Investigations of transformer winding responses to standard full and chopped lightning impulses

Voltage tests of power transformers are performed with usage of full and chopped voltage impulses. Due to the normalized procedures the extension of the tests with lightning voltage impulse have to consider voltage impulses chopped after different time. This specific tests are suggested for transformers which are directly connected to GIS substations by means of oil to SF6 bushings or when the power transformer is protected by rod gaps. Due to actual standardized tests for transformers with nominal voltages below 170 kV chopped wave lightning voltages impulses are described as special tests. For Power Transformers with nominal voltages higher than 170 kV chopped wave lightning voltages impulse are performed as an routine tests. While chopped voltage impulse tests, applied wave of lightning impulse should be chopped after time which is between 3 mu s and 6 mu s. The plan of analysis presented in this article cover measurements of overvoltages appearing inside transformer windings while normalized voltage tests conducted with full wave lightning voltage impulse and chopped impulses after time to chopping in the range from 1 mu s up to 50 mu s. Comparative analysis of internal overvoltages in tested transformer winding are presented on the basis of measured overvoltages waveforms at chosen points inside windings.