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Very fast front transient voltages & dielectric withstand effects on transformer insulation

The behaviour of transformer windings when subjected to very fast front transient (VFFT) voltages produced by gas insulated substations (GIS) is of concern. An internal VFFT voltage was experimentally generated and digital form of this waveform was used to analytically excite the windings of a typical medium power transformer. The influence of these transients on the voltage distribution on different winding sections have been analytically studied. The modelling concept is based on representing each turn of the transformer as an impedance networks which is suitable for MHz range. The practical implications of these results on the dielectric stress of transformer insulation is discussed in relation with those stresses generated under power-frequency (50 Hz) and standard lightning impulse (1.2/50) voltage excitations on which are transformer designs generally based.