Analysis and Simulation of Time-Frequency Characteristics of Dynamic DC Bias Current in Urban Power Grids caused by Metro Stray Current

In order to better understand the dynamic DC bias caused by metro stray current, this paper analyzes it from the perspective of time-frequency characteristics. Firstly, 220kV Foshan Station, which is greatly affected by Foshan Metro, and 220kV Panyu Station, which is greatly affected by Guangzhou Metro, were selected for transformer neutral point current measurement. Then, the time-frequency characteristics of the bias current were extracted with wavelet analysis. Results show that the the dynamic DC bias current are mainly composed of low-frequency components, with a base frequency of 0.005Hz∼0.01Hz. In addition to the base frequency, the bias current also contains a large number of harmonic components, which may be formed by the superposition of stray currents generated by multiple metro trains. It can be inferred that there may be other intrusion paths with lower resistance than the earth that can cause significant superposition of stray currents besides. Simulation model considering the whole metro grounding system was established, and the 24h time-domain DC bias current generated by the simultaneous operation of multiple metro lines was simulated. The time-frequency characteristics of the simulated DC bias current are consistent with that of the measured ones. The results prove that the cable metal sleeve, drainage cabinet, and OVPD provide a low resistance path for stray current intrusion and superposition.