Influence regularity of O 2 on dielectric and decomposition properties of C 4 F 7 N–CO 2 –O 2 gas mixture for medium-voltage equipment

: Fluorinated nitrile (C 4 F 7 N) gas mixture has been introduced as the most promising candidate to replace sulfur hexafluoride using in gas-insulated equipment. In this study, the authors explored the influence of oxygen on the dielectric and decomposition properties of C 4 F 7 N–CO 2 –O 2 gas mixture. The authors found that the dielectric strength of the C 4 F 7 N–CO 2 –O 2 gas mixture with 2, 4, 6, 8 and 10% O 2 was increased by 4.85%, 6.49%, 7.70%, 3.21% and 2.74% compared with C 4 F 7 N–CO 2 . The addition of 2–6% O 2 to the C 4 F 7 N–CO 2 gas mixture could effectively reduce the content of most of the decomposition by-products such as CF 4 , CO, C 2 F 6 , C 3 F 6 , C 3 F 8 , CF 3 CN, C 2 F 5 CN, (CN) 2 . While high content of oxygen (>6%) results in higher decomposition of C 4 F 7 N, which has a negative effect on the stability of C 4 F 7 N gas mixture. Generally, it is recommended to add 2–6% O 2 in the C 4 F 7 N–CO 2 gas mixture to improve its insulation properties as well as inhibit the decomposition of C 4 F 7 N in the discharge


Introduction
Sulfur hexafluoride (SF 6 ) is currently widely used in gas-insulated transmission and distribution network in the power industry due to its great insulation and switching characteristics [1][2][3][4]. SF 6 is a strong greenhouse gas with the global warming potential (GWP) of 23,500 and atmospheric lifetime of 3200 years [5][6][7]. It is reported that about 80% SF 6 produced worldwide goes to power industry [8]. In order to reduce the negative impact of SF 6 on the environment, political developments like the Paris treaty or the coming revision of the European F-gas directive in 2020 indicate that stricter regulations for the application of F-gases (including SF 6 ) might be expected in the future [9]. Thus, seeking for an ecofriendly gas insulating medium has become a hot topic.
Among all kinds of SF 6 alternatives investigated, the fluorinated nitrile (C 4 F 7 N) has been recognised as the potential solution. The dielectric strength of pure C 4 F 7 N is approximately twice that of SF 6 [10,11]. However, C 4 F 7 N has high boiling temperature (−4.7°C) and should be used as the additive to CO 2 , N 2 to avoid the liquefaction of gas at low ambient temperatures. It was reported that the gas mixture with 4, 6, 10% C 4 F 7 N would be suitable for −30, −25 and −10°C minimum operating temperature high-voltage (HV) gas-insulated equipment (GIE). In addition, the GWP values of these gas mixtures are 327, 462 and 690, respectively, which is only 2-3% of that of SF 6 [11]. The LC50 (rats, 4 h) of 10% C 4 F 7 N-90% CO 2 gas mixture is in the range of 95,500 ppm (μL/L)-100,000 ppm, which belongs to the no toxic substance according to relevant EU regulations [11].
Over the past three years, extensive studies have been made on insulation properties [12][13][14][15][16][17], decomposition characteristics [18][19][20][21][22][23][24], material compatibility [25][26][27] and arc quenching properties [28][29][30] of C 4 F 7 N gas mixture, confirming that it has the potential to be used in GIE. Also, CO 2 is mostly chosen as the buffer gas because of its superior arc quenching performance and strong cooperative effect with C 4 F 7 N. As for medium-voltage (MV) application, Zhang et al. studied the breakdown strength and partial discharge (PD) characteristics of C 4 F 7 N-CO 2 and SF 6 under AC voltages. It was found that 15% C 4 F 7 N-85% CO 2 gas mixture has an equivalent dielectric strength to SF 6 at 100 kPa. In non-uniform and highly non-uniform fields, SF 6 exhibits higher dielectric strength than 20% C 4 F 7 N−80% CO 2 mixture. The PD analysis also suggests that SF 6 has much more PDs with lower magnitude and the 20% C 4 F 7 N−80% CO 2 mixture has fewer PDs but with higher magnitude [14]. Relevant results confirm that C 4 F 7 N-CO 2 has the potential for being used in MV equipment to replace pure SF 6 .
In addition, it is reported that oxygen (O 2 ) should be added to the mixture as a second additive gas to improve the switching performance of C 4 F 7 N gas mixture for HV application such as HV-GIS [31,32]. Meyer and Kieffel explored the C 4 F 7 N-CO 2 -O 2 using as a current interruption medium in GIS circuit breaker. The current interruption behaviour of the 6% C 4 F 7 N-5% O 2 -89% CO 2 gas mixture was investigated via a sequence of breaking tests at 40 kA (T100) on 145 kV, 40 kA GIS circuit breaker. It was also reported that the addition of O 2 could avoid carbon deposit on insulators during the breaking operation and help to recombine the generated CO back to CO 2 for HV GIE [32,33]. However, there are few reports on the influence mechanism of oxygen on the insulation and decomposition properties of C 4 F 7 N-CO 2 -O 2 gas mixture for MV GIE at present. The MV GIE such as gas-insulated cabinet, ring main units has the operation pressure in the range of 0.1-0.14 MPa (absolute pressure) [34]. Thus, the C 4 F 7 N content in the gas mixture could be higher than that of HV GIE, making it possible to replace pure SF 6 . On one hand, the addition of oxygen may affect the insulation characteristics of C 4 F 7 N-CO 2 -O 2 gas mixture such as the breakdown voltage. On the other hand, the gas stability and decomposition properties of C 4 F 7 N-CO 2 -O 2 gas mixture would also be affected considering the strong oxidation of oxygen itself. In addition, the optimum oxygen addition content is also missing for MV GIE engineering application.
In this paper, we explored the insulation and decomposition characteristics of C 4 F 7 N-CO 2 -O 2 gas mixture for MV application comprehensively. The AC breakdown voltage of C 4 F 7 N-CO 2 -O 2 gas mixture with different oxygen content was tested first. Then the gas composition of the gas mixture after the breakdown was detected and analysed based on the gas chromatography-mass spectrometry (GC-MS). The influence mechanism of oxygen on the decomposition properties of the gas mixture was discussed and the optimum addition content of oxygen for the practical application was proposed. Finally, the additional reaction pathways of C 4 F 7 N due to the addition of oxygen were proposed and relevant reaction enthalpy, activation energy were calculated based on the density functional theory (DFT). Relevant results provide a significant reference for the engineering application of C 4 F 7 N-CO 2 -O 2 gas mixture. Fig. 1 gives the schematic diagram of the AC breakdown properties test platform. The voltage regulator (0-380 V) is used to control the output HV of the testing transformer which provides AC HV (0-100 kV) around the electrodes. The resistor is used to limit the current in the circuit after breakdown to protect the whole devices. In addition, the capacitive voltage divider could measure the actual HV applied to the electrodes. The cylinder gas chamber with 30 L volume made of stainless-steel could withstand the gas pressure at 0∼0.6 MPa. We choose the sphere-sphere electrodes to simulate the quasi-uniform electric field in the equipment. The radius of the brass sphere is 25 mm and the electrode gap is set to 3 mm.

Test platform
The composition of the gas mixture after the breakdown is detected based on the GC-MS (Shimadzu Ultra 2010plus). The CP-Sil 5CB column (60 m × 8 µm × 0.32 mm) is used to separate the characteristic decomposition products of C 4 F 7 N gas mixture.

Test method
The gas chamber is first cleaned using anhydrous alcohol to remove impurities and then filled with CO 2 (99.999%) to 0.3 MPa. The chamber is pumped to 0 MPa using the vacuum pump and filled with CO 2 again for three times. Also, the C 4 F 7 N-CO 2 -O 2 gas mixture is finally injected to the gas chamber to 0.14 MPa (absolute pressure) for AC breakdown tests. The selection of 0.14 MPa as the gas pressure in the experiment meets the working pressure conditions of the MV equipment. According to reference [35], the content of C 4 F 7 N in the gas mixture should be <18% at 0.2 MPa for the minimum operating temperature at −25°C. It should be noted that C 4 F 7 N plays a major role in the insulation performance of the gas mixture due to its high dielectric strength. Thus, we carried out breakdown tests for gas mixture with 15% C 4 F 7 N. The content of oxygen added in the C 4 F 7 N gas mixture is set to 2, 4, 6, 8 and 10%, as shown in Table 1.
The HV was added across the sphere electrodes using the stepstress test method and the transient breakdown voltage is recorded as the AC breakdown voltage of the gas mixture. AC breakdown tests for each group of the gas mixture are carried out for 100 times and the gas sample was analysed after every 20 breakdown.
The single ion monitoring (SIM) and SCAN methods were used to analyse the composition of the gas mixture. Table 2 gives the characteristic mass-to-charge ratios of main decomposition byproducts of C 4 F 7 N gas mixture, where bold type m/z is chosen as the target particle. The qualitative analysis of the by-products is carried out based on the standard gas, National Institute of Standards and Technology (NIST 14.0) database. In addition, the quantitative analysis is conducted based on the external standard method for CF 4 , C 2 F 6 , C 3 F 6 , C 3 F 8 , CO and peak area integral method for CF 3 CN, C 2 F 5 CN, COF 2 , (CN) 2 due to the reason that the standard gas for these by-products are not applicable at present.
The heat program of the column is given as follows: first, keep the column at 32°C for 7 min. Then the column is heated to 150°C with the temperature increase rate of 60°C/min. Finally, the column is kept at 150°C for 2 min. Fig. 2 shows the gas chromatogram of 15% C 4 F 7 N-85% CO 2 gas mixture after 20 numbers of AC breakdown discharge. It can be found that the characteristic peak of CF 4 , C 2 F 6 , COF 2 , CO, C 3 F 8 , CO 2 , CF 3 CN, C 2 F 5 CN and (CN) 2 exists, confirming that the column and heat program applied is reasonable.

Theoretical method
In order to explore the decomposition mechanism of C 4 F 7 N under the oxygen environment, we conducted DFT calculations to obtain the energy and reaction enthalpy of the proposed pathways.
Geometry optimisation and harmonic frequency calculations were performed based on the generalised gradient approximation with the Perdew-Burke-Ernzerhof (PBE) method [36]. Also, the double numerical plus polarisation is chosen as the basis set. The transition state (TS) of the proposed pathway was searched using linear synchronous transit and quadratic synchronous transit (LST-   QST) method [37]. The activation energy of the reactions with a TS is also obtained.

Influence of oxygen on the breakdown voltage of C 4 F 7 N-CO 2 -O 2 gas mixture
Fig . 3 gives the AC average breakdown voltage of C 4 F 7 N-CO 2 -O 2 gas mixture under different O 2 content conditions. We defined the average value of the first to tenth test results as the AC average breakdown voltage of the gas mixture. It can be found that the AC average breakdown voltage of C 4 F 7 N-CO 2 -O 2 gas mixture increases with the oxygen content <6% first and then decreases. The dielectric strength of the C 4 F 7 N-CO 2 -O 2 gas mixture with 2, 4, 6, 8 and 10% oxygen was increased by 4.85, 6.49, 7.70, 3.21 and 2.74% compared with that of C 4 F 7 N-CO 2 gas mixture (28.06 kV). Actually, the relative critical reduced electric field strength (E/N) crit of CO 2 and O 2 to SF 6 is 0.23 and 0.33 [8], indicating that O 2 has greater dielectric strength than that of CO 2 . Thus, the breakdown voltage of C 4 F 7 N-CO 2 -O 2 gas mixture increases with the increase of O 2 content (the decrease of CO 2 content). The breakdown voltage of the gas mixture shows a decreasing trend when the O 2 content is >6%, which may be due to the decomposition of C 4 F 7 N considering the oxidation reactions. 4 , CO, C 2 F 6 , C 3 F 6 and C 3 F 8 : Generally, arc plasma generated by discharge will lead to complex physical and chemical reactions, which could result in the decomposition of gas molecules and the generation of several byproducts. In order to understand the influence of oxygen on the discharge decomposition properties of C 4 F 7 N-CO 2 -O 2 gas mixture, we conducted repeated AC breakdown tests and analysed the gas in the chamber based on the GC-MS. Fig. 4 shows the content of CO, CF 4 , C 2 F 6 , C 3 F 6 and C 3 F 8 for C 4 F 7 N-CO 2 -O 2 gas mixture after repeated AC breakdown tests. It can be found that the yield of CF 4 ranks the top among all the quantitative products, followed by CO, C 2 F 6 , C 3 F 8 . The content of C 3 F 6 is the lowest. In addition, the yield of CO, CF 4 , C 2 F 6 , C 3 F 6 and C 3 F 8 increases with the breakdown number, confirming that the discharge could cause continuous decomposition of C 4 F 7 N-CO 2 -O 2 gas mixture. Fig. 5 gives the variation feature of CF 4 under different oxygen conditions. In this paper, we defined an effective formation rate (EFR) to reveal the influence of oxygen on the decomposition characteristic of C 4 F 7 N-CO 2 -O 2 gas mixture. Its definition formula is given as follows: (see (1)) , where V P is the EFR, V 20 , V 40 , V 60 , V 80 , V 100 represent the detected gas component content after 20th, 40th, 60th, 80th, 100th breakdown.

Influence of oxygen on the decomposition properties of
According to Fig. 5, the yield and EFR of CF 4 decreases gradually with the oxygen content. The content of CF 4 in C 4 F 7 N-CO 2 gas mixture after 100th breakdown is 4004 ppm, while the corresponding value decreases to 2548 ppm when 2% O 2 is added to the gas mixture. That is to say, the addition of 2% O 2 could effectively inhibit the production of CF 4 . The influence mechanism of oxygen on the generation of CF 4 will be discussed later in the discussion section.
Moreover, the yield and formation rate of CF 4 keep stable when 2-4% oxygen is added in the C 4 F 7 N-CO 2 gas mixture. Then the decreasing trend occurs when the oxygen content in the C 4 F 7 N-CO 2 -O 2 gas mixture is >6%, which may result from the decrease of the breakdown voltage of the C 4 F 7 N-CO 2 -O 2 gas mixture. Fig. 6 describes the yield and EFR of CO under different O 2 content conditions. The content of CO dropped sharply from 2277 to 1414 ppm (100th breakdown) when 2% O 2 is added in the  For the gas mixture with O 2 added, the higher the breakdown voltage, the higher the energy released by the discharge. Also, the decomposition of C 4 F 7 N-CO 2 -O 2 gas mixture will be intensified. In addition, the addition of 2% oxygen could effectively reduce the EFR of CO in the C 4 F 7 N gas mixture. With the increase of oxygen content, the yield of EFR also has a similar change trend of the breakdown voltage.
As shown in Fig. 7, the content of C 2 F 6 decreases when 2% O 2 is added in the C 4 F 7 N-CO 2 gas mixture. The yield of C 2 F 6 in the C 4 F 7 N-CO 2 gas mixture reaches 241 ppm after 100 breakdown tests, while this value is only 145 ppm for the 15% C 4 F 7 N-83% CO 2 -2% O 2 gas mixture. The yield of C 2 F 6 shows an increasing trend with the further increase in oxygen content, especially when the oxygen content is >6%. The formation of C 2 F 6 originates from the recombination of CF 3 particles or C 2 F 5 and F particles, indicating that the amount of these particles is higher in C 4 F 7 N-CO 2 -O 2 gas mixture with oxygen >6%.
The yield and EFR of C 3 F 6 under different O 2 content conditions are given in Fig. 8. The yield of C 3 F 6 after 100th breakdown in C 4 F 7 N-CO 2 gas mixture is only 8.12 ppm, which is lower than the other by-products. With the increase of O 2 content, the yield of C 3 F 6 shows a decreasing trend. Considering the existence of unsaturated C = C bond in the C 3 F 6 molecule, the addition of O 2 may bring about oxidation reaction between them.
According to the variation feature of C 3 F 8 given in Fig. 9, we can find that the addition of 2% oxygen results in the decrease of C 3 F 8 content after repeated breakdown tests. The yield of C 3 F 8 in the C 4 F 7 N-CO 2 gas mixture after 100th breakdown is 12.38 ppm, while this value is only 5.74 ppm for the gas mixture containing 2% O 2 . In addition, an increasing trend of C 3 F 8 content can be found when the oxygen content is >6%. The generation of C 3 F 8 is relative to the C 3 F 7 and F particles, the increase of C 3 F 8 indicates that the decomposition of C 4 F 7 N is accelerated when the oxygen content in the gas mixture is >6%.

Variation feature of CF 3 CN, C 2 F 5 CN, (CN) 2 and COF 2 :
The decomposition of C 4 F 7 N-CO 2 -O 2 gas mixture also generates other by-products including CF 3 CN, C 2 F 5 CN, (CN) 2 and COF 2 . Considering the standard gas of these by-products is unavailable at present, we used the peak area integral method to reveal the influence of oxygen on their generation. The peak area of the characteristic peak could reveal the relative content of the detected substance.
Figs. 10 and 11 gives the yield and EFR of CF 3 CN and C 2 F 5 CN under different O 2 content conditions. We can find that the peak area of CF 3 CN and C 2 F 5 CN has similar variation features with the oxygen content. Their peck area decreases first when 2% O 2 is added into the C 4 F 7 N-CO 2 gas mixture and then shows an increasing trend with the O 2 content. The gas mixture with 2% O 2 has the lowest EFR, indicating that the addition of 2% oxygen could effectively inhibit the decomposition of C 4 F 7 N compared with the C 4 F 7 N-CO 2 gas mixture.
As the O 2 content in the gas mixture increases, the EFR of CF 3 CN, C 2 F 5 CN increases. When the O 2 content in the C 4 F 7 N-CO 2 -O 2 gas mixture is >8%, the peak area and EFR of CF 3 CN and C 2 F 5 CN reach to that of C 4 F 7 N-CO 2 gas mixture level. The formation of CF 3 CN and C 2 F 5 CN need the participation of several particles including CF 3 , CN and F. Thus, the addition of high content O 2 (>8%) has a negative effect on the stability of C 4 F 7 N-CO 2 -O 2 gas mixture.  Fig. 12 gives the influence of oxygen on the peak area of and effect formation rate of (CN) 2 . We can find that there exists a sharp decrease in the (CN) 2 peak area when 2% O 2 is added in the C 4 F 7 N-CO 2 gas mixture. The generation of (CN) 2 requires the CN group, which comes from the decomposition of C 4 F 7 N. Therefore, the addition of 2% oxygen could effectively prevent the decomposition of C 4 F 7 N in the C 4 F 7 N-CO 2 gas mixture. In addition, the yield of (CN) 2 for C 4 F 7 N-CO 2 -O 2 gas mixture with 2-10% content is comparable, indicating that oxygen content has little influence on its generation. Fig. 13 gives the variation feature of COF 2 under different O 2 content conditions. It can be seen that the peak area of COF 2 increases with oxygen content. The content of COF 2 for gas mixture with O 2 <6% is similar, and quite an increasing trend exists when the oxygen content is >6%.
According to Fig. 3, the breakdown voltage of the gas mixture starts to decrease for gas mixture with 6-10% O 2 , indicating that the energy released by the discharge reduces. While the yield of COF 2 increases. Actually, the formation of COF 2 originates from the reactions between CF x and O 2 , O particles. The increase of COF 2 for gas mixture with O 2 higher than 6% is due to the strong oxidising effect of oxygen. High content of O 2 intensifies the reaction between CF x and O particles to form COF 2 . That is to say, the stability of gas mixture with oxygen >6% is inferior and the negative effect of oxygen on the stability of C 4 F 7 N gas mixture can be confirmed.

Influence mechanism of O 2 on insulation properties of C 4 F 7 N-CO 2 -O 2 :
According to the relevant test results, we can find that the addition of oxygen in the C 4 F 7 N-CO 2 gas mixture has quite an influence on its insulation properties. Considering the content of C 4 F 7 N is fixed to 15% in this paper, relevant changes may be attributed to the dielectric difference between O 2 and CO 2 .
Actually, the influence of O 2 on the dielectric properties of CO 2 -O 2 gas mixture has been investigated comprehensively. Uchii et al. [38] pointed out that adding some O 2 to CO 2 can improve its dielectric strength and bring the better thermal interrupting capability to CO 2 . Zhao et al. explored the dielectric breakdown properties of CO 2 -O 2 mixtures by considering electron detachments from negative ions [39]. The reduced ionisation coefficient (α/N), reduced attachment coefficients (η/N), (E/N) crit , breakdown reduced electric fields (E/N) breakdown was calculated. It was found that the increase of O 2 in the CO 2 -O 2 mixture enhances both reduced ionisation coefficients and reduced attachment coefficients. The increase of α/N is due to the ionisation potential of O 2 (12.06 eV) is lower than that of CO 2 (13.3 eV). In addition, the increase of η/N is attributed to the strong electronegativity of O 2 . Furthermore, the attachment cross-section of O 2 is larger than CO 2 for electron energy around 5-10 eV, which could lead to a reduction of the electron kinetic energy [40]. The influence of O 2 on the dielectric properties of CO 2 at the elevated temperatures in discharge explored by Rong et al. [41] also indicates that CO 2 -O 2 mixtures have a much better dielectric strength than both the pure CO 2 and air. It was pointed out that the electron energy distribution function increase with O 2 content due to the vibrational cross-sections of O 2 is smaller than that of CO 2 . The α/N below 90 Td slightly increase with more O 2 , owing to the increase of average electron energy by the excitation reaction, ionisation reaction and attachment reactions. Although the η/N increases markedly with more O 2 than that of α/N. Thus, the (E/ N) crit increases with the increase of O 2 content. In addition, the breakdown electric field in the temperature range from 300 to 1500 K of pure CO 2 is much lower than O 2 . The electric breakdown field in the temperature range from 300 to 3000 K increases markedly with the increase of O 2 , mainly owing to the large attachment cross-sections of O 2 .
As mentioned above, the dielectric property of the CO 2 will be markedly developed by addition of O 2 . For C 4 F 7 N-CO 2 -O 2 gas mixture, the content of C 4 F 7 N which provides the dielectric strength to the mixture is fixed. The difference in the dielectric strength is mainly attributed to the ratio change of CO 2 and O 2 . The larger attachment cross-sections of O 2 might result in the increase of attachment coefficients and critical reduced electric field strength of the C 4 F 7 N-CO 2 -O 2 mixture. Thus, the breakdown voltage of the C 4 F 7 N-CO 2 -O 2 gas mixture with 2-10% O 2 added is higher than that of C 4 F 7 N-CO 2 gas mixture.
In addition, we also found that the decreasing trend of breakdown voltage occurs when the O 2 ratio is >6%. This could be attributed to the strong oxidising characteristic of O 2 . According to the decomposition detected results, the yield of most of the byproducts show an increasing trend when the O 2 content is >6%. That is to say, the decomposition of C 4 F 7 N is accelerated with higher O 2 ratio. According to the literature [29], C 4 F 7 N cannot recombine into itself after decomposition in high-temperature arc. Table 3 gives the relative insulation properties of C 4 F 7 N and main decomposition products. We can find that all the generated byproducts have lower dielectric strength compared with C 4 F 7 N. Thus, the enhanced decomposition of C 4 F 7 N and generation of byproducts with lower dielectric strength under higher O 2 ratio condition cause the decrease of breakdown voltage. The influence mechanism of O 2 on the decomposition properties of C 4 F 7 N will be discussed in Sections 3.3.2 and 3.3.3. CO 2 gas mixture also influences its stability and discharge decomposition properties. The yield of CF 4 , CO, C 2 F 6 , C 3 F 6 , C 3 F 8 , CF 3 CN, C 2 F 5 CN and (CN) 2 shows a certain degree of reduction when 2% O 2 is added in the C 4 F 7 N-CO 2 gas mixture, indicating that the addition of oxygen could prevent the decomposition of C 4 F 7 N in the C 4 F 7 N-CO 2 gas mixture to a certain extent. Generally, the temperature of arc discharge plasma is usually in the range of 300-12,000 K [45]. As for C 4 F 7 N, Wu et al. [28] and Zhong et al. [29] have calculated the plasma compositions. It was pointed out that the predominant species at temperatures <3000 K include CF 2 , CF 4 , C 4 F 6 , C 2 F 3 N, C 4 F 3 N, CO and CO 2 [29]. The initial gas C 4 F 7 N is not observed at high concentration, indicating that C 4 F 7 N cannot recombine into itself after the decomposition in the high-temperature arc. That is the reason why all the detected by-products after discharge show an increasing trend with the breakdown number.

Influence mechanism of O 2 on decomposition
Actually, the arcing time constant of O 2 gas is known to be relatively smaller than that of CO 2 , specifically 1.5 μs, whereas 15 μs for CO 2 [46]. Thus, oxygen has a preferable arc-quenching capability than CO 2 . Uchii et al. [46] explored the quenching properties of pure CO 2 and 85% CO 2 -15% O 2 gas mixture. They found that the post arc current of 85% CO 2 -15% O 2 gas mixture is much smaller than that of pure CO 2 , indicating that the addition of oxygen could make the decaying rate of arc conductivity faster [46].
As for C 4 F 7 N-CO 2 gas mixture, the addition of oxygen could improve the arc-quenching capability of C 4 F 7 N-CO 2 gas mixture, especially improve the decaying rate of arc conductivity [31]. According to the test results, the yield of CF 4 , CO, C 2 F 6 , C 3 F 6 , C 3 F 8 , CF 3 CN and (CN) 2 has decrease trend when 2-6% O 2 is added in the gas mixture, and the content of some by-products starts to increase when 6-10% oxygen is added. The yield of COF 2 and C 2 F 5 CN keeps relative stable when 2-4% oxygen is added, and the increasing trend can also be confirmed with the addition of oxygen >4%.
It should be noted that the formation of these decomposition byproducts mainly comes from the recombination process of several kinds of particles including CF 3 , CN, C 3 F 7 , F. Thus, the generated particles reduce when 2-4% O 2 is added. Relevant theoretical studies on the decomposition mechanism of C 4 F 7 N indicates that the bond-breaking processes to generate CF 3 and C 3 F 4 N requires the lowest reaction enthalpy, which is most likely to occur [18][19][20][21][22]. Although the breakdown voltage is increased when 2-6% O 2 is added in the system, the energy released by the discharge may not increase considering the great arcing characteristics of oxygen. Therefore, we can conclude that the addition of 2-6% oxygen in the C 4 F 7 N-CO 2 gas mixture results in lower by-products generation and C 4 F 7 N decomposition. Also, when the oxygen is added in the higher content (>6%), the negative effect of oxygen can be found. The breakdown voltage of the gas mixture starts to decrease while some of the by-products show an increasing trend with the oxygen content, indicating that the high content of oxygen results in faster decomposition of C 4 F 7 N.
In addition, the decomposition properties of C 4 F 7 N-CO 2 -O 2 gas mixture for HV application have been explored by Meyer and Kieffel [31]. A circuit breaker (145 kV/40 kA) was developed and type-tested according to IEC standard using the 6% C 4 F 7 N-5% O 2 -89% CO 2 gas mixture as the insulating medium. The minimal pressure for insulation and interruption was set to 0.75 MPa, which is 0.2 MPa higher than that of SF 6 -filled circuit breaker. It was pointed out that CO 2 is used as the arc quenching medium, C 4 F 7 N is responsible for enhancing the dielectric withstand. Also, the addition of 5% O 2 could improve the electrical endurance of the circuit breaker by limiting the generation of gaseous and solid decomposition products. Especially for the tests with higher currents, a clear improvement was observed with the addition of 5% O 2 . The decomposition by-products of the gas after shortcircuit tests at 40 kA includes CO, CF 4 , C 2 F 6 , C 3 F 8 , CF 3 CN, C 2 F 5 CN, (CN) 2 , COF 2 , C 2 F 4 , C 3 F 6 . The concentration of these compounds is in the range of 1 to about 200 ppm, except CO, CF 4 , C 2 F 6 and C 3 F 8 [31]. This type of GIS was also installed in Switzerland and Frances in 2017-2018 [33], confirming that 6% C 4 F 7 N-5% O 2 -89% CO 2 gas mixture is suitable for HV applications. Furthermore, Ficheux et al. point out that O 2 plays a major role in the gas chemical decomposition. The influence of O 2 content in C 4 F 7 N-CO 2 is the key factor on gas decomposition and the formation of powders. For instance, the amount of carbon monoxide is lowered by 2 or 3 depending on O 2 ratio and the formation rate of other gaseous by-products is also significantly reduced [47]. In this paper, we also conclude that the addition of 2-6% O 2 to the C 4 F 7 N-CO 2 gas mixture for MV application (0.14 MPa) could effectively reduce the content of most of the decomposition by-products. Relevant conclusions are consistent well with that of HV conditions.

Decomposition mechanism of C 4 F 7 N under O 2 condition:
The decomposition mechanism of C 4 F 7 N has been investigated by Fu et al. [18], Zhang et al. [21] and Li et al. [22] based on the DFT. In this paper, we proposed the additional reaction pathways introduced by the addition of oxygen. It should be noted that the energy released by the AC breakdown will cause the decomposition of C 4 F 7 N and O 2 simultaneously. Then the generated particles by C 4 F 7 N and O 2 including CF 3 , C 3 F 7 , CN, CF 2 , CF 3 CF, O could react with each other to form gaseous byproducts. Table 4 gives the proposed reaction pathways between C 4 F 7 N and its decomposition particles with O. All the reactants and products of the considered pathways were fully optimised and the  reaction enthalpy (Δ r H) could be obtained by subtracting reactant energy from product energy. The activation energy (ΔH) is also calculated using the complete LST-QST TS search protocol. As we can see from Table 4, the reaction between C 4 F 7 N and O particle through path 1 has the reaction enthalpy of −106.52 kcal/ mol, which could generate CF 3 , CF 3 COFCN. This reaction path needs to overcome a barrier height of 47.51 kcal/mol. It was reported that the most likely dissociation path in C 4 F 7 N molecule to generate products of CF 3 , CF 3 CFCN needs to adsorb 73.14 kcal/mol [18,22]. The activation energy of path 1 is quite <73.14 kcal/mol (as shown in Fig. 14), indicating that the participation of O particle makes the bond-breaking processes of C 4 F 7 N much easier to occur. Thus, oxygen shows a negative impact on the stability of C 4 F 7 N.
In addition, the dissociation of CF 3 COFCN through path 2 (generating CF 3 COF and CN) needs 20.54 kcal/mol. Also, the generation of CF 3 and COF by path 3 has the reaction enthalpy of 85.28 kcal/mol without reaction barrier. The combination of CF 3 and O has the negative reaction enthalpy of −104.05 kcal/mol and the generation of COF 2 from COF 3 needs to adsorb 26.56 kcal/ mol. Meanwhile, the COF and F, CF 2 and O could also combine to generate COF 2 through paths 6 and 11 with 120.89 kcal/mol and 116.69 kcal/mol release. The reaction between C 3 F 7 and O to form C 3 F 7 O has negative reaction enthalpy of −101.93 kcal/mol. In addition, the dissociation of C 3 F 7 O through path 9 to generate CF 3 and CF 3 COF need to adsorb 6.53 kcal/mol. The reaction between CF 3 CF and O also has a negative enthalpy of −188.84 kcal/mol.
Overall, most of the reactions between O radical and particles generated by C 4 F 7 N molecule belongs to the exothermic process, which could hinder the generated particles to compound or recombine to C 4 F 7 N. Thus, the addition of oxygen could influence the by-products formation process.

Application suggestions for C 4 F 7 N-CO 2 -O 2 in MV equipment:
As mentioned above, the addition of 2-6% O 2 could improve the insulation performance and inhibit the decomposition of C 4 F 7 N-CO 2 gas mixture. While the added content could not be too high considering its supportive flammability and strong corrosivity. Table 5 gives the toxicity data of characteristic decomposition products of C 4 F 7 N-CO 2 -O 2 gas mixture. We can find that the fluorocarbons (C 2 F 6 , C 3 F 8 ) have little influence on personal safety, which belongs to asphyxiating gas. The LC50 for C 3 F 6 , C 2 F 5 CN is around 3000 ppm, followed by CO (1880ppm). CF 3 CN, (CN) 2 and COF 2 have the LC50 value quite lower than other byproducts, which is only 250 ppm, 175 and 180 ppm. Therefore, it is necessary to limit the production of highly toxic substances. According to the above test results, the addition of 2% oxygen could effectively reduce the generation of most of the by-products. Also, the yield of CO, C 3 F 6 , CF 3 CN, C 2 F 5 CN and (CN) 2 keeps stable or shows little increase trend with 2-6% oxygen added. As for gas mixture with oxygen >6%, the generation of some byproducts starts to accelerate, indicating that high content of oxygen has a negative effect on the stability of C 4 F 7 N gas mixture. Thus, the addition of 2-6% oxygen in the C 4 F 7 N-CO 2 gas mixture is recommended, which could improve the dielectric strength and inhibit the decomposition of C 4 F 7 N.

Conclusion
In this paper, we explored the AC breakdown and discharge decomposition properties of C 4 F 7 N-CO 2 -O 2 gas mixture. The influence of oxygen content on the insulation and decomposition properties of the gas mixture is revealed and discussed. Relevant conclusions can be obtained as follows, which may be useful for engineering application of the new gas.
(i) The addition of oxygen could improve the insulation properties of C 4 F 7 N-CO 2 gas mixture. The dielectric strength of the C 4 F 7 N-CO 2 -O 2 gas mixture with 2, 4, 6, 8 and 10% oxygen was increased by 4.85, 6.49, 7.70, 3.21 and 2.74% compared with C 4 F 7 N-CO 2 .
(ii) The decomposition of C 4 F 7 N-CO 2 -O 2 gas mixture mainly produced CF 4 , CO, C 2 F 6 , C 3 F 6 , C 3 F 8 , CF 3 CN, C 2 F 5 CN, (CN) 2 , COF 2 . The addition of 2-6% O 2 to the C 4 F 7 N-CO 2 gas mixture could effectively reduce the content of most of the decomposition by-products. (iii) The yield of CO, C 3 F 6 , CF 3 CN, C 2 F 5 CN and (CN) 2 keeps stable or shows little increase trend with 2-6% oxygen added to the C 4 F 7 N-CO 2 gas mixture. The generation of some by-products starts to accelerate when the oxygen content is >6%, indicating that high content of oxygen has a negative effect on the stability of C 4 F 7 N gas mixture. (iv) The participation of O particle makes the bond-breaking processes of C 4 F 7 N much easier to occur, which has the reaction enthalpy of −106.52 kcal/mol and the activation energy of 47.51 kcal/mol. The addition of oxygen also has a negative influence on the by-products formation process.
(v) As for MV engineering application, it is recommended to add 2-6% O 2 in the C 4 F 7 N-CO 2 gas mixture to improve its insulation performance as well as inhibit the decomposition of C 4 F 7 N in the discharge.