Dynamic Modeling and Optimal Mitigation of Transformer DC Bias Induced by Multi-Source Metro Stray Currents

With the rapid expansion of urban metro networks, transformer direct current (DC) bias caused by stray currents from multiple metro lines has become increasingly prominent. To evaluate and mitigate DC bias currents under complex coupling conditions, a coupling model of multiple metro lines and the regional power grid is established based on the superposition principle. Field measurements are further used to validate the model and demonstrate the necessity of considering multi-line coupling. Subsequently, dynamic operating conditions involving multiple trains are simulated to evaluate the magnitude and dynamic distribution characteristics of transformer DC bias currents in the regional power grid. To address the pronounced time-varying behavior of these currents and the limited adaptability of conventional static mitigation strategies, a dynamic iterative mitigation model is proposed for the optimized and incremental allocation of DC bias suppression devices. The results show that, under the simulation period and regional power grid conditions specified in this study, the proposed model can reasonably capture the dynamic characteristics of transformer DC bias currents. Compared with a conventional threshold-triggered strategy, the proposed method reduces the number of required suppression devices by approximately one third.