The very fast transient over voltages (VFTO) can cause the stress on insulation and may leads to damage of the insulation. If the maximum peak value of VFTOs is more than the rated lighting impulse withstand voltage then it is necessary to consider the VFTO and mitigation has to be done by employing suitable damping devices. In this paper a power transformer rating of 400MVA has been considered in 765/245kV, gas insulated substation. The parameters of the proposed system have been evaluated using very fast transient nature (VFT) for more accurate evaluation. In this work, the effect of earthing switch and Influence of grid grounding on VFTOs and TEV wereinvestigated. The wavelet transform have employed for the estimation of VFTOs and TEV since the wavelet transform is most powerfull tool fortransient analysis.
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