By Miranda Willson
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May 9th of last year was supposed to be a typical day for solar power in west Texas. But around 11:21 a.m., something went wrong.

Large amounts of solar capacity unexpectedly went offline, apparently triggered by a fault on the grid linked to a natural gas plant in Odessa, according to the Electric Reliability Council of Texas (ERCOT). The loss of solar output represented more than 13 percent of the total solar capacity at the time in the ERCOT grid region, which spans most of the state.

While all of the solar units came back online within six minutes, the incident highlighted a persistent challenge for the power sector that experts warnneeds to be addressed as clean energy resources continue to displace fossil fuels.

“As in Texas, we’re seeing this huge boom in solar technology fairly quickly,” said Ryan Quint, director of engineering and security integration at the North American Electric Reliability Corporation (NERC). “And now, we’re seeing very large disturbances out of nowhere.”

Across the U.S., carbon-free resources make up a growing portion of the electricity mix and the vast majority of proposed new generation. This past summer, solar and battery storage systems helped keep the lights on in Texas and California as grid operators grappled with high power demand driven by extreme heat, according to grid experts.

Even so, while the disturbance last year near Odessa was unusual, it was not an isolated incident. If industry and regulators don’t act to prevent future renewable energy “tripping” events, such incidents could trigger a blackout if sufficiently widespread and damage the public’s perception of renewables, experts say.

The tripping event in Texas — which spanned 500 miles — and other, similar incidents have been tied to the inverters that convert electricity generated by solar, wind and battery storage systems to the power used on the grid. Conventional generators — fossil fuel power plants, nuclear plants and hydropower dams — don’t require inverters, since they generate power differently.

“We’re having to rely more and more on inverter technology, so it becomes more and more critical that we don’t have these systemic reliability risk issues, like unexpected tripping and unexpected performance,” Quint said.

Renewable — or “inverter-based” — resources have valuable attributes that conventional generators lack, experts say. They can ramp up and down much more quickly than a conventional power plant, so tripping incidents don’t typically last more than several minutes.

But inverters also have to be programmed to behave in certain ways, and some were designed to go offline in the event of an electrical fault, rather than ride through it, said Debra Lew, associate director of the nonprofit Energy Systems Integration Group.

“[Programming] gives you a lot of room to play,” Lew said. “You can do all kinds of crazy things. You can do great things, and you can do crappy things.”

When solar and wind farms emerged as a significant player in the energy industry in the 2000s and 2010s, it may have made sense to program their inverters to switch offline temporarily in the event of a fault, said Barry Mather, chief engineer at the National Renewable Energy Laboratory (NREL).

Faults can be caused by downed power lines, lightning or other, more common disturbances. The response by inverter-based resources was meant to prevent equipment from getting damaged, and it initially had little consequence for the grid as a whole, since renewables at the time made up such a small portion of the grid, Mather noted.

While Quint said progress is being made to improve inverters in Texas and elsewhere, others are less optimistic that the industry and regulators are currently treating the issue with the urgency it deserves.

“The truth is, we’re not really making headway in terms of a solution,” Mather said. “We kind of fix things for one event, and then the next event happens pretty differently.”

‘New paradigm’ for renewables?

NERC has sounded the alarm on the threat of inverter-based resource tripping for over six years. But the organization’s recommendations for transmission owners, inverter manufacturers and others on to how to fix the problem have not been adopted universally.

In August 2016, smoke and heat near an active wildfire in San Bernardino County, Calif., caused a series of electrical faults on nearby power lines.That triggered multiple inverters to disconnect or momentarily stop injecting power into the grid, leading to the loss of nearly 1,200 megawatts of solar power, the first documented widespread tripping incident in the U.S.

More than half of the affected resources in the California event returned to normal output within about five minutes. Still, the tripping phenomenon at the time was considered a “significant concern” for California’s grid operator, NERC said in a 2017 report on the incident.

The perception around some of the early incidents was that the affected solar units were relatively old, with inverters that were less sophisticated than those being installed today, said Ric O’Connell, executive director of the GridLab, a nonprofit research group focused on the power grid. That’s why last year’s disturbance near Odessa caused a stir, he said.

“It’s come to be expected that there are some old legacy plants in California that are 10, 15 years old and maybe aren’t able to keep up with the modern standards,” O’Connell said. “But [those] Texas plants are all pretty brand new.”

Following the May 2021 Odessa disturbance, ERCOT contacted the owners of the affected solar plants — which were not publicly named in reports issued by the grid operator — to try to determine what programming functions or factors had caused them to trip, said Quint of NERC. Earlier this year, ERCOT also established an inverter-based resource task force to “assess, review, and recommend improvements and mitigation activities” to support and improve these resources, said Trudi Webster, a spokesperson for the grid operator.

Still, the issue reemerged in Texas this summer, again centered near Odessa.

On June 4th, nine of the same solar units that had gone offline during the May 2021 event once again stopped generating power or reduced power output. Dubbed the “Odessa Disturbance 2” by ERCOT, the June incident was the largest documented inverter-based tripping event to date in the U.S., involving a total of 14 solar facilities and resulting in a loss of 1,666 megawatts of solar power.

NERC has advocated for several fixes to the problem. On the one hand, transmission owners and service providers need to enhance interconnection requirements for inverter-based resources, said Quint. In addition, the Federal Energy Regulatory Commission should improve interconnection agreements nationwide to ensure they are “appropriate and applicable for inverter-based technology,” Quint said. Finally, mandatory reliability standards established by NERC need to be improved, a process that’s ongoing, he said.

One challenge with addressing the problem appears to be competing interests for different parties across the industry, said Mather of NREL. Because tripping can essentially be a defense mechanism for solar, wind or battery units that could be damaged by a fault, some power plant owners might be wary of policies that require them to ride through all faults, he said.

“If you’re an [independent system operator], you’d rather have these plants never trip offline, they should ride through anything,” Mather said. “If you’re a plant owner and operator, you’re a bit leery about that, because it’s putting your equipment at risk or at least potentially at risk where you might suffer some damage to your PV inverter systems.”

Also, some renewable energy plant owners might falsely assume that the facilities they own don’t require much maintenance, according to O’Connell. But with solar now constituting an increasingly large portion of the overall electric resource mix, that way of thinking needs to change, he said.

“Now that the industry has grown up and we have 100 megawatt [solar] plants, not 5 kilowatt plants, we’ve got to switch a different paradigm,” he said.

Sean Gallagher, vice president of state and regulatory affairs at the Solar Energy Industries Association, stressed that tripping incidents cannot be solved by developers alone. It’s also crucial for transmission owners “to ensure that the inverters are correctly configured as more inverter-based resources come online,” Gallagher said.

“With more clean energy projects on the grid, the physics of the grid are rapidly changing, and energy project developers, utilities and transmission owners all need to play a role when it comes to systemwide reliability,” Gallagher said in a statement.

Overall, the industry would support “workable modeling requirements” for solar and storage projects as part of the interconnection process — or, the process by which resources link up to the grid, he added.

‘Not technically possible’

The tripping challenge hasn’t gone unnoticed by federal agencies as they work to prepare the grid for a rapid infusion of clean energy resources — a trend driven by economics and climate policies, but turbocharged by the recent passage of the Inflation Reduction Act.

Last month, the Department of Energy announced a new $26 million funding opportunity for research projects that could demonstrate a reliable electricity system powered entirely by solar, wind and battery storage resources. A goal of the funding program is to help show that inverter-based resources can do everything that’s needed to keep the lights on, which the agency described as “a key barrier to the clean energy transition.”

“Because new wind and solar generation are interfaced with the grid through power electronic inverters, they have different characteristics and dynamics than traditional sources of generation that currently supply these services,” DOE said in its funding notice.

FERC has also proposed a new rule that draws on the existing NERC recommendations. As part of a sweeping proposal to update the process for new resources to connect to the grid, FERC included two new requirements to reduce tripping by inverter-based resources.

If finalized, the FERC rule would mandate that inverter-based resources provide “accurate and validated models” regarding their behavior and programming as part of the interconnection process. Resources would also generally need to be able to ride through disturbances without tripping offline, the commission said in the proposal, issued in June.

While it’s designed to help prevent widespread tripping, FERC’s current proposal could be improved, said Julia Matevosyan, chief engineer at the Energy Systems Integration Group. Among other changes, the agency should require inverter-based resources to inject so-called “reactive power” during a fault, while reducing actual power output in proportion to the size of the disturbance, Matevosyan said. Reactive power refers to power that helps move energy around the grid and supports voltages on the system.

“It’s a good intent. It’s just the language, the way it’s proposed right now, is not technically possible or desirable behavior,” Matevosyan said of the FERC proposal.

To improve its proposal, FERC could draw on language used by the Institute of Electrical and Electronics Engineers (IEEE) in a new standard it developed for inverter-based resources earlier this year, she added. Standards issued by IEEE, a professional organization focused on electrical engineering issues, aren’t enforceable or mandatory, but they represent best practices for the industry.

IEEE’s process is stakeholder-driven. Ninety-four percent of the 170 industry experts involved in the process for developing the latest inverter-based resource standard — including inverter manufacturers, energy developers, grid operators and others — approved the final version, Matevosyan said.

The approval of the IEEE standard is one sign that a consensus could be emerging on inverter-based resource tripping, despite the engineering and policy hurdles that remain, observers said. As the industry seeks to improve inverter-based resource performance, there’s also a growing understanding of the advantages that the resources have over conventional resources, such as their ability to rapidly respond to grid conditions, said Tom Key, a senior technical executive at the Electric Power Research Institute.

“It’s not the sky is falling or anything like that,” Key said. “We’re moving in the right direction.”