Complete analysis of very fast transients in layer-type transformer windings

Abstract — This paper deals with the measurement, modelling and simulation of very fast transient overvoltages in layer-type distribution transformer windings. Measurements were performed by applying a step impulse with 50 ns rise time on a single-phase test transformer equipped with measuring points along the winding. Voltages along the transformer windings were computed by applying multi-conductor transmission line theory for transformer layers. The secondary voltage of the transformer was simulated with another model implemented in ATP-EMTP which takes into account the surge capacitance between the primary and secondary transformer winding. It was found that, when an impulse with a short rise time is applied, internal resonance occurs, which results in a very high secondary voltage. The computations were verified by measurements.