Research on Analytical Calculation Method for DC Bias Characteristic of High Voltage Group Transformer

The calculation methods and characteristics analysis of transformer DC bias (TDB) are fundamental to researching the DC bias problems. Presently, the numerical calculation method is mainly adopted to solve the electromagnetic coupling equation of the transformer, which is unable to obtain the current expression and make precise TDB characteristics analysis. In this paper, the knee point of the magnetization curve is taken as the demarcation point of the calculation cycle. The electromagnetic coupling equation of the group transformer is solved analytically segment by segment, and time-domain expressions of winding current and excitation current in the unsaturated area and saturated area are obtained. The influence of TDB on the winding current and excitation current characteristic components is analyzed. On this basis, A calculation method for the DC bias current that causes critical saturation of the transformer is proposed. Furthermore, achieving the prediction of whether the core occurs saturated phenomenon under TDB. The validity and accuracy of the theoretical analysis are verified using the Matlab/Simulink simulation platform. The conclusions of the paper provide certain references for quantitatively analyzing the characteristics of TDB.