Analytic Computation of the Magnetizing Inductance of Current Instrument Transformers Under Consideration of Eddy Currents

In this paper, an analytical method is presented to compute the frequency-dependent magnetization inductance and iron losses resistance of the conventional equivalent circuit of current instrument transformers (CTs). This model considers the effect of eddy currents, which are more pronounced at higher frequencies. Furthermore, this model considers the characteristics of the commonly used toroidal laminated cores, which occur due to the spiraled windings of the lamination. Magnetic flux in the core is computed based on Maxwell´s equations. To get a more simple and numerically stable equation for the flux, the occurring Bessel functions are approximated. Finally, the parameters of the equivalent circuit are computed using the derived equation of the magnetic flux. The results of the proposed model are presented using the parameters of a CT.