The programme will strengthen the electricity transmission network across Bedfordshire, Hertfordshire and North West London. It will also create enough capacity to connect five new data centres with a combined electricity demand of 1GW.
Laing O’Rourke will deliver new and upgraded substations at Letchmore Heath, Elstree and St John’s Wood.
The Hochtief Murphy JV (HMJV) has been appointed to install new 400kV cables and upgrade the existing transmission tunnel between Elstree and St John’s Wood.
Hyundai Electric will supply transformers and other major electrical equipment, while Siemens Energy will provide shunt reactors at Elstree.
The project will run from Sundon substation in Bedfordshire to Elstree in Hertfordshire and then to St John’s Wood in London.
Works include upgrades at Sundon and a 35km reconductoring programme on the Sundon to Elstree overhead line. The scheme will also extend Elstree substation, build the new Letchmore Heath substation and carry out reinforcement works at St John’s Wood.
More than 200km of overhead line will be upgraded as part of the programme. A further 60km of new cable will be installed through the existing 20km-long Elstree to St John’s Wood tunnel. The tunnel was originally designed to accommodate an additional transmission circuit.
National Grid said the investment will improve network resilience and unlock future electricity connections. It is also expected to reduce long-term electricity constraint costs while supporting rising demand from homes, businesses and digital infrastructure across London.
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