A wide-range ubiquitous wireless power environment, called a ubiquitous Wi-power zone, where any electric devices can be charged by three-dimensional (3-D)-omnidirectional wireless charging, is newly proposed in this paper. By virtue of the proposed direct-quadrature (DQ) transmitting (Tx) coils composed of multiple Tx modules with same size, a ubiquitous Wi-power zone having the transmitting (Tx) coils can be provided over wide-range 3-D space. In this area, a number of electric devices can be simultaneously charged during operation. Therefore, the proposed IPT is expected to be the next generation of wireless power transfer (WPT), as the internet of things (IoT), mobile devices, and sensor technologies evolve. For magnetic shielding behind the Tx coils and high power efficient operation of the proposed DQ Tx coils, providing an evenly distributed magnetic field, a conductive plate is adopted, and the effects of the conductive plate for the dipole coil structure were thoroughly evaluated by a 3-D FEM simulation. To create the proposed ubiquitous Wi-power zone, simulation-based optimum design procedures are established based on four design considerations and the optimized number of modularizations for the DQ Tx coils can be found by an optimum point of figure of merit (FoM). The DQ Tx module composed of the DQ Tx coils and series resonant capacitors was adopted as a standard unit of the proposed IPTso that different sizes and shapes of 3-D space can be utilized for universal use of the proposed ubiquitous IPT; hence, a general solution to create the proposed ubiquitous IPT is provided in this paper. A prototype 5 × 5 modularized IPT composed of 25 DQ Tx modules having the same size of 10 × 10 cm × 2 cm was fabricated and experimentally verified for a 1 m × 1 m × 0.7 m ubiquitous Wipower zone. Experimental results showed that 95.5%, 92.1%, and 98.8% of high magnetic field uniformity for z ₁ = 25 cm, 50 cm, and 75 cm, respectively, and 3-D omnidirectional wireless power delivery have been achieved; thus, meeting essential requirements for the proposed ubiquitous Wi-power zone. Nine Rx coils can be simultaneously charged with 14.2 W of total received load power and 8.2% power efficiency at z ₁ = 70 cm.

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.

New coreless transmitting (Tx) coils with a conductive material plate for wide-range ubiquitous inductive power transfer (IPT) zones, i.e., a U-IPT zones, where electric devices can be wirelessly charged during operation, are proposed in this paper. Because commercialized IPT systems are only available for short distances of several centimeters and one-to-one charging, the demand is increasing for U-IPT zones in which long-distance wireless charging with 3-D free-positioning and simultaneous charging of multiple receiving (Rx) coils can be achieved. To satisfy this demand, U-IPT zones should be constructed to have a uniform magnetic field distribution in a wide space, which also poses no risk to the human body. In addition, to generate a uniform magnetic field in a wide 3-D space, such as a single room, the system requires the installation of many Tx coils above the ceilings and behind the walls of the room, and these coils should be capable of high electrical performance to support the efficiency of the entire IPT system and should be of excellent mechanical quality. Therefore, a U-IPT zone should meet the following four criteria: human safety, free-positioning of Rx coils, magnetic field uniformity, and economization. By virtue of a conductive material plate that cancels out the undesired direction of magnetic flux generated from the return wires, the proposed coreless Tx coil does not need any heavy ferromagnetic cores, which can cause non-uniformity of the magnetic field distribution and difficult construction. A prototype coreless Tx coil with the dimensions of 1 m × 1 m × 10 cm was fabricated and experimentally verified for a 1 m × 1 m U-IPT zone. The experimental results showed that 74%, 83%, and 76% high magnetic field uniformity could be achieved for $z1$ = 25 cm, 50 cm, and 75 cm, respectively, and 3-D omnidirectional wireless power delivery was achieved, while the guideline of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) for human safety was satisfied throughout the entire U-IPT zone. Moreover, the weight of the entire system was only 9 kg, enabling easy installation. In the multiple wireless charging experiments, 47 W of power was consumed in total by load resistors, each of them connected to nine Rx coils, respectively, with 32 % power efficiency at $z1$ = 20 cm.

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.

In this paper, a reinforcement learning algorithm is applied for the first time to find a ferromagnetic core structure with optimal coupling coefficient between transmitting (Tx) and receiving (Rx) coils of a wireless power transfer (WPT) system. Since formula-based theoretical design is not available due to the non-linear magnetic field distortion stems from the presence of the ferromagnetic core in a WPT system, the proposed design has been achieved through finite element analysis (FEA) simulation-based data learning. The proposed design methods are so general that they can be applied to any conventional WPT coil types. We applied the proposed algorithm to the ferromagnetic core structure design of a simple dipole coil first. By training only 2.3 % data out of total possible cases, it is experimentally verified that the core structure obtained by the proposed method has a coupling coefficient 7 % higher than that of the example design level in the case of 98 cm distance between Tx and Rx coils.

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.