Magnetic field created by DC traction power supply system has become a serious man-made perturbation to the operations of geomagnetic observatories near big cities in China. Leakage current produced by DC traction power supply system is the key factor. Based on vacuum line current magnetic field model in railway transit, this paper presents a more general and simple model for quantitative estimation of the magnetic perturbation. It is effective in fast estimating magnetic perturbation value. Using this model we obtained two numerical example results: distance decay of magnetic perturbation and the amplitude variation at the end of railway line caused by motion of locomotive. In order to verify this model, field measurement was carried out in the vicinity of Jinghai, Tianjin. The ground structure in this area is approximately layered.Theoretical calculation and field measurement show a good agreement, indicating that at the distance of 45 km, the perturbation caused by Tianjin subway can reach to 0.5 nT.%直流牵引城市轨道交通系统在运行过程中产生的磁场对我国大城市周边地磁台站的观测数据产生较为严重的干扰.城市轨道交通运行过程中产生泄露电流是产生干扰的重要原因.本文以真空中的线电流磁场计算模型为基础,通过简化的远场磁场干扰模型对天津地铁轨道交通干扰进行理论建模.该模型计算简洁高效.通过对模型的数值模拟,获得了干扰随距离衰减关系曲线和在线路终点的干扰幅值变化曲线.同时,在天津静海开展实地测量天津地铁干扰来验证该理论模型.理论计算与实地测量结果表明,如果该区域地表介质大致均匀的情况下,天津轨道交通干扰磁场的衰减距离到45 km处仍然存在0.3 nT左右的干扰.
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