Iberia blackout caused by ‘overvoltage’ and ‘chain reaction’ says Spain

The Spanish government has released a report on the causes of the April 28 Iberia blackout, saying the event was caused by a ‘multifactoral origin’ that caused a chain reaction of overvoltage disconnections, mitigated by miscalculations from Spain’s system operator, Redeia.

The report, presented by Spain’s vice president and minister for Ecological Transition and the Demographic Challenge, Sara Aagesen, comes nearly two months after the blackout, which hit Spain, Portugal and parts of France.

After analysis of over hundreds of gigabytes of data, the possibility of the incident resulting from a cyberattack has been ruled out, says the report, including at the system operator’s facilities, control centres, or the generation centres analysed.

Rather, it concludes that the incident had a multifactorial origin, with three key elements standing out: insufficient voltage control capacity, oscillations creating difficulties to stabilise voltage and ‘apparently improper’ disconnections of generation plants.

This led to a temporal cascade of events that progressively unbalanced the system and culminated in the peninsular electricity grid being shut down due to overvoltage.

Sequence of events

In the report, the Committee for the Analysis of the Circumstances of the Electricity Crisis of April 28, 2025, identified a sequence of events that establish a chronology of the incident:

• Phase O: Voltage instability.

According to the report, during the days leading up to the incident, there were fluctuations in voltage, and by the morning of the 28th, voltages were fluctuating more intensely than normal.

• Phase 1: System oscillations (12:00–12:30pm)

At 12:03pm, an atypical oscillation of 0.6Hz was recorded, causing large voltage fluctuations for 4.42 minutes.

This oscillation forced the System Operator to implement protocolised mitigation measures, such as increasing grid meshing—restricted by low demand—or reducing the interconnection flow with France.

These actions by Redeia, although dampening the oscillation, had the side effect of increasing tension.

At 12:16pm, the same oscillation was recorded again, although smaller, and at 12:19pm, another oscillation of 0.2Hz. The System Operator applied the same measures to mitigate it, which also contributed to increasing the voltage.

• Phase 2: Generation losses (12.32.57 – 12.33.18)

Voltage began to rise rapidly and steadily, and numerous progressive disconnections of generating facilities were recorded in Granada, Badajoz, Segovia, Huelva, Seville, Cáceres, and other provinces.

• Phase 3: Collapse (12.33.18 – 12.33.30)

The progressive increase in voltage produced a chain reaction of overvoltage disconnections.

According to the report, it was possible to contain this, since each of the disconnections contributed to further increases in voltage.

There was also a drop in frequency, which resulted in the loss of synchronisation with France, the tripping of the interconnection with the rest of the continent, and the peninsular electricity gridlock.

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Multifactoral origin

According to the report’s analysis, the electrical failure origin consists of three key elements:

1. Insufficient voltage control

The system showed insufficient voltage control capacity for two reasons.

First, on the 27th, prior to the incident, the System Operator scheduled the operation of 10 synchronous plants capable of regulating voltage on the 28th, according to their schedule.

The final number of synchronous plants connected was the lowest since the beginning of the year.

Second, several of the plants capable of regulating voltage—and specifically compensated for it, having been programmed for this purpose due to technical restrictions—did not respond adequately to the System Operator’s instructions to reduce it. Some even produced reactive power, the opposite of what was required, contributing to the problem.

2. Oscillations

Oscillations occurred.

The first of these, an atypical one, originated at a facility on the Iberian Peninsula. It required modifications to the system configuration, increasing the difficulty in stabilising the voltage.

After the second oscillation, the System Operator requested the availability of a plant capable of helping regulate the voltage, but it was technically impossible to do so before the collapse.

3. ‘Apparently improper’ disconnection of generating plants

The report says that generating plants were disconnected, some of them improperly.

Namely, some of the power plant disconnections occurred before the voltage thresholds, established by regulations (between 380kV and 435kV on the transmission grid) were exceeded, while other disconnections occurred after these limits were exceeded to protect the facilities.

Once the chain reaction began, the usual protections of the electrical system were unable to stop or contain this process.

Some of these protections, such as load shedding, may have even contributed to the surge phenomenon by further discharging the lines, contributing to the rise in voltages, because they acted to compensate for the drop in generation rather than to manage the voltage.

Responsibility

The report on the incident comes as the Spanish government faces increasing pressure to give an answer to the cause of the event, which shook the Peninsula and sent millions into darkness.

With its release, the report and the Committee attribute large responsibility to Redeia (formerly known as Red Eléctrica), whom it claims failed to calculate the correct mix of energy – one of the factors that led to the blackout.

The Spanish grid operator has disputed the findings, according to Reuters reportage, saying its own investigation had discovered anomalies in the disconnection of power plants on April 28 even though voltage in the system was within legal limits, as well as an anomalous growth in demand from the transport network.

They also dispute claims that their miscalculation was one of the factors hindering the grid’s ability to cope with a surge in voltage.

Redeia announced that they will release its own report on the causes of the outage.

Reaction

Reacting to the report’s publication, Adrian Guggisberg of ABB Electrification warned that grid resilience must now be treated as an economic imperative. The company’s division president of Distribution Solutions commented that, despite the need for a reliable, stable energy supply, grids are being pushed to their limits:

“The rise of decentralised energy sources, electrification of industry and transport, and digitalisation of everything from buildings to mobility are placing unprecedented demands on ageing electrical systems. Managing this variability while maintaining reliability and protection is a major engineering challenge of our time.”

According to Guggisberg, meeting these challenges requires a smarter, more connected approach to power infrastructure. “This includes everything from substations and feeders to automation systems, protection devices, and yes, medium-voltage switchgear — which plays a key role in controlling power flows and isolating faults.

“True grid resilience isn’t about one technology. It’s about a system that can adapt, isolate faults quickly, recover autonomously, and support real-time decision-making.”

In a joint reaction statement to the report, the Global Solar Council, UNEF, APREN, SolarPower Europe and the Global Renewables Alliance emphasise that solar PV – its integration into Spain’s power mix one the believed causes – was not responsible.

“The investigation confirms that managing an electricity system is a complex and multi-faceted undertaking and is of great societal importance. Going forward, the Iberian blackout must be a moment of learning. Solar PV already has the capacity to control voltage, but regulations did not allow its application.

“This is a call for accelerated investment in grid resilience and system flexibility –especially through grid-forming inverters and battery storage. These technologies are already available and are key to supporting stable voltage levels, managing variability, and delivering renewable-powered energy security.”

The report’s committee also proposes a series of actions to prevent a recurrence of the blackout in Iberia.

These include strengthening supervision and verification of compliance with obligations by all stakeholders in the electrical system, as well as technical measures to strengthen voltage control and protection against fluctuations in the system.

They also propose to increase demand and the flexibility of the electricity system.

Regarding cybersecurity, they propose to expedite the transposition of European regulations, and say that network controls and segmentation must be implemented, as well as event detection and correlation systems that provide a higher level of surveillance.

This post has been updated to include commentary from Adrian Guggisberg of ABB Electrification.