Optimized Cable-Armor Blocking for Metro Stray Current Mitigation in Urban Power Grids

Metro DC traction systems cause return current to leak from the running rails into the earth. This stray current intrudes into urban power grids through metallic paths such as cable armors, driving transformer DC bias that can cause core saturation, increased losses, vibration, and accelerated insulation aging. Conventional mitigation measures install blocking devices at transformer neutral points only after the current limit is exceeded; this single-point treatment protects the treated transformer but may redistribute stray current to neighboring substations, requiring repeated device installations. To overcome this limitation, this paper proposes a coordinated mitigation method for metro stray current in urban power grids based on cable armor path blocking. An equivalent nodal admittance model incorporating cable armor blocking is established to calculate transformer neutral-point currents following current redistribution. A constrained optimization model is formulated and solved by NSGA-II to jointly minimize the maximum neutral-point current and the number of blocking devices, and Monte Carlo sampling of train positions is used to determine the selection frequency of each candidate location across repeated optimization outcomes and identify priority installation locations. Simulation results show that, in a practical coupled metro–power grid case, the proposed method effectively suppresses transformer neutral-point currents throughout the network with fewer devices than conventional single-point neutral-point blocking under the examined operating conditions. Multi-scenario analysis further identifies cable armor paths that consistently have high placement priority across different train positions, supporting coordinated siting and phased deployment of blocking devices.