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.

The series-connected hybrid diode rectifier (SCH-DR) based high voltage direct current transmission system (HVdc) for integrating offshore wind farm (OWF) confronts challenges in startup, which comprises of a largecapacity DR and a small-capacity auxiliary modular multilevel converter (A-MMC) in series-connection. To achieve DR’s inputting onto OWF’s grid in zero dc current while maintaining energy balance of offshore system, this study proposes a startup strategy based on coordinated energy balance method. Based on this method, OWF needs to generate certain active power to meet consumption of offshore necessary auxiliary devices. To dissipate the surplus power, the proposed method parallels an energy-consuming resistor on AMMC’s dc side. Hence, through its adjustable dc voltage capability, A-MMC can actively and smoothly regulate the dissipated power, thus realizing the coordinated energy balance. The proposed startup strategy exhibits feasibility and engineering practicality, without much escalating investment cost or space requirements. The effectiveness of the proposed startup strategy is verified in MATLAB/Simulink using a scenario of a 2000 MW/±500 kV SCH-DR-HVdc system.

Hybrid DC circuit breakers (HDCCBs) are among the most promising solutions for DC breaking. HDCCBs based on forced commutation have the shortcomings of conduction loss or high cost, which hinders their large-scale application. HDCCBs based on natural commutation, despite their lower cost and smaller volume, are limited by the arc voltage. Therefore, this paper proposes a type of multibreak mechanical switch applicable for medium-voltage natural commutation HDCCBs. Possessing the advantages of compact size and high arc voltage, the switch consists of a four-break gas chamber and a vacuum chamber, the former of which realizes the series connection of multiple arcs through a bridge contact structure. The vacuum chamber is used to bear the turn-off overvoltage. Combined with high-speed observation methods, the mechanical characteristics and arc characteristics of the multibreak mechanical switch are analyzed, and the influence mechanism of different factors on the arc characteristics of the four-break chamber is revealed. On this basis, a principle prototype of the 15 kV medium-voltage natural commutation HDCCB was developed, and breaking tests were carried out. The breaking current can reach 15 kA, and the breaking time is less than 3 ms.