The electric power industry is witnessing a sea change in how the grid works. With ambitious goals for decarbonization and current installations in some parts of the world providing 75%–90% instantaneous generation from renewables, the physics of the grid are shifting.
Today, most renewable generating sources, storage batteries, and a variety of electricity-powered devices like electric vehicles (EVs) connect to the grid using inverters. Inverters are programmable electronic equipment that mimic traditional grid-connected generators and devices in their operation. These are called “grid-following” inverters because, as the name implies, they simply follow the grid and inject power into it.
Under the existing framework for power system operations, automatic controllers respond to physical signals produced by spinning turbines. But grid-following inverter-based resources do not produce those same signals. And we already know that grid-following inverters may not behave correctly in a variety of circumstances. For example, they may cause installations to withdraw from the grid during faults or large changes in load, threatening an outage. Are we reaching a tipping point when the 20th-century approach to controlling power on transmission and distribution networks is no longer ideal for 21st-century systems?
A group of power system experts from around the world debated this topic at the second general meeting of the Universal Interoperability of Grid-Forming Inverters Consortium (UNIFI). With funding from a U.S. Department of Energy (DOE) grant, UNIFI participants have spent the past two years querying industry, constructing models, testing theories, and observing systems to understand the following:
- How quickly we could reach a very high penetration of renewables on our grids.
- What types of problems to anticipate.
- Whether and when we need a new generation of inverters — called “grid-forming” or GFM inverters — to ensure stability, optimal operations, and black start capability.
- How to achieve interoperability among a wide array of devices that will connect to our future grid.
Image 1 — Participants at UNIFI General Meeting in Orlando, Florida, July 2023
UNIFI’s external advisory board, which includes experts from Europe, Asia, and the Americas, participated in the meeting to critique the project and help the consortium carry out its work. As a nontechnical guest at the meeting, I observed presentations and interviewed participants, and I noted that the idea of a tipping point came up frequently. Based on my lived experience as a Texas resident, I realize it is approaching quickly.
The Texas Tipping Point? Heat, Renewables, Outages, and New Rules
In 2023, Texans endured one of the hottest summers in recent memory. Day after day, temperatures exceeded 100 degrees Fahrenheit across much of the state. Demand for electricity broke records, then broke records again. On Aug. 1, 2023, for example, demand broke the previous August record by more than 5,000 MW, and then broke that record again on Aug. 7, then Aug. 9, then Aug. 10. And the Electric Reliability Council of Texas (ERCOT), which operates a grid that provides power to 90% of the state’s customers, met that demand with generation exceeding past amounts as well.
At the most recent UNIFI meeting, Ben Kroposki, organizational director of UNIFI and director of the Power Systems Engineering Center at the National Renewable Energy Laboratory, reported that the ERCOT grid has periodically hit more than 70% instantaneous generation by renewables. In other words, the penetration of renewables on the ERCOT grid is increasing rapidly, and wind and solar power — when available — are increasingly responsible for meeting peak load demands, especially in Texas’ hot weather. We are relying more and more on inverter-based resources in Texas.
The grid-following inverters that link renewables to the ERCOT network have failed on occasion. During the Odessa disturbance events in May 2021, June 2021, and June 2022, the Texas grid lost generation from solar installations when the control devices responded to faults elsewhere on the system. The inverters, acting as programmed, removed solar power from the grid, which was not the intended result.
Though each incident was slightly different, together they reflected shortcomings in the way grid-following inverters are designed and programmed to perform on interconnections. While the outages did not directly affect customers, they were significant enough to entail analysis by ERCOT, the Texas Reliability Entity (which monitors and enforces ERCOT compliance with national reliability standards), and the North American Electric Reliability Corporation (NERC). The agencies noted in the report on the 2022 disturbance that this type of outage posed a significant risk to the bulk power system. Both reports called for improved compliance with NERC recommendations for reliability, and the later report underscored the urgent need for updated NERC standards for inverter-based resources connected to the grid.
In response to the Odessa disturbances and similar problems elsewhere in the world, ERCOT recently issued a proposal to require upgraded grid-following inverter technology to avoid these types of outages in the future. The proposal calls for all inverter-based resources, both currently connected and planned for, to upgrade their grid-following connection technologies by 2025.
While representatives from the renewables industry expressed support for the new standard in concept, the cost of making the upgrades, some ambiguity about how well the changes will work, and the speed with which the transition would have to take place is causing great consternation. The ERCOT system has been at the leading edge of integrating renewables. ERCOT collaborates with UNIFI and other initiatives to investigate how grid-following and grid-forming inverters affect the grid as they become more numerous. For example, ERCOT’s Inverter-Based Resource Working Group has already been conducting tests to determine how GFMs could mitigate the problems previously experienced with grid-following inverters. ERCOT put forward the current proposal to address an immediate concern, and it reflects the challenges all systems will face as the they reach an inverter-tipping point: What kind of changes are needed, and how fast? What will they cost, and will they work?
Taken together, the heat, role of renewables, outages, and new rules seem to indicate that the tipping point is upon us. The history of electrification in the United States illustrates similar transitions in the past — for example, the shifts from direct current (DC) to alternating current (AC) generation and transmission and from discrete central station networks to large, interconnected power systems. But the work of the UNIFI consortium to manage the current transition reflects a few significant differences in the challenges we face going forward.
A Historical Truism
Every expansion of electric power systems, introduction of new technologies, and advance in efforts to control and optimize interconnections results in the discovery of nuances in the behavior of electricity. In 1903, when discussing the great opportunities presented by long-distance transmission using high-tension power lines, Charles F. Scott — president of the American Institute of Electrical Engineers, an engineer for Westinghouse Electric Corporation, and Nikola Tesla collaborator — wrote, “Each increase in voltage introduces new problems, reveals new classes of phenomena and presents difficulties more complex.” At the UNIFI general meeting in July 2023, 120 years later, presenters despaired over the limits of their models to predict what will actually happen when the number of inverters on the grid passes the yet-to-be-determined tipping point. The truism holds. But the timescale of change is radically different.
Since Scott’s presentation in 1903, transmission distances have grown, voltages have increased, and the use of electricity has dramatically expanded. Yet over the past 120 years, transitions from one technology to another evolved slowly. For example, Thomas Edison introduced central station service for incandescent lighting using DC in 1882. Edison’s Pearl Street Station provided electricity for a 1-square-mile area. Eleven years later, George Westinghouse demonstrated the efficacy of AC for longer distances and multiple uses. Rotary converters, invented by Charles Bradley in 1888, facilitated the eventual use of both AC and DC on single networks through the end of the century.
By the early 1900s, most new power systems used AC. By the early 1960s, New York City’s power network was part of the giant Eastern Interconnection, which reached every state east of the Rocky Mountains and parts of Canada. But it was not until 2007 that New York abandoned its last operating DC network. One hundred and twenty-five years of transition.
In the case of integrating inverter-based resources, the transition is happening right now, and fast. As Deepak Divan, professor of electrical and computer engineering at Georgia Tech and part of the UNIFI team, explained to me, the push is to “deploy, deploy, deploy.” But the power electronics experts — those who design and program inverters — still aren’t entirely sure how to make inverter-rich systems work.
During the meeting, Kroposki described the timeline for integrating inverter-based resources into power systems, from 2001 to 2023. Engineers began to work out guidelines for grid-following inverters in 2001, and the industry agreed to simplified rules for grid-following inverters in 2015. Since 2020, a mere five years later, engineers have published numerous papers offering specifications for grid-forming inverters, including a proposed UNIFI specification and a proposed standard from the Institute for Electrical and Electronic Engineers. And as Deepak Ramasubramanian, senior technical leader at the Electric Power Research Institute (EPRI) and co-technical lead of UNIFI, explained during the meeting, UNIFI is evaluating whether present industry standards pose a barrier to adopting GFM. Under the old framework, the industry spent a century tinkering to keep the grid working; now, we have a handful of years to get it right.
What Can, What May, What Must Work
At the UNIFI consortium meeting, multiple teams reported in. We learned from small island systems what canwork. We learned from those modeling approaches and implementations what may work. We learned from regulators what must work.
What Can Work
On installations in the United States, Australia, and Great Britain, large-scale storage batteries connected to the grid with GFM have demonstrated success in facilitating black start (restarting a system after a complete outage), islanding (separating an area still operating from a segment of the system that has experienced an outage), and maintaining stability (keeping the grid in a state of equilibrium) during grid disturbances.
In a key case study, Kauai Island Utility Cooperative reached 90% instantaneous generation by renewables reliant on GFM. Some small island systems powered 100% by inverter-based resources operate successfully today with GFM. Systems are already in operation and heavily reliant on renewables and storage batteries, which increases the urgency with which the UNIFI consortium is proceeding.
What May Work
Grid-forming inverters, unlike grid-following ones, could participate in essential grid service markets, provide black-start support, and maximize resilience and reliability on larger systems. UNIFI teams are modeling these opportunities, offering specifications, and defining the value proposition for manufacturers, grid operators, and investors.
For example, engineers at the Pacific Northwest National Lab (PNNL) operate a 380-MW renewable energy facility in Wheatridge, Oregon. This facility combines wind, solar, and battery storage in one location. With DOE funding and in collaboration with Portland General Electric and inverter manufacturers, PNNL will test linking this system to the Western Interconnection using both grid-forming and grid-following inverters.Experiments will address providing grid services using GFM.
At the UNIFI meeting, Wei Du, from PNNL, reported on this first-of-its-kind project and other modeling and simulation approaches underway. Nonetheless, participants raised concerns about scaling up from small island grids and models to large interconnected systems. There is much to learn about how GFMs will actually perform on large interconnected systems.
What Must Work
From the perspective of power customers, regulators, and grid operators, the power must stay on. While acknowledging that scaling up was a “wide-open problem,” presenters at the UNIFI meeting offered multiple paths to a stable, inverter-rich grid. As Divan explained to me, the work of the consortium mirrors efforts in the past to standardize Bluetooth. Today, all devices relying on Bluetooth technology, regardless of company of origin or intended use, are able to connect over common network protocols. A similar consensus is needed for GFM. Legislators and regulators, at the end of the day, will require a reliable and affordable power system.
Risks and Benefits
The challenge for the stakeholders outside this highly technical community of experts is to start incorporating grid-forming inverters into our discussions. Market designs, grid connection requirements, and manufacturing standards will all bear on the success of transitioning to a high density of inverter-based resources on the grid.
There are opportunities to maximize the benefits of those resources through grid-forming inverter technologies, but there may be enormous costs — either in terms of grid stability or in terms of replacing and retrofitting devices — if policies fail to consider GFMs.
What are the risks and rewards? If policies fail to address the role of GFMs on future power systems, we risk instability, outages, and unsatisfied customers. If policies lag the capabilities and necessary functionality of GFMs, we may face very high costs to upgrade legacy devices. Regulations that limit the potential benefits of GFMs may leave us with less efficient, less reliable, and more expensive electricity.
On the other hand, rules that promote untested technologies may likewise result in a poorly functioning grid. Policies that require costly investments may be out of step with the benefits of distributed generation, renewable generation resources, EV fleets, storage batteries, and other inverter-connected devices. Yet, if policies facilitate development and deployment of maximally functional GFMs, we should expect more efficient, reliable, and sustainable energy systems.
How should we approach this as we near a tipping point? The answer: with deliberation and agile solutions that can adjust to new and better knowledge about what grid-forming inverters can deliver.
 Miranda Wilson, “Why solar ‘tripping’ is a grid threat for renewables,” E&E News, September 20, 2022, https://www.eenews.net/articles/why-solar-tripping-is-a-grid-threat-for-renewables/.
 Universal Interoperability of Grid-Forming Inverters Consortium (UNIFI), “Unifing interters & grids,” https://sites.google.com/view/unifi-consortium/home. For an essay following the first general meeting of UNIFI in July 2022, see Julie A. Cohn, “Power Systems on the Cusp of Fundamental Transformation: From Spinning Machines to Power-electronics” (Houston: Rice University’s Baker Institute for Public Policy, August 22, 2022), https://doi.org/10.25613/QC45-VR60.
 UNIFI, Value Proposition for Grid-Forming (GFM) Inverters, https://drive.google.com/file/d/1p1nhm2Ueimiimp9rSHCpCNs8mJ5JhSA6/view.
 North American Electric Reliability Corporation (NERC), Odessa Disturbance — Texas Events: May 9, 2021 and June 26, 2021 Joint NERC and Texas RE Staff Report (Atlanta: NERC, September 2021), https://www.nerc.com/pa/rrm/ea/Documents/Odessa_Disturbance_Report.pdf; NERC, 2022 Odessa Disturbance — Texas Event: June 4, 2022, Joint NERC and Texas RE Staff Report (Atlanta: NERC, December 2022), https://www.nerc.com/comm/RSTC_Reliability_Guidelines/NERC_2022_Odessa_Disturbance_Report%20(1).pdf.
 Jason Plautz, “How a grid rule change could derail Texas renewables,” E&E News, August 8, 2023, https://www.eenews.net/articles/how-a-grid-rule-change-could-derail-texas-renewables/; Electric Reliability Council of Texas (ERCOT), “Inverter-Based Resource (IBR) Ride-Through Requirements,” https://www.ercot.com/mktrules/issues/NOGRR245#summary.
 Charles F. Scott, “High-tension transmission lines,” Transactions of the American Institute of Electrical Engineers XXI (1903): 230, https://doi.org/10.1109/T-AIEE.1903.4764297. For a short biography of Scott, see J.C. Brittain, “Charles F. Scott: A pioneer in electrical power engineering,” IEEE Industry Applications Magazine 8, no. 6 (2002), https://doi.org/10.1109/MIA.2002.1044199.
 Paul A. David and Julie Ann Bunn, “The Economics of Gateway Technologies and Network Evolution: Lessons from Electricity Supply History,” Information Economics and Policy 3, no. 2 (1988).
 Deepak Divan, interview by Julie A. Cohn, July 15, 2023.
 D. Venkatramanan et al., “Grid-forming Inverter Technology Specifications: A Review of Research Reports & Roadmaps,” UNIFI Technical Report, UNIFI-2022-1-1, November 2022, https://drive.google.com/file/d/1YRwF1mNH2pA-WKlQ_HVbEYCQy4iWsAB_/view; IEEE Standards Association, IEEE Standard for Interconnection and Interoperability of Inverter-Based Resources (IBRs) Interconnecting with Associated Transmission Electric Power Systems, https://standards.ieee.org/ieee/2800/10453/.
 Julia Matevosyan, “A Unique Window of Opportunity: Capturing the Reliability Benefits of Grid-Forming Batteries” (Reston: Energy Systems Integration Group, March 2023), https://www.esig.energy/wp-content/uploads/2023/03/ESIG-GFM-batteries-brief-2023.pdf.
 UNIFI, “Publications,” https://sites.google.com/view/unifi-consortium/publications#h.1sis3yuf459k.
 U.S. Department of Energy (DOE), “Solar and Wind Grid Services and Reliability Demonstration Funding Program,” https://www.energy.gov/eere/solar/solar-and-wind-grid-services-and-reliability-demonstration-funding-program.
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