Comparison of Initial Voltage Distributions in Layer and Disc Type Power Transformer Windings

Power transformers are subjected to surge overvoltages which can lead to failure of the windings insulation system. The amplitude of the overvoltages in the windings is dependent on the parameters of the surge voltage (duration, waveform, steepness, etc.) and the design of the windings, thus various winding designs perform differently when surge voltage is applied to their terminals. The transformer winding vulnerability to surge phenomena can be estimated based on the initial voltage distribution. In practice, the initial voltage distribution can be improved by increasing the series or decreasing the ground capacitance of the winding, by design changes, or by application of electrostatic shields. This article presents the analysis of the initial voltage distributions in power transformer windings, comparing layer and disk designs. The analysis was done based on the results of measurements of surge voltage waveforms in real-scale winding models, while a lightning impulse is applied to their terminals. The experimental analysis was extended by simulations of the electrostatic potential distribution in the winding models, and the influence of selected factors (shielding, interleaving turns) on the initial voltage distribution was analyzed and discussed. The article is focused on the comparison of initial voltage distribution in transformer windings of disk and layer designs based on quantitative and qualitative analysis.