Thermal and magnetic analyses of gas-insulated lines

This paper presents the factors affecting current rating "ampacity" of buried three-phase, naturally cooled isolated-phase gas-insulated lines (GIL) with flat formation and finite-element simulation of double-circuit, three-phase GIL inside a tunnel to calculate the magnetic-flux and current densities. In the former analysis, the depth of GIL burial, interspacing of phases and soil thermal resistivity are investigated to show their impacts on the GIL ampacity. In the latter analysis, the effects of electrical and physical arrangements of GIL phases on the magnetic-flux density inside and outside the tunnel, and the current density in few installed liquefied natural gas (LNG) pipelines, which run in parallel to the GIL inside the tunnel, are also studied. In any GIL configuration, the magnetic field impact is always extremely low. Localized hot spots along the periphery of the GIL enclosure are found and corrosion can be accelerated due to temperature gradient. Moreover, there is a possibility of galvanic corrosion of the enclosures when they are bonded together and with the gallery steel reinforcement at very short length intervals. The simulation results also give the best location of the LNG pipelines or any cables to have the lowest electromagnetic induction. The configuration that has the best "magnetic behavior" is distant-placed arrangement (DPA) with (abc-cba) phase arrangement. The induced eddy-current density in the LNG pipelines is very low and hence its thermal effects can be neglected.