Transient Behavior of Large Transformer Windings Taking Capacitances and Eddy Currents Into Account

Transformer windings, as resonant systems, respond to impulse excitation by oscillating voltages and currents. In this paper, a detailed transient circuit analysis of an auto-transformer exposed to a full and a chopped standard lightning impulse has been performed, taking into account each single turn of the two windings. The corresponding inductance and capacitance matrices used in the circuit analysis are obtained by finite-element method (FEM)-calculations. To get the resistances of the turns, eddy currents have to be considered. Since the turns (continuously transposed conductors) consist of a certain number of single strands to keep the eddy current losses low, an appropriate eddy current calculation by FEM has to be performed, taking account of each single strand of the turns. The resulting resistances of the turns are typically several hundred times higher than the respective dc resistances. This has a significant influence on the damping behavior of the high-frequency oscillations of the system.