The very fast Transient over-voltage generated due to the switchgear in GIS switching off or on is very harmful to transformer insulation. The model presented is a winding hybrid lumped parameter model which combines the branch model based on parallel cables in one turn with grouped-turns model of two discs to efficiently simulate the very fast transient in the coil. Under quasi-stationary electromagnetic field, network topology and element size are derived from the geometry and properties of the transformer, whose inductance and capacitance are assumed to be passive and to have linear characteristics, however which have an inherent feature of a medium, which propagates waves. The formula of parameters are obtained. Based on the built winding model, the voltage gradients between discs and turns are found out, according to the simulation results, the reasonable winding insulation design can be made.