DC, Interrupted

By Simon Morrow

As John Shen likes to say, Thomas Edison was right—he was just 100 years or so too early.

In the late nineteenth century, Edison said that direct current (DC) is a better form of electricity than alternating current (AC). Shen, the Grainger Professor of Electrical and Computer Engineering at Illinois Institute of Technology, agrees.

“DC has always been more efficient and stable compared to AC electricity,” says Shen. “Unfortunately, at the time, people didn’t know how to raise the voltage level in DC for long-distance transmission, but they did for AC. That’s why AC won the war. But that’s changed.”

Around a decade ago, Shen started to see signs that emerging technologies would benefit from greater use of DC power, so he set out to eliminate one key remaining barrier: a lack of effective, cost-efficient DC circuit breakers.

“You can buy an AC circuit breaker from a Home Depot for $5 or $10 that is very reliable, but you can’t use those for DC power networks,” Shen says. Since Shen started working with DC electricity, the Advanced Research Projects Agency Energy (ARPA-E), of the United States Department of Energy, has awarded him $2.65 million to develop DC fault protection technology and has started a new funding program dedicated to developing medium-voltage DC circuit breakers, credited in part to his work.

A prototype of the iBreaker, a 380 volt solid-state circuit breaker for DC data centers

DC in Your Home or Office: iBreaker

Shen and his research team have been working since December 2017 to develop state-of-the-art versions of solid-state circuit breaker technologies.

While many conventional circuit breakers work by melting a fuse or triggering a flip to switch, solid-state circuit breakers contain electronics that monitor the flow of current and use algorithms to decide when to switch it off. These semiconductor switches are more precise, reliable, and durable, and stop the flow of electricity much faster.

Among the prototypes that the Illinois Tech graduate students on Shen’s research team have developed include a 380-volt iBreaker for DC data centers and a new version of Smart Plug devices, which control home appliances via Wi-Fi while also protecting against fire or shock hazards in homes and offices.

DC power has an inherent efficiency advantage over AC because it constantly transmits peak voltage and current while AC constantly fluctuates in a waveform, alternating between a maximum and minimum.

“With DC, you’re utilizing the hardware resources constantly, versus AC, where you’re only utilizing [the hardware resources] a fraction of the time,” says Shen. “For the same voltage level, you get 40 percent more DC power using the same set of cables than AC.”

This makes it the preferred power method for high-voltage applications like electric cars, aircraft, and boats, and already almost all of our electronic loads such as computers and printers run on DC power.

Many of our renewable power sources such as wind turbines and solar panels already generate DC power. But with the average wall outlet still set up to transmit AC power, there is often still an intermediate step.

“By using DC power directly,” Shen says, “you’d eliminate the DC-AC-DC power conversion stages that hurt your energy efficiency.”

The Future of Flight

While he continues to develop iBreaker, Shen has taken the lessons from that project to move on to bigger things. Where iBreaker is an improved version of the standard ideas of how DC circuit breakers should be made, Shen is now developing proprietary technology called superconducting momentary circuit interrupters (SMCI).

“It breaks the previous paradigm and is very different from anything in the market or anything that previously existed in the technical literature,” he says.

Shen was awarded a $779,374 grant in March 2021 from ARPA-E to develop this technology for the use of circuit protection in turboelectric aircraft, which require massive power outputs to run.

Turboelectric aircraft are expected to be the new design for commercial aircraft within the next 20 years. Unlike current jets, which burn fuel to power propellers directly, turboelectric aircraft will burn fuel to generate electricity and then use that electricity to power propellers, which turns out to have huge efficiency benefits.

“Jet technology hasn’t changed much in the last 70 years,” says Shen. “But turboelectric aircraft, being propelled by many electric fans distributed across the body of the aircraft instead of just at the wings, represents a fundamental and exciting shift in air travel as we know it, with the potential to reduce air travel emissions by up to 90 percent. This technology will be a critical step to make that a reality.”

Each turboelectric plane will be powered by up to 50 megawatts of electricity, enough to power a small city, so ensuring these planes continue to function safely while in the air is critical.

Grainger Professor of Electrical and Computer Engineering John Shen

To develop the SMCI concept, Shen considered the limitations of existing technology. Solid-state circuit breakers can shut off a fault current quickly, but they tend to produce a lot of heat when a current flows through the semiconductor material. For high-powered turboelectric aircraft, this is a major efficiency and weight drawback.

Hybrid circuit breakers exist with lower power loss, but their slow response time would allow the fault current to rise dangerously, causing excessive stress to the power system.

To achieve the best of both worlds, Shen has developed a design to temporarily interrupt the current flow to buy time for a mechanical isolation switch to open safely.

When everything is running normally, current flows through superconducting materials in the SMCI, resulting in ultralow power loss of less than one watt, even lower than hybrid designs that tend to lose a few watts.

When a problem occurs, the SMCI interrupts by injecting a voltage into the system that counteracts the normal flow of electricity, driving it to zero within 10 microseconds, even faster than solid-state circuit breakers, which tend to take around 100 microseconds.

The counter voltage produced by the SMCI holds the current at or near zero for a few hundred microseconds while a series of mechanical switches open, isolating the problem. This idea of using a counter voltage is brand new, offering a paradigm shift in the way electricity is controlled in the system.

Shen has preliminary proof of concept results showing that the design works. “Now we’re working to increase the power level and perfecting the art,” he says. ●