Tuesday, August 15, 2017

Getting to the Heart of Circuit Breaker Arcs

If you want to see a stunning demonstration of nature colliding with modern technology, do a simple image search for lighting strikes a power line. A chance strike can wreak havoc on the daily lives of those nearby and on the wallets of those responsible for restoring power. Most of us lucky enough to live with stable electric grids take for granted the traffic lights, internet connections, refrigerators, air conditioning, lights, coffee makers, and credit card readers that are essential to our way of life. A major interruption to the grid is a serious and often dangerous issue.

Storms are rolling in.
Image Credit: Artondra Hall (CC BY 2.0).

In order to protect overhead power lines from lightning strikes, falling trees, and other events that can cause short-circuits or current overloads, high voltage circuit breakers are a key component of the grid. Like a fuse that blows when you overload an electrical outlet, the purpose of a circuit breaker is to quickly stop the flow of current in a circuit. Unlike a fuse, however, a circuit breaker automatically resets once things are back to normal. A circuit breaker can turn a potentially dangerous, damaging event into a simple flickering of the lights.

The heart of a circuit breaker lies in the rapid separation of two electrical contacts within the circuit. As they draw apart, an electric arc forms between them and is contained, cooled, and extinguished by insulating gas that fills the circuit breaker. Once this is done, the contacts can close the circuit again and service resumes. Most modern high voltage circuit breakers extinguish the arc with a supersonic blast of SF6 (sulfur hexafluoride) gas.

The kind of current running through a power line is so strong that it can continue to flow through air even once the contacts have been separated, necessitating fast jets of insulating gases to "cut" the arc. In this video, arc-prevention and suppression systems have failed.

A team of researchers from ABB Corporate Research Center in Switzerland recently performed an experiment that will help scientists better understand this process and assess the validity of existing theoretical models. Their results are published in the American Physical Society’s journal Physical Review Applied.

While the idea is simple, the reality is not. “A brief interruption of the flow of current—barely noticed by users and essentially consisting of the simple separation of two contacts to draw an arc between them—is actually a complex process that involves physics spanning the range from mechanics (pistons, levers, and gears), to rocket science (supersonic gas flow through nozzles), to non-equilibrium plasma physics,” says Jan Carstensen, the scientist who led the experimental study.
Electric arcs have captivated the interest of scientists going back to Benjamin Franklin, but we still don’t have a complete understanding of the process. One key aspect of arc physics that scientists are striving to understand is the transition that takes place as the hot, ionized core of an arc comes into equilibrium with the surrounding gas. Currently there are simulations and models of this process, but not a lot of experimental data to compare them too.

“During the last thirty years, circuit breaker research focused more and more on complex computer simulations and modeling, and less effort was spent on improving and applying experimental techniques,” according to Carstensen. “As a result, today there is a lack of high quality experimental data to benchmark simulations and to verify new arc models.”

To meet this need, Carstensen and his colleagues at ABB Corporate Research Center set out to map three important properties of a high-voltage circuit breaker arc: the density of electrons, the density of ionized and neutral atoms and molecules (called heavy particles), and the temperature. By mapping these properties along the cross section of a three-dimensional arc, researchers can more easily distinguish between the hot core of an arc and the surrounding gas—and compare the experimental reality to the results of simulations and models.

Although they created and studied an arc just a few millimeters in diameter, the custom-designed experimental setup weighed more than one ton (the weight limit of the crane in their lab) and involved a steel tower almost 10 feet high. Assembling the test device safely took careful planning along with helping hands from interns and colleagues.

An image of the experimental arc. To see a video of the arc forming as the contacts are separated, visit the paper’s homepage on Physical Review Applied.
Image Credit: J. Carstensen, P. Stoller, B. Galletti, C. B. Doiron, and A. Sokolov, Phys. Rev. Applied 8, 024002 – Published 2 August 2017, https://doi.org/10.1103/PhysRevApplied.8.024002 (CC BY 4.0).

The team used a technique called interferometry to create a visualization of the cross section of the three-dimensional arc and the blast of cooling gas. Interferometry is a way to measure tiny distances based on the interference pattern of waves. Employing a variation that uses two different colored lasers, the researchers determined the electron density, heavy-particle density, and temperature profiles of a cross section of the arc at varying blast pressures and currents that are typical of high voltage circuit breakers immediately before the current is interrupted. These results can help scientists refine their models and, in doing so, shed light on the complex process by which circuit breaker arcs are extinguished.

When there is trouble in the electrical grid, the results can include dangerous electrical activity (like the high voltage arc in the video above, or this vaporizing lightning strike), costly equipment damage, and serious interruptions of daily life with wide-ranging consequences. With this new work, we are a big step closer to understanding the complex physics at play when a high voltage circuit breaker trips, and to designing safer and more reliable systems.

Kendra Redmond

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