Gas Insulated Lines (GIL) are a method of burying high-voltage cables. GIL consists of a tubular aluminium conductor to carry the current, enclosed in a rigid metallic tube that is filled with an insulating gas (sulphur hexafluoride or a mixture of nitrogen and sulphur hexafluoride gases). It has particular advantages where very high powers are required and has been used in a few places worldwide, though not yet in the UK.
With GIL, a return current, almost equal and opposite to the load current in the central conductor, flows back through the sheath. This provides a large degree of cancellation of the magnetic field. Because the sheath current is not exactly equal and opposite, the cancellation is not perfect, and there is still a resultant magnetic field, but it is lower than from a conventional underground cable where the sheath currents are small. The following graph shows this for a single circuit cable carrying a standard 500 A, using figures for the sheath currents calculated by the manufacturer, Siemens, and compared to a standard UK direct buried cable, also single-circuit carrying 500 A.
Fields are calculated from data in "Electromagnetic Fields Of Gas Insulated Lines (GIL)", paper 49, International Colloquium Power Frequency Electromagnetic Fields ELF EMF, Sarajevo 2009, Stephan Poehler and Steffen Schmidt, Siemens AG. Phase spacing 1.15 m, depth of central conductor approx 1.3 m, field calculated for 1 m above ground. 500 A is our standard current for comparing fields from different technologies, but note that in practice, GILwould probably not be used unless the current were somewhat higher.
Photo courtesy of Siemens.