National Grid cites lightning as trigger for blackout

A lightning strike was the initial trigger for a series of events that culminated in the UK's largest power cut since 2008, transmission system operator National Grid said.

In an interim report to regulator Ofgem, National Grid said there was a lightning strike on a transmission circuit in Cambridgeshire at 16:52 local time on 9 August. Automatic protection systems operated as planned to clear the lightning and the line returned to normal operations in under 20 seconds, although around 500MW of embedded generation connected to the low-voltage distribution grid was disconnected.

But immediately following this, two large generation sites — the Hornsea offshore wind farm and the Little Barford gas plant — both suddenly reduced output to the grid, with a combined fall of 1,378MW.

It was this loss of generation that caused the frequency on the grid to fall below its normal range and then triggered some automatic disconnections of demand at local distribution grids, known as low frequency demand disconnection (LFDD).

Lightning strikes on the transmission grid are a regular occurrence, with more than 1,000 a year in the UK. National Grid said the decline in output at Little Barford and Hornsea "occurred independently of one another but each [was] associated with the lightning strike".

Large generation units would not be expected to trip as a result of a lightning strike and so this "appears to represent an extremely rare and unexpected event", it said. National Grid will make a final report to Ofgem next month, with more detailed information.

The operators of the plants described different circumstances that led to their plants tripping, making it unclear what exact role the lightning strike played.

Danish utility Orsted, which owns Hornsea, said that the plant had immediately ramped down from 799MW to 62MW after a system voltage fluctuation "with unusual characteristics" that was coincident with the lightning strike.

Little Barford, owned by German utility RWE, saw its 244MW steam turbine trip, followed automatically by a gas turbine because of a change in steam pressure. A second gas turbine was manually tripped by staff in response to high steam pressure 30 second later, taking 641MW off line.

National Grid had 1,000MW of capacity on rapid response standby at the time but the combined decline of 1,378MW was too large for it to handle, leading to the frequency deviation. The capacity available included 472MW of battery storage and all of this was dispatched automatically in response to the frequency deviation.

As electricity system operator, National Grid is obliged to hold enough immediate frequency response and reserve capacity to ensure that the system remains stable following the loss of the single largest unit without the disconnection of any demand, but the combined effect of two simultaneous outages proved too much.

The most widespread disruption caused by the power cut was to rail services around London and the southeast of England. Trains halted as the frequency deviation caused an automatic shutdown, rather than because of a total loss of power. Two makes of train "reacted unexpectedly to the electrical disturbance" and half of these trains could not be restarted by their drivers, requiring visits from engineers, which extended the effective shutdown of rail services for several hours. This will be investigated further by Network Rail, which owns most of the railway network, and train operator Govia Thameslink.

A final report is due to be presented to Ofgem by 6 September.