Initial voltage distributions in transformer windings at ultra fast stresses

Transformers operating in power grid or in industrial environment are subjected nowadays also to transients or stimulus with ultrafast wavefronts. Traditionally, in case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such effects are stressed due to the voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non uniform character stress concentration on certain parts of the winding. Winding resonance might occur, if the spectrum of incoming surge voltage matches the winding one, the corresponding resonance will be excited. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. The paper presents results of voltage distribution measurements performed on disk and layer winding subjected to ultra fast stresses. The spatial distribution of the stress as a function of surge rise time and winding position will be presented. Special focus is paid on the analysis of the surge rise time and steepness on the winding stress. The presented results might be used both for design and optimization of transformers windings.