This paper proposes a novel DC power electronic transformer (DCPET) topology for locomotive, AC/DC hybrid grid, DC distribution grid and other isolated medium-voltage and high-power applications. Compared with conventional PET topology, the proposed DCPET has fewer power semiconductor devices and high-frequency isolation transformers, which can improve the power density and reliability. Fault handling or redundancy design can be achieved to further improve the reliability when some DC-DC modules break down. In addition, input voltage sharing (IVS) control can be omitted to simplify the control system and improve the stability. Meanwhile, soft switching is guaranteed for all the switches, which is beneficial to increase switching frequency and improve power density. In this paper, the principle, evolution, and control of the proposed DCPET are respectively presented and studied in detail. Finally, a prototype of the proposed DCPET is built and the experimental results verify the validity and superiority of the proposed topology.

Due to the low output voltage of photovoltaic cells and fuel cells, a high voltage gain DC-DC converter is indispensable in the renewable energy power generation systems. In this paper, a series of high voltage gain capacitor clamped DC-DC converters is constructed based on interleaved Boost structures by considering the requirement of low input current ripple in fuel cells application. Further, the construction method of capacitor clamped DC-DC converters is summarized according to the construction processes of these converters. Finally, the capacitor clamped DC-DC converter with the voltage gain of 3/(1-D) is analyzed in detail. Then, a 250W prototype is built to validate the theoretical analysis and the construction method.

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.

The low-voltage ride through condition (LVRT) attracts wider attention with the increased power of central grid-connected photovoltaic (PV) systems. The conventional double-loop-based control strategy designed for the nominal voltage is slow and potentially unstable under the LVRT operation. In this article, a novel sliding-mode control-based feedback linearization is proposed to improve response speed of the PV system under LVRT condition. To increase the robustness of the control to parameter variations, sliding-mode control is combined with the feedback linearization in this article. Further, the complete linearization of the PV system can be achieved over the whole operating range. Moreover, the dc-link voltage operating point of the PV panel can be dynamically adjusted based on the drop depth of the grid voltage, which will reduce the load changes during the LVRT and subsequently ensure the ac current levels to stay within the design limits in any fault situation and enhance the LVRT capability of the central grid-connected PV system. Finally, a 100-kW PV inverter is built to validate the feasibility and effectiveness of the proposed strategy for LVRT.

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.