With the ever-growing challenges of federal and local reporting and SF6 emission reduction, several alternatives have been presented to the industry as replacements for SF6 gas. As the industry continues to review and consider alternatives, we should not limit our solutions, and the alternative of turning to the existing stockpile of SF6 gas offers several benefits. However, what preventative measures must be taken so that the industry can continue using this insulating medium safely and in compliance with best practices and standards?
Discovered in 1901 by French Chemists Henri Moissan and Paul Lebeau, sulfur hexafluoride is a man-made gas that has a greater dielectric strength and density than air (Figure 1). In pure form, SF6 is inert, non-flammable, non-toxic, and thermally stable, and has unmatched arc-quenching capabilities. Most interesting are the molecule’s self-healing abilities. The gas only begins to break down at a temperature of 380 degrees F; once the heat source is removed, the atoms regenerate like new. The chemical properties of SF6 make it ideal for use within circuit breakers and switchgear.
Figure 1: Comparison of Dielectric Breakdown Strength of SF6 Gas versus Air and Oil
The use of SF6 within circuit breakers has offered the electrical industry several benefits. Oil circuit breakers (OCBs), which were popular in the early 1900s, required frequent upkeep as the medium became easily polluted following arcing events and posed a potential fire hazard. Oil is also difficult to recondition. The introduction of sulfur hexafluoride offered a non-flammable resolution for the insulation of switches. In addition, the shift toward SF6 changed the design of circuit breakers by allowing a more compact infrastructure due to the dielectric strength of the gas. About 80 percent of manufactured SF6 is used in high-voltage equipment like switchgears and circuit breakers.
Unfortunately, the same properties that make SF6 such a great insulating gas also make SF6 the most potent of all greenhouse gases. One molecule of SF6 can trap 23,900 times more heat than carbon dioxide over a 100-year period, and it remains in the atmosphere for thousands of years.
Alternatives
Some proposed alternatives fall short of SF6 and would create an enormous economic burden on utilities because it would require a complete overhaul of their gas insulated switchgear (GIS) equipment. We make the case that the industry has an SF6 handling problem, not an SF6 problem.
The industry could make two changes to drastically decrease SF6 emissions:
- Use reconditioned SF6 instead of importing virgin gas.
- Ensure that trained and certified personnel are using best management and safety practices when handling SF6.
The United States has a large stockpile of SF6 gas. From an environmental perspective, reconditioning gas could lower the carbon footprint by removing the need to manufacture new product. Sulfur hexafluoride is 100 percent recyclable, and there is no chemical difference between virgin SF6 gas and certified reconditioned SF6 gas. Any contaminants that occur due to the intrusion of air, moisture, or generation of arc by-products can be removed with the proper process.
Reconditioning
The SF6 reconditioning process ensures readily available supply and eliminates the need for importing virgin product (the manufacturing process for SF6 is a known source of emissions). A limited number of North American suppliers can recondition SF6 gas to meet the standards set by CIGRE, IEEE, ASTM, and IEC while ensuring zero emissions of SF6. The process requires specific handling procedures to ensure zero emissions of SF6 gas, and the overall properties of the gas must be respected. When the process is followed correctly, CIGRE, IEEE, ASTME, and IEC standards can easily be exceeded. An SF6 gas separator, which uses a three-stage cryogenic process to remove non-reactive gases, can recondition SF6 gas to a guaranteed 99.99 percent purity and less than 40 ppm moisture.
An SF6 gas separator uses a cryogenic process to recondition contaminated SF6 gas to >99.99 percent purity.
The reconditioning process includes increasing the storage pressure of the gas to allow the start of SF6 gas liquefaction followed by lowering the temperature by using two to three temperature changes below -40 degrees C. This further helps the liquefaction of SF6 gas, thus separating other vapors such as N2, O2, and CF4. Upon the start of the reconditioning process, the contaminated SF6 gas must be processed through a pre-filter system ensuring oil, moisture, and SF6 gas by-products are captured and scrubbed from the gas. This process is similar to the final process in the production of virgin SF6 gas.
Training
While the capability of recycling stockpiles of SF6 for utility and industrial users is beneficial to the environment, the composition of the gas remains the same. Handlers are still required to use SF6 in a closed cycle to prevent emissions while performing maintenance on circuit breakers and switchgear. Even after recycling, the global warming potential and the lengthy atmospheric lifespan (3,200 years) SF6 remains the same. However, the dielectric strength, thermal stability, and self-healing capabilities of the gas also remain. Choosing to reuse is just a step in the right direction. Ultimately, the only way companies can ensure that their choice is beneficial to the environment is to guarantee field staff are properly versed in SF6 handling methods.
SF6 Safety and Handling Training
Proper field training is essential to safeguarding an organization from unintentionally leaking SF6 into the atmosphere. According to the Environmental Protection Agency, “training raises awareness of emissions, environmental and health impacts of SF6 and by-products, and potential reduction options, but training also enables employees to follow procedures and protocols properly.”
Basic methods on preventing emissions during the circuit breaker maintenance process include:
- Recovery. When recovering SF6 using a gas cart, it is imperative to reach the lowest level of vacuum or blank-off pressure possible. Generally, reducing the amount of gas in a breaker to atmospheric pressure alone leaves a startling 20 percent of residual gas within the gas insulated enclosure. Starting additional maintenance while residual gas is still present can cause an emission.
- Filling. Check the OEM’s manual or nameplate for accurate SF6 capacity information. Accidently overfilling GIE could result in an SF6 emission by engaging a pressure relief valve. Tracking gas weights by documenting SF6 movements and using a cylinder weight scale can aid in preventing both positive and negative emissions. The Mass Balance Equation is used to determine leakage rates:
User Emissions = (Decrease in Storage Inventory) + (Acquisitions) – (Disbursements) – (Net increase in Total Nameplate Capacity of Equipment Operated)
Using the above equation, it can easily be determined that underestimating the capacity of the retiring equipment (for example, if the true value is 300 lbs., but the nameplate label reads 280 lbs.) will lead to showing a negative SF6 emission, and the emission rate will subsequently be underestimated. - Testing. SF6 analysis to determine gas quality prior to performing maintenance is standard practice. It is important to use a collection bag when taking gas samples if your test equipment is not equipped with a self-recovery feature.
- Fittings and adaptors. Ensure fittings used for connection to GIE and other accessories (manifolds, analyzers, cylinders, etc.) are leak tested. Refrain from using threaded connections, which are prone to leak over time.
Conclusion
Turning to the existing stockpile of SF6 gas offers an environmentally friendly alternative solution to importing virgin SF6 gas from overseas. The implementation of reconditioned SF6 that has been properly treated to meet or surpass IEEE/CIGRE standards helps reduce the carbon footprint and eliminates SF6 emissions that are the direct result of the manufacturing process. Awareness and preventative measures will allow the industry to continue using this insulating medium safely, provided the benefits of reuse are shared and understood.
References
Lukas Rothlisberger. “SF6 Nameplate Inaccuracies and their Impact on Greenhouse Gas Reporting,” May 2014. Available at: www.epa.gov/sites/production/files/2016-02/documents/nameplate_issues_final.pdf.
United States Environmental Protection Agency. “Overview of SF6 Emissions Sources and Reduction Options in Electric Power Systems,” August 2018. Available at:
www.epa.gov/sites/production/files/2018-08/documents/12183_sf6_partnership_overview_v20_release_508.pdf.
Danielle White is a Marketing Representative and DILO’s Account Manager for the Canadian Region. She is committed to DILO’s Zero SF6 Emissions philosophy. Danielle is responsible for assisting customers in establishing comprehensive SF6 maintenance and management plans through on-site trainings, articles in DILO’s newsletter and other publications, as well as co-coordinating DILO’s annual SF6 Gas Management Seminar.
Lina Encinias, a Marketing Specialist for DILO Company, Inc., has a passion for sustainability. She helps spread awareness on SF6 emissions, regulations, and handling issues through DILO newsletters, the annual SF6 Gas Management Seminar, and industry trainings. Lina graduated from the University of South Florida with a BS in environmental science and a minor in mass communications.