Surge characteristics of two-winding transformers

THE object of this paper is to show the performance of transformers with two windings in the rapid-transient state when surges develop in their conductors. On the one hand, we will determine the characteristic constants for traveling waves, as surge-impedance and wave velocity, from the data of the transformer. On the other hand, we will consider the rapid interaction of the primary and secondary windings, with different numbers of turns, which for travelling waves is very different from the coupling under quasi-steady-state conditions.^1,2,3 In a previous paper¹¹ it is shown that the propagation of traveling waves of moderate frequency in coils and windings is similar to the propagation in smooth lines and that the values of surge-impedance and traveling-velocity of such waves can be derived from the total self-inductance and the total ground-capacitance of the windings under steady state conditions. Furthermore, very steep parts of incident waves, corresponding to the highest frequencies, are cut off and converted within the winding into a flattened front, the length of which depends on the internal capacitance between the turns or coils. In this paper these results for moderate frequencies will be used but the influence of internal capacitances for very high frequencies will not be considered, as this would only obscure the principal objective of the paper.