<figure><img src="image_1.png" /></figure>

HAL
open science

# Research status of replacement gases for SF 6 in power industry

Shuangshuang Tian, Xiaoxing Zhang, Yann Cressault, Juntai Hu, Bo Wang, Song Xiao, Yi Li, Narjisse Kabbaj

► To cite this version:

Shuangshuang Tian, Xiaoxing Zhang, Yann Cressault, Juntai Hu, Bo Wang, et al.. Research status of replacement gases for SF 6 in power industry. AIP Advances, 2020, 10 (5), pp.050702. 10.1063/1.5134727 . hal-03304270

HAL Id: hal-03304270

https://hal.science/hal-03304270v1

Submitted on 3 Jul 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Research status of replacement gases for SF₆ in power industry

Shuangshuang Tian Ⓔ ; Xiaoxing Zhang Ⓔ ; Yann Cressault Ⓔ ; Juntai Hu; Bo Wang; Song Xiao Ⓔ ; Yi Li Ⓔ ; Narjisse Kabbaj

<figure><img src="image_3.png" /></figure>

Check for updates

AIP Advances 10, 050702 (2020)

https://doi.org/10.1063/1.5134727

<figure><img src="image_4.png" /></figure>

View
Online

<figure><img src="image_5.png" /></figure>

Export
Citation

## Articles You May Be Interested In

* Study on the decomposition mechanism and insulation stability of HFO-1336mzz(E)/air mixtures as a new environment-friendly insulation medium under AC voltage

* AIP Advances (April 2023)

* Study of the dielectric breakdown properties of hot SF₆-CF₄ mixtures at 0.01–1.6 MPa

* J. Appl. Phys. (August 2013)

* Research on the electric life evaluation technology of the arc extinguishing chamber of the 550 kV circuit breaker

* AIP Advances (May 2022)

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>
# Research status of replacement gases for SF₆ in power industry

Cite as: AIP Advances 10, 050702 (2020); doi: 10.1063/1.5134727

Submitted: 4 November 2019 • Accepted: 29 March 2020 •

Published Online: 7 May 2020

<figure><img src="image_9.png" /></figure>

View Online

<figure><img src="image_10.png" /></figure>

Export Citation

<figure><img src="image_11.png" /></figure>

CrossMark

Shuangshuang Tian,<fnref n="1" /> ID Xiaoxing Zhang,<fnref n="1" />,ª ID Yann Cressault,<fnref n="2" /> ID Juntai Hu,<fnref n="3" /> Bo Wang,<fnref n="3" /> Song Xiao,<fnref n="4" /> ID

Yi Li,<fnref n="4" /> ID and Narjisse Kabbaj<fnref n="2" />

## AFFILIATIONS

## ABSTRACT

SF₆ is widely used in the industrial field due to its stable structure and excellent properties. It is mainly used in electrical insulation equipment. Due to the boiling point of SF₆, its use in extremely cold regions has been limited. It is harmful to the health of practitioners due to the toxicity of decomposition products. The gas has limited its wider use because of its strong greenhouse effect. As a result, researchers and electrical equipment manufacturing companies around the world are gradually searching for new environmentally friendly gases and have conducted research and exploration on theory and experiment. In this paper, the current status and existing problems of SF₆ are summarized. The research contents and research methods of SF₆ alternative gas direction are reviewed from the aspects of insulation performance, interrupter performance, and decomposition performance. The existing research results of the natural gas, SF₆ mixed gas, perfluorocarbons, and CₙFₘX gas are summarized, and the future development trend of alternative gas for SF₆ in the electrical industry is proposed.

© 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5134727

## I. INTRODUCTION

In 1900, an artificial inert gas called sulfur hexafluoride (SF₆) was synthesized by two French chemists, Moissan and Lebeau. SF₆ is composed of the most active elements in the halogen, fluorine (F) and sulfur (S) atoms.

The S atom is bonded by the sp3d2 hybrid orbit, and the molecular structure is an octahedron, in which the six F atoms are at the fixed position and the S atom is in the central position. S and F atoms are linked with covalent bonds, as shown in Fig. 1.<fnref n="1" /> It has a stable chemical structure and strong insulation capacity, and the arc-extinguishing capacity of SF₆ is 100 times that of the air, and the insulation capacity is 2.5 times that of the air. It is a new ultra-high voltage dielectric insulation gas, which is superior to air and oil. Due to the excellent insulation properties and arc

extinguishing performance, it is used in electrical equipment as gas-insulation media, such as circuit breakers, high voltage transformers, gas sealed combination capacitors, high voltage transmission lines, and transformers.

High purity SF₆ is an ideal electronic etchant, which is widely used in microelectronic technology. SF₆ is also used as a refrigerant in the refrigeration industry, and the refrigeration range can be between -45 °C and 0 °C. SF₆ is used as an anti-adsorbent in the mining industry to replace oxygen in coal mine dust.

SF₆ is widely used in metal melting (such as magnesium alloy melting furnace protective gas), aerospace applications, medical applications (x-ray machine and laser), meteorological tracer analysis, chemical applications (senior car tires and new fire extinguishers), and so on. With the development of science and technology, the fields covered by SF₆ are constantly expanding more and more,

<footnote n="1">¹Hubei Key Laboratory for High-efficiency Utilization of Solar Energy and Operation Control of Energy Storage System, Hubei University of Technology, Wuhan 430068, China</footnote>

<footnote n="2">²LAPLACE (Laboratoire Plasma et Conversion d'Energie), UPS, INPT, Université de Toulouse, 118 route de Narbonne, CO F-31062 Toulouse Cedex 9, France</footnote>

<footnote n="3">³State Grid Pingdingshan Power Supply Company, Pingdingshan 467001, China</footnote>

<footnote n="4">⁴School of Electrical Engineering, Wuhan University, Wuhan 430072, China</footnote>

<footnote>ªAuthor to whom correspondence should be addressed: xiaoxing.zhang@outlook.com</footnote>
<figure><img src="image_12.png" /><figcaption>FIG. 1. Molecular structure of SF$_{6}$.</figcaption></figure>

such as basic fields and science and technology fields. With the con-
tinuous development of the power grid and the gradual increase in
the voltage level of the equipment, the usage of SF₆ is also gradu-
ally increasing. However, since SF₆ is a typical greenhouse gas and
the demand for limiting its use is getting higher and higher, the
research on alternative gases is going deeper and deeper. Of course,
in addition to the greenhouse effect, the toxicity of SF₆ decomposi-
tion products and the boiling point are also factors that limit their
use.

This paper reviewed the research status of SF₆ replacement gases in order to help researchers study the new substitute gases. This paper is organized as follows: Sec. II introduces the properties of SF₆, which is the important reason that it can be used widely in the power industry, and points out the disadvantages, which is the reason for looking for substitute gases. Section III focused on the main evaluation indices of alternative gases, such as the gas insulation, arc extinguishing, and decomposing properties. Section IV lists the research results of natural gases, SF₆ mixtures, perfluorocarbons, and other gases based on the evaluation indices in Sec. III. Section V shows the conclusion and outlook about the SF₆ replacement gases.

II. SF<sub>6</sub>

A. The use and development of SF<sub>6</sub> in power industry

1. *Insulation properties*

SF₆ is not permanently broken down as solid insulation, and the
insulating properties can be comparable to those of oil if the pres-
sure is large enough, so it can be used in gas-insulated equipment.
Under severe discharge and high temperatures, pure SF₆ is rela-
tively stable and self-recovery performance is better, and no harmful
solid impurities are precipitated by gas decomposition. Camilli and
Chapman² compared various gases (SF₆, N₂, CF₂Cl₂, CF₃Cl, CF₄,
CHClF₂, and CHF₂) by testing the lightning impulse voltage and
the 60 Hz AC voltage. SF₆ was found to be the most ideal insulat-
ing gas in terms of the boiling point and insulation properties. It can
maintain good insulation properties even at lower pressure. Camilli
et al.³ also explored the feasibility of using gas-insulated media in

high-voltage transformers by analyzing the electrical properties and
chemical properties. Compared with the transformer oil, the gas
cooling capacity is relatively poor, so the application of SF₆ in the
transformer may require an additional cooling system or increase in
the volume of equipment. Menju et al.⁴ studied the insulation prop-
erties of SF₆ in the gap between the ball electrode and the coaxial
cylinder. By measuring the breakdown time T and the breakdown
voltage V, it is considered that the VT curve (the abscissa represents
T, and the ordinate represents V) of SF₆ is relatively flat, and the gas
can well protect the device from over-voltage for a long time. This
provides a theoretical basis for the application of SF₆ in gas-insulated
transmission lines.

As the voltage level is gradually increased, the advantages of DC transmission equipment are obvious. The demand for gas-insulated equipment at DC voltage is driven. Menju and Takahashi⁵ tested the DC breakdown characteristics of different electrodes and concluded that the combination of SF₆ gas and epoxy resin can be effectively used in the insulation system of a high voltage DC gas insulated substation. Rizk et al.⁶ studied the movement characteristics of conductive particles between coaxial cylindrical electrodes and their effect on the DC breakdown voltage by experiments. They improved the model of particle motion in a coaxial field, considering that particles were subjected to charge accumulation and centrifugal forces.

For the microscopic parameters of gas discharge, Boyd and Crichton⁷ measured the ionization coefficient and attachment coefficient of the SF₆ molecule, and these parameters provide important references for the study of the gas discharge process. In the range of 25 °C–125 °C, the temperature does not affect the electron attachment process of the gas molecules, but the increase in temperature would promote the diffusion process, thus changing the space charge distribution and eventually causing the breakdown voltage to rise.⁸

2. Arc extinguishing properties

The interruption ability of SF₆ was significantly higher than that of air.⁹ In 1956, Cromer and Friedreich¹⁰ found that the interruption ability of SF₆ was significantly higher than that of air, the arc extinguishing capacity of plain-break arcs at AC voltage was about 100 times of that of air, and the limiting current value of the arc extinguishing current had a rough linear relationship with the arc length and the gas pressure. The SF₆ circuit breaker could cut off the current of 5000 A at 115 kV. In 1957, Leeds et al.¹¹ measured the current change in SF₆ during quenching. By installing a cross-blast and arc-splitter, the interrupting current can be increased as demonstrated by Kane and Colclaser in 1963¹² in a 69-kV SF₆ common-tank breaker. They concluded that SF₆ as the interrupter medium results in a perfect switching and interruption effect. In 1964, Easley and Telford¹³ designed a new type of circuit breaker, which was quenched by a new jet-type interruption structure. The interruption and impulse withstand tests of the new circuit breaker reached the standards of the American Standards Association (ASA).¹⁴

In 1965, the first SF₆ 500 kV circuit breakers were started to
be used in the USA.¹⁵ 1550-kV basic insulation level (BIL) cir-
cuit breakers with SF₆ as the insulating gas also appeared one
after another. ¹⁶ ASA (American Standards Association) reported
the use of 115 kV-345 kV circuit breakers with SF₆ as the
arc-extinguishing medium in terms of the mechanical structure, working principle, and arc extinguishing performance, and extended it to the corresponding extra high voltage (EHV) transmission lines.<sup>17-21</sup>

Due to its superior performance, SF₆ was considered as an insulating medium and arc-extinguishing medium in electrical equipment.²² SF₆ is today widely used in the power industry in gas insulated circuit breakers (GCBs), gas insulated switchgears (C-GISs), gas insulated circuits (GILs), and gas insulated transformers (GITs),²³⁻³⁰ and the current production of SF₆ is mainly used in the power industry, accounting for about 80% of the total production.³¹

Since then, SF₆ gas-insulated equipment has been developed and perfected for decades. SF₆ occupies an important position in the gas-insulated equipment, and the theoretical and experimental research on SF₆ has never been interrupted. For example, SF₆ is used as the insulation medium and arc extinguishing medium in the 1000 kV GIS, and the rated short-time withstand current reaches 50 kA. With the continuous development of the power grid and the gradual increase in the voltage level of the equipment, the usage of SF₆ is constantly increasing despite its disadvantages.

## B. The disadvantages

In the recent decades, it has been found that there are mainly three problems about the use of SF₆ in electrical equipment. The first one is the boiling point, which cannot be used in electrical equipment in extremely cold areas. The second one is toxicity since SF₆ may be decomposed in the equipment to produce some toxic gases such as SO₂, SO₂F₂, SOF₂, and S₂F₁₀, which can react with metals to produce various metal sulfides. Finally, SF₆ is a typical greenhouse gas. With the increase of usage, the impact on the environment cannot be neglected. The content of SF₆ in the atmosphere increased by 20% from 2011 to 2016, which is also the most important reason for the urgent need to find a new type of green gas instead of SF₆. According to forecasts, by 2030, SF₆ emissions from the power sector alone will reach 64 × 10⁶ metric tons of carbon dioxide equivalent (MtCO₂e).³¹ So, limiting their use and emissions has become the current requirement for environmental protection and sustainable development.

### 1. The boiling point

The boiling point of SF₆ is -65 °C (0.1 MPa). This temperature meets the operating requirements of most equipment, but liquefaction still persists in extremely cold regions, which needs a lower boiling point. In the initial gas-insulated equipment design and development phase, a variety of insulating gases were considered within the scope. All kinds of insulating gases were tested, and theoretical analysis was performed by research workers and electrical equipment companies such as General Electric (GE). The initial purpose of exploration has always been simpler, considering the insulating properties and the boiling point as the main purpose.

In 1980, Devins³² from General Electric (GE) Company conducted research on the key parameters such as dielectric strength and boiling point of thousands of gases and screened out relatively high-performance insulating media. Most of the gases in the insulation strength and boiling point cannot reach the SF₆ level at the same time, but the impact on the environment is ignored. The research on the SF₆ gas mixture started earlier and almost at the same time as

the research on SF₆ gas. It has been a hot topic, and it has been a breakthrough for the SF₆ gas mixture until 2000. The study found that the appropriate reduction in the SF₆ mixing ratio can meet the needs of equipment operation and solved the media liquefaction in the equipment. The SF₆ mixed gas was begun to be used in the GIL.

### 2. The toxicity of decomposition products

Even if a new SF₆ gas insulated device can be non-toxic, it may contain toxic gases after a certain period of operation.³³ Despite the good chemical inertness, the decomposition of SF₆ still occurs under the arc with low-fluoride sulfides (SF₂ and SF₄) produced by breaking the S–F bond. Under the conditions of high voltage and high temperature during discharge, SF₆ decomposes to form various free radicals, which can react with water, oxygen, and solid insulating material epoxy resin [(C₁₁H₁₂O₃)ₙ] in equipment to form SO₂, SO₂F₂, SOF₂, and so on. A very small amount of water molecules and oxygen molecules (ppm level) is inevitable in high purity SF₆. Vanroggen *et al*.³⁴ noted that the reaction of SF₆ with other gases can produce toxic low-fluoride sulfides, as well as oxide SOx, and even the highly toxic S₂F₁₀, HF, and others. Vijk explored the reaction of SF₆ with the metal in the circuit breaker by simulating the environment of the circuit breaker and measuring the consumption of different metal electrodes.³⁵ Due to the impurities contained in SF₆, as well as metal and solid insulation materials in electrical equipment, the decomposition products also include CS₂, WF₆, and SiF₄.

Some substances are toxic gases and cause health problems to workers in the power industry. Kraut and Lilis³⁶ reported that six electrical workers were exposed to SF₆ decomposition products during electrical service for more than 6 h (within 12 h) without protection. Most notably, the decomposition gas has a significant effect on the lung function. The threshold values (TLVs) of the main decomposition products SF₄, S₂F₁₀, SO₂, SOF₂, SO₂F₂, and HF are also given. Pilling and Jones³⁷ described a series of symptoms of dyspnea, hemoptysis, and cyanosis when two power workers inhaled SF₆ decomposition products for on-site maintenance and pointed out that the main decomposition products of SF₆ has a direct effect on the alveolar capillary membrane, which might lead to pulmonary edema, hypoxemia, arrhythmia, and mild tissue acidosis based on the clinical observation. Table I shows the possible damage to the human body caused by the main gas decomposition products of SF₆.

### 3. The greenhouse effect

With the global warming, the use and emission of greenhouse gases have drawn more and more attention from all over the world. Obama pointed out in “Science” that if the greenhouse gas emission is not limited, the global average temperature may increase by 4 °C or more by 2100.⁴¹ The Kyoto Protocol in 1997 classified the types of greenhouse gases. It was clear that SF₆ is one of six greenhouse gases. It was required that the use of SF₆ should be gradually reduced by 2020.⁴² The GWP value of SF₆ is about 23 900 times that of CO₂ with a lifetime of 3200 years. The content of SF₆ gas in the atmosphere has increased by 8.7% per year.⁴³ So far, the SF₆ gas accounts for more than 15% of the total greenhouse gas emissions. In 2005, the emissions of SF₆ and other F-containing gases reached 0.95 pg, which would reach 3.7 pg in 2050 keeping with the current
<table><caption>**TABLE I.** Main clinical symptoms caused by decomposition products of SF<sub>6</sub>.<sup>38–40</sup> Notation: the TLV is a time-weighted average concentration at which no adverse health effects are expected, for exposure during 8 h/day, for up to 40 h/week, in ppm<sub>vol</sub>.</caption><thead><tr><th>The decomposition gases</th><th>Health effects</th><th>Threshold limit value (TLV) (ppm)</th></tr></thead><tbody><tr><td>SF<sub>2</sub></td><td>Not toxic<br>Corrosive: Inhalation, skin, eye, and ingestion. It can cause lung damage, which affects the respiratory system; its toxicity is similar to phosgene toxicity</td><td>...</td></tr><tr><td>SF<sub>4</sub></td><td></td><td>0.1</td></tr><tr><td>S<sub>2</sub>F<sub>2</sub></td><td>Toxic and pungent smell. Destructive to the respiratory system</td><td>0.5</td></tr><tr><td>SOF<sub>4</sub></td><td>Corrosive: Inhalation, skin, and eye<br>Vascular-shock</td><td>0.5</td></tr><tr><td>S<sub>2</sub>F<sub>10</sub></td><td>Lungs, thorax, or respiration-acute pulmonary edema<br>Blood—changes in the cell count (unspecified)<br>Strong irritation to the upper respiratory tract and lungs.</td><td>0.01</td></tr><tr><td>SO<sub>2</sub></td><td>It is mainly used in the upper respiratory tract. A large number of acute exposure can cause pulmonary edema and respiratory paralysis.</td><td>2</td></tr><tr><td>SOF<sub>2</sub></td><td>It can cause severe pulmonary edema and mucous membrane irritation.</td><td>1.6</td></tr><tr><td>SO<sub>2</sub>F<sub>2</sub></td><td>Inhalation of this odorless gas can quickly result in death.</td><td>5</td></tr><tr><td>HF</td><td>Very hazardous in the case of skin contact (corrosive and irritant), eye contact (irritant and corrosive), and ingestion. Inhalation of the spray mist may produce severe irritation of the spray mist may produce severe irritation of the respiratory tract, characterized by coughing, choking, or shortness of breath.</td><td>3</td></tr><tr><td>CS<sub>2</sub></td><td>Extremely hazardous in the case of skin contact (irritant), ingestion, and inhalation.</td><td>10</td></tr><tr><td>CO</td><td>It combines with hemoglobin in the blood to cause hypoxia.</td><td>50</td></tr><tr><td>WF<sub>6</sub></td><td>Very toxic and intensely irritating<br>Inhalation, skin, eye, and mucous membrane contact. High concentration exposures may produce pulmonary irritation, and low concentration exposures may produce generalized effects of fluorine exposure</td><td>0.1</td></tr><tr><td>SiF<sub>4</sub></td><td></td><td>0.6<br>0.6</td></tr></tbody></table>

emission levels.<sup>44</sup> Since 1997, the USA, Japan, and the EU countries have formulated policies and measures to limit the use of SF<sub>6</sub>. California proposed that the use of SF<sub>6</sub> in the electrical field should be reduced year by year from 2020 and the EU plans to reduce SF<sub>6</sub> emissions to 2/3 of 2014 by 2030. Spain and Australia have increased the tax of SF<sub>6</sub> emissions. It can be said that the main reason for the research on SF<sub>6</sub> alternative gas still comes from its threat to the environment. In addition, the policy of restricting the use of SF<sub>6</sub> in various countries gradually makes the study of alternative gases a hot issue in the global power industry.

Based on the above reasons, it has become an important research subject in the power industry to look for a new type of environmentally friendly gas with good insulation ability and safety. Research institutes across the world are dedicated to finding suitable gas alternatives to SF<sub>6</sub> for use in gas-insulated power equipment such as GISs and GILs.

### III. EVALUATION INDICES OF ALTERNATIVE GASES

Alternative gases should be safe, harmless, and environmentally friendly and should meet the needs of equipment operation, such as the boiling point, insulation properties, interrupter performance, and decomposition characteristics. At present, the gas with alternative potential is mainly studied in terms of the insulation performance, the arc-extinguishing performance, and the

decomposition characteristics. The summary of the research is shown in Fig. 2.

Based on the environmental parameter (GWP), insulation properties, safety, and other aspects, the main alternative gases and basic parameters are shown in Table II. The research focuses mainly on natural gases (N<sub>2</sub>, CO<sub>2</sub>, He, Ar, and so on), SF<sub>6</sub> gas mixtures, perfluorocarbons (c-C<sub>4</sub>F<sub>8</sub>, C<sub>3</sub>F<sub>8</sub>, and C<sub>2</sub>F<sub>6</sub>), CnFmX gases, and other gases such as HFOs.

#### A. Insulation performance

The macroscopic parameters of the insulation performance mainly include the breakdown voltage, the partial discharge (PD) inception voltage, and the volt-second characteristic of different gases tested, which can be measured in the test. The voltage type (DC, AC, or lightning impulse voltage), electric field uniformity, gas pressure, and mixing ratio of the insulated gas are all important factors to judge the insulation properties, as shown in Fig. 3. The experimental method of breakdown voltage is shown in Fig. 4. The measurement of partial discharge (PD) inception voltage needs to be combined with the voltage measurement device while measuring the impedance to collect the PD signals presented in the oscilloscope.

Among the influencing factors, the electric field uniformity is determined by the electrode structure, and the non-uniformity factor *f* of the electric field is given by
<figure><img src="image_13.png" /><figcaption>FIG. 2. Evaluation indices of alternative gases and research resource.</figcaption></figure>

$$f = \frac{E_{max}}{E_{av}}. \qquad (1)$$

In Eq. (1), $E_{max}$ represents the maximum electric field strength and $E_{av}$ represents the average electric field strength. The electric

field distribution around the electrodes can be calculated by software such as COMSOL. Different gases have different sensitivities to electric field uniformity.

The influence of the gas mixture ratio on the insulation properties mainly refers to the synergistic effect of the mixed gases. When

<table><caption>TABLE II. Physical and environmental characteristics of common dielectric gases.45–50</caption><thead><tr><th>Type</th><th>Insulation strength relative to SF<sub>6</sub></th><th>Boiling temperature (°C) (1 bar)</th><th>Lifetime (year)</th><th>GWP</th></tr></thead><tbody><tr><td>SF<sub>6</sub></td><td>1</td><td>-63.9</td><td>3 200</td><td>23 500</td></tr><tr><td>N<sub>2</sub></td><td>0.36</td><td>-196</td><td>0</td><td>0</td></tr><tr><td>CO<sub>2</sub></td><td>0.30</td><td>-78.5</td><td>Infinity</td><td>1</td></tr><tr><td>Air</td><td>0.30</td><td>&lt;183</td><td>Infinity</td><td>≈0</td></tr><tr><td>CF<sub>4</sub></td><td>0.39</td><td>-186.8</td><td>6 300</td><td>50 000</td></tr><tr><td>C<sub>2</sub>F<sub>6</sub></td><td>0.78–0.79</td><td>-78</td><td>10 000</td><td>9 200</td></tr><tr><td>C<sub>3</sub>F<sub>8</sub></td><td>0.96–0.97</td><td>-37</td><td>2 600</td><td>7 000</td></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub></td><td>1.25, 1.31</td><td>-6(-8)</td><td>3 200</td><td>8 700</td></tr><tr><td>CF<sub>3</sub>I</td><td>1.23</td><td>-22.5</td><td>0.005</td><td>1–5</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N</td><td>2.2</td><td>-4.7</td><td>0</td><td>2 100</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O</td><td>2.0</td><td>27</td><td>0</td><td>1</td></tr><tr><td>C<sub>6</sub>F<sub>12</sub>O</td><td>2.5</td><td>49</td><td>0</td><td>1</td></tr><tr><td>CF<sub>3</sub>NSF<sub>2</sub></td><td>2.41</td><td>-6</td><td>...</td><td>...</td></tr><tr><td>HFO1234zeE</td><td>0.98</td><td>-9</td><td>12</td><td>0.02</td></tr><tr><td>R12</td><td>0.9</td><td>-29.8</td><td>12</td><td>2 400</td></tr></tbody></table>
<figure><img src="image_14.png" /><figcaption>FIG. 3. Diagram of the breakdown voltage measurement.</figcaption></figure>

two kinds of gases (at least one is a kind of gas with good insulation performance) are mixed, the insulation performance of the mixed gas shows three different variations with the increase in the insulating gas content: negative synergistic effect, linear relationship, and positive synergistic effect. Mixed gas with a negative synergistic effect is rare, which is not suitable as an insulating gas. The mixed gases with a positive synergistic effect will show great advantages such as the same insulating property as pure gas at a certain mixing ratio. Therefore, finding the characteristics of the synergistic effect is helpful to find the most suitable mixing ratio,

$$C = \frac{k(V_m - V_1)}{(1 - k)(V_m - V_2)}, \qquad (2)$$

where $V_1$ and $V_2$ are the breakdown voltage of the two pure gases 1 and 2 ($V_1 > V_2$) in kV, $V_m$ is the breakdown voltage of the mixed gas

in kV, k is the mixing ratio of gas 1, and C represents the degree of synergy of two gases. A smaller value of C indicates that the two gases have the better synergistic effect.⁵¹ The basic synergy relationship of mixed gas: negative synergistic effect is shown in Fig. 4.

The microscopic parameters for characterizing the gas insulation properties can also be measured experimentally or calculated theoretically. The experimental measurement process is shown in Fig. 5, which is generally performed under a steady-state Townsend (SST) test or a pulsed Townsend (PT) test (generally, the gas pressure is small at about 100 Pa).⁵² The measured parameters include the effective ionization coefficient $\alpha$ of the molecule and the effective adhesion coefficient $\eta$. It is generally considered that the critical electric field $E_{cr}/N$ can be obtained when the two coefficients are equal, which can be used as a reference value for measuring gas breakdown. According to Townsend discharge theory, $\alpha$ and $\eta$ can be obtained by changing the electrode spacing $d$ and measuring repeatedly current $I$, according to the following equation:

$$I = I_0 \left[ \frac{\alpha}{\alpha - \eta} e^{(\alpha - \eta) d} - \frac{\eta}{\alpha - \eta} \right]. \qquad (3)$$

<figure><img src="image_15.png" /><figcaption>FIG. 4. Basic synergy relationship of mixed gas: negative synergistic effect $C > 1$ (curves 1 and 2), linear relationship $C = 1$ (curve 3), and positive synergistic effect $C < 1$ (curves 4 and 5).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>FIG. 5. Diagram of measuring microscopic parameters.</figcaption></figure>
In theory, the electron swarm parameters (such as effective ionization coefficient, electron drift velocity, and particle diffusion coefficient) between gaseous molecules and electrons at the gas micro-level are theoretically calculated to comprehensively evaluate the insulating properties of the gas. The first step to obtain these parameters is the calculation of the collision cross section. More information about collisions in the main gases can be found in Ref. 53. There are two main methods of calculation: (1) the Monte Carlo simulation, which is the method with many iterations and computational complexity, and (2) solving the Boltzmann equation using some approximations. Indeed, the transport state and energy distribution of electrons in the discharge process should be clear. When the electron density is unevenly distributed, the electron diffuses from a high concentration point to a low concentration point, which is caused by the thermal motion of electrons. At the same time, under the action of the electric field, it accelerates the movement along the electric field to cause the collision ionization to form an electronic collapse. This complex state of motion results in different distributions of electron energy in the discharge space. In order to determine the transport state of charged particles, a self-consistent model need to be established that reflects the interaction between the electric field and the particles. In this complex model, both the elastic and the inelastic collision between electrons and gas molecules should be properly considered and the effect of the external electric field on charged particles should be taken into consideration. Therefore, some simplifications and approximations are usually needed to obtain electron swarm parameters and transport parameters during gas discharge. In a weakly ionized gas, the velocity distribution function $f(r, v, t)$ of electrons in a six-dimensional phase space obeys Boltzmann's equation,

$$ \frac{\partial f}{\partial t} + v \cdot \nabla_{rf} f + a \cdot \nabla_{vf} f = C[f(r, v, t)], \quad (4) $$

where $a = eE/m$ is the acceleration of the electron under the action of an electric field, in which $e$ and $m$ denote the charge and mass of the electron, respectively. $E$ is the electric field strength; $v$ is the electron velocity; and $r$ is the electron position parameter. $C[f(r, v, t)]$ refers to the collision process of electronic collision process, including the elastic collision and inelastic collision.

The general solution of Eq. (4) is obtained using the Legendre polynomial sphere expansion to construct a series of equations equivalent to Eq. (4) and to introduce the correlation coefficients. In the case of higher order expansion, it is usually necessary to introduce at least six parameters. However, for the second order, the equation can be greatly simplified,⁵⁴,⁵⁵ but the corresponding assumptions need to be made.

(1) The distribution of the electric field in space is uniform.

(2) Collisions are stable over the mean free path dimension.

(3) The velocity distribution of the electron velocity distribution function $f$ in the direction of the electric field is distributed symmetrically.

(4) In the time dimension, only the distribution of the electric field is considered stable and the electrons are in a state of stable distribution or high-frequency oscillation.

By assuming that the equation is simplified, the electron energy distribution function (EEDF) is obtained by approximately

solving the Boltzmann equation, and the ionization coefficient and the attachment coefficient are obtained. When the ionization coefficient and the attachment coefficient are equal, the breakdown field strength is the critical breakdown field strength, (E/N)cr, and the parameters such as the electron drift speed and the average energy can also be obtained.⁵⁶,⁵⁷

In addition, some parameters are calculated based on quantum chemical calculation, such as the molecular ionization energy (IE), binding energy, surface area, and molecular polarization properties, and the gas insulation properties can also be predicted.⁵⁸

## B. Arc extinguishing performance

The main parameters for the experimental measurement are the gas arc time and the cut-off current value. Tests are generally carried out in the interrupter of the circuit breaker. According to Mayr's arc model,⁵⁹ the time constant and power loss coefficient of arc extinguishing can be obtained by changing the arc voltage (V) and arc current (I), based on the following formula:

$$ \frac{1}{g} \frac{dg}{dt} = \frac{1}{\theta} \left\{ \frac{V_i}{N_0} - 1 \right\}, \quad (5) $$

where $v$ is the arc voltage, $i$ is the arc current, $g$ is the arc conductance, $\theta$ is time constant, and $N_0$ is the power loss coefficient.

To judge the arc performance of the gas and finally to evaluate the arc extinguishing performance, the thermodynamic parameters and transport characteristics of the plasma formed during the arc process have to be determined depending on its thermodynamic state. When the temperature is very high, the plasma has a uniform thermodynamic temperature, and it is in a thermodynamic equilibrium state. At this point, the particle velocity is in accordance with the Maxwell-Boltzmann distribution, the population density of the ground state and excited state is in accordance with Boltzmann distribution, and the ionization process can be established based on the Saha equation. However, the complete thermodynamic equilibrium plasma can hardly exist in both natural and laboratory environments. For arc plasma, it is usually assumed that it is in the local thermodynamic equilibrium. Under this state, the particle collisions replace the radiation and occupy the dominant position in various reactions. The electron temperature is approximately equal to the temperature of heavy particles. The population density approximates the Boltzmann distribution, and the Saha equation is also approximately qualified. However, with the decrease in the arc temperature, the electron density decreases and the electron collisions gradually decay. The electron temperature deviates from the temperature of the heavy particles, and the arc plasma is in a non-thermodynamic equilibrium state. In addition, in the arc plasma, the rate of chemical reactions, such as decomposition, ionization, recombination, and adsorption, is limited. When the relaxation time of chemical reactions is less than the characteristic time of physical movement, such as particle convection and diffusion, the plasma reaches the local chemical equilibrium; otherwise, it is in the non-chemical equilibrium state.⁶⁰,⁶¹ In the study of non-chemical equilibrium, the speed of chemical reactions is the standard to judge the degree of chemical non-equilibrium. Therefore, the rate constant of chemical reactions becomes the key parameter to study the chemical reaction process. At present, an effective means of
obtaining the reaction rate constant is by means of quantum chemical calculation.⁶²

Due to the non-chemical equilibrium, the original statistical physics method cannot accurately describe the particle distribution and internal processes of the plasma. Instead, a control equation containing both the chemical reaction process and the physical process needs to be established for each particle so that the modeling and the computational difficulty are greatly increased. Most of the calculations are based on the local thermodynamic equilibrium and the two-temperature model under chemical equilibrium.⁶³ The calculation modeling is shown in Fig. 6. Under equilibrium conditions, there are two theories for the calculation of equilibrium compositions: one is based on the Saha equation and the Guldberg-Waage equation, combined with the Dalton partial pressure law, the conservation of chemical metrology and the conservation of charge to obtain the equilibrium compositions, and the other is solved by the minimum Gibbs free energy. Both of these computational theories are mathematically equivalent with all particles in the gaseous phase. The Saha equation is no longer applicable if the non-gaseous phase (e.g., solid, liquid, and molten) particles are considered, and the minimum Gibbs free energy method is applied. This method simplifies the modeling process without considering specific ionization and decomposition reactions, and it is widely used in the solution of equilibrium particle components.⁶⁴,⁶⁵

### C. Decomposition characteristics

Insulation defects in the device can cause decomposition of the gas and further reaction with traces of impurities in the device to produce more complex products. The detection of discharge or

high temperature decomposition products and the theoretical calculation of the decomposition process of mixed gas have also become an important issue in studying alternative gases. The decomposition characteristics are considered for the following reasons: First, the decomposition process and decomposition mechanism directly affect the gas self-recovery performance. Second, the products of decomposition, especially the gas and solid products that may be generated by breakdown, partial discharge, and electric arcs under different conditions, may have an impact on the environment and the safety of the human body. Finally, the decomposing characteristics of the alternative gas also provide a test method for the maintenance and operation after it has been applied to the equipment. The exploration of the decomposition process is also generally carried out through a combination of experiments and theoretical calculations. In the recent years, the influence of metal particles (such as Cu, Fe, and Ag), a small amount of gas impurities (such as H₂O and O₂), and temperature on the insulation properties has been deeply researched and explored. Even SF₆ decomposition has become an important diagnostic method for detecting SF₆ gas-insulated equipment. Under different insulation defects, SF₆ decomposition products showed different rules. The use of chemical means (GC-MS, infrared, UV, and nano-sensors) to detect these decomposition gases has also become an important study to determine the internal fault of the device and improve equipment maintenance.⁶⁶⁻⁷⁸

For the experiment, the gas in the chamber after the breakdown or partial discharge is collected and detected by the testing equipment such as the GC-MS or infrared spectrometer, as shown in Fig. 9. The main components of the decomposed gas can be obtained qualitatively or quantitatively. Generally, the

<figure><img src="image_17.png" /><figcaption>FIG. 6. Modeling of a decaying arc under local thermodynamic equilibrium.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>FIG. 7. Measurement process of decomposition products.</figcaption></figure>

possible products can be obtained through the qualitative analysis of the mass spectra or spectra, and then, the content of the products can be determined through the calibration of the standard gas. Through the XRD (x-ray diffraction) detection on the electrode surface, the atoms involved in the solid reaction can be clear during the gas decomposition process. The measurement process is shown in Fig. 7.⁷⁹

In theoretical calculations, based on density functional theory (DFT), energy changes in chemical reactions, frequency

characteristics, and transition state search, the basic properties of decomposition products are calculated by means of chemical molecular calculation software (Gaussian or Material Studio). In addition, using molecular dynamics in a constant temperature and constant volume system, we can calculate the decomposition process of gases at different temperatures to obtain the concentration of various free radicals. The reaction of gas molecules with H₂O or O₂ and the route of product generation are also important parts of the research.

<table><caption>TABLE III. Evaluation indices of alternative gases to have good properties.</caption><thead><tr><th>Properties</th><th>Indices</th><th>Require values</th><th>Method</th></tr></thead><tbody><tr><td rowspan="10">Insulation performance</td><td>Breakdown voltage</td><td>High</td><td>Measure</td></tr><tr><td>Partial discharge inception voltage</td><td>High</td><td>Measure</td></tr><tr><td>Synergistic effect</td><td>Low (less than 1)</td><td>Calculation based on the measured voltage</td></tr><tr><td>Electron drift velocity</td><td>Low</td><td>Calculation</td></tr><tr><td>Diffusion coefficient</td><td>Low</td><td>Calculation</td></tr><tr><td>Effective ionization coefficient</td><td>Low</td><td>Calculation or measure</td></tr><tr><td>Ionization coefficient</td><td>Low</td><td>Calculation or measure</td></tr><tr><td>Attachment coefficient</td><td>High</td><td>Calculation or measure</td></tr><tr><td>Critical electric field E<sub>cr</sub>/N</td><td>High</td><td>Calculation</td></tr><tr><td>Molecular ionization energy</td><td>High</td><td>Calculation or measure</td></tr><tr><td rowspan="7">Arc-extinguishing performance</td><td>Interrupting current</td><td>High</td><td>Measure</td></tr><tr><td>Time constant of arc</td><td>Low</td><td>Measure</td></tr><tr><td>Specific heat</td><td>High</td><td>Calculation</td></tr><tr><td>Thermal conductivity</td><td>High</td><td>Calculation</td></tr><tr><td>Electrical conductivity</td><td>Low</td><td>Calculation</td></tr><tr><td>Net emission coefficient</td><td>Low</td><td>Calculation</td></tr><tr><td>Conductance of the decaying arc</td><td>Low</td><td>Calculation</td></tr><tr><td rowspan="3">Decomposition characteristics</td><td>Toxicity of decomposition products</td><td>Low</td><td>...</td></tr><tr><td>Formation rate of products</td><td>Low</td><td>Measure</td></tr><tr><td>GWP and ODP of products</td><td>Low and 0</td><td>...</td></tr></tbody></table>
## D. The main evaluation indices

The main evaluation indices of the insulation performance of gas-insulated media are shown in Table III, which may be helpful to evaluate the properties of new gases.

# IV. THE RESEARCH RESULTS OF ALTERNATIVE GASES

## A. Natural gases (N₂, CO₂, and air) and mixed gases

The natural gases with much lower liquefaction temperature and lower GWP than SF₆ have got relatively stable physical and chemical properties and are cheap. Therefore, it is better to use natural gases in gas-insulated equipment, especially in medium- and low-voltage equipment. Currently, natural gases used as insulating gases are N₂, CO₂, and air. Although the insulation performance is weaker than that of SF₆, these gases are initially considered as gases for insulation due to the simple molecular structure, and theoretical studies also started earlier. Until the introduction of alternative gases, these gases were focused and studied in more detail, both theoretically and experimentally. The related equipment is designed to produce and put into use. However, due to the relatively low insulation performance, it is necessary to take reasonable measures to improve the insulation properties such as increasing the gas pressure of the gas chamber, expanding the size of the equipment, and adopting the gas-solid insulation method.

### 1. Breakdown voltage

Husain and Nema⁸⁰ extended the Townsend breakdown formula to obtain Paschen curves for air, N₂, and SF₆, completing the evaluation of the three gas breakdown voltages. Hara et al.⁸¹ studied the breakdown behavior of N₂ and N₂/O₂ gas mixtures in the presence of metallic particles at the solid-insulator interface of solid insulators. The effects of the cone insulator angle, particle size, and pressure were analyzed. Mizuno et al.⁸² proposed a combination of the gas and solid insulation method. By wrapping rubber on the surface of the electrode, the breakdown voltage of N₂ can be increased by 1.5 times at 0.2 MPa, and the results can be used in gas-insulated switch devices. Philp⁸³ experimentally measured the breakdown voltage of the N₂ + CO₂ mixture in a ball grid electrode, which is only one-third that of SF₆.

Meijer et al.⁸⁴ compared the breakdown voltage of N₂ and CO₂ on the AC voltage of a plate electrode by experiments. Although the difference between the breakdown values of the two gases is not significant, CO₂ is more stable and the breakdown voltage of CO₂ linearly increases with the pressure value. In order to test the ability of the gas to withstand over-voltage in the field, Ueta et al.⁸⁵⁻⁸⁷ tested the breakdown characteristics of CO₂ and N₂ under non-standard lightning impulse voltage and proposed a method to evaluate the performance of CO₂ gap insulation of non-standard lightning impulse waves. Pfeiffer and Schoen⁸⁸ experimented and studied the development of the streamer current before the breakdown of N₂ and N₂O mixed gases. It is believed that the use of the mixed gas in large-scale high-pressure equipment has obvious advantages. Pelletier et al.⁸⁹ measured the breakdown field strength of N₂ and He mixed gases under a uniform field and obtained the standard formula of the breakdown voltage and mixing ratio, Olivier's equation. Lim and Bae⁹⁰ studied the flashover voltage and the partial discharge inception voltage of dry air, N₂, O₂, and mixed gas on

the surface of the GIS insulator. Considering economy and insulation performance, it shows that dry air is suitable for gas insulated switchgear.

Rokunohe et al.⁹¹ found that the insulation property of air at 0.5 MPa is greater than that of pure N₂ and CO₂, and is comparable to that of N₂/O₂ (the mixing radio of O₂ is 20%). They pointed out that the dielectric strength of air at 0.6 MPa is 95% of SF₆/N₂ (the mixing radio of SF₆ is 5%). Pontiga found that N₂ and O₂ mixed gases produced N₂O under negative corona discharge.⁹² Saitoh et al.⁹³ studied the partial discharge and breakdown characteristics of air and N₂ at the plate electrode. The partial discharge voltages of the two gases are almost the same, but the breakdown voltage of air is larger than pure N₂. Without changing the size of the equipment, using 1.0 MPa N₂ and solid insulation can replace 0.5 MPa SF₆.⁹⁴ Zhang et al.⁹⁵ proposed an improved design method of air gap and interface insulation. Based on the small or no SF₆ experiment in medium-pressure C-GIS, a low-pressure N₂ gas tank was used as the insulation gas to replace SF₆. A low-pressure N₂ gas tank 40.5 kV-1250 A three-phase C-GIS was developed. It has a vacuum interrupter and the same area covered as the SF₆ C-GIS.

### 2. Electron swarm parameters

Zhao et al.⁹⁶,⁹⁷ calculated the critical breakdown field (E/N)cr of CO₂ mixed O₂ and H₂. Compared to pure CO₂ and 50% CO₂-50% H₂, the 50% CO₂-50% O₂ mixture has better breakdown characteristics. The (E/N)cr of the 50% CO₂-50% O₂ mixed gas almost does not change when the gas temperature is up to 2500 K. Although there is a significant decrease with the temperature rising to 3500 K, it is still higher than that of pure CO₂. Chen et al.⁹⁸ calculated the critical breakdown strength of CO₂ and mixed gases (CO₂, CO₂/O₂, CO₂/CH₄, CO₂/He, and CO₂/N₂) over a temperature range of 300 K-4000 K. The results show that the mixtures CO₂/O₂ and CO₂/CH₄ have a high critical breakdown field, which makes it possible to further improve the insulation properties of pure CO₂. Yousfi et al.⁹⁹ measured the electron swarm coefficients (drift velocity, diffusion coefficient, and effective ionization coefficient) of N₂, CO₂, and O₂, and mixed gases using a pulse Townsend discharge. The hindering effect of the two gases on the electrons was verified by solving the Boltzmann equation. Moruzzi and Price¹⁰⁰ measured the ionization coefficient of air in the range of 90-260 Td. With the addition of a small amount of CO₂, the adsorption of air was significantly enhanced, but the electron attachment process of N₂ could not be detected. Raju and Gurumurthy¹⁰¹ calculated the parameters of N₂ by solving the Boltzmann equation numerically. The electron average energy, drift velocity, and mean effective field intensity (E<sub>avef</sub>) are obtained from the distribution function, and the field strength of the electron population is described when the electric and magnetic fields coexist. Kucukarpaci and Lucas¹⁰² calculated the electron swarm parameters of CO₂ and N₂ over a large electric field (14-3000 Td) using the Monte Carlo algorithm using the latest collision cross section parameters, and the calculated results were consistent with the experimental measurements.

### 3. Interruption property

Stoller¹⁰³ tested the interrupting current of CO₂ as an interrupter medium in circuit breakers and calculated it by using a hydrodynamic model and found that the thermal disruption performance
of CO₂ is lower than that of SF₆ but higher than air. Optimizing the circuit breaker structure may result in the CO₂ substitution of SF₆ as an arc extinguishing medium. Jonsson et al.¹⁰⁴⁻¹⁰⁷ tested the ability of air to break in a 630 A/24 KV medium voltage load short-circuit switch. Optimization of the contact and nozzle structure helps improve the ability to interrupt, while the length and diameter of the nozzle have little effect on the ability to interrupt. Uchii et al.¹⁰⁸ tested the ability of CO₂ to break off in a GCB (gas circuit breaker). At the same gas pressure, the thermal shutdown capacity of CO₂ is about 50% of SF₆. By blowing more heat through the improved mixing chamber, the cut-off capacity of CO₂ can be increased by 30%.

Chen et al.¹⁰⁹,¹¹⁰ established the arc model to simulate the interruption ability of the vacuum-CO₂ mixed circuit, and the results show that with the combination of vacuum interrupters, the current interruption capacity of CO₂ gas can be increased. Matsumura et al.¹¹¹ studied the current interruption ability of CO₂ mixed with N₂, O₂, He, and air. It was found that the conductance of the residual arc of the CO₂ mixture of O₂ or He (the mixing ratio of CO₂ is 20%) decreased more rapidly and the breaking capacity improved significantly. In theoretical calculation, Zhong et al.¹¹² studied the thermal physical properties of plasma and the effect of the copper vapor from the electrode surface of the circuit breaker on the CO₂-N₂ mixture. The behavior of gas during the quenching process was analyzed based on the thermodynamic parameters and transport characteristics. The existence of copper vapor in the equipment was also analyzed. The influence of copper vapor on the thermodynamic properties of the mixed gas was evidently influenced by the ionization properties of the copper atom itself, atomic density, as well as partition function.

The advantages of natural gas are low cost, harmless to the environment, and no security threat to ecology and human resources. At present, there are already finished products in middle and low voltage equipment (ABB, Schneider, and other companies) using conventional gas as an insulation medium. The main properties are shown in Table V of the Appendix. CO₂ is likely to be used as an arc extinguishing medium in circuit breakers. The use of conventional gas in the switch equipment needs to optimize the structure of the equipment reasonably, and the mechanical structure and electrical performance have greater space to improve. Simply relying on increasing the pressure and increasing the size will reduce the safety of the equipment and increase the production cost of the equipment. It is also an effective means to use the solid insulation material. However, solid insulation materials exist in the atmosphere for a long time and are difficult to degrade. Adding solid insulation materials to the equipment will undoubtedly increase the environmental burden on the other hand. However, this kind of gas still has a great prospect of development and use, which has a great exploration value.

## B. Mixtures of SF₆ with natural gases

Although the study of the SF₆ gas mixture started earlier, there is no interruption in the exploration of insulation and arc extinguishing ability. At this stage, research focuses more on practicality. The SF₆ gas mixture, which generally refers to the mixture of SF₆ and some buffer gas with relatively poor insulation and a very low boiling point (air, N₂, CO₂, N₂O, CF₄, H₂, and some inert gases), not

only meets the equipment insulation requirements but also reduces the amount of SF₆.

## 1. Breakdown voltage

Malik and Qureshi¹¹³,¹¹⁴ compared the breakdown voltage of SF₆ mixed with N₂, CO₂, and air. It was considered that SF₆ mixed N₂ was the most ideal mixed gas. The mixture is non-toxic and non-flammable, and when the SF₆ mixing ratio reaches 50%-60%, the insulation performance can reach 85%-90% of the pure SF₆. When the lightning impulse voltage was applied, the addition of a small amount of SF₆ to N₂ increased the positive lightning impulse breakdown voltage of N₂. This is due to the positive space charge resulting in an anode shield.¹¹⁵ Farish et al.¹¹⁶ investigated the effect of the surface roughness of the SF₆ and N₂ mixed gas on the breakdown voltage with a coaxial electrode. Compared with pure SF₆, the mixed gas is not sensitive to the electric field roughness. Graf and Boeck¹¹⁷ studied the sensitivity of the SF₆/N₂ mixed gas to non-uniform electric fields by setting protrusions on the electrodes. The mixed gas is more sensitive to non-uniform fields than SF₆ at the same dielectric strength. Woo et al.¹¹⁸ measured the breakdown performance of the SF₆/N₂ mixture at the lightning impulse voltage and AC voltage. As the electric field uniformity increased, the slope of the breakdown voltage decreased. In the lightning impulse test, the positive breakdown voltage was higher than the negative breakdown voltage. Guerroui and Lemzadmi¹¹⁹ by experiment measured the initial partial discharge voltage of the SF₆ and N₂ mixed gas under a non-uniform field. The initial voltage increases with an increase in pressure, and the positive polarity was higher than the negative polarity. Sato et al.¹²⁰ designed a gas insulated bus with SF₆/N₂ as gas-insulation. Compared with 0.4 MPa pure SF₆ gas insulated busbars, if a 0.6 MPa 10%SF₆-90% N₂ mixed gas is used, the diameter of the tank needed to be increased by 15%. Hoshina et al.¹²¹ tested the insulation properties of the SF₆/N₂ mixture on a full-scale gas-insulated busbar model and analyzed the effect of contaminated particles on the breakdown characteristics of AC voltage and impulse voltage. The mixed gas at 0.3 MPa and 0.5 MPa showed a positive synergistic effect, and the partial discharge inception voltage of the mixed gas showed a negative synergistic effect. Guo et al.¹²² studied the lightning impulse breakdown voltage of the SF₆/N₂ mixture, which showed a negative synergistic effect at low gas pressure. Rizk and Eteiba¹²³ measured the breakdown voltage and time of a mixture of SF₆ and N₂ under the coaxial electrode and validated it theoretically. Mizobuchi et al.¹²⁴ observed the streamer development of SF₆-N₂ mixed gas discharge by experimental tests and verified by numerical calculation. The electric field was kept constant during the development of the streamer by single steep square high voltages in the measurements. At different mixing ratios, the development trend of the flow-injection process was similar. Jiasen et al.¹²⁵ tested multi-channel discharge characteristics of SF₆-N₂ or SF₆-Ar gas with different mixing ratios in a coaxial distorted electric field using 40 kV-78 kV pulse power. The results showed that the discharge channels of the SF₆-Ar or SF₆-N₂ mixture decreased with the increase in the SF₆ mixing ratio and then tended to be saturated. Hirata et al.¹²⁶ tested the creeping discharge characteristics of the mixed gas. The flashover voltage of 3% SF₆ mixture was lowest with a positive polarity impulse voltage. The mixtures of SF₆ and N₂ gases have been used in GIL. The first GIL that used the SF₆/N₂ gas mixture as the insulation medium
developed by Siemens was put into operation at Geneva International Airport in Switzerland in 2001.¹²⁷ Currently, the SF₆/N₂ gas mixture GIL has been successfully applied in the 245 kV-550 kV line.

The SF₆-air mixture has a more stable corona for positive DC voltages.¹²⁸ Li et al.¹²⁹ measured the breakdown characteristics of the gap between coaxial electrodes of various impulse power sources for SF₆ (1%) with air and N₂ gas mixtures in the range of 100 kPa-500 kPa. The results showed that the influence of the waveform on the breakdown voltage was great. Under the coaxial electrode, little change was caused in insulation properties with adding 1% of the SF₆ in the air, which is different from the situation under a non-uniform field.

Qiu et al.¹³⁰⁻¹³² compared the dielectric properties of SF₆/N₂ and SF₆/CO₂ mixtures and concluded that the dielectric properties of the SF₆/N₂ mixed gas are better than that of SF₆/CO₂ in uniform or quasi-uniform fields, but SF₆/CO₂ on the contrary is better than SF₆/N₂ under the non-uniform fields. By testing the breakdown voltage of 50% SF₆ mixed CO₂ and N₂ at 60 Hz alternating current, it was found that adding a small amount of CO₂ into the 50% SF₆/N₂ mixture can significantly improve the breakdown voltage. The breakdown voltage of 50% SF₆-49% N₂-1% CO₂ is 1.35 times that of 50% SF₆-50% N₂ and 1.15 times that of pure SF₆.¹³³ By testing the breakdown behavior of mixed gases under the rod-plate electrode, it was found that the breakdown voltages of SF₆-N₂ and SF₆-air were similar under the negative impulse voltage.¹³⁴ The SF₆-CO₂ breakdown voltage is higher than the other two. As the gas pressure is higher than 0.1 MPa, the three kinds of mixed gases produced a leader before the breakdown. With the increase in the SF₆ mixing ratio and pressure, the leader showed a gradual development.

Cookson and Wootton¹³⁵ tested the corona and breakdown voltage at AC voltage with a mixture of SF₆ and H₂. The addition of a very small amount of SF₆ (0.002%) to H₂ increases the breakdown voltage of pure H₂ by 70%. The starting voltage of pure SF₆ is about 2.7-3 times that of pure H₂. Adding a small amount of SF₆ (within 1%) to H₂ does not affect the corona onset voltage. Farish et al.¹³⁶ tested the breakdown voltage of the SF₆ and H₂ mixed gas at negative polarity lightning impulse voltage. For H₂, the breakdown voltage of the negative lightning impulse is 60%-100% higher than that of the positive polarity. However, adding a small amount of SF₆ (within 1%) to H₂ increases the rate of increase in breakdown voltage less than the positive polarity.

Akbar and Malik¹³⁷ measured the breakdown voltage of several mixed gases at DC voltage. The measurement results showed that the mixed gas containing N₂O was less sensitive to electrode surface defects. Park et al.¹³⁸,¹³⁹ tested SF₆/CF₄ mixed gas insulation performance, including the lightning impulse test, AC frequency test, and partial discharge test in a 25.8 kV three-phase GIS tank. The experimental results showed that the breakdown voltage increased with the SF₆ content, which showed that the mixing ratio of SF₆ had little effect on the starting voltage. Berg et al.¹⁴⁰⁻¹⁴² studied the insulation properties of SF₆ and CF₄ gas mixtures at DC voltage, 60 Hz AC voltage, and standard lightning impulse voltage. The SF₆ and CF₄ mixed gas was more suitable than the SF₆-N₂ mixed gas to be used as an arc extinguishing medium. Lee et al.¹⁴³ compared the breakdown characteristics of N₂, SF₆, and CF₄ in high-voltage bushing at low temperatures. Due to its outstanding insulating properties

and low boiling point, CF₄ has shown its obvious superiority in cold environments.

Siddagangappa et al.¹⁴⁴ proposed a simple calculation formula to predict the breakdown voltage of SF₆ and mixed gases under a uniform electric field. Nema et al.¹⁴⁵ proposed a breakdown voltage calculation formula of the SF₆ mixed gas under a uniform and non-uniform electric field, which was used to infer the insulating properties of the mixed gas. The calculation results were consistent with the experimental test results. The limitation of the formula is that the parameters in different gas formulas are different and not universal.

## 2. Electron swarm coefficients

Maller and Naidu¹⁴⁶ measured the ionization and attachment coefficients of the SF₆ mixed air and nitrogen. In the meantime, using the calculation method, the breakdown field strength of the three mixed gases of SF₆-N₂, SF₆-air, and SF₆-CO₂ was calculated in the pressure range from 100 kPa to 500 kPa, and the insulation properties were predicted.¹⁴⁷ Govinda Raju calculated the breakdown field strengths of SF₆/N₂ and SF₆/N₂O mixed gases by solving Boltzmann parameters.¹⁴⁸ Kline et al.¹⁴⁹ calculated the electron swarm parameters of SF₆ mixed with He and N₂. The critical breakdown field strength of the mixed gas was obtained from the ionization coefficient and the attachment coefficient. Dincer and Govinda Raju¹⁵⁰ measured the ionization and attachment coefficients of SF₆ and N₂ mixed gases at steady state Townsend discharge. Govinda Raju et al.¹⁵¹ used the steady-state Townsend apparatus to measure the ionization of the SF₆ and N₂ mixed gas and obtained the critical breakdown field strength of the mixed gas. The breakdown field strength does not show a simple linear relationship with the increase in the SF₆ content, and a synergistic effect existed. Lee¹⁵² measured the relative parameters of a mixture of SF₆ and CO₂. Fréchette¹⁵³ used the SST apparatus to measure the ionization coefficients of the SF₆/N₂ and SF₆/CC₁₂F₂ mixed gas to obtain the limited breakdown field strength. For the SF₆/CCl₂F₂ mixed gas, the ionization coefficient was not linear with the SF₆ mixing ratio, and there was a synergistic effect. The breakdown strength of the mixed gas with a SF₆ mixing ratio of 25% was 5% higher than that of pure CCl₂F₂. Using the latest collision cross section data, Phelps and Van Brunt¹⁵⁴ calculated the electron swarm parameters (transport, ionization, adhesion, and diffusion coefficients) of SF₆ and N₂, O₂, and He mixtures by solving two Boltzmann equations approximately. The electron swarm process of the SF₆ and CO₂ mixture under PT discharge was calculated, as well as the ionization, adsorption, and dissociation coefficients.¹⁵⁵ Adding a small amount (2%-5%) of SF₆ into CO₂ under the condition of pulsed Townsend discharge has no obvious effect on the electron drift speed and diffusion coefficient function. However, it had a significant impact on the effective ionization process. Under the same mixing ratio, the critical breakdown strength of SF₆-CO₂ was lower than that of SF₆-N₂.¹⁵⁶ Urquijo measured the electron drift, diffusion, and effective ionization processes and attachment coefficients for SF₆/CHF₃ and SF₆-CF₄ mixed gases under PT discharge conditions. The electron drift and diffusion process had no relation with the SF₆ content, but the ionization coefficient depends on the SF₆ content. The critical breakdown fields of both gases were lower than SF₆-N₂.¹⁵⁷ Xiao et al.¹⁵⁸ and Liu and Xiao¹⁵⁹ calculated the electron swarm parameters of SF₆ and CF₄ using the Monte Carlo method. With the increase in the SF₆ content,
the breakdown voltage increased the effective ionization coefficient decreased. The electron diffusion coefficient of CF₄ was larger than SF₆. The improved Monte Carlo algorithm was used to calculate the electron swarm parameters of the SF₆/N₂ mixed gas, and the calculation was further improved.¹⁶⁰ Zhao *et al.*¹⁶¹ calculated the critical breakdown field strength of mixed gas of 0.01–1.6 MPa SF₆ and N₂ with the temperature of 300–3000 K. The minimum Gibbs free energy was used to calculate the equilibrium composition of particles. The electron energy distribution function was obtained using various particle data analyzed by the Boltzmann equation. At higher gas temperatures, the addition of N₂ to SF₆ gas reduced the kinetic energy of the electrons and increased the breakdown field strength. Increasing the N₂ concentration effectively increased the (E/N)cr of SF₆–N₂ mixtures at gas temperatures around 2000–3000 K. The same method was used to calculate the SF₆–CF₄ gas mixture. At low temperatures, the breakdown field of pure SF₆ was higher than that of the mixed gas, but the breakdown strength of the mixed gas was higher than that of SF₆ at temperatures greater than 4000 K (0.4 MPa).¹⁶² For the SF₆–CO₂ mixed gas, a large amount of CO₂ could reduce the number of high-energy electrons. When the temperature was higher than 1740 K, the addition of CO₂ to SF₆ could improve the insulation performance. However, adding too much CO₂ at low temperatures may cause the insulation performance to decrease. At 0.8 MPa, the dielectric properties of the mixed gas increased with an increase in pressure.¹⁶³ Larin discussed the synergistic effect of SF₆/CF₄ gas mixtures based on solving the Boltzmann equation.¹⁶⁴

### 3. The interruption performance

Lee and Frost¹⁶⁵ studied the interruption ability of SF₆ and its mixed gas and evaluated the arc extinguishing performance of 15 gases and mixed gas at 60 Hz AC. It was showed that the arc extinguishing performance of these gases and mixed gases was lower than that of pure SF₆. Tomita *et al.*¹⁶⁶ studied the interruption capability of 20% SF₆ and 80% Ar mixed gas in circuit breakers. The development of arc quenching was analyzed, and the electron density was measured. The arc extinguishing speed of mixed gas was faster than that of pure Ar. Gleizes *et al.*¹⁶⁷ calculated the transport parameters of the SF₆ and N₂ mixed gas plasma at temperatures from 1000 K to 15 000 K. With the temperature below 8000 K, the conductivity of the 50% SF₆–N₂ mixed gas was similar to SF₆, and much larger than pure N₂, which was due to the lower ionization coefficient of the S atom. At temperatures greater than 4000 K, the thermal conductivity of the mixed gas was greater than that of pure SF₆. In order to evaluate the arc-extinguishing ability of SF₆-CF₄ and SF₆–C₂F₆ mixed gas, Chervy *et al.*¹⁶⁸,¹⁶⁹ calculated the main parameters of the plasma based on the local thermodynamic equilibrium, which mainly referred to the thermodynamic parameters and transport parameters. The arc-extinguishing performance of SF₆–CF₄ and SF₆–C₂F₆ mixed gases was better than that of SF₆–N₂, and SF₆–CF₄ was superior to SF₆–C₂F₆. The thermodynamic properties of CF₄ with a better ability to interrupt were more stable than those of C₂F₆. When the mixing ratio of SF₆ exceeded 50%, the interruption performance of the mixed gas approached SF₆. With the mixing ratio of SF₆ below 25%, the performance is close to CF₄. 40% SF₆ mixed with CF₄ is a more appropriate mixing ratio. The arc extinguishing performance of the SF₆–CO₂ mixture used in circuit breakers was evaluated by calculating the mixed gas

transport parameters (thermal conductivity and conductivity).¹⁷⁰ Wang *et al.*¹⁷¹ calculated the effective ionization coefficient and the critical breakdown field strength of the SF₆ and He mixed gas at a high temperature (300 K–3500 K) in a high voltage circuit breaker. The mixed gas showed better interruption ability because of the He with a higher ratio thermal and thermal conductivity. CF₄ also had a good arc-extinguishing performance to be used in equipment such as a 115 kV/40 kA high voltage circuit breaker with 50%SF₆/50%CF₄ at 0.7 MPa gas pressure applied in the Manitoba hydropower station of Canada. SF₆/CF₄ circuit breakers with a rated voltage of 550 kV, a nominal current of 4 kA, and a breaking capacity of 40 kA operated at the Dorsey Converter Station.¹⁷² In the plasma calculation, Yang *et al.*¹⁷³,¹⁷⁴ calculated the thermodynamic properties and transport parameters of the plasma of SF₆ mixed with N₂ and CO₂ at high temperatures, as well as the influence of metal particles. In the non-thermodynamic equilibrium, the plasma behavior has been analyzed and discussed. Under the non-thermodynamic equilibrium condition, the difference of the conductivity between the mixed gas and N₂ was larger. The thermal conductivity of mixed gases also appeared significantly different, especially below 2000 K.

### 4. Decomposition characteristics

Pradayrol *et al.*¹⁷⁵ examined the major decomposition products of 50%SF₆ mixed 50%CF₄ at 50 Hz AC, including SOF₂ + SF₄, SOF₄, SO₂F₂, S₂F₁₀, S₂OF₁₀, S₂O₂F₁₀, and S₂O₃F₆. The addition of CF₄ to SF₆ had little effect on the major decomposition products. When H₂O and O₂ were present, the formation of the major products (SF₄ + SOF₂) and S₂F₁₀ was reduced. In the presence of solid insulation, the products of the mixed gases were similar to pure SF₆. When H₂O, O₂, and insulators exist, they can interfere with each other.¹⁷⁶ Under negative corona discharge, O₂ and H₂O (less than 0.2% H₂O and less than 1% O₂) enhanced the production of all sulfur oxyfluoride. At lower pressures, product generation increased. The presence of 50%CF₄ inhibited the production of all compounds.¹⁷⁷ SF₆/O₂ under spark discharge decomposed SOF₄, SO₂F₂, SOF₂, and SO₂.¹⁷⁸ The main products of SF₆ and N₂ included SOF₄, SO₂F₂, SF₄ + SOF₂, SO₂, S₂OF₁₀, S₂F₁₀, S₂O₂F₁₀, S₂O₃F₆, SF₅NF₂, NF₃, and (SF₅)₂NF during AC corona discharge. Adding a small amount of water molecules would affect the rate of formation of all products. The production of SF₅NF₂, NF₃, and (SF₅)₂NF was minimal compared to other products. N₂O and CO₂ were also produced at higher pressures.¹⁷⁹,¹⁸⁰ SF₆ and N₂ produce the same decomposition when AC spark discharges.¹⁸¹ When a small amount of CO₂ was added to the mixture of SF₆ and N₂, the generation of SOF₄ + SO₂F₂, COF₂, CO, and S₂F₁₀ was reduced under negative DC corona.¹⁸² Since SF₆ and N₂ have been used in the GIL, their decomposition products in the case of partial discharges and their discharge characteristics were helpful for the detection of partial discharges and insulation monitoring. However, the decomposition products of mixed gases are more complicated than pure SF₆, and the difficulty in insulation monitoring is further increased.

Mixed gases are suitable for equipment in extremely cold areas, such as northern China. However, the recycling and reuse of mixed gases still need to be improved and optimized for existing SF₆ equipment.¹⁸³,¹⁸⁴ ABB proposed a standard for the use of SF₆ gas mixtures in equipment, proposing that the assessment of the environmental
<figure><img src="image_19.png" /><figcaption>FIG. 8. PFC molecular structures.</figcaption></figure>

impact needs to take into account the time period. The mixed gas should be reused in the field without generating other gas components and be easy to recycle separation. The existing SF₆ recovery equipment needs to be rebuilt and upgraded to recover the mixed gas.¹⁸⁵

The use of SF₆ gas mixtures in electrical equipment has the obvious advantage of lowering the boiling point and maintaining good insulation properties. The main properties of SF₆ mixtures are shown in Table VI of the Appendix.

### C. Mixtures of perfluorocarbons (PFCs) with natural gases

In order to completely replace SF₆, some electronegative gases with a low greenhouse effect and reliable insulation have become the main research objects. Compared with SF₆, the PFCs have better insulating properties and the GWP value is lower relative to SF₆. The typical electronegative gases studied today are c-C₄F₈, C₃F₈, C₂F₆, and CF₄ based on the boiling point, safety, and insulation properties. The molecular structures are shown in Fig. 8. The insulating gas and the buffer gas are generally mixed (N₂, CO₂, etc.), which reduces the gas cost and lowers the boiling point of the mixed gas under the premise of ensuring the dielectric strength.

Takuma et al.¹⁸⁶ calculated the boiling point of the mixed gas as a function of the mixing ratio according to the van der Waals formula and tested the breakdown voltage of the mixed gas of c-C₄F₈ and N₂. The insulation performance of the c-C₄F₈ gas under a uniform electric field is 1.18–1.25 times that of the SF₆ gas. Although the price of c-C₄F₈ is ten times that of SF₆, the actual usage is much smaller than that of SF₆. The use of c-C₄F₈ as an arc-extinguishing medium may produce carbon particles that affect the insulation properties. Adding a buffer gas such as CO₂ and N₂ can ease the formation of particles. Wada et al.¹⁸⁷ tested the breakdown behavior of the four perfluorocarbon gases, c-C₄F₈, 1-C₃F₆, C₃F₈, and C₂F₆, under AC voltage at the plate–plate electrode and combined them with CO₂, N₂, and CF₄, respectively. It was found that PFCs and buffer gas mixtures have shown a positive synergistic effect. At the same time, the gas mixture was analyzed in terms of the boiling point and GWP. When the ambient temperature is above –20 °C, the insulation performance of the

mixed gas can reach 70% that of SF₆ and the GWP is reduced by 30%.

Zhao et al.¹⁸⁸ studied the synergistic effect of the c-C₄F₈/N₂ mixed gas at lightning impulse voltage. The effect of gas pressure on the synergistic effect is not significant, and the obvious polar effect appears in the discharge characteristic of c-C₄F₈/N₂. The breakdown voltage under negative impulse was lower than that under positive impulse. This was because the polarity of the lightning impulses affects the speed and kinetic energy of free electrons. The free electrons in a negative lightning strike had a higher velocity, making the c-C₄F₈ to absorb fewer free electrons, so the breakdown voltage under negative lightning impulses was lower than the breakdown voltage under positive impulses.

Whitman¹⁸⁹ studied the impulse breakdown characteristics of C₃F₈ and compared it to air. At both positive and negative impulse voltages, the breakdown voltage of C₃F₈ was about 2.5 times that of air with both electrodes (ball and plate electrodes). Okubo et al.¹⁹⁰ tested the partial discharge inception voltage and breakdown voltages of C₃F₈ and C₂F₆ with the needle plate electrode. The synergistic effect of the C₃F₈/N₂ and C₂F₆/N₂ mixed gases was lower than that of SF₆/N₂. The breakdown voltage of the mixed gas is arranged in descending order of SF₆/N₂, C₃F₈/N₂, and C₂F₆/N₂.

de Urquijo and Basurto¹⁹¹ used the PT test to measure the C–C₄F₈ electronic parameters such as the ionization coefficient, attachment coefficient, and diffusion coefficient, resulting in a critical breakdown field strength of 439.5 Td (1 Townsend = 10⁻¹⁷ Vcm²), which is larger than that of SF₆ (369Td). Itoh et al.¹⁹² calculated the electron transport parameters of the mixed gas of SF₆ and c-C₄F₈. The insulation characteristics of the mixed gas are better than that of pure gases. Wu et al.¹⁹³ and Liu et al.¹⁹⁴ calculated the critical breakdown field strength of the mixed gases of c-C₄F₈ and N₂ under pulsed discharges using Monte Carlo. The calculated results showed that the insulating properties of the mixed gases of c-C₄F₈ and N₂ were comparable to those of the SF₆/N₂ mixed gas, but the GWP value decreased significantly. Liu calculated the insulation properties of the mixed gas of c-C₄F₈ and CO₂ in the same way. As a result, the critical breakdown strength of the mixed gas of c-C₄F₈ and CO₂ was greater than that of the mixed gas with SF₆/CO₂. Li et al.¹⁹⁵ calculated electronic swarm parameters of the c-C₄F₈ mixed gas. The critical breakdown field strength of c-C₄F₈–N₂ and c–air were
similar and significantly higher than other mixed gases. The insula-
tion performance of C₄F₈ can reach levels of pure SF₆ when the con-
tent exceeds 80%. Deng *et al.*¹⁹⁶ used the same calculation method
to analyze the insulation properties of C₃F₈ mixed N₂ and CO₂.
Under the same mixing ratio, the critical breakdown strength of
the C₃F₈–N₂ gas mixture is greater than that of C₃F₈–CO₂. The
C₃F₈–N₂ gas mixture containing 20% C₃F₈ can reach 60% of the
dielectric strength of pure C₃F₈. Xiaoling *et al.*¹⁹⁷ proposed a method
for evaluating the electronegativity of a gas by calculating the elec-
tron transport parameters of the molecule. The electron attachment
property of the molecule was obtained by the Boltzmann equation
under steady state Thomson discharge. It was found that the ability
of c-C₄F₈ to attach electrons was still weaker than that of SF₆ and its
e electronegativity coefficient was at least three orders of magnitude
lower than that of SF₆.

Koch and Franck¹⁹⁸ studied the characteristics of the C₃F₈ gas during partial discharge and breakdown. It is found that the discharge characteristics of the SF₆ gas were approximately similar from the results of discharge capacity, flow propagation time, and time interval, which may be related to (E/N)cr, and ionization coefficients were closely related. The same model was used to predict the breakdown voltage and PD voltage of the CF₄ gas.¹⁹⁹ By calculating the breakdown parameters of C₃F₈–CF₄, C₃F₈–CO₂, C₃F₈–N₂, C₃F₈–O₂, and C₃F₈–Ar of C₃F₈ in the temperature range 300–3500 K, Wang *et al.*²⁰⁰ found that the ionization coefficient of C₃F₈–N₂ was higher, and the attachment coefficient of C₃F₈–CF₄ was the largest. Under a normal temperature, the insulation property of C₃F₈/N₂ was outstanding. However, the critical breakdown strength of C₃F₈ decreased with the increase in temperature. The same method was used to calculate the critical breakdown field of the CF₄ plasma and predict the insulating properties.²⁰¹

Lisovskiy et al.²⁰² measured the DC breakdown voltage of CF₄ at low pressure and calculated the electron swarm parameter of the gas at a small PL value (PL < 2 Torr cm, where P is pressure and L is distance). Wang et al.²⁰³,²⁰⁴ calculated the thermodynamic parameters and transport parameters of the main PFCs (CF₄, C₂F₆, and C₃F₈) in the temperature range of 300–30 000 K, including the electron diffusion coefficient, viscosity, thermal conductivity, and conductivity, extending to the third order approximate (viscosity of the second order) Chapman–Enskog method. Theoretical calculations were made to analyze the arc-extinguishing properties of the three gases. The thermodynamic properties of olefinic gases (C₂F₂, C₂F₄, C₃F₆, etc.) containing C=F double bonds were also analyzed.

Kang et al.²⁰⁵ studied the decomposition products of c-C₄F₈ and mixed gas with N₂ at 50 Hz AC corona discharge. The products of pure c-C₄F₈ gas discharge mainly included CF₄, C₂F₄, C₃F₆, C₂F₆, and C₃F₈, and C₂F₃N was produced in the mixed gas.

The insulation properties of c-C₄F₈, C₃F₈, and C₂F₆ all reach or exceed the level of SF₆, and the GWP values are obviously smaller than those of SF₆ (8700, 7000, and 9200, respectively). However, due to the limitation of the boiling point, it is necessary to mix with the buffer gas. With the addition of the buffer gas, the environmental impact obviously decreases, and the insulating property of the mixed gas also decreases. The main properties of PFCs mixtures are shown in Table VII of the Appendix. There is no obvious advantage in dielectric strength, which restricts the research and exploration process.

D. C<sub>n</sub>F<sub>m</sub>X mixtures

1. Mixtures of CF3I with natural gases

One of the F atoms in CF₄ is replaced by the iodine atom from the CF₃I molecule, as shown in Fig. 9, and the performance of the molecule is greatly changed. Initially, CF₃I was considered as an alternative to the Halon 1301 fire extinguishing agent because CF₃I has no effect on the environment and is non-flammable and non-explosive.<sup>206,207</sup>

The initial study found that the CF₃I gas has good electron attachment capacity. The calculation of the electron swarm parameter showed that the critical breakdown field strength was higher and even higher than SF₆. From 2010, one after another, researchers began to explore the possibility to apply it in electrical equipment.

The AC breakdown voltage of the CF₃I/N₂ mixed gas in the non-uniform field was investigated, and the effects of pressure, electrode spacing, and mixing ratio were analyzed. When the mixing ratio was 30%, the breakdown voltage of the CF₃I/N₂ mixture was about 85%–90% of that of SF₆ at a pressure of 0.3 MPa, but the cost of gas was three times higher than the SF₆ one.²⁰⁸ At the same time, the initial voltage of partial discharge of the CF₃I/CO₂ mixed gas was measured. The partial discharge initial voltage of the CF₃I/CO₂ mixed gas is 0.9–1.1 times that of SF₆/CO₂. The PD performance of the CF₃I/CO₂ mixed gas, with a CF₃I volume fraction of 30%–70%, was about 0.74 times of that of pure SF₆. The CF₃I/CO₂ mixed gas showed a good synergistic effect, with a value of 0.53.²⁰⁹

Toyota et al.²¹⁰ used square pulse voltage as a transient over-
voltage to detect the (V-t characteristic) relationship between the
breakdown voltage and the breakdown time of the CF₃I–N₂ and
CF₃I–air mixture at the plate electrode. The test results showed that
the breakdown voltage of pure CF₃I was 1.2 times that of SF₆ at the
same pressure. The breakdown voltage of the CF₃I–N₂ and CF₃I–air
mixture increased with the increase in the CF₃I mixing ratio. When
the mixing ratio of CF₃I reached 60%, the dielectric strength of the
mixed gas can reach the level of pure SF₆.

Yuan et al.²¹¹ measured the gas–liquid equilibrium data (CF₃I + CO₂) at 243.150 K. The experimental data were modified by Peng-Robinson (PR) on the classical van der Waals formula to get the rule that the vapor pressure of the CF₃I and CO₂ gas mixture and its mole fraction change with the operating environment of GIS/GIL.

<figure><img src="image_20.png" /><figcaption>FIG. 9. Molecular structure of CF<sub>3</sub>I.</figcaption></figure>
Kamarudin et al.<sup>212</sup> tested the breakdown performance of a mixture of CF<sub>3</sub>I and CO<sub>2</sub> (30%:70% mixing ratio) under three kinds of electrodes (rod, plate, and coaxial electrodes) under a lightning impulse voltage. The tests were conducted, and it was assumed that CF<sub>3</sub>I gas mixtures could be used in medium voltage (MV) equipment without involving arc extinguishing equipment. Youping et al.<sup>213</sup> tested the breakdown characteristics of the mixed gas of CF<sub>3</sub>I and N<sub>2</sub> in a uniform electric field and concluded that the decomposition products of CF<sub>3</sub>I would affect the insulation properties. The insulation performance of 30% CF<sub>3</sub>I-70% N<sub>2</sub> at 0.1 MPa was similar to that of 20% SF<sub>6</sub>-80% N<sub>2</sub> by DC voltage and lightning impulse voltage. Widger et al.<sup>214</sup> tested the insulation properties of a 30% CF<sub>3</sub>I-CO<sub>2</sub> mixture in a ring-grid cabinet with a vacuum circuit breaker. It was demonstrated by laboratory tests that the gas mixture at this ratio met the requirements for medium-voltage equipment. Chen measured the 50% breakdown voltage of the 30/70% CF<sub>3</sub>I/N<sub>2</sub> mixture at the lightning impulse voltage of a coaxial electrode. In order to explore the feasibility of the CF<sub>3</sub>I-CO<sub>2</sub> mixture in a 400 kV gas-insulated power transmission line (GIL), the experiment was conducted under the equivalent similarity model with the same electric field distribution.<sup>215</sup> The possibility of the CF<sub>3</sub>I/CO<sub>2</sub> mixture for insulation equipment was analyzed from the breakdown performance. It was considered that this gas was not suitable for the arc extinguishing medium, but it can be considered for use in the GIL.<sup>216</sup> The breakdown voltage of CF<sub>3</sub>I mixed with CO<sub>2</sub> and N<sub>2</sub> showed a ternary mixed gas and no significant synergistic effect, and under the same conditions, the CF<sub>3</sub>I and CO<sub>2</sub> mixed gas has better breakdown characteristics than CF<sub>3</sub>I-N<sub>2</sub>. This phenomenon was explained by measuring the decomposition products, and calculations showed that CO<sub>2</sub> could provide C atoms for the self-recovery of CF<sub>3</sub>I molecules.<sup>217</sup>

de Urquijo<sup>218</sup> measured the electron swarm parameters of the CF<sub>3</sub>I and N<sub>2</sub> mixed gas, including the ionization coefficient, attachment coefficient, drift velocity, and diffusion coefficient. The (E/N)<sub>cr</sub> of CF<sub>3</sub>I was about 437T<sub>d</sub>, much larger than that of SF<sub>6</sub>. The breakdown field of the 70%CF<sub>3</sub>I-N<sub>2</sub> mixture is equal to that of SF<sub>6</sub>. Kawaguchi et al.<sup>219</sup> calculated these parameters using the Monte Carlo algorithm and found good agreement with the experimental results. Kimura and Nakamura<sup>220</sup> calculated the electron swarm parameters of the mixed gas of CF<sub>3</sub>I with CO<sub>2</sub>. Li et al.<sup>221</sup> calculated the electronic swarm parameters of CF<sub>3</sub>I mixed with a variety of gases such as CF<sub>4</sub>, CO<sub>2</sub>, N<sub>2</sub>, O<sub>2</sub>, and air. CF<sub>3</sub>I mixed with N<sub>2</sub> and air has more significant advantages, and when the mixing ratio of CF<sub>3</sub>I exceeded 70%, the breakdown field can exceed pure SF<sub>6</sub>. Yun-Kun and Deng-Ming<sup>222</sup> and Deng and Xiao<sup>223</sup> have done a similar calculation. Combining the boiling point and the greenhouse effect, the feasibility of using CF<sub>3</sub>I in high-pressure equipment was analyzed. It was considered that the mixed gas containing 70% CF<sub>3</sub>I had the same dielectric strength as SF<sub>6</sub>. Li<sup>224</sup> established the model to calculate the discharge process of CF<sub>3</sub>I and calculated the time and process of the formation of the streamer current from the electron avalanche to the electron reaching the cathode.

Taki et al.<sup>225</sup> studied the interruption capability of CF<sub>3</sub>I. First, the arc time constant and arc power loss coefficient were obtained by using Mayr's equation. The order of the arc time constant was SF<sub>6</sub>, CF<sub>3</sub>I, CO<sub>2</sub>, H<sub>2</sub>, Air, and N<sub>2</sub>, while that of the arc loss coefficient was H<sub>2</sub>, SF<sub>6</sub>, CO<sub>2</sub>, Air, N<sub>2</sub>, and CF<sub>3</sub>I; the short circuit fault interruption performance of CF<sub>3</sub>I is tested, which can reach 90%

of SF<sub>6</sub>. When the volume ratio of CF<sub>3</sub>I exceeded 20%, the interruption ability of CF<sub>3</sub>I mixed with CO<sub>2</sub> approached pure CF<sub>3</sub>I levels. The Breaker Terminal Failure (BTF) interruption capabilities of CF<sub>3</sub>I and the mixed gas were tested. The BTF interruption capacity of CF<sub>3</sub>I-CO<sub>2</sub> (30%-70%) was about 67% of SF<sub>6</sub>.<sup>226,227</sup> Yokomizu et al.<sup>228</sup> calculated the electrical conductivity and thermal conductivity of CF<sub>3</sub>I mixed with CO<sub>2</sub> at high temperatures and calculated transient parameters during quenching. When the temperature was above 10 000 K, the conductivity was greatly affected by the content of CF<sub>3</sub>I. At 7000 K, the thermal conductivity was greatly affected by the content of CF<sub>3</sub>I. The calculated results showed that the decay rate of the CF<sub>3</sub>I mixed gas with CO<sub>2</sub> was weaker than that of SF<sub>6</sub>, and the breakoff capacity was worse than SF<sub>6</sub> as a whole. Cressault et al.<sup>229</sup> calculated the plasma thermodynamic parameters (mass density, enthalpy, and specific heat) and transport parameters (viscosity, thermal conductance, and conductance) of CF<sub>3</sub>I mixed with CO<sub>2</sub>, N<sub>2</sub>, and air. The calculated results showed that CF<sub>3</sub>I mixed CO<sub>2</sub> is superior to the other two mixed gases because CO<sub>2</sub> plasma has better heat dissipation ability under a high temperature.

Ngoc et al.<sup>230</sup> tested the DC breakdown voltage of CF<sub>3</sub>I and mixed N<sub>2</sub> at a quasi-uniform electric field (ball-ball electrode). The breakdown voltage of pure CF<sub>3</sub>I was greater than that of SF<sub>6</sub>, but the synergistic effect with N<sub>2</sub> was not obvious. With the increase in the CF<sub>3</sub>I mixing ratio, the breakdown voltage also increased. When the mixing ratio of CF<sub>3</sub>I was less than 50%, the breakdown voltage of the mixed gas was lower than the SF<sub>6</sub>-N<sub>2</sub> mixed gas with the same ratio. After the breakdown experiment, it was found that solid iodine was deposited on the electrode surface. Kasuya et al.<sup>231,232</sup> tested the decomposition products of the CF<sub>3</sub>I-CO<sub>2</sub> (30%-70%) mixed gas during quenching and improved iodine deposition by adding adsorbents. Continuous breakdown of CF<sub>3</sub>I for 1300 cycles resulted in a 11% reduction in breakdown voltage.<sup>233</sup> The solid iodine also was generated by CF<sub>3</sub>I-CO<sub>2</sub> gas mixture breakdown at the lightning impulse voltage.<sup>234-236</sup> Jamil et al.<sup>237</sup> quantitatively detected C<sub>2</sub>F<sub>6</sub>, C<sub>2</sub>F<sub>4</sub>, C<sub>2</sub>F<sub>5</sub>I, and a few C<sub>3</sub>F<sub>8</sub>, CHF<sub>3</sub>, C<sub>3</sub>F<sub>6</sub>, and CH<sub>3</sub>I within 224 h of CF<sub>3</sub>I partial discharge under a non-uniform field. The presence of H<sub>2</sub>O and O<sub>2</sub> would have an impact on the formation of decomposition products during discharge of CF<sub>3</sub>I and accelerates the decomposition of CF<sub>3</sub>I and produce toxic substances such as CF<sub>2</sub>O.<sup>238,239</sup>

Although CF<sub>3</sub>I has high insulation properties, it cannot be used alone in equipment due to the boiling point. Under the premise of ensuring the boiling point, the mixed gas with the buffer gas added cannot reach the level of pure SF<sub>6</sub>, and there is no obvious arc extinguishing advantage. After the gas discharge, the obvious solid main atoms are generated (I and C), which is very detrimental to gas self-recovery in the use of the device. The oxidation of materials due to the presence of iodine could not be ignored. Its acute toxicity is acceptable, but this gas is genotoxic, so the safety of practitioners is a threat in the long-term operation of the device. Although a great deal of exploration has been done, all these achievements are carried out in the laboratory, and it is unlikely that the gas will be used in the equipment at present.

## 2. Mixtures of C<sub>4</sub>F<sub>7</sub>N with natural gases

In addition to the above gases, the bond in the carbon-fluorine bond is broken and replaced by atoms such as N to form some new
<figure><img src="image_21.png" /><figcaption>FIG. 10. Molecular structure of C₄F₇N.</figcaption></figure>

derivatives such as C₄F₇N, the molecular structure of which is shown in Fig. 10.

In 2015, Alstom and 3M collaborated to test the power frequency dielectric strength of the C₄F₇N–CO₂ mixture in a 145 kV GIS. The study found that when the content of C₄F₇N is 18%–20%, it can achieve the same dielectric strength as pure SF₆.⁴⁷ The frequency and lightning impulse breakdown of the C₄F₇N/CO₂ mixed gas under different electric field types were tested under the influence of the mixing ratio, electrode distance, and gas pressure by Nechmi *et al.* from the French National Academy of Sciences AMPERE Lab.²⁴⁰ It was found that the synergistic effect of C₄F₇N and CO₂ at a concentration of 3.7% was obvious. At present, the C₄F₇N/CO₂ mixed gas (g3) is tested in a 420 kV GIL and 245 kV current transformers. Owens²⁴¹ conducted the breakdown test of C₄F₇N with CO₂, N₂, and dry air mixtures at different pressures on a plate–plate electrode and found that the mixture had the best dielectric strength with CO₂. Preve *et al.*²⁴² tested the dielectric strength of the C₄F₇N/dry air mixture and found that its insulation properties were superior to SF₆ at impulse voltage but slightly inferior to SF₆ at AC voltage. Kieffel *et al.*²⁴³ also conducted a continuous 100-s breaking test on C₄F₇N/CO₂ (the mixing ratio of C₄F₇N is 4%), with an average arcing time of 12 ms, which was lower than that of pure SF₆ (15 ms) at the same pressure.

C₄F₇N has a very high dielectric strength and is currently the most promising alternative to SF₆ in existing gases. However, the presence of a cyano-group (C≡N) in the molecule can be a hazard to the safety of the gas used in the equipment and the toxicity of the pure gas is not yet demonstrated, especially regarding neurotoxicity. Its main decomposition products and reaction with the molecules in the environment have also drawn attention.²⁴⁴,²⁴⁵ For C₄F₇N, there may be some highly toxic by-products, such as CO and PFIB (C₄F₈), after 100 C–O 630 A/24 kV, the LC50 (4 h on mice) in an 11-l epoxy bulb of C₄F₇N/air or C₄F₇N/CO₂ mixture is inferior to 225 ppm.²⁴⁶ Sulbaek Andersen *et al.*²⁴⁷ measured the possible chemical reactions of C₄F₇N with OH, Cl, and O₃ in the atmosphere and found the products to be COF₂ and CF₃COF. The GWP of the gas was evaluated based on the infrared absorption spectrum. Due to the low content of the gas, the decomposition products did not adversely affect the environment. Blázquez *et al.*²⁴⁸ calculated the reaction of gas molecules with OH radicals using the

quantum chemical calculation method and obtained the reaction rate at 278–358 K.

The calculation of the molecular structure based on DFT mainly refers to the binding energy, ionization energy and molecular orbital gap, and decomposition process of the gas molecules.²⁴⁹ The breakdown voltage of C₄F₇N/N₂ at power frequency voltage for 30 times does not change much, but a series of products are produced, such as CF₄, C₃F₈, CF₃CN, C₂F₄, C₂F₅CN, and C₃F₆.²⁵⁰ It is noteworthy that the formed CF₃CN and C₂F₅CN are highly toxic gases and the toxicity of the gas C₄F₇N also needs further research and confirmation.²⁵¹ C₄F₇N would further react with H and OH, and the main production are CF₂HCN, CFH₂CN, CF₃CHFCN, and CF₃CH₂CN.²⁵² The computational decomposition process was studied, and the reaction of the molecules within the device with the metal Cu and Al surface was considered.²⁵³,²⁵⁴

The gas is currently a research hotspot and also has great potential for the gas medium used in high-voltage equipment. Due to the current use of less gas, the price of gas is relatively expensive. The gas greenhouse effect is low for high voltage applications and still high for medium voltage applications, while maintaining a low mixing ratio can still meet the equipment requirements of the insulation operation. However, due to the cyano group (C≡N) contained in the molecular structure, the free radical may react with others or atoms and even the metal in the device during discharge to generate toxic substances such as HCN and CuCN. This section needs further research and exploration to avoid the health threats.

### 3. Mixtures of perfluoroketones (C₅F₁₀O and C₆F₁₂O) with natural gases

In 2015, ABB proposed to consider the use of ketones C₅F₁₀O or C₆F₁₂O (boiling points of 26.5 °C and 49 °C, respectively) in combination with the buffer gas as an insulating medium.²⁵⁵ The molecular structures are shown in Fig. 11.

For the insulation properties, Mantilla *et al.*²⁵⁵ found that the 50% positive lightning impulse breakdown voltage of a mixture of 5% C₅F₁₀O and dry air at 0.7 MPa reached that of pure SF₆ at 0.4 MPa. Simka and Ranjan²⁵⁶ tested the breakdown frequency field of C₅F₁₀O and dry air under a quasi-uniform electric field and found that the critical breakdown strength of the mixed gas containing 5.2% C₅F₁₀O at 0.7 MPa can reach 95% of pure SF₆ at 0.45MPa % and 80% of pure SF₆ at 0.6 MPa. For the engineering applications, Saxegaard *et al.*²⁵⁷ found that the lightning impulse withstand level increased by 60% and the power frequency voltage level increased by 44%, by filling a 12 kV air switch cabinet with the C₅F₁₀O–air mixture. Hyrenbach and Zache²⁵⁸ has produced a switchgear insulated with the C₅F₁₀O mixed gas, which was commissioned in 2015 in a substation of Zurich. The switchboard passed the IEC62271-200 standard requirements of the full voltage test, and at this stage, C₅F₁₀O mixed with air can be used for medium voltage equipment (6–40.5 kV). The switchboard with C₅F₁₀O as the insulating medium is developed by ABB, and the gas chamber is made of stainless steel to prevent the metal materials inside the equipment from being oxidized.

By calculating the thermodynamic parameters and transport parameters of plasma at a high temperature of C₅F₁₀O gas molecules,
<figure><img src="image_22.png" /><figcaption>FIG. 11. C₅F₁₀O and C₆F₁₂O.</figcaption></figure>

it is considered that it has a better arc extinguishing property than SF₆ from the nature of transport. Part of the particle structure is complex, so its parameters (such as partition function and polarizability) have no reference data, which are calculated by computational chemistry.²⁵⁹ However, Zhong *et al.*²⁶⁰ calculated the thermodynamic properties of C₅F₁₀O mixed with CO₂ and O₂. The calculation results showed that due to decomposition of C₅F₁₀O in the process of quenching, the gas at the end of quenching cannot completely recover. The main products were CF₄, CO₂, C₄F₆, and graphite carbon. O₂ were added to the mixture without affecting the thermodynamic parameters and transport parameters. Therefore, the existing equipment is used in a mixture of C₅F₁₀O and air.

Tatarinov *et al.* explored the decomposition products of C₅F₁₀O-dry air mixed gases under the application of 10 kV dielectric barrier discharge (DBD) and found that C₄F₁₀, C₆F₁₄, C₃F₆, C₅F₁₂, and other substances were produced.²⁶¹ Based on the optimization of the molecular model, the carbonyl carbon atom is the active site of C5-PFK molecules, and the decomposition of C5-PFK may take place centered on the carbonyl carbon atom.²⁶²⁻²⁶⁴

At the same time, the impact of C5 on the environment and personal safety also attracted attention. ABB tested the gas composition of the GIS with AIRPLUS gas (C₅F₁₀O and air) during normal operation and gas leakage.²⁶⁵ The mixture is not to be a threat to the environment or human body, but the LC50 (4 h) for mice after breaking tests lead to 2100 ppm, which is far lower than the one of gas mixture before breaking.²⁶⁶

For C₆F₁₂O, because of the high boiling point of the gas, the mixing ratio is seldom used in the equipment. Under the frequency test, adding 3% C₆F₁₂O to N₂ can increase the breakdown voltage of pure N₂ almost by 1 time.⁷⁹ The products after breakdown are PFCs and thus have no effect on the insulation properties. The content is low, which will not affect the environment. It is considered that the gas is more suitable for use in medium and low voltage equipment. By adding a small amount of C₆F₁₂O in N₂, the insulation level of the equipment is improved. Existing studies have shown that the C₆F₁₂O/CO₂ mixed gas has the potential for application in a 10 kV switchgear.²⁶⁶,²⁶⁷

The gas insulation far exceeds SF₆, but the problem of boiling point has also become very prominent. At present, C₅F₁₀O is expected to be the main insulating gas for medium voltage equipment.

The main properties of CₐFₘX mixtures are shown in Table VIII of the Appendix.

### E. Others

Since C atoms and F atoms have good electronegativity, the gas molecules, formed by the combination of C and F with other atoms such as H, Cl, S, and N, generally also have good insulation properties. Devins in 1980 summarized the basic insulation properties of gases,³² such as CFₙCl₃⁻n, CHFₙCl₃⁻n, and CₙH₂n+2; their insulation properties gradually increased with the increase in n, but the boiling point was also gradually obvious. However, some gases have the greenhouse effect, and some were toxic gases such as CF₃CN. Therefore, there was not much gas available for reference at the moment.

CF₃NSF₂ was also considered to have high-performance gas insulation, the insulation performance of about 2.41 times the SF₆, and the boiling point of -6 °C. Cheng *et al.* calculated the reaction mechanism of this molecule with OH.²⁶⁸ The literature for the study of this gas is still relatively less, and the field of its application is relatively vague now. The gas products available are difficult to supply. There is not much literature on the experimental research of its various properties.

CCl₂F₂ (R12) is a Freon gas with a GWP of 2400 that is far less than that of SF₆. The experimental study found that the insulation strength of the gas was about 90% of that of SF₆. The mixed gas showed good self-stability through mixed air and N₂ breakdown experiments.⁴⁸ However, this gas has a large damage to the ozone layer, with the ozone depletion potential (ODP) up to 1.0.

HFO1234zeE (CF₃CH=CHF) is non-toxic, non-flammable, and non-destructive of the ozone layer, with a GWP value of less than 0.02 and the lifetime of 1 day. It is generally used as a refrigerant. Through the application of AC and positive and negative lightning impulse voltage, the breakdown test was studied under the uniform field and extreme non-uniform field. HFO1234zeE showed very similar insulating properties to SF₆. It can therefore be considered as a substitute for medium-voltage equipment.⁴⁹ For the electron swarm parameter of the HFO1234zeE molecule under PT discharge, the gas was found to have strong electron attachment capacity.²⁶⁹ In medium- or high-voltage applications, the HFO1234zeE/N₂ mixture would show more excellent electrical strength in the range of 100 kPa to 1 MPa than
currently obtained at 10 kPa. HFO1234zeE has comparable electrical strength to SF₆ at 130 kPa. There is no synergistic effect in the HFO1234zeE/N₂ and HFO1234zeE/CO₂ mixtures.²⁷⁰ The main characteristics of this gas demonstrates that it can be used as a dielectric medium for the MV switchgear, but it cannot be used in breaking.²⁷¹

Chen²⁷² analyzed the molecular structure of OCCHCN and concluded that it was not easily ionized and excited based on quantum chemistry calculation, but there was no experimental data to validate it.

The insulation properties of gases can be predicted to filter potential alternative gases based on the calculation results. The parameters that affect the gas insulation performance are the relative molecular mass M, the dipole moment μ, the polarizability α, and so on. The ionization collisions increase the number of carriers, thereby weakening the gas dielectric strength. The electron attachment collision does not increase the number of carriers but can slow down carriers, which is helpful to strengthen the dielectric strength. Molecules with higher ionization energy are more susceptible to ionization collisions. More electronegative molecules attach electrons more easily, so insulating gas should have higher ionization energy (IE) and electronegativity.

Yu⁵⁸ builds the new structure-activity relationship (SAR) models to calculate the molecular insulation properties by combining the surface area, polarizability, electronegativity, and molecular hardness of a molecule or functional group. Through the calculation of the existing molecules, it is found that the functional group CF₃ has outstanding dielectric strength in the molecular structure due to its large surface area and polarizability. At the same time, taking into account the impact of the boiling point, the authors believe that SF₅CN and SF₅CFO have greater potential to replace SF₆.

# V. CONCLUSION AND OUTLOOK

## A. Conclusion

Table IV shows the main alternative gases and their properties. The main studies of several major alternative gases now draw the following conclusions:

(1) For natural gas, the insulation performance is only 40% of SF₆ or even lower, but safety and low cost become the advantages. By optimizing the structure of the equipment, increasing pressure, and combining solid insulation, the natural gas could be used in medium and low voltage equipment.

(2) The research results of SF₆ mixtures are quite mature. SF₆/N₂ is a relatively stable combination for the moment and has been used in GILs and switchgear cabinets. However, studies have shown that CO₂ shows obvious thermodynamic properties at high temperatures despite the slightly lower insulating properties of CO₂ mixtures, and the sensitivity of the mixed gas to the electric field uniformity is lower than that of the N₂ mixed gas. For interrupter properties, the SF₆-CF₄ mixed gas has also been used in circuit breakers. As for decomposition performance, the decomposition products of the mixed gas are more complicated than pure SF₆. However, the reduced use of SF₆ also inhibits the production of toxic sulfur compounds.

(3) PFCs have some advantages over SF₆ in terms of GWP, and both theoretical calculations and experimental measurements show that the insulating properties of pure gas are superior to that of SF₆. Affected by the boiling point, the insulating properties of the mixture cannot reach the level of SF₆.

(4) Despite its superior gas-insulating properties and no greenhouse effect, the decomposition of CF₃I after discharge is the

<table><caption>TABLE IV. Basic properties of main alternative gases. G presents that the property is better than that of SF₆. NG presents that the property is worse than that of SF₆. E presents that the property is comparable to that of SF₆. - presents that the gases were studied but have no real application.</caption><thead><tr><th>Gases</th><th>Boiling point</th><th>GWP/ODP</th><th>Insulation</th><th>Interruption</th><th>Decomposition</th><th>Application</th></tr></thead><tbody><tr><td>SF<sub>6</sub></td><td>Good</td><td>Not good</td><td>Very good</td><td>Very good</td><td>Not good (toxic but in low concentration)</td><td>GIS/GIL/GCB/GIT</td></tr><tr><td>N<sub>2</sub></td><td>G</td><td>G</td><td>NG</td><td>NG</td><td>...</td><td>C-GIS(72 kV)</td></tr><tr><td>Air</td><td>G</td><td>G</td><td>NG</td><td>NG</td><td>...</td><td>C-GIS(40 kV)</td></tr><tr><td>CO<sub>2</sub></td><td>G</td><td>G</td><td>NG</td><td>NG</td><td>...</td><td>40 kV C-GIS<br>70 kV GCB</td></tr><tr><td>SF<sub>6</sub> + N<sub>2</sub></td><td>G</td><td>G</td><td>E</td><td>NG</td><td>G</td><td>245-550 kVGIL</td></tr><tr><td>SF<sub>6</sub> + CO<sub>2</sub></td><td>G</td><td>G</td><td>E</td><td>NG</td><td>G</td><td>...</td></tr><tr><td>SF<sub>6</sub> + CF<sub>4</sub></td><td>G</td><td>G</td><td>E</td><td>E</td><td>G</td><td>550 kV GCB</td></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub> mixtures</td><td>NG</td><td>G</td><td>E</td><td>NG</td><td>NG</td><td>...</td></tr><tr><td>C<sub>3</sub>F<sub>8</sub> mixtures</td><td>NG</td><td>G</td><td>E</td><td>NG</td><td>...</td><td>...</td></tr><tr><td>C<sub>2</sub>F<sub>6</sub> mixtures</td><td>NG</td><td>G</td><td>NG</td><td>NG</td><td>...</td><td>...</td></tr><tr><td>CF<sub>4</sub></td><td>NG</td><td>G</td><td>NG</td><td>NG</td><td>...</td><td>...</td></tr><tr><td>CF<sub>3</sub>I mixtures</td><td>NG</td><td>G</td><td>E</td><td>NG</td><td>NG</td><td>...</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N + CO<sub>2</sub></td><td>NG</td><td>G</td><td>E</td><td>E</td><td>NG in MV equipment than SF<sub>6</sub></td><td>420 kV GIL/245 kV CT</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O + air</td><td>NG</td><td>G</td><td>E</td><td>E</td><td>NG in MV equipment than SF<sub>6</sub></td><td>MV GIS(6-40.5 kV)</td></tr><tr><td>C<sub>6</sub>F<sub>12</sub>O mixtures</td><td>NG</td><td>G</td><td>E</td><td>...</td><td>G</td><td>C-GIS(12 kV)</td></tr><tr><td>HFO1234zeE</td><td>G</td><td>G</td><td>E</td><td>NG</td><td>G</td><td>C-GIS (24 kV)</td></tr></tbody></table>
<table><caption>TABLE V. Basic properties of natural gases and mixtures.</caption><thead><tr><th>Gases</th><th>Breakdown voltage/partial discharge voltages</th><th>Voltage/electrode</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="3">N<sub>2</sub></td><td>The breakdown voltage can be increased by 1.5 times at 0.2 MPa by composite solid insulation.</td><td>AC/rod-plane</td><td>0.1–0.6</td><td>82</td></tr><tr><td>The breakdown voltage is lower for positive polarity waveforms in the presence of a bias voltage.</td><td>Non-standard-LI rod-plane</td><td>0.7</td><td>87</td></tr><tr><td>The breakdown voltage of N<sub>2</sub> at 1.0 MPa can be equal to that of 0.5 MPa SF<sub>6</sub>.</td><td>LI/spherical-plane</td><td>0.6–2.0</td><td>94</td></tr><tr><td rowspan="2">CO<sub>2</sub></td><td>The breakdown values of CO<sub>2</sub> are more stable than those of N<sub>2</sub>.</td><td>AC and LI plane-plane</td><td>0.7–1.1</td><td>84</td></tr><tr><td>The breakdown voltage is lower for positive polarity waveforms in the presence of a bias voltage.</td><td>Non-standard-LI/ rod-plane</td><td>0.7</td><td>86</td></tr><tr><td rowspan="2">Air</td><td>The dielectric strength of air is higher than that of N<sub>2</sub> and CO<sub>2</sub> at 0.5 MPa based on the initial flashover voltage and the 50% flashover voltage.</td><td>LI/coaxial-cylindrical and parallel plate</td><td>0.1–0.5</td><td>91</td></tr><tr><td>The partial discharge voltages of air and N<sub>2</sub> are almost the same, but the breakdown voltage of air is larger than that of pure N<sub>2</sub>.</td><td>LI/rod-plane</td><td>0.1–0.4</td><td>93</td></tr><tr><td>N<sub>2</sub>O/N<sub>2</sub></td><td>The dielectric strength for 20/80%N<sub>2</sub>O/N<sub>2</sub> at 0.6 MPa showed a synergy effect.</td><td>DC/needle-plane</td><td>0.1–1.0</td><td>88</td></tr><tr><td>N<sub>2</sub>–He</td><td>The breakdown voltage of N<sub>2</sub>–He rises rapidly and then follows a linear behavior with the mixing ratio of N<sub>2</sub>.</td><td>AC/plate-plate</td><td>&lt;0.6</td><td>89</td></tr><tr><td>N<sub>2</sub>/CO<sub>2</sub></td><td>The breakdown voltage of the 50%N<sub>2</sub>/50%CO<sub>2</sub> mixture is only one-third that of SF<sub>6</sub>.</td><td>LI/sphere-plane</td><td>01–30</td><td>83</td></tr><tr><th>Gases</th><th>Electron swarm parameters</th><th>Method</th><th>E/N (Td) T (K)</th><th>P (MPa)</th><th>Reference</th></tr><tr><td rowspan="5">Mixtures</td><td>Small additions of CO<sub>2</sub> (5%) produced a significant attachment in the air-CO<sub>2</sub> mixtures.</td><td>Measure</td><td>90–260 ...</td><td>10–100 Torr</td><td>100</td></tr><tr><td>In the mixtures of CO<sub>2</sub> with both 50% O<sub>2</sub> and 50% H<sub>2</sub>, (E/N)<sub>cr</sub> clearly shows higher values than in pure CO<sub>2</sub>.</td><td>Calculation</td><td>0–200 300–3500</td><td>0.1 MPa</td><td>96</td></tr><tr><td>Adding a small amount of O<sub>2</sub> to CO<sub>2</sub> can effectively improve the value of (E/N)<sub>cr</sub> with a clear synergistic effect.</td><td>Calculation</td><td>0–200 ...</td><td>0.1 MPa</td><td>97</td></tr><tr><td>The reduced critical breakdown field of CO<sub>2</sub> with O<sub>2</sub> and CH<sub>4</sub> show higher values.</td><td>Calculation</td><td>0–150 300–4000</td><td>0.1 MPa</td><td>98</td></tr><tr><td>The values of (α – η)/N for CO<sub>2</sub>–N<sub>2</sub> mixtures are about 30 times smaller than those for CO<sub>2</sub>–O<sub>2</sub> mixtures.</td><td>Measure/ calculation</td><td>0.01–1000 293–298</td><td>0.6–600 Torr</td><td>99</td></tr><tr><th>Gases</th><th>Interruption ability</th><th>Method</th><th>P (MPa)</th><th>Reference</th></tr><tr><td>Air/CO<sub>2</sub></td><td>For a fixed pressure, the gases were ranked in terms of arc interruption performance as follows, in increasing order: air, CO<sub>2</sub>, and SF<sub>6</sub>.</td><td>Measure/calculation 300–4000 K</td><td>0.5</td><td>103</td></tr><tr><td>Air</td><td>A low nozzle-to-contact diameter ratio is favorable in terms of requiring less upstream overpressure for successful interruption of air.</td><td>Measure/calculation Load currents: 300–900 A</td><td>0.05–1.4 bar (upstream)</td><td>107</td></tr><tr><td>CO<sub>2</sub></td><td>The thermal interruption capability of CO<sub>2</sub> could be estimated to be about 50% of that of SF<sub>6</sub>.</td><td>Measure 84 kV GCB</td><td>...</td><td>108</td></tr></tbody></table>
<table><caption>TABLE VI. Basic properties of SF₆ mixtures.</caption><thead><tr><th>Gases</th><th>Breakdown voltage/partial discharge voltages</th><th>Voltage/electrode</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="10">SF<sub>6</sub> + N<sub>2</sub></td><td>SF<sub>6</sub>-N<sub>2</sub> mixtures containing 50%-60% of SF<sub>6</sub> have a dielectric strength of up to 85%-90% that of pure SF<sub>6</sub>.</td><td>AC/DC/LI<br>Plate-plate/rod-sphere</td><td>0.1-0.5</td><td>113</td></tr><tr><td>A simplified streamer criterion proposed for SF<sub>6</sub> can be used to calculate the discharge inception levels in SF<sub>6</sub>-N<sub>2</sub> mixtures.</td><td>DC<br>Sphere-plane/<br>sphere-sphere</td><td>0.1-0.5</td><td>114</td></tr><tr><td>The addition of 0.1% of SF<sub>6</sub> into N<sub>2</sub> results in an increase in the breakdown voltage at pressures below 2.5 bars.</td><td>PI Rod-plane</td><td>0.1-0.5</td><td>115</td></tr><tr><td>The dielectric strength of the 50% SF<sub>6</sub>/N<sub>2</sub> mixture is about 8-12% higher than that of the 20% SF<sub>6</sub> gas mixture.</td><td>LI and AC<br>Sphere-plane/rod-plane</td><td>0.3-0.7</td><td>118</td></tr><tr><td>The increase in corona onset voltages with the increase in the amounts of SF<sub>6</sub>. The values of positive polarity are higher to those obtained with negative polarity.</td><td>DC Tip-plane</td><td>0.3-1.5</td><td>119</td></tr><tr><td>An increase in the GIS tank diameter of about 15% with the 10% SF<sub>6</sub>-N<sub>2</sub> mixture at 0.6 MPa compared to pure SF<sub>6</sub> at 0.4 MPa.</td><td>LI/SI/AC<br>Coaxial cylinder</td><td>0.2-0.6</td><td>120</td></tr><tr><td>N<sub>2</sub>-SF<sub>6</sub> mixtures with a SF<sub>6</sub> content of 1%-20% and pressure of 0.71 MPa-1.24 MPa are more sensitive to fixed protrusions than pure SF<sub>6</sub> at 0.5 MPa of the same dielectric strength.</td><td>AC/LI/SI<br>Needle-plane</td><td>0.5-1.24</td><td>144</td></tr><tr><td>The negative synergistic effect of the SF<sub>6</sub>/N<sub>2</sub> mixture appeared with low gas pressure.</td><td>LI Rod-plane</td><td>0.2-0.5</td><td>122</td></tr><tr><td rowspan="2">SF<sub>6</sub> + CO<sub>2</sub></td><td>With a single large protrusion on the electrode, the breakdown strength of the 50%SF<sub>6</sub>-CO<sub>2</sub> mixture is slightly higher than that of SF<sub>6</sub>.</td><td>AC Cylindrical</td><td>0.1-0.4</td><td>132</td></tr><tr><td>With adding CO<sub>2</sub> to the N<sub>2</sub>/SF<sub>6</sub> gas mixture, the breakdown values rise significantly, particularly at high pressures.</td><td>AC/LI<br>Needle-plane</td><td>0.1-0.6</td><td>133</td></tr><tr><td rowspan="2">SF<sub>6</sub> + Air</td><td>The rate of rise impulse wave voltage has a significant effect on the breakdown voltage of 1% SF<sub>6</sub>/air mixture.</td><td>LI/SI Coaxial<br>cylinder/rod-sphere</td><td>0.1-0.5</td><td>129</td></tr><tr><td>SF<sub>6</sub>-air mixtures have corona stabilization and is higher than the corresponding values for SF<sub>6</sub>-CO<sub>2</sub> and SF<sub>6</sub>-N<sub>2</sub> mixtures.</td><td>DC Rod-plane</td><td>0.1-0.5</td><td>128</td></tr><tr><td rowspan="4">SF<sub>6</sub> + CF<sub>4</sub></td><td>The PD inception voltages have hardly any effect on mixed rate of SF<sub>6</sub> in 25.8 kV GIS</td><td>AC/LI<br>Sphere-sphere</td><td>0.05-0.3</td><td>138</td></tr><tr><td>SF<sub>6</sub>-CF<sub>4</sub> mixtures under uniform fields have breakdown voltages that vary linearly with the SF<sub>6</sub> content. Under highly non-uniform fields, a corona stabilization may occur under positive impulse, positive DC and AC voltages leading to high breakdowns for the mixtures 10% SF<sub>6</sub>-90% CF<sub>4</sub> to 25%SF<sub>6</sub>-75%CF<sub>4</sub>.</td><td>AC/LI/DC<br>Sphere-sphere</td><td>0.2-0.4</td><td>140</td></tr><tr><td>The AC corona inception occurred at lower voltage levels than DC. The average time to breakdown corresponding to negative lighting impulse breakdown voltages are generally shorter than those of positive</td><td>AC/DC/LI<br>Sphere-sphere</td><td>0.3-1.0</td><td>142</td></tr><tr><td>The addition of 0.002% SF<sub>6</sub> to H<sub>2</sub> resulted in a substantial increase in AC breakdown voltage of typically 70%.</td><td>AC Rod-plane/<br>rod-rod</td><td>0.1-0.4</td><td>135</td></tr><tr><td>SF<sub>6</sub> + H<sub>2</sub></td><td>The increase in negative breakdown voltage with the addition of small quantities (&lt;1%) of SF<sub>6</sub> is smaller than for positive-impulse conditions</td><td>LI Rod-plane</td><td>0.05-0.3</td><td>136</td></tr></tbody></table>
<table><caption>TABLE VI. (Continued.)</caption><thead><tr><th>Gases</th><th>Electron swarm parameters</th><th>Method</th><th>E/N (Td)<br>T (k)</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td>SF<sub>6</sub> + N<sub>2</sub><br>SF<sub>6</sub> + N<sub>2</sub>O</td><td>The breakdown is governed by the mean energy of SF<sub>6</sub> for low concentrations of SF<sub>6</sub> up to 10%, and only for still lower concentrations, the mean energy of electrons corresponds to that of N<sub>2</sub>.</td><td>Calculation</td><td>700 273</td><td>...</td><td>148</td></tr><tr><td>SF<sub>6</sub> + N<sub>2</sub></td><td>The variation of the effective ionization coefficient is non-linear with the mixture ratio.</td><td>Measure SST</td><td>182–258<br>293</td><td>0–100 Torr</td><td>150</td></tr><tr><td>SF<sub>6</sub> + N<sub>2</sub></td><td>The variation of the effective ionization coefficient does not vary linearly with the ratio of the two gases in the SF<sub>6</sub>–N<sub>2</sub> mixture. With an increasing SF<sub>6</sub> content in the mixture, a reduction in the apparent secondary ionization coefficient is observed.</td><td>Calculation</td><td>100–450 V cm<sup>-1</sup><br>Torr<sup>-1</sup></td><td>5–40 Torr</td><td>151</td></tr><tr><td rowspan="2">SF<sub>6</sub> + CO<sub>2</sub></td><td>The rate of increase in the attachment coefficient with an increase in the percentage of SF<sub>6</sub> in the SF<sub>6</sub>–CO<sub>2</sub> mixtures was much larger than the rate of change in the first ionization coefficient.</td><td>Measure (SST)<br>sphere-sphere/<br>rod-rod</td><td>...</td><td>0.4</td><td>152</td></tr><tr><td>CO<sub>2</sub> in SF<sub>6</sub>–CO<sub>2</sub> mixtures can change the transport properties, but the effect of detachment is relatively slight on the properties of electrical discharge.</td><td>Calculation (PT)<br>parallel plate</td><td>270–370</td><td>...</td><td>155</td></tr><tr><td rowspan="2">SF<sub>6</sub> + CF<sub>4</sub></td><td>The influence of SF<sub>6</sub> in the SF<sub>6</sub>–CO<sub>2</sub> mixture is strongly apparent in the values for the effective ionization coefficients. The value of critical field strength E/N<sub>cr</sub> is smaller than that measured for the SF<sub>6</sub>–N<sub>2</sub> mixtures.</td><td>Measure (PT)</td><td>100–700<br>293–302</td><td>0–0.001</td><td>156</td></tr><tr><td>At temperatures above 1750 K, an addition of CO<sub>2</sub> to SF<sub>6</sub> gas can enhance dielectric breakdown performances, while at low temperatures, too much CO<sub>2</sub> added into mixtures can reduce dielectric breakdown abilities.</td><td>Calculation</td><td>...</td><td>0.01–1.0</td><td>162</td></tr><tr><td rowspan="3">SF<sub>6</sub> + He</td><td>After addition CF<sub>4</sub> to SF<sub>6</sub>, the electron drift velocity in SF<sub>6</sub> and CF<sub>4</sub> increases, effective ionization coefficient decreases with the increase in the SF<sub>6</sub> content, and diffusion of CF<sub>4</sub> is stronger than that of SF<sub>6</sub>.</td><td>Calculation<br>(Monte Carlo)</td><td>150–500<br>293 K</td><td>1 Torr</td><td>158</td></tr><tr><td>For higher gas temperatures (i.e., T > 2200 K at 0.4 MPa), the (E/N)cr in SF<sub>6</sub>–CF<sub>4</sub> mixtures are obviously higher than that in pure SF<sub>6</sub>.</td><td>Calculation<br>(Boltzmann)</td><td>...</td><td>0.01–1.6</td><td>162</td></tr><tr><td>The mixture of SF<sub>6</sub> and CF<sub>4</sub> exhibits a suppressed CF<sub>4</sub> ionization and an increased SF<sub>6</sub> ionization in the mixture relative to pure CF<sub>4</sub> and SF<sub>6</sub>.</td><td>Calculation</td><td>140–600<br>293 K</td><td>1 Torr</td><td>159</td></tr><tr><td>SF<sub>6</sub> + He</td><td>The addition of He to the gaseous mixture decreases the reduced critical electric field.</td><td>Calculation<br>(Boltzmann)</td><td>...</td><td>0.1–1.6</td><td>171</td></tr><tr><th>Gases</th><th>Interruption ability</th><th>Method</th><th>P (MPa)</th><th>Reference</th></tr><tr><td>SF<sub>6</sub> + N<sub>2</sub></td><td>Under two-temperature assumption, the electron conductivity of the SF<sub>6</sub>–N<sub>2</sub> mixture is distinctly lower than that of pure N<sub>2</sub>.</td><td>Calculation LTE/NLTE<br>300–40 000 K</td><td>0.1</td><td>173</td></tr><tr><td></td><td>Mixing CO<sub>2</sub> with SF<sub>6</sub> at a certain ratio, compared to the case of pure CO<sub>2</sub>, the largely preserved thermal cooling effect of SF<sub>6</sub> can favor the thermal recovery of circuit breakers during current interruption.</td><td>Calculation in LTE<br>300–30 000 K</td><td>0.01 and 1.6</td><td>170</td></tr></tbody></table>
<table><caption>TABLE VI. (Continued.)</caption><thead><tr><th>Gases</th><th>Interruption ability</th><th>Method</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td></td><td>The arc interruption capabilities of the SF<sub>6</sub>-CO<sub>2</sub> mixture increase greatly with an increase in SF<sub>6</sub> concentration and are about 40%, 45%, and 70% that of SF<sub>6</sub> at a SF<sub>6</sub> content of 0%, 20%, and 50% in the applied puffer GCB test prototype and at a short-circuit current of 10 kA.</td><td>Measure short-circuit current: 10 kA</td><td>...</td><td>174</td></tr><tr><td rowspan="2">SF<sub>6</sub> + CF<sub>4</sub></td><td>The net radiation of a SF<sub>6</sub>-CF<sub>4</sub> or SF<sub>6</sub>-C<sub>2</sub>F<sub>6</sub> mixture is always greater than that of pure SF<sub>6</sub>. The SF<sub>6</sub>/CF<sub>4</sub> mixture has interruption properties very close to those of pure SF<sub>6</sub> as long as the proportion of SF<sub>6</sub> is about 50%.</td><td>Calculation in LTE 300-30 000 K</td><td>0.1</td><td>168,169</td></tr><tr><td>An SF<sub>6</sub>/CF<sub>4</sub> filled circuit breaker often retains its breaking capacity down to the lowest ambient temperatures.</td><td>Measure TFT/SFT</td><td>0.5,0.7</td><td>172</td></tr><tr><td>SF<sub>6</sub> + Ar</td><td>The diameter of the Ar/SF<sub>6</sub> (20%) arc was smaller than that of the Ar arc. Before interrupting the arc current, the electron density of the Ar/SF<sub>6</sub> (20%) arc was larger than that of Ar. After the current interruption, however, the electron density of the Ar/SF<sub>6</sub> (20%) arc decreased rapidly.</td><td>Measure (LTS)</td><td>0.1</td><td>166</td></tr><tr><th>Gases</th><th>Decomposition products</th><th>Method</th><th>P (MPa)</th><th>Reference</th></tr><tr><td>SF<sub>6</sub> + CF<sub>4</sub></td><td>The gaseous by-products of the spark decomposition of mixtures included SOF<sub>2</sub> + SF<sub>4</sub>, SOF<sub>4</sub>, SO<sub>2</sub>F<sub>2</sub>, S<sub>2</sub>F<sub>10</sub>, S<sub>2</sub>OF<sub>10</sub>, S<sub>2</sub>O<sub>2</sub>F<sub>10</sub>, and S<sub>2</sub>O<sub>3</sub>F<sub>6</sub> with O<sub>2</sub> and H<sub>2</sub>O.</td><td>AC sparks discharge Point-insulator-plane</td><td>0.1</td><td>176</td></tr><tr><td>SF<sub>6</sub> + N<sub>2</sub></td><td>The decomposition products SOF<sub>4</sub>, SO<sub>2</sub>F<sub>2</sub>, SF<sub>4</sub> + SOF<sub>2</sub>, SO<sub>2</sub>, S<sub>2</sub>OF<sub>10</sub>, S<sub>2</sub>F<sub>10</sub>, S<sub>2</sub>O<sub>2</sub>F<sub>10</sub>, S<sub>2</sub>O<sub>3</sub>F<sub>6</sub>, SF<sub>5</sub>NF<sub>2</sub>, NF<sub>3</sub>, and (SF<sub>5</sub>)<sub>2</sub>NF were assayed.</td><td>DC/AC corona discharges Point-plane</td><td>0.4</td><td>179</td></tr><tr><td>SF<sub>6</sub> + O<sub>2</sub></td><td>The yields of SOF<sub>4</sub>, SO<sub>2</sub>F<sub>2</sub>, SOF<sub>2</sub>, and SO<sub>2</sub> have been measured as a function of O<sub>2</sub> content in SF<sub>6</sub>/O<sub>2</sub> mixtures.</td><td>Spark discharges Cylindrical</td><td>0.133</td><td>178</td></tr><tr><td>SF<sub>6</sub> + CO<sub>2</sub></td><td>The presence of CO<sub>2</sub> in SF<sub>6</sub> and SF<sub>6</sub>-N<sub>2</sub> mixtures leads to a considerable increase in the formation of (SOF<sub>4</sub> + SO<sub>2</sub>F<sub>2</sub>) and decreased formation of S<sub>2</sub>F<sub>10</sub>.</td><td>Corona discharges Point-plane</td><td>0.4</td><td>182</td></tr></tbody></table>

biggest drawback, which also limits its use in equipment. The solid iodine and carbon deposited on the surface of electrodes, which is detrimental to the gas self-healing, so the gas is not suitable for use in the interrupter medium. Its genotoxicity is also a major obstacle.

(5) C<sub>3</sub>F<sub>7</sub>N has a great insulating property so that the mixed gas (at a very low mixing ratio) can still reach the insulation level of SF<sub>6</sub> for high voltage equipment. Mixed gases are currently used in GIS and circuit breakers. It is noteworthy that the decomposition of gas discharge may produce some of the toxic gases or solid products, which requires further study and verification.

(6) Perfluoroketones, mainly C<sub>5</sub>F<sub>10</sub>O, have similar insulation properties of C<sub>4</sub>F<sub>7</sub>N. However, using mixed gas is more suitable for medium voltage equipment due to the higher boiling point than C<sub>4</sub>F<sub>7</sub>N and the too high value of GWP of C<sub>4</sub>F<sub>7</sub>N. Air is used in the mixture to prevent oxidation inside the equipment. The decomposition product after

discharge will not affect the environment and the human body.

(7) For other gases such as HFO1234zeE, some research studies have not reached a mature conclusion, so their potential for substitution remains to be further studied.

## B. Outlook

Through the above research on the current alternative gases, the research on alternative gases mainly focuses on the insulation performance, the arc-extinguishing performance, and the decomposition characteristics.

(1) For insulation properties, the experimental parameters measured are relatively easy to obtain. The gas insulation can be evaluated by comparing with the SF<sub>6</sub>. Setting different electrodes, voltage type, pressure, and mixing ratio has become common experimental means to comprehensively investigate the insulation properties of gas. The inception voltage
<table><caption>TABLE VII. Main properties of PFCs and mixtures.</caption><thead><tr><th>Gases</th><th>Breakdown voltage/partial discharge voltage comment</th><th>Voltage/electrode</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td>c-C<sub>4</sub>F<sub>8</sub>/1-C<sub>3</sub>F<sub>6</sub>/<br>C<sub>3</sub>F<sub>8</sub>/C<sub>2</sub>F<sub>6</sub> + N<sub>2</sub>/<br>CO<sub>2</sub>/CF<sub>4</sub></td><td>Positive synergism was observed for gas mixtures with<br>a PFC gas (c-C<sub>4</sub>F<sub>8</sub>, 1-C<sub>3</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>8</sub> and C<sub>2</sub>F<sub>6</sub>) and<br>any of N<sub>2</sub>, CO<sub>2</sub>, and CF<sub>4</sub> gases.</td><td>AC Rod-plane</td><td>0.10-0.40</td><td>180</td></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub>/N<sub>2</sub></td><td>The sparkover voltage of c-C<sub>4</sub>F<sub>8</sub> is about 1.3 times<br>and 1.3-1.4 times higher than SF<sub>6</sub>.</td><td>AC/LI<br>Parallel-plane</td><td>0.1</td><td>186</td></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub>/N<sub>2</sub></td><td>The breakdown voltage of the c-C<sub>4</sub>F<sub>8</sub>/N<sub>2</sub> mixture under<br>negative lighting impulse is lower than that under positive<br>polarity. c-C<sub>4</sub>F<sub>8</sub>/N<sub>2</sub> mixtures show the synergistic effect.</td><td>LI Sphere-plane</td><td>0.1-0.3</td><td>188</td></tr><tr><td>C<sub>3</sub>F<sub>8</sub>/N<sub>2</sub><br>C<sub>2</sub>F<sub>6</sub>/N<sub>2</sub></td><td>In C<sub>3</sub>F<sub>8</sub>/N<sub>2</sub> and C<sub>2</sub>F<sub>6</sub>/N<sub>2</sub> mixtures, the synergistic effect<br>in electrical insulation performance was less remarkable<br>than that in SF<sub>6</sub>/N<sub>2</sub> mixtures.</td><td>AC Needle-plane</td><td>0.1</td><td>190</td></tr><tr><td>C<sub>3</sub>F<sub>8</sub>+ air</td><td>Both breakdown ratios of C<sub>3</sub>F<sub>8</sub> to air approach a minimum<br>of about 2.5 at the sphere-plane electrode system.</td><td>LI Sphere-plane/<br>point-plane</td><td>0.1</td><td>189</td></tr><tr><td>CF<sub>4</sub></td><td>CF<sub>4</sub> has a smaller critical field strength<br>and is less attaching than SF<sub>6</sub>.<br>At large values, pL > 2 Torr cm, the rate of growth<br>of breakdown voltage with pressure essentially<br>increases due to the attachment.</td><td>AC/DC/LI<br>Point-plane</td><td>0.1-0.4</td><td>199</td></tr><tr><td></td><td></td><td>DC Plane-plane</td><td>0.1-10 Torr</td><td>202</td></tr><tr><th>Gases</th><th>Electron swarm parameters</th><th>Method</th><th>E/N (Td)<br>T (K)</th><th>P (MPa)</th><th>Reference</th></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub></td><td>A critical field strength E/Ncr of c-C<sub>4</sub>F<sub>8</sub> is 439.5 td.</td><td>Measure (PT)<br>12 Td-43 Td</td><td>330-600<br>292-300 K</td><td>1-7.5Torr</td><td>191</td></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub> + SF<sub>6</sub></td><td>The limiting E/N of 20% SF<sub>6</sub> and 80% c-C<sub>4</sub>F<sub>8</sub> can<br>exceed the values for the respective pure gases.</td><td>Calculation</td><td>282-707<br>273 K</td><td>1 Torr</td><td>192</td></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub> + CO<sub>2</sub></td><td>The limiting field (E/N)<sub>lim</sub> of the c-C<sub>4</sub>F<sub>8</sub>/CO<sub>2</sub> mixtures<br>is higher than that of SF<sub>6</sub>/CO<sub>2</sub> mixtures.</td><td>Calculation<br>(Monte Carlo)</td><td>150-450<br>293 K</td><td>1 Torr</td><td>194</td></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub> + N<sub>2</sub><br>c-C<sub>4</sub>F<sub>8</sub> + air</td><td>c-C<sub>4</sub>F<sub>8</sub>-N<sub>2</sub> and c-C<sub>4</sub>F<sub>8</sub>-air have very similar (E/N)cr<br>values, higher than those of the other three mixtures<br>(CO<sub>2</sub>, CF<sub>4</sub>, and O<sub>2</sub>), and superior even to that of<br>pure SF<sub>6</sub> for c-C<sub>4</sub>F<sub>8</sub> concentrations above 80%.</td><td>Calculation</td><td>150-600<br>300 K</td><td>0.1</td><td>195</td></tr><tr><td>C<sub>3</sub>F<sub>8</sub> + N<sub>2</sub></td><td>The limiting field strength of the C<sub>3</sub>F<sub>8</sub>-N<sub>2</sub> gas mixture is<br>greater than that of C<sub>3</sub>F<sub>8</sub>-CO<sub>2</sub>.</td><td>Calculation</td><td>1-1000 293</td><td>1 Torr</td><td>196</td></tr><tr><td>CF<sub>4</sub></td><td>Replacing SF<sub>6</sub> with CF<sub>4</sub> brings an increase in the critical<br>reduced electric field strength for temperatures above 2200 K.</td><td>Calculation</td><td>0-1000 T<br>300-350</td><td>0.01-1.6</td><td>201</td></tr><tr><td>C<sub>3</sub>F<sub>8</sub> + N<sub>2</sub></td><td>The C<sub>3</sub>F<sub>8</sub>-N<sub>2</sub> mixture has the largest (E/N)cr of the cold C<sub>3</sub>F<sub>8</sub>-CF<sub>4</sub>,<br>C<sub>3</sub>F<sub>8</sub>-CO<sub>2</sub>, C<sub>3</sub>F<sub>8</sub>-N<sub>2</sub>, C<sub>3</sub>F<sub>8</sub>-O<sub>2</sub>, and C<sub>3</sub>F<sub>8</sub>-Ar. The hot C<sub>3</sub>F<sub>8</sub> gas<br>gas has much poorer dielectric performance than hot SF<sub>6</sub> because the<br>(E/N)cr of C<sub>3</sub>F<sub>8</sub> decreases significantly above room temperature.</td><td>Calculation</td><td>100-600<br>300-3500</td><td>0.01-1.6</td><td>200</td></tr><tr><th>Gases</th><th>Decomposition</th><th>Method</th><th>P (MPa)</th><th>Reference</th></tr><tr><td>c-C<sub>4</sub>F<sub>8</sub><br>c-C<sub>4</sub>F<sub>8</sub> + N<sub>2</sub></td><td>The main identified gaseous by-products are CF<sub>4</sub>, C<sub>2</sub>F<sub>6</sub>, C<sub>2</sub>F<sub>4</sub>,<br>C<sub>3</sub>F<sub>8</sub>, and C<sub>3</sub>F<sub>6</sub> for pure c-C<sub>4</sub>F<sub>8</sub> gas and CF<sub>4</sub>, C<sub>2</sub>F<sub>6</sub>, C<sub>2</sub>F<sub>4</sub>,<br>C<sub>3</sub>F<sub>8</sub>, C<sub>3</sub>F<sub>6</sub>, and C<sub>2</sub>F<sub>3</sub>N for the c-C<sub>4</sub>F<sub>8</sub>/N<sub>2</sub> mixture.</td><td>50 Hz ac corona discharge<br>Needle-plane</td><td>0.1</td><td>205</td></tr></tbody></table>
<table><caption>TABLE VIII. Main properties of C<sub>n</sub>F<sub>m</sub>X mixtures. DBD-Dielectric Barrier Discharge. 1 Torr = 133.3Pa.</caption><thead><tr><th>Gases</th><th>Breakdown voltage/partial discharge voltages</th><th>Voltage/electrode</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="4">CF<sub>3</sub>I/N<sub>2</sub></td><td>The optimum ratio of CF<sub>3</sub>I/N<sub>2</sub> to replace SF<sub>6</sub> gas as an insulating medium is 30% at 0.3 MPa based on the breakdown characteristics.</td><td>AC Needle-plate</td><td>0.1–0.3</td><td>208</td></tr><tr><td>Under 0.1 MPa and 0.2 MPa, 30%CF<sub>3</sub>I/70%N<sub>2</sub> present similar insulation properties as 20%SF<sub>6</sub>/80%N<sub>2</sub>.</td><td>DC/LI<br>Plane-plane</td><td>0.1 and 0.2</td><td>213</td></tr><tr><td>Based on the 50% breakdown voltage of the CF<sub>3</sub>I/N<sub>2</sub> gas mixture. The CF<sub>3</sub>I mixture has the potential to replace SF<sub>6</sub> as an insulation medium in GIL.</td><td>LI Coaxial<br>cylindrical</td><td>0.1–0.3</td><td>215</td></tr><tr><td>The breakdown voltage of CF<sub>3</sub>I–N<sub>2</sub> mixtures increases proportionally to the CF<sub>3</sub>I content without the synergistic effect. The breakdown voltages for mixtures of 50% CF<sub>3</sub>I are lower than with SF<sub>6</sub> at the same ratio.</td><td>DC Sphere-sphere</td><td>0.1–0.6</td><td>230</td></tr><tr><td rowspan="3">CF<sub>3</sub>I/CO<sub>2</sub></td><td>When the RMU vacuum circuit breakers are insulated with a 30%:70% CF<sub>3</sub>I–CO<sub>2</sub> mixture, no disruptive discharges are detected for a 50 impulse test series.</td><td>LI RMU</td><td>0.14</td><td>214</td></tr><tr><td>The 50% breakdown strength of the 30:70% mixture of CF<sub>3</sub>I–CO<sub>2</sub> for the coaxial electrode was more than two times higher than air.</td><td>LI Rod-plane/<br>plane-plane/<br>coaxial cylindrical</td><td>0.1</td><td>212</td></tr><tr><td>The PDIV+ of CF<sub>3</sub>I/CO<sub>2</sub> is higher than that of SF<sub>6</sub>/CO<sub>2</sub> when k is 10%–40%. The PDIV+ of CF<sub>3</sub>I/CO<sub>2</sub> (30%–40%) at 0.15–0.2 MPa can reach the level of pure SF<sub>6</sub> at 0.1 MPa.</td><td>AC Needle-plate</td><td>0.1–0.3</td><td>209</td></tr><tr><td>CF<sub>3</sub>I–air</td><td>Pure CF<sub>3</sub>I gas has 1.2 times as high a dielectric strength as pure SF<sub>6</sub> gas at the same pressure. V–t characteristics in CF<sub>3</sub>I–air gas mixtures are almost the same as those in CF<sub>3</sub>I–N<sub>2</sub> gas mixtures.</td><td>Steep-front<br>square voltage<br>Rod-plane</td><td>0.1</td><td>210</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N + CO<sub>2</sub></td><td>The content of C<sub>4</sub>F<sub>7</sub>N was 18%–20%, and it can achieve the same dielectric strength as pure SF<sub>6</sub>.</td><td>AC inhomogeneous<br>field</td><td>0.15, 0.19</td><td>47</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N + CO<sub>2</sub></td><td>The synergistic effect of C<sub>4</sub>F<sub>7</sub>N and CO<sub>2</sub> at a concentration of 3.7% is obvious, and the mixture in uniform electric field reaches 72% of the dielectric breakdown of pure SF<sub>6</sub> at 0.55 MPa.</td><td>AC/LI plane-plane/<br>sphere-plane/rod-plane/<br>sphere-sphere</td><td>0.1–1.04</td><td>240</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N +<br>CO<sub>2</sub>/n<sub>2</sub>/air</td><td>The dielectric breakdown strength of the gas mixture increases at higher pressures and increased C<sub>4</sub>F<sub>7</sub>N concentration.</td><td>Disk electrodes</td><td>0.1–1.1</td><td>241</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O + air</td><td>The air PFK-max mixture at 0.7 MPa has about 95% the withstand capability of 0.45 MPa SF<sub>6</sub>.</td><td>Step-DC<br>Sphere-sphere</td><td>0.1–0.7</td><td>250</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O + air<br>C<sub>6</sub>F<sub>12</sub>O + air</td><td>The breakdown performance of the investigated mixtures can approach SF<sub>6</sub> values at increased total filling pressures.</td><td>AC/LI<br>Sphere-sphere</td><td>0.1–0.7</td><td>249</td></tr><tr><th>Gases</th><th>Electron swarm parameters</th><th>Method</th><th>E/N (Td)<br>T (K)</th><th>P (MPa)</th><th>Reference</th></tr><tr><td rowspan="2">CF<sub>3</sub>I + N<sub>2</sub></td><td rowspan="2">The limiting field strength of CF<sub>3</sub>I was found to be E/Nlim = 437 td. For the CF<sub>3</sub>I–N<sub>2</sub> mixture with 70% CF<sub>3</sub>I, this E/Nlim value was found to be essentially the same as that for pure SF<sub>6</sub>.</td><td>Measure</td><td>100–850</td><td rowspan="2">0.4–20 Torr</td><td rowspan="2">218</td></tr><tr><td>PT</td><td>293–301</td></tr><tr><td>CF<sub>3</sub>I + CF<sub>4</sub>/<br>N<sub>2</sub>/CO<sub>2</sub>/<br>O<sub>2</sub>/air</td><td>The (E/N)cr of CF<sub>3</sub>I–N<sub>2</sub> and CF<sub>3</sub>I–air are obviously higher than that of SF<sub>6</sub>–N<sub>2</sub> at the CF<sub>3</sub>I ratio higher than 65% and even higher than 65% and even higher than that of pure SF<sub>6</sub> at the CF<sub>3</sub>I ratio higher than 70%.</td><td>Calculation</td><td>0–600<br>300</td><td>...</td><td>221</td></tr><tr><td>CF<sub>3</sub>I + N<sub>2</sub>/<br>CO<sub>2</sub></td><td>For the mixtures with 70% CF<sub>3</sub>I, the values of (E/N)lim are essentially the same as that for pure SF<sub>6</sub>.</td><td>Calculation<br>SST</td><td>100–1000<br>293</td><td>1 Torr</td><td>222</td></tr></tbody></table>
<table><caption>TABLE VIII. (Continued.)</caption><thead><tr><th rowspan="2">Gases</th><th>Breakdown voltage/partial discharge voltages</th><th colspan="4">Voltage/electrode</th></tr><tr><th>Electron swarm parameters</th><th>Method</th><th>E/N (Td)<br>T (K)</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td>CF<sub>3</sub>I + Ar/<br>Xe/He/<br>N<sub>2</sub>/CO<sub>2</sub></td><td>Among the mixtures of CF<sub>3</sub>I with Ar, Xe, He, N<sub>2</sub>, and CO<sub>2</sub>, CF<sub>3</sub>I-N<sub>2</sub> present the greatest insulation strength with 20%-90% CF<sub>3</sub>I content based on (E/N)cr.</td><td>Calculation<br>SST</td><td>100-800<br>293</td><td>1 Torr</td><td>223</td></tr><tr><td></td><td>Interruption ability</td><td>Method</td><td></td><td>P (MPa)</td><td>Reference</td></tr><tr><td>CF<sub>3</sub>I + CO<sub>2</sub></td><td>The interruption performance of the CF<sub>3</sub>I/CO<sub>2</sub> mixture approximated that of pure CF<sub>3</sub>I when the ratio of CF<sub>3</sub>I exceeds 20%.</td><td>Measure/SLF frequency<br>of oscillating<br>current: 45 kHz.</td><td></td><td>0.2<br>(upstream)</td><td>218</td></tr><tr><td></td><td>In BTF interruption, CF<sub>3</sub>I-CO<sub>2</sub> is superior to CF<sub>3</sub>I-N<sub>2</sub> and the performance approximates to that of pure CF<sub>3</sub>I when the proportion of CF<sub>3</sub>I exceeds 30%.</td><td>Measure/BTF<br>Peak of current:<br>3.0 kA.</td><td></td><td>0.2</td><td>225</td></tr><tr><td></td><td>The arc conductance attenuated more rapidly for CO<sub>2</sub> mixed with CF<sub>3</sub>I at concentrations above 0.9 than for pure CO<sub>2</sub>.</td><td>Calculation in LTE<br>300-30 000 K</td><td></td><td>0.1,0.5</td><td>228</td></tr><tr><td>CF<sub>3</sub>I + CO<sub>2</sub>/<br>N<sub>2</sub>/air</td><td>CF<sub>3</sub>I has a rather small low-temperature electrical conductivity (lower than SF<sub>6</sub>).</td><td>Calculation LTE<br>300-50 000 K</td><td></td><td>0.1-3.2</td><td>229</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N + CO<sub>2</sub></td><td>The arcing time is stable over the 100 operations, and the average arcing time is about 12 ms compared to a typical value of 15 ms for SF<sub>6</sub>.</td><td>Measure<br>420 kV disconnector</td><td></td><td>0.55</td><td>243</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O + air</td><td>The pressure peak is up to 30% higher compared to SF<sub>6</sub> and the peak for SF<sub>6</sub> is reached in 17 ms, while for the mixture, it is reached in 10-12 ms.</td><td>Measure GIS<br>short circuit<br>current: 40 kA</td><td></td><td>...</td><td>253</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O</td><td>The arc interruption performance and electric strength of C<sub>5</sub>-PFK are comparable to or better than those of SF<sub>6</sub>.</td><td>Calculation<br>300-30 000 K</td><td></td><td>0.1-1.0</td><td>254</td></tr><tr><td></td><td>Decomposition characteristics</td><td>Method</td><td></td><td>P (MPa)</td><td></td></tr><tr><td>CF<sub>3</sub>I</td><td>The partial discharge by-products of C<sub>2</sub>F<sub>6</sub>, C<sub>2</sub>F<sub>4</sub>, C<sub>2</sub>F<sub>5</sub>I, C<sub>3</sub>F<sub>8</sub>, CHF<sub>3</sub>, C<sub>3</sub>F<sub>6</sub>, and CH<sub>3</sub>I were obtained.</td><td>Experiment<br>AC/needle-plane</td><td></td><td>0.1</td><td>237</td></tr><tr><td></td><td>CF<sub>3</sub>I gas generates iodine after current interruption.</td><td>Experiment<br>SLF and BTF</td><td></td><td>0.2</td><td>231</td></tr><tr><td>CF<sub>3</sub>I-CO<sub>2</sub></td><td>Iodine is deposited on the electrode along with carbon and oxygen after CF<sub>3</sub>I-CO<sub>2</sub> gas mixtures discharge under a positive impulse polarity.</td><td>Experiment<br>LI/rod-plane</td><td></td><td>0.1-0.2</td><td>234</td></tr><tr><td>CF<sub>3</sub>I + H<sub>2</sub>O</td><td>The decomposition products of CF<sub>3</sub>I include the main components C<sub>2</sub>F<sub>6</sub>, I<sub>2</sub> and other components such as C<sub>2</sub>F<sub>4</sub>, C<sub>2</sub>F<sub>5</sub>I, C<sub>3</sub>F<sub>8</sub>, HF, H<sub>2</sub>, COF<sub>2</sub>, CF<sub>3</sub>H, and CF<sub>3</sub>OH with a small amount of H<sub>2</sub>O.</td><td>Calculation</td><td></td><td>...</td><td>238</td></tr><tr><td>CF<sub>3</sub>I + O<sub>2</sub></td><td>The breakdown decomposition rate of CF<sub>3</sub>I is accelerated, and COF<sub>2</sub> is produced in the presence of O<sub>2</sub>.</td><td>Measure and<br>Calculation/AC</td><td></td><td>...</td><td>239</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N + air<br>C<sub>4</sub>F<sub>7</sub>N + CO<sub>2</sub></td><td>CO and C<sub>4</sub>F<sub>8</sub> are produced after the breaking test, and the LC50 (4 h on mice) after 100 C-O 630 A/24 kV in an 11-l epoxy bulb is inferior to 225 ppm.</td><td>Measure<br>100 C-O 630 A/24 kV</td><td></td><td>0.174</td><td>246</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N + air</td><td>The sole atmospheric degradation products appear to be NO, COF<sub>2</sub>, and CF<sub>3</sub>C(O)F.</td><td>Measure and<br>calculation/296 ± 1 K</td><td></td><td>700 Torr</td><td>247</td></tr></tbody></table>
<table><caption>TABLE VIII. (Continued.)</caption><thead><tr><th rowspan="2">Gases</th><th>Breakdown voltage/partial discharge voltages</th><th colspan="3">Voltage/electrode</th></tr><tr><th>Interruption ability</th><th>Method</th><th>P (MPa)</th><th>Reference</th></tr></thead><tbody><tr><td>C<sub>4</sub>F<sub>7</sub>N + CO<sub>2</sub></td><td>The decomposition rate of C<sub>3</sub>F<sub>7</sub>CN and generation rate of the products increase with the increase in ambient temperature.</td><td>Calculation<br>(2 000–3 000 K)</td><td>0.1</td><td>249</td></tr><tr><td>C<sub>4</sub>F<sub>7</sub>N + Cu/Al</td><td>The adsorption energy of C<sub>3</sub>F<sub>7</sub>CN adsorbed on Cu (1 1 1) and Al (1 1 1) is both below 0.8 eV.</td><td>Calculation</td><td>...</td><td>253</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O + Air/<br>N<sub>2</sub>/CO<sub>2</sub></td><td>After the DBD processing of two test-gases, a large quantity of toxic C<sub>3</sub>F<sub>6</sub> was found in pure C<sub>5</sub>F<sub>10</sub>O.</td><td>Measure DBD</td><td>0.1</td><td>258</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O + Air</td><td>CO and C<sub>4</sub>F<sub>8</sub> are produced after the breaking test, and the LC50 (4 h on mice) after 100 C–O 630 630 A/24 kV in an 11-1 epoxy bulb is inferior to 2100 ppm.</td><td>Measure<br>100 C–O 630 A/24 kV</td><td>0.125</td><td>246</td></tr><tr><td>C<sub>5</sub>F<sub>10</sub>O + N<sub>2</sub></td><td>The amount of decomposition products of C<sub>5</sub>F<sub>10</sub>O (CF<sub>4</sub>, C<sub>2</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>8</sub>, C<sub>4</sub>F<sub>10</sub>, and C<sub>6</sub>F<sub>14</sub>), increased with the increased number of discharges.</td><td>Measure calculation<br>AC/sphere-sphere</td><td>0.11</td><td>262</td></tr><tr><td>C<sub>6</sub>F<sub>12</sub>O + N<sub>2</sub></td><td>The breakdown decomposition products of C<sub>6</sub>F<sub>12</sub>O + N<sub>2</sub> were CF<sub>4</sub>, C<sub>2</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>8</sub>, C<sub>4</sub>F<sub>10</sub>, and C<sub>5</sub>F<sub>12</sub>.</td><td>Measure calculation<br>AC/sphere-sphere</td><td>0.1</td><td>79</td></tr></tbody></table>

measurement for partial discharge is already there. The calculation of the discharge process of PD or discharge by Townsend discharge theory has also been verified.

The measurement process is generally at a low pressure (1 Torr), and there is a deviation of the calculation results of the gas insulation performance under high pressure. The calculated value is derived from the collision cross section of gas molecules. According to the collision cross section data, the corresponding parameters can be calculated, and the critical breakdown field strength can be obtained from the ionization coefficient and the attachment coefficient. At present, the data of the collision cross section of gas are limited and are constantly replenished and improved. It is difficult to measure and calculate the collision cross section, which is an important factor that hinders the new gas research. The current micro-parameter calculations are based on some assumptions and simplifications. There is still a need for further research and exploration on the microscopic calculation of gas discharge to macroscopic processes.

(2) Extinguishing performance: Gas arc performance can be obtained through the interruption test in the laboratory or the circuit breaker equipment. By optimizing the structure of the arc extinguishing device, changing the size of the contact, and adding solid insulating materials, the gas arc extinguishing effect can be improved. According to the data in Table IV, it can be seen that there is not any arc-extinguishing medium, so looking for gases with excellent interruption performance is still the difficult point at present.

For the calculation of the arc extinguishing performance, the calculation of the arc conductance through thermodynamic performance, transport parameters, and radiation has been a relatively mature process. However, the calculation

process is still in an ideal condition. The details of the calculation are still worth exploring. For example, the difference between the electron temperature and the heavy ion temperature in Non Local Thermodynamic Equilibrium (NLTE) and various particles of the chemical reaction rate in non-chemical equilibrium need to be considered in the calculation of equilibrium composition. In-depth exploration of the theory is still a research direction.

(3) For decomposition performance: The decomposition products of various gases in different discharge conditions can be measured by the experiment. However, for the theoretical calculation, at present, only the possible reaction path can be specified to calculate the reaction energy, and the calculations have some subjectivity. The chemical process is not calculated on the physical conditions, so the rate of formation of decomposition products cannot be calculated quantitatively according to the experimental conditions of discharge.

So far, SF<sub>6</sub> is still the most stable and reliable insulation and interrupter medium. The existing alternative gas as an insulation medium can be used in different equipment. In contrast, the interrupter medium needs further exploration. The new gas is still in constant exploration.

The gas-insulated medium is not a single kind of gas. Different gases may be used as the insulating medium in different equipment. Because there is no single insulating gas that can completely replace the use of SF<sub>6</sub> in electrical equipment, mixed gases have become the mainstream of development. There are still many unexplored areas for the gas currently studied. The molecular structure of the ideal alternative gas by computational chemistry can be designed. The calculation can predict the boiling point, insulation properties, and other aspects of the new insulating gas, which is a way to look for alternative gases.
At present, there is still no gas that can substitute for SF₆ completely, and the gas with good insulating property needs to be mixed with the buffer gas (except HFO1234zeE and CF₃I, which can be used alone as an insulating medium in the MV equipment). A series of problems (such as the leakage, the supplementary gas, the recovery, and the decomposition) brought by the use of the mixed gas need to be considered in the future, and the relevant industries are still being explored. Research on alternative gases still faces greater challenges and requires significant time and economic investment.

## ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (Program No. 51707137).

## APPENDIX

Tables V–VIII contain the current research conclusions and references of major environmentally friendly gases.

## REFERENCES

¹H. Moissan and P. Lebeau, Compt. Rend. **130**, 869 (1900).

²G. Camilli and J. J. Chapman, “Gaseous insulation for high-voltage apparatus,” Trans. Am. Inst. Electr. Eng. **66**(1), 1463–1470 (1947).

³G. Camilli, G. S. Gordon, and R. E. Plump, “Gaseous insulation for high-voltage transformers,” Trans. Am. Inst. Electr. Eng., Part 3 **71**(1), 348–357 (1952).

⁴S. Menju, H. Aoyagi, K. Takahashi, and H. Qhno, “Dielectric breakdown of high pressure SF₆ in sphere and coaxial cylinder gaps,” *IEEE Trans. Power Appar. Syst.* **PAS-93**(5), 1706–1712 (1974).

⁵S. Menju and K. Takahashi, “DC dielectric strength of a SF₆ gas insulated system,” *IEEE Trans. Power Appar. Syst.* **PAS-97**, 217–224 (1978).

⁶F. A. M. Rizk, C. Masetti, and R. P. Comsa, “Particle-initiated breakdown in SF₆ insulated systems under high direct voltage,” *IEEE Trans. Power Appar. Syst.* **PAS-98**(3), 825–836 (1979).

⁷H. A. Boyd and G. C. Crichton, “Measurement of ionisation and attachment coefficients in SF₆,” *Proc. Inst. Electr. Eng.* **118**(12), 1872–1877 (1971).

⁸D. Berg and C. N. Works, “Temperature effect in the electrical breakdown of SF₆,” in *1957 Conference on Electrical Insulation* (IEEE, 1957), p. 31.

⁹H. J. Lingal, A. P. Strom, and T. E. Browne, “An investigation of the arc-quenching behavior of sulfur hexafluoride [includes discussion],” *Trans. Am. Inst. Electr. Eng., Part 3* **72**(2), 242–246 (1953).

¹⁰C. F. Cromer and R. E. Friedrich, “A new 115-kv 1,000-mva gas-filled circuit breaker,” *Electr. Eng.* **75**(12), 1085 (1956).

¹¹W. M. Leeds, T. E. Browne, and A. P. Strom, “The use of SF₆ for high-power arc quenching,” *Electr. Eng.* **76**(9), 788–791 (1957).

¹²R. Kane and R. Colclaser, “A 69-Kv SF₆ common-tank breaker rated 5,000 Mva,” *IEEE Trans. Power Appar. Syst.* **82**(69), 1076–1082 (1963).

¹³G. J. Easley and J. M. Telford, “A new design 34.5 to 69 Kv intermediate capacity SF₆ circuit breaker,” *IEEE Trans. Power Appar. Syst.* **83**(12), 1172–1177 (1964).

¹⁴R. C. Van Sickle and R. N. Yeckley, “A 500-kV circuit breaker using SF₆ gas,” *IEEE Trans. Power Appar. Syst.* **84**(10), 892–901 (1965).

¹⁵C. F. Sonnenberg and J. J. Brado, “Insulation coordination qualities of 500-kV SF₆ circuit breakers,” *IEEE Trans. Power Appar. Syst.* **84**(10), 851–863 (1965).

¹⁶R. Yeckley and C. Cromer, “New SF₆ EHV Circuit breakers for 550 kV and 765 kV,” *IEEE Trans. Power Appar. Syst.* **PAS-89**(8), 2015–2023 (1970).

¹⁷R. Kane, R. Newsome, and C. Wagner, “New generation of oilless circuit breakers 115 kV-345kV I. System requirements,” *IEEE Trans. Power Appar. Syst.* **PAS-90**(2), 628–635 (1971).

¹⁸E. Kane and Z. Neri, “New generation of oilless circuit breakers 115 kV-345kV III.-Insulation systems,” *IEEE Trans. Power Appar. Syst.* **PAS-90**(2), 647–651 (1971).

¹⁹R. Crookston, W. Fischer, and R. Kane, “New generation of oilless circuit breakers 115 kV-345kV IV-The mechanical system,” *IEEE Trans. Power Appar. Syst.* **PAS-90**(2), 652–659 (1971).

²⁰T. F. Garrity, R. Matulic, and G. Rhodes, “Installation and field testing of 138kV SF₆ gas insulated station and transmission line,” *IEEE Trans. Power Appar. Syst.* **94**(5), 1589–1598 (1975).

²¹L. Berkebile, C. Cromer, R. Kane, and F. Reese, “New generation of oilless circuit breakers 115 kV-345kV II-The interrupting system,” *IEEE Trans. Power Appar. Syst.* **PAS-90**(2), 636–646 (1971).

²²F. S. Cooper, U.S. Patent No. 2,221,671, U.S. Patent and Trademark Office, Washington, DC (12 November 1940).

²³P. Parvopassu, F. Solimando, L. M. Rolandin, and E. Occhini, “150 kV EPM cable connections between transformers and SF₆ insulated switchgear for 5 substations in Rome,” *IEEE Trans. Power Appar. Syst.* **96**(1), 178–183 (1977).

²⁴K. Morii, S. Matsumura, T. Ushio, S. Tominaga, and Y. Shinozaki, “SF₆ gas circuit breaker, new solution for generator main circuit switching,” *IEEE Trans. Power Appar. Syst.* **PAS-98**(3), 759–769 (1979).

²⁵P. Bolin, A. Cookson, P. Kommineni, and K. Yoon, “Mechanical design and test of 1200 kV semi-flexible SF6 insulated transmission line,” *IEEE Trans. Power Appar. Syst.* **PAS-101**(6), 1630–1637 (1982).

²⁶Y. Harumoto, Y. Kabayama, Y. Kuroda, Y. Yoshida, H. Kan, Y. Miura, E. Tamaki, and T. Hakata, “Development of 275 kV EHV class gas-insulated power transformer,” *IEEE Trans. Power Appar. Syst.* **PAS-104**(9), 2501–2508 (1985).

²⁷S. P. Walldorf and P. A. Gnadt, “Development of 1200-kV compressed-gas-insulated transmission and substation equipment in the United States,” *IEEE Trans. Power Delivery* **2**(2), 374–383 (1987).

²⁸J.-y. Koo, J.-g. Seong, J.-s. Hwang, B.-w. Lee, and S.-h. Lee, “Design of 154 kV extra-high-voltage prototype SF6 bushing for superconducting electric power applications,” *Jpn. J. Appl. Phys., Part 1* **51**(9S2), 09ME01 (2012).

²⁹T. R. Bjortuft, E. Attar, M. Saxegaard, A. Sitko, O. Granhaug, and H. Landsverk, “Dielectric and thermal challenges for next generation ring main units (RMU),” 22nd International Conference on Electricity Distribution Stockholm, 10–13 June 2013, Paper 0463.

³⁰D. Koch, SF6 Properties, and Use in MV and HV Switchgear, Cahier Technique, 2003, p. 188.

³¹X. Zhang, H. Xiao, J. Tang, Z. Cui, and Y. Zhang, “Recent advances in decomposition of the most potent greenhouse gas SF₆,” *Crit. Rev. Environ. Sci. Technol.* **47**(18), 1763–1782 (2017).

³²J. Devins, “Replacement gases for SF₆,” *IEEE Trans. Electr. Insul.* **EI-15**(2), 81–86 (1980).

³³S. Wock, “On the toxicity of SF₆ insulating gas,” *IEEE Trans. Electr. Insul.* **EI-19**(2), 156 (1984).

³⁴A. Vanroggen, P. F. Pugh, and K. P. Brand, “On the toxicity of SF₆ insulating gas,” *IEEE Trans. Electr. Insul.* **EI-18**(1), 93 (1983).

³⁵A. Vijk, “The nature of metal-electrodes/SF₆ reactions in SF₆ decomposition due to direct-current interruption under simulated circuit-breaker conditions,” *IEEE Trans. Electr. Insul.* **EI-11**(4), 157–160 (1976).

³⁶A. Kraut and R. Lilis, “Pulmonary effects of acute exposure to degradation products of sulphur hexafluoride during electrical cable repair work,” *Occup. Environ. Med.* **47**(12), 829–832 (1990).

³⁷K. J. Pilling and H. W. Jones, “Inhalation of degraded sulphur hexafluoride resulting in pulmonary oedema,” *Occup. Med.* **38**(3), 82–84 (1988).

³⁸C. Liu, S. Palanisamy, S. Chen, P. Wu, L. Yao, and B. S. Lou, “Mechanism of formation of SF₆ decomposition gas products and its identification by GC-MS and electrochemical methods: A mini review,” *Int. J. Electrochem. Sci.* **10**(5), 4223–4231 (2015).

³⁹G. D. Griffin, M. G. Nolan, C. E. Easterly, I. Sauers, and P. C. Votaw, “Concerning biological effects of spark-decomposed SF₆,” *IEE Proc., Part A: Phys. Sci., Meas. Instrum., Manage. Educ.* **137**(4), 221–227 (1990).

⁴⁰F. Jakob and M. B. A. Nicholas Perjanik, Sulfur hexafluoride: A unique dielectric, Analytical ChemTech International, Inc., 1998.

⁴¹B. Obama, “The irreversible momentum of clean energy,” *Science* **355**(6321), 126–129 (2017).
<footnote n="42">⁴²M. Grubb, C. Vrolik, and D. Brack, *The Kyoto Protocol: A Guide and Assessment* (Royal Institute of International Affairs Energy and Environmental Programme, 1997).</footnote>

<footnote n="43">⁴³S. Xiao, “Research on insulation performance of SF₆ substitute CF₃I/CO₂ under power frequency voltage and the influence of micro-moisture on CF₃I,” Ph.D. dissertation (Université de Toulouse, Université Toulouse III-Paul Sabatier, 2016).</footnote>

<footnote n="44">⁴⁴P. Purohit and L. Hoglund-Isaksson, “Global emissions of fluorinated greenhouse gases until 2050: Technical mitigation potentials and costs,” in *EGU General Assembly Conference Abstracts* (EGU, 2016), Vol. 18.</footnote>

<footnote n="45">⁴⁵S. Xiao, X. Zhang, J. Tang, and S. Liu, “A review on SF₆ substitute gases and research status of CF₃I gases,” *J. Energy Rep.* **4**, 523–535 (2018).</footnote>

<footnote n="46">⁴⁶J. D. Mantilla, N. Gariboldi, S. Grob, and M. Claessens, “Investigation of the insulation performance of a new gas mixture with extremely low GWP,” in *2014 IEEE Electrical Insulation Conference (EIC) (IEEE, 2014)*, pp. 469–473.</footnote>

<footnote n="47">⁴⁷Y. Kieffel, F. Biquez, and P. Ponchon, Alternative Gas to SF₆ for Use in High Voltage Switchgears: G3, CIRED Paper No. 230, 2015.</footnote>

<footnote n="48">⁴⁸R. Ullah, A. Rashid, A. Rashid, F. Khan, and A. Ali, “Dielectric characteristic of dichlorodifluoromethane (R12) gas and mixture with N₂/air as an alternative to SF₆ gas,” *High Voltage 2*(3), 205–210 (2017).</footnote>

<footnote n="49">⁴⁹M. Koch and C. M. Franck, “High voltage insulation properties of HFO1234ze,” *IEEE Trans. Dielectr. Electr. Insul.* **22**(6), 3260–3268 (2015).</footnote>

<footnote n="50">⁵⁰A. Beroual and A. Haddad, “Recent advances in the quest for a new insulation gas with a low impact on the environment to replace sulfur hexafluoride (SF₆) gas in high-voltage power network applications,” *Energies* **10**(8), 1216 (2017).</footnote>

<footnote n="51">⁵¹L. C. Zhang, *Study on Insulation Characteristics of c-C₄F₈ and its Gas Mixtures Substituting SF₆* (Shanghai Jiao Tong University, Shanghai, China, 2007).</footnote>

<footnote n="52">⁵²D. A. Price, J. Lucas, and J. L. Moruzzi, “Ionization in oxygen-hydrogen mixtures,” *J. Phys. D: Appl. Phys.* **5**(7), 1249 (1972).</footnote>

<footnote n="53">⁵³See http://www.lxcat.laplace.univ-tlse.fr for more information about collisions.</footnote>

<footnote n="54">⁵⁴M.-x. Hou, W.-g. Li, C.-y. Yuan, Y.-x. Yang, and J. Ouyang, “Breakdown characteristics of CF₄ and CF₄/N₂ hybrid gas in refrigeration temperature range,” *J. Cent. South Univ.* **24**(4), 861–865 (2017).</footnote>

<footnote n="55">⁵⁵A. Banković, S. Dujko, S. Marjanović, R. D. White, and Z. L. Petrović, “Positron transport in CF₄ and N₂/CF₄ mixtures,” *Eur. Phys. J. D* **68**(5), 127 (2014).</footnote>

<footnote n="56">⁵⁶H. Itoh, T. Matsumura, K. Satoh, H. Date, Y. Nakao, and H. Tagashira, “Electron transport coefficients in SF₆,” *J. Phys. D: Appl. Phys.* **26**(11), 1975 (1993).</footnote>

<footnote n="57">⁵⁷M. J. Pinheiro and J. Loureiro, “Effective ionization coefficients and electron drift velocities in gas mixtures of SF₆ with He, Xe, CO₂ and N₂ from Boltzmann analysis,” *J. Phys. D: Appl. Phys.* **35**(23), 3077 (2002).</footnote>

<footnote n="58">⁵⁸X. Yu, H. Hou, and B. Wang, “Prediction on dielectric strength and boiling point of gaseous molecules for replacement of SF₆,” *J. Comput. Chem.* **38**(10), 721–729 (2017).</footnote>

<footnote n="59">⁵⁹O. Mayr, “Beiträge zur theorie des statischen und des dynamischen Lichtbogens,” *Arch. Elektrotech.* **37**(12), 588–608 (1943).</footnote>

<footnote n="60">⁶⁰G. Yang and J. Heberlein, “Anode attachment modes and their formation in a high intensity argon arc,” *Plasma Sources Sci. Technol.* **16**(3), 529 (2007).</footnote>

<footnote n="61">⁶¹T. Amakawa, J. Jenista, J. Heberlein, and E. Pfender, “Anode-boundary-layer behaviour in a transferred, high-intensity arc,” *J. Phys. D: Appl. Phys.* **31**(20), 2826 (1998).</footnote>

<footnote n="62">⁶²R. G. Parr, “Density functional theory,” *Annu. Rev. Phys. Chem.* **34**(1), 631–656 (1983).</footnote>

<footnote n="63">⁶³A. Gleizes, J. J. Gonzalez, and P. Freton, “Thermal plasma modelling,” *J. Phys. D: Appl. Phys.* **38**(9), R153 (2005).</footnote>

<footnote n="64">⁶⁴A. B. Murphy, “Thermal plasmas in gas mixtures,” *J. Phys. D: Appl. Phys.* **34**(20), R151 (2001).</footnote>

<footnote n="65">⁶⁵W. Z. Wang, M. Z. Rong, J. D. Yan, A. B. Murphy, and J. W. Spencer, “Thermophysical properties of nitrogen plasmas under thermal equilibrium and non-equilibrium conditions,” *Phys. Plasmas* **18**(11), 113502 (2011).</footnote>

<footnote n="66">⁶⁶X. Zhang, J. Zhang, Y. Jia, P. Xiao, and J. Tang, “TiO₂ nanotube array sensor for detecting the SF₆ decomposition product SO₂,” *Sensors* **12**(3), 3302–3313 (2012).</footnote>

<footnote n="67">⁶⁷J. Tang, F. Liu, Q. Meng, X. Zhang, and J. Tao, “Partial discharge recognition through an analysis of SF₆ decomposition products part 2: Feature extraction and decision tree-based pattern recognition,” *IEEE Trans. Dielectr. Electr. Insul.* **19**(1), 37–44 (2012).</footnote>

<footnote n="68">⁶⁸J. Tang, C. J. Chen, X. X. Zhang, F. Liu, and X. L. Ren, “Influence of trace-level O₂ on SF₆ decomposition characteristics under partial discharge,” *Gaodianya Jishu/ High Voltage Eng.* **37**(1), 8–14 (2011).</footnote>

<footnote n="69">⁶⁹X.-x. Zhang, W.-t. Liu, J. Tang, and P. Xiao, “Study on PD detection in SF₆ using multi-wall carbon nanotube films sensor,” *IEEE Trans. Dielectr. Electr. Insul.* **17**(3), 833–838 (2010).</footnote>

<footnote n="70">⁷⁰S. Xiao, X. Zhang, J. Tang, F. Zeng, C. Pan, and Y. Gui, “Comparative study of materials to SF₆ decomposition components,” in *Nanomaterials Based Gas Sensors for SF₆ Decomposition Components Detection* (IntechOpen, 2017).</footnote>

<footnote n="71">⁷¹X. Zhang, H. Zhou, C. Chen, X. Li, Z. Cui, S. Xiao, and J. Tang, “Ultraviolet differential optical absorption spectrometry: Quantitative analysis of the CS₂ produced by SF₆ decomposition,” *Meas. Sci. Technol.* **28**(11), 115102 (2017).</footnote>

<footnote n="72">⁷²X. Zhang, Y. Gui, and Z. Dai, “Adsorption of gases from SF₆ decomposition on aluminum-doped SWCNTs: A density functional theory study,” *Eur. Phys. J. D* **69**(7), 185 (2015).</footnote>

<footnote n="73">⁷³F. Zeng, J. Tang, X. Zhang, J. Pan, Q. Yao, and X. Hou, “Influence regularity of trace H₂O on SF₆ decomposition characteristics under partial discharge of needle-plate electrode,” *IEEE Trans. Dielectr. Electr. Insul.* **22**(1), 287–295 (2015).</footnote>

<footnote n="74">⁷⁴J. Tang, F. Zeng, X. Zhang, J. Pan, Q. Yao, X. Hou, and Y. Tang, “Relationship between decomposition gas ratios and partial discharge energy in GIS, and the influence of residual water and oxygen,” *IEEE Trans. Dielectr. Electr. Insul.* **21**(3), 1226–1234 (2014).</footnote>

<footnote n="75">⁷⁵F. Zeng, J. Luo, J. Tang, Q. Zhou, and Q. Yao, “Influence regularity of aluminum, copper and stainless-steel on SF₆ PD decomposition characteristics components,” *J. Electr. Eng. Technol.* **12**(1), 295–301 (2017).</footnote>

<footnote n="76">⁷⁶J. Tang, Z. Cao, F. Zeng, Q. Yao, Y. Miao, and N. Qiu, “Influence of metallic materials on SF₆ decomposition components under positive DC partial discharge,” *Plasma Chem. Plasma Process.* **37**(6), 1523–1534 (2017).</footnote>

<footnote n="77">⁷⁷Y. Fu, M. Rong, K. Yang, A. Yang, X. Wang, Q. Gao, D. Liu, and A. B. Murphy, “Calculated rate constants of the chemical reactions involving the main byproducts SO₂F, SOF₂, SO₂F₂ of SF₆ decomposition in power equipment,” *J. Phys. D: Appl. Phys.* **49**(15), 155502 (2016).</footnote>

<footnote n="78">⁷⁸F. Zeng, J. Tang, Q. Fan, J. Pan, X. Zhang, Q. Yao, and J. He, “Decomposition characteristics of SF₆ under thermal fault for temperatures below 400 C,” *IEEE Trans. Dielectr. Electr. Insul.* **21**(3), 995–1004 (2014).</footnote>

<footnote n="79">⁷⁹X. Zhang, S. Tian, S. Xiao, Z. Deng, Y. Li, and J. Tang, “Insulation strength and decomposition characteristics of a C₆F₁₂O and N₂ gas mixture,” *Energies* **10**(8), 1170 (2017).</footnote>

<footnote n="80">⁸⁰E. Husain and R. S. Nema, “Analysis of Paschen curves for air, N₂ and SF₆ using the Townsend breakdown equation,” *IEEE Trans. Electr. Insul.* **EI-17**(4), 350–353 (1982).</footnote>

<footnote n="81">⁸¹M. Hara, K. Adachi, H. Tobata, M. Akazaki, and F. L. Mao, “Particle-initiated breakdown characteristics of conical insulator in N₂ gas and N₂/O₂ mixture under DC voltage,” *IEEE Trans. Electr. Insul.* **EI-22**(1), 87–96 (1987).</footnote>

<footnote n="82">⁸²T. Mizuno, K. Morita, Y. Kurata, and H. Miyagawa, “The electrical performance of air or nitrogen gas with solid insulation and the application for switchgears,” in *IEEE/PES Transmission and Distribution Conference and Exhibition (IEEE, 2002)*, Vol. 3, pp. 1797–1801.</footnote>

<footnote n="83">⁸³S. Philp, “Compressed gas insulation in the million-volt range: A comparison of SF₆ with N₂ and CO₂,” *IEEE Trans. Power Appar. Syst.* **82**(66), 356–359 (1963).</footnote>

<footnote n="84">⁸⁴S. Meijer, J. J. Smit, and A. Girodet, “Comparison of the breakdown strength of N₂, CO₂ and SF₆ using the extended up-and-down method,” in *2006 IEEE 8th International Conference on Properties & applications of Dielectric Materials* (IEEE, 2006), pp. 653–656.</footnote>

<footnote n="85">⁸⁵G. Ueta, J. Wada, and S. Okabe, “Evaluation of breakdown characteristics of CO₂ gas for non-standard lightning impulse waveforms-method for converting non-standard lightning impulse waveforms into standard lightning impulse waveforms,” *IEEE Trans. Dielectr. Electr. Insul.* **18**(5), 1724–1733 (2011).</footnote>

<footnote n="86">⁸⁶J. Wada, G. Ueta, and S. Okabe, “Evaluation of breakdown characteristics of CO₂ gas for non-standard lightning impulse waveforms-breakdown characteristics in the presence of bias voltages under non-uniform electric field,” *IEEE Trans. Dielectr. Electr. Insul.* **20**(1), 112–121 (2013).</footnote>
<footnote n="87">⁸⁷J. Wada, G. Ueta, and S. Okabe, "Evaluation of breakdown characteristics of N₂ gas for non-standard lightning impulse waveforms-breakdown characteristics under single-frequency oscillation waveforms and with bias voltages," *IEEE Trans. Dielectr. Electr. Insul.* **18**(5), 1759-1766 (2011).</footnote>

<footnote n="88">⁸⁸W. Pfeiffer and D. Schoen, "Prebreakdown phenomena in 20/80% N₂O/N₂ gas mixtures for vft-voltage stress," in *Proceedings of the XV International Conference on Gas Discharges and their Applications* (GD, 2004), pp. 359-362.</footnote>

<footnote n="89">⁸⁹J. M. Pelletier, Y. Gervais, and D. Mukhedkar, "Dielectric strength of N₂-He mixtures and comparison with N₂-SF₆ and CO₂-SF₆ mixtures," *IEEE Trans. Power Appar. Syst.* **PAS-100**(8), 3861-3869 (1981).</footnote>

<footnote n="90">⁹⁰D.-Y. Lim and S. Bae, "Study on oxygen/nitrogen gas mixtures for the surface insulation performance in gas insulated switchgear," *IEEE Trans. Dielectr. Electr. Insul.* **22**(3), 1567-1576 (2015).</footnote>

<footnote n="91">⁹¹T. Rokunohe, Y. Yagihashi, F. Endo, and T. Oomori, "Fundamental insulation characteristics of air; N₂, CO₂, N₂/O₂, and SF₆/N₂ mixed gases," *Electr. Eng. Jpn.* **155**(3), 9-17 (2006).</footnote>

<footnote n="92">⁹²F. Pontiga and A. Castellanos, "Nitrogen oxides generation induced by negative corona discharge in N₂+O₂ mixtures," in *2006 IEEE Conference on Electrical Insulation and Dielectric Phenomena* (IEEE, 2006), pp. 264-267.</footnote>

<footnote n="93">⁹³H. Saitoh, K. Morita, T. Kikkawa, N. Hayakawa, and H. Okubo, "Impulse partial discharge and breakdown characteristics of rod-plane gaps in air and N₂ gases," *Electr. Eng. Jpn.* **148**(3), 36-43 (2004).</footnote>

<footnote n="94">⁹⁴H. Goshima, H. Shinkai, and M. Yashima, "Lightning impulse breakdown characteristics of high-pressure N₂ as an alternative insulation gas to SF₆," in *Gaseous Dielectrics IX* (Springer, Boston, MA, 2001), pp. 359-364.</footnote>

<footnote n="95">⁹⁵Q. H. Zhang, Y. Tan, and J. G. Wu, "40.5 kV non SF6 C-GIS research," in *2011 1st International Conference on Electric Power Equipment-Switching Technology* (IEEE, 2011), pp. 172-177.</footnote>

<footnote n="96">⁹⁶H. Zhao, X. Li, S. Jia, and A. B. Murphy, "Prediction of the critical reduced electric field strength for carbon dioxide and its mixtures with 50% O₂ and 50% H₂ from Boltzmann analysis for gas temperatures up to 3500 K at atmospheric pressure," *J. Phys. D: Appl. Phys.* **47**(32), 325203 (2014).</footnote>

<footnote n="97">⁹⁷H. Zhao, Y. Deng, and H. Lin, "Study of the synergistic effect in dielectric breakdown property of CO₂-O₂ mixtures," *AIP Adv.* **7**(9), 095102 (2017).</footnote>

<footnote n="98">⁹⁸Z. Chen, C. Niu, H. Zhang, H. Sun, Y. Wu, F. Yang, M. Rang, and Z. Xu, "Investigation on the reduced critical breakdown field of hot CO₂ gas and CO₂-based mixtures," in *2015 3rd International Conference on Electric Power Equipment-Switching Technology (ICEPE-ST)* (IEEE, 2015), pp. 36-39.</footnote>

<footnote n="99">⁹⁹M. Yousfi, J. de Urquijo, A. Juarez, E. Basurto, and J. L. Hernandez-Avila, "Electron swarm coefficients in CO₂-N₂ and CO₂-O₂ mixtures," *IEEE Trans. Plasma Sci.* **37**(6), 764-772 (2009).</footnote>

<footnote n="100">¹⁰⁰J. L. Moruzzi and D. A. Price, "Ionization, attachment and detachment in air and air-CO₂ mixtures," *J. Phys. D: Appl. Phys.* **7**(10), 1434 (1974).</footnote>

<footnote n="101">¹⁰¹G. R. G. Raju and G. R. Gurumurthy, "Electron transport coefficients in N₂ in crossed fields at low E/p," *IEEE Trans. Plasma Sci.* **4**(4), 241-245 (1976).</footnote>

<footnote n="102">¹⁰²H. N. Kucukarpaci and J. Lucas, "Simulation of electron swarm parameters in carbon dioxide and nitrogen for high E/N," *J. Phys. D: Appl. Phys.* **12**(12), 2123 (1979).</footnote>

<footnote n="103">¹⁰³P. C. Stoller, M. Seeger, A. A. Iordanidis, and G. V. Naidis, "CO₂ as an arc interruption medium in gas circuit breakers," *IEEE Trans. Plasma Sci.* **41**(8), 2359-2369 (2013).</footnote>

<footnote n="104">¹⁰⁴E. Jonsson, N. S. Aanensen, and M. Runde, "Current interruption in air for a medium-voltage load break switch," *IEEE Trans. Power Delivery* **29**(2), 870-875 (2014).</footnote>

<footnote n="105">¹⁰⁵N. S. Aanensen, E. Jonsson, and M. Runde, "Air-flow investigation for a medium-voltage load break switch," *IEEE Trans. Power Delivery* **30**(1), 299-306 (2015).</footnote>

<footnote n="106">¹⁰⁶E. Jonsson and M. Runde, "Interruption in air for different medium-voltage switching duties," *IEEE Trans. Power Delivery* **30**(1), 161-166 (2015).</footnote>

<footnote n="107">¹⁰⁷N. S. Stoa-Aanensen, M. Runde, E. Jonsson, and A. D. O. Teigset, "Empirical relationships between air-load break switch parameters and interrupting performance," *IEEE Trans. Power Delivery* **31**(1), 278-285 (2016).</footnote>

<footnote n="108">¹⁰⁸T. Uchii, T. Shinkai, and K. Suzuki, "Thermal interruption capability of carbon dioxide in a puffer-type circuit breaker utilizing polymer ablation," in *IEEE/PES Transmission and Distribution Conference and Exhibition (IEEE, 2002)*, Vol. 3, pp. 1750-1754.</footnote>

<footnote n="109">¹⁰⁹Z. Q. Chen, X. Cheng, L. Y. Jiao, and G. W. Ge, "Simulation on the high current interruption principle for hybrid circuit breaker of vacuum interrupter and CO₂ gas interrupter," in *2016 27th International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV)* (IEEE, 2016), Vol. 2, pp. 1-4.</footnote>

<footnote n="110">¹¹⁰X. Cheng, Z. Chen, G. Ge, Y. Wang, M. Liao, and L. Jiao, "Dynamic dielectric recovery synergy of hybrid circuit breaker with CO₂ gas and vacuum interrupters in series," *IEEE Trans. Plasma Sci.* **45**(10), 2885-2892 (2017).</footnote>

<footnote n="111">¹¹¹T. Matsumura, Y. Yokomizu, D. Kanda, T. Kumazawa, T. Furuhata, and K. Mitsukuchi, "Effect of magnetic field strength and admixture gas on current interrupting capability of a CO₂ rotary-arc load-break switch," *Electr. Eng. Jpn.* **167**(2), 21-27 (2009).</footnote>

<footnote n="112">¹¹²L. Zhong, X. Wang, M. Rong, and Y. Cressault, "Effects of copper vapour on thermophysical properties of CO₂-N₂ plasma," *Eur. Phys. J. D* **70**(11), 233 (2016).</footnote>

<footnote n="113">¹¹³N. Malik and A. Qureshi, "A review of electrical breakdown in mixtures of SF₆ and other gases," *IEEE Trans. Electr. Insul.* **EI-14**(1), 1-13 (1979).</footnote>

<footnote n="114">¹¹⁴N. Malik and A. Qureshi, "Calculation of discharge inception voltages in SF₆-N₂ mixtures," *IEEE Trans. Electr. Insul.* **EI-14**(2), 70-76 (1979).</footnote>

<footnote n="115">¹¹⁵A. Yializis, N. H. Malik, A. H. Qureshi, and E. Kuffel, "Impulse breakdown and corona characteristics for rod-plane gaps in mixtures of SF₆ and nitrogen with less than 1% of SF₆ content," *IEEE Trans. Power Appar. Syst.* **PAS-98**(5), 1832-1840 (1979).</footnote>

<footnote n="116">¹¹⁶O. Farish, O. E. Ibrahim, and B. H. Crichton, "Effect of electrode surface roughness on breakdown in nitrogen/SF₆ mixtures," *Proc. Inst. Electr. Eng.* **123**(10), 1047-1050 (1976).</footnote>

<footnote n="117">¹¹⁷R. Graf and W. Boeck, "Defect sensibility of N₂-SF₆ gas mixtures with equal dielectric strength," in *2000 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (Cat. No. 00CH37132)* (IEEE, 2000), Vol. 1, pp. 422-425.</footnote>

<footnote n="118">¹¹⁸S.-Y. Woo, D.-H. Jeong, K.-B. Seo, and J.-H. Kim, "A study on dielectric strength and Insulation property of SF₆/N₂ mixtures for GIS," *J. Int. Council Electr. Eng.* **2**(1), 104-109 (2012).</footnote>

<footnote n="119">¹¹⁹M. A. Guerroui and A. Lemzadmi, "Polarity effect on corona discharge voltages in SF₆-N₂ gas mixtures," *Eur. Sci. J.* **9**(21), 72-75 (2013).</footnote>

<footnote n="120">¹²⁰H. Sato, K. Morita, T. Kikkawa, N. Hayakawa, and H. Okubo, "A fundamental study on electrical insulation of N₂/SF₆ gas-insulated electric power apparatus," *Electr. Eng. Jpn.* **139**(4), 9-16 (2002).</footnote>

<footnote n="121">¹²¹Y. Hoshina, M. Sato, H. Murase, M. Toyada, and A. Kobayashi, "Dielectric properties of SF₆/N₂ gas mixtures on a full scale model of the gas-insulated busbar," in *2000 IEEE Power Engineering Society Winter Meeting. Conference Proceedings (Cat. No. 00CH37077)* (IEEE, 2000), Vol. 3, pp. 2129-2134.</footnote>

<footnote n="122">¹²²C. Guo, Q. Zhang, and T. Wen, "A method for synergistic effect evaluation of SF₆/N₂ gas mixtures," *IEEE Trans. Dielectr. Electr. Insul.* **23**(1), 211-215 (2016).</footnote>

<footnote n="123">¹²³F. Rizk and M. Eteiba, "Impulse breakdown voltage-time curves of SF₆ and SF₆-N₂ coaxial-cylinder gaps," *IEEE Trans. Power Appar. Syst.* **PAS-101**(12), 4460-4471 (1982).</footnote>

<footnote n="124">¹²⁴T. Mizobuchi, H. Toyota, S. Matsuoka, A. Kumada, and K. Hidaka, "Nanosecond electrical discharge development in SF₆-N₂ gas mixtures," *IEEJ Trans. Fundam. Mater.* **125**(8), 629-635 (2005).</footnote>

<footnote n="125">¹²⁵C. Jiasen, W. Hu, Z. Qiaogen, and Q. Aici, "Multichannel discharge characteristics of gas switch gap in SF₆-N₂ or SF₆-Ar gas mixtures under nanosecond triggering pulses," *Plasma Sci. Technol.* **13**(6), 719-723 (2011).</footnote>

<footnote n="126">¹²⁶T. Hirata, H. Ueno, and H. Nakayama, "Characteristics of N₂/SF₆ mixture gas in creeping discharge developing in narrow gap with backside electrode," *Electr. Eng. Jpn.* **158**(2), 31-38 (2007).</footnote>

<footnote n="127">¹²⁷H. Koch and M. Hopkins, "Overview of gas insulated lines (GIL)," in *IEEE Power Engineering Society General Meeting* (IEEE, 2005), pp. 940-944.</footnote>

<footnote n="128">¹²⁸N. H. Malik, A. H. Qureshi, and Y. A. Safar, "DC voltage breakdown of SF₆-air and SF₆-CO₂ mixtures in rod-plane gaps," *IEEE Trans. Electr. Insul.* **EI-18**(6), 629-636 (1983).</footnote>

<footnote n="129">¹²⁹Z. Li, R. Kuffel, and E. Kuffel, "Volt-time characteristics in air, SF₆/air mixture and N₂ for coaxial cylinder and rod-sphere gaps," *IEEE Trans. Electr. Insul.* **EI-21**(2), 151-155 (1986).</footnote>
<footnote n="135">¹³⁵Y. Qiu, S. Y. Chen, Y. F. Liu, and E. Kuffel, "Comparison of SF₆/N₂ and SF₆/CO₂ gas mixtures based on the figure-of-merit concept," in *Annual Report: Conference on Electrical Insulation and Dielectric Phenomena* (IEEE, 1988), pp. 299-304.</footnote>

<footnote n="131">¹³¹Y. Qiu and E. Kuffel, "Comparison of SF₆/N₂ and SF₆/CO₂ gas mixtures as alternatives to SF₆ gas," *IEEE Trans. Dielectr. Electr. Insul.* **6**(6), 892-895 (1999).</footnote>

<footnote n="132">¹³²Y. Qiu and I. D. Chalmers, "Effect of electrode surface roughness on breakdown in SF₆-N₂ and SF₆-CO₂ gas mixtures," *J. Phys. D: Appl. Phys.* **26**(11), 1928 (1993).</footnote>

<footnote n="133">¹³³S. Ohtsuka, S. Nagara, M. Koumura, Y. Hashimoto, M. Nakamura, and M. Hikita, "Insulation properties of a CO₂/N₂ 50% SF₆ gas mixture in a nonuniform field," *Electr. Eng. Jpn.* **140**(3), 34-43 (2002).</footnote>

<footnote n="134">¹³⁴Y. Safar, N. Malik, and A. Qureshi, "Impulse breakdown behavior of negative rod-plane gaps in SF₆-N₂, SF₆-air and SF₆-CO₂ mixtures," *IEEE Trans. Electr. Insul.* **EI-17**(5), 441-450 (1982).</footnote>

<footnote n="135">¹³⁵A. H. Cookson and R. E. Wootton, "AC corona and breakdown characteristics for rod gaps in compressed hydrogen, SF₆ and hydrogen-SF₆ mixtures," *IEEE Trans. Power Appar. Syst.* **PAS-97**(2), 415-423 (1978).</footnote>

<footnote n="136">¹³⁶O. Farish, S. J. Dale, and A. M. Sletten, "Impulse breakdown of negative rod-plane gaps in hydrogen and hydrogen-SF₆ mixtures," *IEEE Trans. Power Appar. Syst.* **PAS-97**(1), 118-124 (1978).</footnote>

<footnote n="137">¹³⁷M. Akbar and N. H. Malik, "Electrical breakdown of N₂O-SF₆, N₂O-CCl₂F₂ and N₂O-CO₂ gas mixtures," *IEEE Trans. Electr. Insul.* **EI-20**(3), 581-585 (1985).</footnote>

<footnote n="138">¹³⁸S. W. Park, C. H. Hwang, N. R. Kim, K. T. Lee, and C. S. Huh, "Breakdown characteristics of SF₆/CF₄ mixtures in test chamber and 25.8 kV GIS," *Eng. Lett.* **15**(1), 145-148 (2007).</footnote>

<footnote n="139">¹³⁹C. H. Hwang, B. T. Lee, C. S. Huh, N. R. Kim, and Y. M. Chang, "Breakdown characteristics of SF₆/CF₄ mixtures in 25.8 kV," in *2009 International Conference on Electrical Machines and Systems* (IEEE, 2009), pp. 1-4.</footnote>

<footnote n="140">¹⁴⁰J. M. Berg, *Experimental Studies of SF₆-CF₄ Mixtures for Use as Gaseous Dielectrics in Power System Applications* (The University of Manitoba, Manitoba, Canada, 1995).</footnote>

<footnote n="141">¹⁴¹J. Berg and E. Kuffel, "Breakdown voltage characteristics of SF6/CF4 mixtures in uniform and non-uniform field gaps," in *Proceedings of 1995 Conference on Electrical Insulation and Dielectric Phenomena* (IEEE, 1995), pp. 126-129.</footnote>

<footnote n="142">¹⁴²K. Toufani, *Experimental Studies of Breakdown Characteristics of SF₆/CF₄ Mixtures in Highly Non-uniform Fields for High-Voltage Applications* (The University of Manitoba, Manitoba, Canada, 1997).</footnote>

<footnote n="143">¹⁴³S. H. Lee, J. K. Seong, S. H. Oh, W. J. Shin, U. A. Khan, and B. W. Lee, "Breakdown characteristics of liquefied gases in liquid nitrogen for high voltage bushings in a cryogenic environment," *IEEE Trans. Appl. Supercond.* **21**(3), 1430-1433 (2011).</footnote>

<footnote n="144">¹⁴⁴M. Siddagangappa, C. Lakshminarasimha, and M. Naidu, "Prediction of breakdown voltages of binary gas mixtures in uniform electric fields," *IEEE Trans. Electr. Insul.* **EI-19**(4), 359-361 (1984).</footnote>

<footnote n="145">¹⁴⁵R. Nema, S. Kulkarni, and E. Husain, "Calculation of sparking potentials of SF₆ and SF₆-gas mixtures in uniform and non-uniform electric fields," *IEEE Trans. Electr. Insul.* **EI-17**(1), 70-75 (1982).</footnote>

<footnote n="146">¹⁴⁶V. N. Maller and M. S. Naidu, "Ionization and breakdown in SF₆-air and freon-nitrogen mixtures," *IEEE Trans. Plasma Sci.* **3**(2), 49-54 (1975).</footnote>

<footnote n="147">¹⁴⁷N. Malik and A. Qureshi, "Breakdown gradients in SF₆-N₂, SF₆-Air and SF₆-CO₂ mixtures," *IEEE Trans. Electr. Insul.* **EI-15**(5), 413-418 (1980).</footnote>

<footnote n="148">¹⁴⁸G. R. Govinda Raju and R. Hackam, "Breakdown field strength of SF₆, N₂O, SF₆+N₂, and SF₆+N₂O," *J. Appl. Phys.* **52**(6), 3912-3920 (1981).</footnote>

<footnote n="149">¹⁴⁹L. E. Kline, D. K. Davies, C. L. Chen, and P. J. Chantry, "Dielectric properties for SF₆ and SF₆ mixtures predicted from basic data," *J. Appl. Phys.* **50**(11), 6789-6796 (1979).</footnote>

<footnote n="150">¹⁵⁰M. S. Dincer and G. R. Govinda Raju, "Ionization and attachment coefficients in SF₆+N₂ mixtures," *IEEE Trans. Electr. Insul.* **EI-19**(1), 40-44 (1984).</footnote>

<footnote n="151">¹⁵¹G. R. Govinda Raju and M. S. Dincer, "Measurement of ionization and attachment coefficients in SF₆ and SF₆+N₂," *J. Appl. Phys.* **53**(12), 8562-8567 (1982).</footnote>

<footnote n="152">¹⁵²Z. Y. Lee, "Measurements of breakdown potential and ionization and attachment in SF₆-CO₂ mixtures," *IEEE Trans. Electr. Insul.* **EI-18**(6), 637-641 (1983).</footnote>

<footnote n="153">¹⁵³M. F. Fréchette, "Experimental study of SF₆/N₂ and SF₆/CCl₂F₂ mixtures by the steady-state Townsend method," *J. Appl. Phys.* **59**(11), 3684-3693 (1986).</footnote>

<footnote n="154">¹⁵⁴A. V. Phelps and R. J. Van Brunt, "Electron-transport, ionization, attachment, and dissociation coefficients in SF₆ and its mixtures," *J. Appl. Phys.* **64**(9), 4269-4277 (1988).</footnote>

<footnote n="155">¹⁵⁵D. M. Xiao, X. G. Li, and X. Xu, "Swarm parameters in SF₆ and CO₂ gas mixtures," *J. Phys. D: Appl. Phys.* **34**(24), L133 (2001).</footnote>

<footnote n="156">¹⁵⁶J. L. Hernández-Ávila, E. Basurto, and J. de Urquijo, "Electron transport and swarm parameters in CO₂ and its mixtures with SF₆," *J. Phys. D: Appl. Phys.* **35**(18), 2264 (2002).</footnote>

<footnote n="157">¹⁵⁷J. de Urquijo, E. Basurto, and J. L. Hernández-Ávila, "Measurement of electron drift, diffusion, and effective ionization coefficients in the SF₆-CHF₃ and SF₆-CF₄ gas mixtures," *J. Phys. D: Appl. Phys., Part 1* **36**(24), 3132 (2003).</footnote>

<footnote n="158">¹⁵⁸D. M. Xiao, J. L. Yang, and X. Xu, "Electron swarm parameters in SF₆ and CF₄ gas mixtures," *Jpn. J. Appl. Phys., Part 2* **43**(3A), L369 (2004).</footnote>

<footnote n="159">¹⁵⁹X. Liu and D. Xiao, "Monte Carlo simulation of electron swarm parameters in the SF₆/CF₄ gas mixtures," *Jpn. J. Appl. Phys., Part 1* **46**(4R), 1663 (2007).</footnote>

<footnote n="160">¹⁶⁰W. Feng, W. Pfeiffer, N. Kouzichine, Q. Yuchang, E. Kuffel, and L. Kenli, "Calculation of electron swarm parameters of SF₆/N₂ in a uniform field using an improved Monte Carlo collision simulation method," *Plasma Sci. Technol.* **10**(1), 39 (2008).</footnote>

<footnote n="161">¹⁶¹H. Zhao, X. Li, S. Jia, and A. B. Murphy, "Dielectric breakdown properties of SF₆-N₂ mixtures at 0.01-1.6 MPa and 300-3000 K," *J. Appl. Phys.* **113**(14), 143301 (2013).</footnote>

<footnote n="162">¹⁶²X. Li, H. Zhao, S. Jia, and A. B. Murphy, "Study of the dielectric breakdown properties of hot SF₆-CF₄ mixtures at 0.01-1.6 MPa," *J. Appl. Phys.* **114**(5), 053302 (2013).</footnote>

<footnote n="163">¹⁶³L. Zhong, A. Yang, X. Wang, D. Liu, Y. Wu, and M. Rong, "Dielectric breakdown properties of hot SF₆-CO₂ mixtures at temperatures of 300-3500 K and pressures of 0.01-1.0 MPa," *Plasmas Plasmas* **21**(5), 053506 (2014).</footnote>

<footnote n="164">¹⁶⁴A. V. Larin, N. Meurice, D. N. Trubnikov, and D. P. Vercauteren, "Theoretical analysis of the synergism in the dielectric strength for SF₆/CF₄ mixtures," *J. Appl. Phys.* **96**(1), 109-117 (2004).</footnote>

<footnote n="165">¹⁶⁵A. Lee and L. S. Frost, "Interruption capability of gases and gas mixtures in a puffer-type interrupter," *IEEE Trans. Plasma Sci.* **8**(4), 362-367 (1980).</footnote>

<footnote n="166">¹⁶⁶K. Tomita, D. Gojima, K. Nagai, K. Uchino, R. Kamimae, Y. Tanaka, K. Suzuki, T. Iijima, T. Uchii, and T. Shinkai, "Thomson scattering diagnostics of decay processes of Ar/SF₆ gas-blast arcs confined by a nozzle," *J. Phys. D: Appl. Phys.* **46**(38), 382001 (2013).</footnote>

<footnote n="167">¹⁶⁷A. Gleizes, M. Razafinimanana, and S. Vacquie, "Transport coefficients in arc plasma of SF₆-N₂ mixtures," *J. Appl. Phys.* **54**(7), 3777-3787 (1983).</footnote>

<footnote n="168">¹⁶⁸B. Chervy, H. Riad, and A. Gleizes, "Calculation of the interruption capability of SF₆-CF₄ and SF₆-C₂F₆ mixtures. I. Plasma properties," *IEEE Trans. Plasma Sci.* **24**(1), 198-209 (1996).</footnote>

<footnote n="169">¹⁶⁹B. Chervy, J.-J. Gonzalez, and A. Gleizes, "Calculation of the interruption capability of SF₆-CF₄ and SF₆-C₂F₆ mixtures-Part II: Arc decay modeling," *IEEE Trans. Plasma Sci.* **24**(1), 210-217 (1996).</footnote>

<footnote n="170">¹⁷⁰W. Wang, M. Rong, Y. Wu, and J. D. Yan, "Fundamental properties of high-temperature SF₆ mixed with CO₂ as a replacement for SF₆ in high-voltage circuit breakers," *J. Phys. D: Appl. Phys.* **47**(25), 255201 (2014).</footnote>

<footnote n="171">¹⁷¹W. Wang, X. Tu, D. Mei, and M. Rong, "Dielectric breakdown properties of hot SF₆/He mixtures predicted from basic data," *Phys. Plasmas* **20**(11), 113503 (2013).</footnote>

<footnote n="172">¹⁷²R. L. Middleton and P. Eng, "Cold-weather application of gas mixture (SF₆/N₂, SF₆/CF₄) circuit breakers: A utility user's perspective," in 1st Conference on SF₆ and Environment: Emission and Reduction Strategies, November 2000.</footnote>

<footnote n="173">¹⁷³F. Yang, Z. Chen, Y. Wu, M. Rong, A. Guo, Z. Liu, and C. Wang, "Two-temperature transport coefficients of SF₆-N₂ plasma," *Phys. Plasmas* **22**(10), 103508 (2015).</footnote>

<footnote n="174">¹⁷⁴H. Zhao, X. Li, K. Zhu, Q. Wang, H. Lin, and X. Guo, "Study of the arc interruption performance of SF₆-CO₂ mixtures as a substitute for SF₆," *IEEE Trans. Dielectr. Electr. Insul.* **23**(5), 2657-2667 (2016).</footnote>

<footnote n="175">¹⁷⁵C. Pradayrol, A. M. Casanova, A. Hernoune, and J. Casanova, "Spark decomposition of and mixtures," *J. Phys. D: Appl. Phys.* **29**(7), 1941 (1996).</footnote>
<footnote n="176">¹⁷⁶A. M. Casanovas, I. Coll, C. Pradayrol, and J. Casanovas, "Influence of a solid insulator on the spark decomposition of SF₆ and 50%SF₆+50%CF₄ mixtures," *J. Phys. D: Appl. Phys.* **31**(20), 2835 (1998).</footnote>

<footnote n="177">¹⁷⁷C. Pradayrol, A. M. Casanovas, C. Aventin, and J. Casanovas, "Production of SO₂F₂, SOF₄, (SOF₂ + SF₄), S₂F₁₀, S₂OF₁₀ and S₂O₂F₁₀ in SF₆ and (50–50) SF₆–CF₄ mixtures exposed to negative coronas," *J. Phys. D: Appl. Phys.* **30**(9), 1356 (1997).</footnote>

<footnote n="178">¹⁷⁸I. Sauers, "By-product formation in spark breakdown of SF₆/O₂ mixtures," *Plasma Chem. Plasma Process.* **8**(2), 247–262 (1988).</footnote>

<footnote n="179">¹⁷⁹L. Vial, A. M. Casanovas, I. Coll, and J. Casanovas, "Decomposition products from negative and 50 Hz AC corona discharges in compressed SF₆ and SF₆/N₂ (10:90) mixtures. Effect of water vapour added to the gas," *J. Phys. D: Appl. Phys.* **32**(14), 1681 (1999).</footnote>

<footnote n="180">¹⁸⁰L. Vial, A. M. Casanovas, J. Diaz, I. Coll, and J. Casanovas, "Decomposition of high-pressure (400 kPa) SF₆ and SF₆/N₂ (10:90) mixtures submitted to negative or 50 Hz ac corona discharges in the presence of water vapour and/or oxygen," *J. Phys. D: Appl. Phys.* **34**(13), 2037 (2001).</footnote>

<footnote n="181">¹⁸¹I. Coll, A. M. Casanovas, L. Vial, A. Gleizes, and J. Casanovas, "Sparking-induced decomposition of 10% SF₆-90% N₂ mixtures: Effect of a solid organic insulator, oxygen and water," *J. Phys. D: Appl. Phys.* **33**(11), 1348 (2000).</footnote>

<footnote n="182">¹⁸²A. M. Casanovas and J. Casanovas, "Decomposition of high-pressure (400 kPa) SF₆-CO₂, SF₆-CO, SF₆-N₂-CO₂ and SF₆-N₂-CO mixtures under negative dc coronas," *J. Phys. D: Appl. Phys.* **38**(10), 1556 (2005).</footnote>

<footnote n="183">¹⁸³W. Liu, Z. Su, J. Qi, F. Zhu, Y. Zhao, F. Ma, X. Yuan, and Y. Chen, "Application and recycling of SF₆ gas mixtures in gas-insulated circuit breaker in northern China," *IOP Conf. Ser.: Earth Environ. Sci.* **69**(1), 012002 (2017).</footnote>

<footnote n="184">¹⁸⁴I. Cha, S. Lee, J. D. Lee, G.-w. Lee, and Y. Seo, "Separation of SF₆ from gas mixtures using gas hydrate formation," *Environ. Sci. Technol.* **44**(16), 6117–6122 (2010).</footnote>

<footnote n="185">¹⁸⁵L. Niemeyer, "Cigre guide for SF₆ gas mixtures: Application and handling in electric power equipment," in *Proceedings of the 1st Conference on SF₆ and Environment: Emission and Reduction Strategies* (CIGRE, 2000), pp. 2–3.</footnote>

<footnote n="186">¹⁸⁶T. Takuma, S. Hamada, and O. Yamamoto, "Application of a gas mixture with c-C₄F₈ in gas insulation," in *1999 Eleventh International Symposium on High Voltage Engineering* (IET, 1999), Vol. 3, pp. 197–200.</footnote>

<footnote n="187">¹⁸⁷J. Wada, G. Ueta, S. Okabe, and M. Hikita, "Dielectric properties of gas mixtures with per-fluorocarbon gas and gas with low liquefaction temperature," *IEEE Trans. Dielectr. Electr. Insul.* **23**(2), 838–847 (2016).</footnote>

<footnote n="188">¹⁸⁸S. Zhao, J. Jiao, X. Zhao, H. Zhang, D. Xiao, and J. D. Yan, "Synergistic effect of c-C₄F₈/N₂ gas mixtures in slightly non-uniform electric field under lightning impulse," in *2016 IEEE Electrical Insulation Conference (EIC)* (IEEE, 2016), pp. 531–534.</footnote>

<footnote n="189">¹⁸⁹L. Whitman, "Impulse voltage tests on air and C₃F₈," *IEEE Trans. Electr. Insul. EI-1*(2), 44–48 (1965).</footnote>

<footnote n="190">¹⁹⁰H. Okubo, T. Yamada, K. Hatta, N. Hayakawa, S. Yuasa, and S. Okabe, "Partial discharge and breakdown mechanisms in ultra-dilute SF₆ and PFC gases mixed with N₂ gas," *J. Phys. D: Appl. Phys.* **35**(21), 2760 (2002).</footnote>

<footnote n="191">¹⁹¹J. de Urquijo and E. Basurto, "Electron attachment, ionization and drift in c-C₄F₈," *J. Phys. D: Appl. Phys.* **34**(9), 1352 (2001).</footnote>

<footnote n="192">¹⁹²H. Itoh, T. Miyachi, M. Kawaguchi, Y. Nakao, and H. Tagashira, "Electron transport coefficients in SF₆ and c-C₄F₈ mixtures," *J. Phys. D: Appl. Phys.* **24**(3), 277 (1991).</footnote>

<footnote n="193">¹⁹³B.-T. Wu, D.-M. Xiao, Z.-S. Liu, L.-C. Zhang, and X.-L. Liu, "Analysis of insulation characteristics of c-C₄F₈ and N₂ gas mixtures by the Monte Carlo method," *J. Phys. D: Appl. Phys.* **39**(19), 4204 (2006).</footnote>

<footnote n="194">¹⁹⁴X. Liu, J. Wang, Y. Wang, Z. Zhang, and D. Xiao, "Analysis of the insulation characteristics of c-C₄F₈/CO₂ gas mixtures by the Monte Carlo method," *J. Phys. D: Appl. Phys.* **41**(1), 015206 (2007).</footnote>

<footnote n="195">¹⁹⁵X. Li, H. Zhao, S. Jia, and A. B. Murphy, "Prediction of the dielectric strength for c-C₄F₈ mixtures with CF₄, CO₂, N₂, O₂ and air by Boltzmann equation analysis," *J. Phys. D: Appl. Phys.* **47**(42), 425204 (2014).</footnote>

<footnote n="196">¹⁹⁶Y. Deng, B. Li, and D. Xiao, "Analysis of the insulation characteristics of C₃F₈ gas mixtures with N₂ and CO₂ using Boltzmann equation method," *IEEE Trans. Dielectr. Electr. Insul.* **22**(6), 3253–3259 (2015).</footnote>

<footnote n="197">¹⁹⁷Z. Xiaoling, J. Juntao, I. Bing, and X. Dengming, "The electronegativity analysis of c-C₄F₈ as a potential insulation substitute of SF₆," *Plasma Sci. Technol.* **18**(3), 292–298 (2016).</footnote>

<footnote n="198">¹⁹⁸M. Koch and C. M. Franck, "Partial discharges and breakdown in C₃F₈," *J. Phys. D: Appl. Phys.* **47**(40), 405203 (2014).</footnote>

<footnote n="199">¹⁹⁹M. Koch and C. M. Franck, "Prediction of partial discharge and breakdown voltages in CF₄ for arbitrary electrode geometries," *J. Phys. D: Appl. Phys.* **48**(5), 055207 (2015).</footnote>

<footnote n="200">²⁰⁰X. Wang, L. Zhong, J. Yan, A. Yang, G. Han, G. Han, and M. Rong, "Investigation of dielectric properties of cold C₃F₈ mixtures and hot C₃F₈ gas as Substitutes for SF₆," *Eur. Phys. J. D* **69**(10), 240 (2015).</footnote>

<footnote n="201">²⁰¹Y. Wu, W. Wang, M. Rong, L. Zhong, J. Spencer, and J. Yan, "Prediction of critical dielectric strength of hot CF₄ gas in the temperature range of 300-3500 K," *IEEE Trans. Dielectr. Electr. Insul.* **21**(1), 129–137 (2014).</footnote>

<footnote n="202">²⁰²V. A. Lisovskiy, V. A. Derevianko, and V. D. Yegorenkov, "DC breakdown in low-pressure CF₄," *J. Phys. D: Appl. Phys.* **48**(47), 475201 (2015).</footnote>

<footnote n="203">²⁰³W. Wang, M. Rong, Y. Wu, J. W. Spencer, and J. D. Yan, "Transport coefficients of high temperature CF₄, C₂F₆, and C₃F₈ as candidate of SF₆," in *2011 1st International Conference on Electric Power Equipment-Switching Technology* (IEEE, 2011), pp. 621–625.</footnote>

<footnote n="204">²⁰⁴W. Wang, Y. Wu, M. Rong, L. Éhn, and I. Černušák, "Theoretical computation of thermophysical properties of high-temperature F₂, CF₄, C₂F₆, C₂F₆, C₃F₆ and C₃F₈ plasmas," *J. Phys. D: Appl. Phys.* **45**(28), 285201 (2012).</footnote>

<footnote n="205">²⁰⁵L. Kang, N. Wenhao, L. Tao, Z. Guoqiang, and X. Haijiang, "Study of the decomposition of c-C₄F₈ and its mixture with N₂ under 50 Hz ac corona discharge," in *2013 Annual Report Conference on Electrical Insulation and Dielectric Phenomena* (IEEE, 2013), pp. 1229–1232.</footnote>

<footnote n="206">²⁰⁶S. Solomon, J. B. Burkholder, A. R. Ravishankara, and R. R. Garcia, "Ozone depletion and global warming potentials of CF₃I," *J. Geophys. Res.: Atmos.* **99**(D10), 20929–20935, https://doi.org/10.1029/94jd01833 (1994).</footnote>

<footnote n="207">²⁰⁷M. K. Donnelly, R. H. Harris, and J. C. Yang, *CF₃I Stability under Storage* (US Department of Commerce, Technical Administration, National Institute of Standards and Technology, 2004).</footnote>

<footnote n="208">²⁰⁸S. Xiao, X. Zhang, Y. Han, and Q. Dai, "AC breakdown characteristics of CF₃I/N₂ in a non-uniform electric field," *IEEE Trans. Dielectr. Electr. Insul.* **23**(5), 2649–2656 (2016).</footnote>

<footnote n="209">²⁰⁹X. Zhang, S. Xiao, Y. Han, and Q. Dai, "Analysis of the feasibility of CF₃I/CO₂ used in C-GIS by partial discharge inception voltages in positive half cycle and breakdown voltages," *IEEE Trans. Dielectr. Electr. Insul.* **22**(6), 3234–3243 (2015).</footnote>

<footnote n="210">²¹⁰H. Toyota, S. Matsuoka, and K. Hidaka, "Measurement of sparkover voltage and time lag characteristics in CF₃I-N₂ and CF₃I-air gas mixtures by using steep-front square voltage," *IEEJ Trans. Fundam. Mater.* **125**, 409–414 (2005).</footnote>

<footnote n="211">²¹¹Z. Yuan, Y. Tu, C. Wang, S. Wang, Y. Cheng, and X. Dong, "Vapor-liquid equilibria of (CF₃I+ CO₂) at the temperature of 243.150 K," in *2015 IEEE 11th International Conference on the Properties and Applications of Dielectric Materials (ICPADM)* (IEEE, 2015), pp. 660–663.</footnote>

<footnote n="212">²¹²M. S. Kamarudin, L. Chen, P. Widger, K. H. Elnaddab, M. Albano, H. Griffiths, and A. Haddad, "CF₃I gas and its mixtures: Potential for electrical insulation," in *Proceedings of the CIGRE Session (CIGRE, 2014)*, Vol. 45.</footnote>

<footnote n="213">²¹³T. Youping, L. Yan, W. Cong, L. Shan, and C. Yangchun, "Breakdown characteristics of CF₃I and CF₃I/N₂ gas mixtures in uniform field," in *2015 IEEE 11th International Conference on the Properties and Applications of Dielectric Materials (ICPADM)* (IEEE, 2015), pp. 520–523.</footnote>

<footnote n="214">²¹⁴P. Widger, A. Haddad, and H. Griffiths, "Breakdown performance of vacuum circuit breakers using alternative CF₃I-CO₂ insulation gas mixture," *IEEE Trans. Dielectr. Electr. Insul.* **23**(1), 14–21 (2016).</footnote>

<footnote n="215">²¹⁵L. Chen, P. Widger, M. S. Kamarudin, H. Griffiths, and A. Haddad, "Potential of CF₃I gas mixture as an insulation medium in gas-insulated equipment," in *2015 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP)* (IEEE, 2015), pp. 868–871.</footnote>

<footnote n="216">²¹⁶L. Chen, P. Widger, M. S. Kamarudin, H. Griffiths, and A. Haddad, "CF₃I gas mixtures: Breakdown characteristics and potential for electrical insulation," *IEEE Trans. Power Delivery* **32**(2), 1089–1097 (2017).</footnote>