The very fast transient over-voltage caused by three operation modes of isolating switch of extra-high voltage GIS in a 550 kV substation is investigated.The transformer winding’s end to ground VFTO waveforms and the voltage waveforms of transformer winding pies between 0 and 1 are obtained.The over-voltage of making and breaking short cable of transformer

over-voltage of longitudinal calculating point to ground of transformer winding

and winding pies longitudinal VFTO are calculated

respectively.The results show that the maximum gradient voltage of 1 to 3 winding pies will exceed the critical value under the condition where the isolating switch operates in mode 3 and the transformer is charged.Therefore

the operation mode 3 should be avoided.