When it comes to medium and high voltage circuit breakers, there is no better electrical insulator than sulfur hexafluoride (SF₆). The manufactured gas is most commonly used to fill circuit breakers within utility substations to prevent and quench arcing events. Transmission substations transfer high voltage power to distribution substations, which subsequently disperse the incoming electricity to lower voltages suitable for consumers on power grids. Both transmission and distribution circuit breakers are vulnerable to arcing events, which can be triggered by power surges, partial discharges and lightning strikes.

SF₆’s impeccable dielectric strength, arc quenching capabilities and resilience to remain thermally stable are just a few reasons why this gas has remained a staple in the transmission and distribution (T&D) industry. It has been the preferred gas in switchgears for over 50 years.

Two Types of SF₆

Sulfur hexafluoride is available in two types: virgin SF₆ and reconditioned SF₆. Virgin sulfur hexafluoride is generated overseas in Europe and Asia. Its production is a known source of greenhouse gas emissions. According to the Environmental Science & Technology Report, SF₆ production in China contributed to 10 percent of gas emissions from the country between 1990 and 2010. This is a huge environmental concern due to the long atmospheric lifespan of SF₆ – an astounding 3,200 years.

Moreover, virgin SF₆ impacts climate change as the gas is highly potent. SF₆ has a global warming potential (GWP) of 22,800. To help put this into perspective, one pound of virgin SF₆ released into the atmosphere has the same effect as 11 tons of carbon dioxide. With the results of multiple research studies regarding the effects of emissions and by-products, it is surprising to learn that some original equipment manufacturers (OEMs) for circuit breakers and electrical utilities are proponents for utilizing only virgin SF₆.

Reconditioned SF₆, on the other hand, offers overwhelming benefits. After undergoing the correct cleansing/separation process, used SF₆ can be transformed to a new condition and offers the following advantages:

• Unlike air and oil – the first used insulating mediums in the industry – reconditioned SF₆ is 100 percent recyclable, which provides an ecological benefit in the reduction of the industry’s carbon footprint.
 
• An alternative to product disposal without increasing budgetary costs.

Additionally, virgin SF₆ poses a risk to the environment if emitted into the atmosphere. Preventing SF₆ emissions is possible with properly trained staff, reliable maintenance equipment and utilization of reconditioned gas.

Although SF₆ production is unavailable in the United States, there are a few suppliers who can provide reconditioned gas at premium grades that exceed the necessary standards for use in gas insulated equipment (GIE). Sourcing SF₆ locally can help greatly reduce the carbon footprint in North America without compromising the product quality or durability of GIE.

Figure 1: SF₆ gas separator, which uses a cryogenic process to recondition contaminated SF₆ gas to >99.99% purity.

SF₆ Reconditioning Programs

Generally, contaminated SF₆ goes unnoticed until a breaker fault is reported or during routine maintenance. SF₆ that does not meet International Electrotechnical Commission (IEC) and Institute of Electrical and Electronic Engineers (IEEE) standards is usually set aside for disposal. A more cost-effective and eco-friendly option is to reuse by participating in an SF₆ reconditioning program. These programs guarantee locally available product to the end user while being mindful to the environment. Choosing to recondition gas in place of disposal discourages importation of virgin gas and, in turn, helps reduce SF₆ emissions from not only the creation process, but from exhaust or fuel emissions that take place during transport to North America.

An additional advantage for sourcing reconditioned SF₆ is cost savings. Virgin gas is typically sold at a premium and guarantees a greater than 99.99 percent purity rating. What many users do not realize is that technology is currently available that can separate by-products and air from tainted SF₆. The filtration process makes it possible for used SF₆ to reach federal and international standards for tech grade gas, all at a lower cost. This cost savings does not compromise quality. Whether your SF₆ supply is restocked with greater than 99.9 percent or 5/9 gas (99.999 percent), the result is the same: a circuit breaker filled with a reliable product that requires minimal maintenance (provided best handling techniques are practiced).

Alternative Gases

As of late, alternative gases are a hot topic in the electrical sector. The negative impact of SF₆ emissions has pushed the industry to develop an alternate medium that is much safer for the environment. While implementing a substitute gas is possible, there are still various factors that make the elimination of SF₆ impractical.

The most notable alternative gases are comprised of gas mixtures. These special formulas require a special housing different from SF₆ compartments, necessitating a complete overhaul of T&D grids across North America. Most importantly, very little research has been completed regarding procedures for handling alternative gases that have been exposed to arcs. There are no known methods on how to recycle the gas mixtures for repeated use.

Figure 2: Analysis equipment measures purity and contamination levels in SF₆ gas. 

Figure 3: Cylinders used to store SF₆ gas. 

Conclusion

From a technological view, there are no significant differences in the makeup of engineered virgin SF₆ and reconditioned SF₆ that has undergone a cryogenic process to remove by-products. Circuit breaker manufacturers and electrical utilities should consider the value of choosing to reuse and eliminate the specification for virgin gas in operational and maintenance procedures. The cultivation of alternative gases does show promise for the electrical industry’s future, however, further examination of the by-products, maintenance procedures and recycling capabilities is needed before discounting completely.  

Reusing properly treated SF₆ removes the need for generating new product and, more importantly, helps lower the carbon footprint in the United States. As long as there is an option to reuse, there are no sound reasons for using virgin gas in circuit breaker applications. For now, reconditioned SF₆ is the alternative gas solution.

References

1. Hu Jianxin, et al. “Sulfur Hexafluoride (SF₆) Emission Estimates for China: An Inventory for 1990-2010 and a Projection to 2020.” Environmental Science & Technology, April 2013, Vol. 47, Issue 8, pp 3848−3855. https://folk.nilu.no/~fang/Publication/Fang-2013-EST-Sulfur%20hexafluoride%20(SF6)%20emission%20estimates%20for%20China%20an%20inventory%20for%201990-2010%20and%20a%20projection%20to%202020.pdf

2. European Fluorocarbons Technical Committee (EFCTC). Products & Applications of Sulfur Hexafluoride (SF₆):  https://www.fluorocarbons.org/products-applications-sulphur-hexafluoride-sf6/

3. Ponchon, Philippe. “G3 – In the Air.” Think Grid. December 22, 2017. http://www.think-grid.org/g3-in-the-air

4. Nyberg, Dave. “3M™ Novec™ Dielectric Fluids SF₆ Alternatives for Power Utilities.” Workshop for SF₆ Emission Reduction Strategies, January, 2017. https://www.epa.gov/sites/production/files/2017-02/documents/nyberg_presentation_2017_workshop.pdf

5. United States Environmental Protection Agency. Electric Power Systems Partnership: https://www.epa.gov/f-gas-partnership-programs/electric-power-systems-partnership