THE COMPARISON OF THE METHODS USED FOR ESTIMATION OF CURRENT TRANSFORMER METROLOGICAL PROPERTIES

In the paper, research results on applying field methods in the process determining the parameters of a current transformer equivalent circuit have been published. The metrological characteristics were determined by solving a non‐linear electrical circuit, that is, the equivalent circuit of a current transformer, assuming different values of the forcing current and load. The method gave satisfactory results (the curve 1 in Figure 1) but was found to be time and labour‐consuming. The determination of non‐linear characteristics of RFe and Xµ elements of the equivalent circuit of a current transformer, using field methods, required carrying out a number of calculations of field distributions at the current transformer no‐load state, with different forcing currents and allowing for the magnetization characteristic of the core as well. The total magnetic energy at no‐load state is related to the main flux which permits to determine the Xµ magnetizing reactance, for a given saturation state of the core. On the basis of the calculated distribution of the vector potential, the Bmax magnetic flux density may be calculated in each element of the core and then, the total core losses and the RFe resistance may be calculated using the p = ƒ(Bmax) characteristics of the core sheet loss. Using field methods makes it possible to determine the X2r leakage reactance of the secondary winding, on the basis of the magnetic energy related to the leakage flux of the secondary winding, independently from the shape or localization of the core and the windings as well, while using the analytic method, the formulas used are true only for cylindrical coaxial windings located on the same column of the core. These formulas have been derived with the assumption that the distribution of the flux in the gap between the windings is uniform and that the flux is divided equally between the primary and the secondary windings, which is not true actually. The obtained characteristics of the RFe and the Xµ non‐linear elements, approximated by n‐degree polynomial, and also X2r were used as the input data for solving the non‐linear circuit, in order to determine the error characteristics.