Probabilistic Evaluation Method of Transformer Neutral Direct Current Distribution in Urban Power Grid Caused by DC Metro Stray Current

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.