In high-power DC transformer systems, large-capacity medium-frequency transformers serve as critical components for energy conversion and electrical isolation, with their electrical performance and structural design directly influencing overall system efficiency and reliability. To address the challenges of increased losses, localized heating, and insulation stress under medium-frequency, non-sinusoidal excitation, this paper presents the optimized design and prototype development of a 4.1 MVA, 1 kHz MFT. The study focuses on material selection and structural refinement, including the use of amorphous alloy cores, copper foil windings, insulation configuration, and thermal management. Based on the proposed design methodology, a dry-type, high turns-ratio MFT prototype rated at 1.5 kV/10.5 kV, 4.1 MVA, and 1 kHz was successfully developed. Experimental results show that the transformer achieves an efficiency exceeding 99.2% under full-power operation, demonstrating the effectiveness of the proposed design approach.

This paper investigates the loss behavior of the clamping circuit in a high step-up resonant DC transformer (IGCT-SRDCT) based on integrated gate commutated thyristor (IGCT) and series-connected diodes. The proposed topology features a simple structure, high voltage gain, and high efficiency, making it highly suitable for medium-voltage DC collection in renewable energy systems. Due to the zero-current switching (ZCS) characteristics of the IGCT-SRDCT, the clamping circuit is triggered only during the zero-current turn-on of IGCT, where it suppresses the currents caused by the parasitic junction capacitor of the IGCT and diode. A theoretical analysis is conducted to examine the triggering behavior and energy dissipation of the clamping circuit under varying load current rise rates (di/dt). A mathematical model is developed based on the peak current of the clamping circuit to estimate the clamping losses. The analysis reveals that increasing di/dt following zero-current turn-on of IGCT significantly reduces clamping loss. A prototype of the 1.5kV/30kV/10MW IGCT-SRDCT was developed to validate the proposed theory, demonstrating that the clamping loss is limited to only 72 W.

With green hydrogen projected to supply over 60% of global hydrogen production by 2050, large-scale alkaline electrolyzers (>5 MW, 1000 Nm3/h) require highly efficient and scalable power conversion. MCSCs, featuring single-stage buck-type operation and low-harmonic output, outperform conventional two-stage VSC-based solutions. However, under carrier phase-shifted pulse width modulation (PWM) with large module counts, conventional zero-vector substitution schemes may result in divergent current behavior between submodules (SMs), posing risks of current imbalance and increased switching frequency. To address this critical challenge, a Global Zero-Vector Substitution (GZVS) method is proposed, which extends zero-vector substitution from a single SM to a pair of SMs selected based on current sorting and comparison results, thereby ensuring current convergence for both current polarities. Experimental results on a MCSC prototype demonstrate the effectiveness and scalability of the proposed strategy. It enables stable operation with 18 SMs while ensuring current balancing. An DC output current of 19kA/6.65MW is achieved with less than 3% total harmonic distortion (THD) on the AC side, paving the way for the industrial deployment of next-generation green hydrogen production systems.

A power electronics transformer (PET) based on dual active bridge (DAB) with input series and output parallel (ISOP) cascaded topology is studied in this paper. Fe-based nanocrystalline alloys transformer cores and Silicon Carbide (SiC) power MOSFETs are adopted to improve the power density and efficiency. Mathematical derivation of the input voltage distribution is deduced with the conclusion that the voltage equalization cannot be achieved autonomously. A feedback control method is proposed to balance the input voltage. The measurement of the dynamic B-H loops of the nanocrystalline core under the excitation of square wave and sinusoidal wave is conducted. The high frequency isolation transformer (HFIT) is designed based on the loss performance calculated using the dynamic B-H loop and Ohm's law. The optimized operating flux density is around ±0.68 T at 20 kHz, which is much higher than the empirical value around ±0.3 T to take full advantage of the nanocrystalline core. Finally, a 13.2 kVA prototype is built to verify the above analysis and design.

This paper presents a method to analyze the dynamic physical limits of a phase-shifted full bridge (PSFB) circuit used for the power supply of Magnetic Resonance Imaging (MRI) gradient amplifiers. Based on the time-optimal control theory, the detailed optimal transient response process under a sudden load current change is analyzed considering the sample time delay, the minimum voltage undershoot and the minimum recovery time are calculated. The analytic results are validated by simulations and experiments on a 120 V/30 A PSFB converter.