The stray current of rail transit system causes DC bias issues of transformers in urban power grid. Under the action of dynamic stray current, the transformer presents complex DC bias characteristics. Based on the equivalent nonlinear magnetic circuit model, a method to deduce the no-load excitation current of transformer from the bias flux density has been proposed, and the mapping relationship between the bias DC magnetic flux and the DC bias current is established. A calculation method of transformer DC bias caused by dynamic stray current invasion has also been proposed. The calculation results show that under the action of dynamic stray current, the dc bias characteristics of the transformer also present dynamic changes. The biased DC flux density varies with the stray current. Through the FFT analysis of the stray current in the field, the upper limit frequency of stray current does not exceed 0.6 Hz. Furthermore, the biased DC flux density by the dynamic stray current has been analyzed, and the results show that the upper limit of flux density is about 0~0.6 Hz.

DC bias; dynamic stray current; transformer; biased dc flux method; simulation