National Grid’s preliminary investigation into the blackout that caused widespread disruption in England and Wales last week has raised the possibility that it was caused by the world’s largest offshore wind farm accidentally going offline.

The provisional report, which was submitted to regulators on Friday, suggests for the first time that the Hornsea offshore wind farm, which is owned and run by Denmark’s Orsted, may have tripped offline seconds before an outage at a smaller, gas-fired station.

The findings, which were relayed to the Financial Times by people briefed on the report, suggest the blackout may have been avoided if not for an error at the wind farm.

Investigators had originally thought the shutdown of the Little Barford gas-fired plant in St Neots, west of Cambridge, had triggered a domino effect across the network that led to the blackout.

Investigators now suspect the problems on the grid started when lightning hit part of the network near Cambridge. This caused 300MW-400MW of capacity in the local electricity network, which normally means small-scale renewable power, to go offline. Such a small outage should not have caused any problem for the wider grid. Lightning strikes are common on National Grid infrastructure, which is hit on average three times per day, and they rarely cause serious problems.

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But the strike coincided with the almost instantaneous total loss of supply from the Hornsea wind farm, which lies off the coast of Yorkshire. The facility, which is still under construction, was generating as much as 800MW for the grid last Friday afternoon before cutting to 0MW in less than a second.

National Grid’s report is expected to say this detail is significant because it was previously believed the loss of power took approximately 60 seconds. The instant shutdown suggests the safety systems at Hornsea could have taken the plant offline accidentally.

Analysts have speculated that Hornsea may have disconnected from the grid if its safety systems were configured too sensitively to drops in frequency — a measure of stability of an electricity system. The UK grid is designed to be held in a narrow band near 50 hertz but power plants should not disconnect unless the frequency oscillates sharply.

Orsted acknowledged on Friday that a “technical fault” had meant that the wind farm “rapidly de-loaded”, but declined to comment on whether Hornsea had failed before Little Barford.

The company said it had since made adjustments to “the relevant part of the system”.

“We are fully confident should this extremely rare situation arise again, Hornsea One would respond as required.”

The report is expected to indicate that the frequency of the grid did not plunge to 48.8 hertz — a level that triggers National Grid’s automated system to cut off electricity supply to around 5 per cent of demand — until the first generator tripped at Little Barford, joining Hornsea offline. Blackouts followed as the automated system kicked in.

Other preliminary findings in the report are that the National Grid had only 1,000MW of rapid-response emergency supply at the time, just two-thirds of the amount that was eventually lost.

A broader government investigation is expected to focus, in part, on whether the grid should have more of this reserve supply in the future.

The report is also expected to show that eight rail-signalling networks lost power supplies across the country, leading to much of the chaos on the railways in England and Wales.

Power to overhead lines on train tracks were, however, largely maintained, but the issue was exacerbated by a number of trains that shut down automatically and could not be restarted by their drivers. Govia Thameslink, which had as many as 60 trains affected, said it had to call out engineers to restart half of those that failed.

Steve White, chief operating officer at the train group, said that their own preliminary investigation had established a drop in electrical frequency on the overhead power lines, which triggered safety mechanisms on the shut down trains.

National Grid declined to comment.

Additional reporting by Janina Conboye in London

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