How do you explain that something like this has happened?

‘Until all the information about the event is available, it is not possible to determine the cause of the event in a definite way, although it is possible to guess some of the reasons why it happened. The European transmission network operates with the N-1 safety criterion, which means that the system can continue to operate safely even if a network element fails. This means that the cause is not due to a single failure, but to a concatenation of events that led to the blackout. REE has indicated that the cause was a severe frequency oscillation, which occurs when the balance between the energy entering the system (that produced by the generators) and that leaving the system (that which is consumed, plus losses) is lost. This oscillation (whose causes have not yet been clarified), at a time when demand was low, photovoltaic generation exceeded 55% and there was a reduced contribution from generators that could provide inertia to the system (only 3% from combined cycles and 10% from hydro, with nuclear at half gas), meant that the system could not cope with the frequency variation and a chain disconnection of generators was triggered which, in the end, caused the blackout’.

Was it unlikely to happen?

‘The probability of this type of event is low, but it is possible that it could happen. In fact, the system operator (REE) has a protocol, which has been activated for the restoration of the electricity system after ablackout. However, REE has a simulator at the Tres Cantos control centre where operators are trained to resolve this type of situation, and others, should they occur. Thanks to this, service has been restored in a relatively short period of time despite the seriousness of the situation’.

Why did this happen on the mainland, and is it a more vulnerable region in Europe?

‘The peninsula has a limited interconnection with the rest of the European electricity system through France (just 4 GW), which puts it in a weak position compared to other more interconnected electricity systems. In addition, the fact that photovoltaic production is so high at certain times means that the system's inertia (rolling generation) and reactive energy management capacity to cope with frequency and voltage fluctuations is lower, which reduces its capacity to react’.

Could this happen again in the next few days and in the medium term?

‘It is not foreseeable that it will happen in the next few days, as I understand that the system operator is acting accordingly (today, for example, photovoltaic generation is half of what it was yesterday, although we do not know if this is because they have limited it for this reason or for other reasons), but it is an event that could happen again in the medium term if measures are not taken to prevent the conditions that produced the blackout from recurring. This will require a detailed analysis of the causes and action to be taken accordingly.

What needs to change so that it doesn't happen again?

‘In the short term, it would probably be necessary to limit photovoltaic production (which is already being done by REE's renewable control centre) to lower thresholds in periods of low demand, in order to increase rolling generation that provides inertia to the system to cope with frequency variations. In the medium term, the solution would probably involve strengthening the interconnection with France (and thus with the rest of the European electricity system) and installing frequency and voltage stabilisers in the transmission grid to compensate for the loss of inertia in the system due to the high penetration of renewables (especially photovoltaic). Another measure would be to improve the sectorisation capacity of the transmission grid to isolate faults and prevent them from affecting other areas, although this point is not obvious given the rapid propagation of this type of event. However, the rapid action of the protections in the interconnection with France is what has prevented the rest of the European electricity system from suffering the same effects as we have had on the peninsula’.

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.

How do you explain that something like this happened

‘An ‘absolute zero’ is an extremely serious situation in which the power grid completely loses voltage, i.e. the entire system shuts down. It is as if a switch were flicked that suddenly disconnects the entire electricity supply. This widespread blackout on the Iberian peninsula occurred because, in just five seconds, more than half of the electricity generation capacity was lost. The system, 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. This is a self-protection measure that, paradoxically, implies a total cut-off’.

Was it unlikely to happen

‘Yes, this type of event is considered highly unlikely. The Spanish electricity grid has multiple safety mechanisms and protocols that act automatically to avoid precisely this type of collapse. However, the speed and magnitude of the loss of generation that occurred today exceeded the usual margins for manoeuvre. Even experts from the system operator itself (Red Eléctrica) had ruled out in the past that there could be an ‘absolute zero’ on the peninsula. This shows that, although the system is prepared for many contingencies, it is not infallible’.

Why did this happen on the Iberian peninsula? Is it a more vulnerable region in Europe?

‘The Iberian peninsula has a peculiar position in the European electricity system because it is poorly connected to the rest of the continent. Its electricity interconnections are limited, so in practice it functions almost like an energy island. This makes it more vulnerable to internal disturbances: if a major failure occurs within the peninsular system, it cannot receive sufficient external support to stabilise itself. In addition, in recent years there has been a large increase in the presence of renewable energies, such as solar and wind, which are variable and weather-dependent. This can make the system more difficult to control in real time, without sufficient backup or storage’.

Can it happen again in the next few days? And in the medium term?

‘In the next few days it is unlikely that an outage of the same magnitude will occur again, especially as the system will now be on high alert. In the short term, the operator will take very strict preventive measures. However, in the medium term, if the exact causes are not well understood and possible structural failures are not corrected, the risk does not disappear completely. It is essential to thoroughly investigate what caused such a rapid loss of generation in order to prevent a recurrence’.

What needs to change so that it doesn't happen again?

‘There are several key lines of improvement The most important is to increase electricity interconnections with France and other European countries, so that the peninsula is no longer so isolated’. It is also necessary to improve the flexibility of the electricity system, incorporating more storage (such as batteries or hydraulic pumping systems) that can compensate for the variability of renewable energy. In addition, control and forecasting systems should be reinforced, and more demanding simulations should be carried out to contemplate extreme scenarios such as the one we experienced today. All this requires investment, planning and a clear strategy for a safe energy transition’.

How do you explain that something like this has happened?

‘It is a combination of incidents in the electricity grid. A succession of cascading or joint factors that have caused the collapse of voltages in the grid. It is still too early to know the causes and we will have to wait for official information from Red Eléctrica de España. This type of combination of factors is highly improbable in a grid as strong, as robust and as reliable as the Spanish and European electricity grid, despite the fact that paradoxically today we have had this failure’.

Was it unlikely to happen?

‘The European electricity system, and the Spanish one within it, are enormously robust and a failure like the one that happened today is enormously unlikely. The system works by being prepared for the worst possible failure at all times, so an event like today's must be multifactorial, something caused by many critical events in conjunction. Despite this, the system knows what to do in these cases and the restoration of the system is being carried out as expected’.

Why has this happened on the Spanish mainland, and is it a more vulnerable region in Europe?

‘Spain (or rather the Spain-Portugal peninsular electricity system), due to its low interconnection capacity with the rest of Europe (only a small interconnection with France), is considered in many cases an isolated energy system. This makes it more vulnerable to a combination of events such as the one we have experienced today, despite the fact that an energy zero is highly improbable.

Could it happen again in the next few days, and in the medium term?

‘In the short, medium and long term, this type of event is very unlikely to happen again because of the above. The fact that the system is being properly restored means that the system is under control and we can continue to rely on the very high quality standards of the electricity grid’.

What needs to change so that it doesn't happen again?

‘Without a doubt, analysing the causes of this event will help us to ensure that it does not happen again in the future. For this we must wait for the official report from the system operator (Red Eléctrica de España). From what we are seeing today in the system's replenishment, relying more on renewables, and establishing new grid operation protocols and procedures that include renewables more in the active participation of grid management, is crucial to continue building one of the most reliable and robust grids in the world, as we already are’.