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Lightning strike revealed as cause of 9 August UK black out

Image: Getty.

Image: Getty.

A lightning strike that caused two separate losses of generation – described by National Grid ESO as an “extremely rare and unexpected event” – has been revealed as the cause behind Britain’s power cut on 9 August 2019.

This morning the electricity system operator published its interim report into the incident, revealing that a lightning strike at around 4:52pm on Friday 9 August was the trigger for a cascade of events which resulted in around 1.1 million customers losing their power supply.

It states that lightning hit a transmission circuit – the Eaton Socon – Wymondley Main. But while the grid’s protection systems operated normally and cleared the lightning within 0.1 seconds, shortly after there was a near simultaneous loss of load from both the Little Barford CCGT power station and Hornsea One offshore wind farm.

Those trips, National Grid ESO has concluded, were entirely independent of each other – dispelling a previous theory that a trip at one plant caused the other to de-load – but both were connected to the lightning strike.

The lightning strike also caused some losses from embedded generators in the area of the lightning strike, equivalent to around 500MW, after the Loss of Mains protection system kicked in.

The total loss of load amounted to 1,378MW and occurred within seconds of the lightning strike, triggering the stark drop in frequency from a normal operating state to 48.8Hz.

Lightning strikes do not usually cause generators to de-load – National Grid ESO has further said that its infrastructure was hit by lightning multiple times of Friday 9 August, with protection systems operating as normal in all cases – leading it to conclude that this was an “extremely rare and unexpected event”.

National Grid ESO said it had 1GW of reserve power in place at the time – the level approved by Security and Quality of Supply Standards – but this fell short of the capacity required to offset the loss of load, triggering the call on distribution network operators (DNOs) to initiate Low Frequency Demand Disconnection (LFDD) procedures.

Approximately 1GW of demand was turned off under LFDD and, combined with the grid’s reserve capacity kicking in, grid frequency was returned to a “normal stable state” by 5:06pm.

National Grid ESO said it would now work alongside RWE and Orsted, the respective owners of Little Barford and Hornsea, to better understand the exact failure mechanisms, while it is also keen to collaborate with DNOs to better understand the impacts the incident had on local demand.

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