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.

I. Introduction

Inrush Current is a form of over-current that occurs during energization of a transformer and is a large transient current which is caused by part cycle saturation of the magnetic core of the transformer. For power transformers, the magnitude of the first peak of inrush current is initially several times the rated load current but slowly decreases by the effect of oscillation damping due to winding and magnetizing resistances of the transformer as well as the impedance of the system it is connected to until it finally reaches the normal exciting current value. This process typically takes several minutes. As a result, inrush current could be mistaken for a short circuit current and the transformer is erroneously taken out of service by the over-current or the differential relays. The transformer design and station installation parameters affect the magnitude of the inrush current significantly. Therefore, it is important to have an accurate calculated value of the magnitude and other parameters of inrush current in order to design the relaying to properly differentiate between inrush and short circuit incidents. Also, a proper calculation of the minimum % ratio of 2^nd harmonic content of inrush current is an especially important parameter for this differentiation.