Portuguese grid operator REN will allow utilities to resume imports of electricity from Spain, but the transmission capacity will be limited to around one-third of the levels seen before the huge power blackout in the two countries on April 28.

Spain’s high-voltage electricity grid suffered more power generation disruptions than previously known ahead of the country’s largest ever blackout, the government said.

Spain experienced a widespread power outage that left millions without electricity for nearly a full 24 hours. While power has largely been restored, the cause of the failure is still under investigation.