With the coupling of the DC metro network and urban power grid, the DC metro stray currents flowing into transformers have caused serious DC bias. There is, therefore, a need to evaluate transformer neutral direct currents caused by the DC metro stray current. To achieve this, an evaluation model considering the coupling of DC metro network and urban power grid is proposed. In the modeling principle, metro lines are coupled with grounding grids of transfer stations. The DC metro system and the urban power grid are coupled with the soil and grounding systems. The train operations on DC metro lines are simulated using the Monte Carlo method. And, considering the conditions of one-way conduction devices (OWCDs) and over-voltage protection devices (OVPDs), transformer neutral direct currents are calculated. Moreover, two probability indexes are proposed to evaluate distributions of transformer neutral direct currents. The applicability of the proposed model is verified by comparing the calculation results with test data of the transformer neutral direct current. Using the evaluation model, transformer neutral direct currents are evaluated. The result indicates that substation supplying power to the DC metro network has a large probability of a high-level neutral direct current.

中文翻译：

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直流地铁杂散电流引起的城市电网变压器中性点直流分布概率评估方法


随着直流地铁网络与城市电网的耦合，流入变压器的直流地铁杂散电流造成严重的直流偏压。因此，需要评估直流地铁杂散电流引起的变压器中性点直流电流。为此，提出了考虑直流地铁网络与城市电网耦合的评估模型。建模原理中，地铁线路与换乘站接地网耦合。直流地铁系统和城市电网与土壤和接地系统耦合。使用蒙特卡罗方法模拟直流地铁线路上的列车运行。并且，考虑单向导通装置（OWCD）和过电压保护装置（OVPD）的情况，计算变压器中性点直流电流。此外，提出了两个概率指标来评估变压器中性点直流电流的分布。通过计算结果与变压器中性点直流试验数据的比较，验证了所提模型的适用性。使用评估模型来评估变压器中性点直流电流。结果表明，向直流地铁网供电的变电站中性点直流高电平的概率较大。