Enhanced Analytical Method for the Calculation of the Maximum Inrush Currents of Single-Phase Power Transformers

In this paper, an accurate analytical method is proposed to compute the maximum inrush currents for single-phase transformers. The worst case scenarios for inrush currents caused by switching at the voltage zero crossing with the highest possible residual flux are investigated. Hysteresis is modeled with piece-wise linear approximations. Hence, it is assumed that the transformer is always operating on linear sections of the magnetizing curve. The maximum inrush current is computed with simple steps from the solution of the linear governing differential equations. The method is simple and, thus, suitable to be included in the transformer design program (or even using a calculator). Comprehensive studies are carried out to evaluate the proposed method versus laboratory measurements and Electromagnetic Transients Program simulations. The results indicate a significant improvement from published formulae in the literature. The principal advantages of the technique are its accuracy and simplicity. A step-by-step numerical example illustrates how easy it is to estimate the maximum inrush currents.