Reacción a "Massive power outage on the Iberian peninsula leaves millions without power supply"

A ‘blackout’ or national zero is an extraordinary and highly unlikely circumstance in modern, developed electricity grids such as Spain's. It is defined as a total loss of supply throughout the electricity system. It is defined as the total loss of supply throughout the electricity system, a catastrophic situation, as we have seen today, and is declared by the system operator (REE).

To better understand what has happened, we can imagine that the electricity grid works like a hydraulic network of interconnected pipes through which water circulates. At some points in the network there are hydraulic pumps (the electrical generators), which provide flow (active power) and generate pressure (electrical voltage), driving the flow to the points of consumption. At other points, we have taps or drains, where water is extracted (the demand points). The water flows through pipes (power lines), and its movement depends on the pressure generated, the relative head of the points and the losses of the system.

In this analogy, the elevation of a hydraulic node - i.e. its relative head - represents the electrical voltage at that point, in the sense that a node with a higher elevation drives the flow towards a node with a lower elevation. Hydraulic pressure can also be associated with voltage, as both determine the ability to generate that flow. Most importantly, the pressure gradient or elevation between two points drives the water flow, just as the voltage difference drives the power flow between electrical nodes.

The objective is to keep the water flow (the active power) balanced between what is injected and what is consumed. To visualise this balance, we can imagine a fictitious central reservoir in the network. If the pumps inject more water than is withdrawn, the tank level rises; if more is consumed than is injected, it falls. This reservoir level is analogous to the electrical frequency of the system: a global magnitude reflecting the instantaneous balance between generation and demand. Although there is no real reservoir in the grid, the frequency behaves as if there were one, since its value rises or falls according to the mismatches, reflecting whether the system is accumulating or losing energy.

Thus, if the level (frequency) goes up, it means that we are generating more than is consumed; if it goes down, we are generating less. This ‘invisible reservoir’ is an excellent simile for understanding how the grid reacts to any imbalance. The system operator constantly monitors this level, as major deviations can trigger automatic protections or, in extreme cases, lead to a generalised collapse.

It is important to note that there are limits to this analogy. A conventional hydraulic network behaves more like a direct current (DC) network: there is neither a frequency-analogue magnitude nor a synchronised oscillation phenomenon. In contrast, in an alternating current (AC) network, frequency is a fundamental property of the system: it represents the joint oscillation speed of all the generators coupled to the network and acts as a direct indicator of the balance between generation and consumption in real time. Its control is essential to ensure system stability.

Finally, it is worth remembering that this analogy focuses on explaining the behaviour of active power. Reactive power, which also plays a key role in the regulation of local voltages, has no simple equivalent in this hydraulic model and has therefore been omitted for the sake of conceptual clarity.

Normally, the system operates in a safe state, i.e., with all variables within appropriate margins, even considering foreseeable contingencies (e.g., a pipe break, equivalent to the unavailability of a power line). If the variables are still within their margins, but the safety criteria are not met, we speak of a state of alert, in which the operator makes urgent corrections to return to a safe state. If these measures are not sufficient, the system enters a state of emergency: one or more variables (frequency, voltage, etc.) go out of their admissible operating margins, increasing the risk of catastrophic failure. In this state, extraordinary procedures are applied to re-establish stability or disconnection of the elements (blackout) to avoid damage.

In the case of a complete system disconnection (national zero), the so-called replenishment plans are activated, progressively re-energising the system while balancing generation and demand.

A key difference between this hydraulic simile and the real electricity grid is the limited storage capacity of the latter: there are no large reservoirs to cushion sudden variations. The only comparable elements are reversible pumped storage plants and battery systems, which together account for barely 2.65% of the peninsular's installed capacity (data as of 31 December 2024).

Three mechanisms are available to maintain grid stability:

• Meshing: a more meshed grid offers more alternative routes to distribute flows and avoid overloads.
• Interconnection: connection to neighbouring grids allows electricity to be received or exported as needed.
• Synchronous generators: these types of generators (hydraulic, thermal) provide mechanical inertia that helps to absorb fluctuations, acting as small energy reservoirs.

In short, a large, well meshed grid with strong interconnections and abundant synchronous generators will be more stable and less prone to failures.

The Spanish peninsular grid has historically been robust and reliable thanks to its high degree of high and very high voltage meshing, as well as its large synchronous generation capacity (hydro and thermal plants). However, its weak point has always been its limited international interconnection, conditioned by the geographical barrier of the Pyrenees. Currently, exchange capacity with Europe is barely 3% of installed capacity (3,977 MW out of 132,343 MW), far from the 15% target set for 2030 in the EU's Energy and Climate Change Policy Framework.

The energy transition towards renewable sources is radically transforming the generation profile in Spain. According to the National Integrated Energy and Climate Plan (PNIEC), the target is to reach 81% renewable generation by 2030. By the end of 2024, renewables already accounted for 66% of installed capacity and produced 58.95% of the electricity generated. Wind (37.53%), solar photovoltaic (37.85%) and hydro (20.40%) are the main current renewable technologies.

However, unlike hydro or thermal generators, wind and photovoltaic systems do not have inertia, as they are connected to the grid via power electronics (inverters). This characteristic means that the higher the renewable penetration, the lower the robustness of the grid.

Consequently, with a low interconnection capacity and a high share of inverter-based renewable generation, our grid is today more vulnerable and has less room to react to disturbances.

Regarding the blackout on Monday 28 April, little official information is yet available, although some sources point to a disturbance in the French grid caused by the sudden disconnection of a very high voltage line (400 kV). If confirmed, the closure of this connection would be, in our hydraulic simile, equivalent to closing a valve linking two grids, seriously unbalancing the Spanish system, which is more vulnerable due to its lower interconnection and lower level of synchronous generation (in contrast to France, where 32.67% of installed power is nuclear, providing high inertia).

The problem was exacerbated by the context: at 12 noon on the day of the blackout, 73% of the expected demand (27 GWh at central busbars) was to be covered by solar PV, increasing exposure to possible voltage and frequency fluctuations. The abrupt variation of the derived voltage could have caused the cascading of generation plants (installations are protected, among others, against overvoltages which, in the hydraulic simile, would be similar to a pressure surge), causing a large imbalance between generation and consumption (decrease in the level of the control reservoir), thus accelerating the collapse of the system.

The solution to this type of vulnerability is complex: increasing interconnection capacity is not trivial. However, a new 5,000 MW link between Spain and France (Gatika-Cubnezais), planned for the end of 2027, is already under implementation. This is a direct current (HVDC) link that will decouple voltage and frequency fluctuations between the two systems, as well as almost doubling the exchange capacity.

Finally, in addition to strengthening interconnections, it will be essential to deploy energy storage and inertia provision systems (synthetic inertia) for both voltage and frequency. It would also be interesting to develop microgrids capable of isolating themselves from the main grid in the event of failure, self-supplying through distributed generation (photovoltaic, mini-wind, cogeneration, batteries, etc.). These solutions will increase the flexibility and resilience of the grid, although they still require greater technological maturity and decisive regulatory support.