The Resonance Effect of Oscillating System Overvoltages on Transformer Windings

This paper investigates theoretically and experimentally the conditions for and the effects of internal transformer winding resonances produced by oscillating exciting voltages. In particular, it demonstrates that both short-line faults and energizations can cause high voltage stresses between points in the windings. Calculations and test results on internal transformer voltage gain under resonance conditions, critical line length, surge impedance ratio and other parameters are presented. The frequency characteristics of the transformer terminal impedance and of the internal voltage gain are displayed and the merits of terminal measurements discussed. At the present state of the art, only limited protection against oscillating excitation is available.