Heathrow power outage prompts UK government to review substation design

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THE UK government has promised to review design standards at critical national infrastructure, in response to an electrical substation fire that caused a power outage at London Heathrow Airport in March.

In a policy paper published this week, the Department of Energy Security and Net Zero (DESNZ) said its review will evaluate how to best implement all 12 recommendations made by the National Energy System Operator (NESO) this summer following its investigation into the fire, and that it will inform an “energy resilience strategy” expected next year.

NESO’s investigation found that the fire at the North Hyde substation in West London was most likely caused by moisture entering a bushing in one of the site’s three transformers, causing it to short circuit and ignite the surrounding insulating oil. The fire caused a temporary closure of the nearby Heathrow Airport – one of the world’s busiest airports – and power outages for more than 70,000 domestic and commercial customers.

The 275 kV “extra high voltage” substation was built before industry standards came into force which say there should be barriers and as much physical distance as possible between oil-filled equipment in substations. DESNZ said its review will assess current design standards of critical infrastructure including substations.

Insulating oil is used to immerse the copper windings and cores of transformers to dissipate waste heat. It also prevents oxidation in the cellulose-based insulators wrapped around the electrical conductors in the transformer. The buildup of moisture and contamination in insulating oil degrades performance over time.

NESO found that an elevated moisture reading had been recorded in oil samples taken from North Hyde in 2018, which National Grid Electricity Transmission guidance says is a sign of “an imminent fault” and requires bushings to be replaced. However, despite basic maintenance of a faulty transformer taking place in 2022, the bushings remained untouched.

There was also no site-specific risk assessment for the North Hyde substation, NESO found, while across the sector there is “no common approach” to considering site-wide impacts of fires. In the case of North Hyde, “there was no consideration of the proximity of other oil-filled assets or the suppression system’s ability to prevent fire spread”.

In response, DESNZ said it will review the effectiveness of fire safety standards alongside energy regulator Ofgem and the Health and Safety Executive, paying particular attention to electrical safety regulations from 2002.

NESO said it is “pleased that all our recommendations are being taken forward, and that NESO will play a role in building a better understanding of the dependencies between critical national infrastructure and the energy system”.