Calculation of transformer DC-bias Current in AC Power Systems Considering Metro Stray Current

Since the rail-soil conductance exists, DC traction current can leak from rails, and it becomes the stray current in the earth. The stray current leads to the result that the ground potential rises and the current flows into ground systems of the metro system and the AC network. Through the ground potential offset of substations and the earthing systems, the stray currents can get into the neutral point of transformers, which may cause transformers the consequence of DC bias. With the situation of dynamic trains, the distribution of the DC-bias current is complex and has not been clarified i n t he AC n etworks. F or evaluating the level of the DC-bias current, a numerical simulation model is obtained. Based on the obtained model, the DC distribution influencing the AC networks is calculated. And the results show when the train accelerates near the transformer, the DC bias of the earthing transformer becomes a more serious problem. During the different situation of dynamic train operation, DC-bias current of the transformer changes in accordance with the train operation.