The National Grid says the disruption was caused by the loss of two generators but energy watchdog Ofgem called for an investigation
Energy regulator Ofgem has called for an urgent report into the blackout that saw homes and businesses go without power on Friday.
More than a million people were impacted by the power cut, which caused chaos for commuters across England and Wales.
The National Grid explained the issue was caused by failures at two power generators.
However, energy watchdog Ofgem called for an immediate investigation of what went wrong.
A statement said: “In any incident the priority is to get power restored to customers as quickly as possible. National Grid has now informed Ofgem that the system has been restored.
“However, Ofgem understands the frustration this power cut has caused consumers.
“Ofgem has asked for an urgent detailed report from National Grid so we can understand what went wrong and decide what further steps need to be taken.
“This could include enforcement action.”
National Grid operations director Duncan Burt told the BBC: “We saw an incredible event yesterday evening, the near-simultaneous loss of two large power stations.
“The normal protection systems we have responded to try and manage that, but the loss was so large that we went into our secondary systems.
“What they do is disconnect small amounts of electrical demand around the country, in order to protect the national demand, and the power cut we saw last night were related to those protection systems firing.”
In a statement, the National Grid said: “We appreciate the disruption caused by yesterday’s power outage and investigations have continued overnight to better understand the situation.
“The root cause of yesterday’s issue was not with our system but was a rare and unusual event, the almost simultaneous loss of two large generators, one gas and one offshore wind, at 16.54pm.
“We are still working with the generators to understand what caused the generation to be lost.
“Following the event, the other generators on the network responded to the loss by increasing their output as expected.
“However due to the scale of the generation losses this was not sufficient, and to protect the network and ensure restoration to normal operation could be completed as quickly as possible, a backup protection system was triggered which disconnects selected demand across.
“Following the incident, the system was secured, and the Electricity System Operator gave the all clear to power companies who are responsible for supply at a local level, within 15mins, so that they could start to restore demand.
“All demand was reconnected by [power companies] by 17.40pm. We appreciate the disruption cause and will continue to investigate, with the generators involved and wider stakeholders, to understand the lessons learned.”
Travel chaos continues
Although power was restored on Friday, officials said that it would have “knock on impacts on travel,” with further delays and cancellations across parts of the rail network on Saturday morning.
National Rail warned: “Although power has been restored, trains remain disrupted. We expect some trains will be cancelled or amended until 12:00 today.”
A government spokesperson said: “We’re working hard with Network Rail and others to ensure systems are up and running as quickly as possible, so that everyone can complete their journeys safely.”