The Guardian had this explanation:
The risks posed to electrical systems by big variations in temperatures are well known in the industry, but it is rare for it to cause problems on the scale of the power outage in Spain and Portugal on Monday.
Transmitting power through wires comes with technical challenges. The air around conductors ionises, causing visible and audible electrical discharges known as corona discharges. That energy is lost. Extreme temperature variations cited by Portugal’s Redes Energéticas Nacionais can cause bigger problems by affecting metal wires at a molecular level, changing the frequency of the electrical current.
“Due to the variation of the temperature, the parameters of the conductor change slightly,” said Taco Engelaar, managing director at Neara, a software provider to energy utilities. “It creates an imbalance in the frequency.”
Once frequency drops below a certain level, power generators will automatically turn off in order to avoid overloading the grid.
Engelaar said that – if the atmospheric cause is confirmed – it would be a “pretty spectacular” wake-up call for other countries that they need to invest in resilience against climate risks.
“To see it at a national level, across two countries, I’ve never seen it in my career,” he said.
I am missing the braincells to understand this, but maybe someone else can 😀