Transient Calculation of Return-Flow Safety Parameter Distribution in Urban Rail Transit

With the rapid development of DC traction power supply systems in urban rail transit, return-flow safety has become a crucial concern. Running rails inevitably present longitudinal resistance, and complete insulation from ground is difficult under complex environments, resulting in stray currents and elevated rail potential, which trigger frequent protection actions, and accelerate electrochemical corrosion of metallic infrastructures. Existing studies mainly rely on resistance models, while transient characteristics are often overlooked. However, traction current frequently fluctuates with train operation and regenerative braking, making rail current a time-varying process influenced by inductance and capacitance. To address this gap, this paper analyzes the transient behavior of the return-flow system and develops a transient return-flow model using an electromagnetic transient algorithm with equivalent inductance and capacitance. The model is used to simulate and evaluate the distribution of return-flow safety parameters under sudden traction current changes, varying regenerative braking, and additional operating scenarios. Results reveal transient patterns of rail potential and stray currents, offering theoretical insights into abnormal potential rise and informing effective mitigation strategies.