Studies on Different Dielectric Properties of Different Insulating Gases and Gas Mixtures as an Alternative to SF6 -Review

Studies on Different Dielectric Properties of Different Insulating Gases and Gas Mixtures as an Alternative to SF₆ -Review

Sowmya K. R., Dr. Ravi Prasad D.

Electrical and Electronics Engineering, Sri Siddhartha institute of technology, Tumkur- 572105

DOI: https://doi.org/10.51584/IJRIAS.2025.10060079

Received: 25 June 2025; Accepted: 29 June 2025; Published: 12 July 2025

ABSTRACT

SF₆ gas is considered as one of the most noxious kinds of atmospheric greenhouse gases with global warming potential (GWP) 23,500[10] times higher than that of CO₂ and shelf life of 3200 years in the atmosphere. With threat of global climate change, policy makers all over the world have made regulations which promote renewable energy technologies. Developing countries including India are increasingly emphasising the need to rapidly restructuring of their energy system. A replacement for SF₆ has been in testing and development for over a decade and have come up with different combination of gas and gas mixtures like N₂, CO₂, CF₃I (Trifluoroiodo-methane), C₄-fluoronitrile and C₅-Fluoroketones along with different combination of the later with SF₆. In recent years many gases are being investigated for breakdown voltage and partial discharge characteristics, such as C₅F₁₀O (C5 per uorinatedketone), C₅F₁₀O/N₂ mixture, SF₆ and CO₂ gas mixture, Pd₃-TiO₂(101) (Titanium dioxide), C₄F₇N/CO₂ mixed gases (Fluro nitrile and carbon dioxide), HFO-butene/ CO₂ gas mixture (Hydrofluroolefin).

In the present paper an attempt is made to compare different potential alternatives for SF₆ along with their dielectric properties.

Keywords: CF₃I (Trifluoroiodo-methane), C₄-fluoronitrile and C₅-Fluoroketones, C₅F₁₀O (C5 per uorinatedketone), C₄F₇N/CO₂ mixed gases (Fluro nitrile and carbon dioxide), HFO-butene/ CO₂ gas mixture (Hydrofluroolefin).

INTRODUCTION

The environmentalist and power engineers have sternly considered the benefaction of SF₆ gas to depletion of ozone and the global greenhouse effect. In spite of its good electrical insulation properties, questions regarding use of SF₆ gas. When this gas is subjected to electrical discharges, is believed to form highly toxic and corrosive compounds. SF₆ and its disintegration products generate CuF_2, AIF₃ and other noxious substances, when they react with copper,   aluminium and other materials of metallic composition, there by affecting the properties of metallic materials. Due to corona discharge and spark discharge, SF₆ emitting SF₄ gas. This SF₄ gas reacts with O₂ to form SOF, SOF₂,SO₂F₂ other harmful substances in electrical apparatus.

SF₆ is an electronegative gas and it has dielectric strength three times that of air, the outstanding properties of SF₆ have resulted in its extensive use as an insulating gas in high voltage equipment. On the other hand, it is a highly potent greenhouse gas due to its high global warming potential,_. Alternative insulating gases to replace SF₆ have been investigated in recent decades. As the insulating gas of Gas Insulated Switchgear (GIS), SF₆/N₂ gas mixture can not only reduce the consumption of SF6 in power system, but also effectively alleviate SF6 greenhouse effect and solve the problem of high liquefaction temperature[10].

The research, so far, into alternative gases has shown that CF₃I and its gas mixtures have promising dielectric properties comparable to those of SF₆. It was found that, for various gap geometries (rod plane and plane-plane electrodes) and lengths, CF₃I mixtures exhibit promising breakdown characteristics comparable to those of SF6 gas based on the measured 50% breakdown voltage (U50). These encouraging results led to a trial of CF₃I as the insulation gas on practical 11 kV low-current switches and circuit breakers[2].

(HFO-butene) is regarded as a promising eco-friendly insulating gas to replace the sulfur hexafluoride (SF₆) used in medium-voltage gas insulated equipment (MV-GIE)[8].

The Pd₃-TiO₂(101) surface exhibits high gas sensitivity and selectivity to SOF2 and SO2F2 molecules due to decrease in the conductivity of the material. The most stable configuration for Pd cluster on TiO₂ (101) surface is triangular structure[5].

Comparison of Different Insulating Gases

Comparison of SF₆/N₂ mixture[3]

SF₆/N₂ gas mixture can not only reduce the consumption of SF₆ in power system, but also effectively alleviate SF6 greenhouse effect. Since SF₆/N₂ can decompose under the alternating current partial discharge (PD), its decomposition characteristics are closely related to PD attributes. Therefore, the fault diagnosis method can be established through its PD decomposition characteristics. SF₆/N₂ gas mixture under PD can decompose to SOF2, SO2, SO2F2, SOF4 CF4, CO2, NO, NO2 and NF3. The gas production laws of these decomposed components are quite different under different PD intensity. The content of NO₂ and (SO2F2 SOF2 SOF4 SO2) increases linearly with discharge quantity, which can be used as the characteristic quantity to judge the whole process of PD. In short-term discharge, the content of SO₂F₂ increases greatly with time, which can be used as the characteristic quantity to judge the early PD. conventional GIS, GIS using SF6/N2 gas mixture will inevitably be damaged during manufacturing, transportation, installation and operation, resulting in some internal insulation defects. These insulation defects will deteriorate gradually in the long-term operation of GIS, and when they reach a certain degree, they will induce partial discharge (PD) in the equipment.

Comparison of conducting particles in N₂:SF₆

From the above graphs it can be seen that copper particle at 10 mm gap distance has a higher inception voltage in N₂:SF₆ gas mixtures up to 0.2MPa. Higher the PD inception voltage, better is the performance of the insulating media.

Comparison of SF6/CO₂ mixture[10]

Though CO₂ is no match for SF₆, the later is being investigated with other elements, as an alternative with other gas mixtures because of high global warming potential of SF₆ gas. The particle contamination in CO₂ are the major cause for the partial discharges which in turn cause the insulation failure. The particle contamination inside the GIS may occur because of the manufacturing process, from mechanical vibrations, moving parts of the system such as breakers. It can also be from the negligence during the maintenance inside the GIS or from corrosion or decomposition of the metallic products. The study of PD characteristics for different gas pressure and different particle contaminants can give a real picture of the dielectric strength of insulating medium used in GIS as an alternative for SF_6. The study of PD characteristics for air at different pressures and particles can be taken as reference and compared with other gases like CO₂ and its mixtures.

Comparison of conducting particles in CO₂:SF₆

From the above graphs it can be seen that copper particle at 10 mm gap distance has a lower PD inception voltage in air and CO₂:SF_6.  Higher the PD inception voltage, better is the performance of the insulating media.

Behaviour of 3 gases

The Fig.2.2.2 shows the behaviour of the three gas insulating media in the presence of Aluminium particles at different pressures at 10 mm gap spacing. The CO₂: SF₆ gas mixture has the better performance with respect to PD inception voltage even bettering N₂: SF₆ gas mixture of the same proportion.

Comparison of SF₆ and CF₃I mixture[2]

CF₃I mixture and its potential to replace SF₆in high voltage equipment. 50% breakdown tests conducted on three electrode configurations (rod-plane, plane-plane and coaxial) were used to characterise 30:70% mixture of CF₃I -CO₂. The breakdown strength of the mixture for coaxial electrode was more than two times higher than air. In comparison, breakdown strength of pure SF₆ is about three times higher than air. The insulation capability makes CF₃I a feasible alternative to SF₆ in a GIL system where arc quenching is not required.

Partial pressure of CF₃I in the mixture is selected by a trade-off between three basic factors; boiling point of the gas mixture, insulation strength, and the by-products of the gas mixture upon each electrical discharge.

Saturation Vapour Pressure

Figure 2.3.1.1 : Saturation vapour pressure curve of SF6, CF3I, CO2 and 30:70% CF3I-CO2 mixture.

Typically, in a GIL system, SF6 gas is pressurised at 0.7 MPa. It can be seen from Figure 2.3.1.1 that the boiling point of CF₃I at 0.7 MPa is 38°C, an indication that a buffer gas such as carbon dioxide (CO₂) needs to be added to CF₃I in order to reduce the boiling temperature.

Ionisation Coefficients

Effective ionisation coefficients of different gases and gas mixtures were computed using Bolsig+ software which applies the two-term approximation of Boltzmann equation [4]. Figure 2 shows the 1 2 pressure-reduced ionisation coefficient (α – η) / p as a function of E / p for different pure gases and CF₃I mixtures. It can be seen from Figure 2 that the critical reduced field strength at which (α – η) = 0 for CF₃I is 108 kV/cm bar compared to 89 kV/cm bar in SF6 [5]. which indicate that pure CF₃I has a dielectric strength of around 1.2 times higher than that SF6.

Figure 2.3.2.1: Effective ionisation coefficients in pure gases (Air, SF6, CF3I and CO2) and CF3I-CO2 mixtures (10%-90%, 20%-80% and 30:70%)

Figure 2.3.2.1 shows that 30:70% mixture ratio has a higher reduced field strength E/p compared to CF₃I CO₂ mixtures with low CF₃I contents. The 30:70% mixture ratio is considered to be most appropriate for gas-insulated switchgear (GIS) applications. the interruption capability of CF₃I -CO₂ mixtures is far superior to that of CF3I-N2 mixtures. With only 30% of CF₃I in the CF₃I -CO₂ mixture, the insulation performance was reported to be approximately 0.75 to 0.80 times that of SF6. The 30:70% mixture ratio, therefore, offers a reasonably high dielectric strength while been able to sustain its gaseous form at 0.7 MPa with a boiling temperature of mixture of around –4°C. Furthermore, by-products produced during arcing such as iodine can be reduced substantially using 30:70% mixture, It is important to minimise iodine deposition as it can compromise CF₃I insulation performance.

Comparison of C₅F₁₀ O/N₂ mixture[1]

It is found that the AC breakdown voltage of the C₅F₁₀O gas mixtures increases gradually with both the gas pressure and the content of C₅F₁₀O. As the partial pressure of C₅F₁₀O increases, the relative insulation strength increasing trend of the C₅F₁₀O gas mixtures becomes less obvious with the increase in the gas pressure. The AC breakdown voltage of C₅F₁₀O /Air gas mixture is higher than that of C₅F₁₀O /N2 gas mixture under the same conditions. The breakdown voltage of C₅F₁₀O /Air gas mixture is less affected by gas pressure. When the partial voltage of C₅F₁₀O is greater than 15kPa, the breakdown voltage of C₅F₁₀O /Air gas mixture increases with the partial voltage of C₅F₁₀O at a rate similar to that of C₅F₁₀O /N2 gas mixture.

F, Figure 2.4.1: Relation between breakdown voltage and gas pressure of C₅F₁₀O /N₂ gas mixture.

Figure shows the Relation between breakdown voltage and gas pressure of C₅F₁₀O /N₂ gas mixture. When the gas pressure is in the range of 0.1-0.4MPa, the breakdown voltage of each gas mixture increases linearly with the gas pressure. While the increase rate of the AC breakdown voltage of C₅F₁₀O gas mixtures and SF6 are reduced, showing a certain saturation trend at gas pressures higher than 0.4MPa.

Comparison of C₄F₇N /CO₂ mixture[6]

Research indicates that adding an appropriate amount of O₂ to a C₄F₇N /CO₂ mixture to form a ternary gas mixture helps suppresdeep decomposition and the formation of solid byproducts. Therefore, a thorough investigation into the electrical performance of such mixtures is essential the application of the C₄F₇N /CO₂/O₂ ternary mixture warrants further attention to the specific properties of the gas-solid interface. Additionally, due to the relatively lower stability of C₄F₇N compared to SF₆, ionization and decomposition may occur under discharge or overheating conditions.

The addition of O₂ can enhance the insulation performance and chemical stability of C₄F₇N/CO₂ mixed gases during partial discharge to a certain extent. However, the influence of O2 on the surface flashover characteristics at gas-solid interfaces, as well as the compatibility and interaction mechanisms between gaseous and solid materials, remain poorly understood. The surface flashover characteristics of epoxy resin in C₄F₇N/CO₂ /O₂ ternary mixed gases, exploring the differences in solid surface ablation characteristics under various conditions of gas pressure, C₄F₇N mixture ratio, and O2 concentration. The peak current of the flashover discharge channel increases significantly with higher C₄F₇N concentration and gas pressure, while the peak current remains relatively stable across different O₂ concentrations.

Fig 2.5.1: Variation of flashover voltage of mixed gas with O2 concentration.

Figure 2.5.1 illustrates the variation in flashover voltage with oxygen con centration in C₄F₇N/CO₂ /O₂ ternary mixtures at pressures of 0.1 MPa and 0.2 MPa, under a slightly non-uniform electric field. At a pressure of 0.1 MPa, the oxygen concentration significantly affects the flashover voltage of the gas mixture. As the oxygen content increases, the flashover voltage generally decreases. When the C₄F₇N concentration is 20%, the reduction in flashover voltage is most pronounced with increasing oxygen content.

Fig: 2.5.2 Variation of flashover voltage of mixed gas with C₄F₇N concentration.

Figure 2.5.2 illustrates the variation in flashover voltage with C₄F₇N concentration under different oxygen contents. It is evident that the flashover voltage of the ternary gas mixture increases overall with the rise in C₄F₇N concentration. At lower pressures, the increase in flashover voltage with C₄F₇N concentration is relatively gradual.

Comparison of HFO(E)/CO₂ gas mixture[8]

HFO(E)/CO₂ gas mixture exhibited a linear-saturation rising pattern with both mixing ratio and gas pressure, mirroring the trend observed in SF₆ when the HFO(E) gas concentration ranged between 25% and 30%. The HFO-butene/ CO₂ with low HFO-butene content and gas pressure, which is ascribed to the shielding effect of the stable “space charge layer” nearby the needle electrode that improves the non-uniformity of the electric field. The corresponding results revealed the PD characteristics of HFO-butene/ CO₂ gas mixture. The HFO(E) and CO₂ gas mixture have equivalent break down and PD insulation strength to SF₆ at a pressure range of 0.15 MPa. Therefore, the findings of this research conclude that the HFO(E)- CO₂ gas mixture, more precisely (30/70)% ratio, can successfully substitute SF₆ gas in medium voltage gas insulated switchgear applications. The partial discharge characteristics were conducted using the UHF method for PD inception and extinction voltages.

Fig 2.6.1: PDIV and PDEV of HFO(E) and CO2 gas mixture.

An important parameter known as partial discharge extinguishing voltage PDEV defines the difficulty of gas mixture PD extinction. PDEV is typically less than PDIV; the higher PDEV value of the gas mixture indicates the challenges associated with PD extinguishing. In conjunction with SF₆ gas, Figure shows the HFO(E)andCO2 PDIV and PDEV properties for various mixing ratios at different pressure levels. It has been noted that the SF₆ PDIV and PDEV difference between 0.1 and 0.3 MPa is nearly constant at 3 kV. However, the difference between HFO(E) and CO₂ obtained for 10 % HFO(E) is averaged at 2.4 kV; for 20%, it is 2.2 kV, and for 30%, the difference is 2.8 kV. It is important to note that the difference between the gas mixture’s PDIV and PDEV is not significantly affected by changing the mixing ratio of HFO(E) content.

Comparison of Pd₃-TiO₂(101)[5]

Comparing with the intrinsic anatase TiO₂ (101) surface, the energy gap of the Pd3-dopedanatase TiO₂ (101) surface decreases dramatically, signifying an increase of conductivity. Based on the conductivity change of adsorption structures, Pd3- TiO₂ (101) can effectively distinguish the type and concentration of SOF2 and SO2F2. Therefore, the Pd3-doped TiO₂ (101) surface can be applied in the development of sensing material for gas detection.

CONCLUSION

• Firstly, SF6/N2 gas mixture can not only reduce the consumption of SF6 in power system, but also effectively alleviate SF6 greenhouse effect and solve the problem of high liquefaction temperature.
• While studying SF₆ / CO₂ the particle contamination in CO₂ are the major cause for the partial discharges which in turn cause the insulation failure. Because CO₂-SF₆ gas mixture was not good in arc interruption. As the voltages were increased near breakdown the arc persisted in all the experiments conducted. Therefore CO₂-SF₆ gas mixture may not be feasible to be used in circuit breakers which needs better arc interruption properties.
• The research, so far, into alternative gases has shown that CF3I mixtures exhibit promising breakdown characteristics comparable to those of SF6 gas based on the measured 50% breakdown voltage.
• As the partial pressure of C₅F₁₀O increases, the relative insulation strength increasing trend of the C₅F₁₀O gas mixtures. The AC breakdown voltage of C₅F₁₀O /Air gas mixture is higher than that of C₅F₁₀O /N2 gas mixture under the same conditions. C₅F₁₀O has a high liquefaction temperature (26.9C) at normal pressure, its dielectric strength reaches twice that of SF6. The Global Warming Potential (GWP) value of C₅F₁₀O is only 1 and its atmospheric lifetime is about 15 days.C₅F₁₀O (C5 per uorinatedketone) has received extensive attention due to its great insulation and eco-friendly performance, which has the potential to replace SF6 usage in power industries.
• Due to the relatively lower stability of C₄F₇N compared to SF6, ionization and decomposition may occur under discharge or overheating conditions. C₄F₇N has emerged as one of the most promising eco-friendly insulating gases. The power frequency breakdown voltage of pure C₄F₇N is approximately twice that of SF6, while its GWP is 2100, significantly lower than that of SF6. In practical applications, to reduce the liquefaction temperature of C₄F₇N, it is often mixed with buffer gases such as N₂, CO₂, or dry air, which further reduces the GWP of the gas mixture.
• This paper conclude that the HFO(E)-CO₂ gas mixture, more precisely (30/70)% ratio, can successfully substitute SF gas in medium voltage gas insulated switchgear applications. The HFO-butene/ CO₂ with low HFO-butene content and gas pressure, which is ascribed to the shielding effect of the stable “space charge layer” nearby the needle electrode that improves the non-uniformity of the electric field.
• Due to the high specific surface area, high symmetry, electronic properties, and other outstanding advantages of TiO₂ nanotubes, it has become a research hotspot as gas detection material, and shows broad application prospect. Noble metal doping on TiO₂ nanotubes surface has proven to be able to narrow the energy gap, and enhance the gas-sensing ability to specific gas molecules

By studying different dielectric properties of different insulating gases and gas mixtures as an alternative to SF_6. All the gases exhibit some partial discharge characteristics in a good manner some dielectrics having poor properties to exhibit partial discharge characteristics.

ACKNOWLEDGMENT

I want to acknowledge the invaluable assistance from the Ph.D guide Dr. RAVI PRASAD D sir, internet resources and references that enriched my work’s quality and depth. These resources broadened my understanding and added significant value to my paper.

REFERENCES

1. YUE ZHANG1, XIAOXING ZHANG GUOZHI ZHANG 1,2, YI LI 1,YALONG LI1, QI CHEN1, SONGXIAO1, AND JU TANG1 “AC Breakdown and Decomposition Characteristics of Environmental Friendly Gas C5F10O/Air and C5F10O/N2” IEEE, June 19, 2019.
2. M. S. KAMARUDIN, L. CHEN*, P. WIDGER, K. H. ELNADDAB, M. ALBANO, H. GRIFFITHS AND A. HADDAD, Advance High Voltage Engineering Research Centre, Cardiff University United Kingdom “CF3I Gas and Its Mixtures: Potential for Electrical Insulation”, IEEE 2014.
3. CHEN LI 1, JUTANG1, (Member, IEEE), ZHIQIANG ZHAO1, HAOTIAN LI1, YULONG MIAO2, AND FUPING ZENG 1,(Associate Member, IEEE) “Decomposition Characteristics of SF6/N2 Under Partial Discharge of Different Degrees”, IEEE November 2, 2020.
4. FENG-CHANG GU Department of Electrical Engineering, National Chin-Yi University of Technology, Taichung 41107, Taiwan, Identification of Partial Discharge Defects in Gas-Insulated Switchgears by Using a Deep Learning Method”, IEEE September 18, 2020.
5. LINGNA XU1, WENLONG CHEN1, YINGANG GUI 1, ANDQIAOMEI LI2 1College of Engineering and Technology, Southwest University, Chongqing 400715, China “Influence of Pd Clusters Doping on Gas Sensing Properties of TiO2(101) Nanotubes to SF6 Decomposition Products”, IEEE November 23, 2020.
6. DONGWEI SUN1, FENG WANG LIPENG ZHONG 2,3, (Senior Member, IEEE), KAIBIN LIANG 2,(Member, IEEE), HENG YI2, NIAN TANG1, AND ZHI LI1 1Electric Power Research Institute, Guangdong Power Grid Company Ltd., Guangzhou 510080, China “Investigating the Influence of O2 on Surface Flashover Characteristics and Gas-Solid Interface Compatibility in C4F7N/CO2 Mixtures Under Negative DC Voltage”, IEEE 20 March 2025.
7. AHMADDARWISH 1,SHADYS. REFAAT2, HAMID A. TOLIYAT 1,ANDHAITHAMABU-RUB 1Electrical Engineering Department, Texas A&M University, College Station, TX 77843, USA “On the Electromagnetic Wave Behavior Due to Partial Discharge in Gas Insulated Switchgears: State-of-Art Review”, IEEE June 24, 2019.
8. RIZWAN AHMED ZAHID ULLAH 1,RAHISHAM ABD-RAHMAN 1,(Member, IEEE), 2,(Graduate Student Member, IEEE), RAHMAT ULLAH3, MOHD FAIROUZ MOHD YOUSOF AND KALEEM ULLAH 4 1,(Member, IEEE), 1Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, Batu Pahat 86400, Malaysia “Partial Discharge Characterization of HFO(E) Gas Using Ultra-High Frequency (UHF) Antenna for Medium Voltage Switchgear Application”, IEEE 17 June 2024.
9. AMMARSALAH MAHDI, ZULKURNAIN ABDUL-MALEK AND RAI NAVEED ARSHAD, (Senior Member, IEEE), Institute of High Voltage and High Current, School of Electrical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru 81310, Malaysia “SF6 Decomposed Component Analysis for Partial Discharge Diagnosis in GIS: A Review”, IEEE March 15, 2022.
10. “Particle contamination and its effect on the PD characteristics of 90:10 CO₂ and 10SF₆ gas mixture” is published in vol.2 issue 2,pp.39 to 54,ISSN:2582-9890,SSAHE-journal of interdisciplinary research by: Ravi prasad D, B. Rajesh