtures could achieve more than 80% that of SF 6 over 0.2 MPa in power-frequency breakdown tests [17]. Thus, from the perspective of the value of (E/N ) lim , the mixtures of 9.00% ∼ 13.00% C 4 F 7 N in CO 2 have potential for using as alternative gases to SF 6 . As shown in Figure 4, there is a synergism of (E/N ) lim in C 4 F 7 N/CO 2 mixtures measured in this work, which also has a good agreement with the results obtained by PT method [10]. However, the results obtained in this work and Chachereau's work [10] were lower than that reported in Nechmi's [7] work. According to the report, the reason to this differences could be the scatter in the data measured by Nechmi. Moreover, it could be found that the comparison of (E/N ) lim of the C 4 F 7 N/CO 2 mixture and the C 4 F 7 N/N 2 mixture [13] shows that C 4 F 7 N/N 2 has a better insulation ability than C 4 F 7 N/CO 2 with almost the same concentration of C 4 F 7 N, which shows similar phenomena in SF 6 /N 2 and SF 6 /CO 2 mixtures as reported in the literature [18]. According to previous studies [16], the value of effective ionization coefficients of CO 2 is higher than that of N 2 for the same E/N , which indicates that it is easier for CO 2 to show the ionization characteristics in discharge. Therefore, for the same concentration of electronegative gas like C 4 F 7 N, the mixtures with N 2 are more stable to be ionized, which means that the mixtures with CO 2 have weaker insulation performance in this situation.
It was reported that the breakdown voltage of C 4 F 7 N/CO 2 gas mixture is higher than that of C 4 F 7 N/N 2 gas mixture at high pressure [19], which is opposite to the SST experimental results mentioned above. Meanwhile, the electric strength in C 4 F 7 N/CO 2 gas mixture is higher than the (E/N ) lim measure by SST method. This interesting phenomenon were found by Chachereau [10], [11] with PT experiments as well, and the different gas pressure used in these experiments were considered as an important factor to effect the result.
In order to investigate the influence of gas pressure on (α−η)/N and (E/N ) lim at low pressure in SST experiments, the 9.00%C 4 F 7 N/91.00%CO 2 were chosen and the gases pressure were chosen as 500, 1000 and 2000 Pa respectively in this work. As reported, in order to avoid the influence of secondary ionization process, the gas pressure should not be over 2000 Pa in the SST experiments. The effective ionization coefficients (α−η)/N measured were plotted in Figure 5 and critical electric field (E/N ) lim were listed in Table 2. It could be found that the effect of different gas pressure at 500, 1000 and 2000 Pa on the results is not significant, even could be negligible within the range of the SST experimental condition. Comparing to the dielectric strength obtained by breakdown tests at high gas pressure, the insulation ability in 9.00%C 4 F 7 N/91.00%CO 2 is weaker at low gas pressure obtained in SST experiment. However, due to the limitation of SST method, it is impossible to carry out the experiments at higher gas pressure over 2000 Pa.
According to Chachereau's researches [10], [11], the different results of dielectric strength measured in SST  experiment and breakdown test could be explained with the ion kinetics of C 4 F 7 N at different gas pressure. As reported, there are three different negative ions in C 4 F 7 N discharges, and all these three ions could have electron detachment process. While, at low gas pressure and small geometric distance, which is the SST experimental condition in this study, the electron detachment could be ignored. However, electron detachment could affect the discharge more strongly by forming a significant current from detached electrons when the gas pressure is high. Moreover, it should be noticed that the detachment of short-lived anion could only be observed at low pressure and the detaching anions do no contribute to the electric strength at high gas pressure. Then, the ion kinetics could have an effect on discharge process at different gas pressures. Therefore, the dielectric strength evaluated by the (E/N ) lim obtained by SST experiments could be different from the results obtained in the breakdown test at high gas pressure.

E. THE BOILING POINT IN C 4 F 7 N/CO 2 MIXTURES
The liquefaction temperature (i.e. boiling point) of the mixed gas at high pressure is an important indicator that cannot be ignored, when considering the actual operating conditions. In the high voltage applications such as GIL, the gas pressure of insulation gas is at least 0.3 MPa usually. According to the Antoine equation [20], [21], the relationship between the saturated vapor pressure p and the boiling point T could be VOLUME 8, 2020 FIGURE 6. Comparison of the liquefaction temperature at different operation pressure in C 4 F 7 N/CO 2 mixtures with different content of C 4 F 7 N. described in equation (2) as follows, where A i , B i , C i (i=1,2) are the Antoine constants for different gases and y is the mole ratio for the gaseous. The boiling point at different operating pressure with different content of C 4 F 7 N mixed in CO 2 gas are plotted in Figure 6. It could be found that the boiling point of C 4 F 7 N/CO 2 increased with the increasing concentration of C 4 F 7 N. Furthermore, the boiling point of mixtures with same content of C 4 F 7 N become higher at higher operating pressure. Among the investigated mixtures, the 9.00%C 4 F 7 N/91.00%CO 2 mixtures, which could still keep gaseous state at -15 • C under 0.7 MPa, would have better possibilities to be considered as candidate insulation gases to replace the SF 6 used in high voltage applications.

IV. CONCLUSION
In this work, both the α/N and η/N values in C 4 F 7 N/CO 2 mixtures with C 4 F 7 N concentrations of approximately 4.99, 7.00, 9.00, 13.13 and 20.02% have been measured using the SST method for E/N in the range from 100 to 550 Td, and the effective ionization coefficients of C 4 F 7 N/CO 2 gas mixtures are compared with that of SF 6 and CO 2 . The results show that the C 4 F 7 N/CO 2 mixtures with C 4 F 7 N concentrations greater than 20.02% could achieve a similar insulation performance than SF 6 .The critical electric field (E/N ) lim of C 4 F 7 N/CO 2 increases with increasing concentration of C 4 F 7 N. Considering the application scope and the discharge processes of SST experiment, the gas pressure in SST experiments cannot be over 2000 Pa. The values of (E/N ) lim in 9.00%C 4 F 7 N/91.00%CO 2 gas mixtures were obtained at 500, 1000 and 2000 Pa respectively. It could be found that the gas pressure have little effect on the effective ionization coefficients and (E/N ) lim on the SST experimental conditions since the electron detachment could be ignored at low gas pressure and small geometric distance.
The value of (E/N ) lim in 13.13%C 4 F 7 N/86.87%CO 2 is 292.44 Td, which is approximately 80% that of SF 6 . However, when the liquefaction temperatures of the gases are considered, the C 4 F 7 N/CO 2 gas mixtures with 9.00% C 4 F 7 N contents, which has the critical electric field over 70% of SF 6 gas, can be considered as potential candidate insulation gases to replace SF 6 in the high voltage engineering field in the future.