Continental Spain and Portugal transmission collapse from voltage control gaps

In April 2025, shortly after midday local time, the transmission system of Continental Spain and Portugal suffered a complete blackout. The collapse propagated across Spain and Portugal in about 30 seconds and briefly affected a small area of neighbouring Southwest France through interconnection. Tens of millions of consumers lost supply, and in some areas restoration took most of the remainder of the day. The cascade developed far faster than in earlier European blackouts. Early oscillations were followed by a rapid voltage rise in the south of the peninsula. Protection on numerous solar photovoltaic plants tripped on overvoltage, reducing generation and pushing voltages higher in a reinforcing loop. Within about twenty seconds of the initial rise the overvoltage had become extreme, and the interconnectors to the neighbouring systems to the north and to the south tripped on protection settings. System defence plans activated but did not prevent the collapse. Voltage collapsed and frequency fell, resulting in a total blackout of the Continental Spain and Portugal system. It was the most severe European blackout in more than two decades. A regional expert panel drawn from transmission system operators, regional coordination centres and European and national regulatory bodies, chaired from two systems not affected by the event, published a factual report in October 2025 and a final report in March 2026.

According to the expert panel's final report, published in March 2026, the blackout resulted from a combination of around fifteen interacting factors. The principal ones reported were: (1) oscillations that were not adequately damped, with a low-damped inter-area mode in the wider synchronous area not effectively mitigated; (2) gaps in voltage and reactive power control, with substantial reactive capacity from shunt reactors available but not activated during the voltage rise preceding the event; (3) differences in voltage regulation practice between operators in the affected area; (4) rapid output reductions and generator disconnections, where protection at collector substations produced cascading disconnection under sustained overvoltage conditions that were not observable to the transmission operators; and (5) uneven stabilisation capability across the system. The panel stated that a high share of renewable generation was not the root cause, and that the central issue was voltage control regardless of the type of generation. It reported that renewable plants were operating in a fixed power control mode and that distributed rooftop solar experienced voltage-related disconnection through inverter protection. The panel also reported that some generator protection settings, for example overvoltage thresholds measured away from the point of connection, or instantaneous trips without time delay, could disconnect generation unnecessarily during voltage oscillations. The event was described as the first documented case in that synchronous area in which cascading generation disconnection under sustained overvoltage led to a system-wide blackout.

Lessons drawn from the expert panel's final report:

1. treat voltage and reactive power control as a primary reliability priority on systems with a high share of inverter-based generation, since a frequency-led security doctrine alone is no longer sufficient;
2. confirm that reactive reserves listed as available will respond automatically to a rising voltage, and design and test that automatic activation rather than relying on manual action;
3. provide high quality real-time measurement data so that inter-area oscillations can be detected and located, and so that system-wide damping controls can be tuned, validated and monitored;
4. review generator protection settings so that overvoltage thresholds are referenced to the point of connection and instantaneous trips without time delay are avoided, to reduce the risk of cascading disconnection during voltage excursions;
5. close the visibility gap between distribution-connected collector substations and transmission control rooms, so that local voltage conditions driving mass disconnection are observable in real time;
6. assess the system stability effects of rapid changes in injected power from generation, storage and demand, including fast reductions in photovoltaic output driven by market prices;
7. apply the harmonised operating voltage range consistently across interconnected systems and remove local derogations that permit operation outside it;
8. review technical connection rules and grid codes at regular intervals against the current generation mix, since rules written for an earlier plant population may no longer deliver the intended behaviour;
9. include sustained overvoltage and cascading generation disconnection as scenarios in defence plan design, restoration planning and operator training.

entsoe.eu/publications/blackout/28-april-2025-iberian-blackout