Analysis of short circuit and inrush transients in a current transformer using a field-circuit coupled FE formulation

Abstract

The existing models based on the lumped-circuit approach consider the constant leakage inductance during transient simulations. However, leakage inductance of the current transformer (CT) may not remain constant. In fact, it may vary with the degree of saturation of the core. During severe saturation conditions, leakage inductance variation in fact may result in significant difference between the calculated and the measured secondary currents. In this work, a nonlinear, field-circuit coupled finite element model of a current transformer (CT) is presented. The model considers the variation in leakage inductance during the simulation of transient conditions. Moreover, since geometry of the transformer was taken into account, it was possible to estimate forces associated with the short-circuit current. Hysteresis characteristic of the core was taken into account by using Jiles–Atherton model in conjunction with fixed-point iteration method. The formulation was realized using a code developed in MATLAB and implemented for a 69 kV, 200/5 A current transformer. The normal operation and other cases such as short-circuit, and transformer sympathetic inrush current through the primary of the CT are presented. These cases demonstrate modeling flexibility of the field-circuit coupled formulation.