What’s the trick to keep grids humming along on solar, wind and batteries alone?

Someday, power grids could run entirely on solar, wind, batteries and other sources of carbon-free energy — but the grids will need a new type of beating heart at their center. Testing and demonstrating the technology necessary for that job is no simple task.

Since electrical grids were first built, they have been anchored by massive spinning turbines powered by fossil fuels, hydropower or nuclear power. These central generators provide a stable frequency ​“pulse” for the alternating-current (AC) power that energizes the grid.

To even get close to 100 percent carbon-free energy, that pulse needs to be provided by another means. Enter digitally controlled inverters that can be programmed to do the job. Inverters are equipment used to turn the direct-current output of wind turbines, solar panels, batteries and other resources into grid-friendly alternating current. Grid operators will need to fortify a few of these inverters to set the pulse — these are called ​“grid-forming” inverters because they form the backbone of a grid.

Showing how this can be done at scale is the goal of the U.S. Department of Energy’s new Universal Interoperability for Grid-Forming Inverters Consortium, known as Unifi, launched last month. Members of the consortium include dozens of research organizations, national labs, utilities, grid operators and inverter manufacturers.

“It’s a way to get the entire industry on board with this massive transformation that’s about to happen on the power grid,” Ben Kroposki, director of the National Renewable Energy Laboratory’s Power Systems Engineering Center, said at an August event unveiling the program.

Unifi is being funded by $25 million from DOE, which will be matched by $10 million from project partners. The investment needed to upgrade inverters is a drop in the bucket compared to the massive investment needed to build out solar, wind, batteries and other resources to reconfigure power grids to run solely or primarily on clean energy — but it’s critical.

The consortium’s work over the next five years — including a 1-megawatt test of multiple companies’ technologies and a 20-megawatt demonstration at a real-world utility test site — could be a pivotal step in giving utilities and grid operators the confidence that the transition can happen without major disruptions, Kroposki said.

Building this confidence is important for hitting the clean energy targets of countries around the world, including the Biden administration’s goal of a zero-carbon grid by 2035, he said. Some mainland grids are already reaching renewable penetration levels approaching half of their total resource mix, and some islands are exceeding that.

“But in order to get above 50 percent — and basically to operate at 100 percent at some times of the day — that’s really where the grid-forming inverters need to kick in,” he said. 

From grid-following to grid-forming: Providing stability for the clean grids we need 

To better understand what a grid-forming inverter does, it’s important to understand the ​“grid-following” role played by almost every inverter today. Think of a grid-forming inverter as a conductor, setting the tempo of the music the orchestra is playing. The grid-following inverters are akin to musicians who follow the conductor’s lead.

Here’s how Rick Wallace Kenyon, a member of NREL’s power systems engineering center team, described a grid-following inverter during an August episode of The Energy Transition Show podcast:

You have this inverter, and it uses a device called a phase-locked loop. […] What it does is at the point of interconnection — literally, the terminals of the inverter — it’s measuring the voltage waveform at that point, and it’s locking on. […] All of these inverters on the power system are sort of presupposing the presence of voltage and frequency. They are explicitly relying on something else to create this energized power system. 

That ​“something else” has always been massive spinning generators, Kenyon said. The steady ​“heartbeat” of these rotating steel cores has been built into every aspect of how grids operate, from managing the constant balancing of electricity supply and demand, to preventing system collapses like the Eastern U.S. 2003 blackout or the one that came within minutes of taking down the Texas grid in February 2021.

That’s not to say that inverters can’t be programmed to operate independently of the grid. In fact, grid-forming inverters have been sending energy through stand-alone and isolated power systems for years now, said Deepak Ramasubramanian, a technical leader at the Electric Power Research Institute, which is involved in the Unifi project.

The challenge now is scaling it up to large networks, while ensuring that when you have different inverter manufacturers,” their inverters ​“still work together,” he said. 

“We’re not coming at this from the cold,” Ramasubramanian emphasized. He highlighted the example of Australia’s Energy Storage for Commercial Renewable Integration project. Escri uses a 30-megawatt/8-megawatt-hour battery with a grid-forming inverter to balance about 90 megawatts of wind power on a peninsula with about 5 megawatts of load and only a single connection to the larger grid.

Hitachi ABB Power Grids designed Escri’s control architecture to make its battery inverters mimic what a spinning generator would normally provide to the grid. Similar approaches allow smaller-scale microgrids to run on 100 percent renewable power or switch between clean energy and spinning generators without disruptions, said John Glassmire, a senior adviser with Hitachi’s grid-edge solutions group.

“We’ve technologically solved this,” he said. “[But] there are some elements of getting everybody to play together — that’s important.” 

That’s because things get more complicated as the scope of this grid-balancing task starts to expand to broader areas and encompasses greater numbers of inverters. At larger scales, if not designed and coordinated appropriately, multiple grid-forming inverters may start to ​“fight” each other to control the frequency of the grid they’re connected to, Ramasubramanian said.

This means that at wider scales, ​“every grid-forming resource, to a certain extent, also has to be grid-following — to see what’s happening on the grid and adapt itself based on that,” he said. ​“So the definition of ​‘grid-forming’ will change” depending on what the inverters are being asked to do.

For example, Unifi plans to test inverters to ​“black-start” power grids after they’ve collapsed, a task that today requires rotating generators with special equipment and tight coordination between power plants and grid operators.

“There’s no signal from the grid, so the inverter has to generate its own signal to start up,” Ramasubramanian said. ​“But that’s a nuanced operating scenario, which we can hopefully minimize with improved planning.”

More common will be circumstances when parts of the grid experience dramatic rises and falls in inverter-based inputs, as wind and solar output ramps up and down, grid connections are broken or spinning generators are unexpectedly tripped offline. In these cases, inverters will need to adapt to conditions at subsecond speeds, and with an understanding that every other inverter on the system is also making its own simultaneous adjustments, he said.

That will require the application of ​“group control” methods, according to Ramasubramanian. In simple terms, this concept calls on individual devices to act in ways that won’t end up fighting each other, but instead will target a compromise approach ​“that all individual [inverters] can settle on,” he said.

Ideally, the end result is a system that looks more like the right side of the following graphic, with the inverters connecting solar and wind farms, rooftop solar systems, batteries and electric vehicle chargers all working together to stabilize the grid.

This graphic comes from a 2020 NREL report that details the multiple technical approaches that could achieve this outcome.

But ​“at the end of the day, the control architecture is proprietary [to the] vendor making the inverters, and we want to protect that,” Ramasubramanian said. ​“At the same time, we want to allow system planners to get the type of services they may need inverters to provide.”

Coordinating grid equipment for a common goal 

That’s where the Unifi consortium comes in. Utilities rely on ​“accurate models of their grid” and need to ​“ensure the technologies react the same way in the field as they do inside their simulation programs,” Kroposki said. ​“Right now, there’s a lot of uncertainty still about how best to model these technologies.”

One of the first tasks will be working with the inverter makers involved — Danfoss, Eaton, Enphase Energy, General Electric, Hitachi ABB Power Grids, Siemens and SMA — to define the technical standards at play. Existing guidelines, including the Institute of Electrical and Electronics Engineers 1547-2018 standard, already define the ​“smart inverter” capabilities required in California and a handful of other jurisdictions, but grid-forming standards add another layer of complexity.

The first real-world tests of inverter interoperability will come in a 1-megawatt pilot project with inverters ​“demonstrating they’ll operate without a lot of advanced controls,” Kroposki said. The team will also test inverter-based black-start capabilities, which ​“has not been done at significant scale across the system,” he said. NREL’s Energy Systems Integration Facility and Flatirons Campus will provide the venues to test different combinations of wind, solar, batteries, inverters and grid controls.

At the end of the project, Unifi plans to demonstrate a combination of grid-forming inverters at ​“20-megawatt-plus” scale, which is expected to be hosted by one of the participating utilities, he said. Those include utilities in solar-rich territories such as Hawaiian Electric, Pacific Gas & Electric and Southern California Edison, as well as Commonwealth Edison, the New York Power Authority, PacifiCorp, Portland General Electric and Southern Co. Transmission grid operators ISO New England and the Midcontinent Independent System Operator are also participating, representing the entities that are in charge of maintaining grid reliability across entire regions.

Hawaiian Electric is ​“one of the leaders in putting inverter-based technology onto the grid,” with enough wind and solar power in its territory to push its island grids beyond 50 percent renewable energy for significant periods of time, Kroposki said. California’s utilities also manage grids that get most of their power from solar at times.

“We have already been working with PV inverter manufacturers and wind inverter manufacturers to help maintain grid stability,” Kroposki said. First Solar has used solar farms to mitigate transmission grid fluctuations in California and Chile, and wind developer Avangrid has done similar work with wind turbines in California. The Electric Power Research Institute has been working with Texas grid operator ERCOT and Midwestern grid operator Southwest Power Pool to determine how regions with growing wind capacity could help set frequency and voltage on stressed portions of the grid.

The inverters connecting rooftop solar, batteries and EV chargers can ​“also provide an array of grid support,” Kroposki said. ​“It depends on where they’re deployed and what market exists for those services.” So far, however, efforts to use distributed energy inverters to provide grid services are still in their infancy, he said. 

The next five years could see record-high levels of renewable energy expand beyond island grids to large swaths of the U.S., Ramasubramanian said. Just because these high levels of renewables are already happening on power grids without major incidents doesn’t mean this will always be the case, he said.

“Sometimes you can have 100 percent solar if the grid is not connected to anything else, and just by chance, the load on the network and the solar power were exactly matched at that point in time,” he said. ​“But if there was one small disturbance on that network, the grid could have gone down.”