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.

中文翻译：

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地铁轨道电位与附近变压器中性直流电的关系分析

在直流地铁系统动态运行过程中，交流电力系统中会出现接地变压器中性点直流（NDC）过大的现象，目前尚未明确。严重的是，高级NDC导致变压器处于直流偏置下，影响变压器和系统的寿命和安全性。为分析动态地铁列车接地变压器的NDC，提出了直流地铁系统牵引电流和轨道电位与附近变压器NDC的同步监测方案。分析并介绍了地铁轨道潜力与 NDC 之间的关系。根据监测数据分析，RSP和NDC的方向和幅度呈负相关。为了解释这种关系的原因，数值模拟是基于被监测的地铁线路和变电站的结构提出的。此外，通过耦合模拟模型验证了该关系的普遍性。结果表明，当轨道电位为正时，NDC 通常流入变压器。此外，在动态列车的加速和制动条件下，NDC 可能会超过变压器的允许阈值。