Dual-active-bridge (DAB) converter is an attractive topology to be used as a dc transformer (DCT) in the dc power distribution system. Because the voltage gain for dc transformer is relatively fixed, single-phase-shift (SPS) control is popularly adopted for the DAB converter to achieve simple power control. Aiming at increasing the light load efficiency of high-frequency DAB converter under SPS control, this letter proposes a concept to achieve full load range zero-voltage-switching (ZVS) with minimum rms current in certain voltage variation range. In the proposed method, the added magnetizing inductance creates the possibility to achieve ZVS for all power switches. The voltage gain of the converter is specially controlled by turns ratio, which is no longer the conventional unity voltage gain condition. The key parameters optimization, including the turns ratio, magnetizing inductance, and leakage inductance of the transformer, is introduced based on ZVS and current stress analysis. A 250 kHz, 400/48 V, 1000 W GaN-based DAB prototype is built, which shows performance improvements at light load.

It is a challenge for dual-active-bridge (DAB) converters to take a good balance between full-load-range zero-voltage switching (ZVS) and control complexity with a wide voltage range. Dual-phase-shift (DPS) control can extend the ZVS range greatly compared with single-phase-shift (SPS) control, and it has fewer control variables with reduced operation modes compared with triple-phase-shift (TPS) control. However, with conventional DPS control, at least a pair of switches will be operated under hard switching within a large power range, which severely reduces the efficiency, especially at the high switching frequency. To solve this problem, based on the Lagrange multiplier method (LMM), an optimized DPS control utilizing magnetizing current is proposed. In this article, the magnetizing inductance is designed to create the possibility of full-load-range ZVS achievement for all power switches. Based on ZVS analysis considering switch junction capacitors and power characteristics analysis, a power control law premised on full-load-range ZVS achievement is proposed to achieve low current stress. Finally, the effectiveness of the proposed solution is verified by experimental results obtained with a 500-W, 250-kHz gallium–nitride (GaN)-based prototype.

The modular multilevel converter (MMC) is attractive for medium or high-power applications because of the advantages of its high modularity, availability, and high power quality. Reliability is one of the most important challenges for the MMC consisting of a large number of submodules (SMs). The capacitor monitoring is one of the important issues in the MMC. This paper proposed a reference submodule (RSM)-based capacitor monitoring strategy for the capacitance estimation in the MMC, where the capacitances in the monitoring SMs can be estimated based on the capacitor voltage relationship between the RSM and the monitoring SMs. The proposed monitoring strategy does not rely on the information of all capacitor voltage and current, which effectively simplifies the algorithm for capacitance estimation. The simulation studies with the time-domain professional tool PSCAD/EMTDC are conducted and a down-scale MMC prototype is also tested in the laboratory with the proposed capacitor monitoring strategy. The study results confirm the effectiveness of the proposed capacitor monitoring strategy.

The frequency-domain passivity theory offers an effective way to assess the stability of inverters connecting to complex grid conditions, e.g., inductive grid, capacitive grid, and system comprising multiple inverters. In this article, an admittance model for the grid-side current-controlled LCL-type inverter with capacitor voltage feedforward active damping (CVF-AD) is built to facilitate the passivity-based stability assessment and controller parameter design. Especially, it is found that if the antiresonance frequency of the inverter-side inductor and the filter capacitor is designed in a certain region, i.e., (0.66f _c + 0.034f ₈ , 0.22f ₈ ), all frequencies' passive output admittance of the inverter can be achieved via a modified CVF-AD scheme with simple digital implementation. Thanks to the proposed methods, the inverter can be connected to the grid regardless of the grid impedance, which means a robust plug-and-play functionality suitable for microgrid applications. In addition, the controller parameters are analytically designed; time-consuming iterations are, hence, avoided. The allowable tolerance of power circuit parameters for a valid controller parameter design is as high as 64% or more. The validity of the theoretical analysis and effectiveness of the proposed method are verified by using experimental results on a laboratory prototype.

Operation efficiency is one of important concerns for power conversion system. Efficiency issue of microgrids with multiple paralleled inverters has been paid slightly concerns. Therefore, this paper presents an efficiency analysis method of microgrid in system level and an efficiency-prioritized droop control strategy to improve operation efficiency of microgrids under different load profiles. Efficiency model of microgrid is first established to reveal time-varying efficiency characteristic under different load profiles. Then, optimum solution of efficiency model is derived by Lagrange Multiplier Method, and optimum conditions to capture maximum efficiency are obtained. Furthermore, an efficiency-prioritized droop control strategy is proposed to improve operation efficiency according to the established efficiency model. In addition, stability analysis of the proposed droop controller is performed, from which the guideline for controller design is given. Simulation and experimental results are provided to validate the proposed efficiency-prioritized droop control strategy. The proposed droop control strategy is able to improve overall efficiency of microgrid under different load profiles, which also preserves advantages of conventional droop control strategy.

A repetitive controller (RC) is one of the most promising candidates for harmonic compensation when designing the current controller of grid-connected inverters. The discrete-time closed-loop transfer function based frequency response method can be used to shape RC, which is applicable for a single inverter connecting to well-known inductive-impedance grids, but cannot guarantee the stability in the case of capacitive-impedance grids or complex systems, such as microgrids. To address this issue, this article presents a new passivity-based design method for an RC for LCL-type grid-connected inverters with either inverter-side or grid-side current control. With the proposed design guidelines, the output admittance of the RC-controlled inverter is tuned to be passive in all frequencies, so that it can be plug and play connected to a grid regardless of grid impedance. Meanwhile, thanks to infinite gains of an RC at the fundamental frequency and its multiples as the high-quality grid injected current in accordance with IEC 61000-3-4 standard is ensured even in the presence of distorted grid conditions. Finally, experimental results with an established lab prototype are provided to verify the effectiveness of the proposed approach.