National Grid has published a technical report into the outage that left more than a million customers without power and caused travel chaos last month.
So what happened?
- Before the power cut occurred on Friday the system was operating as normal, with around 30% of generation from wind, 30% from gas, 20% from nuclear and 10% imported from interconnectors
- The weather was not unusual for the time of year, with heavy rain and some lightning storms around the transmission network north of London
- A lightning strike occurred on a transmission circuit at 4.52pm but systems operated correctly, and the line returned to normal in around 20 seconds. There was a loss of 150 megawatts of small generation connected to the distribution network as a protection, which would be "expected" in a lightning strike
- Immediately following the lightning strike, Hornsea offshore wind farm, in the North Sea, reduced its energy supply to the grid by 737 megawatts, and in Bedfordshire, Little Barford gas power station's steam turbine tripped, reducing supply to the grid by 244 megawatts. National Grid said the generation would not be expected to trip or "de-load" in response to a lightning strike and therefore it "appears to represent an extremely rare and unexpected event"
- The loss of a total of 1,131 megawatts of generation caused a rapid fall in the frequency of the grid, which caused another 350 megawatts of local generation to disconnect under protection measures
- There was automatic "back-up" power to cater for the loss of 1,000 megawatts, the level required under regulatory standards, but the total loss was 1,481 megawatts, which meant the frequency of the grid fell very quickly to outside the normal range
- All the back-up power and tools the Electricity System Operator had were used to stop the frequency fall and bring it back into its normal levels
- But just as it was recovering, there was a further trip of a gas turbine at the Little Barford power station, causing supply to drop a further 210 megawatts, so that the overall loss of generation totalled 1,691 megawatts
- All of the back-up power had been deployed and the total loss of generation meant the frequency fell to a level at which secondary backup systems acted automatically to disconnect around 5% of demand. Following the implementation of the scheme, the second gas turbine at Little Barford tripped, so that the total loss of generation was 1,878 megawatts
- The scheme automatically disconnected customers on the network "in a controlled way", taking off around 1,000 megawatts of demand, and leaving 1.1 million customers without power for between 15 and 45 minutes
- The system returned to a normal state by 4.57pm and supply was returned to all customers by 5.37pm
- As well as 1.1 million customers left without electricity, there was major disruption to parts of the rail network, in a large part due to a type of train operating in the South East where 60 trains shut down unexpectedly when the frequency dropped, half of which required a visit from a technician to restart
- Other critical facilities were affected, including Ipswich Hospital and Newcastle Airport