The very fast transient overvoltage (VFTO) produced during isolator switching operation in EHV GIS substations could impose a potential risk of insulation breakdown in the equipment connected directly to the GIS. A quantitative assessment of the expected VFTO against the VFTO withstand capability of the concerned EHV equipment is necessary for power companies in assessing the risk and formulating appropriate remedial measures to mitigate the risk without unduly limiting the flexibility of switching operations.
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