Propagation of high frequency overvoltages in transformers

In modern grids and networks more and more switching and reconfiguration operations are present. In addition external stresses like lightning may cause overvoltages. Thus, such transients may subject the electric insulation system of power equipment. The crest value of overvoltages appearing at the transformer terminals are limited by protective devices such as surge arresters to the level determined by the insulation coordination. Those values are usually much higher than maximum nominal voltage. In some cases overvoltages may influence transformers despite of overvoltage protection systems. Usually overvoltages contain oscillating dumped components in broad frequency range. Thus, often the internal overvoltages formed inside the insulation system of high voltage equipment are caused by internal resonance in windings. Particularly some switching transients of oscillating character may cause high amplitude internal overvoltages. Overvoltages manifesting such character and having lower amplitude than the protection level of the surge arresters are not attenuated at the transformer terminal. They arrive to the transformer input practically without any changes both in amplitude and waveform. The paper will present comparison of overvoltage propagation in different types of transformer design, taking into account also impact of steel and amorphous core. Both resonance and transfer overvoltages will be analyzed.