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.

A dual active-bridge (DAB) dc-dc converter is widely used in bidirectional high-power conversion, such as renewable energy system or train traction system. The open-circuit fault of DAB may threaten the system safety operation and should be eliminated in time. In this article, each fault mode of DAB is analyzed in detail. The transient operations and the load-carrying capacity are carefully analyzed. Based on the analysis, a DAB fault-tolerant method called “primary-side lower power secondary-side bypass arm” (PLP-SBA) is proposed. When the fault occurs in the primary side, the power of DAB is cut down, and when the fault occurs in the secondary side, the entire fault bridge arm is bypassed. The PLP-SBA method can ensure a certain load-carrying capacity after an open-circuit fault. A 5-kW DAB experimental platform is built to verify the correctness and effectiveness of the DAB mode analysis and PLP-SBA method.

Power electronic traction transformer (PETT) is a new topology for a traction drive system, which can realize the high voltage and power conversion through multilevel converters and high-/medium-frequency transformer link. Reliability becomes a key issue for PETT adopting the module-cascaded structure. In this paper, an improved fault-tolerant method based on reverse-charging control of dual active bridge is proposed, which is suitable for PETT and other two-stage structure converters. The control strategy and operating process of an improved fault-tolerant method are introduced. Comparisons among improved and traditional fault-tolerant methods are also presented. Simulation and experimental results are provided to show the correctness and effectiveness of the improved fault-tolerant method.

Solid-State transformer for onboard traction application is also addressed as power electronic traction transformer (PETT), which is used to replace the line frequency traction transformer and four-quadrant converter to further improve efficiency, save installation space, and reduce weight of the onboard equipment. However, based on the conventional PETT configuration with the modular series-parallel structure, it is hard to guarantee the high efficiency and high power density at the same time due to the limitations caused by the switching characteristics of the semiconductor devices, transformer design, and high-voltage insulation design. To deal with the mentioned problem, a PETT configuration and the parameters' design method for onboard traction application are proposed in this paper to replace the conventional dc/dc stage with the low-voltage insulated gate bipolar translator (IGBTs). By using the proposed configuration, the high-voltage IGBTs can be replaced by the low-voltage IGBTs, which can help reduce switching loss. Meanwhile, zero-current switching or zero-voltage switching for all the IGBTs can be guaranteed, and the number of the medium-frequency transformers (MFTs) will not increase. Therefore, a multiple optimization of PETT, including switching losses, the number of MFTs and switches, and costs, can be finally achieved. Finally, an experimental prototype is built to verify the performance of the proposed PETT configuration.

An LLC-LC type bidirectional control strategy for an LLC resonant converter is proposed in this paper. It can achieve bidirectional power flow automatically without any power flow detection procedures. Meanwhile, the soft-switching can be always guaranteed all the time. In order to adapt to the proposed control strategy, a no-load-point-based design method is put forward in this paper, which can rapidly obtain optimal parameters of an LLC resonant converter. What is more, by using the proposed control strategy, the starting characteristic in no-load condition can be improved, output voltage rising and current surge risk can be avoided. Finally, an experimental platform of power electronic traction transformer power cell (consisting of a single-phase rectifier and a bidirectional LLC resonant converter) is built to verify the validity of the proposed control strategy.

This paper proposes a new auxiliary power supply (APS), which is used to feed the controllers, drivers, and sensors of the sub-modules for medium-/high-voltage and high-power solid state transformer (SST). In medium-/high-voltage and high-power applications, the APS is usually connected to the dc bus of the sub-module to obtain electric energy. The voltage of the sub-module's dc bus is often as high as several thousand volts, so that the APS must have the ability to handle such high input voltage. An input-series capacitor structure, which makes the voltage stress of the power semiconductor devices reduced, is adopted in the proposed APS. Therefore, the high input voltage can be handled by the low voltage power semiconductor devices. Furthermore, the input voltage sharing (IVS) can be achieved without any special control strategy, which makes the control system simplified and the stability improved. Meanwhile, zero-current switching (ZCS) is guaranteed for all the switches, so that the power density and efficiency can be improved. Finally, the simulation model and prototype are built, by which the feasibility and effectiveness of the proposed APS are verified.