Alternative Environmentally Friendly Insulating Gases for SF₆

Abstract

Sulfur hexafluoride (SF₆) shows excellent insulation performance as an insulating gas. It is suitable for various climate conditions due to its low boiling point (−64 °C). Therefore, it has been widely used in power grid equipment. However, its global warming potential (GWP) is 23,500 times higher than that of CO₂. Thus, it is imperative to find an environmentally friendly insulating gas with excellent insulation performance, lower GWP, and which is harmless to equipment and workers to replace SF₆. In this review, four possible alternatives, including perfluorocarbons, trifluoroiodomethane, perfluorinated ketones, and fluoronitrile are reviewed in terms of basic physicochemical properties, insulation properties, decomposition properties, and compatibility with metals. The influences of trace H₂O or O₂ on their insulation performances are also discussed. The insulation strengths of these insulating gases were comparable to or higher than that of SF₆. The GWPs of these insulating gases were lower than that of SF₆. Due to their relatively high boiling point, they should be used as a mixture with buffering gases with low boiling points. Based on these four characteristics, perfluorinated ketones (C₅F₁₀O and C₆F₁₂O) and fluoronitrile (C₄F₇N) could partially substitute SF₆ in some electrical equipment. Finally, some future needs and perspectives of environmentally friendly insulating gases are addressed for further studies.

1. Introduction

In high-voltage transmission systems, gas insulation has the advantages of being light weight, cost-effective, having simple manufacturing construction, and recyclability when compared with liquid or solid insulation. Thus, it has been widely applied in power grids all over the world. At first, a mixture of CCl₄ vapor and air was used as insulating gas. Herb and Rodine found that CCl₄ vapor and air can synergize and enhance the dielectric strength, especially with a lower CCl₄ concentration [1]. Charlton and Cooper found that CCl₂F₂ and CF₄ also showed better dielectric strength than N₂ [2,3]. SF₆ was first patented as an insulating gas by Cooper in 1938 (Figure 1). Since then, it has been studied systematically. It has been noted for its arc quenching capability and insulating properties. The excellent arc quenching capability is because of its high heat capacity, dissociation, and reassembly properties. The high dielectric strength can be attributed to its large molecular weight, complexity, and electron affinity, which affects the reaction between gas molecules and free electrons [4]. The decomposed products of SF₆ can recompose again when the temperature decreases, which ensures that the insulation strength is maintained well. As a result, it decomposes by only about 5% after working at 140 °C for 25 years [5]. Besides, it is non-poisonous, chemically stable, and non-flammable, which provides security for operation in practical applications. Considering the dielectric strength, cost, stability, toxicity, and liquefaction temperature, SF₆ stands out as the best insulating gas. It has been widely used in air-insulated switchgear (AIS) and gas-insulated switchgear (GIS) since the 1960s [6].

However, SF₆ has also caused serious environmental problems. It was identified as one of the seven greenhouse gases in the Kyoto Protocol. It shows a remarkable absorption at infrared frequency and absorbs upward radiance 42,000 times more effectively than CO₂, thus causing a great greenhouse effect [7]. The global warming potential (GWP) of SF₆ is 23,500 times higher than that of CO₂ over a 100 year integration time horizon according to the report of Intergovernmental Panel on Climate Change (IPCC) in 2013, and its lifetime in the atmosphere reached to 850 years with an uncertainty range of 580–1400 years [8]. With the rapid development of electrical insulation media, large amounts of SF₆ have been leaking or discharged into the atmosphere. In fact, the concentration of SF₆ in the atmosphere increased by 20% from 2010 to 2015 (Figure 2) [9]. It was estimated that the emissions will reach 4270 ± 1020 t in 2020 [10]. The greenhouse effect caused by SF₆ will be incalculable. Besides, when water vapor exists in the insulating equipment containing SF₆, the reaction generates SOF₄, SO₂F₂, S₂F₁₀, SF₄, HF, and SO₂. Among the products, SO₂F₂, S₂F₁₀, and SF₄ are highly toxic. HF and SO₂ are corrosive for insulating equipment [11]. Thus, the use and emission of SF₆ should be seriously restricted. One approach to reduce the emission of SF₆ was to replace some of the SF₆ with other inert substances with lower GWPs. For example, SF₆/N₂ was selected. Although the dosage of SF₆ is decreased, the GWP of a SF₆(10%)/N₂ mixture by volume is still unacceptable, at 8650 [9].

Therefore, an environmental alternative to completely replace SF₆ is necessary and urgent. It should meet the features of low GWP, no ozone depletion potential (ODP), it should be non-toxic or hypotoxic, and have high dielectric strength, good thermal conductivity, low boiling point, good compatibility with switchgear materials, etc. [5,9]. Herein, we have reviewed the pioneered studies about environmentally friendly insulating gases, including perfluorocarbon, trifluoroiodomethane, perfluorinated ketones, and fluoronitrile (

Table 1. Basic properties of compounds used in electrical insulation. GWP: global warming potential; ODP: ozone depletion potential.

Chemical Formula	GWP/100-Years	Lifetime/Years	Dielectric Strength Relative to SF6	Boiling Point/°C	Toxicity	ODP	Flammability
SF6	22,800	850	1	−64	Non-toxic	0	Non-flammable
CF4	9200	50,000	0.4	−128	Low-toxicity	0	Non-flammable
C2F6	12,200	10,000	0.76	−78.1	Non-toxic	0	Non-flammable
C3F8	8830	2600	1.01	−36.7	Non-toxic	0	Non-flammable
c-C4F8	8700	3200	1.3	−8	Non-toxic	0	Non-flammable
CF3I	0.4	0.0055	1.23	−22	Non-toxic	0	Non-flammable
C5F10O	1	0.044	1.5–2	27	Non-toxic	0	Non-flammable
C6F12O	1	0.014	2.7	49	Non-toxic	0	Non-flammable
C4F7N	2100	22	2	−4.7	Non-toxic	0	Non-flammable
CF4	6630	50,000	0.4	−128	-	-	Non-flammable
CO2	1	-	0.32–0.37	−79	Non-toxic	0	Non-flammable
N2	-	-	0.34–0.43	−196	Non-toxic	0	Non-flammable
air	-	-	0.37–0.40	−193	Non-toxic	0	Non-flammable
He	-	-	0.02–0.06	−268.9	Non-toxic	0	Non-flammable
Ar	-	-	0.04–0.10	−186	Non-toxic	0	Non-flammable

). For each alternative, its basic physicochemical properties, insulation properties, decomposition properties, metal compatibility, and influence of trace H₂O or O₂ on its insulation performance are reviewed in detail in order to provide a better understanding of these compounds’ insulation performances.

2. Perfluorocarbons

Due to the electronegativity of fluorine, it is believed that perfluorocarbon has good insulation performance. Therefore, perfluorocarbons have attracted a great deal of attention as new insulation gases. The mainly proposed perfluorocarbons are CF₄, C₂F₆, C₃F₈, and C₄F₈. Their basic properties are listed in Table 1. Their GWPs are all lower than that of SF₆, they show no ozone depletion potential, comparable dielectric strength, and relatively lower GWPs relative to SF₆.

2.1. Perfluoromethane (CF₄), Perfluoroethane (C₂F₆), and Perfluoropropane (C₃F₈)

CF₄, C₂F₆, and C₃F₈ have the potential to be used in gas insulation equipment because of their strong electronegative property. However, the lifetimes of CF₄ and C₂F₆ are as long as 50,000 and 10,000 years, respectively. Their dielectric strengths are both lower than that of SF₆. Further, CF₄ may cause choking disease. Therefore, CF₄ and C₂F₆ are unsuitable for gas insulation.

Meanwhile, C₃F₈ is harmless to O₃ in the stratosphere. It also has low toxicity, good thermal stability, relatively low boiling point, and comparable dielectric strength to SF₆. The GWP of C₃F₈ is 8830, which is 38.7% that of SF₆. The breakdown voltages of C₃F₈/N₂ or C₃F₈/CO₂ have a significant linear correlation with the ratio of C₃F₈. The GWPs of C₃F₈ (12%)/N₂ (2736) and C₃F₈ (12%)/CO₂ (6612) were found to be 12% and 29% of that of SF₆ (22,800), respectively [12]. The C₃F₈/N₂ mixture exhibited higher dielectric strength than that of C₃F₈/CO₂. When the ratio of C₃F₈ was 20%, the insulation strength of C₃F₈/N₂ reached 60% of that of pure C₃F₈. The insulation strength under 0.79 MPa was comparable to that of SF₆ at 0.5 MPa. Besides, the liquefaction temperature decreased to −30 °C and the GWP also decreased greatly [13]. Thus, it is feasible to apply C₃F₈/N₂ or C₃F₈/CO₂ in practical insulation equipment.

2.2. Perfluorocyclobutane (c-C₄F₈)

Among CF₄, C₂F₆, C₃F₈, and c-C₄F₈, c-C₄F₈ exhibits the highest dielectric strength [14]. The insulation strength is about 1.3 times higher than that of SF₆. The GWP of c-C₄F₈ is 8700, which is 38.2% of that of SF₆. Moreover, c-C₄F₈ also has the features of non-toxicity, no O₃ destruction, and high thermal stability. Thus, it has the potential to replace SF₆ as an environmentally friendly insulating gas [15]. However, due to its high boiling point (−8 °C), it should be used by mixing with CF₄, N₂, CO₂, or air. The dielectric strength of c-C₄F₈/CO₂ is higher than that of SF₆/CO₂, and the GWP of c-C₄F₈/CO₂ is much lower than that of SF₆/CO₂ [16]. Li et al. [17] studied the dielectric strength of c-C₄F₈ with CF₄, CO₂, N₂, O₂, and air mixture by Boltzmann equation. They found that c-C₄F₈/N₂ and c-C₄F₈/air mixtures showed comparable dielectric strength, which were higher than those of c-C₄F₈/CF₄, c-C₄F₈/CO₂, and c-C₄F₈/O₂. When the concentration of c-C₄F₈ exceeded 80%, the dielectric strengths of c-C₄F₈/N₂ and c-C₄F₈/air were comparable to that of pure SF₆. After 30 experimental breakdown tests, the breakdown voltage of the c-C₄F₈/N₂ decreased by only 0.76%, indicating a good self-recovery characteristic. It was also reported that the decomposition rate of c-C₄F₈/N₂ was lower than that of pure c-C₄F₈ at the same temperature, which is more suitable in practical gas insulation systems. The main decomposition path of c-C₄F₈ was from c-C₄F₈ to C₂F₄, and it further decomposed into CF₂:, F·, CF₃·, CF·, C, CF₄, and C₂F₄ [18]. However, when a certain amount of O₂ was added into the mixture gas of c-C₄F₈/N₂, the breakdown voltage decreased more and more observably with the O₂ content increase from 0% to 1%. Then, the breakdown voltage decreased slightly when further increase of the O₂ content. The breakdown voltage decreased by 4.47% after 30 breakdown tests in the presence of 3% O₂. This was mainly attributed to the relatively lower dielectric strength of O₂ and the new produced products [19]. O₂ promotes the decomposition of c-C₄F₈ and generates the very toxic and corrosive COF₂. Thus c-C₄F₈ should be used without O₂ [20].

3. Trifluoroiodomethane (CF₃I)

CF₃I is a colorless, odorless, incombustible, and stable gas. Because of the excellent electronegative property of CF₃I, its dielectric strength is 1.2 times higher than that of SF₆. Besides, the GWP of CF₃I is 1–5, which is far less than that of SF₆. The C–I bond can be easily cracked under UV irradiation. Therefore, its lifetime in atmosphere is less than 2 days, and it does not cause O₃ destruction [21,22]. According to these characteristics, CF₃I has been a potential alternative to SF₆ as a new insulating gas.

Due to its high boiling point of −22.5 °C and the formation of I₂ in pure CF₃I, CF₃I should be mixed with other gases with low boiling point, such as N₂, CO₂, O₂, air, CF₄, Ar, Xe, and He. Among the mixtures, CF₃I/N₂ showed the best insulating strength [22,23]. Li et al. [22] found that the saturated vapor pressure of CF₃I/N₂ was higher than that of c-C₄F₈/N₂, indicating that CF₃I-N₂ can be used under higher pressure. Besides, the dielectric strength of CF₃I/N₂ was higher than that of c-C₄F₈/N₂, and they were both higher than that of SF₆/N₂ [24]. The dielectric strength of CF₃I (20%)/N₂ at 0.79 MPa was 102% of SF₆ at 0.5 MPa at −10 °C. When the CF₃I concentration exceeded 65%, the insulation strength of CF₃I/N₂ was higher than that of SF₆/N₂. It was even higher than that of pure SF₆ when CF₃I concentration exceeded 70% [23]. Regarding CF₃I/CO₂, the CF₃I and CO₂ can act synergistically and enhance the physicochemical properties of CF₃I. When the ratio of CF₃I or SF₆ was 10%–30% at 0.1–0.3 MPa, the partial discharge inception voltage of CF₃I/CO₂ was 0%–20% higher than that of SF₆/CO₂. The insulation strength of CF₃I/CO₂ was comparable or even higher than that of SF₆/CO₂ [25]. In this case, both the boiling point and insulation strength could satisfy the practical requirements. The breakdown performance of CF₃I/CO₂ was also superior to that of CF₃I/N₂. In quasi-uniform and highly non-uniform electric fields, the breakdown voltages of CF₃I/CO₂ were 84% and 65% of pure SF₆, which were both higher than that of CF₃I/N₂ [26,27]. The 50% breakdown voltages of CF₃I (30%)/CO₂ and CF₃I (20%)/CO₂ under 0.1 MPa were 67.1 and 66.6 kV, respectively. For CF₃I (30%)/N₂ and CF₃I (20%)/N₂, they were 60.5 and 50.1 kV, respectively [28]. After 20 breakdown experiments, less CF₃I decomposed in CF₃I/CO₂ mixture than that in CF₃I/N₂ mixture. It was explained that CO₂ could provide an additional C source for the reaction system to maintain the C balance, which suppressed the decomposition of CF₃I [26].

According to density functional theory (DFT), the reactions of CF₃I to CF₄, C₂F₆, C₂F₄, and C₂F₅I were more energetically favorable than that to C₃F₈, C₃F₆, and I₂. Thus, the decomposition products were mainly C₂F₆, C₂F₄, and I₂. It can be clearly seen that the transparent glass changed to tawny after several experiments, indicating the formation of I₂. The products in partial discharge were stable after 20 h test [29]. Although the products cannot reassemble to CF₃I completely after discharge, there is a dynamic equilibrium among CF₃, CF₂, I, F·, and CF₃I. Thus, the insulating strength can be maintained well for pure CF₃I [30].

However, in the presence of O₂, the O· from O₂ consumes free radicals (CF₃, CF₂:) from CF₃I and generates COF₂ (Figure 3), which is a highly toxic irritant for respiratory mucosa and skin. What is more, it destroys the dynamic equilibrium among CF₃, CF₂, I, F·, and CF₃I, hindering the regeneration of CF₃I. As a result, the CF₃I content and insulation performance decreased with the extension of discharge time [31]. In order to ensure the insulation strength and safety, the O₂ content in CF₃I cannot exceed 7% and 20%, respectively [32]. Therefore, it is impracticable to use O₂ and air as buffer gases with CF₃I in GIS.

Moreover, the free radicals H· and HO produced from H₂O destroy the balance between CF₃I and free radicals, which aggravates the decomposition of CF₃I and generated C₂F₆, I₂, C₂F₄, C₂F₅I, C₃F₈, HF, H₂, COF₂, CF₃H, CF₃OH. As a result, the partial discharge initial voltage and insulating strength decrease gradually [33,34]. So, it is vital to control the content of H₂O in insulating systems.

Zhang et al. [35] studied the influence of metal particles (Cu, Al, and Fe) on the insulation property of CF₃I. They found the metal particles—especially Cu and Al—could increase the electrical conductivity and decrease the insulation strength of CF₃I. The breakdown voltages decreased with the increase of metal particles. Therefore, the metal particles in insulation equipment should be well-covered by an insulating varnish or sleeve to avoid the interaction between CF₃I and metal particles.

Zhang et al. [36] studied the feasibility of CF₃I/N₂ in gas insulating equipment. They concluded that CF₃I (30%)/N₂ at 0.3 MPa could be applied in some low-pressure insulating equipment. By increasing the total pressure or the partial pressure of CF₃I, the CF₃I/N₂ can also be applied in apparatuses requiring high insulation strength. Tan et al. [37] applied a CF₃I (20%)/N₂ mixture in 126 kV GIL (gas-insulated line). The insulation performance was 83% of that of SF₆ (20%)/N₂ and 59% of that of pure SF₆. When the pressure of CF₃I (20%)/N₂ exceeded 0.7 MPa, it could meet the insulating and safety requirements. However, authors did not consider the safety and feasibility over a long time period.

4. Perfluorinated Ketones (C₅F₁₀O and C₆F₁₂O)

Recently, it was found that perfluorinated ketones (C_nF_2nO:C₅F₁₀O and C₆F₁₂O) can act as new eco-friendly and promising insulating gases. They were initially applied in fire extinguishing applications due to their incombustibility [38,39]. Their physical property parameters can be seen in Table 1. C_nF_2nO shows high insulation capacity and its dielectric strength is 1–3 times higher than that of SF₆. Moreover, the atmospheric lifetime is just 7 days because of its instability under UV radiation, and it does not cause any damage to O₃. Therefore, it causes low greenhouse effect and other atmospheric environmental damage. However, the boiling point of C_nF_2nO (n = 5, 6) is above 27 °C, making it easy to liquify under natural conditions. Therefore, it is infeasible to apply pure fluoroketones as insulating gases, but only as additives to other buffer gases with low boiling points, such as N₂, air, and CO₂.

4.1. Perfluoropentanone (C₅F₁₀O)

Zhang et al. [40,41] studied the decomposition mechanism of C₅F₁₀O products by gas chromatography-mass spectrometry (GC-MS) and density functional theory (DFT). The reaction paths of C₅F₁₀O are shown in Figure 4. According to the relative energy change, the breakage of the C–C bond between carbonyl carbon and α-carbon atom was more likely to occur and generate CF₃CO· and C₃F₇ (Reaction A1) or C₃F₇CO and CF₃ (Reaction B1). They reacted further to generate CF₄, C₂F₆, C₃F₈, C₃F₆, C₄F₁₀, C₅F₁₂, and C₆F₁₄. The decomposition rate increased with the increase of breakdown tests, generating more products with weaker dielectric strength relative to C₅F₁₀O. The products cannot reassemble into C₅F₁₀O when the environment temperature cools down [42]. As a result, the breakdown voltage decreased gradually. Besides, they also found that when the temperature was over 625 K and 825 K, the decomposition of C₃F₇CO· and CF₃CO· was enhanced. Reactions A1 and B1 would change to spontaneous. Among the products, C₂F₆, C₃F₆, and C₄F₈ have choking, bronchitis, anesthetic, and pneumonia effect. However, the content of C₅F₁₀O in practical application is below 20% and the concentration of products is extremely low. It has been reported that during arc discharge, the product of C₃F₆ was 50 ppm and just 6.5 ppb may have leaked into the air, which was far less than the exposure threshold of 0.1 ppm [43]. Although the GWPs of CF₄, C₂F₆, C₃F₈, C₄F₁₀, and C₆F₁₄ are 7390, 12,200, 8830, 8860, and 9300 (much higher than that of C₅F₁₀O), it should be noted that the concentration of deposited products is extremely low under normal working conditions [44]. Therefore, C₅F₁₀O is safe as an insulating gas. The application in GIS does not pose a threat to the environment or human health.

As an additive, the ratio of C₅F₁₀O in mixtures is usually less than 20%. For C₅F₁₀O/N₂ mixtures, the GWPs are lower than 0.7. However, these mixtures generate some CO and CF₃CN with high toxicity. For C₅F₁₀O/air mixtures, less CO was generated and more oxygenated chemicals were generated, decreasing the toxicity of the products. However, more C₅F₁₀O would decompose in C₅F₁₀O/air mixtures than that in C₅F₁₀O/N₂ mixtures [45]. For C₅F₁₀O/CO₂ mixtures, the breakdown voltages of C₅F₁₀O (10%)/CO₂ mixtures can reach to 62% of SF₆ under 200 kPa. When the percentage of C₅F₁₀O increased to 20%, the breakdown voltage increased by 32.5% [46].

Zhang et al. [47] studied the compatibility between C₅F₁₀O and Cu by theoretical calculation. Due to the high activity of the carbonyl group in C₅F₁₀O, it could be strongly absorbed on Cu (1 1 1) surfaces by chemical bonding. However, the interaction between the F atom and Cu is weak, which contributed to physical adsorption. Besides, they studied the compatibility between C₅F₁₀O and Al or Ag. The strong interaction between C₅F₁₀O and Al (1 1 1) was chemical adsorption. The weak adsorption on Ag (1 1 1) resulted from van der Waals force. Thus, they considered that Ag is more compatible with C₅F₁₀O than Cu and Al [48].

4.2. Perfluorohexanone(C₆F₁₂O)

When adding 3% C₆F₁₂O into N₂, the liquefaction temperature was −26 °C. The breakdown voltage of the mixture gas was 1.7 times higher than that of the pure N₂, which was equal to that of SF₆ (10%)/N₂. The decomposed products of C₆F₁₂O/N₂ were mainly CO, CO₂, CF₄, C₂F₆, C₂F₄, C₃F₈, C₃F₆, CF₃CN, C₂HF₅, C₄F₁₀, C₅F₁₂, and C₆F₁₄. Similar to C₅F₁₀O/N₂, the reaction generated CF₃CN, which causes mortal danger [49]. Besides, the products (e.g., C₂F₆, C₃F₈, and C₄F₁₀) showed high insulation strength. Thus, the breakdown voltage of C₆F₁₂O (3%)/N₂ was maintained even after 100 voltage breakdown tests. With the increase of total pressure, the breakdown voltage of the mixture gas decreased gradually [50,51]. For the mixture of C₆F₁₂O and air, the generation of CF₃CN is avoided. The products are mainly CO₂, CF₄, C₂F₆, C₃F₈, and C₂O₃F₆, among which the content of CO₂ is the highest [49]. When the temperature exceeded 475 °C, the decomposition of C₆F₁₂O/CO₂ was enhanced. The possible decomposition paths are shown in Figure 5. The strength of the C–C bond is weaker than that of C–F and C=O bonds. Thus, C₆F₁₂O firstly decomposed into C₃F₇COCF₂ and C₂F₅COCF₂CF₃. Then, they further decomposed into fragments such as F, CF₃·, CF₂, CF·, C₃F₇·, CO, COF₂, and C. These free radicals cannot recombine to C₆F₁₂O. The final products were mainly CF₄, C₂F₆, C₃F₈, C₃F₆, and C₅F₁₂ and their contents decreased as follows: C₂F₆ > C₃F₆ > C₃F₈ > CF₄ [52].

In the presence of trace water, the produced HO and H aggravate the decomposition of C₅F₁₀O and produce more new products, such as C₃F₇COH, C₃F₇OH, HF, and CF₂O. The ionization parameters of the new formed products are lower than that of C₅F₁₀O, thus resulting in decreased dielectric strength. Furthermore, the newly formed CF₂O has an irritative effect on the skin and respiratory mucosa. HF can cause aggressive corrosion to equipment and irritation to humans [53]. Therefore, the presence of water negatively impacts the insulation performance of C₅F₁₀O.

During practical application, the insulating gas and equipment must have good compatibility to maintain security. Zhang et al. [54] systematically studied the compatibility between C₆F₁₂O and metal materials by combining experimental tests and theoretical calculation. C₆F₁₂O can be absorbed on the surface of Cu and Al due to chemical adsorption between C=O and C₆F₁₂O, generating metal oxide. The interaction between C₆F₁₂O and Ag is attributed to physical adsorption, and thus less C₆F₁₂O was absorbed. Overall, from their SEM images, it was seen that C₆F₁₂O did not cause serious corrosion to the surface of Cu, Al, or Ag even after reaction for 125 days. Thus, the compatibilities between C₆F₁₂O and Cu, Al, and Ag were excellent.

5. Fluoronitrile (C₄F₇N)

Heptafluoro-iso-butyronitrile (C₄F₇N), one kind of fluoronitrile, was firstly prepared and commercialized by the 3M^TM Company. It has the features of low toxicity and high thermal conduction. Its lifetime in the atmosphere is 22 years. The GWP is 2100, and its insulation strength is about one-fold higher than that of SF₆ under normal pressure. Its insulation property makes it a promising alternative to SF₆ in electrical insulation systems.

However, C₄F₇N cannot be applied alone due to its relatively high boiling point of −4.7 °C. Thus, it is necessary to add other buffer gases to decrease the boiling point of insulating gas mixtures in practical applications. The breakdown voltage of C₄F₇N (12%)/N₂ at 0.4 MPa was comparable to pure SF₆ at 0.2 MPa. Increasing the ratio of C₄F₇N in gas mixtures can effectively enhance the insulation strength Considering the minimum temperature of −25 °C in practical application, the breakdown voltages of C₄F₇N (5%)/N₂ at 0.3, 0.4, 0.5, and 0.6 MPa were 63.4%, 54.6%, 49%, and 56.4% of that of pure SF₆, respectively. Moreover, the negative partial discharge inception voltages reached 80.4%, 66.9%, 62.8%, and 68.8% of that of pure SF₆, respectively. The insulation strength of C₄F₇N/N₂ in a uniform electric field was higher than that in a non-uniform electric field [55]. The insulation strength and breakdown voltage of C₄F₇N (5%)/N₂ were 83.34% of that of pure SF₆. The breakdown voltage was maintained at 33.6 kV after 30 breakdown tests, indicating an excellent self-recovery property. The GWP of C₄F₇N (5%)/N₂ was less than 600, which was far less than that of SF₆ (22,800). The probable decomposition pathways are shown in Figure 6. C₄F₇N mainly decomposed to four free radicals (CF₃, CN, F, and C₃F₇) and the path from C₄F₇N to C₃F₄N and CF₃ was the most energy favorable. The free radicals react with each other form different products. CF₃ can react with CN, F·, and other free radicals to generate products. Among the products, C₂F₆, CF₄, and CF₃CN are dominant. Although the products (e.g., CF₃CN and C₂F₅CN) were toxic, their concentrations were extremely low. Overall, the toxicity of the products was lower than that of the products of SF₆ decomposition, and was acceptable. Besides, N₂ was more likely to decompose than C₄F₇N. Thus, it acted as buffer gas and avoided the excessive decomposition of C₄F₇N, which ensured the insulation performance [56]. For C₄F₇N, the decomposed products were mainly C₃F₇, CN, CNF, CF₃, CF₂, CF, CF₃CFCN(C₃NF₄), F, other free radicals, and CF₄. Their amounts increased with the increase of temperature. The free radicals recombined with each other and generated products such as CF₄, C₂F₆, C₃F₈, CF₃CN, CO, and so on. However, less C₄F₇N decomposed and less products were generated in C₄F₇N/CO₂ mixture due to the buffer action of CO₂. At 2400 K, the amount of products in pure C₄F₇N was 96%, while that in C₄F₇N/CO₂ was 58%. Besides, the amount of CF₄ and C decreased after introducing CO₂, thus avoiding the formation of precipitate carbon and other products with relatively inferior insulation performance [57,58].

The H· and HO· radicals generated from H₂O decomposition are active in reacting with free radicals decomposed from C₄F₇N during discharge (Figure 7). the activation energy in all possible paths with H₂O is lower than that without H₂O. Thus, the decomposition of C₄F₇N was accelerated, and the insulation performance was weakened. With the catalysis of H, the reactions generating HF, HCN, CF₃H, and other small molecules were more likely to occur. In the presence of HO·, the reaction generating CF₃OH occurred easily. The products, including CF₂O, HF, HCN CF₃CH₂CN, CF₂HCN, and CH₂FCN, are toxic, which would cause damage to operation personnel. HF and HCN would also cause severe corrosion to the equipment [59].

Zhang et al. [60] studied the compatibility of C₄F₇N with Cu (1 1 1) and Al (1 1 1). They found that the interaction between C₄F₇N and Cu (1 1 1) or Al (1 1 1) was weak. The N atom was more likely than the F atom to react with Cu (1 1 1) or Al (1 1 1) and form a weak chemical bond. So, the compatibility of C₄F₇N with Cu (1 1 1) or Al (1 1 1) was good. They also studied the compatibility of decomposition products of C₄F₇N with Cu (1 1 1) and Ag (1 1 1). C₂F₅CN, CF₃CN, COF₂, and CF₄ could be adsorbed on Cu (1 1 1) and Ag (1 1 1) surfaces by van der Waals force. The adsorption energies of products on Ag (1 1 1) surface were weaker than those on Cu (1 1 1) surface. Overall, the compatibility of decomposition products of C₄F₇N with Cu and Ag were excellent [61].

6. Challenges and Perspectives

Most of the related studies were conducted based on theoretical calculation (e.g., DFT), and few experimental studies have been done to investigate the insulation performance in practical application—especially with the existence of trace H₂O or O₂, which is more close to reality. A certain amount of H₂O or O₂ may lead to the severe deterioration of insulation performance.

The compatibilities between the insulating gases or their decomposed products and metals were simulated and studied based only on a certain crystal face of metals. In electrical equipment, every crystal face can be exposed to the insulating gas. Thus, the actual compatibility between the insulating gases or their decomposed products and the insulation equipment may be very complex. It is significant to consider the compatibility through systematic experiments.

Common adsorbents such as γ-Al₂O₃ can not only absorb the hazardous products, but also the insulating gas. It would be interesting to design a novel adsorbent which could absorb the harmful products exclusively. This could ensure the safety of employees and equipment and maintain the insulation performance.

7. Conclusions

The GWPs of C₂F₆, C₃F₈, and c-C₄F₈ are still too high to show significant advantages compared with SF₆. CF₃I shows distinguished low GWPs and dielectric strength, however, it has been identified as a cancerogenic substance, and its stability and compatibility with the materials of electric equipment should be further studied. The GWPs of perfluorinated ketones (C₅F₁₀O and C₆F₁₂O) and fluoronitrile (C₄F₇N) are low, and they show high dielectric strengths and low toxicity, and therefore they have the potential to partially replace SF₆ in some electric insulation equipment. However, the compatibility of these insulating gases with the equipment materials, and the leaking rate obtained by using the conventional sealing materials should also be well studied. The adsorbents used to eliminate H₂O and O₂, which can accelerate the decomposition of the insulating gas, should also be screened or developed to ensure the safe operation of the equipment.