National Grid reinforces London grid for data centres

National Grid has launched a major reinforcement of the North West London transmission network that will enable five new Hertfordshire data-centre connections equivalent to around 1GW of combined electricity demand.

The National Grid North West London Upgrade runs from Sundon substation in Bedfordshire through Elstree in Hertfordshire to St John’s Wood in London, combining new substations and cable circuits with reinforcement of infrastructure already serving the capital.

At Sundon, existing substation equipment will be upgraded before the 35km Sundon–Elstree overhead-line route is reconductored. National Grid says more than 200km of overhead-line infrastructure will be upgraded across that part of the programme, increasing the capability of existing transmission corridors rather than building an entirely separate route.

Elstree will receive an extension to its existing 400kV gas-insulated switchgear substation, while a new 132kV GIS installation will be built alongside it at Letchmore Heath. The new substation is intended to provide the connection point for five data-centre customers whose combined requirement is equivalent to approximately 1GW.

The southern section makes use of infrastructure designed with future expansion in mind. National Grid will install an additional 400kV circuit through the existing Elstree–St John’s Wood cable tunnel, a route of more than 20km that was built with space for another transmission circuit.

Around 60km of new cable infrastructure will be installed within that tunnel, reflecting the multiple conductors required for the three-phase circuit rather than the geographical length of the route. Tunnel and headhouse equipment will also be upgraded, alongside reinforcement at St John’s Wood substation.

The programme is significant because it combines new digital demand with strategic transmission renewal rather than treating each data-centre application as a standalone connection. At around 1GW collectively, the five customers represent an electrical load closer in scale to a group of major industrial facilities than to conventional commercial development.

That changes the network-planning requirement. The transmission system has to deliver sufficient power during normal operation and through credible outages, maintaining voltage, thermal limits, fault levels, and system security even when part of the surrounding network or substation equipment is unavailable.

Data centres also impose demanding continuity requirements behind the grid boundary. Operators typically use multiple transformers, switchboards, UPS systems, batteries, and backup generation to prevent a single equipment failure from interrupting computing loads, meaning the transmission connection feeds another highly redundant electrical system rather than a simple building supply.

The North West London Upgrade exploits existing infrastructure where possible because creating entirely new transmission corridors around the capital is slow, expensive, and disruptive. Reconductoring allows more capacity to be extracted from an established overhead-line route, while the spare circuit position in the existing cable tunnel avoids another large underground civil-engineering project.

High-voltage cable circuits create different operating conditions from overhead lines, particularly over a route of more than 20km. Cable capacitance produces reactive power, making voltage control and shunt compensation important when the circuit is lightly loaded and influencing the design of the associated substations.

National Grid has appointed Siemens Energy to supply shunt reactors at Elstree, while Hyundai Electric will supply and install major equipment including transformers. Laing O’Rourke is responsible for substation works at Letchmore Heath, Elstree, and St John’s Wood, with the Hochtief-Murphy joint venture delivering cable installation and tunnel upgrades.

The mix of contractors illustrates the number of specialist workstreams involved in a transmission programme of this type. Civil engineering, high-voltage cables, transformer procurement, switchgear, protection, controls, overhead-line work, tunnelling infrastructure, and commissioning all have to converge without compromising supply from the existing network.

Equipment lead times add another constraint. Large transformers, GIS, reactors, cables, and protection systems are being ordered into a market where transmission operators, renewable developers, industrial projects, and data-centre owners are all pursuing grid infrastructure at the same time.

The data-centre load arrives alongside wider electricity-demand growth from transport, heating, industry, housing, and other digital services. National Grid therefore has to distinguish between reinforcement needed for identifiable customer connections and investment required to keep the wider network resilient as demand increases across the region.

Its North West London programme is being delivered in stages through to 2030 and sits alongside London Power Tunnels 2 and the North London Reinforcement scheme. The capital is consequently seeing several major transmission programmes progress simultaneously rather than relying on individual incremental upgrades.

The new Letchmore Heath substation is particularly instructive because the existing Elstree site is already at capacity. Adding a new connection point beside an established 400kV node allows the grid to serve additional demand without pretending that the existing substation can accommodate another gigawatt simply because the physical transmission lines are already nearby.

That distinction is becoming more important as data-centre developers compete for power. A proposed site may sit close to high-voltage infrastructure yet still lack a viable connection because the relevant circuits, transformers, switchgear, or fault-level margins have no spare capacity.

North West London therefore demonstrates how AI and cloud computing are beginning to influence the physical architecture of the transmission system. Five data-centre connections are large enough to justify new substations, additional 400kV cable capacity, and extensive reinforcement of an existing overhead-line corridor — an infrastructure consequence considerably more tangible than another forecast of future digital electricity demand.