Transient Behaviour of Large Transformer Windings Taking Capacitances and Eddy Currents into Account

Transformer windings, as oscillatory systems, respond to impulse excitation by starting to oscillate. In the paper, a detailed transient circuit analysis of an auto-transformer responding to a standard lightning impulse (LI) 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 FEM-calculations. To get the resistances of the turns, eddy currents have to considered. Since the turns (continuous transposed conductors, CTC’s) 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, considering each single strand of the turns. In the present example, 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 behaviour of the high frequency oscillations of the system.