Unified magnetic equivalent circuit-based model of two-core Sen transformer for low-frequency electromagnetic transients and its numerical calculation

•A topology of two-core Sen transformer (TCST) is introduced, and the low-frequency electromagnetic transient model of TCST is proposed.•The energization transient and the steady-state performance of TCST are analyzed.•There is a numerical study on the differences of control accuracy and winding current between TCST and single-core ST (SCST). This paper proposes a low-frequency electromagnetic transient model of two-core Sen transformer (TCST) based on unified magnetic equivalent circuit (UMEC) and also presents its numerical calculation procedure for revealing the internal electromagnetic characteristics of TCST. Firstly, the magnetic equivalent circuits of the two parts of TCST including series unit and exciting unit are established according to their associated iron-core structures. Secondly, the electromagnetic transient models about the two parts are deduced by the electromagnetic coupling relationship. Finally, the electromagnetic transient model of TCST can be obtained through the combination of the series unit and the exciting unit on the basis of the internal electrical connections between them. The saturation and hysteresis characteristics of the iron core are represented by the J-A model, and a cascaded R-L lumped circuit is used to depict the eddy current effect. The case studies have been carried out in the environment of MATLAB and PSCAD/EMTDC. The comparisons between the results from the numerical calculation and the time-domain simulation reveal the effectiveness of the proposed model. Furthermore, it is found that the current flowing through the tap-changer in TCST is about 50% of the corresponding current in single-core ST (SCST), which implies the former may have lower cost and higher reliability. Moreover, the actual step length of TCST is about 0.5 times smaller than that of SCST, which may lead to more accurate control performance for TCST.