The Iberian blackout shows the dangers of operating power grids with low inertia

Last week there was a widespread blackout across Iberia which also briefly affected the south of France. All power was lost on the peninsula and it was many hours before it was fully restored after a rare black start. Eight people are reported to have died – one in Portugal and seven in Spain – as a result of the blackout, highlighting the seriousness of the situation.

What happened

At 12:33 the Spanish grid experienced an as yet un-identified fault followed 1.5 seconds later by what appears to be a large trip. This caused frequency instability. 3.5 seconds later, the Iberian Peninsula grid disconnected from the rest of Europe, tripping the interconnectors to France – rated at 2.5 GW. All nuclear reactors connected to the grid also tripped off.

Although the chart indicates 10 GW of generation available after the trip, this reflects scheduled output, not real output. The grid went to zero and a full black start was required which took until the following morning to complete.

The cause of the initial fault has not been disclosed, although Spanish grid operator Red Eléctrica de España (“REE”) almost certainly would have known within hours if not minutes what happened. There has been a good deal of speculation, from cyber attacks to rare atmospheric phenomena (the Portuguese grid operator, Rede Eletrica Nacional (“REN”) apparently suggested the cause was an “induced atmospheric vibration” but this seems highly unlikely. There was nothing unusual about the weather in Iberia that day, and such a phenomenon would have required a highly localised, high amplitude temperature oscillation which does not seem realistic. In fact, REN later said it had not claimed this and it seems to have been a misunderstanding by a Reuters journalist, which illustrates the way the media was trying to grapple with unfamiliar concepts. I spent much of Monday and Tuesday last week explaining inertia to journalists and giving TV and radio interviews trying to explain it to the public.

There are three more realistic explanations: the trip of a large source of generation, likely solar; a grid fault, possibly arcing due to smoke from a fire reducing the insulating properties of air; or inverter oscillation amplification where small grid disturbances are amplified when they bounce between inverters. Of these I think a grid fault is probably the most likely – fire was reported in the region believed to be the area of the initial fault – southwest France on the Alaric mountain range – which damaged a 400 kV line between Perpignan and eastern Narbonne, which would tie in with the theory that arcing caused a major circuit to trip. REN identified this fire as a possible cause, however the French grid operators, Réseau de Transport d’Électricité (“RTE”) denied there had been a fire in the region, but there may have been a fire elsewhere on the line.

The next question is why did a grid fault cause a complete loss of the entire Iberian power grid and part of France. This is probably due to the low inertia operation of the Spanish and Portuguese power grids which have large amounts of solar generation, and also decent amounts of wind power connected. In fact, just a couple of weeks ago, REE boasted about running the Spanish grid entirely with renewable sources, with wind contributing 46% of the total output, solar PV at 27%, hydro at 23%, solar thermal at 2%, and other renewable sources including renewable waste at 2%.

However, even with the hydro, that was pretty low inertia operation. There had been warnings earlier this year when the Spanish competition authority published an analysis of voltage control within its annual review of network charging. In its report, the regulator pointed out that the growing integration of renewable energy and falling demand is causing high oscillations in the voltage levels that may result in blackouts. Redeia/REE also pointed out in a report last February that the planned nuclear exit would create “problems due to the excess of renewables.” There were particular concerns about low demand periods such as lunchtime (solar generation connected to distribution networks creates demand reduction on the transmission system).

Low inertia means that grid frequency changes more than it would in a high inertia context for the same fault, making it more likely that other grid equipment trips as its protection measures activate, leading to a cascading grid failure. France, with its grid that is dominated by spinning mass – nuclear and hydroelectric turbines – has very high inertia, which may explain why the blackout in France was limited to a small area and with power being quickly restored. Similarly, the August 2019 blackout in Great Britain was contained without causing a system-wide collapse, due to the higher inertia of the GB grid compared with Spain.

Here is a description of the Iberian blackout from an anonymous expert quoted on X (make of it what you will)

Sadly eight people lost their lives. Of the seven people who died in Spain, one was in a house fire linked to the use of candles, two were from the failure of generators powering respirators (breathing machines) and three died when the generator powering their home respirator gave off carbon monoxide. The cause of the other death is unclear. The fatality in Portugal was also linked to a respirator.

Some basics about our power grids

With apologies to my expert readers, I think it’s useful to set out the basics as many people have been asking and it’s not widely understood.

Much of the way in which the electricity system operates is driven by the physical characteristics of electricity. In the most basic sense, a generator is one magnet rotating while inside the influence of another magnet’s magnetic field. The generator consists of stationary magnets – the stator – which create a powerful magnetic field; and a rotating magnet – the rotor – which distorts and cuts through the magnetic lines of flux of the stator, generating electricity. Due to the rotation of the magnet, the direction of the current periodically reverses – this is known as alternating current and is the basic output of a generator. For this reason, generators are also known as alternators.

This is illustrated here. A bent wire is placed between the poles of a magnet and set to rotate. As the loop of wire turns, each side will sometimes be travelling up and sometimes down – when it moves up, the current moves in one direction, and when it moves down the current moves the in the other direction. When the loop is perpendicular to the lines of magnetic flux, the current is zero, while when it is parallel to the lines of flux it is at a maximum. This is the origin of the sine waveform of the current and voltage.

An ac generator can be modified to generate direct current, creating  by replacing the metal rings in the ac generator with a commutator which consists of a hollow metallic cylinder split into two halves that are insulated from each other. Two brushes are in contact with each part of the cylinder at the moment when the coil is perpendicular to the magnetic field. During the first half of the cycle, one brush touches one half of the cylinder and the other touches the other half. As the current reverses direction, the brushes swap to touch the other half of the cylinder – this reverses the external direction of the current so it is the same as in the first half of the cycle. The result is known as a dynamo.

A small detour into the history of power grids

The history of how we come to use alternating current is also interesting. In September 1882, Thomas Edison opened the Pearl Street Power Station in Manhatten. This plant, fired by coal, began with six dynamos, and had an initial load of 400 lamps at 82 customers. By 1884 it was serving 508 customers with 10,164 lamps. This was also the world’s first co-generation plant, combining the provision of heating as well as power with the waste steam being piped to nearby buildings.

Edison favoured direct current because he wanted to power electric motors as well as lights, and at the time there were no good ac motors available. However, direct current was limited because it could not be transported over long distances because too much of the energy would be lost through the wires as heat. This meant that power stations had to be located close to consumers. Alternating current on the other hand can be transported over longer distances since the voltage could be increased to a level that reduced the heating effect through the use of transformers: a “step-up” transformer increases the voltage of the electricity at the power station, and a “step-down” transformer reduces it again for distribution into homes and businesses.

In 1883 Nikola Tesla invented the first transformer, known as the “Tesla coil”. The following year, Tesla went on to invent the first electric alternator for the production of alternating current. In 1887 he patented a complete electrical system, including a generator, transformers, a transmission system, a motor for use in appliances, and lights. The scheme caused a sensation and the patents were bought by George Westinghouse.

What followed has come to be known as the War or Battle of the Currents. On the one side were Tesla and Westinghouse and on the other, Edison. The stakes were high, with the opportunity to secure the rights to electrify American cities and earn potentially huge patent royalties.

The use of ac spread rapidly, leading Edison to begin a mis-information campaign against the technology. He claimed that the high voltages used to transport alternating current were hazardous, and that the design was inferior to, and infringed on the patents behind, his direct current system. The spring of 1888 saw a media outcry over fatalities caused by pole-mounted high-voltage ac lines, attributed to the greed and callousness of the arc lighting companies that used them.

Harold P. Brown, a New York electrical engineer, claimed that alternating current was more dangerous than direct current and tried to prove this by publicly killing animals with both currents, with technical assistance from Edison. They also contracted with Westinghouse’s main ac rival, the Thomson-Houston Electric Company, to make sure the first electric chair was powered by a Westinghouse ac generator. The dirty tricks continued until the World Fair in Chicago in 1893, which was lit with an ac system and the competition between the two was effectively over. Modern electricity systems are almost all run on alternating current.

Impact of the energy transition on power grids

The energy transition has seen a major deployment of intermittent renewable generation across the world. In general, wind and solar energy is converted into direct current…although wind turbines rotate, they do not do so at a constant rate, so are unable to generate ac electricity with a stable waveform without the use of power electronics. Clearly there are no rotational elements to solar power.

In the developed world, electricity grids and all the connected infrastructure from machinery in heavy industry to the devices in our homes, are all built on the fundamental principle that the power delivered to the socket has a consistent, predictable set of characteristics. Those characteristics may differ slightly from country to country – hence the need to adaptors when using devices in another country – but the fundamental concept is the same: alternating current at either 50 Hz or 60 Hz (depending on the country) with that frequency being delivered within a very narrow tolerance band. Outside that band, equipment can fail since it is designed to operate within a narrow frequency range.

Not only do intermittent renewables not deliver stable ac current, they also tend to be “distributed”, particularly in the case of solar. This means instead of grids based around large units of generation connected to high voltage networks we have many smaller generation sources connected to distribution networks, and even “behind-the-meter” ie not technically on the grid at all.

This presents both physical and economic challenges. Physical challenges relate to how the stable electricity expected by end users can be maintained when the means of generation no longer supports that to the same degree, and economic since the growth in self-generation challenges the economic assumptions behind the way in which networks are built and paid for.

Grid operators are having to develop new ways of managing their networks, creating new markets for ancillary services such as fast and dynamic frequency response, reserve and reactive power products, short circuit levels and inertia support. New technologies such as chemical batteries are being used for some of these services, and old technologies such as synchronous condensers are being used in new ways to support changing grid needs.

What is inertia and why is it important?

Generators convert the kinetic energy of the spinning turbines into electrical energy. The principle of conservation of energy governs the relationship between system power and the kinetic energy of the rotors – if there is insufficient supply to meet demand, the rotors will slow down and system frequency will fall, and vice versa. A sudden loss of generation or surge in demand can lead to a rapid change in frequency, which can damage equipment connected to the grid:

• Large frequency drops can damage transformers and induction motors due to the high magnetising currents required for maintaining flux. These devices are widely used in electricity transmission and distribution networks as well as in consumer appliances, so avoiding large frequency deviations is important to avoid damaging both network infrastructure and end-user devices
• Turbine blades are designed to operate in a narrow band of frequencies to avoid mechanical vibrations of blades at their natural frequencies – operation outside this frequency range can damage the turbine
• Drops in frequency cause the air flow in generators and turbines to fall, thus reducing cooling, while generator control systems increase their input power to maintain the generation and demand balance, which may result in an increase of the internal temperature of the turbine and generator windings – as the internal temperature increases, the protection devices cause the generator to disconnect from the grid, increasing the imbalance between demand and generation
• Increases in generator power output can also overload transmission lines, causing them to trip

For these reasons, system operators are required to maintain frequency within narrow limits and, if frequency deviates, to restore it to normal levels within a specified time period. In all global electricity grids, these requirements are part of the system operator’s license conditions.

The level of this challenge is illustrated by this chart which shows demand through the day on two days separated by 6 months. As you can see, winter demand is, as expected, higher than in summer when less heating and lighting is needed.

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Demand also varies through the day with a strong peak in the evening as the nation comes home from work and cooks dinner, and low demand at night when people are sleeping.

Many countries around the world are transitioning their electricity systems from ones dominated by conventional fossil fuel and nuclear generation, where the large, heavy turbines described above rotate in synchronicity to generate electricity at the required frequency, to systems with higher proportions of intermittent renewable generation.

In a conventional electricity system, these large heavy turbines do not just generate electricity at the desired frequency, they inhibit changes in grid frequency through their physical characteristics: large, heavy objects are difficult to move, and resist changes to their movement. Energy is delivered to the turbine causing it to rotate at a constant speed – more energy is required to change that speed if the input energy remains constant. In this way, these turbines resist changes to grid frequency caused by fluctuations in supply and demand, a property which is known as “inertia”.

The growth in intermittent renewable generation disrupts system frequency in two ways:

• Intermittent renewable generation delivers an output that is highly variable in time. Wind speeds are rarely constant, changing in both intensity and direction by the second, and similarly cloud patterns can create significant, instantaneous variations in solar output. Changes in either generation or demand can lead to changes in grid frequency, so highly variable generation patterns make maintaining a stable grid frequency more difficult
• Intermittent renewable generation is increasingly displacing conventional generation in the generation mix, reducing the amount of heavy, rotating turbines on the grid and therefore the amount of inertia they provide

“Operating the system with low inertia will continue to represent a key operational challenge into the future and we will need to ensure we improve our understanding of the challenges this will bring,”
– National Grid ESO

Taken together, these effects are making it increasingly difficult for system operators to maintain grid frequency, and in some regions such as South Australia, solar power is curtailed in order that gas generators can run, purely so that they can provide electricity at the correct frequency and deliver inertia to the grid.

No doubt Thomas Edison would feel vindicated in his support for direct current, but as replacing existing grids would be unfeasibly expensive for the most part, grid operators are looking at different types of inertia provision that can support systems with high levels of intermittent renewable generation, and there are plans to try to get inverter-based renewables such as wind and solar to create the ac waveform through so-called “grid-forming power electronics” (“GFPE”)

Can inverters cause grid voltage oscillations?

The answer to this is yes, under certain conditions, inverter-based resources (“IBR”) such as solar PV, batteries, and wind turbines can cause grid oscillations, although that is not to say this was the cause of the Iberian blackout. To understand why IBR can cause grid oscillations it’s necessary to understand what inverters actually do.

The purpose of an inverter is to convert dc electricity generated by solar panels, batteries and wind turbines (wind turbines generate alternating current, but not with constant frequency so they cannot synchronise to the grid. To get round this, the ac generated is converted to direct current and then back to ac for connection to the grid). Inverters not only convert from dc to ac, but they need to do so in a way that is compatible with the power grid. This means they must:

• Match the grid’s voltage and frequency (eg 240 V, 50 Hz in the UK)
• Synchronise phase angle and wave shape with the grid
• Inject power (active and reactive) into the grid as instructed by controls or markets

To do this, the inverter uses:

• A switching bridge (eg insulated-gate bipolar transistors (“IGBTs”) or metal-oxide-semiconductor field-effect transistors (“MOSFETs”))
• Filters (to smooth out the pulse width modulation (“PWM”) switching)
• A control system (to regulate output)
• A phase-locked loop (“PLL”) to “track” the grid voltage phase/frequency

Inverter controls assume they’re connected to a reasonably strong grid, where voltage is “stiff “(ie doesn’t sag or distort), impedance is low and inductive, and frequency is stable and easy to follow. (People often get stuck on impedance and inductance. Inductors are like capacitors in that they store energy, but in magnetic rather than electric field. Their effects are mainly seen in alternating current systems, not direct current, because inductance depends on changes in current and voltage over time. Impedance is similar to resistance but also accounts for the effects of inductance and capacitance. It represents the overall opposition to current flow in an ac grid, combining conventional resistance (as in dc circuits) with additional effects caused by the changing current in the presence of inductors and capacitors.)

Even without loads, power grids contain both resistance and impedance. Power lines don’t conduct electricity perfectly — some energy is lost as heat due to electrical resistance. Long power lines also behave like inductors because when ac current flows through a conductor, it constantly changes direction, and this changing current generates a changing magnetic field around the wire (by Ampère’s Law). This magnetic field, in turn, induces a voltage (by Faraday’s Law) that opposes the change in current. This opposition is inductive reactance, which is what inductors do. Over a longer conductor, the total magnetic field created is larger, and the effect is stronger so the wire behaves more and more like an inductor the longer it gets. Similarly, the windings in transformers create magnetic fields and are a classic source of inductance in power grids. Then there is the effect of loads such as motors which can be both inductive and contain capacitors.

Impedance causes problems for inverters in a number of ways:

• Instability in control loops: if the inverter thinks the grid is stable but it’s not, its control loop can overshoot, oscillate or lose synchronisation (PLL unlock)
• Harmonic interaction: high impedance at certain frequencies can amplify harmonics – inverters create some harmonics by design (from switching) but normally, they’re filtered out or absorbed by the grid. In a weak grid, these harmonics reflect back and build up creating resonance
• Sub-synchronous resonance: at sub-50 Hz frequencies impedance may create oscillating power flows, current/voltage instability and damage to cables, filters, or transformer windings

Returning to the question about inverters causing grid voltage oscillations these typically happen through control interactions, lack of damping, or fast-acting power electronics responding poorly to weak grid signals. The key mechanisms of such oscillations are:

• Phase-locked loop (“PLL”) instabilities: inverters need to synchronise with grid voltage and frequency via PLLs. In a weak or noisy grid, PLLs can lose lock or misinterpret disturbances, leading to unstable current injections and oscillatory feedback
• Low inertia and fast response: unlike synchronous machines, inverters don’t possess inertia, ie they don’t naturally resist changes in grid frequency. However, their response is not necessarily passive – their speed of response can sometimes amplify grid disturbances instead of smoothing them as a heavy spinning mass would
• Resonance with grid impedance: inverters interacting with certain line or transformer impedances can excite harmonic or sub-synchronous resonances particularly in weak grids. In real world power grid applications, oscillations are seen in offshore wind farms due to long cables.
• Poor coordination: If multiple inverters or parks use similar fast-reactive controls (eg droop response, virtual inertia), they may reinforce each other’s actions and create a positive feedback loop

Not only can inverters create grid voltage oscillations they can also amplify existing oscillations. This is actually more common than them creating oscillations in the first place. If a grid is already oscillating due to a disturbance such as a sudden large generator or interconnector trip, inverters can:

• Misinterpret voltage/frequency swings, causing over- or under-reaction
• Inject reactive power too aggressively (or with delay), adding energy to the oscillation rather than damping it
• Enter anti-islanding or fault ride-through modes erratically — some will stay connected, others may trip too fast, worsening the imbalance

Modern grid-forming inverters are designed to damp such oscillations, but almost all inverters currently deployed in western power grids are grid-following and were not designed to operate under stressed or low-inertia conditions.

There is precedent for inverters causing blackouts. In the 2016 South Australia blackout, wind farm inverters misread a sequence of faults and collectively disconnected, precipitating grid collapse. According to the official report by AEMO:

“On Wednesday 28 September 2016, tornadoes with wind speeds in the range of 190–260 km/h occurred in areas of South Australia. Two tornadoes almost simultaneously damaged a single circuit 275 kilovolt (kV) transmission line and a double circuit 275 kV transmission line, some 170 km apart. The damage to these three transmission lines caused them to trip, and a sequence of faults in quick succession resulted in six voltage dips on the SA grid over a two-minute period at around 4.16 pm.

As the number of faults on the transmission network grew, nine wind farms in the mid-north of SA exhibited a sustained reduction in power as a protection feature activated. For eight of these wind farms, the protection settings of their wind turbines allowed them to withstand a pre-set number of voltage dips within a two-minute period. Activation of this protection feature resulted in a significant sustained power reduction for these wind farms. A sustained generation reduction of 456 megawatts (MW) occurred over a period of less than seven seconds.

The reduction in wind farm output caused a significant increase in imported power flowing through the Heywood Interconnector. Approximately 700 milliseconds (ms) after the reduction of output from the last of the wind farms, the flow on the Victoria–SA Heywood Interconnector reached such a level that it activated a special protection scheme that tripped the interconnector offline.

The SA power system then became separated (“islanded”) from the rest of the NEM. Without any substantial load shedding following the system separation, the remaining generation was much less than the connected load and unable to maintain the islanded system frequency. As a result, all supply to the SA region was lost at 4.18 pm (the Black System). AEMO’s analysis shows that following system separation, frequency collapse and the consequent Black System was inevitable.”

While the initial grid fault in Spain was probably not inverter-related in this case (although this remains a possibility), the low inertia operation of the grid is likely to have been the reason the fault was not contained until it reached France, and there was a full blackout on the Iberian peninsula.

Are GFPE the answer?

With its large amounts of solar power, the Australian system operator, AEMO is at the forefront of the work on grid-forming power electronics. Its trials have been progressing, albeit with some challenges. In January 2024, AEMO released a “Core Requirements Test Framework” for grid-forming inverters, building upon their 2023 voluntary specifications. These documents outline the desired capabilities of grid-forming inverters, such as voltage source behaviour, inertial response, and fault ride-through capabilities.

While these specifications provide a foundation, the practical implementation has revealed complexities. For instance, achieving the necessary energy headroom for inertial response can be challenging, especially for offshore wind power plants where curtailing output to provide headroom may not be economically viable. Additionally, ensuring interoperability among devices from different manufacturers and adapting to varying grid conditions require further research and development.

More specifically, GFPE face the following challenges:

• Technical risk: GFPEs need to mimic inertial and voltage-source behaviour in real time — this requires fast, stable, robust control, which is still being refined
• Commercial hesitation: developers and investors are cautious because there are few clear grid codes or performance standards and regulatory frameworks are still evolving. Energy headroom requirements reduce revenue potential
• Interoperability concerns: GFPE units from different vendors need to co-operate without destabilising the grid which, is not straightforward given issues around commercial confidentiality
• Operator caution: TSOs are reluctant to rely on GFPEs until there are more data from field-testing
  

AEMO is working on these issues with industry stakeholders – its efforts are focusing on enhancing testing methodologies, improving control strategies, and developing standards to facilitate the integration of grid-forming inverters into the Australian power grid (NEM). It wants up to 30–50% of new inverters to be grid-forming by 2030.

Notable projects GFPE projects in Australia include:

• Hornsdale Power Reserve (South Australia): this 150 MW /194 MWh battery system, co-located with the Hornsdale Wind Farm, has been upgraded to provide grid-forming capabilities. It delivers synthetic inertia and fast frequency response, enhancing grid stability in a region with significant renewable energy resources
• Dalrymple ESCRI Battery (South Australia): a 30 MW/8 MWh battery that supports a 100% renewable microgrid on the Yorke Peninsula. It enables the region to operate independently during grid disturbances, showcasing the practical application of GFPE in enhancing energy resilience
• Victorian Big Battery (Victoria): with a capacity of 300 MW/450 MWh, this facility has been tested for grid-forming capabilities, contributing to system strength and reliability in Victoria’s energy network

Beyond Australia, GFPE technologies are being implemented in various Western power grids:

• United States: grid-forming inverters are being integrated into projects like the Los Angeles Department of Water and Power’s initiatives to enhance grid stability amidst increasing renewable energy adoption
• Europe: countries such as Germany and Great Britain are exploring GFPE to support their energy transition goals, with pilot projects demonstrating the technology’s potential in maintaining grid reliability. In Britain, NESO has used its Pathfinder projects to explore commercial procurement of inertia, and recently connected a grid forming battery in Scotland
• Middle East: GFPE is being considered in regions aiming to diversify their energy mix and incorporate more renewable sources, with pilot projects assessing their feasibility and benefits

The deployment of grid-forming power electronics in Spain remains limited, despite the large amounts of inverter-based renewables, particularly solar power, that has connected to the Spanish power grid.

In summary, no western country currently uses grid-forming power electronics outside limited trial scenarios, and it may be several years before widespread deployment is seen.

Weird voltage behaviours on the GB grid the day before the Iberian blackout

In the early hours of 27 April 2025, the day before the Iberian blackout, the GB power system experienced unusual voltage disturbances, triggering protective trips at the Keadby 2 CCGT and the Viking interconnector with Denmark. The incident occurred just after 3:00 am. The Viking REMIT notice specified “Control and Protection Systems” as the reason for its unplanned trip. Keadby had been returning from a longer outage.

Clear oscillations in frequency are visible shortly after 3am between ~49.92 Hz and ~50.05 Hz. The oscillation frequency appears to be in the 100–200 second range (0.005–0.01 Hz), suggestive of inter-area or mode instability. These are not normal load-following frequency variations, but a sign of poor damping of an active oscillatory mode — often linked to:

• HVDC control interactions (e.g. interconnectors)
• Generator control system interactions
• Weak system areas connected via long corridors

There is then a drop to 49.85 Hz at around 3:30am which suggests a large, fast disconnection (like a power station or interconnector tripping).

The oscillations before 03:30 imply a system already under stress or poorly damped. The event at 03:30 appears to have snapped the system into a new state, possibly triggering protection schemes or tripping logic due to instability.

Inertia at the time was not unusually low, which suggests this was likely a control-mode instability, not raw RoCoF (Rate of Change of Frequency)-driven collapse.

According to NESO, these events were resolved swiftly and did not lead to consumer disconnections – which is not surprising given they occurred in the middle of the night.

While the grid was operationally intact it was dynamically unstable, which suggests a need to review the tuning and coordination of grid-following assets (eg interconnectors, inverters), mode damping contributions from synchronous plant, or the effectiveness of frequency stability services (like dynamic containment) under oscillatory conditions.

Later in the day, around 6pm, further unusual oscillations were observed. There were notable frequency spikes and sharp reversals just before and after 18:00, with several steep drops followed by rapid rebounds.

This pattern suggests sudden generation or interconnector disconnection or possible control loop interaction for example, response overshooting from frequency response providers. As there were no reported large generator or interconnector trips at the time, more likely causes are:

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• Control or dispatch error: a sudden imbalance caused by poor dispatch timing (eg under-delivery of frequency response or over/under ramping of flexible assets) which could involve distribution-connected assets, which aren’t captured in REMIT
• Embedded generation behaviour: a sharp solar PV cutout (eg due to irradiance loss or inverter group behaviour) might affect net load shape without showing up in REMIT, which could explain a short, steep imbalance without a formal “trip”
• Frequency response overshoot or interaction: dynamic containment or low-frequency response from storage could have overreacted, causing the kind of spike–rebound signature seen at around 6pm

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The GB grid was not running with low inertia on 27 April, so these oscillations are unusual. Fortunately, the high inertia meant the impact of the oscillations was limited and there was no loss of load, but on a low inertia day, the outcome could have been different. There is no suggestion that these oscillations were related to the Iberian blackout.

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While we may not know the true cause of the Iberian blackout for some time, we have a reasonable idea about why it was so widespread. Full system-wide outages are rare, as are the black starts that follow, but with the growing reliance on inverter-based renewables, electricity grids are becoming less stable. No doubt system operators around the world will be reviewing their approach to maintaining inertia in the aftermath of this incident.

Blackouts kill. We should not risk people’s lives in the quest for net zero.