An analytical electromagnetic model of “Sen” transformer with multi-winding coupling

Abstract

In flexible AC transmission systems, the “Sen” transformer (ST) is an economical and effective method for voltage regulation and power flow control, which can achieve independent control of active and reactive power. The ST is a three-phase multi-winding transformer based on the on-load tap-changing technology. In order to reveal the internal characteristics of the ST, this paper proposes an analytical electromagnetic model considering the multi-winding coupling in the ST with the three-phase three-limb structure. Firstly, according to the electromagnetic coupling relationship in the ST, a magnetic equivalent circuit model consisting of self-inductance coefficients and mutual inductance is derived via the unified magnetic equivalent circuit (UMEC). Secondly, an electric circuit model for the internal voltage and current of the ST is established based on the electrical connection between the ST and the external system. Finally, an analytical electromagnetic model considering multi-winding coupling for the ST is obtained by combining the magnetic equivalent circuit model and the electric circuit model. Two case studies have been carried out on a three-phase three-limb model of the ST with the help of MATLAB. The effectiveness of the proposed model is verified by comparing the obtained analytic results with the existing results from time-domain simulation about the series compensating voltage of the ST. In addition, the voltages and currents of windings in the ST in the conditions of different transformer core structures and ferromagnetic materials are investigated for evaluating the influence of magnetic coupling on the results.