Impact of oil and temperature on initial voltage distributions in transformer windings at ultra fast stresses

Evolution and operation of modern grids and networks leads to reconsideration of several aspects. For example, transformers operating in a power grid or in industrial environment are nowadays also subjected to transients or stimuli with ultrafast wavefronts. Traditionally, in the 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 of stress, results in focus on certain parts of the winding. The impact of oil and temperature on initial distribution attributes will be presented in this paper. Winding resonance might occur, if the spectrum of incoming surge voltage matched the winding one, and the corresponding resonance was excited. Therefore external transients occurring in power systems might trigger out internal overvoltages with high maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model winding subjected to ultra fast stresses. On a special setup the influence of oil and temperature on the stress distribution along the winding could be investigated. The distribution of stress as a function of surge rise time and winding position will be presented, while oil impact and temperature will be additional parameters. 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 designing and optimization of transformers windings.