The 2025 European Power Outage Incident: day 4

Day 4: Is the Iberian power production systems properly designed for dynamic regulation?

On day 4, there are still no conclusive reports on the cause of the April 28 outage.

Various sources attribute the event to two rapid “disconnection events” in the electrical grid, likely originating in southwestern Spain, specifically Extremadura. These events led to a loss of about 15 gigawatts of electricity generation—equivalent to 60% of Spain’s power demand at the time—within seconds, triggering a collapse of the Iberian Peninsula’s grid.

While Spanish authorities have denied that the country’s high reliance on renewable energy production could have contributed to the April 28 outage, the composition of energy production types might influence the ability to respond to changes in demand. This note investigates further the “System or operational causes” mentioned in the initial note on this topic.

In the European transmission grid (synchronous interconnected grid), electric power is transferred at a nominal frequency of 50 Hz. According to ENTSO-E, the frequency must remain within ±0.05 Hz. During normal operation, the grid frequency will fluctuate within this range based on varying load. A frequency below 50 Hz (49.95–50 Hz) indicates slight underproduction, and automatic primary regulation ensures that additional energy is transferred to generators (for example from turbines), increasing the rotational speed of the generator to compensate for higher demand. Likewise, when the frequency is above 50 Hz, generator speed is decreased to allow the electric load to consume excess energy, restoring the energy balance between mechanical rotational speed and electric energy. This dynamic regulation is called Frequency Containment Reserve (FCR), or primary reserve, and intervenes within a few seconds, with activation time within 30 seconds. There are also secondary reserves (aFRR, 5-minute activation time) and tertiary reserves (mFRR, 12.5-minute activation time).

FCR must be symmetric, meaning plants must be able to ramp production up or down according to demand. Below is an overview of plant types and their contribution to total energy production in Spain:

Type	FCR	aFRR/mFRR	Contrib. (%)	Comments
Wind	❌	❌	23.2	Variable production, but new technology increase capability.
Nuclear	❌	❌ (limited)	20.0	Serves as base load.
Solar photovoltaic	❌	❌	17.0	Depends on available sunlight. No dynamic capability.
Combined cycle	✅	✅ (partly)	13.6	Improved FCR over traditional gas turbines
Hydro	✅	✅	13.3	Low response time, and good precision
Cogeneration	✅ (partly)	✅ (partly)	6.2	Capability depends on capacity
Other	-	-	6.7

Table: Contribution to energy generation in percent, for the year 2024, from various energy production plants in Spain.
Source: REE total energy production

We see that the overall largest contributors (wind, nuclear, and solar) do not have good FCR capability. However, it is important to keep in mind that these numbers are averaged over a year (2024) and vary greatly with season and time of day.

we see that solar generation was increasing rapidly. Increase in solar contribution is expected during hours of available solar radiation. Wind (brown colour), nuclear (gray colour), and hydro (pink colour) remained at stable levels before the incident. Hydro-pumped storage generation decreased before 11:00, but this follows the same pattern as other days in the same period.

The diagram confirms, however, that FCR-capable energy generation sources had only a moderate contribution to overall power production.

Some reports have emerged showing the grid frequency around the time of the incident.

These show two oscillations that occurred from 12:03:15 to 12:07:40 and from 12:19:01 to 12:22:03. According to various online sources, these oscillations supposedly led to the disconnection of tie-lines, and by 12:33:16 CET, the technical separation of Spain and Portugal from the Central European system had taken place.

However, this is raw data of unclear origin and needs to be verified. If the data is valid, it is clear that during the oscillations the frequencies went beyond the nominal range. Were there problems with the activation of reserve capacity in Spain?

We have not yet found verified and quality-assured data on the actual transmission frequency. Such data might reveal whether there were issues with primary regulation prior to the incident. It would also be relevant to investigate whether the composition or contribution of the various power plants could have contributed to the incident.

This note is part of a series of notes on the subject of the Iberian power blackout that occured on April 28 2025, with focus on the technical details and capacity of the energy systems, as well as evaluation of possible causes.