Series Hybrid Flexible Traction Substation for AC Electric Railways With Reduced Converter Capacity

The flexible traction power system (TPS) is a key development trend for ac electric railways. However, the substantial converter investment cost of its core component, the flexible traction substation (FTSS), limits its widespread application. To reduce this cost, this article proposes a series hybrid FTSS (SFTSS), whose power converter handles only a fraction of the total load power. Because the SFTSS retains the existing traction transformer, it can seamlessly integrate into widely used TPSs that operate at the utility grid frequency. Based on analyzing the working principle, internal power flow, and negative sequence current (NSC) characteristics of the SFTSS, a biobjective control method is designed to stabilize dc-bus voltages and suppress emitted NSC. Furthermore, an average consensus-based distributed coordinated voltage control method is proposed to ensure efficient, stable networked operation of multiple SFTSSs. Hardware-in-the-loop tests were conducted to verify the effectiveness of the proposed SFTSS and control methods. Moreover, existing FTSSs with a 30 MVA load capacity typically require a total converter capacity of 60 MVA. In contrast, the proposed SFTSS requires only 31.46 MVA, achieving a 47.56% reduction, which significantly reduces investment cost while maintaining comparable performance in NSC suppression and load supply.