With its chemically stable composition and ability to withstand extreme conditions, sulfur hexafluoride is a man-made gas used in particle accelerators, semiconductors, cellphone components, eye surgery, common consumer products and even some manufacturing processes. Since the 1950s, the U.S. electric power industry has managed the high voltages traveling from generating plants to customer load centers by using sulfur hexafluoride (SF6) in circuit breakers, gas-insulated substations (GIS) and gas-insulated lines (GIL).
In fact, approximately 80% of the global use of SF6 is in the transmission and distribution of electricity, including the generation and storage of renewable energy. Medium- and high-voltage electrical equipment contains SF6 to insulate live electrical parts and quench arcs during power switching events. SF6 is a compact and economic approach to safely performing these tasks.
However, while SF6 is a highly effective electrical insulator, it also is a potent greenhouse gas. A relatively small amount of SF6 can impact the climate, as it is 23,500 times more effective at trapping heat than an equivalent amount of carbon dioxide (CO2). Even in low concentrations, it remains in the atmosphere for approximately 3200 years. SF6 can be leaked by aging or defective equipment as well as inadvertently during gas handling in any stage of the equipment’s life cycle.
Identifying emission sources is the first step to better managing SF6 gas in power systems. EPRI’s controlled laboratory tests and field trials aid electric utilities in emission reduction by informing leak detection and mitigation approaches. In addition to protecting the environment, reducing SF6 emissions saves money and increases grid reliability.
Lab Testing Leaks
Current practices for sealing SF6 leaks are costly, often difficult to implement in the field and may require removing equipment from service.
EPRI performed a controlled lab experiment to seal simulated leaks and tested a collection of adhesives using custom-built pressure cells. From there, at its SF6/GIS lab in Charlotte, North Carolina, U.S., the research organization conducted outdoor leak sealing tests on pressurized, out-of-service gas-insulated equipment (GIE) containing SF6. The GIE was exposed to multiple weather conditions, such as direct sunlight, wind, precipitation and temperature fluctuations. For two years, the project team tracked the performance of seals on pipes, threaded fittings, flanges and threaded bolts, determining the most successful sealing techniques for different types of leaks.
In evaluating sealing materials and developing application techniques for gas-filled substation equipment, the EPRI team adhered to criteria important to grid operators:
- Use cost-effective materials that are commercially and readily available
- Ensure any sealant technique is easy to apply, remove and reapply by utility personnel
- Avoid the need for specialized tools or clamps
- Prevent the system from needing to be depressurized
- Contain or significantly reduce a leak until a permanent repair can be installed.
Leak-Sealing Field Trials
The next step involved real-world experience by field-testing to determine if sealing methodologies would perform well on in-service equipment. Since 2019, EPRI project team members have applied cutting-edge SF6 leak-sealing techniques in eight field trials globally, resulting in an overall reduction of 2685 lb (1218 kg) of SF6 that otherwise would have been emitted. This reduction is equivalent to avoiding 28,618 metric tons of greenhouse gas emissions from 6811 gasoline-powered cars for one year.
Some participants in the field trials include New York Power Authority, FirstEnergy, Consolidated Edison, UK Power Networks and Saudi Electricity Co. (SEC), part of National Grid Saudi Arabia (SA). EPRI is currently initiating new field trials with three additional utilities.
In one trial, SEC enlisted EPRI’s assistance with the research design and implementation of SF6 leak sealing at one of its National Grid SA substations. The project team applied a lab-tested technique that did not require an outage or reduction in equipment pressure and used materials that were easy to procure, apply and remove. After sealing the leak in the field, the team observed a considerable reduction in leakage, leading to decreased requirements for refilling SF6 gas annually. Based on the success achieved, National Grid SA is planning to apply the technology to substations across its service territory. Sufficient hands-on training and knowledge transfer from the EPRI team ensured National Grid SA staff could apply the techniques without relying on outside vendors.
Another participant in the field trials, UK Power Networks enlisted EPRI’s assistance to test innovative SF6 leak-sealing materials and techniques at one of its 132-kV substations. UK Power Networks tested an adhesive and vent-pipe method for leaks in bolts and threaded fittings. The trial was successful and the technique subsequently applied at an 11-kV primary substation, which enabled the line to stay energized during the heart of winter when demand is highest. The seal held until a more permanent repair could be implemented. The method is expected to be used widely throughout UK Power Networks to significantly support the utility’s goals of decreasing SF6 emissions, improving electric system reliability, and reducing operating and maintenance costs.
Replacing SF6
Work is also underway in the electric industry to eliminate or significantly reduce the use of SF6. However, replacing it presents technical challenges, particularly in high-voltage equipment. EPRI is researching effective alternatives to SF6, whether by adopting other gases, vacuum technology or a different approach. Each new technology brings a combination of benefits and challenges.
In analyzing alternatives, EPRI is leading a 12-month project to rapidly derisk the application of new SF6-free technologies in circuit breakers. Using a research substation at EPRI’s high-voltage lab in Lenox, Massachusetts, U.S., the team is deploying a range of technologies, including vacuum/clean-air, CO2+O2, and fluoronitrile-mixture solutions to simulate the full life cycle of circuit breaker operations.
Researchers are gaining insights on circuit breaker performance in weather extremes, wet environments, and lightning and overvoltage events. The team also plans to introduce intentional leaks to better understand gas detection and performance. The lab enables researchers to perform all the practical tasks a utility would need to perform with new technologies, such as commissioning, leak detection, analysis, recycling and disposal.
EPRI’s research on potential strategies for SF6 replacement also extends to tracking and understanding the regulatory developments affecting SF6 and alternate technologies.
Research up Next
Leak monitoring is crucial for reducing SF6 emissions. Monitor suppliers offer various commercial approaches, each with different physical and mechanical constraints, sensor types and proprietary algorithms to interpret data. This complexity makes it challenging to compare the value of different monitors.
To address this, EPRI is analyzing commercially available online monitors to understand their capabilities more fully. Work in 2024 focused on gathering utility use cases, documenting experiences from deployed monitors and using that information to update EPRI’s circuit breaker guidebook. Assessments have continued into 2025, focusing on technologies of interest to electric utilities.
EPRI is also investigating innovative leak detection technologies, such as acoustic imaging, unmanned aerial vehicles and robotics. Further work will concentrate on technologies and techniques that can provide early warnings before significant volumes of SF6 are released.
Ongoing research with EPRI’s SF6 leak-sealing techniques includes monitoring field trial performance, investigating sealants that cure at lower temperatures, scaling existing techniques to large flanges and exploring effective methods used by other industries. For successful lab and field-text techniques, EPRI has developed application guides, such as its annually updated GIS and GIL guidebook, workshops, how-to videos, webinars and technical reports.
Areas of exploration in 2025 include C4-fluoronitrile mixtures, in-depth vacuum technologies, noninvasive mechanical wear detection techniques, and further interpretation of component deficiencies, irregularities and degradation. These efforts aim to inform the industry on reducing SF6 usage and emissions, ultimately helping grid owners reduce costs, increase system reliability and protect the environment.
