Determination of VFTO waveform in GIS according to simplified structure of GIS bus and circuit parameters

Since there is no standard waveform of VFTO (very fast transient overvoltage) so far for power transformer manufactures, the designers need to use simulation method to obtain VFTO waveform and then calculate VFTO distributions in the windings and verify the abundance of the insulation design. In this paper, the analytical expressions of VFTO waveform are theoretically developed using transmission line equivalent circuit and Thevenin´s theorem for the GIS bus systems without branch and with one-branch in the gas insulated substation (GIS). The effects of bus length, bus characteristic impedance, and transformer external equivalent capacitance on VFTO waveform are also analyzed and some results of practical importance are obtained. The simulation results using PSCAD software are consistent with the analytical results.