The insulation system of power and distribution transformers is subjected to fast and very fast overvoltages. They may lead to non-linear voltage distribution along the winding causing additional stresses. In addition, if the frequency spectrum of incoming surge voltage would match the resonant frequencies of the windings, resonant amplification of voltage within the winding will take place. Therefore external transients occurring in power systems may trigger internal overvoltages of large peak value. Novel aspect of the paper is related to the assessment of winding insulation system in the frequency domain. The transformer diagnostic methods are systematically being improved and extended due to growing requirements for reliability of power systems in terms of uninterrupted power supply. The detection of winding faults in transformers, both in the exploitation and during transportation is an important aspect of power transformer failure prevention. The frequency response analysis method, more and more frequently used in electric power engineering, has been applied for investigations and signature analysis based on the admittance and transfer function. The paper presents an approach to mitigate the impact of transients using specially designed serial choke. The performance of the serial choke is assessed by the frequency characteristics of the transformer windings with and without the protective choke element. The presented approach might be used in optimization of winding insulation systems.
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