2025-April-28: Iberian Electricity Network Blackout

On 28 April 2025 at 12:33 CET, the power systems of Spain and Portugal suffered an important blackout. A small area in France, close to the border with Spain, was also affected by the incident for a very limited duration.

Power demand

(Source: REE)

Published cronology

  • 12:03 – 12:07 CET power and frequency swings in the Continental European synchronous area
  • 12:19 – 12:21 CET power and frequency swings in the Continental European synchronous area
  • The TSOs of Spain (Red Electrica) and France (RTE) took actions to mitigate these oscillations
  • Before the incident, the international exchange programs of Spain were 1,000 MW to France, 2,000 MW to Portugal and 800 MW to Morocco, all in the exporting direction
  • Starting at 12:32:57 CET and within 20 seconds afterwards, presumably a series of different generation trips were registered in the south of Spain, accounting to an initially estimated total of 2200 MW. No generation trips were observed in Portugal and France. As a result of these events the frequency decreased and a voltage increase is observed in Spain and Portugal.
  • Between12:33:18 and 12:33:21 CET, the frequency of the Iberian Peninsula power system continued decreasing and reached 48,0 Hz. The automatic load shedding defence plans of Spain and Portugal were activated.
  • At 12:33:21 CET, the AC overhead lines between France and Spain were disconnected by protection devices against loss of synchronism.
  • At 12:33:24 CET, the Iberian electricity system collapsed completely and the HVDC lines between France and Spain stopped transmitting power.

(Source: ENTSO-E)

However, observing the power demand curve, that same day there were previous possible loss of supply, or interconnectors disconnection, incidents (for example at 11:00 – 11:20, 1GW).

In this sense, aeléc (the Spanish Association of Electric Utilities) has published several network voltage snapshots showing values nearing the threshold levels hours before the blackout:

  • ST-Lancha 220kV: Starting at 10:00 a.m. voltages fluctuate and, shortly before the blackout, they reach values around 250kV
  • ST-Badajoz 400kV: Voltages oscillating below the contingency limit but above normal operating values after 10:30 a.m. and exceeding the threshold level just before the blackout

(Source: aeléc)

Generation mix

(Source: REE)

Interconnectors: Physical Flow

(Source: IESOE)

Legal requirements for generators

Generators are required to operate maintaining almost constant frequency, increasing generated power when consumption increased and vice versa, and also to adjust voltage to the level required by the System Operator (REE in Spain, REN in Portugal). However, when voltage or frequency moves out of the admissible range, generators disconnect from the network. When too many generators disconnect simultaneously, a black-out happens.

Under the current regulation, both Synchronous Power generators (most of them classical thermal and hydraulic power plants) and Power-Electronics-Interfaced-Power-Sources (typically Solar and Wind power plants) must provide frequency restoration services and voltage control services. Both must withstand frequency deviations (in the order of 1.5 Hz) and voltage deviations (in the order of 10%) [See figure below]

  • Commision Regulation (EU) 2017/1485 establishing a guideline on electricity transmission system operation (Source: EUR-lex.Title 5. Frequency Containment Reserves):
    • for the CE synchronous area, the reference incident shall be 3000 MW in positive direction and 3000 MW in negative direction
  • REE-Operational Procedure PO1.1 Operation and safety criteria (Source: REE. 4.3.2. System behavior against contingencies): “The system must maintain its control parameters within the limits for the following contingencies: Simple failure (criterion N-1), Loss of double-circuit lines, Simultaneous failure of the largest generator in an area and of a line connecting it to the rest of the system. In addition, it must be ensured that there is no voltage instability that could lead to a voltage collapse.”
  • REE-Operational Procedure PO1.5. Reserve for frequency-power regulation (Source: REE):
    • The Primary Control reserves (Frequency Containment Reserves, FCR) must withstand an instantaneous imbalance between generation and demand, due to a sudden loss of generation, loss of demand, or interruption of international exchanges.
    • The Secondary Control reserves (Automatic Frequency Restoration Reserves, aFRR) automatically corrects deviations from the schedule and system frequency deviations. Its operational timeframe must not be delayed more than 30 seconds and must be able to sustain a period of 15 minutes.
    • The Tertiary Control (Manual Frequency Restoration Reserves, mFRR) resolves deviations between generation and demand, and restore the used secondary control reserves. It can be maintained for 15 minutes or up to 29 minutes.
    • Replacement Reserves (RR) restore the Secondary and Tertiary control energies. It’s manually activated within a maximum of 30 minutes.

One piece of the puzzle: The Spanish Grid Voltage Control problem

  • There are thousands of hours each year with voltage nearing the regulation threshold. (Source: REE)
  • Previous voltage stability related events include, for example, the automatic shutdown of a nuclear power plant in January 2025. (Source: CNS)
  • REE is analysing market solutions to avoid this problem (Source: REE)
  • And the CNMC has approved an extension of this project (Source: CNMC)
  • Grid node voltage control is achieved inyecting or consuming reactive power, or using On-Load Tap-Changer transformers
  • Both Synchronous Power generators (most of them classical thermal and hydraulic power plants) and Power-Electronics-Interfaced-Power-Sources (typically Solar and Wind power plants) can provide voltage control by reactive power control (adjusting the field current the former, or the electronics control the latter).
  • However, not all of them are configured to do so because, although the previously mentioned EU Network Code and DG Energy Grid Connection Code stablish that both types of generators must provide that service, the legislation in force when they came into operation only considered classic generators (thermal, hydraulic) as systemic important generators. All other generators were considered secondary, because they were newcomers and small, and, consequently, to improve electrical safety, it was prioritized that they disconnect from the network in the event of any incident, so they were not required to control either voltage or frequency or provide any other service such as black-start capability.
    • REE-Operational Procedure PO1.7 Complementary voltage control service of the transmission network (Source: REE). ‘Generators belonging to the special regime [that is Solar, Wind, etc] will be service providers when the regulation allows it.’
    • RD_413-2014 Production of electrical energy from renewable energy sources, cogeneration, and waste (Source: BOE). ‘Installations with an installed capacity equal to or greater than 5 MW may voluntarily participate in the voltage control service’

Another piece: The Inertia Problem

The traditional value of the System Inertia is about 5 to 6 s

However Spain is under 2 s (Source: ENTSOE)

Both, Synchonous Generators and Power-Electronics-Interfaced-Power-Sources can provide inertia (from moving masses the former, from batteries the latter). But not all of them are configured to do so.

For comparison: Other power outages

Power restoration process

  • At 12:44 CET, a first 400 kV line between France and Spain was re-energised (Western part of the border).
  • At 13:04 CET, the interconnection between Morocco and Spain was re-energised.
  • From the start of the restoration until approximately 13:30 CET, several hydro power plants in Spain with black-start capability launched their black-start processes to initiate the restoration of the system.
  • At 13:35 CET, the eastern part of the France-Spain interconnection was re-energised.
  • At 16:11 and 17:26 CET, the two power plants with black start capability in Portugal succeeded their start up process after unsuccessful previous attempts, allowing to initiate the restoration process in Portugal with two islands.
  • At 18:36 CET, the first 220 kV tie-line between Spain and Portugal was re-energised, allowing to speed up the restoration of the Portuguese system.
  • At 21:35 CET – the southern 400 kV tie-line between Spain and Portugal was re-energised.
  • At 00:22 CET on 29 April 2025, the restoration process of the transmission grid was completed in Portugal.
  • At around 04:00 CET, the restoration process of the transmission grid was completed in Spain.

(Source: ENTSO-E)

Reports

The April 28th blackout: physical analysis of its possible causes; and preliminary proposals for a robust and stable grid, compatible with the energy transition (Source: FFII)