Iberian blackout, six months on


12:33:16.460. The voltage in the 400 kV grid at the time of this trip was 435.4 kV but this value, due to the way PMUs calculate and timestamp phasors, could already be influenced by the generation loss, the expert panel observes. In the second substation, the trip occurred at 12:33:16.820. The reasons for these two trips are not known. After that, several trips occurred between 12:33:17 and 12:33:18.020, with disconnection of wind and solar generation in Segovia, Huelva, Badajoz, Sevilla and Caceres totalling around 930 MW (or perhaps more than 1100 MW, as suggested by frequency variation). Some of these trips occurred due to over-voltage protection, but the cause of most of these trips is not known, the expert panel says.


The voltage increased up to a level beyond 435 kV during this sequence of generation trips in Spain, amounting to more than 2.5 GW in total as of 12:33:18.020.


No generation trips were observed in Portugal and France within the 12:32:00 – 12:33:18 timeframe.


As some generation units were consuming reactive power with the effect of reducing the voltage, the disconnections of these units without adequate compensation of loss of reactive


power by other resources in the system with the capability to inject/absorb reactive power meant that voltages in the system increased, not only in Spain but also in Portugal, the expert panel says, and the frequency decreased.


Between 12:33:18 and 12:33:21, voltage in the south of Spain sharply increased, and consequently also in Portugal. The over-voltage triggered a cascade of generation losses that caused the frequency of the Spanish and Portuguese power system to decline. It is the first time that a cascading series of disconnections of generation components along with voltage increases has been part of the sequence of events leading to a blackout in the Continental Europe Synchronous Area. At 12:33:19, the power systems of Spain and Portugal started losing synchronism with the rest of the European system.


Between 12:33:19 and 12:33:22, the automatic load shedding and system defence plans of Spain and Portugal – implemented in accordance with Commission Regulation 2017/2196 of 24 November 2017 establishing a network code on electricity emergency and restoration (NC ER) – were activated but unable to prevent the collapse of the Iberian power system.


At 12:33:20.473, the AC interconnection to Morocco tripped due to underfrequency. At 12:33:21.535, the AC overhead lines between France and Spain were disconnected by protection devices against a loss of synchronism. After this AC separation of the Iberian Peninsula, the power imbalance continued to increase, causing the frequency to further decline. Finally, at 12:33:23.960, the electrical separation of the Iberian system was completed by the tripping of the HVDC lines that transmitted power from Spain to France and all system parameters of the Spanish and Portuguese electricity systems collapsed.


In comparison to the blackout in Spain and Portugal, France was only marginally affected by the incident. Besides a loss of approximately 7 MW of load, one nuclear power plant tripped due to the incident.


Among aspects of the blackout needing further investigation, the expert panel mentions the following: voltage management instruments available; assessment of grid users’ behaviour with respect to voltage control and disconnections; performance of the system defence plan and scope for improvement; and analysis of the local oscillations.


An excitation control specialist’s perspective


Aasha Istiaq, power systems engineer at excitation control specialist Excitation & Engineering Services (EES), explores the ramifications of the Iberian blackout for power system operators in Europe and beyond


The Iberian outage highlighted the vulnerability of modern grids with high proportions of renewable energy and reduced conventional synchronous generation.


The ENTSO-E (European Network of Transmission System Operators for Electricity) expert panel’s factual report was released on 3 October 2025 (see pp 20-22), but the the root cause of the outage has yet to be fully determined and will be the subject of a further report to be published in early 2026. Lightly loaded transmission lines, high reactive power conditions and insufficient system inertia are all among likely potential contributors that produced a rapid sequence of trips and load shedding.


Restoration required co-ordinated intervention across multiple operators and operators used black start procedures to bring generation centres back online in stages before they could be reconnected to the grid.


For power engineers, the Iberian blackout was a jarring reminder of the structural challenges facing electricity systems undergoing the transition to net zero. By viewing this event through the lens of inertia, reactive power control


and grid stability, we can better understand how to anticipate and mitigate future risks.


How the blackout happened At 12:32:57 CEST on 28 April, a 400/220 kV generation evacuation power transformer in the Granada region of Spain tripped. This transformer transfers power from several wind and solar power plants to the Spanish transmission system.


Where transmission lines are lightly loaded and a large share of generation is inverter-based, the network can experience elevated voltages and stressed reactive power balance. The voltage on the 220 kV side reached 242.9 kV, activating an over-voltage protection relay. Crucially, this unit, and subsequent ones, were operating in reactive power consumption (absorption) mode to manage the already-high system voltage.


The loss of this reactive power absorption capacity led to an uncompensated surge in system voltage, exceeding 435 kV in some areas. This severe over-voltage then triggered a cascade of further trips. The simultaneous loss of generation and the voltage excursions induced


22 | October 2025| www.modernpowersystems.com


a rapid fall in frequency, activating automatic load-shedding schemes and precipitating wider disconnection and islanding.


Publicly available operational data and technical analyses, as well as the expert panel factual report, show the following pattern. In the hour before the major outage, there were measurable inter-area/low-frequency oscillations. Generators and phasor measurement units recorded those oscillatory modes, but power system stabilisers (PSS) only damped them intermittently.


The official inquiry, in its report of June 2025, found that certain generation facilities did not provide the dynamic voltage support or reactive power absorption capability required by their respective grid codes, exacerbating the voltage excursions and forcing subsequent protection actions.


What grid conditions allowed this failure?


In Spain and Portugal on 28 April, a combination of high variable renewable energy (VRE) output and the network’s operating point created conditions with small voltage regulation


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