In practice two types of impulse voltages can occur (i) full wave and (ii) chopped impulse wave.If the lightning surge travels along the transmission line,it results in full wave impulse voltage reaching to transformer.Whereas if the surge voltage causes a flash over just before reaching to transformer it results in chopped impulse wave.

The full wave impulse because of its longer duration causes major oscillations of its longer duration causes major oscillation to develop in the winding and stresses the inter-turn and inter-section insulation throughout the winding.It also develops high voltages across large portion of the winding,between winding and earth.The chopped wave due to its short duration does not develop high voltage across large portion of the winding.But due to its faster rate of rise and the rapid change in the voltage after flashover it produces higher inter turn and inter section stresses particularly near the winding end.