A fire at a Heathrow airport substation, owned by the National Grid, was caused by a problem first detected seven years ago but never fixed, according to a final report by the National Energy System Operator (NESO).

On 20 March, a fire at the North Hyde 275kV substation, which feeds most of Heathrow, resulted in the airport’s closure. 

Aviation chaos ensued, with more than 1,300 flights cancelled and hundreds of thousands of passengers affected. More than 70,000 other businesses, homes and services were also affected, including Hillingdon Hospital.

Energy watchdog Ofgem commissioned NESO to conduct an investigation, assessing the reasons the fire started in the first place as well as a broader assessment about the UK’s energy resilience and actions that could be taken to prevent similar incidents in future.

The report found that at 11.21pm on 20 March one of three supergrid transformers (SGT3) at the North Hyde substation and its associated circuit tripped. Shortly after, the other transformers (SGT1 and SGT2) tripped. The cause was a catastrophic failure on one of SGT3’s high-voltage bushings, which caused the transformer to catch fire.

This was most likely caused by moisture entering the bushing, causing an electrical fault. 

It found that an elevated moisture reading in the bushing had been detected in oil samples taken by the National Grid in July 2018.

According to the National Grid’s relevant guidance, such readings indicate “an imminent fault and that the bushing should be replaced”.

While this elevated moisture reading was recorded in its online system, “mitigating actions appropriate to its severity were not implemented”, the report found.

In addition, in 2022 a decision was made to defer basic maintenance on SGT3.

Fintan Slye, NESO’s chief executive, told the BBC’s Today programme that while the report “did not set out to apportion blame”, the National Grid was responsible for the maintenance of the equipment and ensuring it was fit for operation.

“So when they identified those moisture levels back in 2018, what should have happened is that the transformer should have been taken out of service for a short period of time and that fault effectively repaired.

“That was missed. The moisture got worse over time and ultimately that’s what caused the fire to happen. The story of our report really is probably one of missed opportunities.” 

According to the BBC, National Grid said it had “taken further action since the fire”, including “re-testing the resilience of substations that serve strategic infrastructure”.

Following the report's findings, Ofgem has now launched an investigation into National Grid.
The report also found that because of the way Heathrow’s electricity network is configured, the loss of power from the substation – one of three supply points for the airport – led to a power outage to some of the airport’s critical systems.

The report states that “Heathrow airport have a plan to deal with this which includes reconfiguring their internal electrical distribution network to take power from the other two supply points”.  

It also found that “energy network operators are not generally aware whether customers connected to their networks are Critical National Infrastructure”.

The report makes a number of recommendations to reduce the likelihood and impact of a similar event in the future. These include resilience of infrastructure with multiple supply points, maintenance actions, electricity safety, quality and continuity regulations, and incident management protocols.

Slye said: “The power outage and closure of Heathrow airport were hugely disruptive, and our report seeks to improve the way parties plan for and respond to these incidents, building on the underlying resilience of our energy system. 

“All parties involved are focused on working together to deliver these important recommendations, and much of this work is already under way with NESO’s full support.”

Last month, the Spanish government found that “poor planning” by grid operator REE was to blame for the April blackout in Spain and Portugal.