A lightning strike and subsequent power loss at two plants caused the blackouts earlier this month, which led to more than a million people losing electricity, an interim report has found.
The "extremely rare and unexpected" outages happened almost simultaneously at Hornsea off-shore windfarm and Little Barford gas power station just after the lightning strike on August 9.
The scale of generation loss meant the level of "backup" power required under regulations was insufficient to cover the loss, the report from National Grid Electricity System Operator said.
The system automatically disconnected customers on the distribution network with about 5% of electricity demand being turned off to protect the other 95%, meaning around a million people were without power.
Timeline of events which led to power cut - Friday August 9
4.52pm: Lightning strike on transmission circuit north of London, resulting in a small loss of generation
4.52pm: The lightning strike triggered two large generators to lose supply - Hornsea off-shore windfarm and Little Barford gas power
4.53pm: Back-up power is kept in the event of a outage, but only enough to cover the loss of the single biggest generator to the grid. However not enough to cover the scale of the loss of generation in this instance
4.53pm: At this point, the secondary back-up system kicks in and customers on this network experience an outage - around one million homes
4.57pm: The system returns to a "normal, stable state"
5.06pm: Power starts returning to customers
5.37pm: Distribution network operators return power to all customers, however there is an ongoing impact on customers
“Two almost simultaneous unexpected power losses at Hornsea and Little Barford occurred independently of one another – but each (was) associated with the lightning strike," the interim report said.
“As generation would not be expected to trip off or de-load in response to a lightning strike, this appears to represent an extremely rare and unexpected event.”
The report said the lightning strike was one of many that hit the grid on the day and strikes are “routinely managed as part of normal system operations”.
It said protection systems “operated correctly to clear the lightning strike and the associated voltage disturbance was in line with what was expected”.
Regulator Ofgem said its investigation would examine National Grid's requirements to hold sufficient back-power in the event of a loss in generator supply.
Ofgem also said it would be “looking at whether the companies made the right decisions both in the numbers of customers disconnected and whether those customers disconnected were the right ones”.
The regulator has the power to fine companies up to 10% of UK turnover and said it was looking into whether the Grid and other electricity companies breached their licence conditions.
It said it was working with rail authorities to establish why the blackout had a knock-on effect on passengers.
Rail passengers in the south-east were left stranded on trains during the outage on August 9, which also left a million homes without power and traffic lights to go down.
Jonathan Brearley, Ofgem’s executive director of systems and networks, said: “It’s important that the industry takes all possible steps to prevent this happening again.
“Having now received National Grid ESO’s interim report, we believe there are still areas where we need to use our statutory powers to investigate these outages.
“This will ensure the industry learns the relevant lessons and to clearly establish whether any firm breached their obligations to deliver secure power supplies to consumers.”