The DC-DC converter serves as a core component in medium-voltage DC collection systems, undertaking crucial tasks such as voltage boosting, isolation and power transmission. An innovative half-bridge zero-current soft-switching medium-frequency converter is proposed, which utilizes an auxiliary transformer and is designed through the sharing of two switching devices by two half-bridge circuits. This converter employs a mixed-frequency modulation strategy, enabling the main circuit to operate in a medium-frequency state to undertake the primary power transmission tasks. Meanwhile, the auxiliary circuit utilizes the high-frequency chopping control technology to regulate the circuit’s power and voltage. Through a reasonable parameter configuration, the medium-frequency switching devices are ensured to achieve zero-current switching across the full-load range, thereby significantly reducing the switching losses and enhancing the system’s overall efficiency. The working principle for the converter is introduced in detail, and the design method for key parameters is analyzed. Finally, a 240 V/3 000 V/7 200 W prototype was developed to verify the performance of the proposed converter.

The four-switch Buck-Boost converter applied to a fuel cell system can theoretically achieve a higher efficiency under the traditional two-mode control. However, in fact, the dynamic performance of the system is poor due to the influence of a voltage gain blind zone in the process of transition between the two modes, which greatly reduces the operation stability of the converter. In high-power applications with large current, the interleaving technology is usually used to reduce the inductor current ripple and increase the power density of the converter. Therefore, the smooth mode transition of a four-phase interleaved four-switch Buck-Boost converter is taken as a control objective. First, the topology of the converter is introduced, and its basic mathematical relationship is obtained. Then, the reason for the voltage gain blind zone of the converter is analyzed using a graphical method, and a control strategy is proposed to eliminate the blind zone and realize the smooth mode transition based on the traditional three-mode control strategy. In the dynamic process, the changes in duty cycle are minimized and the inductor ripple current is also optimized, which improves the efficiency and stability of the converter. Finally, the effectiveness of the proposed control strategy was verified by experimental results.

To minimize the current stress of a dual active bridge (DAB) converter in its full power range, a global optimization strategy for the current stress under triple phase shift (TPS) control is studied. First, the operation principle and process of the converter under TPS control are analyzed, and a model of the relationship among its output power, current stress and phase-shift ratio is established. On this basis, the TPS control is optimized by seeking the optimal phase shift ratio combination with the goal of global minimization of current stress. In addition, the optimization results are compared with those under the dual phase shift (DPS) control in terms of current stress and reactive power. Finally, verification was performed by carrying out an experiment. The theoretical and experimental results show that the TPS control with the global optimization of current stress further reduces the current stress and reactive power in the full power range.

Due to the effects of the droop equation and equivalent impedance, the adoption of traditional droop control as a strategy for parallel-operated inverters will cause steady-state errors in the output voltage amplitude, which is not applicable to scenarios requiring high precision in voltage regulation. To solve this problem, a voltage amplitude compensation strategy based on hierarchical cooperative control for a system of parallel-operated inverters is proposed. The robust droop control is employed as the primary control instead of the traditional resistive droop method, which effectively constrains the amplitude error within a 5% margin and thus ensures the system’s capability to regulate voltage under different operation conditions. On the basis of analyzing the primary control error, the secondary voltage control is carried out utilizing a low-bandwidth communication system, and the voltage amplitude is tuned to the reference value to improve the compensation accuracy. Experimental results verified the feasibility and effectiveness of the proposed scheme.

Three-level inverters have attracted widespread attention in medium- and high-voltage applications. However, traditional continuous pulse width modulation (CPWM) methods suffer from drawbacks such as high switching losses, significant common-mode voltage (CMV) fluctuations and neutral-point voltage imbalance. To solve these problems, based on the analysis of the generation mechanism for CMV, the suppression conditions for CMV are derived based on discrete pulse width modulation (DPWM), and a modified DPWM modulation strategy is proposed, which can reduce switching losses under DPWM0~DPWM3 modulations while suppressing CMV. In addition, based on the proposed DPWM modulation strategy, the control method for neutral-point voltage is explored, and the multi-objective optimal control of the three-level inverter is realized ultimately. Simulation and experimental results verified the validity of the proposed DPWM control strategy.

Compared with the continuous pulse width modulation scheme, the discontinuous pulse width modulation (DPWM) scheme can effectively reduce switching losses and is more suitable for converters with a high switching frequency. However, as the switching frequency increases, the proportion of dead time in the switching period rises, significantly affecting the harmonic performance of the converter. A dead-time compensation method for Si/SiC hybrid three-phase three-level grid-tied converters with DPWM is proposed. When the bridge-leg voltage is high-frequency switching, the impact of dead-time on the switching state will be eliminated by offsetting the modulation wave. When the bridge-leg voltage is clamped, the comparison logic between the modulation wave and the carrier wave will be improved to avoid unnecessary switching actions in the clamped phase and ensure sufficient dead time during the transition of clamping modes. Finally, an experimental platform of a three-phase three-level active clamped converter was set up to verify the feasibility and effectiveness of the proposed dead-time compensation method.

Five-level inverters are widely used in the fields of energy storage and electric drive systems, especially in the high-voltage and high-power applications. As the power level increases, the capacity of one single inverter unit cannot meet the demand. Therefore, inverters need to be operated in parallel. Compared with the two-level or three-level topology, the circulating current between single-phase modules of parallel five-level inverters is more prominent due to the existence of floating capacitors. An improved current balance open-loop control method for a public DC bus structure is proposed. With a comparison of output current from each power module, a time stamp is set and the time shift control for vector switching is implemented, so that a positive or negative unbalanced current is artificially introduced to quickly eliminate the original circulating current. A set of 6 kV parallel high-voltage inverters were tested, and experimental results validated the effectiveness of the proposed method and the impact of voltage difference of the floating capacitor on the circulating current. The correction process only took 0.1 ms, and the current balance can be quickly and accurately completed.

To address the problems of lower power density and fewer application scenarios of a traditional multi-stage AC-DC bidirectional converter, a novel AC-DC bidirectional converter with adjustable frequency output on AC side is proposed. A matrix frequency conversion circuit is used instead of a full-bridge circuit, and the two working mode control methods of rectification and inversion are combined to realize a single-stage AC-AC frequency conversion function of the converter without DC links, so that the converter can add a flexible energy outgoing function of new energy electric vehicles on the basis of satisfying the vehicle-to-grid energy interaction strategy. At the control level, when the converter works in an inversion mode, proportional resonance control will be used to obtain lower total harmonic distortion and adjustable frequency characteristics, so as to cover more AC-side electrical requirements. When the converter works in a rectification mode, the average current control based on CCM mode can ensure a high value of power factor on the AC side and a stable voltage output on the DC side. Finally, a MATLAB/Simulink simulation model and a hardware experimental platform were built to verify the scheme, and results proved the correctness and effectiveness of the proposed topology.

The modeling and stability analysis methods for a fractional-order Cuk converter closed-loop system are proposed. First, the transfer function of a PWM-controlled fractional-order Cuk converter closed-loop system is analyzed, and the nonlinear links of its control system are modeled to obtain a describing function model for the fractional-order Cuk converter closed-loop system. Then, the effect of fractional-order component order on the stability of the converter closed-loop system is analyzed using an improved Nyquist stability criterion. Finally, the accuracy of the modeling and stability analysis methods for the fractional-order converter closed-loop system was verified by simulations and an RT Box platform.

With the continuous increase in new energy penetration, the impedance interaction between a grid-connected inverter (GCI) and a weak grid is easy to induce small-signal oscillation instability. To solve this problem, the limitations of the existing voltage feedforward impedance reshaping strategy are analyzed in this paper at first. Then, an adaptive voltage feedforward impedance reshaping strategy is proposed. This strategy utilizes simulated anneal-particle swarm optimization algorithm to optimize the feedforward path parameters off-line under sample conditions of different values of output power and grid impedance, and polynomial fitting is used to cover the continuous full operating range. By deploying the fitting relationship in the inverter, the feedforward path parameters can be adjusted adaptively according to real-time power and grid impedance. The proposed adaptive strategy can ensure a better stability margin of GCI under conditions of different values of output power and grid impedance, and the dynamic performance of the system is considered to some degree. Finally, experiments were carried out on an OPAL-RT semi-physical experimental platform to verify the effectiveness of the proposed strategy.