Latest Development Trends of SF6 Alternative Gas-Based High-Voltage Circuit Breakers

1. Introduction
SF₆ is widely used in electric power transmission and distribution systems, such as gas-insulated switchgear (GIS), circuit breakers (CB), and medium-voltage (MV) load switches. It possesses unique electrical insulation and arc-quenching capabilities. However, SF₆ is also a potent greenhouse gas, with a global warming potential of approximately 23,500 over a 100-year time horizon, and therefore its use is regulated and subject to ongoing discussions regarding restrictions. Consequently, research into alternative gases for power applications has been conducted for about two decades.

The "Club Zéro" (CZC), in cooperation with CIGRE, recently launched an initiative to assess the state-of-the-art of SF₆ alternative gases for switching applications. A survey was carried out to collect all available recent literature on this topic. The results were presented and discussed at a joint session during the CIGRE Session in 2016. This paper presents the main findings of that survey. Since vacuum switching technology constitutes a separate ongoing activity, it will not be covered in this review.

2. Alternative Gases

Following the adoption of the Kyoto Protocol in 1997, research into alternative gases intensified and has further increased over the past decade. Key requirements for alternative gases have been identified as: low global warming potential (GWP), zero ozone depletion potential (ODP), low toxicity, non-flammability, high dielectric strength, high arc-quenching and heat dissipation capability, chemical stability, material compatibility, and market availability.

Among various naturally sourced gases investigated, CO₂ has proven to be the most promising arc-quenching gas, with its performance potentially enhanced by additives such as O₂ or CF₄. However, studies have shown that both the interrupting and insulating performances of CO₂ are inferior to those of SF₆. Other interesting candidates have been identified among fluorinated gases, such as CF₃I, hydrofluoroolefins (HFO-1234ze and HFO-1234yf), perfluoroketones (e.g., C₅F₁₀O), perfluoronitriles (C₄F₇N), fluorinated ethers (HFE-245cb2), fluorinated epoxides, and hydrochlorofluoroolefins (HCFO-1233zd).

Considering all requirements, the most promising current candidates are C₅ perfluoroketone (CF₃C(O)CF(CF₃)₂ or C₅-PFK) and iso-C₄ perfluoronitrile ((CF₃)₂CF-CN or C₄-PFN). For pure gases, dielectric performance is proportional to boiling point—i.e., gases with high dielectric strength typically also have high boiling points. At 0.1 MPa, the boiling points of C₅-PFK and C₄-PFN are 26.5°C and –4.7°C, respectively. Therefore, for switching equipment applications requiring sufficiently low boiling points to meet low-temperature operational demands, buffer gases must be added. Due to its good arc-quenching capability, CO₂ is selected as the buffer gas in high-voltage applications. In medium-voltage applications, air has also been reported as a buffer gas used in combination with C₅-PFK for insulation purposes.

3. Properties of Pure and Gas Mixtures

Table 1 presents the properties of selected alternative gases relative to SF₆. The GWPs of these gases vary significantly: C₄-PFN exhibits a much higher GWP than CO₂ or C₅-PFK, both of which have GWPs of approximately 1. All candidate gases of interest are non-flammable, have zero ODP, and are reported as non-toxic according to technical and safety data sheets provided by chemical manufacturers. The dielectric strength of pure C₄-PFN and C₅-PFK is nearly twice that of SF₆. The dielectric withstand voltage of CO₂ is comparable to that of air—that is, significantly lower than that of SF₆.

Table 1: Comparison of Pure Gas Properties with SF₆

Table 2 shows the characteristics of gases and gas mixtures when used in switchgear. The concentrations of C₄-PFN and C₅-PFK in mixtures with buffer gases are given in the second column, typically below 13% (molar concentration). It should be noted that for the use of C₅-PFK in CO₂, oxygen additives have also been reported, as the presence of oxygen can reduce the formation of harmful by-products (such as CO) and solid by-products (such as soot).

Table 2: Characteristics/Performance of Pure Gases and Gas Mixtures in Medium- and High-Voltage Switchgear Applications

Due to the reduced dielectric withstand voltage of the mixtures compared to SF₆ at the same pressure (Column 6), the minimum operating pressure for C₅-PFK and C₄-PFN with CO₂ as the buffer gas in high-voltage applications needs to be increased to approximately 0.7–0.8 MPa. For medium-voltage applications using air/C₅-PFK mixtures, a pressure of 0.13 MPa can be maintained, achieving a dielectric withstand voltage close to that of SF₆.

The high dielectric withstand voltage achieved with relatively low blending ratios of C₄-PFN or C₅-PFK can be explained by a synergistic effect—i.e., the dielectric strength increases nonlinearly with the additive concentration, a phenomenon previously observed in SF₆/N₂ mixtures. The GWP of C₅-PFK mixtures is negligible, but this comes at the cost of a higher minimum operating temperature. Low-temperature applications (e.g., –25°C) can be addressed using either pure CO₂ or CO₂ + C₄-PFN mixtures, albeit with trade-offs: significantly reduced dielectric withstand voltage in the case of pure CO₂, or a substantially higher GWP when using C₄-PFN mixtures.

4. Switching Performance of Alternative Gases
Table 3 compiles preliminary information on the switching performance of pure CO₂ and CO₂-based mixtures, with SF₆ performance provided for comparison. By increasing the operating pressure relative to SF₆, the cold dielectric strength—used, for example, as a metric for capacitive switching performance—can be brought up to the level of SF₆.

Table 3: Comparison of Switching Performance of Gases and Gas Mixtures at Elevated Operating Pressures versus SF₆ in High-Voltage Applications

In the reviewed literature, only qualitative statements regarding the switching performance of C₄-PFN and C₅-PFK mixtures could be found. For CO₂, some quantitative comparisons are available. Generally speaking, with pure CO₂ at an increased filling pressure of approximately 1 MPa, insulation and short-line fault (SLF) interrupting performance of about two-thirds that of SF₆ can be expected.

By adding O₂ to CO₂ (with mixing ratios up to 30%), an improvement in SLF interrupting performance and a slight increase in dielectric strength can be anticipated. Adding C₄-PFN or C₅-PFK to CO₂ enables dielectric performance approaching that of SF₆. Studies report that the SLF switching performance of CO₂/O₂/C₅-PFK mixtures is about 20% lower than that of SF₆. In contrast, circuit breakers specifically adapted for CO₂/C₄-PFN mixtures have been claimed to achieve SLF performance comparable to SF₆.

However, there are also studies directly comparing pure CO₂ with CO₂/C₄-PFN and CO₂/C₅-PFK mixtures under identical geometry and pressure conditions, which show similar near-zone (thermal) interrupting performance for CO₂ with or without additives. With minor design modifications or modest derating, the new mixtures have successfully passed IEC test duties L90 (SLF) and T100 (100% terminal fault), indicating that their switching performance is not significantly inferior to SF₆. This has also been demonstrated for the breaker’s interrupting function.

Further improvements in switching performance through dedicated design optimizations are expected in the future. An important issue is the toxicity of gases after arcing. C₅-PFK and C₄-PFN are complex molecules that begin to decompose above approximately 650 °C in the case of C₄-PFN. Upon decomposition, these molecules do not recombine into their original structures but form smaller fragments. A decomposition rate of 0.5 mol/MJ has been reported for CO₂/O₂/C₅-PFK mixtures under high-current interruption. For partial discharges, the decomposition rate was observed to be more than one order of magnitude lower than the above value.

The decomposition behavior of these new gases is not directly comparable to that of SF₆, which decomposes primarily due to chemical reactions with ablated contact and nozzle materials. For the new gases, decomposition over the equipment lifetime is not considered a critical issue, but gas concentration within the equipment should be monitored or periodically checked. The most toxic decomposition products in high-pressure applications (i.e., mixtures with CO₂) are CO and HF. Arc by-products of these mixtures are considered to have toxicity similar to or lower than that of arc-decomposed SF₆. Therefore, handling procedures similar to those used for arc-exposed SF₆ are recommended.

However, it must be noted that the above statements are based on limited knowledge of the toxicity of these new gases. More experience is needed regarding the post-arc toxicity of potential SF₆ alternatives. Other reported concerns include material compatibility (e.g., effects on seals and greases), gas sealing integrity, and gas handling procedures. Consequently, existing high-voltage equipment should not be expected to operate safely with these new gases without appropriate design or material modifications.

Internal arc tests have been performed with all mixtures, and no serious issues have been reported. The thermal conductivity of the mixtures is slightly inferior to that of SF₆, which may necessitate moderate derating or design adjustments for current-carrying capacity. CO₂ live-tank circuit breakers have already gained field experience, with deployments beginning several years ago, and CO₂-filled breakers are now commercially available.

High- and medium-voltage pilot installations using C₅-PFK mixtures have been operating successfully in Switzerland and Germany since 2015. Pilot projects using CO₂/C₄-PFN mixtures are planned or underway in several European countries, including a 145 kV indoor GIS in Switzerland, a 245 kV outdoor current transformer in Germany, and outdoor 420 kV GIL systems in the UK and Scotland.

5. Conclusions and Outlook
Published information on SF₆ alternative gases for switching applications has been reviewed. At the current stage, this research is still in its early phases and far less extensive than the decades-long body of work on SF₆. Available manufacturer data indicate that new gases—such as C₅-PFK and C₄-PFN—are viable options that, when blended with CO₂ as a buffer gas, can partially match SF₆ performance, though they may not fully replicate all of SF₆’s capabilities.

Key differences lie in insulation and interruption performance, as well as boiling point—which determines the minimum specified operating temperature of the switchgear. A low minimum operating temperature (e.g., –50 °C) can be achieved with pure CO₂. However, CO₂ appears to exhibit generally lower interruption performance, particularly in terms of recovery voltage peak withstand and interruption capability, compared to gas mixtures containing C₄-PFN or C₅-PFK.

An advantage of CO₂/C₅-PFK mixtures over CO₂/C₄-PFN mixtures is their negligible GWP (~1 vs. 427/600 for C₄-PFN). Conversely, CO₂/C₄-PFN mixtures offer a lower minimum operating temperature (approximately –25 °C) compared to CO₂/C₅-PFK mixtures (approximately –5 °C).

6. 40.5kV 72.5kV 145kV 170kV 245kV Dead tank Vacuum Circuit-Breaker

Description ：

The 40.5kV, 72.5kV, 145kV, 170kV, and 245kV Dead tank Vacuum Circuit-Breakers are essential protective devices for high-voltage power systems. Employing vacuum as the arc-extinguishing and insulating medium, they boast exceptional arc-quenching capabilities, swiftly interrupting fault currents and effectively preventing arc re-ignition to ensure stable power system operation. The dead tank design offers a compact footprint and robust mechanical stability, facilitating installation and maintenance. Equipped with highly reliable spring operating mechanisms, these circuit breakers have a mechanical lifespan exceeding 10,000 operations, delivering rapid and precise responses. With outstanding environmental adaptability, they can operate stably under harsh outdoor conditions. Widely applied in substations, transmission lines, and other scenarios, they provide efficient and secure power switching control and reliable protection across various voltage levels.

Main function introduction:

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