Understanding the April 2025 Iberian Peninsula Blackout: Early Analysis and Lessons Learned

On April 28, 2025, at 12:33 p.m. local time, a significant blackout affected the entire Iberian Peninsula, plunging Spain and Portugal into darkness. During a webcast on May 6, Sean McGuinness, Transmission and Distribution Protection Research Program Manager with EPRI, provided background on the Spanish and Portuguese power grids, and an overview of the events leading up to and after the widespread power outage. McGuinness is based in Dublin, Ireland, and is very familiar with the continental European power system. He has reportedly been involved in several blackout investigations EPRI has done on other systems over the past several years.

To open the session, Daniel Brooks, EPRI’s Senior Vice President for Energy Delivery and Customer Solutions, pointed out that investigations are still in progress surrounding the event, and that McGuinness would not be speculating on unconfirmed details. Nonetheless, the information McGuinness put forward in his presentation should remain pertinent regardless of the ultimate findings. The following summarizes what is currently known about this event, drawing from preliminary analysis and the context of past incidents in the region.

Overview of the Spain and Portugal Grids

Spain’s electrical grid serves a population of approximately 44 million people with a peak load of about 40 GW. The system boasts an impressive installed capacity of more than 125 GW, with renewable energy playing a significant role—approximately 32 GW from solar PV and 31 GW from wind. The remainder comes mostly from gas, hydro, and nuclear generation. Spain’s transmission network consists of roughly 25,000 miles of high-voltage lines (400 kV and 220 kV) and maintains interconnections with three neighboring regions: France (2.8 GW import capacity), Portugal (2.1 GW import and 3.9 GW export capacity), and Morocco (600 MW import and 900 MW export capacity).

Portugal’s smaller grid serves a population of about 11 million with a peak load of 9 GW. The Portuguese system is predominantly hydro-based, with approximately 8.3 GW of hydro capacity and significant pumped-storage resources. Its transmission network spans roughly 6,000 miles and is well-integrated with Spain through multiple 400-kV interconnections, allowing the two countries to operate in a closely synchronized manner.

2021 Event Provides Relevant Historical Context

McGuinness noted that a significant incident in 2021 provides important context for understanding the recent blackout. In that earlier event, wildfires in southeast France caused smoke-related tripping of a 400-kV transmission line carrying approximately 2.5 GW of power. Before system operators could fully respond, a second line on the same circuit also tripped, leading to overloading of the remaining tie lines between France and Spain, and causing power swings. These conditions ultimately led to the tripping of all interconnectors between France and Spain.

The cascade continued as 2.6 GW of generation in Spain subsequently tripped, with a notable 1 GW failing due to overvoltage conditions—an unusual factor in cascade failures. System defense mechanisms activated automatically, including the disconnection of 2.3 GW of pumped storage that was in pumping mode, disconnection of industrial customers, and the shedding of 3.6 GW of end-user load through under-frequency load-shedding relays.

A key insight from the 2021 event was the unusual voltage behavior observed. The initial line trips caused voltage drops in northeast Spain, with levels falling as low as 337 kV on the 400-kV grid. Shunt reactors switched out to address this under-voltage condition. However, as interconnectors tripped and load was shed, the voltage rapidly increased to as high as 450 kV. This dramatic voltage swing contributed significantly to generator tripping.

Spain has documented ongoing challenges with voltage regulation. Most wind and solar plants in the country operate in fixed-power-factor control mode, rather than actively controlling voltage. The 400-kV and 220-kV grids frequently operate above normal voltage ranges for several hours each month, and incidents of generator tripping due to over-voltage have been increasing from 2021 through 2023.

The April 2025 Blackout

At the time of the April 28 event, the Iberian Peninsula was experiencing relatively normal operating conditions. The load was approximately 25 GW in Spain and 8 GW in Portugal. Weather conditions were favorable—sunny with no faults, though there were high winds in the southern peninsula. The system had benefited from a rainy spring, resulting in high hydro reservoir levels. However, several large generators were undergoing seasonal maintenance. Meanwhile, there was significant solar PV generation being supplied to the grid at the time of the incident.

Spain was exporting power on all its interconnectors before the event, with particularly notable trading activity causing flow changes on the France-Spain interconnector. This trading had reduced the export to France from about 1 GW to nearly zero just prior to the disturbance, indicating that the system was not operating in a steady state but was experiencing fluctuations due to market activities.

McGuinness again emphasized that not all of the data needed to make final conclusions has been made available publicly. Therefore, the information that he was about to present was preliminary and root cause findings could ultimately change.

Sequence of Events. The blackout cascade developed with surprising rapidity, but preliminary data shows warning signs appeared well before the system collapsed. Approximately 30 minutes before the blackout, monitoring systems detected a notable 0.2 Hz frequency oscillation in Spain that persisted for about five minutes before damping out. Then, 17 minutes before the blackout, a smaller 0.06 Hz oscillation appeared for roughly two minutes, followed closely by a return of the 0.2 Hz oscillation, which lasted about two more minutes.

McGuinness noted, however, that oscillations themselves aren’t enough to bring down a grid. “An oscillation in frequency can happen, but you do have to push the grid outside of its normal frequency range in order for protection to actually operate and disconnect devices, or for the oscillation to result in the voltage going out of normal operating range and voltage protection operating,” he said.

Four minutes before the blackout, a sudden small frequency drop of about 0.05 Hz to 0.06 Hz occurred precisely at 12:30 p.m. local time. This may have been due to scheduled market trading activity at the bottom of the hour. The first confirmed generation trip occurred 19 seconds before the main blackout, though this was relatively small, causing only a 0.01 Hz to 0.02 Hz frequency drop.

The main event began with the tripping of a large generator in southwest Spain. Immediately, the Spanish frequency began falling faster than the rest of the European grid, indicating the beginning of a loss of synchronism. What followed was an extraordinarily rapid cascade, between 1.5 seconds and 5 seconds after the first large generator trip, a second large generator in Spain tripped, accompanied by what officials described as a “massive” disconnection of renewable energy sources throughout Spain. The tie lines between France and Spain also tripped during this period, as did a 1.3 GW unit at the Golfech nuclear power plant in southern France. Within just 5 seconds after the first large generator trip, Spain was experiencing a complete system blackout.

System Restoration. “Immediately after the blackout occurred, all of the system operators began to kick into restoration mode, assessing what was available, operating breakers, performing the switching, getting crews out to site, and so on,” explained McGuinness. “Black-starting an entire 40-GW grid from scratch is obviously not easy,” he said, noting that it requires a lot of switching actions and a lot of coordination between the system operator, distribution operators, and other significant entities and stakeholders.

The restoration efforts in both countries were relatively successful, considering the scale of the blackout. In Spain, the first priority was re-establishing the interconnection with France. This connection was quickly leveraged to import up to 1.5 GW, providing vital support for the restoration process. Hydro units were prioritized next, followed by combined cycle units as the grid was progressively rebuilt.

The restoration metrics for Spain show that 62% of substations were re-energized after 9.5 hours, with all substations restored after approximately 15.5 hours. Full-load restoration was achieved within about 24 hours of the initial event.

In Portugal, restoration relied on two main black-start resources: a 330-MW gas turbine plant in the north and a 346-MW hydro facility in central Portugal. The restoration proceeded somewhat faster than in Spain, with 96% of transmission substations restored within 10.5 hours and all load reconnected within approximately 12 hours.

Key Lessons and Considerations

While the exact causes of the April 2025 blackout remain under investigation, several critical factors warrant attention in grid planning and operations, particularly for systems with high renewable penetration.

Voltage Control. Voltage control emerges as a primary concern. Both static and dynamic voltage control are critical for grid stability, and sufficient reactive power capability must be available and properly leveraged. System operators need to maintain awareness of reactive power source limits and assess their vulnerability to elevated voltage conditions, especially as renewable penetration increases.

Protection Coordination. Protection coordination represents another vital consideration. Power plant protection settings can create common mode failures if not properly coordinated. When multiple plants set protection limits near the same grid code or interconnection requirements, they may all trip simultaneously during abnormal conditions. This means that sustained abnormal voltage events can potentially disconnect more capacity than standard contingency planning assumes, leading to cascading failures.

Oscillations. System oscillations, while not direct causes of blackouts, can push systems outside normal operating ranges. Understanding existing oscillatory modes and their damping characteristics is crucial for system stability. As grids evolve and new interconnections are added, new oscillatory modes can emerge that require careful monitoring and mitigation.

Synchronizing Torque. Beyond the often-discussed issue of inertia, synchronizing torque deserves special attention. This factor determines how well synchronous generators remain tied together during disturbances and becomes increasingly critical as systems incorporate fewer synchronous generators. Adequate synchronizing torque is essential for preventing loss of synchronism during system events.

Ride-Through Capability. Ride-through capability requirements for generators during abnormal conditions must be clearly defined and, importantly, verified through performance testing. Setting requirements is only the first step; ensuring that equipment actually performs as expected during disturbances is equally important.

Inertia. Inertia management is increasingly critical for grids with high renewable penetration. Many such systems now implement inertia floors to limit the maximum rate of change of frequency during disturbances. While inertia is often considered primarily for frequency stability, it also plays a crucial role in preventing loss of synchronism between different parts of the grid. As conventional synchronous generation decreases, careful monitoring and management of system inertia becomes essential to maintain stability during disturbances.

Details Still Emerging

McGuinness emphasized that the extent to which any of these factors contributed to the April 28 event is still uncertain. “We’re not saying any one of these was the cause or the trigger, but we want to give them as context and lessons learned for your own grids to review where you are and identify if any of these are potentially an important factor you may want to revisit or look at on your own grid, if you haven’t done so in detail already,” he said.

The April 2025 Iberian Peninsula blackout demonstrates the complexity of modern power systems, especially those with high renewable penetration. While the exact causes of this event are still being investigated, it highlights the importance of comprehensive system planning, protection coordination, voltage management, and dynamic stability assessment.

As resource mixes evolve, dynamic system assessment becomes increasingly vital. Modern power systems require rigorous reliability standards and regular security assessments in both planning and operational timeframes. System operators need to understand how close their system is to potential cascade points and implement appropriate mitigations. The lessons from this event will be valuable for grid operators worldwide as they navigate similar transitions in their own systems.

—Aaron Larson is POWER’s executive editor. This summary is based on preliminary information available as of May 6, 2025. A complete investigation report is expected to provide more definitive findings and recommendations.