An actively cooled high power, high frequency transformer with high insulation capability for use in a high power multilevel converter is discussed. The transformer is designed for a power level of 350 kW and is realized with amorphous core material and coaxial windings. Special attention is paid to the insulation problem, since the dielectric losses and the influence of the voltage waveform with very steep edges have to be investigated more in detail to guarantee a long lifetime of the device.

An actively cooled high power, high frequency transformer with high insulation capability

Solid state transformer (SST) exhibits good features such as high power density, small volume and weight, controlled power factor, voltage sag ride through, etc. compared with traditional line frequency transformer. The 7.2kV AC to 120V/240V AC 20kVA solid state transformer is a key component of the future renewable electric energy delivery and management (FREEDM) systems as the interface between the 7.2kV distribution grid and the low voltage residential micro-grid. Three cascaded 6.7kVA high-voltage high-frequency transformers operating at 3kHz are employed to convert voltage from 3800V high voltage DC link of each cascaded stage to 400V low voltage DC link. The transformer is required to withstand at least 15kV high frequency voltage insulation continuously. Transformer magnetic core materials were reviewed and compared. Winding layout alternatives for leakage, magnetizing inductance and insulation were compared. An insulation strategy based on split core and separate winding structure with inserted insulation layer between the C cores was proposed. One 6.7kVA high voltage high frequency transformer prototype was built and the test results were reported.

High-voltage high-frequency transformer design for a 7.2kV to 120V/240V 20kVA solid state transformer

With the growing adoption of electric vehicles (EVs), there is a pressing need for constructing charging infrastructures. In this context, wireless charging technology has been under development for the past few decades. Dispensed with awkward plugs and wires, wireless power transfer (WPT) technology is very safe, convenient and easy-to-use. Researchers have taken sustained efforts to make it an improved technology and automakers also work to provide the wireless charging option for their customers. In this paper, the basic principles of resonant inductive power transfer that is common-used for wireless electric vehicle charging (WEVC) are elaborated. Then, with a different emphasis on WEVC technologies, the key issues in academia and industry are discussed respectively. The core technologies of a fully-function WEVC system in academia are summarized and a comparative study is conducted among the selected WEVC industry standards. In addition, based on the possible technical development and currently valid policies, the author envisions the future of WEVC, followed by some conclusions and helpful advice.

The state-of-the-arts of wireless electric vehicle charging via magnetic resonance: principles, standards and core technologies

Silicon carbide (SiC) is more favorable than Silicon (Si) to build high voltage devices due its wider band-gap and higher critical field strength. Especially, the SiC MOSFETs are finding their niche in 1 kV range, which is currently dominated by Si IGBTs. This paper aims at demonstrating high power and high frequency operation of the SiC MOSFETs, as a means to evaluate the feasibility of using SiC MOSFETs for high power density applications. The sample devices chosen for this study are 1200 V, 20 A, SiC MOSFETs co-packed with 10 A JBS diodes — manufactured by the CREE Inc. A dual active bridge (DAB) converter has been built to validate the suitability of SiC devices for high power density converters. The design details of the DAB hardware, and the high frequency transformer used for interfacing both the bridges are given. Experimental results on the DAB at 100 kHz switching frequency are presented. Finally, the device switching waveforms up to 1 MHz are given.

High-frequency design considerations of dual active bridge 1200 V SiC MOSFET DC-DC converter

This paper proposes a three-port three-phase bidirectional DC-DC converter suitable for high-power applications. The converter combines a slow primary source and a fast storage to power a common load (e.g., an inverter). Since this type of system is gaining popularity in sustainable energy generation systems and electrical vehicles, the proposed topology is of practical interest The proposed converter consists of three high-frequency inverter stages operating in a six-step mode, and a high-frequency three-port three-phase symmetrical transformer. The converter provides galvanic isolation and supports bidirectional power flow for all the three ports. An arbitrary power flow profile in the system can be achieved by phase shifting the three inverter stages. Thanks to the three-phase structure, the current handling capability of the circuit is larger and the ripple currents at the dc sides are much lower owing to the interleaving effect of the three- phase, and thus the VA rating of the filter capacitors is much lower. The operating principle and, in particular, the transformer design which is based on conventionally and coaxially wound structures are presented. Circuit simulation results are included to verify the proposed converter topology and the dual-PI-loop control strategy.

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High-Power Three-Port Three-Phase Bidirectional DC-DC Converter

Previous papers on coaxial winding transformers have focused on designs that use a copper tube to form a single- turn outer winding, and litz wire for a multiple-turn inner winding. In high-power applications, the increased resistance of a solid outer winding due to skin effects can be a limiting factor in achieving good performance, especially at frequencies above several hundred kilohertz. A single-turn outer winding leads to large core cross section areas at lower frequencies and the turns ratio is limited to a ratio of &#x1d4c3;: 1, where &#x1d4c3; is an integer. This paper demonstrates several methods for achieving multiturn outer windings to improve design flexibility for the coaxial winding transformer. Experimental measurements on transformers that have multiturn outer windings are included to confirm analytical results and demonstrate the modified designs.

Robert R. Bacon

Papers

Multiturn High-Frequency Coaxial Winding Power Transformers