The outage was "the biggest and worst we have seen in Europe's electricity system in over 20 years," said Damian Cortinas, CEO of the European industry organization ENTSO-E, which commissioned the investigation.
It knocked out internet and telephony, stopped elevators and darkened cities for up to ten hours.
The investigation confirms that several cases of overvoltage were important links in the chain of events, but they had different causes. A network should be able to handle such events, but the safety systems failed.
"There is no single cause. This was a 'perfect storm' with many factors contributing to the outage," Cortinas said.
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