Research on Modeling Method of Transformer DC Bias Caused by Metro Stray Current

The metro stray current will flow through the neutral point of the transformer from the grounding grid in the soil, causing the transformer DC bias phenomenon. In order to effectively evaluate the influence of metro stray currents on the transformers, the traction current in metro, the "rail-current drainage net-earth" grounding network structure, and the DC bias current loop were equivalent, so a resistance model for the transformer DC bias caused by stray current was built. Then the dynamic equation of transformer neutral point current with time and metro train position was established, and the neutral point current can be solved by LU decomposition. The software CDEGS was used to build train and transformer simulation model. By comparative analyzing the transformer neutral point current between the simulation results and the calculation results of the proposed modeling method, the model error was small, which can be effectively applied to the evaluation of transformers affected by metro stray current.