Why is Grid Inertia Still Important?

Without sufficient synchronous grid inertia, the grid becomes unstable and a blackout occurs. The 28 April 2025 Iberian Peninsula Blackout underscored how important grid inertia is.

March 4, 2024

Inertia refers to a system’s capability to resist change. For a power grid, greater synchronous inertia confers greater ability to resist frequency changes.

An electric power grid supplies alternating current with a specified frequency (60 Hertz or 60 cycles per second in the United States) and a specified voltage level such as 120 AC Volts (within a tolerance band.) Precise control of frequency (and phase) allows a group of geographically separated generators to function in unison. The North American Electric Reliability Corporation (NERC) specifies the tolerance band as a function of time for voltage and frequency for the United States electric power grid. For the specifics, please refer to NERC Standard PRC-024-3 [1] Since grid - connected equipment could be harmed by excursions outside the tolerance band, protective relays may disconnect the equipment if the frequency and/or voltage excursion is outside the tolerance band. A group of disconnections may trigger a blackout, such as the large-scale North American blackout which began on August 14, 2003.

In contrast to gigantic 2,256 megawatt nuclear power plants such as Diablo Canyon Power Plant (DCPP) near San Luis Obispo, California which provide very large amounts of synchronous grid inertia, so-called inverter-based resources (IBRs) such as solar powered generators, wind power generators, and batteries supply negligible amounts of synchronous grid inertia. Within the past decade, political decision makers in states such as California have been advocating for increasingly larger penetrations of IBRs without understanding the adverse impact those energy policy choices are having on electrical grid reliability. The purpose of this article is to provide those policy makers and interested members of the public an introduction to the importance of electrical grid inertia.

A Paris, France - based international standards group has been developing electric power grid standards since 1921. The English translation of their name is, “The International Council on Large Electric Systems.” (French: Le Conseil international des grands systèmes électriques - or its acronym CIGRE.) In 2021, CIGRE released an article with the title, “Impact of high penetration of inverter-based generation on system inertia of networks.” The Catholic University of Louvain, Belgium has made a copy of this technical brochure available. [2] CIGRE’s 2021 article is the focus of this article.

            IBRs have no intrinsic frequency, unlike synchronous generators such as DCPP. For IBRs to be synchronized to a power grid, they must receive a synchronizing signal from this power grid which controls the frequency and phase of the IBR. If this synchronizing signal has errors in frequency and/or phase, the IBR may absorb rather than contribute alternating current (AC) to the grid, which could destroy the IBR's electronic components.

            Prior to the introduction of significant penetrations of IBRs, each power grid's synchronous generators (coal and natural gas-fired generators, large hydroelectric dams, geothermal plants, and nuclear power plants) had sufficient synchronous grid inertia to assure power grid stability. The synchronous generators have a large amount of rotational inertia as a consequence of having massive rotating turbines and massive rotating generator rotors. (See photograph below.)  

            As a simplified example, each of the pair of DCPP’s generators have rotating components which weigh in excess of a million pounds (500 tons.) DCPP’s turbines rotate 30 times per second. The rotating magnetic field induces the 60 cycle per second (Hertz) AC voltage (25,000 Volts) and AC current (45,120 Amperes) in the stator windings of each unit. In response to perturbations in grid frequency, the rotational kinetic energy can be instantaneously converted to changes in the output power of the generator which tend to stabilize the generator’s output frequency and voltage.

Workers inspect the area near the bottom of one of the pair of massive DCPP generators. The top of the generator is even with the top of the low-pressure steam turbine in the background. (Since large amounts of steam pass through the steam turbines, hearing protection is required on the DCPP turbine deck.) The generator is about 20 feet tall. (Photo courtesy of PG&E and the Diablo Canyon Independent Safety Committee.)

            The CIGRE's technical bulletin notes that, "In some power systems such as in Ireland and Northern Ireland, Great Britain, ERCOT, South Australia, a minimum inertia constraint is applied. " That means, a group of synchronous generators must run at all times to provide the stabilizing synchronous grid inertia.  All of these power systems have minimal or no interconnection to other power systems.

            In contrast, California is well-interconnected with other generators throughout the American west. California is distinguished by importing the greatest amount of power of any state. During the past two decades, California's annual power consumption  has been about 300 terawatt-hours (TWh) per year, with about 100 TWh being sourced from out of state. 1 TWh is a billion kilowatt-hours.

            Despite the need for a minimum synchronous grid inertia, the California Independent System Operator (CAISO) continues to misleadingly advocate for a high penetration of IBRs. CAISO publicizes short intervals, typically on weekends in the spring when there are large amounts of IBRs being utilized while it is sunny. [3] At the time when CAISO claimed, "97.6 percent of electricity on the grid came from clean, renewable energy," synchronous generators were actually supplying between 4,955 MW and 5,145 MW, or between 25.02% and 25.85% of the total demand of 19,801 MW and 19,907 MW being consumed between 3:35 PM and 3:40 PM on Sunday, April 3, 2022. Furthermore, electricity exports were between 3,070 MW and 3,232 MW and batteries were being charged between 241 M and 81 MW. These statistics show the large mismatch between CAISO's public relations claims and the actual CAISO power production statistics.

            A far more meaningful set of statistics showing the importance of synchronous generation are demonstrated when the CAISO grid was experiencing a load peak. The CAISO peak demand of 52,061 MW occurred on a hot summer weekday - Tuesday, September 6, 2022 at 4:57 PM. Synchronous generators were supplying a far more robust 33,029 MW and 33,656 MW, or between 62.94% and 64.36% of the total 52,479 MW and  52,296 MW being consumed  between 4:55 PM and 5:00 PM. (In contrast, power was being imported and batteries were being discharged.) [4]

            In summary, synchronous generation provides the "muscle" for a power grid. A proposed theoretical model for grid forming IBRs requires huge energy storage devices that mimic the functionality of large synchronous generators. (These proposed devices do not yet exist.) Despite the fervent wishes of renewable power advocates, synchronous generation will be a necessary part of power grids for the foreseeable future. DCPP will continue to be needed. If safe, reliable, abundant, cost-effective and emission-free DCPP is needlessly closed in 2030, it will likely be replaced with heavily-polluting coal-fired power from synchronous generators mostly in Wyoming. [5] Such a replacement will eviscerate important California environmental statutes such as SB 1386 (Perata, 2006) codified as PUC § 8340 and PUC § 8341.

---

[1]  https://www.nerc.com/pa/Stand/Reliability%20Standards/PRC-024-3.pdf

[2]  https://lirias.kuleuven.be/retrieve/706089 

[3]  "California ISO hits all-time peak of more than 97% renewables," April 14, 2022, California Independent System Operator News Release.

http://www.caiso.com/Documents/California-ISO-Hits-All-Time-Peak-of-More-Than-97-Percent-Renewables.pdf

[4]  The power production statistics were from CAISO's website http://www.caiso.com/TodaysOutlook/Pages/supply.html  . Data is reported at five-minute intervals. The methodology for tracking natural gas power production data was updated. Data posted prior to December 13, 2023  is not comparable. (However, the September 6, 2022 peak demand data point has been adjusted.)

[5]  "Closing Diablo Canyon spurs fears over replacement power," Gene Nelson, Ph.D., April 5, 2022, Capitol Weekly.

https://Tinyurl.com/DCPP-VERSUS-COAL