Accurate modeling of capacitively graded bushings for calculation of fast transient overvoltages in GIS

Two simulation models (distributed and lumped parameter models) to analyze the behavior and adequacy of design of capacitor-type bushings for GISs (gas-insulated substations) during very fast transients are proposed. The first model is formed by a cascade of multiphase transmission lines that reproduces the inductive and capacitive behaviour of the bushing well. The second model is formed by a cascade of lumped parameters, in which the shunt branches are realized by means of series RC elements. Both models are suited for the EMTP program by the Bonneville Power Administration, Portland, Oregon. They have been validated by comparing calculation results to laboratory measurements performed on an actual bushing. Either of the models can be chosen according to the user´s needs. They differ in the accuracy level, computation burden etc. In any case, both models can be set up by only using the geometric and electrical design characteristics of the actual bushing