An improved magnetic circuit model of power transformers under DC bias excitation

Direct current (DC) flowing into a power transformer from the directly earthed neutral of star-connected winding affects the magnetizing current, resulting in several undesirable effects on a power transformer. An improved magnetic circuit model is presented in the paper for the DC biased power transformers, which takes into account of the core geometry, coil flux leakage, nonlinear characteristic of ferromagnetic material, and the load level. A new formula for the core reluctance had been derived, to consider the effect of the eddy currents in the lamination, and appears to be nonlinear and frequency dependent. An equivalent magnetic circuit of series Foster type was used to integrate the new core reluctance into the time-domain computation. The computational model was applied to a 150 kVA power transformer of three-phase core-type construction on various values of the DC bias. Measurement and calculation agree well, and demonstrate the effectiveness of the model presented in the paper.