Proving out DC Fast Charging — in the Extreme
Solid-state transformers enable medium voltage connections in this next-gen charging system.
The ability to charge electric vehicles at a very fast rate is a key to electrifying mobility across the U.S. It’s a focus of the U.S. Dept. of Energy’s Extreme Fast Charger project, whose potential was demonstrated convincingly on a production GMC Hummer EV at the American Center for Mobility (ACM) in Ypsilanti, Michigan, last fall.
The Hummer EV observed by SAE Media featured an 800-volt electrical architecture –considered state-of-the-art for DC “extreme fast” charging. Both GM and Delta Electronics (Americas) Ltd., which provided the charging system, are partnering with DoE on the program. The results of the demonstration were “phenomenal,” said Jim Khoury, senior manager of global electrification at GM.
The parameters for the extreme fast charger were established in an earlier test that also involved a Hummer EV. That test, spanning nine minutes at 500 amps at an average 725 volts, yielded nearly 55 kWh before the EV’s battery started to limit the current.
“While other DC fast chargers can also charge at 500 amps, the power capability is lower, so it power-limits sooner in the charge cycle. Our power limit is 400 kW,” noted Dr. Charles Zhu, VP of the Automotive Business Group for Delta Electronics and the principal investigator for the DoE project. He said that generally, the Delta extreme fast charging system can deliver 66.7 kWh in 10 minutes.
Delta Electronics developed the solid-state transformer (SST), power cabinet and the charging stand/dispenser that are vital to the project. The SST converts the “medium voltage” (13,200V AC) into 1000V DC. The power cabinet uses the DC voltage to create a current source, providing up to 500 amps. The dispenser communicates with the vehicle and provides current to the battery pack.
Current-generation fast chargers are considered low-voltage. Tesla’s Supercharger, for example, runs at 480V. The move to medium voltage presented hurdles, explained Zhu. “Medium voltage can jump a wider gap because of its higher potential energy,” he said, noting special wire, materials and designs were needed to accommodate the higher voltages. From a safety standpoint, the DoE-Delta extreme fast charger has a medium-voltage switch to isolate the system from the grid. A licensed electrician with proper personal protection equipment must close the switch.
“If something goes wrong, the ‘blast radius’ is about 30 ft [91.4 m], which is why all others must stand clear when the switch is actuated,” Zhu said.
There are strong reasons to opt for medium voltage, according to Zhu. “The advantages that we gain from pulling directly from medium voltage include higher efficiency, as a conventional transformer can be about 95% efficient,” he noted. A conventional transformer needs to have energy pass through a conversion stage — another loss — to create the DC current needed to charge a vehicle battery, he added. The medium-voltage system provides an approximate 96.5% output and eliminates the conventional transformer from the charging process.
“13.8 kW medium voltage and charging current up to 500 amps are key features to enable energy efficient and highly scalable extreme fast charging,” Zhu said
Michael Standing, Delta Electronics program manager for the extreme fast-charger system, said that eliminating a traditional transformer in favor of a power conversion via a solid-state transformer equates to about a 3% efficiency gain.
“When you’re talking about 400 kW, three percent starts to be significant. The losses go out in heat, and you pay for the electricity that you’re wasting,” Standing said. He noted that the extreme fast-charging system would eventually be a less-expensive way to charge a vehicle.
Delta Electronics has multiple extreme fast-charger system patents relating to power-conversion topology and control. To reach production readiness, the system would need to undergo additional development, system integration and testing. Regulatory certification also required.