St John's Wood

Upgrade and installation of new Super Grid Transformer, Shunt Reactors and site assets on a congested site located in a residential area

Modifications to the substation to accept new connections from London Power Tunnels

Bespoke and sustainable solutions developed to work with the site

Securing the power supply for Kensington, Chelsea and Westminster, including the Houses of Parliament

Client: National GridInternational OEMMorgan Sindall


A complex project within a constricted site in London city centre, the upgrade and replacement of the Super Grid Transformer (SGT) and associated assets at St John’s Wood ensured increased power demands were able to be met. It saw the existing 275kV SGT replaced with a new 400kV SGT, which is connected by a 400kV cable to the existing Gas Insulated Switchgear (GIS) building. Additional asset upgrades were also made across the site, including foundations, buildings, cable troughs and security fences and gates.

BakerHicks provided civil and structural engineering and HV underground cable design on this project. Key services supplied included:

  • Structural analysis and modelling of SGT foundations, DH15 assessment
  • Design of SGT, Marshalling Kiosk and Neutral Earth Resistor foundations, noise enclosure, elevated access road and retaining wall, HV underground cabling, cable troughs and associated site infrastructure, including ISS security fences and gates
  • TP137 verification of the new 400kV cable connections from two SGTs and two new Shunt Reactors

Specialised solutions

With the site surrounded by residential buildings and a medical centre adjacent, space was constrained and strict noise pollution restrictions had to be taken into account. Our engineers created a number of bespoke solutions to overcome these obstacles. For example, the fence design included self-contained strain panels to reduce the size and in many cases remove the need for fence foundations. This allowed feasible solutions for the required fence positions where the ground was too congested to allow for new foundations to be installed. Our team also designed a temporary noise enclosure to help keep disturbance to a minimum. Aggregated deliveries and effective traffic management plans also helped with noise pollution control and reducing the impact on the local community.

The site has been heavily developed over the past 100 years and its previous uses meant there were a number of redundant services and foundations, as well as varying levels of made ground and a heavily congested site. Large concrete obstructions in the ground were identified during the piling process for the Cable Sealing End and Gas Insulated Busbar foundations. Due to the constricted nature of the site, removing the obstructions was not an option. Working closely with the site team, our engineers produced a ground bearing foundation design coupled with a settlement monitoring specification using a specially designed kentledge to replicate the equipment loading. Our engineers also designed modifications to the existing HV cable trough to allow the new 400kV cable to pass through it without issues. These bespoke solutions not only reduced project risk, but brought programme and cost savings.

Sustainable solutions were used wherever possible throughout the site, reducing the visual, noise and chemical pollution produced by the works. DH15 assessments carried out by our team identified where existing structures could be reused on the site. This reduced the amount of demolition works required resulting in much less pollution, as well as making huge savings in raw materials and the amount of CO2 produced as less concrete was required. Where the use of structural concrete was necessary, our team specified the use of GBBS cement replacement, delivering CO2 emission savings.

Completed to schedule and with cost savings, the upgrading works across the St John’s Wood site delivered a modern substation able to efficiently and effectively meet power demands for Kensington, Chelsea and Westminster.