Last year’s paralysing power blackout in Spain and Portugal was caused by a “perfect storm of multiple factors,” according to a final report by an expert panel published on Friday.

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The 28 April outage raised doubts about Spain’s high dependence on renewable energy sources and planned phaseout of nuclear energy, but the leftist government and some experts have rejected claims that they exposed the power grid to a blackout.

The report commissioned by the association of electricity grid operators ENTSO-E cited the Iberian electricity system’s inability to control overvoltage events as a “key” factor, but stressed it was not the only one.

“There is no single cause. It was a perfect storm of multiple factors that contributed to the outage,” Damian Cortinas, the president of the association, said during a presentation of the report by 49 European experts.

Overvoltage occurs when there is too much electrical voltage in a network, overloading equipment.

It can be caused by surges in networks due to oversupply or lightning strikes, or when protective equipment is insufficient or fails.

The massive blackout cut internet and telephone connections, halted trains, shut businesses and plunged cities into darkness across Spain and Portugal for up to 10 hours. It also briefly affected southwestern France.

It was “the largest and most severe blackout we have ever experienced in the European electricity system in more than 20 years,” Cortinas said.

The report described a series of voltage fluctuations that led to widespread disconnections of power generation in Spain, particularly among converter-based systems commonly used in renewable energy.

These installations, the report said, were too rigid in operation to adapt to sudden increases in voltage.

Grid operators such as Spain’s REE were faulted for a lack of real-time monitoring, with the report noting that no risk was identified even as voltage levels approached critical thresholds.

The findings confirm the conclusions of a preliminary report which the experts issued in October.