Steady state and transient ampacities of gas-insulated transmission lines

This paper presents a thermal model for predicting the steady state and transient ampacities of gas-insulated transmission lines (GILs). Using the thermal model, a computer program has been formulated such that it can estimate the ampacity of a GIL for any time-varying or constant current loading and any variation in environmental conditions. Relations for the convection and radiation heat transfer coefficients for a GIL filled with sulfur hexafluoride (SF₆), nitrogen, or a gas mixture of both gasses are presented. The validity and accuracy of the ampacity model were verified by comparing the predicted temperature of core, gash and enclosure of the GIL with measured temperatures reported in the literature under the same conditions. Analytically predicted and experimentally measured values both show close agreement to each other. Moreover, using a 50%-50% mixture of SF₆ and nitrogen resulted in a conductor temperature: of only less than 1°C hotter than SF₆ alone.