An interim report from National Grid Electricity System Operator (NGESO) says that a lightning strike and the sudden loss of two large electricity generators was to blame for this month's blackout.
The lightning strike at 4.52pm on Friday 9 August on the Eaton Socon, Wymondley Main transmission circuit caused the loss of 500MW of embedded generation.
However, immediately following the lightning strike and within seconds of each other Hornsea offshore windfarm and Little Barford gas power station both reduced their energy supply to the grid.
The total generation lost from these two connected generators was 1,378MW. This unexpected loss of generation meant that the frequency fell very quickly and went outside the normal range of 50.5Hz - 49.5Hz.
Energy regulator Ofgem has launched an investigation into the power cuts which left 1.1 million customers without power for between 15 and 20 minutes.
The investigation will seek to establish what lessons can be drawn from the power cut to ensure that steps can be taken to further improve the resilience of Britain's energy network.
It will also seek to establish whether any of the parties involved - National Grid ESO, National Grid Electricity Transmission, 12 distribution network operators in England and Wales, as well as generators RWE Generation (Little Barford Power station) and Orsted (Hornsea) - breached their licence conditions.
At the time of the incident National Grid ESO was keeping 1,000MW of automatic "backup" power - this level is what is required under the regulatory approved Security and Quality of Supply Standards (SQSS) and is designed to cover the loss of the single biggest generator to the grid.
All the "backup power" and tools the ESO normally uses and had available to manage the frequency were used (this included 472MW of battery storage). However, the scale of generation loss meant that the frequency fell to a level (48.8Hz) where secondary backup systems were required to disconnect some demand (the Low Frequency Demand Disconnection scheme) and these automatically kicked in to recover the frequency and ensure the safety and integrity of the network.
The report added that the system "automatically disconnected customers on the distribution network in a controlled way and in line with parameters pre-set by the distribution network operators." In this instance c. 5% of GB's electricity demand was turned off (c. 1GW) to protect the other 95%.
National Grid described the incident as "an extremely rare and unexpected event" and something that has not happened in over a decade.
According to the report, the disconnection of demand along with the actions of the ESO Control Room to dispatch additional generation returned the system to a normal stable state by 5:06pm. The DNOs then commenced reconnecting customers and supply was returned to all customers by 5:37pm.
The blackout caused major travel disruption on the rail network while other facilities were affected including Ipswich hospital and Newcastle airport.
The report, which was submitted to Ofgem on Friday (16 August), insists that the transmission system operated in line with security standards and the Grid Code.
National Grid will submit a final report by 6 September and this will seek to understand the exact failures at Little Barford and Hornsea and update on work with the distribution network operators (DNOs) to review the impact on demand and priority of disconnections.
Ofgem's investigation will initially focus on NGESO's requirements to hold sufficient back-up power to manage the loss of generation supplies both in terms of inertial and frequency response.
It will also examine whether distribution network operators complied with their Low Frequency Demand Disconnection (LFDD) obligations and the circumstances leading to the loss of power to critical infrastructure. Ofgem will 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.
Ofgem is also supporting the government's Energy Executive Emergency Committee investigation into the power failure and its consequences.
Jonathan Brearley, Ofgem's executive director of systems and networks, said: "The power cuts of Friday 9 August caused interruptions to consumers' energy and significant disruption to commuters. 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."
According to Ofgem, the opening of this investigation does not imply that the regulator has made any findings about non-compliance for any of the companies mentioned at this early stage of the investigation. However, if evidence does emerge in the course of the investigation enforcement action could follow.