Power transformer insulation systems are subjected to many stresses during normal operation due to lightning and switching. If the spectrum of incoming surge voltage matches the winding one, the corresponding resonance will be excited. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. Overvoltages having such character have been the root cause of many power transformer failures. The paper presents an approach to the identification of sensitive zones in the transformer windings based on the measurements of overvoltages inside windings and frequency dependences of admittance of the power transformer. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The presented approach might be used both for design optimization and diagnostics of distribution and power transformers.
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