What we know about Monday's sweeping power outage in Spain and Portugal

MADRID — The sweeping power outage that hit Spain and Portugal this week has raised questions about the electricity grid in a region not normally known for blackouts.

Monday's outage, one of the worst ever in Europe, started in the afternoon and lasted through nightfall, affecting tens of millions of people across the Iberian Peninsula. It disrupted businesses, hospitals, transit systems, cellular networks and other critical infrastructure.

Authorities in Spain and Portugal are still investigating exactly what caused the failure, though some information has emerged about happened. Here's what to know:

What happened?

Spain and Portugal lost most of their electricity early Monday afternoon.

Shortly after 12:30 p.m. local time, Spain lost 15 gigawatts of electricity — or roughly 60% of demand in the country of 49 million, Prime Minister Pedro Sánchez said.

There had been two significant ''disconnection events'' — or interruptions in power — before the outage, Spain's electric grid operator Red Eléctrica de España said Tuesday.

While Spain's grid managed to recover from the first event, systems operations director Eduardo Prieto said that the second was more damaging, progressing to the point of interruptions from France's electrical system and producing ''a massive, temporary disconnection.''

About three hours before the outage, power quality sensors in homes in the Madrid area showed warning signs of an unstable grid — there were small fluctuations in voltage around 9:30 a.m. local time, Whisker Labs CEO Bob Marshall said Tuesday. The Maryland-based software developer has a couple dozen sensors in homes in and around Madrid, testing the technology for use in Europe for home fire prevention and grid monitoring.

Instead of normal, steady voltage, Marshall said the data shows there were oscillations whose frequency and magnitude increased over the next three hours until the grid failed. He does not know what caused the instability.

Around noon, there was a big jump in the magnitude of the fluctuations, with the voltage measured going up and down by about 15 volts every 1.5 seconds, Marshall said.

''The way I would interpret our data," Marshall said about Monday's events, "is that the grid is struggling. Something's wrong. And it's showing increasing signs of instability.''

What don't we know?

We don't know exactly what caused Monday's fluctuations and eventual failure.

''There's a variety of things that usually happen at the same time, and it's very difficult for any event to say ‘this was the root cause,''' said Eamonn Lannoye, managing director at the Electric Power Research Institute, Europe.

Lannoye said there was a range of events that can explain grid failures, including that electric grid lines or generators are switched off in some locations for maintenance.

''This could be a really complex event, I think it's fair to say,'' Lannoye said.

What causes power outages?

Power outages can have several triggers, including natural disasters and extreme weather, human-caused disasters, equipment failures, overloading transformers and wires and so on.

Electric grids are sensitive to imbalances in the amount of energy generated versus what's used.

Enric Bartlett, an energy expert and professor of public policy at Spain's Esade business school, likened a grid's supply and demand balance to a tandem bike.

''To avoid falling, everyone must pedal at the same cadence,'' Barlett said.

''Electrical grids are large interconnected systems, and their stability is related to a very close balance between electricity generation and demand," said Grazia Todeschini, an engineering researcher at King's College London.

''If one area is disconnected, it can cause knock-on effects in nearby areas,'' she said, adding that while grids have measures in place to limit the impact of outages to small areas, when the imbalance is large, disruptions can spread quickly and far.

What have Spain and Portugal's governments said?

Authorities in Spain and Portugal have downplayed the idea that a cyberattack was responsible. Spain's High Court said it would investigate to find the cause.

On Tuesday, there was renewed attention on Spain's renewable energy generation. The southern European nation is a leader in solar and wind power generation, with more than half of its energy last year having come from renewable sources. Portugal also generates a majority of its energy from renewable sources.

Questions remain about whether Spain's heavy renewable energy supply may have made its grid system more susceptible to the type of outage that took place Monday. The thinking goes that non-renewable energy sources, such as coal and natural gas, can better weather the type of fluctuations observed Monday on Spain's grid.

That is because renewable energies like solar and wind provide intermittent supply, relying on when the sun is shinning and wind is blowing. However, in recent years batteries that store energy, along with other methods, have helped regulate changes in electricity supply from renewables.

Lannoye said it was too early to draw a straight line between Monday's event and Spain's solar power generation.

''I think there's some putting the cart before the horse to say this was solar,'' Lannoye said, simply because there was solar power on the grid at the time of the disruption.

Why was France partially affected?

Europe's electric grid is highly connected, meaning that it can pool power between countries. That can make the system more resilient, experts say. It also means a disruption in a major transmission artery or frequency imbalance can trigger cascading protective shutdowns across countries, according to Shreenithi Lakshmi Narasimhan, member of the Institute of Electrical and Electronics Engineers.

Spain and Portugal are connected to Europe's main electric grid through France. Spain's grid operator on Tuesday said that the sudden drop in power caused an interconnector between Spain and France to trip.

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Associated Press journalist Jennifer McDermott contributed to this report from Providence, Rhode Island.