Alternatives to SF6 Gas: What are the Available Options?

The Future of SF₆ Gas

Alternatives to SF₆ gas have made great strides in the last few years. SF₆ gas has been the preferred gas in electrical switchgear for over 50 years due to its dielectric strength, arc quenching capability, and thermal stability. The dielectric strength of SF₆ increases with pressure, therefore smaller circuit breakers can be used at higher voltages allowing a reduced overall footprint compared to other insulating mediums. Additionally, sulfur hexafluoride is non-toxic and self-healing gas. It takes an immense amount of heat to break the bonds between the molecules, and once the heat source is removed the atoms will recombine immediately.  

Unfortunately, SF₆ gas is also the most potent greenhouse gas known to man. It has a global warming potential 23,500 times greater than CO₂. The Environmental Protection Agency estimates that the Electrical Industry is responsible for 80% of SF₆ emissions. Despite an increase in awareness and implementation of state and federal emission limits and regulations on SF₆ gas, the proportion of SF₆ in the atmosphere has risen continuously worldwide since the year 2000 (see Figure 1 below).

There are many ways that SF₆ gas users can reduce or eliminate SF₆ emissions. Training, proper SF₆ gas handling procedures, and utilizing emission-free gas handling equipment greatly reduces the incidence of SF₆ emissions. However, due to the GWP of SF₆, a need for alternative solutions has been recognized in the industry. Advances in technology and years invested in R&D have allowed Original Equipment Manufacturers to start producing switchgear that operate with various alternatives to SF₆ Gas.

The move away from SF₆ switchgear is inevitable, but it will be a slow process that occurs as existing SF₆ switchgear ages out and becomes decommissioned. Utilities looking to make the switch to Alternatives to SF₆ gas will have to make large investments to replace existing SF₆ switchgear. It is important to note: alternative di-electrics currently on the market cannot be used with existing SF₆ switchgear. Hitachi has made an announcement for EconiQ, which is expected to hit the market in late 2022 and is designed to replace SF6 gas in existing equipment. SF₆ service carts are not designed to be used with any Alternative Gases.

Available Alternative Gases That Are Making Strides

There are two main technologies that are winning out to eventually replace SF₆ gas that will be explained further in this blog post: gas mixtures containing C4-FN or C5-FK and Synthetic Air applications, which include “CleanAir” and “DryAir” technology. For simplification, C4-FN will be referred to as C4 and C5-FK will be referred to as C5 hereafter. 

Alternative gases consist of gas mixtures of two or more single gas components that form the insulating medium. This is different from SF₆ gas, which is usually the single component. Alternative Gases have a considerably lower GWP to SF₆ gas, which can be seen in Figure 2 below. Additionally, alternatives to SF₆ gas have a much lower atmospheric life span than SF₆ gas.

The properties of Alternative Gases will be discussed briefly below, but more details can be found in DILO’s 4-part White Paper Series on Alternative Gases. There are different options for the mixing and providing of Alternative Gases with various advantages and disadvantages. These are described in detail in DILO’s White Paper Gas Mixtures and Mixing of Gases: Part 2 whitepaper. There are major differences in gas handling for C4/C5 and Synthetic Air compared to SF₆ gas. These are discussed in detail in DILO’s White Paper Handling Alternative Gases: Part 3. Options and technical details for re-use, reconditioning, and disposal of Alternative Gases is discussed in DILO’s White Paper Measuring Gas Quality and Sustainable Reconditioning and Reuse of Alternative Gases.

C4 and C5 Gas Mixtures

In their pure forms, C4 is in a liquid state at pressures above 250 kPA and C5 is in a liquid state at atmospheric pressure. In contrast, SF₆ gas remains in a homogenous, gaseous state in ambient conditions (>-50^oC; <20bar_abs). We speak of a homogeneous state when the medium has only one aggregate state (gaseous, liquid or solid) and there is no partial liquefaction. Therefore, C4 and C5 must be mixed with nitrogen or carbon dioxide as these gases require higher temperatures and pressures for liquification. Mixing C4 and C5 with these carrier gases provides more stability in ambient conditions. In extremely cold temperatures, there may be partial liquification of specific components which would affect the mixing ratio and insulating capacity of the gas.

C4 or g3 (pronounced g-cubed) gas applications have the same compact design as SF₆ switchgear but with a 99% reduced global warming potential. C4 gas mixtures also have the same electrical ratings as SF₆ gas. Additionally, C4 products are able to operate in the same temperature range as SF₆ products, down to -30^oC. 

Extensive testing has been conducted on this gas mixture and found that the homogeneity and composition of the gas remains stable over time. Another important quality of C4 is that it has no voltage limitations.

Synthetic Air Applications

Synthetic Air applications, for example Siemen’s CleanAir technologies. Synthetic Air consists of a mixture of 20% oxygen and 80% nitrogen. Synthetic Air is mainly used in vacuum switching technology up to and below 145 kV. Vacuum switching technology has been around for decades and recent technological advances have allowed for a reduced footprint in vacuum breakers closer to that of SF₆ breakers. Synthetic Air remains gaseous until temperature lower than -183 °C and remains in a homogenous state at room temperature under high pressure. In this way, Synthetic Air and SF₆ gas are similar in that they remain in a homogenous, gaseous state under ambient conditions (> − 50 °C; < 20 bar_abs). Synthetic Air is non-toxic and is not subject to any environmental regulations at this time.

Summary of Available Alternatives to SF6 Gas

The major driving force of seeking alternatives to SF₆ gas is its Global Warming Potential. Aside from a drastically reduced GWP, all alternatives to SF₆ currently on the market are inferior to most technical and electrical performance characteristics of SF₆, especially for operating voltages greater than 145 kV. As George Becker from Power Engineers pointed out at the 2021 SF₆ Gas Management Seminar, the market potential for SF₆ alternatives above 145 kV is relatively small when compared to the cost of research and development for alternative gases. It will be interesting to see what developments occur in the market for higher voltage switchgear over the next few years.   

Looking to Learn More About Alternative Gases?

Join us at our Annual SF6 Gas Management Seminar in Tampa, FL on November 8-9. Part of talking about all things SF₆ is helping utilities and gas users learn about new alternative to SF₆ technologies on the market. This year, we will be featuring TWO hands-on break-out sessions on Alternative Gas Topics.

The Mixing of Alternative Gases Break-Out Session will help users identify and achieve the correct ratio/mixture of alternative gases. This session will also feature how to prevent cross contamination during mixing and the differences between the liquid and vapor states of alternative gases.

The Alternative Gas Handling Break-Out Session will feature leak detection and gas analysis equipment for Alternative Gases. This session will focus on the similarities and differences between SF₆ and Alternative Gas Handling with tips on how to maintain a homogenized mixture of gases.

Early bird registration pricing ends August 26, so be sure to register early and save!

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