Researchers this month will begin testing a high-voltage circuit breaker that can quench an arc and clear a fault with supercritical carbon dioxide fluid. The first-of-its-kind device could replace conventional high-voltage breakers, which use the potent greenhouse gas sulfur hexafluoride, or SF6. Such equipment is scattered widely throughout power grids as a way to stop the flow of electrical current in an emergency.
“SF6 is a fantastic insulator, but it’s very bad for the environment—probably the worst greenhouse gas you can think of,” says Johan Enslin, a program director at U.S. Advanced Research Projects Agency–Energy (ARPA-E), which funded the research. The greenhouse warming potential of SF6 is nearly 25,000 times as high as that of carbon dioxide, he notes.
If successful, the invention, developed by researchers at the Georgia Institute of Technology, could have a big impact on greenhouse gas emissions. Hundreds of thousands of circuit breakers dot power grids globally, and nearly all of the high voltage ones are insulated with SF6.
A high-voltage circuit breaker interrupter, like this one made by GE Vernova, stops current by mechanically creating a gap and an arc, and then blasting high-pressure gas through the gap. This halts the current by absorbing free electrons and quenching the arc as the dielectric strength of the gas is increased.GE Vernova
On top of that, SF6 byproducts are toxic to humans. After the gas quenches an arc, it can decompose into substances that can irritate the respiratory system. People who work on SF6-insulated equipment have to wear full respirators and protective clothing. The European Union and California are phasing out the use of SF6 and other fluorinated gases (F-gases) in electrical equipment, and several other regulators are following suit.
In response, researchers globally are racing to develop alternatives. Over the last five years, ARPA-E has funded 15 different early-stage circuit breaker projects. And GE Vernova has developed products for the European market that use a gas mixture that includes an F-gas, but at a fraction of the concentration of conventional SF6 breakers.
Reinventing Circuit Breakers With Supercritical CO2
The job of a grid-scale circuit breaker is to interrupt the flow of electrical current when something goes wrong, such as a fault caused by a lightning strike. These devices are placed throughout substations, power generation plants, transmission and distribution networks, and industrial facilities where equipment operates in tens to hundreds of kilovolts.
Unlike home circuit breakers, which can isolate a fault with a small air gap, grid-scale breakers need something more substantial. Most high-voltage breakers rely on a mechanical interrupter housed in an enclosure containing SF6, which is a non-conductive insulating gas. When a fault occurs, the device breaks the circuit by mechanically creating a gap and an arc, and then blasts the high-pressure gas through the gap, absorbing free electrons and quenching the arc as the dielectric strength of the gas is increased.
In Georgia Tech’s design, supercritical carbon dioxide quenches the arc. The fluid is created by putting CO2 under very high pressure and temperature, turning it into a substance that’s somewhere between a gas and a liquid. Because supercritical CO2 is quite dense, it can quench an arc and avoid reignition of a new arc by reducing the momentum of electrons—or at least that’s the theory.
Led by Lukas Graber, head of Georgia Tech’s plasma and dielectrics lab, the research group will run its 72-kV prototype AC breaker through a synthetic test circuit at the University of Wisconsin-Milwaukee beginning in late April. They group is also building a 245-kV version.
The use of supercritical CO2 isn’t new, but designing a circuit breaker around it is. The challenge was to build the breaker with components that can withstand the high pressure needed to sustain supercritical CO2, says Graber.
The team turned to the petroleum industry to find the parts, and found all but one: the bushing. This crucial component serves as a feed-through to carry current through equipment enclosures. But a bushing that can withstand 120 atmospheres of pressure didn’t exist. So Georgia Tech made its own using mineral-filled epoxy resins, copper conductors, steel pipes, and blank flanges.
“They had to go back to the fundamentals of the bushing design to make the whole breaker work,” says Enslin. “That’s where they are making the biggest contribution, in my eyes.” The compact design of Georgia Tech’s breaker will also allow it to fit in tighter spaces without sacrificing power density, he says.
Replacing a substation’s existing circuit breakers with this design will require some adjustments, including the addition of a heat pump in the vicinity for thermal management of the breaker.
If the tests on the synthetic circuit go well, Graber plans to run the breaker through a battery of real-world simulations at KEMA Laboratories‘ Chalfont, Penn. location—a gold standard certification facility.
The Georgia Tech team built its circuit breaker with parts that can withstand the very high pressures of supercritical CO2.Alfonso Jose Cruz
GE Vernova Markets SF6-alternative Circuit Breaker
If Georgia Tech’s circuit breaker makes it to the market, it will have to compete with GE Vernova, which had a 20-year head start on developing SF6-free circuit breakers. In 2018, the company installed its first SF6-free gas-insulated substation in Europe, which included a 145 kV-class AC circuit breaker that’s insulated with a gas mixture it calls g3. It’s composed of CO2, oxygen and a small amount of C4F7N, or heptafluoroisobutyronitrile.
This fluorinated greenhouse gas isn’t good for the environment either. But it comprises less than 5 percent of the gas mixture, so it lowers the greenhouse warming potential by up to 99 percent compared with SF6. That makes the warming potential still far greater than CO2 and methane, but it’s a start.
“One of the reasons we’re using this technology is because we can make an SF6-free circuit breaker that will actually bolt onto the exact foundation of our equivalent SF6 breaker,” says Todd Irwin, a high-voltage circuit breaker senior product specialist at GE Vernova. It’s a drop-in replacement that will “slide right into a substation,” he says. Workers must still wear full protective gear when they maintain or fix the machine like they do for SF6 equipment, Irwin says. The company also makes a particular type of breaker called a live tank circuit breakerwithout the fluorinated component, he says.
All of these approaches, including Georgia Tech’s supercritical CO2, depend on mechanical action to open and close the circuit. This takes up precious time in the event of a fault. That’s inspired many researchers to turn to semiconductors, which can do the switching a lot faster, and don’t need a gas to turn off the current.
“With mechanical, it can take up to four or five cycles to clear the fault and that’s so much energy that you have to absorb,” says Enslin at ARPA-E. A semiconductor can potentially do it in a millisecond or less, he says. But commercial development of these solid state circuit breakers is still in early stages, and is focused on medium voltages. “It will take some time to get them to the required high voltages,” Enslin says.
The work may be niche, but the impact could be high. About 1 percent of SF6 leaks from electrical equipment. In 2018, that translated to 9,040 tons (8,200 tonnes) of SF6 emitted globally, accounting for about 1 percent of the global warming value that year.
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