Medium-Voltage Converters Can Replace Grid Transformers

Power Electronics INSIDER

To electrify everything from vehicles to heating systems to stovetops, the U.S. grid must expand by about 57% and get more flexible, too. Solar and wind energy are the renewables most likely to dominate a future clean energy grid. But they are found primarily in remote areas, far from the hubs that need their power, which is a burden on the transmission system.

Over the last four years, the NREL team has built a testbed to study back-to-back medium-voltage (1 kV to 35 kV) converters, which could replace medium voltage transformers at a fraction of the weight and cost.

“This is a new type of equipment that opens up a whole new way for a utility to manage their distribution systems,” said Barry Mather, a researcher at NREL. “Much higher levels of distributed energy resources could be put onto a circuit if you had this extra device within the system.”

“The U.S. grid is little unique,” Mather said. “We have medium-voltage distribution that ends up pretty close to our customers.” The medium voltage feeders bring power to a local transformer that reduces it to standard household levels of 208/120 volts.

Say a tree limb falls on a power line, cutting off electricity to a neighborhood. Today, a utility worker can open and close switches in a transformer to move power from a nearby line into the disrupted one and restore at least some power while workers remove the tree. But if we can replace a transformer with a back-to-back converter, power could be redirected automatically, reducing outage times.

Power sharing is not just beneficial in emergencies. Many of today’s rooftop solar panels must first convert their energy to low voltages and then to a medium voltage. With medium-voltage converters, that energy can skip an unnecessary hurdle.

“It’s kind of like arbitrage of power across the distribution system,” Mather said. “It allows you to mitigate a lot of the impacts of distributed resources, specifically solar.”

In short, medium-voltage converters could help utilities safely control how and where electricity moves through local grids. And that could help make a future clean energy grid both resilient and reliable.

Medium-voltage converters are still a relatively new technology. That means they must undergo extensive testing to ensure they are efficient, affordable, and safe enough to introduce to the U.S. power grid.

But it is not easy to build or study them. “The checklist for startup, operation, and shutdown procedures had to be written and rewritten,” said Ramanathan Thiagarajan, a research engineer at NREL.

For the NREL testbed, the team constructed a replica of the kind of power system that could benefit from their medium-voltage converters. Safety — for the crew but also the prototype — was the team’s top priority. The whole testbed, which takes up a space about the size of a city bus, is enclosed in a black fence.

The team’s academic partners built one prototype: a 10-kilovolt silicon-carbide-based power converter that is designed to work back-to-back with its twin. When the researchers were ready to plug the prototype into their grid replica, they nudged the voltage up bit by bit until they reached the medium-voltage range. The prototype was too precious (and expensive) to risk damaging with a voltage surge.

“We did get up to the 5-kilovolt range,” Mather said. “It’s not a massive number, but that’s higher than anybody else has operated at.”

Next, they plan to open their testbed to more research on how medium-voltage devices could help the grid adapt or rely on hydrogen as another source of clean energy. The NREL team also hopes to reach even higher voltages in an entirely new facility. “Medium-voltage power conversion in general looks like its day is rapidly approaching,” Mather said. “We’re going to see more and more applications and more and more people interested in this.

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