Computer studies of the performance of transformer windings against chopped impulse voltages

The authors present a numerical study of the behaviour of a transformer winding with one grounded end, when stressed by standard impulse voltage waves chopped at different instants on the front as well as on the tail of the wave. This is basically an extension of the investigation undertaken by the authors on surge performance of a transformer winding. The mathematical model considered takes into account the series and shunt capacitances and inductance of the winding elements, including the effect of their mutual inductances. Conductor and dielectric losses are neglected for the sake of simplicity and to highlight the most critical potential swing and mounting of electrical stresses in winding discs that are likely to be promoted along the winding during the course of free oscillations going undamped in the absence of a damping factor. The authors suggest the propriety of finding the critical instant of chopping determined with regard to the design and rating of the transformer under test