Improved Power Transformer Model for DC Biasing Analysis Considering Transient Leakage Reluctances

This paper presents an improved power transformer model for analyzing the performance of power system under DC bias. To study the influences of DC current on both the power transformer and the system, the characteristics of transformer leakage paths are required to be analyzed in detail. The field computation by time-stepping finite element method is indirectly coupled with electromagnetic circuit simulation to obtain the equivalent transient leakage reluctances of the on-line power transformer. A simplified system with a 240 MVA/500 kV single-phase power transformer has been studied as an example. The simulation results show that the flux and energy in the leakage paths of DC biased power transformer would change noticeably when transient leakage reluctances are considered. It would improve the simulation accuracy by taking transient leakage reluctances into account when evaluating DC bias phenomenon, especially for the cases caused by leakage flux.