Research on the Evaluation Model of Urban Main Transformer DC Bias Caused by Metro Stray Current

During the actual operation of metro locomotives, some traction current leaks and injects into the ground, forming stray currents, resulting in surface potential differences along the line and causing DC bias of the main transformer in the urban area. Reasonably evaluating the degree of influence of transformer DC bias can effectively guide the suppression of transformer DC bias. In response to the above issues, this article first established a coupling model between stray current and DC bias of the main transformer in urban substations, and further modelled the distribution of transition resistance during subway operation, which achieved dynamic calculation of stray current; By selecting indicators such as neutral point DC, excitation current DC component, and harmonic distortion deviation, effective evaluation of the degree of DC bias influence on the main transformer along the subway line was achieved. The simulation results show that as the ground potential difference increases, the DC flow at the neutral point of the transformer increases. Along with the degree of hysteresis loop distortion in the iron core increases, the DC component of the excitation current and the deviation of harmonic distortion rate gradually increase. It is show that DC bias of the transformer becomes more severe.