The rails are generally employed as the return path of traction current in urban rail transit. As the rails are not totally earth isolated, there exist the rail potential and stray current issues inevitably, which may lead to the electric shock, corrosions, and other hazards. In the past decades, different mitigation measures have been adopted to solve these issues. However, there are some limitations in the existing mitigation measures, such as poor performance, weak applicability, and so on. Therefore, the dc autotransformer (DCAT)-based traction power supply system is proposed in this article for the rail potential and stray current issues. The configurations, principles, and current distributions of DCAT system are analyzed in this article. Then, the comparisons between the DCAT system and the existing system for the rail potential and stray current issues are discussed, considering the different grounding schemes. Finally, the correctness and effectiveness of DCAT system on the rail potential and stray current issues are validated by both simulation and experimental results. Moreover, it is shown that DCAT system transfers the traction current from the rail to other lines and solves the rail potential and stray current issues much effectively.

Modular multilevel converter (MMC) is one of the excellent selections for high voltage and high power conversion applications without transformers. It provides the advantages such as high modularity, availability, low generation of harmonics, etc. However, the circulating current distorts the leg currents and increases the rated current of power devices, which further increases system cost. By controlling the role of circulating current as current controlled voltage sources (CCVS), this paper derived the circulating current model of MMC, which shows that both the upper and lower legs of the same phase can be controlled separately as two independent cascaded multilevel converters. Then the quantitative relationship between the circulating currents and dc-bus current had been derived and modified considering the actual characteristics of the circulation system. Further, this paper also revealed that MMC is suitable for the need to achieve negative sequence balancing applications.

As one of the nonneglectable challenges in urban rail transit, rail potential and stray current issues may cause metallic corrosions and electronic shocks, which endangers the safety of the traction power system. Hence, a zero-resistance converter system (ZRCS), parallel to a conventional traction power system (CON-TPS), is introduced for mitigating rail potential and stray current. By establishing zero-resistance loops for the train current, its return path on running rails is shortened. Since multiple trains operate in the up-line and down-line, this article proposes the ZRCS share method in both lines. The control strategy based on rail-section detection and relevant operation principles of ZRCS is analyzed in detail with considering multiple trains under multiple conditions. Moreover, ZRCS does not affect the operational reliability of CON-TPS considering its independence from CON-TPS. The locations of switch units (SUs) in ZRCS are optimized to improve the mitigation performance of ZRCS. Both simulation and experimental results prove that zero-resistance loops are established by ZRCS in the up-line and down-line, and their rail potential and stray current are mitigated obviously.

In this article, the resonant switched-capacitor converter (RSCC) is applied to the auxiliary power system of electric locomotive to improve its efficiency and power density. Phase shift control is generally applied to the RSCC for voltage regulation. But the dead time and parasitic parameters impose the negative effects on the switching transit, the resulted hard-switching process may cause extra switching loss. An optimized soft- switching realized method is proposed under phase shift control. First, transient analysis of resonance is carried out by state trajectory, and steady-state characteristics are discussed. Then, the influence of parasitic parameters on resonant current and output power is discussed. Moreover, the dead-time effect and soft-switching condition are analyzed; therefore, a high-efficiency control strategy with zero-voltage switching is proposed. Experimental results show that the proposed optimization control strategy can improve the efficiency up to 99.4%.