Relationship Analysis Between Metro Rail Potential and Neutral Direct Current of Nearby Transformers

During the dynamic operation of the dc metro system, excessive neutral direct current (NDC) of the grounding transformer appears in the ac power system, which has not been clarified until now. Seriously, the high-level NDC has led the transformer under dc bias, which affects the lifespan and safety of transformers and systems. To analyze the NDC of the grounding transformer with the dynamic metro train, the synchronous monitor schemes of the traction current and rail potential of the dc metro system and the NDC of the nearby transformer are proposed. The relationship between the metro rail potential and NDC is analyzed and presented. Based on the monitor data analysis, the negative correlations are found in the directions and magnitudes of the RSP and the NDC. To explain the reasons for the relationship, the numerical simulation is proposed based on the structure of the monitored metro line and substation. Moreover, the universality of the relationship is verified through the coupled simulated model. Results show that the NDC is commonly flowing into the transformer as the rail potential is positive. Moreover, the NDC may exceed the allowable threshold of transformers in acceleration and brake conditions of the dynamic train.