The causes of the power outage that left millions in Spain and Portugal without electricity on Monday have yet to be fully determined, though service has now been restored across 99 percent of the Iberian peninsula. Red Eléctrica, the public company in charge of operating Spain’s transmission infrastructure, has preliminarily ruled out a cyberattack, human error, or unusual weather or atmospheric conditions as a cause of the outage. The company points out that the incident could have originated from two “disconnections of generation,” possibly linked to the inherent volatility of renewable sources.
Specialists emphasize that this type of total blackout—an exceptional and infrequent event—is also a security mechanism of the electricity system itself. For a grid to operate stably, energy production must be kept in balance with consumption; imbalances can cause blackouts as well as potentially damage infrastructure.
Maintaining grid balance is the responsibility of the system operator, who monitors parameters such as electrical frequency, voltage, and load from substations in real time. When there are significant discrepancies between generation and demand, automatic disconnections are activated in specific areas of the grid to avoid imbalances. In the most serious situations, the impacts of these triggered disconnections can extend to the entire network.
“This generalized blackout occurred because, in just five seconds, more than half of the electricity-generation capacity was lost,” Álvaro de la Puente Gil, professor of electrical engineering at the School of Mining Engineering of the University of León, said in comments to the Science Media Centre (SMC) in Spain. The grid, unable to balance such a sharp drop between generation and demand, protected itself by automatically disconnecting both internally and from the rest of the European grid.
In comments to the SMC, Miguel de Simón Martín, professor of electrical engineering at the University of León, explains that balance on a grid is typically guaranteed by three things. First is a complex network of interconnected lines, known as meshes, that distribute electrical flows across the grid to prevent overloads. Second, there are interconnections with neighboring countries’ grids, which allow energy to be imported or exported as needed to balance generation and demand.
Finally, there is something called “mechanical inertia.” Synchronous generators—the large spinning machines that generate electricity in power stations—also store a lot of energy in their very large rotating parts. Imagine, say, a coal-fired power station. Even if it stops burning coal to generate more power, the huge, heavy turbines it uses to create electricity will continue spinning for some time because of the energy stored up in them. Known as mechanical inertia, this phenomenon can act as a buffer against abrupt fluctuations in the grid. When there are imbalances between energy generation and demand, synchronous generators can speed up or slow down their rotational speed to balance things out, essentially acting as a shock absorber to the grid by absorbing or releasing energy as needed.
“A large, well-meshed grid, with strong interconnections and abundant synchronous generators, will be more stable and less prone to failures,” says De Simón Martín “The Spanish peninsular power grid has historically been robust and reliable thanks to its high degree of meshing at high and very high voltage, as well as its large synchronous generation capacity. However, its weak point has always been its limited international interconnection, conditioned by the geographical barrier of the Pyrenees.”
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