Solid-state transformers (SST) are emerging technologies to replace the conventional low-frequency main transformer (MTR) in railway vehicles. In order to isolate the load side from the AC 25 kV high-voltage (HV) side and to reduce the whole size of the SST, HV insulation and high-frequency transformers should be used in DC/DC converters. Furthermore, due to the severe constraint of installation space and difficulty of heat dissipation management in the bottom of railway vehicles, high-power density and minimum transformer loss should be considered. To satisfy such design requirements of the proposed SST for railway vehicles, high efficiency integrated transformers, which inherently have leakage inductance, are proposed for dual-active-bridge (DAB) converters in this paper. In order to obtain appropriate equivalent leakage inductance for the DAB converters, a calculation methodology for the leakage inductance has been introduced and verified by simulation. Two separated winding structures in the center-core are adopted to improve the core loss, and appropriate winding turns with 60 kV HV insulation capability are selected, according to the proposed design procedure of the integrated transformers. In order to compare the proposed transformers with the conventional transformer having additional inductors and to verify the major characteristics of the transformers, two 125 kW proposed transformers and conventional transformers have been built and evaluated by an 250 kW SST module, operating at 10 kHz switching frequency. Under a 125 kW full-load condition, 99.7% transformer efficiency was obtained by the proposed integrated transformers, which are 4.7% smaller and 7.0% lighter than the conventional transformer. As a result, 26°C maximum temperature improvement has been obtained and 60k V HV insulation capability has been satisfied by the proposed integrated transformers.

Inductive power transfer (IPT) systems based on segmented transmitter (Tx) coils are newly proposed in this article. These IPT systems can deliver wireless power to multiple transport robots in an electrified monorail system (EMS) for continuous transportation missions. Considering the one-dimensional long-line driving characteristics of the transport robots having receiver (Rx) systems, a segmented Tx coil structure is introduced so that flexible Tx installation, easily Tx coil fabrication, and insensitive IPT characteristics are possible. In this case, in order to remove the magnetic dead-zones at the adjacent sections between the adjacent Tx coils, folded Tx coils, and an optimally sized Rx coil are applied to the proposed IPT system in this article. Thus, continuous wireless powering to multiple transport robots in the EMS and flexible extending of the Tx length are possible by the proposed IPT systems. For the soft-switching operations of the proposed IPT systems, a gyrator-based static circuit modeling is presented in order to select an appropriate resonant capacitance. By the proposed gyrator-based circuit analysis, it is found that high efficiency Tx circuit operation can be achieved, regardless of the number of Rx and the load conditions. The proposed segmented multiple Tx systems and the 30 W Rx systems for the transport robots were fabricated and verified by experiments for various operating scenarios. Compared with the conventional 2-m-long Tx coil having 69.1% dc power efficiency, the two-segmented 1-m-long Tx coil systems have no powering dead zone and achieved 74.2% dc power efficiency for one 30 W Rx system. As a result, the proposed two segmented 1-m-long Tx systems have 5.1% higher dc efficiency than the conventional 2-m-long Tx coil under the 2-m-long driving route in EMS application.

The solid-state transformer (SST) can be a promising solution to replace the conventional 50- or 60-Hz main transformer (MTr) for railway vehicles. Unlike the existing SST technologies for stationary power applications, the SST applied to railway vehicles requires severe constraint of installation space and intensive management of heat dissipation. To optimally design the SST module applied to the railway vehicles, which is one of the major components in the whole SST system, the optimal magnetic component designs for high-efficiency and thermal stable operations in the confined installation space are proposed in this article. In order to meet the SST system requirements for ac 25-kV-to-dc 1.5-kV power conversion, an overall SST topology and its control schemes are established. Based on the proposed SST topology, considering the operating drive profile of the urban railway vehicles, the operating sequence of the proposed SST is introduced and verified by simulation. By virtue of the proposed magnetic component design, the optimal number of turns for the inductor and transformer in dual-active-bridge converters can be appropriately determined for 50-kV high-voltage insulation and high-efficiency operation when the installation space for the magnetic components is given. For targeting the 3.0-MW maximum power transfer capability of the proposed SST system, the 300-kW SST modules, operating at 500 Hz and 10 kHz for ac/dc converters and dc/dc converters, respectively, were fabricated and verified by experiment. By two experimental SST module sets, the static performance of the power converters in the SST module and the dynamic operating sequence has been successfully implemented. For the 300-kW full-load operation, 99.7% of power transfer efficiency for the proposed magnetic components and 97.2% of total power efficiency were obtained. As a result, the measured temperature characteristics for the proposed SST module, including the magnetic components, were managed below 125 °C under a severe experimental environment.

In this paper, an integrated transformer design methodology for dual-active-bridge (DAB) converters, which basically utilize leakage inductance so as to substitute a cumbersome inductor in the conventional DAB converters, is newly proposed. The proposed integrated transformer with integrated leakage inductance inherently includes leakage inductance and magnetizing inductance, whose values can be appropriately selected by modulating the number of turns and the center-core air-gap. Based on the theoretical analysis of the DAB converters, the major parameters, i.e., primary and secondary current levels and its phase difference, can be derived, and these values are utilized for magnetic analysis of the proposed integrated transformer. Considering core loss and copper loss of the proposed integrated transformer in the DAB converters, optimal values of the center-core air-gap $l_{g}$ and the optimal number of turns $N_{\mathrm {1,}op} ~\&~ N_{\mathrm {2,}op}$ can be determined so that high power efficiency is achieved at a normal operating point of the DAB converters. The 20W prototypes of the DAB converter with EER2834 and EER3019 of the ferrite cores were fabricated and verified by simulation and experiment. The design results showed that the optimal number of turns at $l_{g} =2.0$ mm are found to be 11 and 10, for the EER2834 and EER3019, respectively. Compared to 94.5% and 96.3% of AC efficiencies for the conventional transformer and inductor set, the proposed integrated transformer achieved 95.8% and 96.9% of AC efficiencies at maximum power transfer point of $\theta _{p} = \pi $ /2 for EER2834 and EER3019, respectively.

In this study, we present a 1 MW-class wireless power transfer (WPT) system and test the operational data by applying it to an actual light-rail system for over 40,000 km. The data demonstrate that there are no reliability issues when applying the WPT system to light-rail systems. The power capacity of the WPT system was designed based on actual operating scenarios of light-rail systems. We propose a suitable transmitter and pick-up structure for real vehicle operation, and analyze the input/output voltages, current, power, and efficiency of the WPT system during operation. The applied WPT system achieved a maximum efficiency of 89%, which is considered high. The one-year operation data are expected to provide significant assistance in the future commercialization of wireless charging applications for railways.

The high-power wireless power transfer (WPT) system in railways does not require physical contact to transfer electrical power, is electrically safe, and reduces maintenance costs from wear and tear. However, a high-power system generates a strong magnetic field that can result in problems of electromagnetic field (EMF) exposure and electromagnetic interference (EMI). In this study, EMF and EMI were measured at various positions under in-motion environment conditions for a 1-MW WPT light rail transit system. The measured maximum EMF was 2.41 μT, which is lower than the international guideline of 6.25 μT for the various locations with a potential presence of passengers. The measured EMI also satisfied international standards in the frequency range of 150 kHz–1 GHz.