Introduction: A number of options for reducing the footprint of substations using conventional Air-Insulated Switchgear (AIS) have been identified. A compact AIS design would have advantages in applications where restrictions in the available land are encountered. For example, such a design might allow extension bays to be constructed within an existing substation boundary where space is restricted.

Potential applications also arise where a replacement substation is to be constructed off-line on land adjacent to an existing substation, allowing the latter to remain in service during the construction period and avoiding the safety and operational issues associated with bay-by-bay replacement. Applications might also be found in urban areas. In such cases, a compact AIS substation might offer a cost-effective alternative to Gas-Insulated Switchgear (GIS) substation and would reduce dependence on Sulphur Hexafluoride (SF6), the use of which is being questioned on environmental grounds. The identified options for reducing the footprint of AIS substations are being investigated in the present work.

Objectives: The project has the key objective of reducing the size of substations using AIS, which will, in turn, reduce National Grid's dependence on SF6 as an insulating medium. This will have benefits in terms of both reduced environmental impact and reduced costs. The specific target of the project is the ability to build a 400 kV substation with the footprint of a conventional 132 kV substation. As a secondary target, the feasibility of constructing a compact conductor system as a 400 kV transmission circuit within a cable tunnel would be studied.