A lightning strike on a power line caused a massive blackout which caused travel chaos and affected more than one million people.
A report commissioned by energy regulator Ofgem by the National Grid looked into why large parts of the country were plunged into darkness last Friday.
An early investigation has shown that a lightening strike near the Little Barford gas fired power plant in Bedfordshire triggered a shut down shortly before 5pm last Friday.
Moments later the Hornsea One wind farm off the coast of Yorkshire shut down. The loss of the two plants caused a 5 per cent drop in power across the National Grid.
According to a report seen by The Times, both plants should have been resilient enough to withstand the lightening strike.
When the two plants lost power, several other smaller plants automatically tripped their safety systems when they noticed a significant drop in the 50hz frequency of the National Grid.
This led to a series of rolling blackouts, with power companies cutting supplies in order to protect the rest of the National Grid.
Power supplies to rail lines were cut leaving thousands of passengers stranded.
According to the report, Govia Thameslink Railway trains shut themselves down after losing power. In many cases the drivers were unable to restart them when the supply was resumed.
A spokesperson for Orsted, who own the wind farm told The Times: ‘During a rare and unusual set of circumstances affecting the grid, Hornsea One experienced a technical fault which meant the power station rapidly de-loaded – that is it stopped producing electricity.
‘Normally the grid would be able to cope with a lose of this volume (0.8GW). If National Grid had any concerns about the concerns about the operation of Hornsea One we would not be allowed to generate. The relevant part of the system has been reconfigured and we are fully confident should this extremely rare situation arise again, Hornsea One would respond as required.’
According to Professor Tim Green, co-director of the Energy Futures Laboratory, Imperial College London, the first generator to disconnect on Friday was a gas-fired plant at Little Barford in Bedfordshire at 4.58pm.
Two minutes later the Hornsea Offshore wind farm also disconnected.
Power supplies were restored by 5.40pm but rail services suffered disruption for almost 24 hours.
Duncan Burt of National Grid said: ‘What happened then is our normal automatic response mechanisms came in to help manage the event, but the loss of power was so significant that it fell back to a set of secondary back-up systems which resulted in a proportion of electrical demand across the country being disconnected for a short period to help keep the rest of the system safe.
‘Those events happened very, very quickly, in a matter of a few seconds, maybe a couple of minutes maximum.
‘That sequence of events is entirely automatic, we think that worked well, we think the safety protection systems across the industry on generators and on the network work well to secure and keep the grid safe.’
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