Analysis of Simplified Models for Numerical Calculation of VFT in GIS

In order to analyze very fast transients (VFT) caused by the operation of isolator in GIS, the waveforms of voltage and current surges should be obtained, and it is usually needed to construct an exact model according to the equivalent circuit of each GIS component. It is obvious that the exact model is very complex because of the huge number of electrical elements. In this paper, the exact model is simplified by analyzing the geometric shape and dimension of GIS components and studying the relevant results on VFT research in recent years. The effectiveness of the simplified model is verified with a typical GIS compartment system which consists of busbars, isolators, earthing switches, circuit-breakers, current transformers, voltage transformers, line isolators, earthing switches and cable termination etc. The simplified model will be useful to analyze the main factors influencing VFT overvoltages and further more will be helpful to the design of GIS structure.