Heathrow airport said an interim report into a power outage which caused its closure for most of a day “raises important questions.”
No flights operated at the west London airport until about 6 p.m. on March 21 because of a fire at a nearby electrical substation which started late the previous night.
More than 270,000 air passenger journeys were disrupted by the incident.
In the immediate aftermath, Energy Secretary Ed Miliband commissioned the National Energy System Operator (Neso) to conduct a review into what happened.
Neso published its interim report on Thursday, stating that “the root cause of the fire remains unknown whilst forensic fire investigations are ongoing.”
It noted that the Metropolitan Police confirmed on March 25 it had “found no evidence to suggest that the incident was suspicious.”
The report stated that one of North Hyde substation’s three supergrid transformers—devices which enable voltages to be stepped up or down so electricity can be efficiently distributed—became disconnected, known as tripping, at 11:21 p.m. on March 20.
It was later confirmed to have caught fire.
At 11:49 p.m., the adjacent transformer also tripped, which resulted in the simultaneous loss of connection to the remaining transformer, according to the document.
The report said, “The consequence was the loss of all supplies from North Hyde 275kV substation, impacting thousands of customers including Heathrow airport.”
A Heathrow spokesperson said: “Heathrow welcomes the Neso review’s initial report, which raises important questions for National Grid and SSEN [Scottish and Southern Electricity Networks] that we hope the final report will provide answers to, including the cause of the fire.
“Further clarity on how the fire started and why two transformers were subsequently impacted can help ensure greater resilience for the UK’s energy grid moving forward.”
A spokesperson for substation-owner National Grid welcomed the report, adding, “We now look forward to the full findings and will continue to work closely with the National Energy System Operator as its independent review process continues.”
A spokesperson for SSEN Distribution, which is responsible for power distribution in the area, said the report shows it “played its part” in the “significant operational response” to the incident.
Neso said the flow of electricity to all four of Heathrow’s passenger terminals was restarted by 10:56 a.m. on March 21, with power restored to the “wider Heathrow Airport Limited network” by 2:23 p.m. that day.
This was followed by “a period of safety checking” to ensure “safety-critical systems were fully operational prior to passengers arriving at the airport,” according to the report.
Miliband said: “My department took immediate action to investigate the power loss, which impacted Heathrow and the surrounding area [on March 20 and 21], causing major disruption to thousands of people and many businesses.
“The National Energy System Operator’s initial summary of the incident rules out the possibility of any suspicious activity.
“We now await the full report to understand what happened and learn lessons to strengthen UK energy resilience and protect our critical, national infrastructure.”
The full report is expected to be published by the end of June.
