Frequency method for identification of resonance overvoltages in transformer windings

The insulation systems of power transformers are subjected to many transient overvoltages. They are usually generated in power systems during lightning and switching operations. If the spectrum of incoming surge voltage matches the winding one, the corresponding resonance might 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. This 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 transformer. The presented approach might be used both for design optimization and diagnostics of distribution and power transformers.