MW-level medium-frequency transformer (MFT) is crucial for the efficient transmission of large-scale renewable energy. But under the combined constraints of MW-level power, high current, and several-hundred Hz operation, traditional design approaches cause elevated losses, which restricts overall efficiency. To enhance the energy transfer efficiency of MFT, this study proposed a loss-optimized design methodology tailored for MW-level MFT. The approach combines material selection and structural optimization across three main subsystems: magnetic core, winding, and cooling system. A silicon steel core joint structure with a diagonally staggered lamination is designed to homogenize the magnetic density at the corner and reduce the local core loss. Copper foil windings are employed to handle the kA-level current, with the thickness and layer count optimized to suppress eddy current. Based on the leakage magnetic field distribution, the structural dimensions and layout positions of the cooling plate is optimized to reduce additional eddy current losses. Based on the proposed methodology, a 1.5 MVA / 600 Hz MFT prototype was developed using ultra-thin grain-oriented silicon steel and copper foil winding. Experimental results demonstrate a full-load efficiency of 99.57% and a hotspot temperature below 91 °C.

This article proposes an improved trapezoidal modulation (ITM) strategy to achieve zero voltage switching (ZVS) in three arms of a series-connected device-based dc transformer equipped with lossless buffer capacitors. To overcome the challenge of achieving ZVS in arms 1, 2, and 4 under light-load and no-load conditions, the ITM introduces an auxiliary phase-shift angle. By analyzing current stress under various power levels and voltage mismatch conditions, a precise calculation method for the auxiliary phase-shift angle is proposed to minimize reactive current. Due to the asymmetry of the topology, a natural power reversal method is further developed to prevent the failure of ZVS in arm 4 during power direction changes. Finally, a 10 kV/1.5 kV 3 MW prototype has been built and experimentally validated using a back-to-back test platform. The results confirm that all buffer-capacitor-equipped arms maintain ZVS over the full power range, and that the voltage balancing across series-connected devices remains within +/- 5%. The proposed solution is designed for medium-voltage dc distribution networks and features a simple topology with a peak efficiency of 98.3%.

The Dual Active Bridge (DAB) dc-dc converter is a critical component in modern high-power bidirectional energy systems, such as solid-state transformers and electric vehicle chargers. However, its control performance is often compromised by the strong nonlinearity of power transfer characteristics and the sensitivity of system dynamics to operating points. Traditional proportional-integral (PI) controller tuning methods, which rely on linearized models at a single nominal point or exhaustive trial-and-error, frequently fail to deliver optimal transient responses across the full operating range. This paper proposes a robust, model-driven intelligent tuning framework for DAB voltage control loops. We first derive a control-oriented second-order small-signal model that accurately approximates the dominant dynamics of the DAB under Single Phase Shift (SPS) modulation. Subsequently, an Improved Grey Wolf Optimizer (IGWO) is developed to automate the search for optimal PI parameters. The IGWO algorithm distinguishes itself through two key enhancements: (1) Logistic chaotic mapping for population initialization to prevent premature stagnation in local optima, and (2) a nonlinear convergence factor to dynamically balance global exploration and local exploitation. A composite objective function combining the Integral of Time-weighted Absolute Error (ITAE) with a rigorous squared overshoot penalty is formulated to ensure fast settling times with zero overshoot. Extensive numerical validations in MATLAB demonstrate that the proposed IGWO-based tuning significantly outperforms both manual tuning and standard GWO in terms of convergence speed, solution quality, and closed-loop robustness.

Full-power and thermal stability experimental validation of megawatt-level medium-frequency transformers (MFTs) used for dc transformers, especially those with voltage conversion ratios exceeding 1:20, remains a major technical challenge in the laboratory due to the lack of suitable verification topologies and methods. This letter proposes a novel topology and modulation method for experimentally verifying the 1.5 kV/33.4 kV 4.1 MVA MFTs using only a low-voltage small-capacity dc power, without injecting megawatt-scale power into the grid. The proposed topology ensures soft switching for all power semiconductors, thereby reducing switching losses and improving efficiency. In addition, detailed power control and modulation methods have also been proposed. Experimental results demonstrate the effectiveness and feasibility of the method proposed in this letter. Thus, the steady-state behavior of high-ratio megawatt-level MFTs can be verified in a laboratory setting.

The modular multilevel converter (MMC) is widely deployed in medium-voltage dc (MVDC) and high-voltage dc (HVDC) systems but suffers from high cost and bulky submodule capacitors. Voltage source converters (VSCs) offer a compact and cost-effective alternative but require voltage balancing control for series-connected devices in MV/HV high current applications with significant stray inductance. This work advances existing active clamping module (ACM) to self-powered, bypass-capable modular switches (MS) and proposes a dynamic clamping arm (DCA) control for series-connected MSs, enabling soft-switched recovery of turn-off energy and low-bandwidth sorting-based voltage balancing. The commutation process under DCA control is discussed in detail. Analytical models quantify voltage balancing modes and derive design constraints for key parameters of a 10kV/5MW prototype. Performance comparisons of the proposed scheme with MMC and existing balancing methods are discussed. A phase unit of the prototype, employing eight series-connected 1.7kV IGBT-based MSs per arm, is tested in a power-cycling setup. The experimental results demonstrate robust voltage balancing and minimal auxiliary switch stress, confirming the effectiveness of DCA control and its feasibility in MV/HV VSCs.

The dual active bridge converter is promising for dc systems since it provides galvanic insolation and high efficiency. In most situations, the full-bridge ac output and dc voltage are observed, while overlooking the possible high-frequency voltage oscillations (HFVOs) at the transformer port. HFVOs threaten the insulation of transformers. The causes of HFVOs have not yet been clarified. This article finds that HFVOs are caused by the harmonics within the series-resonance frequency band, rather than the single frequency. Moreover, existing solutions mostly rely on optimizing transformer winding structures, which causes a tradeoff between HFVOs, efficiency, and insulation performance. To address this issue, this article proposes a modulation method that reduces HFVOs by optimizing the output harmonic content in the resonance band. The root mean square current of the inductor is also optimized simultaneously to ensure efficiency. The proposed strategy is validated on a 12 kW/50 kHz/750 V prototype. Compared to the method that only optimizes the RMS current, the proposed one achieved the optimal RMS current while reducing voltage oscillations by 77%.

The variable and unpredictable nature of renewable energy creates operational difficulties for off-grid hydrogen production systems seeking economic viability. To address this issue, this research introduces an innovative sizing methodology for off-grid hydrogen production systems, integrating probabilistic scenario analysis and load-follow-source coordination control technique. First, a scenario generation method is employed to identify representative operational scenarios of renewable energy. Second, a load-follow-source control strategy considering the multimodal conversion of alkaline electrolyzers and proton exchange membrane electrolyzers is developed. Subsequently, a optimization model of capacity configuration is formulated, with the objective of minimizing the levelized cost of hydrogen (LCOH), maximizing system operation efficiency, and minimizing the rate of abandoned for renewable energy. An improved generalized normal distribution optimization algorithm is introduced to solve the optimization model. Compared with the existing methods, the load-follow-source strategy proposed in herein achieves a higher hydrogen production rate and has better multi-mode operation flexibility. Meanwhile, the capacity optimization configuration method demonstrates good economic and efficiency performance, that is, the LCOH and the rate of abandoned are the lowest, reaching 19.4 ¥ /kg and 4.5% respectively, and the system operation efficiency is the highest at 89.9%. The researches furnish foundational principles for orchestrating autonomous hydrogen synthesis systems.

As power grids transition toward power-electronics-dominated systems, voltage-source-converter-based high-voltage direct current (VSC-HVdc) systems are increasingly required to provide grid support services, creating an urgent demand for robust short-term overload capabilities in converters. However, conventional solutions relying on global component oversizing result in substantial increases in system cost and volume. To address this challenge, this paper proposes a staged commutation strategy for the modular commutated converter (MCC), in which only a limited number of H-bridge modules bear the initial turn-off stress, while the majority of the remaining modules are turned off after the arm current decays to zero, thereby forming a “quasi-soft” commutation characteristic. In this way, significant short-term overload capability can be achieved without introducing global hardware redundancy. The commutation timing and transient process are analyzed, and key design constraints are derived. The strategy is validated on a ±5 kV/20 MW engineering-scale MCC prototype, demonstrating stable operation under 3.0 p.u. overcurrent for 500 ms. This work provides a practical commutation-level control solution for MCCs requiring short-term overload capability, facilitating their engineering application in future VSC-HVdc systems.

The rapid dissipation of instantaneous surplus energy during AC faults is critical for ensuring fault ride-through (FRT) capability in photovoltaic (PV) medium-voltage direct current (MVDC) systems. This article introduces an integrated scheme that combines active energy recovery with fault current limiting. The proposed approach first ensures fault current limitatio n in the grid-forming converter through power outer-loop blocking and voltage reference adjustment. Following this, the reserve for active energy recovery via the modular multilevel converter (MMC) is evaluated and enhanced through a margin enhancement strategy, thereby demonstrating the feasibility of utilizing the MMC's submodule (SM) capacitor voltage margin. A surplus power recycling control strategy is further proposed to enable efficient recovery of surplus power during grid faults and its smooth release after fault clearance. Finally, the effectiveness of the overall strategy is validated through MATLAB/Simulink simulations under various low-voltage ride-through (LVRT) scenarios induced by AC faults.

Voltage source converters (VSC) with series-connected devices offer a compact and low-cost alternative to modular multilevel converters for medium voltage (MV) applications but suffer from excessive dv/dt. While quasi-two-level (Q2L) operation mitigates dv/dt, it imposes complexity in both circuitry and control to prevent oscillatory overcurrent. This letter proposes an oscillation-free dv/dt suppression control for MV VSCs with series-connected modular switches. With a simple fixed-timing control scheme, output voltage stepping is decoupled from linear arm commutation, mitigating oscillations while preserving voltage balancing, modularity, and scalability. Experiments at 5 kV DC validate the targeted dv/dt suppression with minimized module capacitance among existing solutions.

For large-capacity medium-frequency transformers, open-circuit testing is a critical method for evaluating magnetic characteristics and insulation performance. However, conventional sinusoidal excitation sources are insufficient for testing under non-sinusoidal conditions. When conducting open-circuit tests using an H-bridge square-wave excitation, voltage oscillations tend to occur at the primary terminals, compromising the accuracy of transformer performance assessment. This paper first reveals the mechanism of such oscillations based on experimental waveforms, identifying the source as a series resonance loop formed between the transformer’s magnetizing inductance and parasitic capacitance. Accordingly, a damping injection method is proposed, in which a small-value resistor is inserted between the square-wave source and the transformer to increase the damping and suppress high-frequency oscillations. Experimental comparisons of voltage and current waveforms under various voltage levels (200、500V) demonstrate that the proposed method significantly reduces voltage oscillation—amplitude decreases by 30%, and oscillation duration shortens by over 90%—thus validating its effectiveness.

This letter proposes a novel modular commutation converter with high-overload capability for high-voltage dc transmission. The combination of single-switching-device modules (SDMs) and high-switching SDMs exploits the soft-switching characteristics of the topology and the high surge-current capability of integrated gate-commutated thyristors, enabling the converter to achieve short-term high-overload capability at reduced cost and volume. The topology, operating principle, and device characteristics are presented in detail. A megawatt-level engineering prototype is developed, and experimental results demonstrate the feasibility of the proposal.

This paper presents an IGCT-MMC submodule design based on diodes in parallel connection applied in modular multilevel converter valve. In contrast to traditional MMC submodule design based on single diode, the proposed arrangement with diodes in parallel connection reduces the anode inductance and clamping capacitance which leads to a compact design of IGCT submodule. By analyzing diode electrical characteristics and thermal impact to current sharing performance, proposes the design principle of diodes in parallel connection. IGCT-MMC submodule prototype is developed based on diode in parallel, the pulse test of power device and power cycling test are performed on the IGCT submodules. Safety switching capability of power devices, and long-term operation capability of IGCT submodule are verified as well.

A 1.5kV/30kV/10MW series resonant DC transformer based on integrated gate commutated thyristors and medium frequency isolation (IGCT-SRDCT) is proposed and implemented for Solar-PV DC collection application. The IGCT-SRDCT improves the input current capability by using IGCTs and boosts the output voltage to 30kV through series-connected diodes with buffer capacitors and a cascaded structure comprising three power units. The design guidelines and parameter specifications for the IGCT-SRDCT prototype are presented. Subsequently, the additional resonant effects induced by buffer capacitors on the operating characteristics of the IGCT-SRDCT are analyzed. An optimized dead time is proposed to ensure the zero current turn-on of the IGCT, and the internal phase shift angle is used to suppress overvoltage under ultra-light load. Finally, five IGCT-SRDCT prototypes were implemented and experimentally verified for a 50MW Solar-PV DC collection application. Long-term power tests demonstrate the reliability and stability of the IGCT-SRDCT, achieving a system efficiency of over 98.7% at loads above half capacity and a peak efficiency of 99.02%.

With the rapid development of flexible DC transmission technology, the increasing capacity and voltage level of the grid puts forward higher requirements on bridge arm reactors (BARs) in terms of power density, magnetic field control, and structural optimization. Traditional cylindrical air-core reactors (CARs) limit their feasibility in high-capacity applications due to the large magnetic field radiation range. In order to improve the reactor performance and reliability, this paper proposes a new type of annular air-core reactor (AAR). The design optimizes the magnetic field distribution through the structure and significantly reduces the reactor size, while increasing the power density and reducing the magnetic field radiation. Based on the finite element method (14,M), this paper presents a multi-physics field analysis of the force and acoustic fields, and the results show that the AAR achieves significant improvements in both magnetic field confinement capability and volume reduction compared to the conventional CAR In addition, the mechanical characterization shows that the AAR has superior structural stability and vibration resistance, which can effectively reduce the vibration and noise levels. The research in this paper provides a new theoretical basis and engineering solution for the design of reactors in large-capacity flexible DC transmission systems.

To address the challenges posed by the integration of large-scale new energy into the power grid, this paper conducts a fault analysis of a 100 MW-scale high-voltage direct current transformers (HDCTs) applicable to all-DC transmission renewable energy systems. Focusing on faults such as high-voltage side DC bipolar short-circuit through a smoothing reactor, high-voltage side single bridge arm short-circuit through a bridge reactor, and high-voltage side AC outlet short-circuit. The dynamic characteristics of the fault were theoretically analyzed and verified through PSCAD simulation. The main findings indicate that the protection setting value(lambda) and the blocking delay time(Delta t(delay)) directly determine the magnitude of the bridge arm sub-module blocking current. Longer delay or higher protection setting values result in larger blocking currents, which may exceed the turn-off capability of power devices (e.g., the IGBT turn-off peak current is 1.6 kA). In the case of single bridge arm short-circuit through a bridge reactor, due to the direct injection of current from the bridge arm into the fault point and complex current paths, the maximum blocking current reaches 1.972 kA (when the protection setting value is 1.2 pu and the delay is 100 mu s), posing the greatest challenge to the IGBT turn-off capability. It is recommended to enhance the withstand capability through valve tower structure optimization (such as layout and heat dissipation design).

Active Neutral-Point Clamped (ANPC) converters have been extensively applied to wind power, solar power, grid interconnection of new energy sources, and other fields. ANPC converters with Integrated Gate Commutated Thyristors (IGCTs) characterized by high current-carrying and voltage withstanding capabilities will face a significant current change rate in the circuit during the commutation process. This high di/dt induces substantial overvoltage across the stray inductance in the commutation loop, posing challenges to the safe operation and cost control of the equipment. Hence, there is an urgent need to investigate the overvoltage induced by stray inductance during commutation. This paper first demonstrates the principles for calculating stray inductance in ANPC converters. Next, using the finite element simulation software ANSYS, the paper extracts the partial inductance matrix of the commutation loop and the associated stray inductance for a directly series-connected ANPC three-level converter. Based on this stray inductance analysis, a simplified circuit model of the ANPC commutation circuit taking into account stray inductance is developed. Later on, this paper analyzes the transient behavior during the ANPC circuit’s turn-off process on the basis of the simplified commutation circuit, and calculates the overvoltage spikes caused by stray inductance. Finally, a double-pulse simulation circuit is constructed on the PSCAD platform to validate the analytical and calculation process, as well as the accuracy of the simplified circuit model through simulation.

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.

Large-capacity DC hydrogen production converters stand as a core equipment for large-scale DC hydrogen production systems using renewable energy. To address the fluctuations of wind and solar energy and meet the diverse demands of large-scale hydrogen production, this paper introduces a novel architecture for large-capacity DC hydrogen production converters with multi-electrolyzer hybrid connections. The study analyzes the architecture’s typical topology, operational mechanism, and key parameter design methodology. Based on multiple resonant cavities and a multi-channel independent output structure, this architecture integrates medium-frequency inversion with non-isolated DC/DC cascading to achieve wide-range, low-ripple, and high-efficiency power supply to various types of electrolyzers. At last, the effectiveness and feasibility of the proposed scheme are validated in a hybrid system simulation model incorporating multiple loads.

Integrated Gate Commutated Thyristor (IGCT) is widely used for its advantages such as high surge current ability and high energy capacity. However, because IGCT is current-controlled device, many voltage balance methods are not applicable for IGCT. This article proposed a novel method for IGCT voltage balance achieved by adjusting the turn-off moment of device when used in line commutated converter (LCC) to cut off current. The strategy design methods are showed in the paper, explained how to apply the novel strategy on series IGCTs. Then, a 4 cascaded IGCT experiment platform is designed to verify the effect of the proposed method. The strategy performed an exciting effect on voltage balance, which supports the strategy to be used in real equipment.