Characteristics of Inrush Current of Present Designs of Power Transformers

Accurate calculation of peak and % 2nd harmonic of inrush current is critical to appropriate selection of relay protection of a power transformer. In this paper, a description is given of a rigorous calculation of magnitude and wave-shape of inrush current as a function of the transformer design parameters as well as parameters of the system to which the transformer is connected. A comparison is given between magnitudes of peak inrush currents calculated using the presented calculation method and the old formula commonly used in the industry. Also, advancements made to transformer technology in the past three decades have changed the characteristics of the transformer inrush current. The second part of this paper presents the impact of these design, material, and system parameters on the peak value and minimum % ratio of 2nd harmonic content of inrush current of transformers of small, medium, and large power transformers. Parameters studied are core design induction, core material, core joint geometry, power rating, winding connections, and number of phases. These studies have been performed using the newly developed calculation.