From Darkness to Light: Iberia’s Rapid Grid Recovery Explained

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Last Updated on: 29th April 2025, 08:54 pm

On April 28, 2025, the Iberian Peninsula experienced a significant and sudden blackout, plunging large portions of Spain and Portugal into darkness and disrupting critical infrastructure for millions of people. The outage occurred at around midday, severing approximately 60% of the region’s electricity supply in seconds. The initial cause remains under thorough investigation, with complexities involving interconnected transmission systems, diverse generation sources including nuclear, renewable, gas, and remaining coal capacity, and operational factors that authorities have yet to fully unpack.

Like many people engaged in the global energy transformation, as soon as I heard the news I knew that the usual suspects—delayers and nuclear advocates especially—would be blaming renewables despite the lack of any evidence that they contributed. And lo, it has come to pass. My ‘favorite’ was the people who blamed the lack of nuclear in Spain despite there being 4 GW of operating nuclear on the grid that tripped off due to the blackout. However, while causation remains uncertain and speculative at this stage, the technical process used to restore power—known as a black start—offers clear insights into the resilience and sophistication of modern grid management.

It also provides some indication of the changes that will be occurring as we move into grids dominated by wind, solar, storage and transmission. I spoke with Mark O’Malley, Leverhulme Professor of Power Systems and co-founder and research head of the Global Power Systems Transformation (GPST) consortium recently about a broad range of topics including black start (part 1, part 2).

A black start is fundamentally about restarting a power grid from a completely de-energized state. The analogy often made is jump-starting a car, albeit at a much greater scale and complexity. In practical terms, it involves initiating small, self-sufficient generation units that can independently produce electricity without external grid support. Once these initial generation sources come online, they provide the necessary foundation of stable voltage and frequency that allows progressively larger generating units to reconnect. The eventual goal is a controlled, stepwise re-energization of the entire transmission system, reconnecting individual areas until the grid returns to full operational capability. It’s a carefully orchestrated process, demanding rigorous planning, specialized infrastructure, and exceptional coordination.

In Spain and Portugal’s recent outage, the grid operators—REE in Spain and REN in Portugal—activated a well-rehearsed black start procedure. The initial response required quickly mobilizing power stations equipped for independent restart, a role typically reserved for specific generation assets such as hydroelectric dams, gas turbines configured for rapid and autonomous start-up, and small-scale diesel generators strategically positioned at critical nodes. (Before anyone gets excited about the continued need for fossil generation for black start, battery storage is also excellent for this, and the generators can run on biofuels.) Each of these facilities has specific capabilities enabling them to bootstrap the grid back into life without needing significant external energy inputs.

Hydroelectric plants, especially those with reservoirs or pumped hydro storage, are uniquely suited for black start duties. Their advantage lies in the minimal initial power needed to begin generation, the inherent frequency provision and the large amount of power they can provide. Typically, a hydro plant requires only a very modest energy input—often provided by small on-site battery systems or auxiliary diesel engines—to open intake gates and energize generator fields. According to industry standards, as described in various technical manuals and operational guidelines from grid operators like REE, these systems can move from a dormant state to supplying substantial quantities of power within minutes. The stored potential energy in water behind dams provides a near-instantaneous generation response, making hydroelectricity an essential backbone in emergency restoration plans.

Gas turbines, particularly smaller open-cycle units equipped for rapid start-up, also played a pivotal role in Iberia’s recovery efforts. These turbines require fuel—typically natural gas—but rely on minimal external electrical inputs, provided by batteries or compressed-air starters, to initiate rotation of their internal compressors. Once spinning, the combustion turbine ignites fuel and swiftly transitions from consuming electricity to generating it. Small diesel generator sets offer similar benefits on an even more localized scale, needing only stored diesel fuel and small battery packs for ignition. While individually limited in capacity—often ranging from one to ten megawatts—these diesel sets prove invaluable in energizing local substations and providing initial auxiliary power for larger generation assets.

In contrast, large-scale nuclear plants, combined-cycle gas plants, and coal or biomass-fired thermal units are typically incapable of performing black-start functions. Such plants depend on external electrical power for critical auxiliary services, such as reactor cooling pumps, feedwater systems, control-room operations, and emissions mitigation technologies. For example, per guidelines from the International Energy Agency (IEA), a nuclear plant generally needs about ten percent of its total rated capacity to power essential services before reconnecting safely to the grid. Similarly, combined-cycle plants, involving both gas and steam turbines, require five to ten percent of their rated output to energize feedwater pumps and associated auxiliary equipment. These substantial energy needs illustrate why such large thermal plants remain entirely dependent on prior restoration of stable grid voltage and frequency.

Renewable generation assets, primarily wind and solar, present a unique paradox during black-start events. While capable of nearly instantaneous power delivery when conditions are favorable, legacy inverter-based technologies cannot independently form or stabilize an isolated grid segment. Instead, they require an existing stable frequency reference from the network to synchronize and start injecting power. Thus, despite their negligible auxiliary power needs, their role in initial grid re-energization has been limited to date. Most existing wind and solar farms become relevant only once grid stability is firmly reestablished by other more robust and controllable generation sources.

That’s not inherent to the technology however. Just as the power management systems of wind and solar farms can provide ancillary services if designed and configured to do so, increasingly they are likely to be asked to be grid-forming members and capable of black start re-energizing. They require virtually no power to start and if the sun is shining or the wind is blowing, provide very well understood power. This is part of what O’Malley’s GPST is working on, defining all aspects of the operations of grids heavy in inerter-based resources.

The restoration procedure executed across Spain and Portugal followed a deliberate and carefully phased sequence. Initially, operators quickly formed localized power islands—small, independently stable grids anchored by the activated black-start facilities. Regions such as Aragón-Cataluña, Galicia-León, and the Duero basin rapidly emerged as islands of stability within hours of the outage. Once these islands achieved internal equilibrium in terms of voltage and frequency, the next step involved cautiously reconnecting them, one by one, gradually rebuilding larger sections of the transmission network. This incremental process, detailed explicitly in post-event communications from REE and REN, ensured system stability and prevented dangerous voltage spikes or frequency fluctuations.

With stable grid conditions gradually expanding, operators began reintroducing larger generation assets such as nuclear reactors and combined-cycle gas turbines. For these plants, reconnection required the careful supply of substantial auxiliary power from already energized grid segments, up to 200 MW for some of Spain’s nuclear generation facilities. Imports of electricity from neighboring countries—particularly Morocco via the Gibraltar-Tarifa HVDC interconnection and France through multiple northern links—significantly accelerated and reinforced this phase. The Moroccan interconnection alone provided approximately 900 megawatts of critical assistance, per reports from grid operators, underscoring the strategic value of international interconnections in bolstering grid resilience.

By early morning of April 29, Iberian operators successfully completed the daunting task of full grid restoration. REN reported complete restoration for Portugal overnight, while Spain had approximately 99 percent of loads back online by early morning, achieving full service restoration by midday. The remarkably efficient recovery timeline—a testament to thorough planning and coordinated emergency response protocols—highlights the effectiveness of Iberia’s existing black-start resources and procedures.

This incident underscores broader lessons for grid resilience planning. Diverse, geographically distributed black-start capabilities—especially robust hydroelectric facilities coupled with strategically placed gas turbines and localized diesel generation—proved critically important in the swift restoration of power. Additionally, the event illustrates the increasingly important role of grid-scale battery systems, a technology rapidly becoming an essential consideration in black-start planning globally, though less influential in this specific scenario.

As Iberia reflects on this challenging event, the successful execution of its black-start recovery procedures serves as both reassurance and guidepost for future grid infrastructure planning. Without speculating on initial causes—appropriately left to thorough and methodical investigation—the professional and decisive management by REE, REN, and regional utilities stands as a commendable example of modern grid management and infrastructure resilience. Looking ahead, continued investment in diverse black-start resources, reinforced interconnections, and increasingly integrated battery storage solutions will only enhance the reliability and responsiveness of power grids worldwide.