Suppression of voltage transients across the tap windings of an auto-transformer by means of ZnO varistors

Voltage transients entering autotransformers can cause oscillations within the tapping windings, resulting in overvoltages and severe insulation stresses (inter-turn and turn-to-tank) leading to the dielectric breakdown to ground as well as in the tap selector. Flashovers within the tap selector area cause internal short circuits in the tap winding. In this study, effective means of suppressing these high power frequency voltage and electric field stresses and protecting the tap winding have been investigated, for the purpose of reducing inter-turn as well as turn to ground stresses generated by lightning. From the simulations conducted it appears that the ZnO varistors connected across the tapping leads of a transformer could effectively suppress such transients. Full results is presented in the proposed paper.