Breakthrough technology from an engineering research center could help prevent blackouts while allowing the grid to run on clean energy.
By Tom Gibson
New Yorkers got a harsh warning about their overtaxed power grid this past July, when successive weekend blackouts paralyzed large swaths of Manhattan. The first stranded subway passengers and shuttered Broadway shows, sending performers into the street to entertain disappointed theatergoers. The second left more than 50,000 residents to swelter in darkness during a brutal heat wave. “The truth is this: We simply do not have infrastructure that can respond to climate shocks,” City Controller Scott Stringer wrote in a Daily News op-ed. “And it’s only going to get worse.”
Is he correct? Maybe not. Researchers at North Carolina State University report promising strides toward preventing future blackouts by mitigating the strain on electricity supplies brought by spikes in air conditioning. Chief among innovations to emerge from the school’s FREEDM Systems Center is a novel transformer that could soon make the grid both more manageable and better able to rely on renewable energy.
Established during the dark days of the 2008 financial blackout, the multi-institutional program seeks to “foster a revolution” in the electric-power industry as dramatic as the computer world’s pivot from mainframes to laptops. The problem, investigators contend, isn’t the lack of solar or wind power but the “infrastructure needed to deliver and manage large-scale, distributed renewable energy resources.” By developing breakthrough technologies in key areas like energy storage and transmission, the researchers envision creating an “energy Internet” that will permit seamless control at the distribution level—turning today’s one-way flow into a multilane conduit that can receive and send energy from renewables without frying the wires or interrupting service.
Solid Support System
Now at the end of its $36.3 million award from the National Science Foundation, FREEDM (for Future Renewable Electric Energy Delivery and Management Systems) can point to a number of milestones as it begins its 12th year. Among them: producing 10 start-ups and more than 100 inventions that range from low-cost, plug-and-play residential solar photovoltaics to a mobile energy management system for tents, emergency shelters, and other isolated, off-grid buildings powered by wind, solar, or diesel generators. Membership has expanded, too. Beyond the five original institutions—Arizona State University, Florida A&M and Florida State Universities, Missouri University of Science & Technology, and NC State—the center now counts over 20 industry partners whose roster includes Swiss power-grid colossus ABB, Duke Energy, and the New York Power Authority, plus leaders in fuel cell and energy storage, alternative energy systems, and software development.
Perhaps most critically, the revenue stream has grown and diversified, enabling FREEDM to build a $1.5 million endowment and continue without such heavy reliance on NSF. Staffed by alumni, faculty, and postdoctoral researchers as well as other NC State employees, the center generates some $12.5 million annually in research funding. Current projects range from a U.S. Department of Energy-sponsored effort to develop an intelligent, grid-friendly fast charger for electric vehicles to increasing smart-grid resilience. Meanwhile, strong academic programs for undergraduate and graduate students—who perform most of the work—guarantee both a steady source of lab hands and future industry talent. According to its website, the center has helped train more than 140 Ph.D. and 100 master’s graduates in electric power systems engineering since its inception.
NC State is far from the only institution focused on modernizing America’s aging electrical power generation and distribution system. The topic is deemed such a priority that the National Academies has undertaken, at the request of Congress, a comprehensive evaluation of strategies for improving the grid’s reliability, resiliency, business models, and cybersecurity. Chaired by Granger Morgan, a Carnegie Mellon University professor of engineering and codirector of both the Center for Climate and Energy Decision Making and Electricity Industry Center, the committee has held two meetings, in March and May.
Sustainable energy also is a hot topic on many campuses. The University of Michigan, for example, offers a master’s degree in engineering sustainable systems, while the University of Massachusetts–Lowell has master’s and doctoral degrees in renewable energy engineering. Research hubs and test beds have sprouted on several campuses as well, such as the Energy Systems Innovation Center and Smart Grid Demonstration and Research Investigation Lab at Washington State University and the UCLA Smart Grid Energy Research Center.
Few have enjoyed the working prototype success of the FREEDM center’s signature development: solid-state transformers (SSTs). Invented around 2010 and currently in its fourth generation of improvement, the technology shows increasing promise as “a replacement for conventional transformers,” contends FREEDM Systems Center director Iqbal Husain, the ABB distinguished professor of electrical and computer engineering at NC State. “You can call it a controllable transformer.”
For renewable energy to take hold, researchers must solve a core problem: Today’s power flows one way—from large coal-fired, natural gas, or nuclear plants to consumers. Because service must remain uninterrupted, the grid has a hard time accommodating power from intermittent renewable sources like wind and solar. “It’s the power balance,” explains NC State’s Husain. “The load always has to match the generation.” He cites an example from Hawaii, when an excess of solar energy pouring into the grid during midday caused instability and brought down the whole electric power system. Digital technologies, sensors, and two-way communications offer a way to balance demand with power generation and bring renewables online. Ultimately, a smart grid will allow users to sell electricity back to utilities from solar panels or idling electric vehicles.
The idea of a smart grid that also can handle power flowing in reverse has been around for years. A new FREEDM center computer modeling study indicates the concept could become a reality in the near future, using technology that already exists along with one vital innovation: solid-state transformers. SSTs can distribute the energy in a controlled way so when there’s excess generation in one place, it can shift it to an area that needs power. “But you also need to have some storage capabilities, like batteries,” Husain says. When there’s excess power, the SST can put it in storage and then release it when there’s more energy demand and less renewable energy being generated, for example at night. “The solid-state transformer can manage power distribution so there’s always a balance between the load and the generation,” says Husain.
Few Moving Parts
Often found at substations and utility pole tips, transformers rely on copper wires wrapped around an iron core to step AC voltage up and then down when conveying power from the generator to homes or businesses. SSTs are made of semiconductor devices such as transistors, integrated circuits, and diodes. While SSTs have iron cores, “the components and construction are completely different,” says Husain, adding that they need no oil for lubrication. SSTs also are made of a new material that can operate at higher frequencies—20 or 30 kilohertz compared with 50 or 60 hertz for a regular transformer. Thus, they can be made much smaller, lighter, and more compact while retaining “all the functionalities of a regular transformer stepping up or down,” says Husain, noting that prototype demonstrations in the lab have shown the workability of various concepts.
In simple terms, these controllable transformers act as energy-routing devices between the grid and consumers, redirecting power as needed to address changes in supply and demand. That capacity to store and later tap surplus solar or wind energy is considered key to making renewables viable on a broad scale. A recent FREEDM center study using computer models found that SSTs could indeed create a reliable smart electrical grid—and potentially replace the decades-old electromagnetic and mechanical systems found in today’s behemoth utilities.
“The potential is tremendous because there are several advantages of using solid-state transformers compared to the ones we have today,” says Aranya Chakrabortty, an associate professor of electrical and computer engineering at NC State and control-theory specialist who designed the SST’s controls. But “solid-state transformers are complicated devices” that have “very nonlinear behavior,” he notes. His responsibility on the latest SST iteration involves designing “nonlinear control systems that can make sure the current and voltages and frequencies and other electrical variables associated with the SST are controlled the way they should be, despite the different kinds of disturbances happening in the grid.”
With a team that includes three Ph.D. students and one pursuing a master’s, Chakrabortty is working to refine the design. The team first created high-fidelity models to carry out the multiple simulations needed to ensure the controllers they built “perform as we expect them to.” The students wrote down all the SST’s modeling equations and then coded them in MATLAB and Simulink programs. They also wrote the controller equations and simulated different kinds of operating scenarios that can commonly arise in a distribution grid, such as loads in the system, faults, disturbances. “Once we had confidence about the software part of the simulation, then we implemented it in a realistic hardware model of the transformer in our lab,” Chakrabortty says.
Charged Up for the Future
Reza Ghorbani, an associate professor of mechanical engineering and founding director of the Renewable Energy Design Laboratory at the University of Hawaii–Manoa, offers a firsthand glimpse of the need for SSTs. A few years ago, he notes, rooftop solar panels began popping up everywhere—propelled by a combination of high energy prices, abundant sunshine, and federal and state tax credits available for going solar. As a result, 12 percent of utility customers on the island of Oahu, where most of the state’s population lives, have rooftop solar. That compares with an average of 0.5 percent on the U.S. mainland.
Solar’s popularity—coupled with the intermittent nature of energy output—posed problems for the state’s main utility, however. Concerned that an excess of home-generated power risked fluctuations in the grid’s service or sporadic blackouts, the Hawaiian Electric Company (HECO) proposed a moratorium on granting permits for new solar-panel installations. “There are still issues regarding voltage variability and frequency disturbance on the grid, and managing the stability of the grid is an issue,” observes Ghorbani, who calls SSTs “an important enabling technology for a place like Hawaii.”
SSTs already have proven their worth in one application: electric-vehicle charging stations. “Similar technology can be used in the grid when you have lots of battery storage and DC sources,” explains Ghorbani, noting that SSTs will enable bidirectional power flow, allowing cars to charge from or feed power into the grid. He predicts “we will see more batteries in different parts of the grid—in homes, electrical vehicles connected to the grid or charging stations, commercial buildings, feeders, and substations.”
The FREEDM center has field projects demonstrating these technologies. Last year, for example, researchers built an EV fast charger that is one-tenth the size of existing advanced systems and wastes 60 percent less power while charging. Conventional 50-kilowatt (kW) charging systems include a heavy (1,000 kilograms) distribution transformer, which draws alternating-current (AC) power from a medium-voltage utility line and steps it down to 480 volts, which the fast charger then converts to direct-current (DC) voltage compatible with the EV’s battery. The whole system typically must be installed on a concrete slab. By contrast, the FREEDM center’s device weighs roughly 100 kg and does the work of both the transformer and charger, so it can be mounted on a wall or pole. The team now seeks to create an even more rapid charger that can handle multiple vehicles simultaneously.
“If you use a solid-state transformer, you can switch back and forth between AC and DC connections without causing any kind of instability or other troubles,” explains Chakrabortty. “That’s going to be a major thing because DC is much more efficient and less costly. If there’s any kind of device that lets our homes use both types of connections, it could save a lot on electricity bills.”
Don’t look for widespread slaking at the meter anytime soon, however. Beyond successful field demonstrations, costs would have to fall dramatically for SSTs to become commercially viable, cautions NC State’s Husain. Chakrabortty concurs. “At lab scale studies, we consider it ready, but to translate that into massive deployment in every house and neighborhood, that is going to take another couple of years.”
Still, there has been notable progress toward bringing intelligent grids online sooner rather than later. GridBridge, a FREEDM center spinoff, was recently acquired by ERMCO (Electric Research and Manufacturing Cooperative), a Raleigh, N.C.-based company that manufactures distribution transformers. The firm is installing solid-state transformers in the field for trials. GridBridge’s commercially available Grid Energy Router combines power electronics with software and communication capabilities to improve distribution efficiency, power reliability, and conservation while integrating renewable generation and energy storage.
Heading into the post-NSF era with expanded staff and labs, the FREEDM center vows to remain focused on grid-related research on power electronics, power transmission and distribution systems, and distributed energy resource management. “But as times change, so must our vision,” cautioned Husain in his 2018 annual report. The center’s broader mission, he wrote, will include electric transportation technologies such as new inverters, novel motor topologies, and high-power EV charging.
Meanwhile, don’t be surprised if SSTs start populating smart grids fed by solar panels and wind turbines around the country. A glimpse of that future is on full display at NC State’s Keystone Science Center, where a green energy hub houses a 12-kV loop, SST, and lab for demonstrating the full FREEDM system—including connecting 40 kW of rooftop photovoltaics with four EV charging stations. Knowledge indeed is power.
Tom Gibson is a consulting mechanical engineer and freelance writer specializing in engineering, technology, and sustainability. He publishes Progressive Engineer, an online magazine (www.ProgressiveEngineer.com).
Design by Toni Rigolosi