The Time and Frequency Response of Shielded Transformer Windings

Abstract

A distributed-parameter model is presented for the transient and frequency analysis of power transformers’ windings in the presence of electrostatic shielding, a technique occasionally applied for improving their initial voltage distribution. The s-domain solution takes into account the windings’ circuit parameters with special emphasis on the additional distributed capacitances between the windings’ conductors and the conducting shield which is connected to the windings’ line terminal. The closeness of the resulting initial voltage profile to the ideal case of uniform distribution is suggested as a criterion for assessing the achieved improvement in the windings’ voltage stresses. This leads to the determination of the optimal shielding arrangement. The sensitivity of the modified initial and transient responses to step and double-exponential voltage stimuli is investigated. Moreover, the resulting change in the windings’ series and parallel resonance frequencies is also addressed.

Keywords: Power transformers, equivalent circuit, power system transients, transformer winding, shielding, distributed parameters, initial voltage distribution, high-frequency model, frequency response, resonance.