Alternating Current and Frequency — A Giant Merry-Go-Round the Whole Country Pushes Together

60 Inside the Outlet

When you plug your phone charger into a wall outlet, the electricity inside that hole goes back and forth 60 times a second. Korea's electricity is set that way. This article takes Korea's electricity as a starting point and also covers the case of Europe, which uses 50 times a second.

This number 60 is kept so that it does not stray from a narrow range. It is the same even when countless homes turn on lights all at once in the evening, or when air conditioners run simultaneously on a midsummer afternoon. The work of maintaining that number is an ongoing adjustment between distant power plants and everyone using electricity at this very moment. This article follows how that adjustment operates and what happens when it goes significantly off track.

Alternating Current and Frequency

Electricity from a battery is a flow in one direction, coming out of one end and going into the other. Direct current is electricity that flows in only one direction. Electricity from a wall outlet is different. Alternating current is electricity whose direction of flow reverses back and forth many times per second.

Frequency is a count of how many times per second the direction of the alternating current goes back and forth, measured in a unit called hertz. Electricity in South Korea is 60 hertz, and in continental Europe it is 50 hertz. Electricity in countries around the world mostly uses one of these two as its standard (DOI: 10.1145/3134600.3134639). Going back and forth once means one complete round trip of going forward and returning, so the direction of 60-hertz electricity changes 120 times per second.

This back-and-forth movement is created as the generators in power plants spin. When magnets and bundles of wire rotate against each other inside a generator, the direction of the electricity produced in the wires changes alternately to match the rotation. Therefore, when a generator spins faster, the frequency goes up, and when it spins slower, it goes down.

Power plants are tied together as one. A power grid is a system that connects power plants and places that use electricity with wires to make them operate as one, also called a system in the industry. Large generators attached to the same power grid spin to the same beat. A synchronous generator is a generator whose rotating speed spins in step with the frequency of the power grid. The frequency of the power grid is stabilized together by the weight of the rotation driven by these synchronous generators and the control that adjusts that rotation speed (DOI: 10.1109/pes.2008.4596800). That is why under normal circumstances, measuring the frequency anywhere on the grid yields almost the exact same value (DOI: 10.1109/pesgm.2014.6939306).

Think of a large merry-go-round at a playground. It is a platform that multiple people standing around the edge push together to spin. While pushing a single platform, no one can spin at a different speed alone. The synchronous generators are those people pushing, and the speed at which the platform spins is the frequency.

This analogy has one limitation. An actual power grid is closer to multiple plates connected by a long rubber band rather than a single solid chunk of iron. They normally spin together, but immediately after a major accident, the plates wobble slightly differently and surge out of sync with one another. The surges between regions and the differing rates of frequency change from region to region covered later are examples of this (DOI: 10.24084/reepqj26-376, journal article) (arXiv:2511.21387, preprint).

Balance at Every Single Moment

At seven in the evening, rice cookers and lights turn on in homes everywhere. That electricity was made at almost that exact moment in some power plant. That is because wires and power generation equipment almost nowhere store electricity. A power grid is a system that must match the amount being made and the amount being used at every single moment (DOI: 10.2172/1003830). What is matched here is the intensity at that moment. Power is the amount of energy that electricity moves in one second, measured in a unit called watts. In this article, "the amount being made" and "the amount being used" both mean the power at that moment.

Load is the power that the power supply network must supply at that exact moment. Nothing happens just from one house turning on an electric kettle. Power grid operators forecast the load in advance using models incorporating weather, seasons, and time of day, and this prediction rests on the premise that the jagged fluctuations of countless small users cancel each other out (DOI: 10.1145/3134600.3134639).

Let us go back to a spinning playground merry-go-round. If children get on the board and drag their feet on the ground, the board slows down. The side using electricity is these foot-dragging children. If the pushing force and the dragging force are equal, the board spins at the same speed. If the dragging force is greater, it slows down; if the pushing force is greater, it speeds up. The power grid is the same. When the amount being used becomes greater than the amount being made, the rotation of synchronous generators slows down and the frequency drops, and vice versa.

A moving frequency means that the amount being made and the amount being used are out of alignment. Even after filling the mismatch, the frequency stays out of the baseline for a while before returning through slower adjustments. Therefore, the power grid is operated within a prescribed safe frequency range, and operators prepare facilities to respond to both anticipated mismatches and unanticipated mismatches, returning the deviated frequency to its scheduled value (DOI: 10.2172/1003830). The 60 inside the wall outlet is a sign that all that matching is working properly right now.

Time Bought by Weight

Let's imagine pushing a heavy iron rotor and a light plastic rotor with the same force and then letting go. The heavy plate slows down slowly, and the light plate slows down quickly. The power grid also has something corresponding to the weight of this plate.

System inertia is the rotational energy held by heavy chunks of iron spinning directly connected to the grid, and this slows down the rate at which the frequency changes without any instructions. The large synchronous generator at a power plant spins a heavy chunk of iron, so the more such generators are spinning in the grid, the greater the inertia (DOI: 10.1109/tpwrs.2005.857393). Earlier we said that there is almost nowhere to store electricity in the power grid, and this rotational energy is a part of that "almost."

The effect of inertia appears in how fast the frequency moves. The rate of change of frequency (RoCoF) is a value measuring how many hertz per second the frequency is collapsing, in other words, the speed of frequency change. When inertia decreases, the rate of change of frequency increases for a mismatch of the same size (DOI: 10.3390/en19133063).

Inertia only slows down the drop; it cannot stop it. Instead, as much as it slows it down, time is gained. When the rate of change of frequency increases, the time available to respond before emergency measures to cut off load operate becomes short (DOI: 10.3390/en13040816, simulation). Inertia slows down the speed at which the frequency drops, buying time for other responses to move.

A Hand to Stop the Fall

People who feel a rotating plate slowing down will push a little harder without anyone telling them to. They can do this because they usually do not use all their strength and leave a little in reserve. Power grids are the same.

Primary frequency response is the immediate reaction that generators and electricity users show on their own when the frequency deviates from its standard. This reaction is essential for avoiding the instability that can be triggered when protection devices designed for frequency anomalies activate, leading to equipment damage, localized power outages, or in extreme cases, a blackout. In the past, conventional power generation facilities naturally possessed this response, and in most power grids, the amount available was well above what was needed, but a 2013 paper pointed out the trend of this response decreasing (DOI: 10.1109/tpwrs.2013.2264942).

Users also play a part here. When the frequency drops, some loads naturally draw less power. A study simulating the continental European power grid projects that the share of this effect in frequency stability will become more important in the future (DOI: 10.1049/iet-rpg.2015.0141).

Reserves are the spare generation capacity set aside in advance to make up for sudden electricity shortages. Most primary frequency response comes from these reserves, specifically the portion kept in reserve by generators that are already spinning (DOI: 10.2172/1003830) (arXiv:2010.14016, preprint). It is not enough for the amount to simply be ample; it must come out fast enough to stop the fall. The Western Australian power grid operator was using a tool in its control room (as reported in 2020) to check in real time whether spinning reserves are sufficient and fast enough (arXiv:2010.14016, preprint).

After the fall stops, slower adjustments return the frequency to its scheduled value. The response of inertia, the primary response, and the restoring adjustments — protecting the frequency is divided into these three stages of different speeds (DOI: 10.3390/en19133063).

After a Single Power Plant Stops

Now let's trace an accident from the beginning. One large power plant suddenly stops due to a breakdown. It is as if one person pushing the spinning plate let go of their hands. Because the amount used stays the same while the amount made decreases, the frequency starts to drop. Losing a large generation facility all at once and having the frequency drop is an accident that threatens power grid frequency stability (DOI: 10.1109/tpwrs.2019.2943150).

In the first moment, the speed at which the frequency drops is determined by the size of the lost generation and the weight of the plate, in other words, the grid inertia. While the inertia buys time, the reserve capacity left empty by the generators adds pushing force through primary frequency response. The frequency that was falling stops at some lowest point.

Where that lowest point is matters. Underfrequency load shedding is an action that automatically cuts a predetermined amount of load and reduces the consuming side if the frequency falls below a predetermined line. The job that primary frequency response must do is to stop the fall of the frequency above the highest line where this load shedding begins (DOI: 10.2172/1003830). If it stops there, people using electricity receive it without interruption, and afterwards the frequency is restored to the scheduled value (DOI: 10.2172/1003830).

If it cannot stop it, the spinning plate makes some of the children dragging their feet get off. Traditional underfrequency load shedding is divided into several stages, and the frequency that operates and the amount cut are fixed in advance for each stage (DOI: 10.1049/iet-gtd:20080210). Blackouts come to the places that were cut off. Even so, this action is the main means of preventing the frequency from continuing to fall and spreading into a major blackout (DOI: 10.1109/tpwrs.2019.2943150). Underfrequency load shedding is the last device intended to prevent the entire power grid from collapsing by intentionally darkening a part of it (DOI: 10.1109/pesgm.2014.6939306). It does not always succeed. In the Iberian blackout that appears later, there is an analysis suggesting that the time to correct it was too short, which limited the effect of load shedding (DOI: 10.2139/ssrn.6895119, preprint).

When the Lights All Go Out

Weeks after the August 2003 blackout in the northeastern United States and parts of Canada, Italy went dark. After a storm knocked down power lines near the border between Italy and Switzerland, Italy became disconnected from the European power grid; in less than two minutes, the frequency became unstable, and the entire Italian grid collapsed. In Rome, the subway came to a halt during an all-night festival, and at least four deaths were blamed on the blackout (DOI: 10.4324/9780203126523, secondary account in a book on energy transition).

Italy is a case of frequency collapse. Major blackouts also arrive by other paths. Voltage is the difference that causes electricity to flow between two points, measured in units called volts. Like frequency, voltage must be maintained within a set range. If it drops too low or rises too high, protective devices meant to guard equipment disconnect it from the grid. There is another difference. Frequency is roughly the same everywhere on the grid, but voltage differs from place to place and must be managed close to that spot.

Voltage collapse means that the voltage in a region drops continuously until it cannot be recovered. While drops in frequency were common in blackouts of the 1960s and 1970s, almost all recent major blackouts have had their roots in this voltage collapse. This is the conclusion of an analysis looking mainly at large North American blackouts around 2007. The same analysis suggests that relying solely on under-frequency load shedding can be "too little, too late" because voltage collapse is the signal that warns of trouble first (DOI: 10.1109/psamp.2007.4740897). That is why methods that look at frequency and voltage together to cut load are studied (DOI: 10.1109/tpwrs.2013.2241794).

On April 28, 2025, electricity across Spain and Portugal was cut, affecting about 47 million people (DOI: 10.2139/ssrn.6551460, preprint). According to a final report issued in March 2026 by an expert panel formed by ENTSO-E, an association of European grid operators, the frequency of the 50-hertz grid fell below 48 hertz within a few seconds (ENTSO-E Expert Panel Final Report, 2026).

The start of the Iberian blackout was on the voltage side. This time, the voltage became too high. On a day when several phenomena overlapped, voltage rose rapidly without control until it could no longer be managed; generation output dropped quickly and equipment dropped offline, accelerating this and causing power generation facilities in Spain to drop offline in a cascade (ENTSO-E Expert Panel Final Report, 2026). Overvoltage is when voltage rises higher than its set range. Once it exceeds the range, protective devices disconnect the generation equipment from the grid, and when one piece of equipment drops out, the situation for the remaining equipment worsens, causing the next piece to drop out. This is a cascading trip.

The picture of a cascading trip triggered by overvoltage is the same in a policy analysis based on ENTSO-E's investigation (DOI: 10.2139/ssrn.6551460, preprint) and in an analysis reconstructing the event using public waveforms (DOI: 10.2139/ssrn.6895119, preprint). An editorial that does not separate the start and the course suggests that even though facilities and generation capacity were ample, rapid solar output changes, low demand, and insufficient voltage control overlapped to cause the collapse (DOI: 10.32397/tesea.vol6.n1.911, editorial).

Explanations diverge on the process that magnified the damage and on the root causes. As for the magnifying process, rapid oscillations between regions are singled out. The idea is that these oscillations arose when there were fewer spinning synchronous generators, quickening the frequency drop. This is the sight we saw earlier of multiple plates connected by rubber bands shaking out of alignment with one another (DOI: 10.24084/reepqj26-376, journal article). As for the root causes, things beyond technology—such as regulation, market design, and management systems—are pointed to. The first is that even though renewable facilities certified with the ability to regulate voltage existed, regulations prevented them from using that ability (DOI: 10.2139/ssrn.6551460, preprint).

The final report does not point to a single cause, painting a picture where multiple factors overlap. Included together are the fact that renewable facilities operated with fixed settings unresponsive to voltage changes, a system where existing generators that needed to support voltage faced no penalties even if they fell short of standards, and oscillations that occurred twice in the 30 minutes before the blackout (ENTSO-E Expert Panel Final Report, 2026).

Could this day's collapse have been prevented if inertia had been much greater? Simulations in the final report suggest that even if inertia had been much larger, given the sequence in which this day's events unfolded, the Iberian grid could not have avoided falling out of step with continental Europe. Falling out of step means that the two grids could no longer rotate together at the same frequency, at which point the connection linking the two grids is severed. This is because generators dropped out one after another, and the force with which synchronous generators held the grid to a single beat diminished too quickly. Defense mechanisms, including load shedding, operated as designed but could not prevent the collapse (ENTSO-E Expert Panel Final Report, 2026).

Two reconstructed analyses differ in tone from this, meeting at the point that the time available to correct things was short. One side suggests that the voltage-side events narrowed that time, limiting the effectiveness of load shedding and reserves, and that the time for frequency reserves closed before they could react (DOI: 10.2139/ssrn.6895119, preprint). The other side suggests that reduced inertia drastically cut the time for corrective measures (DOI: 10.24084/reepqj26-376, journal article). The preceding analyses are preprints, an editorial, and a journal article, while the conclusion of the official investigation is the final report.

The Generator That Doesn't Spin

Rooftop solar panels have no rotating parts. Because what panels produce is direct current (DC), a device is needed to change it so it matches the alternating current (AC) in a wall outlet. An inverter is a device that turns direct current into alternating current and feeds it into the power grid. Solar and wind typically connect to the grid through these fast-responding power conversion devices, and on their own, they do not add inertia (DOI: 10.1109/tsg.2018.2871085). Wind turbines have spinning blades, but modern wind turbines generally have their rotation separated from the grid by a conversion device in between, so they do not spin together in time with the grid's rhythm. Therefore, unlike synchronous generators, adding wind power does not bring an increase in inertia along with it (DOI: 10.1109/tpwrs.2005.857393).

Ordinary inverters connecting to the grid today add pushing force, but they cannot act like the weight of the plate. To use the spinning plate analogy, this inverter measures how fast the plate is spinning and then pushes from the side to match it. Because it measures first and then follows along, it cannot automatically hold its ground when the plate suddenly slows down. This way of following someone else's rhythm has been used for quite a long time. There are also ways to push without following, generating a constant rhythm on their own, but as of a 2024 review paper, they were still rarely used (DOI: 10.1049/rpg2.12991).

During hours when solar and wind generation are high, synchronous generators rest by that much, and the plate becomes lighter. If these facilities replace a significant portion of synchronous generators, the total rotational inertia can drop sharply, and frequency can fluctuate wildly and become unstable (DOI: 10.1109/pes.2008.4596800). When wind replaces conventional generation, the rate of change of frequency increases, and the magnitude of frequency deviation after losing generation capacity can also grow. For this reason, reserve operation policies may need to be revised or the inertial response of wind turbines changed, which is especially true in small, isolated power grids (DOI: 10.1109/tpwrs.2005.857393). A 2018 review paper noted that this problem had already shaken smaller grids and viewed larger grids as likely to face it soon (DOI: 10.1109/jestpe.2018.2877766).

Inverters can also be made to move as if they had weight. The key is whether the energy to draw out is kept close by, and how that energy is controlled. By supplementing with a short-duration storage device and properly controlling the conversion device, any power generation facility can mimic inertia for a short time (DOI: 10.1109/pes.2008.4596800). For example, if a battery is made to discharge electricity in proportion to the rate at which the frequency drops, the faster the drop, the harder it pushes to hold its ground like weight. The authors verified this method through simulations and experiments (DOI: 10.1109/tsg.2018.2871085). The same 2018 review considers advancements in semiconductors and control to be what will turn power conversion devices into a means of handling inertia (DOI: 10.1109/jestpe.2018.2877766). These methods of making an inverter act as if the plate is heavy, as well as methods for making it set its own rhythm, are covered next in "The Few Seconds a Generator's Spinning Mass Gave Away for Free — What Gets Harder as Grid Inertia Falls".

Let us trace the reasoning from the previous section again on a light plate. A power plant of the same size stops. Because the plate is light, the frequency drops faster (DOI: 10.1109/tpwrs.2005.857393), and the time left before hitting the load-shedding line becomes shorter (DOI: 10.3390/en13040816, simulation). One way to fill that short gap is with fast-responding storage devices. Island-like isolated grids experience large frequency swings, making automatic load shedding frequently necessary. In simulations of such island grids, storage devices injected electricity instantly right after the loss of generation facilities, holding the fort while reserves from conventional generators ramped up. The authors consider that this device played a role in mimicking inertia (DOI: 10.1109/tste.2012.2205025, simulation).

The required speed depends on the size of the mismatch. One simulation shows that boundary. This is a case where a grid with over 80% inverter-connected generation experiences a large imbalance where generation and consumption mismatch by up to 40%. In this scenario, fast reserves had to come out fully within 0.1 seconds. In both simulated grid sizes, this held true regardless of the synchronous generator side's response. The authors conclude that current frequency measurement methods and the deployment times of fast reserves cannot guarantee stability under such large imbalances. This means that if the mismatch is very large, current equipment cannot prop things up quickly enough. Even when the imbalance was less severe, there was no room for complacency. A Europe-sized grid reached a precarious state where stability limits were hit starting from a 3% imbalance when non-synchronous generation accounted for 60% (DOI: 10.3390/en13040816, simulation).

Actually measured values vary by grid and by time. In high-resolution frequency measurements taken in Austria over two months, even when the share of non-synchronous renewable facilities in Austria and Germany's generation reached 74%, the rate of change of frequency did not rise dangerously, nor did the frequency deviate abnormally (DOI: 10.3390/su10051605, single measurement). However, it is difficult to extrapolate this result as-is. The 74% figure is the share for just those two countries, and those two countries are tied together within the much larger Continental European power grid. Nor does this measurement tell us whether an accident as large as the one in the earlier simulation occurred during the measurement period. The instantaneous share of solar and wind that can be handled cannot be fixed as a single number and must be calculated separately for each time of day. This is the conclusion obtained by simulating the Continental European power grid for every hour of a year across numerous weather scenarios (DOI: 10.1049/iet-rpg.2015.0141).

The situation differs even within a single grid. Inertia is not evenly distributed by region, and in California, the regional rate of change of frequency reached up to 6 times the value for the broader power grid as a whole to which California belongs. This is the result of analyzing actual incidents from 2013 to 2024. The recent recovery in inertia is analyzed to be thanks to the deployment of batteries equipped with functions that mimic inertia (arXiv:2511.21387, preprint).

In the earlier simulation, in the extreme case of a very large imbalance occurring in a grid mostly made up of inverters, the same speed was required in both grid sizes. Being a larger grid offered no help. In less severe cases, the situation varies from grid to grid. When solar and wind increase, frequency can drop faster for an accident of the same size, and the degree of that drop depends on the size of the grid and its generation mix at that moment. How generation connected via power conversion devices is regulated, and how it interacts with the rest of the grid, was a problem not yet fully understood as of the 2018 review (DOI: 10.23919/pscc.2018.8450880).

Where to Read More

The story of what happens after the rotor gets lighter continues in "The Few Seconds a Generator's Spinning Mass Gave Away for Free — What Gets Harder as Grid Inertia Falls". How to make an inverter mimic inertia, and how to have it count its own beat instead of following someone else's, is the job of that article.

The story of the consuming side adjusting its usage to add to the balance continues in "Is a Data Center Really an Inflexible Load? — What Can Move and What Cannot". That is the side where the kids dragging their feet lift their feet themselves. You can see how large the newly attached loads are in "How Much Electricity Does AI Use? — Why Every Source Gives a Different Number".

Source Papers

• "Grid Incident in Spain and Portugal on 28 April 2025 — Final Report" (2026) — ENTSO-E Expert Panel — https://www.entsoe.eu/publications/blackout/28-april-2025-iberian-blackout/ — The blackout occurred as voltage rose rapidly and uncontrollably, leading to cascading trips of generation facilities. Frequency dropped below 48 hertz at 12:33:22. Even with significantly greater inertia, loss of synchronism could not have been avoided, and the defense plan operated as designed but failed to prevent the collapse.
• "Grid Shock" (2017) — DOI: 10.1145/3134600.3134639 — Power grids continuously maintain the balance of supply and demand to keep a reference frequency of 50 or 60 hertz, resting on the premise that consumption is predicted using models incorporating weather, seasons, and time zones, while the jaggedness of small consumers cancels out in the average.
• "Virtual synchronous generators" (2008) — DOI: 10.1109/pes.2008.4596800 — Power grid frequency is stabilized by the rotational inertia and rotational speed control of synchronous generators, and if asynchronous small-scale generation replaces them, the total rotational inertia drops significantly, which can lead to large frequency fluctuations and instability. Short storage devices and converter control can provide virtual inertia to any generator.
• "Power system stability using decentralized under frequency and voltage load shedding" (2014) — DOI: 10.1109/pesgm.2014.6939306 — Load shedding is the final emergency control against voltage and frequency instability and major blackouts, and because frequency is uniform across the entire system, conventional relays can operate in an overlapping manner.
• "Use of Frequency Response Metrics to Assess the Planning and Operating Requirements for Reliable Integration of Variable Renewable Generation" (2010) — DOI: 10.2172/1003830 — Interconnected power grids are operated within a safe frequency range to maintain the instantaneous balance between generation and load, and primary frequency response must arrest the frequency drop above the highest setpoint of under-frequency load shedding.
• "Frequency Control and Wind Turbine Technologies" (2005) — DOI: 10.1109/tpwrs.2005.857393 — The increase in inertia is inherently tied to the addition of conventional generation, but wind power lacks this connection, and if wind replaces conventional generation, the rate of change of frequency and deviation magnitude can increase, especially in small isolated systems.
• "Inertia Response and Frequency Stability in Renewable Energy-Dominated Power Systems: Review of Virtual Inertia Techniques" (2026) — DOI: 10.3390/en19133063 — Reduced inertia increases the rate of change of frequency; a review paper organizing the mechanisms of inertia, primary, and secondary frequency responses.
• "Determination of the Required Power Response of Inverters to Provide Fast Frequency Support in Power Systems with Low Synchronous Inertia" (2020) — DOI: 10.3390/en13040816 — A high rate of change of frequency reduces the response time required before load shedding (simulation study).
• "Market Designs for the Primary Frequency Response Ancillary Service—Part I" (2013) — DOI: 10.1109/tpwrs.2013.2264942 — Primary frequency response is the immediate and autonomous reaction of generation and demand, critical for avoiding relay operation, equipment damage, load shedding, and in extreme cases blackouts; it was once abundant, being inherent to conventional generation, but (as of 2013) has been on a declining trend.
• "Impact of high penetration of variable renewable generation on frequency dynamics in the continental Europe interconnected system" (2016) — DOI: 10.1049/iet-rpg.2015.0141 — The allowable instantaneous share of wind and solar cannot be fixed to a single value and must be calculated for each time window, increasing the importance of the load self-regulation effect.
• "Online Security Assessment of Low-Inertia Power Systems" (2020, preprint) — arXiv:2010.14016 — Western Australia grid operators ran a control-room tool (as reported in 2020) to check in real time whether reserves are sufficient and sufficiently fast.
• "Continuous Under-Frequency Load Shedding Scheme for Power System Adaptive Frequency Control" (2019) — DOI: 10.1109/tpwrs.2019.2943150 — Frequency drops following large-scale generation loss are a threat to frequency stability, and under-frequency load shedding is the main means of preventing cascading drops and blackouts.
• "New centralised adaptive load-shedding algorithms to mitigate power system blackouts" (2008) — DOI: 10.1049/iet-gtd:20080210 — Traditional under-frequency load shedding uses fixed values for stage-by-stage frequency, delay, and shedding amount.
• "Governing the Energy Transition" (2012, book) — DOI: 10.4324/9780203126523 — Less than two minutes after Italy was separated from the European power grid in 2003, the entire system collapsed due to frequency instability, and at least four deaths were attributed to the blackout.
• "Undervoltage load shedding" (2007) — DOI: 10.1109/psamp.2007.4740897 — Almost all recent major blackouts had their roots in voltage collapse, whereas under-frequency was common in the 1960s and 1970s; relying solely on under-frequency load shedding can be too little and too late.
• "Adaptive load shedding based on combined frequency and voltage stability assessment using synchrophasor measurements" (2013) — DOI: 10.1109/tpwrs.2013.2241794 — A load shedding algorithm that utilizes both frequency and voltage information.
• "The 2025 Iberian Peninsula Blackout: Lessons for Modern Power Systems and Policy Implications" (2026, preprint) — DOI: 10.2139/ssrn.6551460 — A blackout affecting 47 million people. Based on the ENTSO-E investigation, it reports cascading trips triggered by overvoltage and traces the root causes to regulation (blocking the use of certified voltage-control renewable facilities), market design, and management structures rather than technology.
• "The Iberian Blackout of 28 April 2025 and Frequency Stability in Power Systems with High Renewable Energy Source Penetration" (2026, preprint) — DOI: 10.2139/ssrn.6895119 — Reconstruction using public waveforms indicates that the disturbance began in the voltage and reactive power domain as trips caused by overvoltage, with frequency falling to about 48.5 hertz; the voltage-side event narrowed the operational time window, limiting the effectiveness of load shedding and reserves, and the window closed before reserves could react.
• "Grid Resilience under high shares of Non‑Synchronous Generation: Insights from the 2025 Iberian Peninsula Blackout" (2026, journal article) — DOI: 10.24084/reepqj26-376 — Analyzed using a layered framework separating the triggering disturbance from the subsequent amplification process. Under reduced synchronous generation, rapid inter-area oscillations appeared, raising the rate of change of frequency and accelerating the decline, while reduced inertia severely limited the time available for corrective action.
• "Challenges of variable energy resource integration and power system security: Lessons from the 2025 Iberian system blackout" (2025, editorial) — DOI: 10.32397/tesea.vol6.n1.911 — An editorial noting that despite robust facilities and dispatchable generation capacity, a combination of rapid solar output variations, low demand, and limited voltage control led to the collapse.
• "Frequency Derivative-Based Inertia Enhancement by Grid-Connected Power Converters With a Frequency-Locked-Loop" (2018) — DOI: 10.1109/tsg.2018.2871085 — Renewable facilities connect to the grid through fast power converters that do not provide inertia, leading to reduced inertia; an inertia emulation method setting battery output proportional to the rate of change of frequency was validated through simulation and experimentation.
• "On the Inertia of Future More-Electronics Power Systems" (2018) — DOI: 10.1109/jestpe.2018.2877766 — Power-electronics-based renewable generation can drastically reduce inertia, having already shaken small-scale systems with large-scale systems soon to face the same. It concludes that advances in semiconductors and control will make power electronics a means of inertia control.
• "Impact of Asynchronous Renewable Generation Infeed on Grid Frequency: Analysis Based on Synchrophasor Measurements" (2018) — DOI: 10.3390/su10051605 — Two months of measurements in Austria showed no dangerous increase in the rate of change of frequency or abnormal deviations even when the asynchronous renewable share between two countries (Austria and Germany) reached a maximum of 74%.
• "Understanding Regional Inertia Dynamics in CAISO from Real Grid Disturbances" (2025, preprint) — arXiv:2511.21387 — An analysis of actual disturbances from 2013 to 2024 showed that regional rates of change of frequency reached up to six times the system-wide value, and recent inertia recovery is attributed to the deployment of batteries with synthetic inertia functions.
• "Dynamic Frequency Control Support by Energy Storage to Reduce the Impact of Wind and Solar Generation on Isolated Power System's Inertia" (2012) — DOI: 10.1109/tste.2012.2205025 — In island power systems, frequency deviations are large and automatic load shedding is frequently required; fast storage devices can bridge the gap while primary reserves ramp up and act like synthetic inertia to reduce the impact (Guadeloupe simulation study).
• "Grid‐forming control for inverter‐based resources in power systems: A review on its operation, system stability, and prospective" (2024) — DOI: 10.1049/rpg2.12991 — Inverter control is divided into grid-following and grid-forming; grid-following has been operated for a long time, whereas the relatively new grid-forming is still deployed sparsely.
• "Foundations and Challenges of Low-Inertia Systems" (2018) — DOI: 10.23919/pscc.2018.8450880 — In the transition replacing synchronous machines with power-electronics-interfaced generation, its regulation and interaction with the grid are not yet fully understood.