A massive blackout partly caused by a fault at the world’s largest operating offshore wind farm – Orsted’s 1.2GW Hornsea 1 – needs further investigation before final conclusions can be drawn, said an interim report from the UK government.

A lightning strike on a transmission line, followed by the loss of 1.4GW from the system due to simultaneous shutdowns at Hornsea 1 and a gas-fired station, affected more than a million people and caused transport chaos on 9 August.

An interim report into the incident from the UK government’s Energy Emergencies Executive Committee said more investigation is needed in several areas ahead of a final report in November, including whether the reserve and response policies of electricity system operator National Grid are “fit for purpose going forward” and the communications protocols in place.

The report noted: “National Grid has stated that the situation was not caused by a systemic risk in wind; further work is required from the electricity sector to ensure continual balance between system security, resilience and the generation mix.”

Of Hornsea 1 specifically, the interim report said loss of generation at the plant off eastern England occurred “after experiencing an unusual voltage fluctuation coincident with the lightning strike. Though the wind farm’s onshore control system performed as expected, the offshore system did not, leading to automated safety mechanisms to shut the wind farm down.

“The operator has identified the issue with the offshore control system and taken action to prevent a reoccurrence of this event in similar circumstances.”

Orsted said earlier that it is confident of a correct response if the “extremely rare” circumstances occurred again.

Like almost every other entity involved in the blackout, Orsted is the subject of a continuing investigation by UK energy regulator Ofgem over whether it broke its licence conditions in relation to outage.

Hornsea 1 was at the time of the blackout still under construction and only partly exporting power.

Orsted said this week that the last of the project’s 7MW Siemens Gamesa turbines has now been installed.