Reliability Concerns Emerge for Renewable Energy, Analysts Say Inverter Upgrades Are Urgent

As the U.S. energy transition continues, grid reliability issues are becoming increasingly prominent. Engineers focused on power system reliability and clean energy point out that a key technology is crucial for maintaining the momentum of the transition.

Engineers from the North American Electric Reliability Corporation (NERC) and the Energy Systems Integration Group (ESIG) state that inverters with "grid-forming" (GFM) capability are needed to support the growing number of inverter-based resources (IBR) such as wind and solar. ESIG is dedicated to advancing the integration and management of higher proportions of renewable energy.

Inverters convert the electricity generated by renewable sources into the alternating current that flows through the U.S. transmission system.

"The heartbeat of the power system is the 60-hertz (cycles per second) current, and all generation on the grid must synchronize with it," said Mark Lauby, Senior Vice President and Chief Engineer at NERC. "As long as inverters from variable renewable resources synchronize their generation with the 60-hertz heartbeat, they can deliver power to the system without worrying about system failures."

But engineers agree that the "grid-following" inverters used by today's renewable resources have limited capability to correct faults that cause current fluctuations, making the deployment of GFM technology in new IBR urgent.

Julia Matevosyan, Chief Engineer at ESIG, noted that the U.S. currently lacks a unified comprehensive standard requiring grid-following inverters, while Europe took 20 years to develop such standards. "We don't have another 20 years to develop GFM standards to avoid reliability failures," she added.

Engineers say that traditional power plants (known as synchronous machines) once stabilized heartbeat fluctuations in the power system that threatened reliability, but these plants are being retired.

To drive GFM deployment, engineers agree that pilots are needed to test GFM performance. Based on pilot results, NERC and system operators should develop standards defining GFM requirements and provide market incentives that protect reliability and reward participants deploying GFM.

The role of GFM

According to a GFM roadmap released by the U.S. National Renewable Energy Laboratory (NREL) in November 2020, coal, natural gas, nuclear, and hydroelectric plants are designed to drive rotating turbines to generate electricity at a 60-hertz heartbeat rate. The "rotating mass" of these synchronous machines has inertia that automatically corrects frequency fluctuations.

NREL explains that as traditional synchronous machines with rotating mass, such as natural gas, coal, and hydro, are replaced by IBR like wind and solar, system inertia decreases, "making frequency fluctuations more risky." IBR using today's grid-following inverters cannot significantly respond to these changes, but GFM can detect these changes and adjust power flow to limit fluctuations.

ESIG's Matevosyan said during a September 20 webinar that without GFM, system operators could reliably integrate 30% to 75% IBR by keeping synchronous generation online. But she added that keeping conventional generation online to protect reliability could lead to costly renewable curtailment and slow the transition to IBR.

Mahesh Morjaria, Executive Vice President and Head of Plant Operations Technology at Terabase Energy, a utility-scale solar project developer, said at the RE+ renewable energy conference in September that GFM can replace synchronous machines because it is "artificially programmed through power electronics, highly controllable, and can provide very fast response to frequency fluctuations."

Engineers say a key to the reliability provided by GFM is "frequency control," which manages frequency fluctuations. Another is "voltage control," which keeps system voltage stable by increasing or decreasing system output.

NREL says GFM can also provide other system protections, called "stabilizing services," that can detect "abnormal grid operating conditions" and mitigate them by "isolating the faulted portion."

Matevosyan said, "If a fault causes a blackout, GFM can perform a system 'black start,' using energy supplied by IBR to restart generation and gradually rebuild normal operation."

But Daniel Dedrick, Senior Vice President of Engineering, Procurement, Construction, and Technical Operations at GridStor, said including black start capability brings design and operational challenges. He added that inverters designed specifically for black start cost 2% to 5% more and require energy to drive the restart.

There are also other questions about the cost of introducing GFM.

The economics of GFM

Engineers and developers say integrating GFM into IBR may add costs, but there may also be offsetting market opportunities.

Terabase's Morjaria said new inverter capabilities "do not currently seem to make IBR more expensive." He added that as market incentives emerge, IBR equipped with GFM "can provide lower-cost reliability services, creating additional revenue because they use zero-cost fuel."

Matevosyan said GFM is "basically inverter software," and reprogramming could add "a few percentage points of marginal cost on top of the inverter cost" for batteries equipped with GFM. She added that equipping new batteries with GFM before deployment would be more cost-effective than upgrading after deployment, which incurs management, engineering, and downtime costs.

"There is also a cost to inaction," Matevosyan continued. "Without GFM deployment, there will continue to be stability challenges, ongoing solar and wind curtailment, and the need for expensive new transmission and other supplementary stabilization equipment."

Lauby agreed: "The costs associated with integrating GFM do not appear to make solar, wind, and batteries uncompetitive." He added that GFM can drive the transition from traditional synchronous machines to IBR.

Lauby, Matevosyan, and others agree that the challenge is to introduce GFM-equipped IBR in a cost-effective way to address the increasing number of events threatening reliability. NERC records show that in 2022 alone, there were three reliability-threatening events related to high IBR penetration, compared to five between 2017 and 2021.

One key solution—GFM-equipped batteries—is gaining attention.

Batteries will lead GFM deployment

Engineers and analysts say the growing momentum of utility-scale battery deployment provides a unique opportunity to leverage policy-driven clean energy construction and achieve GFM at scale.

"DOE is investing tens of millions of dollars, making GFM a top priority, and the industry needs to adopt it," said Becca Jones-Albertus, Director of the Solar Energy Technologies Office at the U.S. Department of Energy (DOE), at the RE+ conference in September.

Matevosyan reported in a March 2023 ESIG paper on GFM and utility-scale batteries that deploying GFM in batteries "is a low-hanging fruit solution." GFM can be integrated into the more than 400 gigawatts of battery capacity in the U.S. interconnection queue, making wind and solar growth "cheaper and faster than building new transmission lines to mitigate stability issues."

Matevosyan added during the September ESIG webinar that energy in charged batteries can be immediately used to provide reliability services. She added that solar and wind may not generate when energy is needed, or may not be able to forgo generation revenue, and wind services could cause mechanical stress on turbines.

Matevosyan reported that the Electric Reliability Council of Texas (ERCOT) will finalize performance requirements for GFM-equipped batteries by the second quarter of 2024. ERCOT will use these requirements to develop market incentives to drive adoption of GFM-equipped batteries in "weak grid" areas with high IBR penetration.

Matevosyan said that where IBR levels are moderate and synchronous machines still provide reliability services, there is "a unique window to procure, test, and gain experience with GFM technology." She added that with well-designed policies and market incentives, GFM-equipped IBR can offset costs through revenue from new or existing stability service markets.

A September NERC paper on GFM-equipped utility-scale battery systems concluded: "It is recommended to begin requiring and enabling GFM in all future battery energy storage system projects." The paper added that it can be added to new battery systems "at a relatively low incremental controller and hardware cost" and "may only require control changes."

Pilots, research, and standards

NERC, ESIG, and NREL agree that simulations and pilots are needed to test GFM performance at scale, and then through a multi-stakeholder process, research and develop GFM standards based on pilot results.

NERC's Lauby said both NERC's 2021 white paper and its September paper on GFM and battery systems call for these important next steps to move beyond the limited experience with large-scale GFM penetration to date.

Lauby continued: "Pilots are also needed to learn how to manage large asynchronous generating units as synchronous machines retire and load grows." He said this is "very complex engineering, possibly involving the use of artificial intelligence, and should be thoroughly modeled and simulated to understand the implications of coordination and control."

Lauby added: "We need to have the same confidence in systems with many grid-following and grid-forming inverters as we have with synchronous machine systems." But Matevosyan responded: "Everyone wants someone else to adopt grid-forming capability first."

DOE's Jones-Albertus said at the RE+ conference: "System operators need to consider operating the grid differently." That is why DOE funded demonstration projects for 15 solar and wind projects providing grid services and reliability.

NREL's roadmap states that after GFM research, development, and field trials, standards can be developed to allow synchronous machines to be fully replaced by GFM-equipped IBR within 10 to 30 years. But it adds that for "the next decade and beyond," the power system will have both synchronous machines and GFM-equipped IBR.

Matevosyan reported that demonstration projects in California in 2016 and 2017 provided reliability services, and a small installation in Michigan built in 2014 continues to stabilize a "weak grid." The Kauai Island Utility Cooperative in Hawaii will soon pilot GFM-equipped batteries, and Florida Power & Light will deploy 2,000 megawatts of batteries over the next decade to evaluate "GFM performance and benefits," other engineers added.

Matevosyan warned: "But the pace of pilot learning needs to accelerate because wind and solar penetration is rising and synchronous machines are retiring."