EXPLAINED - A five-second power drop in the power grid plunged Spain and Portugal into chaos. The most important questions and answers about the blackout

It is currently unclear what caused Monday's power outage in Spain and Portugal. However, several Spanish media outlets are reporting that at 12:33 p.m., a large portion of the grid's power generation suddenly failed. Specifically, 15 gigawatts of power were missing for five seconds. According to Spanish government officials, this amount corresponds to approximately 60 percent of the electricity consumed in the country at that time.

The European power grid is not designed to handle such a large energy outage. According to Veit Hagenmeyer, a professor at the Karlsruhe Institute of Technology, the grid can only handle a shortfall of 3 gigawatts. Hagenmeyer writes to the Science Media Center: The power grid can handle the outage of one large power plant or several smaller ones, "but not this amount."

Several experts now believe that automatic shutdown devices were activated in Spain. These shut down power plants, power lines, and substations to protect them from damage. This triggered a cascade of shutdowns that spread throughout the grid and led to a widespread power outage.

We don't yet know. Many experts say that a power outage of this magnitude would require several extraordinary events or technical errors to occur simultaneously.

Sensors record everything that happens within the power grid. Therefore, it can be assumed that analyzing this data will provide answers.

The most important components are the power plants and the transmission and distribution grids. The electricity generated in power plants is distributed everywhere, even across national borders, via power lines. High-voltage lines form the grid's highways, so to speak. These lines bring the electricity close to users. The electricity then flows into distribution grids, which are comparable to main and secondary roads. They bring the electricity to consumers.

To ensure the grid remains stable, as much electricity as is currently being consumed must be fed into the system at any given time. Whether there is too much or too little electricity in the system can be determined by the frequency, which is measured in hertz. In the European grid, the frequency is 50 hertz. If the frequency drops, it means that more electricity is being consumed than generated. If it rises, more electricity is being fed into the grid than is demanded. Gabriela Hug, Professor of Information Technology and Electrical Engineering at ETH, says, "Both too high and too low a frequency are bad. But it is especially critical when the frequency continues to drop."

The energy supplier makes predictions about how much electrical energy will be needed tomorrow at noon, for example. Based on these predictions and the electricity market, schedules are created for the power plants. Any deviations from these estimates are reflected in real-time frequency changes.

If the frequency drops to 49 hertz due to excessive demand for electricity, a process known as load shedding begins. A portion of the consumers is then cut off from the grid to reduce the overall load and keep the rest of the system stable.

There are various reserves that must be deployed at different speeds. The first, the so-called primary control reserve, is activated a few seconds after the fluctuation, says Hug. These reserves are supplied by power plants that can quickly deviate from their schedules. The primary reserve stabilizes the frequency and thus prevents a blackout. The grid operator then intervenes – in Switzerland's case, that's Swissgrid. It coordinates the deployment of the so-called secondary control reserves so that the frequency returns to 50 hertz. This happens within a few minutes.

First of all, it's important to distinguish between power outages and blackouts. A power outage is a local event, for example, when the power line to a village is cut. Local, small power outages occur somewhere every day. And the nationwide power grid has built-in protective mechanisms that disconnect the affected lines in such a case, so that a power outage can be isolated. A blackout, on the other hand, is more widespread, and the problem takes longer to resolve.

Hug says: "Blackouts usually involve several unforeseen events." For example, a critical power line or power plant fails, and at the same time, another unforeseeable event occurs, or an extreme weather situation such as a storm damages power lines. Power grids are generally operated in such a way that they can cope with the failure of a critical component, such as a transmission line or a power plant.

In principle, one could try to increase redundancy in the system, says Hug. This could include, for example, additional power plants or safety measures in the transmission grid. In Switzerland, for example, the system of high-voltage lines, the power highways, is redundant. If one high-voltage line fails, says Hug, the power is distributed among the other parallel lines.

Redundancies could also be continually increased. But at some point, says Hug, it becomes a question of cost and benefit: "Because you would spend a lot of money preparing for an emergency that rarely occurs."

In 2003, a blackout occurred in Italy, which also affected parts of the canton of Geneva. The reason was the failure of a key power line between Switzerland and Italy.