Contribution analysis of neutral current for the substations invaded by stray current from multiple metro lines

• Synchronized acquisition of neutral current from several substations with time-stamped signals. • Wavelet transform analysis of neutral current. • The relationship between the main fluctuation period and the operating time between metro stations. • Contribution degree solution of two metro lines. As a type of convenient and environmentally friendly transportation tool, urban rail transit has been constructed as vital infrastructure in numerous cities over the world. Currently, direct current (DC) traction power supply mode has been applied as mainstream for most of metro lines. However, along with long-term service, the insulation performance between steel track and concrete sleeper deteriorates gradually, possibly causing traction current leaking from rail to ground, forming as ‘stray current’. It possibly invades into transformers through their grounding poles, causing local thermal surge, abnormal vibration or even power quality degradation. At present, only relying on the passively protecting method via installing fixed impedance at the grounding pole can hardly cope with the transiently-varying stray current. Tracing the origin of stray current especially under the scenario of multiple metro lines becomes essential, as the periodicity of traction current from each metro line can be extracted as ‘fingerprint’. A boundary element model of a region with multiple metro lines is launched based on practical geological data, for evaluating the route and distribution of stray current. Based on the Wavelet Transform (WT) method, the correlation is found precisely via comparing periodic feature between the traction current of different metro lines and the neutral current measured synchronously at substations. Moreover, an origin tracing method for clarifying the contributions made by different metro lines on the substation’s neutral current is proposed via analyzing the local periodicity of traction current and neutral current, for laying the solid foundation of precise suppression from origin.