In modern power grids more and more switching and reconfiguration operations are performed, leading to harmful overvoltages. In addition, external factors causing overvoltages may also be dangerous to insulation systems, especially those which occur during lightning strikes. Overvoltages propagate in power transformers along the windings and between them, influencing the insulation systems of the windings as well as those of other electrical devices. An interesting phenomena which takes place in the networks relates to the transferred overvoltages propagating through distribution transformers. The internal and transferred overvoltages for transformers with a conventional steel core and for those with an amorphous core are analysed in this study. The frequency response of overvoltages which occur when the transformer is exposed to a voltage impulse of rectangular shape form the basis of the analysis. Surges of rectangular shapes simulate transient voltages occurring at surge arresters. The frequency response of overvoltages reveals the transformer response to oscillating transient components of overvoltages generated in the power systems. The coincidence of frequency responses of overvoltages and admittances for transformers is also presented.
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