Calculation of Very Fast Transient in Transformer Windings Based on Transmission Line Theory

To calculate the very fast transient overvoltages in transformer windings, a novel model based on the modified multi-conductor transmission line (MTL) theory is proposed in this paper. In original MTL model, the lengths of all the conductors are considered to be equal and this approximation can cause inaccuracy. In fact, the lengths of the conductors are decreasing from the outside to the interior. This new model can settle this problem. Based on the original transmission line model, equations of curve-shaped MTL are deduced. Two methods are used to calculate the voltage distribution of the winding using the proposed new model. Some measurements are conducted to verify the calculation results. And comparisons between the original MTL model and the modified model are used to illustrate the deviations.