Recent Advances in the Quest for a New Insulation Gas with a Low Impact on the Environment to Replace Sulfur Hexafluoride ( SF 6 ) Gas in High ‐ Voltage Power Network Applications

The  growing  environmental  challenge  of  electrical  energy  systems  has  prompted  a substantial increase in renewable energy generation. Such generation systems allow for significant reduction  of CO2  emissions  compared with  a  traditional  fossil  fuel plant.  Furthermore,  several improvements  in  power  systems  network  configuration  and  operation  combined  with  new technologies have enabled reduction of losses and energy demand, thus contributing to reduction of CO2 emissions. Another environmental threat identified in electrical networks is the leaking of insulating  sulfur  hexafluoride  (SF6)  gas  used  in  electrical  gas  insulated  substations  (GIS)  and equipment. Because of its Global Warming Potential (GWP) of nearly 24,000 and its long life in the atmosphere (over 3000 years), SF6 gas was recognized as a greenhouse gas at the 1997 COP3; since then  its use and emissions  in  the atmosphere have been  regulated by  international  treaties.  It  is expected  that  as  soon  as  an  alternative  insulating  gas  is  found,  SF6  use  in  high‐voltage  (HV) equipment will be banned. This paper presents an overview of the key research advances made in recent years  in  the quest  to  find  eco‐friendly gases  to  replace SF6. The  review  reports  the main properties of candidate gases that are being investigated; in particular, natural gases (dry air, N2 or CO2)  and polyfluorinated gases  especially Trifluoroiodomethane  (CF3I), Perfluorinated Ketones, Octafluorotetra‐hydrofuran, Hydrofluoroolefin (HFOs), and Fluoronitriles are presented and their strengths and weaknesses are discussed with an emphasis on their dielectric properties (especially their  dielectric  strength),  GWP,  and  boiling  point  with  respect  to  the  minimum  operating temperature for HV power network applications.


Introduction
Sulfur hexafluoride, SF6, has been the most common compressed gas used in high-voltage (HV) power transmission line and substation applications since the 1960s. About 80% of the SF6 gas produced worldwide is used in HV circuit breakers (GCB) and in gas insulated switchgear (GIS) because of its excellent properties such as its dielectric strength, which is 2.5 times that of air, good heat transfer capacity, and interruption of electric arcs. Unfortunately, the excessive size, radiative effect, and atmospheric lifetime (several centuries due to the resistance to chemical and photochemical degradation) of the SF6 molecule makes this gas an aggravating agent for the greenhouse effect. The Global Warming Potential (GWP) of SF6 is one of the highest. Table 1 gives a comparison of GWP and lifetimes of SF6 with carbon dioxide (CO2) [1]. Whilst SF6 is chemically stable (non-toxic, inert gas, non-flammable), it can be asphyxiating when its concentration in the air exceeds 12 mg/m 3 [2]. When subjected to an electric arc and when the temperature of the gas exceeds 600 °C, it decomposes and recomposes extremely rapidly when the temperature drops again, allowing the gas to recover its dielectric strength. This recovery property makes it particularly suitable for HV AC circuit breakers. However, at 140 °C, after operating for more than 25 years, it decomposes by approximately 5%. Some breakdown byproducts are known to be of greater toxicity, particularly those that occur during discharges in the presence of oxygen and hydrogen atoms (water). Table 2 reports some decomposition products of SF6 as well as their toxicity depending on their tolerated quantity [2]. Among these byproducts, S2O2F10 and HF are very toxic in addition to being very corrosive. To reduce the amounts of SF6 in HV apparatus, one solution consists of diluting it in N2 or CO2. Many investigations have been conducted on this topic. A well-documented synthesis on SF6-N2 mixtures has been published by L. G. Christophoru and R. J. Van Brunt [3]. These authors reported that such mixtures may help to reduce SF6 rate of release into atmosphere from power-system applications and serve as an intermediate step in the environmental control of this gas. However this solution requires high pressure and does not eliminate the potential contribution of SF6 to global warming.
Thus the various negative characteristics of SF6, indicated above, have led the international community to designate it as a greenhouse gas (the 1997 Kyoto Protocol-COP3) and to regulate its use, encouraging at the same time research into new more environmentally friendly alternatives gases and the reduction of SF6 emissions in the atmosphere [1]. Note that, despite a high GWP, it should be emphasized that SF6 gas contribution to the global greenhouse effect is less than 0.3% because of its low concentration relative to CO2 and this number is steadily decreasing.
Naturally, the initial investigations were focused on natural gases (N2, CO2 [4], and dry air [5]) and the halogenated products that have reduced GWP such as trifluoroiodomethane (CF3I) [6], perfluorinated ketones [7], hydrofluoroolefins (HFOs) [8] and fluoronitriles [9]. The goal of such investigations was to identify new eco-friendly candidates with equivalent dielectric strength to SF6. Unfortunately, polyfluorinated gases have a high boiling point, which is a drawback for their use in low-temperature electrical apparatus applications. To overcome this shortfall, these halogenated gases are mixed with ordinary carrier gases such as dry air, N2, or CO2. Note that the substitute gas or gas mixture must be non-corrosive and must not interact with materials (insulating materials and metals) of the gas-insulated systems in which it is used.
The aim of this paper is to carry out a review of the main characteristics of the candidate gases and gas mixtures with a low environmental impact that have a potential to replace SF6This will allow better understanding of their performance under the conditions of electrical (especially the dielectric strength) and thermal stresses with the complex geometries encountered in GIS equipment

Properties Required for Gas/Gas Mixture Substitutes
The properties required for the use of a gaseous dielectric in HV equipment vary according to the nature of the application and the operating conditions. The intrinsic properties are those inherent to the molecular structure and atomic physics of the gaseous molecule. These properties are independent of the application and environment where the gas is placed. The dielectric strength of the gas/mixture depends mainly on its capability to reduce the density of electrons that are generated when it is subjected to an electric field. For this purpose, the gas must be electronegative to reduce the number of electrons by attachment. This attachment capability is efficient up to an energy level of the electrons and is dependent on the temperature of the gas. Also, the gas must be able to slow down electrons to capture them efficiently at lower energies and thus avoid the generation of other electrons by impact ionization.
Before considering the use of a given insulation gas in industrial applications, it is necessary to understand the secondary process that covers the emission of electrons by impact of ions and photons on the surface of the cathode, with the photonic processes being a factor controlling the development of a discharge in a non-uniform electric field, ion-molecule reactions, dissociation under the action of collision decomposition, reactions with surfaces, and traces of impurities. Furthermore, the gaseous dielectric must also have a high saturation vapor pressure for low temperatures, high thermal conductivity to provide a cooling function, high specific heat, thermal stability at a temperature above 400 K over long periods of time, chemical stability and insensitivity to conductive or insulating materials, non-flammability, non-explosivity, and toxicity acceptable for industrial exposure including epidemiological effects (eco-toxicity, mutagenic or carcinogenic effects on health). If used as a mixture, the candidate gas must have thermodynamic properties suitable for uniformity of composition and separation of the mixtures.
As far as the extrinsic properties are concerned, the gas must not undergo extensive decomposition or polymerization, and no formation of carbon or other types of deposition. Independently, the gas must be ecologically compatible with the environment, not contribute to global warming or stratospheric depletion of ozone, and must not persist in the environment for long periods of time.

Perfluorocarbons (PFCs)
It is accepted that fluorinated compounds have the best insulation performance. This is helped by the presence of fluorine, a very electronegative element. In research efforts, particular attention was paid to some families of fluorinated molecules including perfluorocarbons (PFCs), which are gaseous halogen compounds of the fluorocarbon family (FC). These fluorinated electronegative gases are composed exclusively of carbon and fluorine atoms. The main PFCs proposed as gaseous dielectric are CF4, C2F6 C3F8, and C4F8. These have high dielectric strength (breakdown voltage); Figure  1 gives the variation of the breakdown voltage versus the pressure-electrode gap product (P.d), i.e., Paschen curves of these gases [10]. Table 3 gives the main properties of these gases. Note that among these gases, C4F8 appears to be the most interesting from the point of view of the dielectric strength, which is 1.25 to 1.31 times higher than that of SF6 [11]. However, its high liquefaction temperature makes its use very difficult. Nevertheless, it remains a promising gas component for mixtures intended for gaseous insulation [12,13]. Most of these compounds generate corrosive and toxic decomposition products. To add to these undesirable properties, the GWP of PFCs and HFCs are in the high ranges from 5700 to 11,900 and from 9700 to 12,000, respectively [15], which makes most of these molecules not suitable as candidates as an insulation gas alternatives. Table 3. The main properties of CF4, C2F6 C3F8, and C4F8 [13,16]; the dielectric strengths of these gases is expressed in relative value with respect to that of SF6, which is taken to be equal to 1. To circumvent the problem associated with the high liquefaction temperature and to reduce the GWP, these fluorinated components can be mixed with buffer gases as CO2 and N2. Table 4 gives the boiling temperature and other characteristic parameters of the selected gases. Thus, many investigations have been conducted on the dielectric strength of the above gases when mixed with N2 and CO2 in different electrodes geometry under AC, DC, and lightning impulse voltages [11][12][13]17]. So, it was reported that to obtain 20% C3F8-80% N2, equivalent to 20% SF6-80% N2 in terms of dielectric strength, a pressure of 1.25 times greater is required; the GWP of this mixture is 0.37 times that of 20% SF6-80% N2 [17]. Note that the breakdown voltage of C3F8 alone is comparable to that of SF6. Under the same pressure, a proportion of 50% C3F8-50% N2 is required for a GWP of 0.73 times that of the 20% SF6-80% N2 mixture. When it is desired to obtain 20% C3F8-80% N2 comparable to 100% SF6, always in terms of dielectric strength, the pressure must be increased by 1.59 times for a GWP of 0.009 times. It is a mixture that makes it possible to obtain a weak GWP. It is also a good candidate to replace SF6 within reasonable pressure ranges. Table 4. Some properties of other selected gases [14].

Gas
Relative Also, to obtain a 20% C3F8-80% CO2 mixture comparable to 20% SF6-80% N2 in terms of dielectric strength, a pressure of 1.35 times greater is required for a GWP of 0.40 times that of SF6-N2 [17]. At the same pressure, a proportion of 60% C3F8-40% CO2 mixture is required with a GWP of 0.87 times. Finally, to obtain a 20% C3F8-80% CO2 mixture comparable to pure SF6, a pressure of 1.69 times greater is necessary, which lowers the GWP by 0.10 times. The C2F6 alone has a breakdown voltage of 76% that of SF6. To obtain 20% C2F6-80% N2 comparable to 20% SF6-80% N2 in terms of dielectric strength, the pressure must be 1.35 times greater for a GWP of 0.52 times that of SF6-N2. To obtain the same gas mixture equivalent to pure SF6, the pressure must be 1.69 times greater, for a GWP of 0.13 times that of SF6-N2 [17]. All this shows that C2F6-N2 is better than SF6 from the GWP point of view, but seems slightly less realistic than C3F8-N2 in terms of gas pressure. To obtain a 20% C2F6-80% CO2 mixture comparable to 20% SF6-80% N2 in terms of dielectric strength, the pressure must be increased by 1.49 times for a GWP of 0.58 times that of SF6-N2. And to obtain the same proportions of the previous mixture comparable to 100% SF6, the pressure must be 1.89 times greater, which lowers the GWP by 0.15 times.
The breakdown voltage of C2F6 is lower than that of C3F8 and SF6. Among these gases, the 20% C3F8-80% N2 mixture has attracted a lot of attention because it not only reduces GWP below 1/10 compared to SF6-N2 but also gives good dielectric properties without the addition of SF6. For this mixture, the required gas pressure is 0.79 MPa (realistic value) and the obtained GWP is 9.3%.
It has also been reported that the 2-C4F8-N2 mixtures constitute promising substitutes for SF6 [18]. Such mixtures have extremely low GWP in comparison to SF6. However, for outdoor application down to low temperatures, 2-C4F8 mixtures might not be possible due to the liquefaction.
Although this paper is mainly focused on gases/mixtures without SF6, the SF6-CO2 mixture has to be underlined. Indeed, this mixture has been successfully applied in outdoor circuit breakers designed in Manitoba (Canada) [19]. An equally proportioned SF6-CF4 mixture (50-50%) makes it possible to obtain excellent arc extinguishing properties and reduce the liquefaction temperature. Berg [20] showed that AC and impulse breakdown voltages in a quasi-uniform field (sphere-sphere electrodes) increases when SF6 content is increased. These results have been confirmed by the experimental studies carried out by Kuffel and Toufani [21] at 60 Hz alternating voltage (AC), DC (for both polarities), and impulses in a highly non-uniform field, with SF6-CF4 mixtures and percentages of SF6 varying from 0 to 100% at a pressure of 0.3 MPa. The breakdown voltage of the SF6-CF4 mixture (SF6 content ranging from 0 to 100%) in a highly inhomogeneous field increases under DC (+) and AC voltages when a few percent of SF6 is added. This effect is more pronounced for large inter-electrode distances when the divergence of the field increases and stabilization of the corona effect can occur, leading to high breakdown values. Under impulse voltage without stabilization of the corona effect, the breakdown voltage increases almost linearly with the percentage of SF6. Note that theoretical analyses have been conducted on the synergism in the dielectric strength for SF6-CF4 mixtures [22] and assessment of electron swarm parameters and limiting electric Fields in SF6 + CF4 + Ar gas mixtures [23].
However, it is important to note that the use of the SF6-CF4 mixture in practice requires special attention because the combination of these two compounds could increase the rate of decomposition. Moreover, in countries where the moisture content is very high, it is possible that the formation of the HF compound could lead to high corrosion. Although the decomposition rate of CF4 is unlikely to be greater than that of SF6, it could lead to carbon deposition. As for the formation of the HF compound, due to a very high humidity level, the presence of water in SF6 or in equipment insulated with SF6-CF4 mixtures should be avoided at all costs.
Although molecules without the fluorine atom do not have the same dielectric characteristics, several gases such as N2, CO2, air, H2, and CH4 allow good stabilization and extinguishing of the electric arc. H2 and CH4 are unusable because of the danger of explosion in case of electrical discharge. Carbon dioxide (CO2) has a 35% insulation capability compared to that of SF6. CO2 has some advantages compared to N2 or air due to its better current interruption capabilities. Its reduced dielectric strength can be compensated for either by increasing the operating pressure or by enlarging the dimensions of the high-voltage equipment.

Natural Gases: N2, CO2, and O2
These are freely available in the atmosphere and are abundant poly-atomic gases; Table 5 gives a typical composition of atmospheric air. They are commonly used at high pressure as insulating materials and as a medium for extinguishing the electric arc in HV switches. It should be noted that CO2 and N2 are the most environmentally suitable candidates given their low ozone depletion potential (ODP). Table 6 gives the dielectric strength of these gases [24]. In practice, the dry air insulation and mixtures of N2-O2 (80% N2-20% O2, 60% N2-40% O2 and 40% N2-60% O2) under lightning impulse voltages exhibit better performance (dielectric strength) than those of N2 or CO2 on their own [25]; and the dielectric strength under positive lightning impulse voltages for dry air, CO2, N2, and their mixtures are higher than the negative ones. Also, the dielectric strength of N2 and N2-CO2 mixtures increases quasi-linearly with pressure up to 1.6 MPa under lightning impulse voltage (LI) [4]. This quasi-linearity has been also observed for N2 and CO2 in AC, DC, and lightning impulse voltages by Beroual and Coulibaly [26].
On the other hand, the dielectric strength of CO2 is higher than that of N2 under impulse voltage [27]. Unlike for CO2, the dielectric breakdown in N2 occurs on the tail of the lightning impulse. This phenomenon can be explained by the electropositive nature of N2. Indeed, CO2 is a low electronegative gas, attaching when the electrons have an energy in the range of 7-9 eV or 20-50 eV. The mechanisms of generation of the first avalanche are different for carrier gases, which are weakly attaching or non-attaching (CO2 and N2). Under negative impulse voltage and for non-uniform fields, electrons generated at the cathode surface immediately enter the intense electric field (cathode). However, most of the free electrons generated in the low electric field zone have lower energy and velocity so that their attachment to CO2 molecules is easier. Therefore, additional energy is required to cause the growth of the electronic avalanche in such cases. In addition, it was found that CO2 breakdown values are stable and reproducible, and represent less dispersion than those of N2 [4]. In contrast to SF6, the total attachment of electrons in dry air is extremely low, especially at low energy (0-1 eV). This is due to the effective cross-section of O2 being small and N2, which is the main component of the air (see Table 5), being an un-attaching gas for electrons. On the other hand, an 85% CO2/15% O2 mixture shows a marked improvement of more than 15% compared with CO2 in a divergent field under positive polarity [27].
Extensive experimental testing has demonstrated that CO2 exhibits better repeatability of dielectric performance compared with N2 [4,25]. This is an influencing parameter that should be taken into account during the design process.
It appears from the above that pure CO2 gas and/or its mixtures are suitable candidates for insulation in HV equipment. The dielectric strength can be improved by increasing the gas pressure to obtain a gas/gas mixture having the same static breakdown voltage as that of pure SF6; CO2 pressure needs to be three times than that of SF6 to achieve the same level of insulation performance. It should be emphasized that, with the increase of the working pressure in GIS, a new dimensioning of the mechanical structure would be required to withstand the higher pressure. Thus, the economic impact of such a solution needs to be taken into account.

Trifluoroiodomethane (CF3I) and Mixtures CF3I/CO2
Trifluoroiodomethane (CF3I) has excellent dielectric properties equivalent to SF6. Its GWP is of the same order as CO2, and its ozone depletion potential is less than 0.08 [28]. In addition, it has a very short life time in the atmosphere (less than one day), and hence is considerably more environmentally friendly. It is a colorless, non-flammable electronegative gas. Its molecular geometry is given in Figure 2. It has a molecular weight of 195.91 g/mol. However, CF3I has a boiling point of about −22.5 °C, which is not suitable for use in cold regions. Table 7 lists the main properties of this chemical element.  CF3I is a moderately toxic gas [31]; it is classified as a mutagen type 3 [32]. Table 8 gives the toxicity indicators and their values for CF3I. However, these properties are being investigated again to examine the evidence presented before. Indeed, the C-I bond of the molecule is easily dissociable even at a low intensity of radiation in the UV or visible region. Consequently, CF3I produces di-iodine I2 (acute toxicity) by decomposition, following the appearance of a partial discharge or an electric discharge [33].
If not adequately taken care of, the production of di-iodine can cause pollution of the HV apparatus and considerably affect the interruption (extinguishing) capacity and the dielectric strength of the gas.
2CF3I  2CF3 + 2I  C2F6 + I2 CF3I has better insulation properties than SF6, both in uniform and non-uniform electric fields. The critical field of CF3I is about 1.2 times higher than that of SF6.
Because of its high liquefaction temperature, this gas cannot be used on its own in HV equipment. Indeed, its boiling point is 25°C at 0.5 MPa which is the SF6 filling pressure in GIS. That is why it is used with a gas buffer such as CO2. Such a mixture (CF3I/CO2) has very good insulation performance. Its breakdown voltage increases linearly with the molar fraction of CF3I. A 60% mixture of CF3I with CO2 has similar insulating characteristics to pure SF6 (Figure 3a,b) [34]. The dielectric strength of the CF3I/CO2 mixture (30%/70%) is 0.75 to 0.80 times of that of pure SF6. With this gas mixture, good insulating properties have been demonstrated. Further investigation of its interruption capability needs to be undertaken. Figure 4 shows the effective ionization coefficients of CF3I and its mixtures compared with SF6 and air. It clearly shows the suitability of the dielectric insulating properties of CF3I mixtures.

Fluoroketones (FKs) and Fluoroketone Mixtures
The fluoroketones (FKs) or perfluorinated ketones have a crude formula of the form CnF2nO, in which n is an integer ranging from 3 to 8 ( Figure 5). Table 9 summarizes the important characteristics of two perfluorinated ketone candidates.  The vaporization of perfluoroketone is obtained above 24 °C. Because of this high boiling point, the gaseous state of this chemical element is maintained at a pressure lower than atmospheric pressure. Fluoroketones can only be used as an additive at a pressure below the saturated vapor pressure (Figures 6 and 7). Indeed, by adding fluorine (halogen element) in the chemical formulation, the dielectric strength of the complex molecule is improved. In contrast, the boiling point increases considerably. Therefore, this gaseous compound cannot be used alone for insulation applications where the operating temperature of the GIS may drop to −30 °C or below.  A mixture with a low concentration of a fluoroketone and a buffer gas results in an improvement of the dielectric strength. The analysis of the effective ionization coefficients in C4F8O/CO2 and C4F8O/N2 mixtures shows a strong dependence of the effective ionization coefficient on the complex gas density [39]. For high values of gas density, the improvement of the attachment properties of complex molecules can be explained by the three-body electron attachment mechanism [40]. The critical reduced electric field varies linearly as a function of the C4F8O content for low concentrations. It is also important to take into account the slope of the straight line in the ionization zone in the vicinity of the critical value which corresponds to a zero effective ionization coefficient [40]. The slope plays a role in determining the limit value of the sensitivity of the gas mixture to the surface condition (state) of the electrodes (roughness, particles, etc.). The sensitivity value (pressure × defect height) is inversely proportional to the slope of ionization line (straight). Also, the increase of the attachment rate in the vicinity of the critical field is important for evaluating and comparing the potential of the gas mixture.
The fluoroketones do not exhibit, after ionization in plasma state, a capability to recompose analogous to that present in SF6, and therefore the quantity of fluoroketone, which is initially present in the gaseous state in an extinguishing apparatus of an electric current, decreases as the number of extinctions achieved by this apparatus increases. Fluoroketones C5F10O and C6F12O are substantially non-toxic in the pure state (Table 10) and present high insulation capabilities, in particular a high dielectric strength (or breakdown field strength), and at the same time an extremely low global warming potential (GWP) can be obtained [36,41,42].
In a systematic dielectric test performed on all FKs in different mixtures, Hyrenbach et al. [43] showed that, thanks to a 20 °C lower boiling point, the C5 FK gas mixture's dielectric performance is superior compared to the C6 FK mixture. C4 FK mixtures have shown even better results; however, those mixtures have to be excluded due to their toxicity profile. The dielectric performance of the C5 FK gas mixture is not as good as 100% SF6. Only 12% of the gas mixture is C5 FK.

Hydrofluoroolefins (HFOs)
The general formula of HFO gases is Cn(H,F)2n. These are synthesized molecules of the hydrofluoroolefin family such as HFO-1234ze and HFO-1234yf, or HFO-1336mzz-Z. They have good dielectric strength properties, and their GWP is less than 9; the insulation properties of HFO-1234ze are closed to SF6, with a dielectric strength in the range comprised of 0.8-0.95 that of SF6. Table 11 summarizes the physicochemical properties of the hydrofluoroolefins candidates. The performance of a hydrofluoroolefin is dictated by its large molecule, i.e., it has a large number of carbon and fluorine atoms, and the molecule is branched. However, the larger the hydrofluoroolefin is, the higher its liquefaction temperature will be, i.e., its saturated vapor pressure (SVP) at a given temperature is low; the gas liquefies already at around 0.42 MPa at room temperature. One of the downsides of HFO 1234 yf is its extremely flammable nature and, for HFO-1336mzz-Z, the high boiling point and the very limited SVP are crucial. For this reason, HFO-1234ze is chosen because it is nonflammable. HFO-1234ze can probably be used in its pure state for replacing SF6 in medium-voltage applications (MV); the operating temperature is limited to −15 °C. On the other hand, mixtures of HFO-1234ze with other dilution gases such as nitrogen, carbon dioxide, or dry air are required for high-and medium-voltage applications at lower operating temperatures. However, HFO-1234ze is characterized by a high dielectric strength, (E/N)critic, which is highly dependent on the pressure [47]. The attachment performance of the HFO-1234ze increases gradually with the increasing pressure of the pure gas [48]. The critical stationary value (E/N) = 305 Td (recall that 1 Td = 10 −21 V.m 2 ), which corresponds to 85% of that of SF6, is obtained for pressures greater than 0.165 MPa.
Moreover, these molecules decompose in atmosphere to form trifluoroacetic acid (TFA), which is an extremely persistent substance that accumulates in nature without decomposing. On the other hand, a ternary mixture (HFO-1234ze, C5F10O, buffer gas) in gaseous form, which is suitable for lowtemperature applications, improves the dielectric strength. However, because the molecules of fluoroketones and hydrofluoroolefins are very similar in composition, the generation of a liquid phase at a temperature above the liquefaction temperature of the gases in contact is noted and explained by Raoult's law. In addition, following a flashover in HFO1234ze, a carbon deposit appears on the solid insulation; this conductive layer can short-circuit the insulator support in GIS equipment.

Fluoronitriles and Fluoronitriles Mixtures
Heptafluoro-iso-butyronitrile or fluoronitrile gas belongs to the family of the fluorinated nitriles of the general formula CnF2n+1CN. The fluoronitrile dielectric fluid was synthetized, patented, and commercialized by 3M TM Company (Saint Paul, MN, USA) under the name of NOVEC TM 4710 according to the molecular model (CF3)2CFCN shown in Figure 8 [49]. Unfortunately, fluoronitrile has a high liquefaction temperature. Given the minimum temperature at which the MV or HV equipment has to operate, for example −30 °C for temperate zones, the maximum absolute pressure without liquefaction of fluoronitriles (measured at 20 °C) is limited to 0.31 bar abs. Therefore, due to their high boiling point, fluoronitriles should be mixed with other buffer gases such as CO2, dry air, or N2. Fluoronitrile gas mixtures could form an ideal compromise between dielectric performance and minimum operating temperature of the apparatus while providing a considerable reduction of the environment impact. The mixing of fluoronitriles with CO2 is an effective technique that enables us to obtain optimized insulating properties combining the advantageous features of each component: the high dielectric strength of NOVEC TM and the low boiling point of CO2. Therefore, to obtain a given gas mixture for a minimum operating temperature, the partial pressure of the fluoronitriles compound is adjusted to avoid liquefaction. Furthermore, the desired dielectric strength is obtained by adding a sufficient proportion of carbon dioxide. Figure 9 illustrates the variation of the saturating vapor pressure of the studied fluids versus the temperature. Fluoronitrile gas compound is a polar gas that has a specific mass of 8.11 kg/m ; its purity is greater than 98.54%. It is odorless, highly electronegative, colorless, and chemically stable up to a temperature of about 700 °C (the appearance temperature of carbon monoxide) [50]. Tables 12 and 13 summarize the basic chemical and physical properties of fluoronitriles. Figure 10 gives the main degradation products and thermal stability of the dielectric fluid (CF3)2CFCN analyzed by infrared spectroscopy (FTIR) [51]. Carbon monoxide is the main decomposition product; its dielectric strength is about 40% of that of SF6 [51].   and its ozone depletion potential (ODP) is zero. For example, the 4% fluoronitriles/96% CO2 mixture has a GWP that is 98.4% lower than that SF6.
The mixtures of fluoronitriles with CO2 are classified in the lowest risk category according to Regulation (EC) No. 1272/2008 of the European Parliament and of the Council of 16 December 2008 on the classification, labeling, and packaging of substances and mixtures [53]. Indeed, the calculated lethal CO2 concentration is above 300,000 ppmv, and it is 12,000 ppmv for fluoronitriles [53]. The recent tests and analysis performed on the NOVEC 4710 conclude that this gas is not CMR (Carcinogens Mutagenic Reprotoxic) and the TLV TWA (Threshold Limit Values Time Weighted Average) is equal to 65 ppm [54]. Table 14 summarizes the toxicity of degradation products of Fluoronitriles and Table 15 summarizes the main characteristics of fluoronitrile gas compared with SF6 and CO2. Recent studies have shown that fluoronitriles mixed with a buffer gas such as N2 or CO2 could be a potential alternative to SF6 [54]. Such mixtures represent an ideal compromise between the operating temperature, the global warming potential, and a dielectric strength that is comparable to that of SF6. The fluoronitrile/CO2 mixture was presented for the first time at CIGRE 2014 by Alstom Grid (Villeurbanne, France), now GE Grid Solutions (Villeurbanne, France), and at CIRED 2015 as a candidate gas to replace SF6 for HV switchgear insulation [55][56][57].
The fluoronitrile has a dielectric strength 2.7 times greater than that of SF6 in uniform field [58] for both AC and DC voltages. The ratio VFluoronitriles/VSF6 decreases for divergent fields, in particular for the positive impulse voltage. Figure 11 depicts examples of the breakdown voltage characteristics for sphere-to-sphere and rod-to-plane electrodes configurations.  Experiments conducted on gas mixtures consisting of CO2 and fluoronitrile Novec TM 4710 in concentrations ranging 3.7 to 20% with different electrode geometries (namely plane-to-plane, sphere-to-sphere, sphere-to-plane, and rod-to-plane; and different field utilization factors), under AC and lightning impulse (LI) voltages show that in a quasi-homogeneous electric field, the equivalent dielectric strength with 0.1 MPa SF6, at identical total pressure, is reached by a mixture containing 20% fluoronitriles [58]. The 3.7% fluoronitriles/96.3% CO2 mixture constitutes a good compromise and an appropriate gas mixture for high-voltage apparatus insulation in view of the pressure and low ambient temperature applications (−30 °C). This compromise takes into account different criteria including the working pressure and temperature as well as the environmental impact (global warming potential). This mixture, in a uniform electric field, reaches 72% of the dielectric breakdown of pure SF6 at 5.5 bar absolute. In homogeneous and quasi-homogeneous fields, equivalences to 0.55 and 0.65 MPa SF6 are obtained with 3.7% Fluoronitriles/96.3% CO2 mixtures at 0.88 and 1.04 MPa total pressure, respectively, and lead minimum operating temperatures of −30 and −25 °C, respectively. Figure 12 depicts the variation of AC breakdown voltage of SF6 and two fluoronitriles/CO2 mixtures as a function of electrode gap distance at 0.1 MPa abs (absolute pressure) in a sphere-tosphere electrode arrangement referred to the equivalent breakdown (BD) in uniform configuration using the field utilization factor. Figure 13 gives the negative LI breakdown voltages of 3.7% fluoronitriles/96.7% CO2 mixture and pure SF6 at different pressures as a function of the sphere-toplane electrodes gap.
Thus, the addition of a small amount of fluoronitriles (a few percent by volume) to carbon dioxide (CO2) results in an appreciable improvement of the breakdown voltage compared to CO2 (3 times lower than SF6) [60]. At atmospheric pressure, a mixture at a percentage of 20% reaches the same dielectric performance as pure SF6. This mixture offers the possibility to be used below −30 °C without liquefaction and, as such, constitutes an interesting possibility for replacing SF6 as a dielectric medium in MV switchgear.

Conclusions
In this paper, an overview of research into new eco-friendly candidates for replacing SF6 has been presented. The main properties of natural gases and potential candidates have been discussed. These latter include trifluoroiodomethane (CF3I), perfluorinated ketones, octafluorotetra-hydrofuran, hydrofluoroolefin (HFOs), and fluoronitriles. One of the main drawbacks of these polyfluorinated gases is their high boiling point, which limits their use in low-temperature electrical apparatus applications. To overcome this shortfall, these halogenated gases are mixed with buffers (ordinary carrier gases) such as dry air, N2, or CO2. Such mixtures could form an ideal compromise between dielectric performance and the minimum operating temperature of the apparatus while providing a considerable reduction in the environment impact.
Through this synthesis, the following points are highlighted: -The use of dry air, nitrogen, and carbon dioxide requires a significant increase in the pressure and size of equipment. In case of surface defects in equipment, a significant decrease withstand voltage was observed. - The mixtures of trifluoroiodomethade and nitrogen or carbon dioxide (CF3I/CO2 and CF3I/N2) have promising insulation properties but concerns are raised about CF3I due to its carcinogenic, mutagenic, and toxicity properties (type 3). Although CF3I has a high boiling point (−22.5 °C at 0.1 MPa) in its pure form, the temperature can be improved when mixed with CO2 and N2. A mixture of 20% CF3I-80% CO2 gives an equivalent dielectric strength to SF6. - The mixtures of perfluorinated ketones (C5F10O and C6F12O /Technical air or CO2) have a high boiling point (24 °C or 49 °C at 0.1 MPa) due to their higher molecular weight and higher minimum operating temperature compared with SF6. Fluoroketones C5F10O and C6F12O present high insulation capabilities, in particular a high dielectric strength (or breakdown field strength), and at the same time enable an extremely low global warming potential (GWP).
-HFO-1234ze exhibits a higher operation temperature than SF6 (limited to −15 °C). When subjected to breakdown, the gas can decompose, which results in carbon dust being deposited on the electrodes. However, despite the high boiling temperature, the insulation properties of HFO-1234ze are closed to SF6, with a dielectric strength in the range comprised of 0.8-0.95 times that of SF6. - The fluoronitriles could be also a potential alternative to SF6. Unfortunately, they have a high boiling point. Like for CF3I, this can be mitigated by mixing them with buffer gases such as CO2, dry air, or N2. As concerns the dielectric strength at atmospheric pressure, a mixture of 20% fluoronitriles-80% CO2 reaches the same dielectric performance as pure SF6.