Origin, Development, and Design of K-Factor Transformers

Abstract

Large rectifier transformers, because of the harmonics, always had to be specially designed to cope with the additional heating. But at that time there was no market or demand for 480 Delta-208Y1120 volt transformers to cope with the harmonic problem. Many people were aware of the problems with overheating but did not know the cause. In addition, average reading ammeters read too low when harmonic currents flowed in the neutral, and it rook true RMS meters to give accurate results. The problem was how to quantify the heating effect of harmonics. ANSI/IEEE C57.110, a guide for determining the heating effects of nonlinear loads, developed an equation for calculating the heating effects. By squaring the frequency and the per-unit current and multiplying them together, the guide arrived at a number without any designation. This was going to be called C for "constant", but was decided against because of possible confusion with "centigrade". The letter K for "Konstant" seemed reasonable to use, and Underwriters Laboratory accepted this designation in the original submission of a low voltage dry type transformer. K since became the standard measure of the ability of a transformer to withstand nonlinear loads. The features of the K-transformer are briefly discussed.