With the growing fleet of a new generation electric vehicles (EVs), it is essential to develop an adequate high power charging infrastructure that can mimic conventional gasoline fuel stations. Therefore, much research attention must be focused on the development of off-board DC fast chargers which can quickly replenish the charge in an EV battery. However, use of the service transformer in the existing fast charging architecture adds to the system cost, size and complicates the installation process while directly connected to medium-voltage (MV) line. With continual improvements in power electronics and magnetics, solid state transformer (SST) technology can be adopted to enhance power density and efficiency of the system. This paper aims to review the current state of the art architectures and challenges of fast charging infrastructure using SST technology while directly connected to the MV line. Finally, this paper discusses technical considerations, challenges and introduces future research possibilities.

https://www.mdpi.com/1996-1073/12/19/3721/pdf

Extreme Fast Charging Technology—Prospects to Enhance Sustainable Electric Transportation

This article proposes a novel isolated three-port dc/dc converter based on a series resonant converter and a dual active bridge (DAB) converter for electric-vehicle charging stations with fast and slow charging functions. With this three-port structure, the proposed converter has fewer components, which results in lower system cost and volume compared with separate charger systems. A simple control method using phase-shift and frequency modulations was developed to control the output power of the fast and slow charging ports simultaneously. An optimal phase-shift angle was also derived to minimize the transformer current for when only the DAB converter is operated for slow charging. To verify the converter operation, a 5-kW SiC-based prototype with a power density of 2.74 kW/dm3 was built and tested with an input voltage of 600 V. A high-efficiency performance over a wide output voltage range has been achieved, and the peak efficiency is 98.2% at the rated conditions.

High-Efficiency SiC-Based Isolated Three-Port DC/DC Converters for Hybrid Charging Stations

Power electronic transformer (PET) technology is one of the promising technology for medium/high power conversion systems. With the cutting-edge improvements in the power electronics and magnetics, makes it possible to substitute conventional line frequency transformer traction (LFTT) technology with the PET technology. Over the past years, research and field trial studies are conducted to explore the technical challenges associated with the operation, functionalities, and control of PET-based traction systems. This paper aims to review the essential requirements, technical challenges, and the existing state of the art of PET traction system architectures. Finally, this paper discusses technical considerations and introduces the new research possibilities especially in the power conversion stages, PET design, and the power switching devices.

Evolution of Power Converter Topologies and Technical Considerations of Power Electronic Transformer-Based Rolling Stock Architectures

A dynamic charging strategy with hybrid fast charging station for electric vehicles

In this paper, quarter-turn planar transformer structure is proposed and implemented in an LLC resonant converter for server power supply applications. Main power losses of high-frequency resonant converters are secondary-side winding loss and transformer core loss. Conventional transformer winding structures limit the minimum number of turns on the primary and secondary sides, such that the core size and secondary-side conduction loss cannot be reduced by increasing the frequency. To achieve high efficiency and high power density, a fractional-turn transformer structure can reduce core volume and conduction loss in the windings. The optimized transformer design also enables the turn ratio of the fractional-turn to achieve normal coupling of primary and secondary sides without generating flux imbalance. The effects of different fractional-turn windings on efficiency are examined, and Maxwell simulation results are used to verify the structure and operation of the planar transformer. The printed circuit board layout is analyzed using Q3D Extractor to reduce the uneven current generated by parasitic components on the secondary-side windings. Wide-bandgap switches are utilized to implement a resonant converter with 1-MHz operating frequency, 380-V input voltage, 12-V output voltage, 1-kW output power, 37.6-W/cm3 power density, and 97% maximum efficiency.

Quarter-Turn Transformer Design and Optimization for High Power Density 1-MHz LLC Resonant Converter

This paper presents a design procedure and loss estimation for a high-power, medium-voltage series resonant converter (entitled SRC#), intended for application in megawatt medium-voltage dc wind turbines. The converter is operated with a novel method of operation, entitled pulse removal technique, characterized by variable frequency and phase-shift modulation, below the resonant point of the LC tank. A step-by-step design procedure is proposed and circuit component ratings and efficiency are compared for two variants. The new method of operation reduces transformer size to below 50%, whereas losses are kept below 1.5% from zero to nominal power, due to soft-switching characteristics. An experimental setup, rated for 10 kW and 5 kV output was assembled to extract losses and validate the semiconductor loss model.

Design of a High-Power Resonant Converter for DC Wind Turbines

Medium and high frequency, high power transformers play an important role in footprint reduction along with their functions of galvanic isolation, and voltage transformation in all high power converters typically used in traction power systems, offshore wind plant power converters, and solid state transformer based distribution system grids. This state of art report analysis the various materials and design tradeoffs that govern the electromagnetic behavior and loss mechanisms of the medium frequency transformer applications. Typical winding and core geometries that have been reported extensively in the literature are described, and some design procedures and flow charts are analyzed including specific optimization routines. Estimation of core loss at high frequency using Steinmetz method and other modified methods are shown in detail. Thermal modelling including static and dynamic methods available in literature are put forward with references to thermal management methods. FEM analysis for electromagnetic behavior is described and couple of commercially available tools and their limitations are analyzed. Different challenges of relevance are included in different sections and brief comparisons are drawn. Design tools which are available is given a preview and limitations are drawn. A comprehensive literature survey was done and included in the paper in the reference section.

State of art survey for design of medium frequency high power transformer

Medium/high-frequency transformer is an integral part of many power conversion systems. Switching at higher frequency results in lesser volume of magnetics but induces higher winding loss density, on account of increased eddy current effects in conductors. Thus winding resistance is a key parameter to characterize performance of a medium-frequency (MF) highpower (HP) transformer. In this paper, 10 kW, 0.5/2.5 kV, 1 kHz transformer designs are presented employing different winding dispositions (normal and interleaved) and conductor geometries (foil, round). Dowells' and Ferreira's methods, which are computationally cost-effective among other analytical methods, are discussed to calculate winding resistance at higher switching frequency. Nature of ac resistance factor distribution among multi-layer winding is examined in detail and shown that with adaption of interleaved winding, eddy-current effects at higher switching frequency is minimized. Analytically calculated results are validated with FEA (2D) simulation. At medium frequency, with proper selection of core and winding geometry, very close agreement is achieved between analytical and FEA (2D) results, which substantiates aptness and computational efficacy of Dowell's and Ferreira's methods as well as aiding effect of interleaving to reduce ac resistance in multi-layer wining.

Accurate calculation of winding resistance and influence of interleaving to mitigate ac effect in a medium-frequency high-power transformer

A medium/high-power conversion system using power electronic (PE) converter along with a medium/high-frequency transformer, offers many desirable features that are beneficial for present-day power system topologies. Leakage inductance is identified to be one of the key parameters to characterize performances of such a medium-frequency (MF) high-power (HP) transformer. In this paper, existing analytical method to calculate leakage inductance of a concentric winding is further refined employing mean turn length of individual layer and winding porosity factor. Impact of interleaving to reduce leakage inductance is studied. Consequently different winding dispositions (normal and interleaved) and conductor geometries (foil, round) are investigated for a 10 kW, 0.5/2.5 kV, 1 kHz transformer design. Finally generalized method to deduce leakage inductance of fractional-interleaved winding is introduced in scope of this paper. Analytical calculations are corroborated with PEMag simulation results. With proper selection of wire and core geometry, good agreement between analytical and PEMag results is achieved at medium switching frequency, which substantiates aptness of presented methodology.

Accurate calculation of leakage inductance for balanced and fractional-interleaved winding in medium-frequency high-power transformer

A medium/high-power conversion system, using power electronic (PE) converter in conjunction with a medium/high-frequency transformer, has many desirable effects suitably oriented for modern power system architecture. Switching at high frequency results in lesser volume of magnetics but induces higher loss density. Thus design and characterization of a medium-frequency (MF) high-power (HP) transformer has significant ramification on its performance and application. Thermal management of a MF HP transformer is one of key aspects for its characterization. In this paper, an equivalent thermal model of a multi-layer concentrated winding is derived. Core and copper losses are carefully estimated. Thermal resistance and capacitance are accurately calculated. Time-domain response of proposed thermal network is obtained using Heun's method (Modified Euler) and validated with PLECS. Effects of temperature change on thermal properties of material and coolant (transformer oil) are also discussed. Furthermore, accuracy of said thermal network is corroborated through FEM-CFD study of a 10kW, 0.5/2.5kV, 1kHz natural oil-cooled transformer. Close agreement between analytical and simulation results is observed which substantiates proposed thermal model in terms of accuracy and efficacy of computation.

Sriram Vaisambhayana

Papers

Design of a High-Power Resonant Converter for DC Wind Turbines

State of art survey for design of medium frequency high power transformer

Accurate calculation of winding resistance and influence of interleaving to mitigate ac effect in a medium-frequency high-power transformer

Accurate calculation of leakage inductance for balanced and fractional-interleaved winding in medium-frequency high-power transformer

Thermal modeling and transient behavior analysis of a medium-frequency high-power transformer