Abstract
Investigations were conducted on the VFT (Very Fast Transient) surges that propagate into a 500 kV transformer. The disconnector re-striking surge and ground fault surge were discussed. It was supposed that large part of the surge voltage was applied at the just entrance of the transformer winding for the VFT surges. So, an equivalent circuit that models the windings was used for the analysis. The overvoltages that appear between first winding sections at the entrance of the transformer were computed. The followings were clarified. (1) The overvoltage between the first winding sections becomes bigger when the magnitude of the voltage change at the transformer terminal is larger. The overvoltage between the first winding sections is not affected by the magnitude of the transformer terminal voltage. (2) For the disconnectors that are not connected directly to the transformer, the voltage change at the transformer terminal is not so large. (3) In the case of ground fault at the GIS near the transformer, the voltage change at the transformer terminal is the same as that for a disconnector directly connected to the transformer. (4) In actual GIS, the disconnector that is connected directly to the transformer is not used usually. In this situation, the overvoltages that threaten the transformer insulation will not be generated by.the re-striking of the disconnectors.
