ɫ����

Replacing the potent greenhouse gas SF₆ in high-voltage circuit breakers with a clean alternative is critical as the U.S. looks to upgrade its aging electrical infrastructure. 

Although well-known greenhouse gases like carbon dioxide (CO₂) and methane contribute the most emissions, it is a lesser-known greenhouse gas, sulfur hexafluoride (SF₆), that owns the title as the “most potent.” The man-made gas has a global warming potential 23,900 times than that of CO₂ and an atmospheric lifetime persistence of up to 3,200 years.

Like other greenhouse gases, SF₆, plays a significant, albeit indirect, role in everyday life, as it is a key component in high-voltage circuit breakers and switchgear for electric power systems. For the U.S. to effectively decrease carbon emissions to goals set at the 2021 United Nations Climate Change Conference (COP26), the country’s electrical power grid will need substantial updating, which includes finding an alternative to SF₆ electrical equipment

“High-voltage alternating current (AC) SF₆-insulated circuit breakers can be found in most electrical substations in the U.S. and around the world. They are vital mechanisms for a reliable and resilient power grid,” said Lukas Graber, associate professor in the ɫ���� School for Electrical and Computer Engineering. “But any leaks of SF₆ are extremely bad for the environment due to its greenhouse gas effect.”

A team of researchers from ɫ����, led by Graber and in collaboration with Mississippi State University, has recently been awarded nearly $4 million from the Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) to develop a three-phase SF₆-free AC high-voltage circuit breaker. Fittingly, the proposed design is called TESLA (Tough and Ecological Supercritical Line Breaker for AC), acknowledging AC electricity pioneer Nikola Tesla. 

The Impact of SF₆

From 2008 to 2018, the annual emissions rate of SF₆ rose from about 7,300 tons to approximately 9,040 tons, an increase of 24%, according to a 2020 study published by the European Geosciences Union. That amount of SF₆ equates to greenhouse gas emissions of approximately 44 million passenger vehicles driven for one year, or 226 billion pounds of coal being burned.

According to ARPA-E, equipment leaks are a major source of SF₆ emissions from the electrical transport and distribution sector. This is particularly true for aging equipment which, due to natural deterioration, is more prone to gas leaks. Ironically, as the U.S. strives to supplant fossil fuel-derived electricity generation with cleaner wind and solar power, the power grid will become increasingly decentralized, which will require more SF₆ gas-insulated equipment.

“The electrical infrastructure in the US is in desperate need of upgrades to accommodate an increasing share of renewable energy, the electrification of the transportation sector, and improved resiliency against cyberattacks,” said Graber. “Existing electrical substations will require new equipment, and as part of these upgrades, a new eco-friendly generation of circuit breakers should be implemented.” 

Looking to Supercritical Fluids 

Replacing SF₆ is no easy task. While SF₆ has exceedingly high global warming potential, the synthetic gas is an excellent electrical insulator — a material in which electric current does not flow freely. The gas is known for its effectiveness, stability, and intrinsic non-toxic, non-corrosive, and non-flammable nature, and while non-SF₆ equipment has long been available for low to medium-voltage applications, there are no alternatives for high-voltage equipment ready for market.

The team’s research has shown that the key to success may be utilizing recent breakthroughs in the dielectric (or electrical insulating) properties of supercritical fluid. A supercritical fluid is a highly compressed fluid that combines the properties of gases and liquids, and is most frequently used for power generation. The team is currently experimenting with supercritical CO₂, which has ecologically friendly attributes that could be utilized in high-voltage equipment.

“Our preliminary results show that the supercritical fluid is a better dielectric than SF₆,” said Zhiyang Jin, research engineer in Graber’s Plasma and Dielectrics Lab at ɫ����. “The breakdown voltage of supercritical CO₂ is at least three times that of SF₆, and since CO₂ is everywhere, so a man-made gas will no longer be needed.”

Unlike SF₆ circuit breakers, the design pressure needed for supercritical fluid in TESLA is significant — about ten times higher than SF₆ counterparts. Achieving this design means developing a different circuit breaker chamber to maintain structural integrity during and after the fault current interrupting event. Computational fluid dynamics models have already been developed to study the pressure and temperature changes, and the velocity distribution of supercritical fluids for designs of the chamber, nozzle, and contact system.

In addition to the engineering challenge of connection compatibility with existing high-voltage electrical equipment/infrastructure, and the subsequent workforce training that will entail, market adoption is critical hurdle to clear.

“To replace existing circuit breakers, we cannot just show that TESLA passed all required tests,” said Jonathan Goldman, principal at ɫ����’s Venturelab. “Gaining trust from large utility companies is also one of our crucial tasks. We will seek opinions from experts from various backgrounds.”

Goldman and electrical engineering professor Santiago Grijalva will work with several industry partners to guide the design process, explore additional application segments, and advise on the commercialization of TESLA. 

Getting to Work 

The interdisciplinary team will design and build the proposed circuit breaker at a high voltage rating (245 kV, 4 kA) and validate the design and functionality using a synthetic test circuit. The testbed will be modular in design and enable both high-current and high-voltage testing without needing access to a high-power source or generator. According to Graber, the development of such experimental capability is not only important for the TESLA project, but also for the power and energy industry of the U.S.

The three-year ARPA-E-funded project will culminate in the development of a TESLA prototype tested at the Paul B. Jacob High Voltage Laboratory at Mississippi State University — the largest university-operated high voltage facility in North America. The lab is directed by Chanyeop Park, who received his Ph.D. at ɫ����. 

The team also includes Juergen Rauleder, assistant professor in the Daniel Guggenheim School of Aerospace Engineering, and Lauren Garten, assistant professor in the School of Materials Science and Engineering.

Raulder will investigate the fluid dynamics inside the circuit breaker and provide guidance for mechanical designs of a high-pressure tank, contact system, and arc quenching mechanism, while Garten will research metal oxide varistor characteristics for direct current circuit breaker applications. Garten’s research would have an impact on another ARPA-E-funded project at ɫ���� Tech called EDISON led by Graber.

“Edison and Tesla as people never got along with each other, but through advancements in high-voltage circuit breakers, we’re trying to make them good friends,” said Graber. “There is no win or lose for choosing AC or DC nowadays, together they can both make our world a better place to live.”