Spain and Portugal suffered the worst blackout in living memory in Europe this week. About 55 million people were affected and it lasted more than half a day. Some have blamed renewables and net zero emissions targets, as the two countries boast high levels of wind and solar on their electricity grid and lead Europe in the technologies. But is this true?

Some initial reports suggested a “rare atmospheric phenomenon” may have been to blame, citing a statement attributed to the Portuguese grid operator REN. In extreme weather, such as storms and heatwaves, the cables that carry electricity overhead can oscillate, which can cause problems with the conductors, particularly on older and less well maintained equipment. However, the weather in Spain was normal on Monday. On Tuesday REN denied it had been responsible for the statement, but did not give further details.

The Portuguese government blamed an unexplained power transmission issue. “In Portugal, we have no information related to a cyber-attack or a hostile act at this stage,” said a government spokesperson, António Leitão Amaro, who told CNN Portugal there seemed to have been an issue in Spain’s power transmission.

Spain pointed the finger at a breakup in its connection to France. Experts have said it could take days to identify the cause.

Spain and Portugal lead the EU in renewables and were sourcing about 80% of their electricity from solar and wind when the blackout hit on Monday.

Some commentators speculated that the grid could have been overloaded by the amount of wind and sun available, as renewables are more intermittent than other forms of energy and sudden fluctuations in sunshine or wind can cause problems.

However, this does not appear to be the case. Daniel Muir, a senior European power analyst at S&P Global, said: “The nature and scale of the outage makes it unlikely that the volume of renewables was the cause, with the Spanish network more often than not subject to very high volumes of such production.”

The Spanish grid operator said on Tuesday that there had been two events akin to loss of power generation followed by a massive loss of renewable power, which had destabilised the grid. They said it was “very possible” the first of these events was a sudden loss of solar power.

Blackouts can happen regardless of what type of energy powers the grid. In 2003, there was a significant blackout in London when the grid was primarily powered by fossil fuels. It was caused by a failed transformer and an incorrectly installed protection relay.

The 2019 UK blackout was caused by equipment failure at a windfarm and at a gas power station, which both occurred when a lightning strike hit a transmission circuit north of London. In both cases, engineering failures caused the problems.

In 2003 a problem with a hydroelectric power line between Italy and Switzerland caused a major outage across the Italian peninsula for about 12 hours.

Keith Bell, a professor of electronic and electrical engineering at the University of Strathclyde, said: “Events of this scale have happened in many places around the world over the years, in power systems using fossil fuels, nuclear, hydro or variable renewables. It doesn’t matter where you are getting the energy from: you’ve got to get the engineering right in order to ensure resilient supplies of electricity.”

The Spanish grid is undergoing upgrades as it is unable to carry the renewable energy required. Experts have said that the grid upgrades have not matched the pace of renewables being brought online.

Pratheeksha Ramdas, a senior analyst at Rystad Energy, said: “We cannot say that high wind and solar power caused the blackout – in the days before we saw far greater amounts of renewables on the system. But greater amounts of renewables may have made it more difficult to absorb a frequency disturbance. There are many possibilities behind what can cause these: a fault in the system or a weak transmission line. It’s a lesson for other countries: there needs to be greater investment in grid-forming inverters, which can help to stabilise the grid.”

Gas and nuclear-powered electricity grids have high inertia, which is a term for the resistance of a grid to frequency changes. Renewable grids have low inertia, which when unmitigated can cause them to be less resilient to sudden shocks.

David Brayshaw, a professor of climate science and energy meteorology at the University of Reading, said: “Technical changes mean the system now has less inertia, so imbalances must be corrected more quickly. Outage events, when they occur, are likely to become more significant and widespread.

“What stands out is that the power system is changing fast – driven by renewables, electrification, and massive investment. Yet there’s limited research into how climate change will affect future power systems, or how to design grids that are truly robust.”

Battery storage and other technologies such as super-capacitors and flywheels can substitute for traditional forms of inertia by providing capacity in the event of a sudden change in frequency. These can provide breathing space for the operator in times of shock.

So if large volumes of renewables are going to be loaded on to the grid, it needs to be properly designed and invested in.

Grids need to be resilient against blackouts but governments need to balance cost against risk, as disaster-proofing the grid is expensive. Electrical grids are interconnected across countries to lower the costs and increase reliability, but this can mean a problem in one area can cause a domino effect across the entire zone.

Prof Janusz Bialek of Imperial College London said: “The usual mechanism is of a fault causing tripping of a line, which causes overloading of another line leading to a trip, and hence leading to a domino effect propagating trips across a network. To prevent that happening, power utilities apply so-called (N-1) security criterion, meaning that a trip of a single element (transmission line or a power station) should not cause problems. They consider only one element failing, as the probability of two or more failures happening at the same time is very low, and protecting against them would be prohibitively expensive.”